soil organic matter decomposition: effects of … · al qur’an 55: 10-13 . abstract relationships...

SOIL ORGANIC MATTER DECOMPOSITION: EFFECTS OF ORGANIC MATTER ADDITION

ON PHOSPHORUS DYNAMICS IN LATERITIC SOILS

A thesis submitted

by

Fadly Hairannoor Yusran

Ir. (Agriculture), Lambung Mangkurat University, Indonesia

M.Sc. (Agriculture), McGill University, Canada

for

the degree of Doctor of Philosophy

School of Earth and Geographical Sciences

Faculty of Natural and Agricultural Sciences

The University of Western Australia

2005

uÚ ö‘ F{ $# uρ $yγ yè|Ê uρ ÏΘ$tΡF|Ï9 ∩⊇⊃∪

…and the earth: He has put down for creatures.

$pκ Ïù ×π yγÅ3≈ sù ã≅ ÷‚ ¨Ζ9 $# uρ ßN# sŒ ÏΘ$yϑø. F{ $# ∩⊇⊇∪

Therein are fruits, date-palms producing sheathed fruit-stalks.

= pt ø:$# uρ ρèŒ É#óÁyèø9 $# ãβ$ pt ø†§9 $# uρ ∩⊇⊄∪

And also corn with its leaves and stalk for fodder, and sweet-scented plants.

Äd“r'Î6 sù Ï™Iω# u™ $yϑä3 În/ u‘ Èβ$t/ Éj‹s3 è? ∩⊇⊂∪

Then which of the blessings of your Lord will you deny?

Al Qur’an 55: 10-13

ABSTRACT

Relationships between the persistence of organic matter added to soil, the

dynamics of soil organic carbon (C) and phosphorus (P) were examined in four

experiments on lateritic soils of Western Australia. The main objective was to

quantify the release of P following organic matter application in soils which have

high P adsorbing capacity. Another objective was to confirm that due to its

recalcitrant materials, the effect of peat lasted longer in soil than other sources of

organic matter in terms of increasing plant-available P fractions. Three

experiments were conducted under glasshouse conditions for various lengths of

time, with nine- to twelve-month incubations to investigate these hypotheses.

As expected, organic matter with lower C:N ratios than peat (lucerne hay)

decomposed more rapidly compared with peat, and the most active

mineralisation took place within the first three months of incubation. Soil organic-

C (extracted by 0.5 M K2SO4) had a significant positive correlation with P

extracted with 0.5 M NaHCO pH 8.53 . For a higher application rate (120 ton

ha-1), peat was better than wheat straw and lucerne hay in increasing extractable

bicarbonate-P concentrations in soil, especially at incubation times up to 12

months. Throughout the experiment, peat was associated with a steady increase

in all parameters measured. In contrast to peat, nutrient release from lucerne

hay and wheat straw was rapid and diminished over time. There was a tendency

for organic-C (either in the form of total extractable organic-C or microbial

biomass-C) to steadily increase in soil with added peat throughout the

experiment. Unlike wheat straw and lucerne hay, extractable organic-C from

peat remained in soil and there was less C loss in the form of respiration.

Therefore, peat persisted and sequestered C to the soil system for a longer time

than the other source of organic matter.

Freshly added organic matter was expected to have a greater influence on P

transformation from adsorbed forms in lateritic soils than existing soil organic

matter. By removing the existing soil organic matter, the effect of freshly applied

organic matter can be determine separately from that of the existing soil organic

matter for a similar organic-C content. In order to do this, some soil samples

Abstract iii

were combusted up to 450° C to eliminate inherent soil organic matter. The

release of P was greater when organic-C from fresh organic matter was applied

to combusted soils than in uncombusted soils that contained the existing soil

organic matter. The exception only applied for parameters related to soil micro-

organisms such as biomass-C and phosphatase. For such parameters, new soil

organic matter did not create conditions favourable for organisms to increase in

activity despite the abundance of organic matter available. More non-extractable-

P was formed in combusted soils compared to bicarbonate-P and it contributed to

more than 50% of total-P. As for the first experiment, peat also showed a

constant effect in increasing bicarbonate extractable-P in the soil.

To follow the dynamics and loss of soil organic-C in the soil profile, a third

experiment was carried out to assess leaching of dissolved organic-C and its

effect on P status along the soil profile. A small percentage (1.43%) of C

leaching was recorded, but this was only one tenth of the C lost by soil

respiration. Despite the absence of bicarbonate-P and non-extractable-P

leaching, there was a trend of P mobility inside the leaching columns, especially

in sandy lateritic soil receiving higher rainfall. Hence, if lateritic soil is exposed to

simulated tropical rainfall for a longer period of time, P leaching is likely to occur.

Phosphate adsorption in the lateritic soils was significantly reduced by addition of

lucerne hay and to a lesser extent by addition of peat or wheat straw, but these

reductions only existed up to nine months after treatments. Soil combustion

increased phosphate adsorption capacity, but adding organic matter to such soils

released more bicarbonate-P. Bicarbonate-P release into soil solution was

therefore mainly a mineralisation process.

In summary, adding organic matter to lateritic soils appeared to be effective in

reducing the high sorbing capacity for phosphate and providing source of

phosphate by mineralisation. In contrast with more easily decomposed sources

of organic matter such as wheat straw and lucerne hay, a source of organic

matter such as peat, with low C:N content, not only reduced phosphate

adsorption, but also had high persistence in soil and had longer term effects on

the abundance N and P pools in these soils which is likely to prove more

beneficial for plant growth.

Abstract iv

ACKNOWLEDGEMENTS

I wish to express my sincere gratitude to my supervisors Dr. Andrew W. Rate and

Professor Lynette K. Abbott for their excellent supervision, dedicated support,

encouraging guidance, and meticulous advice rendered throughout the course of

this thesis. Their guidance made this research an enjoyable and fruitful

experience.

Thanks also due to all the staff and my friends in Soil Science and the Faculty of

Natural and Agricultural Sciences for their friendship and assistance during my

study. It would have been impossible to run my third experiment without the help

of Dr. Ir. Titiek Yulianti, M.Sc. and Ir. Irnanda Aiko Fifi Djuuna, M.Sc. I am

indebted to Dr. Daniel V. Murphy, Dr. Nui Milton, Dr. Mark G. Whitten, Dr.

Christoph Hinz, Said Munzir, M.Eng.Sc., Kevin Murray, M.Sc., Michael Smirk,

and Wanphen Wiriyakitnateekul for their valuable help and discussions.

Financial support from ADS (Australian Development Scholarship)-AusAID

through the Indonesian Government is gratefully appreciated. For this, Keith

Chambers and Rhonda Haskell provided excellent guidance.

My deepest appreciation goes to my wife, Ir. Hj. Nuri Dewi Yanti, M.Sc. for her

patience, understanding, encouragement, and assistance, despite being a

postgraduate student herself and a mother for Windi Bunga Devita and

Muhammad Hari Diputera. The same appreciation goes to them for being so

brave in dealing with many culture shocks during their childhood.

Last but not least, my lifelong gratitude goes to my parents, Ahmad Yusran

(deceased) and Siti Nurhayati who always support me in every step I make.

Rabbighfir lii wa liwaalidayya wa lilmu’miniina yauma yaquumul'hisaab,

Rabbirhamhumaa kamaa rabbayaanii shaghiiraa.

Acknowledgements v

TABLE OF CONTENTS

ABSTRACT................................................................................................ iii

ACKNOWLEDGEMENT .............................................................................v

CONTENTS ...............................................................................................vi

LIST OF FIGURES .................................................................................. xiii

LIST OF TABLES......................................................................................xx

LIST OF APPENDICES ......................................................................... xxvi

CHAPTER 1

1. INTRODUCTION

1.1 GENERAL INTRODUCTION......................................................... 1

1.2 AIMS OF RESEARCH................................................................... 3

1.3 ORGANISATION OF THE THESIS............................................... 3

CHAPTER 2

2 LITERATURE REVIEW

2.1 INTRODUCTION........................................................................... 5

2.2 LATERITIC SOILS ........................................................................ 6

2.2.1 Some important properties of Ultisols .................................... 6 2.2.2 Some important properties of Oxisols .................................... 7

2.3 SOIL ORGANIC MATTER............................................................. 8

2.3.1 The important of soil organic matter....................................... 9 2.3.1.1 Organic amendments .................................................................. 9

2.3.1.2 Contribution of organic matter to soil nutrients for plants ......................................................................................... 10

Table of Contents vi

2.3.1.3 Peat as a source of organic matter and carbon......................... 11

2.3.1.4 Effect of organic matter addition on soil properties and crop responses ................................................................... 12

2.3.2 Organic matter decomposition and persistence................... 15 2.3.2.1 The age of soil organic matter ................................................... 16

2.3.2.2 Loss and persistence of newly added organic matter................ 18

2.3.2.3 Priming effect............................................................................. 20

2.3.3 Pools of soil organic matter and its modelling...................... 21

2.4 PHOSPHORUS CYCLING IN RELATION TO SOIL ORGANIC MATTER................................................................ 25

2.4.1 Dissolution and precipitation ................................................ 27 2.4.2 Sorption and desorption....................................................... 28 2.4.3 Organic phosphorus............................................................. 30

2.4.3.1 Organic phosphorus classification ............................................. 31

2.4.3.2 Factors affecting organic phosphorus availability ...................... 32

2.4.4 Microbial biomass phosphorus ............................................ 34 2.4.5 Phosphatase........................................................................ 35 2.4.6 New and existing soil organic matter in phosphorus

supply .................................................................................. 35 2.4.6.1 Phosphate displacement ........................................................... 36

2.4.7 Phosphorus leaching ........................................................... 37 2.4.8 Summary points ................................................................... 39

2.5 CONCLUSION ............................................................................ 43

CHAPTER 3

3 TRANSFORMATION OF SOIL ORGANIC MATTER IN AN OXISOL IN RESPONSE TO THE APPLICATION OF VARIOUS SOURCES OF ORGANIC CARBON

3.1 INTRODUCTION......................................................................... 44

3.2 MATERIALS AND METHODS .................................................... 46

3.2.1 Design of the experiment ..................................................... 46 3.2.2 Procedures .......................................................................... 46

3.2.2.1 Soil sampling ............................................................................. 47

Table of Contents vii

3.2.3 Measurements ..................................................................... 47 3.2.3.1 Microbial biomass carbon.......................................................... 47

3.2.3.2 Soil respiration ........................................................................... 48

3.2.3.3 Ammonium and nitrate............................................................... 48

3.2.3.4 Bicarbonate phosphorus............................................................ 48

3.2.3.5 Extractable potassium ............................................................... 48

3.2.3.6 Soil pH ....................................................................................... 49

3.2.3.7 Water content ............................................................................ 49

3.2.3.8 Water holding capacity .............................................................. 49

3.3 RESULTS.................................................................................... 49

3.3.1 Carbon ................................................................................. 50 3.3.1.1 Extractable organic carbon........................................................ 50

3.3.1.2 Microbial biomass carbon.......................................................... 53

3.3.1.3 Soil respiration ........................................................................... 55

3.3.1.4 The loss of carbon as carbon dioxide ........................................ 57

3.3.2 Nitrogen ............................................................................... 58 3.3.2.1 Ammonium................................................................................. 58

3.3.2.2 Nitrate ........................................................................................ 61

3.3.3 Phosphorus.......................................................................... 61 3.3.4 Potassium ............................................................................ 64 3.3.5 Other chemical properties.................................................... 66

3.3.5.1 Soil pH ....................................................................................... 66

3.3.5.2 Water content ............................................................................ 68

3.3.5.3 Water holding capacity .............................................................. 70

3.3.6 Correlation matrices on all parameters ................................ 75

3.4 DISCUSSION.............................................................................. 80

3.4.1 The persistence of organic matter ....................................... 80 3.4.2 Phosphorus availability ........................................................ 81 3.4.3 Mineralisation of nitrogen and release of potassium............ 83

3.5 CONCLUSION ............................................................................ 84

Table of Contents viii

CHAPTER 4

4 EXISTING VERSUS ADDED SOIL ORGANIC MATTER IN RELATION TO PHOSPHORUS AVAILABILITY ON LATERITIC SOILS

4.1 INTRODUCTION......................................................................... 86

4.2 MATERIALS AND METHODS .................................................... 88

4.2.1 Design of the experiment ..................................................... 88 4.2.2 Procedures .......................................................................... 89

4.2.2.1 Soil sampling ............................................................................. 92

4.2.3 Measurements ..................................................................... 92 4.2.3.1 Dissolved organic carbon .......................................................... 92

4.2.3.2 Total phosphorus ....................................................................... 93

4.2.3.3 Bicarbonate phosphorus............................................................ 93

4.2.3.4 Non-extractable phosphorus...................................................... 93

4.2.3.5 Phosphatase.............................................................................. 93

4.2.3.6 Microbial biomass phosphorus .................................................. 94

4.2.3.7 Extractable aluminium and iron ................................................. 94

4.2.3.8 Water content and water holding capacity................................. 94

4.3 RESULTS.................................................................................... 94

4.3.1 Carbon ................................................................................. 94 4.3.1.1 Dissolved organic carbon .......................................................... 94

4.3.2 Phosphorus.......................................................................... 97 4.3.2.1 Bicarbonate extractable phosphate ........................................... 97

4.3.2.2 Total phosphorus ..................................................................... 100

4.3.2.3 Non-extractable phosphorus.................................................... 102

4.3.2.4 Phosphatase............................................................................ 104

4.3.2.5 Microbial biomass phosphorus ................................................ 107

4.3.3 Concentrations of different forms of phosphorus ............... 109 4.3.3.1 Subsurface soil ........................................................................ 109

4.3.3.2 Ultisol ....................................................................................... 109

4.3.3.3 Oxisol....................................................................................... 110

4.3.4 Other parameters............................................................... 114

Table of Contents ix

4.3.4.1 Soil pH ..................................................................................... 114

4.3.4.2 Extractable iron........................................................................ 116

4.3.4.3 Extractable aluminium ............................................................. 119

4.3.5 Correlations between all parameters ................................. 122

4.4 DISCUSSION............................................................................ 129

4.5 CONCLUSIONS........................................................................ 134

CHAPTER 5

5 SOIL ORGANIC CARBON LOSSES: THE BALANCE BETWEEN RESPIRATION AND LEACHING, AND PHOSPHORUS MOBILITY IN LATERITIC SOILS

5.1 INTRODUCTION....................................................................... 135

5.2 MATERIALS AND METHODS .................................................. 137

5.2.1 Design of the experiment ................................................... 137 5.2.2 Procedures ........................................................................ 138

5.2.2.1 Soil sampling ........................................................................... 139

5.2.3 Measurements ................................................................... 140 5.2.3.1 Bicarbonate phosphorus in soil................................................ 140


5.2.3.3 Dissolved reactive phosphorus in leachate ............................. 141

5.2.3.4 Soil respiration ......................................................................... 141

5.2.3.5 Total phosphorus in soil........................................................... 141

5.2.3.6 Total phosphorus in leachate................................................... 141

5.2.3.7 Dissolved organic carbon in soil .............................................. 142

5.2.3.8 Dissolved organic carbon in leachate ...................................... 142

5.2.3.9 Extractable aluminium and iron ............................................... 142

5.2.3.10 Leachate pH ............................................................................ 143

5.3 RESULTS.................................................................................. 143

5.3.1 Organic carbon mobility ..................................................... 143 5.3.1.1 Leachate carbon...................................................................... 143

5.3.1.2 Leachate pH ............................................................................ 145

5.3.1.3 Carbon throughout the leaching column.................................. 146

Table of Contents x

5.3.1.4 Soil respiration ......................................................................... 147

5.3.2 Balance on carbon losses between respiration and leaching ............................................................................. 150

5.3.3 Effect of leaching on phosphorus concentrations .............. 150 5.3.3.1 Phosphorus concentration in columns and leachate ............... 150

5.3.4 Factors affecting phosphorus transformation in the leaching columns ............................................................... 156

5.3.4.1 Extractable aluminium ............................................................. 156

5.3.4.2 Extractable iron........................................................................ 158

5.3.4.3 Soil pH ..................................................................................... 161

5.3.4.4 Correlations between all parameters in the leaching column ..................................................................................... 162

5.4 DISCUSSION............................................................................ 162

5.5 CONCLUSION .......................................................................... 165

CHAPTER 6

6 THE RELATIONSHIP BETWEEN PHOSPHATE ADSORPTION AND SOIL ORGANIC CARBON FROM ORGANIC MATTER ADDITION

6.1 INTRODUCTION....................................................................... 167

6.2 MATERIALS AND METHODS .................................................. 169

6.2.1 Design of the experiments ................................................. 169 6.2.2 Procedures ........................................................................ 170 6.2.3 Measurements ................................................................... 170

6.2.3.1 Phosphate adsorption isotherm............................................... 170

6.2.3.2 Numerical method.................................................................... 171

6.2.3.3 Dissolved organic carbon ........................................................ 172

6.2.3.4 Total phosphorus ..................................................................... 173

6.2.3.5 Bicarbonate phosphorus.......................................................... 173


6.2.3.7 Extractable aluminium and iron ............................................... 173

6.3 RESULTS.................................................................................. 173

6.3.1 The effect of incubation time.............................................. 174

Table of Contents xi

6.3.2 The effect of organic matter sources.................................. 176 6.3.3 The effect of organic matter levels ..................................... 178 6.3.4 The effect of existing and new soil organic matter ............. 180 6.3.5 The phospphate adsorption capacity of the three

soils ................................................................................... 183 6.3.6 Correlation on phosphate adsorption parameters

with dissolved organic carbon, extractable aluminium, and extractable iron ......................................... 186

6.4 DISCUSSION............................................................................ 187

6.5 CONCLUSION .......................................................................... 191

CHAPTER 7

7 GENERAL DISCUSSION AND CONCLUSION

7.1 BRIEF SUMMARY OF THE MAJOR FINDINGS AND THEIR INTERPRETATIONS................................................. 193

7.2 LIMITATIONS AND FUTURE RESEARCH NEEDS.................. 201

REFERENCES

APPENDIX

Table of Contents xii

LIST OF FIGURES

Figure 2.3-1 Steps in carbon cycle and the formation of soil organic matter and humus in aerobic soils. Source: Modifiied from Coyne (1999) and Grunwald (2004)............................................................. 14

Figure 2.3-2 Soil organic matter pools according to the CENTURY model. The soil organic matter submodel is part of the overall model in which carbon cycling is affected by climate, soils, management, and topography. Source: (Brenner et al., 2001)............................... 24

Figure 2.4-1 Soil phosphorus cycle, its components and measurable fractions (adapted from Stewart and Sharpley (1987)). Arrows represent fluxes between reservoirs. ................................................ 27

Figure 3.3-1 Comparative effect of organic matter sources, organic matter levels, and incubation times on extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ...................................................................................... 52

Figure 3.3-2 Comparative effect of organic matter sources, organic matter levels, and incubation times on microbial biomass carbon in an Oxisol from York, Western Australia. = control, = peat,

= wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph............. 54

Figure 3.3-3 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil respiration in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ................................... 56

Figure 3.3-4 Cumulative respiration representing total carbon loss following the application of organic matter during the experiment in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay.................................................... 57

Figure 3.3-5 Interaction effect of organic matter sources, organic matter levels, and incubation times on ammonium (A) and nitrate (B) in an Oxisol from York, Western Australia. = control, = peat,


List of Figures xiii

Figure 3.3-6 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil bicarbonate phosphorus in an Oxisol from York, Western Australia. = control, = peat,


Figure 3.3-7 Interaction effect of organic matter sources, organic matter, and incubation times on extractable potassium in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ................................... 65

Figure 3.3-8 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil pH in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and

= lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. .............................................. 67

Figure 3.3-9 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water content in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph. ................................... 69

Figure 3.3-10 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water holding capacity in an Oxisol from York, Western Australia. = control, = peat,


Figure 3.3-11 The effects of organic matter source on water content measured one week after watering in an Oxisol from York, Western Australia. C = control, P = peat, W = wheat straw, and L = lucerne hay. Solid line is mean of C1-C4. 1 = 40 ton ha-1, 2 = 80 ton ha-1, 3 = 120 ton ha-1, and 4 = 160 ton ha-1. Bar in each graph indicates LSD 5% for organic matter source in every graph................................................................................................. 73

Figure 3.3-12 The effects of the highest organic matter level (160 ton ha-1)

on water content in an Oxisol from York, Western Australia, measured one week after watering and maximum weekly temperature for Perth (Source: Aufdemkampe (2001)). = control, = peat, = wheat straw, and = lucerne hay. Solid line is maximum weekly temperature. Bar indicates LSD 5% for organic matter source. ...................................................................... 74

List of Figures xiv

Figure 4.3-1 Interaction effect of organic matter sources, type of organic matter, and incubation on dissolved organic carbon in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................... 96

Figure 4.3-2 Interaction effect of organic matter sources, type of organic matter, and incubation on bicarbonate phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph .......................................... 99

Figure 4.3-3 Interaction effect of organic matter sources, type of organic matter, and incubation on total phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................................... 101

Figure 4.3-4 Interaction effect of organic matter sources, type of organic matter, and incubation on non-extractable phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ....................................... 103

Figure 4.3-5 Interaction effect of organic matter sources, type of organic matter, and incubation on soil phosphatase in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................................... 106

Figure 4.3-6 Interaction effect of organic matter sources, type of organic matter, and incubation on soil microbial biomass phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ....................................... 108

Figure 4.3-7 Phosphorus status comparison between existing and new organic matter treatments in subsurface soil from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus....................................................................... 111

List of Figures xv

Figure 4.3-8 Phosphorus status comparison between existing and new organic matter treatments in an Ultisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay).

= total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus. .................................................................................... 112

Figure 4.3-9 Phosphorus status comparison between existing and new organic matter treatments in an Oxisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay).

= total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus. .................................................................................... 113

Figure 4.3-10 Interaction effect of organic matter sources, type of organic matter, and incubation on soil pH in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. .................................................................... 115

Figure 4.3-11 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable iron in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ......................................................... 118

Figure 4.3-12 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable aluminium in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph. ....................................... 121

Figure 5.2-1 Leaching column with six layers of soil and one layer of soil + organic matter. Suction apparatus (right part of the graph) was set at 10 cm water........................................................................... 140

Figure 5.3-1 Cumulative dissolved organic carbon in leachate expressed per mass of soil after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors. ........................................................... 144

Figure 5.3-2 Cumulative pH on leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors. ............................................ 146

List of Figures xvi

Figure 5.3-3 Dissolved organic carbon concentration throughout the leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. .............................................. 147

Figure 5.3-4 Soil respiration throughout the experiment with tropical rainfall. = control, = peat, = wheat straw, and = lucerne hay.

Bar in each graph represent LSD 5% for organic matter. ............... 149

Figure 5.3-5 Soil respiration throughout the experiment with subtropical rainfall. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph represent LSD 5% for organic matter.............................................................................................. 149

Figure 5.3-6 Bicarbonate phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. .............................................................. 152

Figure 5.3-7 Cumulative dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors............................................................................................... 153

Figure 5.3-8 Total phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. ................................................................................ 155

Figure 5.3-9 Non-extractable phosphorus distribution in leaching column with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. ................................................................................ 156

Figure 5.3-10 Cumulative dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors............................................................................................... 158

Figure 5.3-11 Cumulative dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors. ......................... 160

List of Figures xvii

Figure 5.3-12 Extractable aluminium and iron distribution in leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error. .............................................................. 161

Figure 5.4-1 Carbon losses (respiration and leaching) and deposition due to tropical rainfall (4000 mm year-1) in soil (Ultisol) column treated with wheat straw (80 ton ha-1). ........................................................ 166

Figure 6.3-1 The effect of incubation and organic matter sources on phosphate adsorption at 80 ton ha-1 organic matter in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Lines are a logarithmic trend of observations and a visual guide only. ............. 175

Figure 6.3-2 Linearised Langmuir (A) and Freundlich (B) plots of incubation effect on phosphate adsorption for peat treatment (80 ton ha-1) in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).................................. 176

Figure 6.3-3 The effect of organic matter sources of and their levels on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Lines are a logarithmic trend of observations and are a visual guide only........................................ 177

Figure 6.3-4 Linearised Langmuir (A) and Freundlich (B) plots of organic matter sources (120 ton ha-1) on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).................................. 178

Figure 6.3-5 Linearised Langmuir (A) and Freundlich (B) plots of peat levels at six months incubation time on phosphate adsorption. = 40, = 80, = 120, and = 160 ton ha-1 in “Balkuling” Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05)................... 179

Figure 6.3-6 The effect of soils, existing and new soil organic matter on phosphate adsorption in peat and lucerne hay treatments at 80 ton ha-1 and six-month incubation time. = subsurface soil, = Ultisol, and = Oxisol. Lines are polynomial trend for observations and are a visual guide only........................................ 182

List of Figures xviii

Figure 6.3-7 Linearised Brunauer-Emmett-Teller equation of soil type (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption in new soil organic matter + peat treatment. = subsurface soil,

= Ultisol, and = Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.01). .................................................................... 184

Figure 6.3-8 Linearised Brunauer-Emmett-Teller equation of soil organic matter type (existing and new) on phosphate adsorption in subsurface soil. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05)................... 185

Figure 6.3-9 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Ultisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05). .................................................... 185

Figure 6.3-10 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Oxisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05)................... 186

List of Figures xix

LIST OF TABLES

Table 2.3-1 Recent literature on extent to which addition of peat affects a range of physical, chemical and biological properties of soils .......... 12

Table 2.4-1 Half life comparison between three groups of organic phosphorus in soils ........................................................................... 32

Table 3.3-1 Some physical and chemical characteristics of the Oxisol (Balkuling soil) from York, Western Australia.................................... 50

Table 3.3-2 Some chemical characteristics of organic matter sources for the three organic matter treatments.................................................. 50

Table 3.3-3 Analysis of variance for extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. Only main factors and their interactions are shown ........................................... 51

Table 3.3-4 Analysis of variance for microbial biomass carbon in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 53

Table 3.3-5 Analysis of variance for soil respiration in an Oxisol from York, Western Australia. Only main factors and their interactions are shown................................................................................................ 55

Table 3.3-6 Total carbon loss (%) during the experiment (48 weeks) in an Oxisol from York, Western Australia, calculated from the initial total carbon in soils ........................................................................... 58

Table 3.3-7 Analysis of variance for ammonium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown................................................................................................ 59

Table 3.3-8 Analysis of variance for nitrate in an Oxisol from York, Western Australia. Only main factors and their interactions are shown ......... 59

Table 3.3-9 Analysis of variance for bicarbonate phosphorus in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 62

Table 3.3-10 Analysis of variance for extractable potassium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 64

List of Tables xx

Table 3.3-11 Analysis of variance for soil pH in an Oxisol from York, Western Australia. Only main factors and their interactions are shown................................................................................................ 66

Table 3.3-12 Analysis of variance for soil water content in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 68

Table 3.3-13 Analysis of variance for water holding capacity in an Oxisol from York, Western Australia. Only main factors and their interactions are shown...................................................................... 71

Table 3.3-14 Correlation matrices on all soil parameters, presented according to incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01) ............................................................... 75

Table 3.3-15 Correlation matrices on all soil parameters overall incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01) ....... 79

Table 4.2-1 Description of soil used and locations in Western Australia where soils were collected ................................................................ 88

Table 4.2-2 Organic matter sources (peat, wheat straw, and lucerne hay) used as treatments in the experiment............................................... 89

Table 4.2-3 Treatment combinations and abbreviations used from three factors of the experiment. Soil (subsurface soil, Ultisol, and Oxisol), source of organic matter (control, peat, wheat straw, and lucerne hay), and type of soil organic matter (existing and new) .................................................................................................. 89

Table 4.2-4 Characteristics of three soils (subsurface soil, Ultisol, and Oxisol) used in the glasshouse incubation....................................... .90

Table 4.2-5 Some characteristics of soil (subsurface soil, Ultisol, and Oxisol) samples before and after combustion. Values after ± are standard errors. .......................................................................... 91

Table 4.3-1 Analysis of variance for dissolved organic carbon in three soils from Western Australia. Only main factors and their interactions are shown ......................................................................................... 95

Table 4.3-2 Analysis of variance for bicarbonate phosphorus in three soils from Western Australia. Only main factors and their interactions are shown ......................................................................................... 98

Table 4.3-3 Analysis of variance for total phosphorus in three soils from Western Australia. Only main factors and their interactions are shown.............................................................................................. 100

List of Tables xxi

Table 4.3-4 Analysis of variance for non-extractable phosphorus in three soils from Western Australia. Only main factors and their interactions are shown.................................................................... 102

Table 4.3-5 Analysis of variance for phosphatase activity in three soils from Western Australia. Only main factors and their interactions are shown.............................................................................................. 105

Table 4.3-6 Analysis of variance for microbial biomass phosphorus in three soils from Western Australia. Only main factors and their interactions are shown.................................................................... 107

Table 4.3-7 Analysis of variance for pH in three soils from Western Australia. Only main factors and their interactions are shown ....... 114

Table 4.3-8 Analysis of variance for extractable iron three soils from Western Australia. Only main factors and their interactions are shown.............................................................................................. 117

Table 4.3-9 Analysis of variance for extractable aluminium in three soils from Western Australia. Only main factors and their interactions are shown ....................................................................................... 120

Table 4.3-10 Correlation matrices on all soil parameters. Presented according to incubation times. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation...................................................... 123

Table 4.3-11 Correlation matrices between phosphorus, carbon, iron and soil pH for different incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation...................................................... 126

Table 4.3-12 Correlation matrices between phosphorus, carbon, iron and soil pH for different incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation...................................................... 127

Table 4.3-13 Correlation matrices between phosphorus, carbon, iron and soil pH, separated into soil types and into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represent a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation .............................................................................. 128

Table 5.2-1 Treatment combinations and their abbreviation from two types of soils (Ultisol and Oxisol), two rainfall simulations (tropical and subtropical), and four sources of organic matter (control, peat, wheat straw, and lucerne hay) ........................................................ 138

List of Tables xxii

Table 5.3-1 Analysis of variance for dissolved organic carbon in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Only main factors and their interactions are shown ... 143

Table 5.3-2 Cumulative carbon loss (%) from leaching following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical) ..................................................................................... 144

Table 5.3-3 Analysis of variance for leachate pH after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment................... 145

Table 5.3-4 Analysis of variance for dissolved organic carbon throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 147

Table 5.3-5 Analysis of variance for soil respiration after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment................... 148

Table 5.3-6 Respiration estimates (mg CO2) during the experiment for every source of organic matter (control, peat, wheat straw, and lucerne) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical).............................................. 149

Table 5.3-7 Cumulative carbon loss (%) from soil respiration following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical) .............................................................................. 150

Table 5.3-8 Balance between respiration and leaching on every treatment, calculated from loss (%) due to respiration divided by loss (%) due to leaching................................................................................ 150

Table 5.3-9 Analysis of variance for bicarbonate phosphorus throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ................................ 151

Table 5.3-10 Analysis of variance for dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ..................................................................... 152

List of Tables xxiii

Table 5.3-11 Cumulative dissolved reactive phosphorus (mg kg-1) from leaching following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical).............................................. 154

Table 5.3-12 Analysis of variance for total phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 154

Table 5.3-13 Analysis of variance for total phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ...................................................................................... 155

Table 5.3-14 Analysis of variance for non-extractable phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown.............................................................................................. 157

Table 5.3-15 Analysis of variance for extractable aluminium throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 157

Table 5.3-16 Analysis of variance for dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ...................................................................................... 159

Table 5.3-17 Analysis of variance for extractable iron throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ... 159

Table 5.3-18 Analysis of variance for dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment ...................................................................................... 161

Table 5.3-19 Analysis of variance for soil pH throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown ................................ 162

List of Tables xxiv

Table 6.3-1 The effect of organic matter sources (80 ton ha-1) and incubation time on phosphate adsorption parameters based on Langmuir and Freundlich equations................................................ 179

Table 6.3-2 The effect of organic matter sources and levels on phosphate adsorption parameters at six months incubation time based on Langmuir and Freundlich equations................................................ 180

Table 6.3-3 The effect of soil organic matter types (existing and new) and soil types (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption parameters based on fitting data to the Brunauer-Emmett-Teller equation................................................................... 183

Table 6.3-4 Correlation between inorganic phosphorus, dissolved organic carbon, extractable aluminium, and extractable iron in uncombusted soils treated with peat and lucerne hay treatments. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correalation ............................................................................ 187

Table 6.3-5 Correlation between phosphorus adsorption parameters and soil chemical properties in “Balkuling” Oxisol. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation.............................. 187

Table 7.1-1 Summary of findings in all experiments in this thesis .................... 195

List of Tables xxv

LIST OF APPENDICES

Appendix A Linearisation of Brunauer-Emmett-Teller equation ....................... 244

Appendix B Article for 17th World Congress of Soil Science, Bangkok 14-21 August 2002.................................................................................... 246

Appendix C Pictures from some experiments .................................................. 257

List of Appendices xxvi

C h a p t e r 1

INTRODUCTION

1.1 GENERAL INTRODUCTION

Marginal soils such as lateritic soils often become the most readily available

alternative to increase the area of agricultural land to maintain food security. This

situation occurs in developing countries of the tropics due to population pressure.

Food shortages occur not only because of the limited production and yield of the

crops, but also due to shrinkage of the production area when productive lands

are used for housing or industrial purposes. One way to address this problem is

to expand agricultural production into areas with lower soil quality, such as

lateritic soils.

Lateritic soils can be low in phosphorus (P) availability for plant growth due to

their high content of aluminium (Al) and iron (Fe)-oxides which are able to adsorb

phosphate from added fertilisers (Buol and Eswaran, 2000; West et al., 1998).

Soil organic matter can decrease the affinity of Al and Fe-oxides for phosphate

and provide biochemical conditions suited for making P more soluble (Dubus and

Becquer, 2001; Haynes and Mokolobate, 2001; Maguire and Sims, 2002).

However, the persistence of organic matter in soil is an important issue as

artificial sources of organic matter of agricultural origin often decompose rapidly,

especially in tropical areas (Chuyong et al., 2000; da Silva and Cook, 2003).

Furthermore, the effectiveness of newly applied organic matter in alleviating P

deficiency is limited, as early decomposition processes are not necessarily

favourable for P to be mineralised or transformed from organic-P to inorganic-P

(Iyamuremye and Dick, 1996).

Peat is an abundant source of organic matter which might be suitable for

reducing the affinity of sesquioxides for phosphate in the longer term because it

is composed of more recalcitrant substances than agricultural organic matter,

allowing it to withstand microbial attack and delay microbial decomposition.

However, the low nutrient content in peat, including P, is a limiting factor for peat

to be applied as organic amendment. Therefore, organic matter sources with

higher nutrient content applied with peat may be a good combination in the soil in

I. General Introduction 1

order to release phosphate by mineralisation or competitive adsorption in the

longer term.

Processes involved in interactions between soil solutes and the soil solution such

as ligand exchange, sorption, and desorption, may be considered as being

directly involved in P transformation in association with organic matter. Some soil

micro-organisms play an important role in mediating P dynamics in soils,

especially where P input from fertilisers is limited (Beck and Sanchez, 1994; Yao

et al., 2002). Immobilisation of soluble phosphate and the promotion of P

mineralisation by production of phosphatase are among the biochemical

processes involving soil organic matter which affect P transformation.

Several studies have concluded that the loss of carbon (C) as dissolved organic-

C via leaching could reach 50 % of the total C loss from soil (Cronan, 1985;

Magill and Aber, 2000). However, these studies mainly concern C loss at the soil

surface. Investigation of C leaching within the soil profile is crucial, especially in

areas with very high annual rainfall. Carbon transfer due to leaching through the

soil profile is important because soluble organic-C may affect soil chemical

properties such as sesquioxide concentrations. For lateritic soils in tropical

areas, heavy rainfall not only causes nutrient leaching (Duwig et al., 2000; Lessa

and Anderson, 1996) but also removes organic substances (Haberhauer et al.,

2002) which may affect sorption and desorption of nutrients such as phosphate.

Soil organic matter can change the phosphate fixing capacity of some soils (Erich

et al., 2002; Iyamuremye and Dick, 1996; Kwabiah et al., 2003; Ohno and

Crannel, 1996). Several mechanisms have been proposed to explain how soil

organic matter affects phosphate adsorption, either due to biotic or abiotic

processes (Iyamuremye and Dick, 1996). In biotic processes, soil organic matter

affects P mineralisation and transformation (Frossard et al., 2000; Magid et al.,

1996), and abiotic processes affect P dynamics via mechanisms such as organic

ligand exchange (Hinsinger, 2001; Violante and Gianfreda, 1993), dissolution

(MacKay et al., 1986; Traina et al., 1986), and desorption (Burkitt et al., 2002;

Rhue and Harris, 1999).

Organic-C leaching may lead to loss of applied organic matter and, at the same

time, may affect P dynamics throughout the soil profile. Interactions between soil

solutes and the soil solution where the process of P sorption and desorption

occur need to be investigated, especially when lateritic soils are used being used.


This is not only because of higher rainfall in the tropics where lateritic soils are

common, but also due to high phosphate sorbing capacity of the soils. Any

novel information within the frame of limiting factors of these soils is needed to be

repeatedly investigated for the final results in the applied knowledge according to

the specific areas.

1.2 AIMS OF RESEARCH

The main aims of this study were:

1. To measure the persistence of soil organic matter with different C:N ratio over

time in lateritic soils, especially after addition organic matter. At the same

time contributions of organic matter addition to the availability of nutrients N,

P, and potassium (K) were also measured, as well as C dynamics were

measured.

2. To study the contribution of recently applied organic matter to the P status of

lateritic soils. In this regard, the question of whether microbial biomass

phosphorus and soil organic-C will generate more inorganic-P in a low P

environment was addressed, as well as whether phosphatase becomes more

active indicating a transformation from organic-P to inorganic-P.

3. To study the effect of simulated rainfall regimes relevant to tropical and

subtropical environments, on the loss of C by respiration and leaching from

the lateritic soil profile, and to measure the effect of leaching on the change in

soil organic-C, phosphate, and extractable-Al and Fe in the soil.

4. To study the long-term effect of added organic matter with contrasting C:N

ratios on phosphate adsorption across soil type, sources, and level of organic

matter applied. In addition, the effect of phosphate adsorption and P

mineralisation after organic matter addition on the process of bicarbonate-P

release and the alteration of dissolved organic-C after organic matter

treatment on closely related factors of phosphate adsorption in soil, e.g. Al,

Fe, and pH was investigated.

1.3 ORGANISATION OF THE THESIS

Chapter 2 reviewed the role of soil organic matter in highly weathered soils, its

rapid decomposition process, and its persistence. It also considered soil organic


matter in relation to tropical conditions as well as the main properties of tropical

lateritics such as Ultisols and Oxisols. Peat was evaluated as a source of applied

organic matter in relation to its effects on soil properties. Chapter 3 investigated

the effect of organic matter sources (control, peat, wheat straw, and lucerne hay),

organic matter levels (40, 80, 120, and 160 ton ha-1), and incubation times (3, 6,

9, 12 months) on organic matter persistence and soil nutrient status. Chapter 4

covered in more detail the effectiveness of freshly added organic matter as a

nutrient source and/or as an agent of transformation in lateritic soil. In order to

study the effect (i.e. separated from intrinsic soil organic matter) of recently

applied organic matter (peat, wheat straw, and lucerne hay), soil samples were

first combusted (450° C) to remove the inherent soil organic matter. Subsurface

regolith material with low intrinsic organic matter, an Ultisol, and an Oxisol were

included in this experiment and organic matter was incubated in the soils for 3, 6,

or 9 months. Phosphorus transformation and dynamics were examined in detail

as the lateritic soils have low capacity to supply P in a form that plants can use.

As soil organic-C can be associated with P transformation (Chapters 3 and 4),

the experiment in Chapter 5 investigated how the process of leaching dissolved

organic-C also might affect P dynamics for different simulated rainfall regimes

(tropical and subtropical). Soil type (Ultisol and Oxisol) and organic matter

sources (peat, wheat straw, and lucerne hay) were used to study C dynamics and

its effects on P solubility in leaching columns. Organic forms of P were

considered as non-extractable-P in this study. The most important mechanism

controlling P availability in lateritic soils, i.e. phosphate adsorption, was discussed

in Chapter 6. This experiment used soil samples from the first and second

experiment to determine the effects of added organic matter on phosphate

adsorption. Factors related to the adsorption of P were investigated in three soils

(Oxisol, Ultisol, and subsurface soil from mining site), different sources of organic

matter (peat, wheat straw, and lucerne hay), and four application rates (40, 80,

120, and 180 ton ha-1). The thesis concludes in Chapter 7 with a General

Conclusion. In this chapter, general findings in the experimental scenarios are

discussed, as well as future research.


C h a p t e r 2

LITERATURE REVIEW

2.1 INTRODUCTION

The loss of soil organic matter in agricultural lands, usually by erosion and/or

rapid mineralisation, is often considered to be the most serious factor in causing

soil degradation (Craswell and Lefroy, 2001; Gregorich et al., 1994; Katyal et al.,

2001; Paustian et al., 1997). This loss can have detrimental effects on soil

physical, chemical, and biological properties. Indeed, maintaining and improving

soil organic matter content is generally accepted as being an important objective

of any sustainable system of agriculture (Gregorich et al., 1994; Kobayashi et al.,

1999; Rodolfi and Zanchi, 2002).

In most developing countries, extending the area of agricultural land is a major

problem because agricultural areas are under the pressure of high rates of

population growth and the expansion of urban areas into productive agricultural

soils. Consequently, soils with low fertility tend to be the most likely alternative

for expanding agricultural development. However, in many developing countries,

the knowledge required for exploiting these soils is far behind that of existing

agricultural soils.

Lateritic soils such as Ultisols and Oxisols, which are commonly low in soil

organic matter, are acidic, have limited cation exchange capacity, and have low

nutrient status could become more suitable for food crops if levels of soil organic

matter are raised. This could be achieved by amending soil with various sources

of organic matter. More specifically, the transformation of soil organic carbon

during the decomposition of organic matter allows soil to provide nutrients for

plants, especially P. In this respect, soil organic carbon (C) has been known to

influence phosphate adsorption (Brennan et al., 1994; Erich et al., 2002; Leytem

et al., 2002) and to be positively correlated with phosphatase activity (Baligar et

al., 1999).

Peat as a source of organic matter may be useful as an organic amendment in

managing unfertile lateritic soils. Broad scale utilisation of peat, i.e. as a growth

medium in forestry nurseries (Gad, 2003; Prasad and Maher, 2004) and as a soil

II. Literature Review 5

conditioner (Pietola and Tanni, 2003), might also resolve the limited content of

soil organic matter in lateritic soils, especially in a tropical country such as

Indonesia. There are abundant peat deposits in Sumatra, Borneo, and

Indonesian Papua (Abdurachman and Suriadikarta, 2000; Jauhiainen and

Vasander, 2002; Page et al., 2002; Whinam et al., 2003). There are also vast but

ignored areas of lateritic soil which might become the next alternatives for

agricultural production to secure food supplies in these areas.

2.2 LATERITIC SOILS

Lateritic soils are very common in tropical regions (Eswaran, 1993). Those in

high rainfall areas are highly weathered with inorganic colloids being mainly

kaolinitic with significant amounts of (hydr)oxides, especially those of iron (Fe)

and aluminium (Al) (Buol and Eswaran, 2000; Eswaran, 1993; West et al., 1998).

Because of the high ambient temperatures throughout the year and the

abundance of water, the turnover of soil organic matter is rapid (Rezende et al.,

1999; da Silva and Cook, 2003; Silver, 1998), with a half life of 9-33 days

(Rezende et al., 1999). However, soil organic matter associated with inorganic

colloids in these soil environments appears to have a considerable turnover time

in soil environments, ranging from 14-275 years (Monreal et al., 1997). This kind

of soil organic matter may have an important role in the reactivities of the soil

colloidal component and in soil fertility, especially in lateritic soils in the tropics.

2.2.1 Some important properties of Ultisols Ultisols are part of a group of soils with an argillic and/or kandic horizon that have

developed in a humid climate (West et al., 1998). Important features of these

soils are: (1) the parent material contains minerals which weather to form silicate

clays, and (2) the climate during soil development characteristically has more

precipitation than evapotranspiration. Ultisols are common in tropical and

subtropical areas between 40° N and 40° S (Eswaran, 1993).

The main characteristic of Ultisols that differentiate them from other soils is that

they must have 35 % or less base saturation in the lower part of the subsoil (Soil

Survey Staff, 1975). This defining characteristic is related to other properties

such as low pH, low cation exchange capacity, and high Al saturation, resulting in

negative effects of the ability of these soils to sustain agricultural plant growth.


Therefore, chemical properties are the most commonly discussed limiting factors

in managing Ultisols for plant production.

With regards to cation exchange capacity, Ultisols have permanent negative

charges from isomorphic substitution of cations within the clay, but they also have

variable charges (Barnhisel and Bertsch, 1989; Wada and Wada, 1985), which

are positive at low pH. Consequently, most Ultisols are expected to have very

limited net negative charge and this influences on nutrient retention, especially for

cations.

2.2.2 Some important properties of Oxisols Oxisols are characterised by the existence of oxic horizons which usually have a

minimum of 15% clay (Buol and Eswaran, 2000; El Swaify, 1980). Physical

properties of Oxisols are determined by their sesquioxides and kaolinite

mineralogy. The fine and very fine granular structure is very porous and leads to

low bulk density which is generally between 1.0-1.3 Mg m-3 (El Swaify, 1980).

Oxic material with high oxide content is generally not sticky, and can be

hydrophobic to some extent. Water moves rapidly through the large pores

between aggregates. The combination of high porosity (Tejedor et al., 2003) and

low wettability (Scott, 2000) can make these soils susceptible to erosion (El

Swaify, 1980; Scott, 2000; Tejedor et al., 2003), leading to loss of organic matter.

Oxisols have a low capacity to retain cations (Buol and Eswaran, 2000;

Krishnaswamy and Richter, 2002). Cation exchange capacity for these soils

arises from kaolinitic clays and organic matter, it is pH dependent, and effective

cation exchange capacity values are less than cation exchange capacity at pH 7.

Oxisols with substantial content of Fe-oxides have a high fixation capacity for

phosphate applied from fertilizers (Haynes and Mokolobate, 2001; Leytem et al.,

2002). Oxisols also have low quantities of essential elements for plant growth

(Melgar et al., 1992; Moraghan and Mascagni, 1991). As a consequence,

Oxisols and Ultisols are usually the next options in the reclamation program for

agricultural development for many countries in tropical regions. Managing these

soils requires comprehensive knowledge in order to gain profits from their

management, not only for agricultural products but also for sustainability of the

soil resource.


2.3 SOIL ORGANIC MATTER

The term ‘soil organic matter’ has been widely used to describe the organic

components in soils. The initial concept of soil organic matter refers to the whole

of the organic material in soils, including litter, light fraction, microbial biomass,

water soluble organics, and stabilised organic matter (humus) (Baldock and

Nelson, 1998; Stevenson, 1994). Soil organic matter is defined as the total of all

organic materials contained within and on soils (Baldock and Nelson, 1998), as

well as non-decayed plant and animal tissues, their partial decomposition

products and the living soil biomass (MacCarthy et al., 1990).

Soil organic matter investigations, have been established for decades. The

importance of soil organic matter as a source and sink of C has also been known

for some time (Lal, 1999; Lal, 2001a). Considerable recent research on soil

organic matter has been focused on the dynamics of dissolved organic-C,

ranging from description and chemical composition (Kaiser et al., 2001; Strobel et

al., 2001), quantification and its role in soil chemistry and pedogenesis (Jansen et

al., 2003; Kaiser and Zech, 1998), and the availability of dissolved organic-C to

soil microflora (Kalbitz et al., 2003; Yano et al., 2000). Moreover, knowledge of

dissolved organic matter, as well as dissolved organic nitrogen (N), is well

documented (McDowell, 2003). However, the relationship between these

processes and soil phosphorus (P) is less well understood, especially dissolved

organic-P. In agricultural soils where fertiliser input was regular, dissolved

organic-P was about 70% of total-P in solution (Chardon et al., 1997), and

dissolved organic matter was responsible for redistribution and loss of P in forest

soils (Donald et al., 1993). Dissolved organic-C is in close contact with soil

particles that adsorb phosphate so it is expected that this pool is important in

controlling P dynamics in soil solid and soil solution interactions.

In lateritic soils, soil organic matter is a major contributor to the soil exchange

capacity (Buol and Eswaran, 2000; Pushparajah, 1998; West et al., 1998). Since

the clays in lateritic soils are mainly kaolinitic (Allen and Fanning, 1983; West et

al., 1998), soil organic matter is a major contributor to the negatively charged

colloids and plays an important role in soil chemical (Pushparajah, 1998;

Schnitzer, 2000) and biological properties (Marinari et al., 2000).


2.3.1 The importance of soil organic matter Organic matter content in highly weathered soils is generally low (Buol and

Eswaran, 2000; Pushparajah, 1998; West et al., 1998). Critical factors include:

climate, where warmer temperature and high rainfall create the fast breakdown of

organic residues, and erosion where soil organic matter is lost (West et al.,

1998). Erosion may also lead to leaching of some forms of organic matter. In

this case, there must be an excess of precipitation in relation to the capacity of

the soil to retain water, so that water will also percolates through the solum

(Miller, 1983). As a consequence, highly weathered soils commonly occur in

warm areas from the humid tropics to humid warm temperate climates.

2.3.1.1 Organic amendments The application of organic matter to soil to improve soil physical, chemical, and

biological properties (Anikwe and Nwobodo, 2002; Khalilian et al., 2002; Larbi et

al., 2002) has been a practice since prehistoric time (Kleber et al., 2003). In the

past, the types of organic matter applied were generally manure, green manure,

compost, crop residues, and to some extent sewage sludge and biosolids.

Today, the term organic amendment has become broader in meaning and

includes materials from various terrestrial and marine sources such as fish bone

meal and crab meal.

The use of organic amendments is increasing with the development of organic

farming and the increase is even higher among conventional farmers (Hartz et

al., 2000). This is not only because of social pressure for healthy food under

conditions that protect the environment, but also as a result of pressure for

recycling organic resources (Thuries et al., 2001).

Among physical properties, organic matter enhances soil particle aggregation for

better water permeability and gas exchange (Poch et al., 2000). Organic matter

increases water retention by preventing shrinking and drying (Hajnos et al., 2003;

Stehouwer, 2003). The black colour of organic matter may facilitate soil warming

in temperate regions by balancing the radiative heat (Schmidt and Noack, 2000).

In relation to chemical characteristics, organic matter increases cation exchange

capacity and buffering capacity to minimise changes in solute concentrations and

pH. Soil organic matter can also adsorb and buffer trace soil components

(Barancikova et al., 2004; Burt et al., 2003; Minkina et al., 2000). In addition, an

improved biological environment in soil results from organic matter addition


increasing microbial activity, leading to mineralisation and enhanced availability

of nutrients such us N, P, potassium (K), and sulphur (S) for plant growth

(Fortuna et al., 2003; Krishna, 2002; Williamson and Wardle, 2003). These

interactive effects of soil organic matter will be discussed in the following

sections.

Overall, organic amendments are considered the most beneficial for soils which

have low organic matter contents since the small amount of application can

improve soil characteristics of importance to agricultural production.

2.3.1.2 Contribution of organic matter to soil nutrients for plants Organic matter affects nutrient availability for plants directly and indirectly

(Stevenson, 1994). Organic matter is a source of N for plants when mineralised

(Parfitt et al., 1998; Russell and Fillery, 1999), a process which also supplies P

(Parfitt et al., 1998) and S (Blair et al., 1994; Eriksen et al., 1995). The amounts

of each element released during mineralisation, and the rate of release, depend

on the content of the element and elemental ratios in the biomass, which reflects

the origin of the organic matter. Indirectly, organic matter contributes to the

mineral nutrition of plants in soils through incorporation of N and S into humic

substances during decomposition, or by complexation of calcium (Ca), Al, and Fe

from their respective phosphates by humic substances to increase concentrations

of soluble phosphate (Stevenson, 1994).

Incorporation of N (Kelley and Stevenson, 1995) and S (Brown, 1986; Xia et al.,

1998) into humic substances, as well as P (Cooper et al., 1998; Singh and

Amberger, 1990), keeps the nutrients from volatilising (except P) and leaching

and also provides those nutrients to plants for longer periods of time.

Furthermore, there is a relationship between those nutrients within organic matter

which has been described as a definite ratio. The C:N ratio of organic matter has

been used as an indicator for the maturity of compost (Contreras-Ramos et al.,

2004; Priya and Garg, 2004) and the stage of C sequestration in soils (Tan et al.,

2004). The average C/N/P/S ratio of 140:10:1.3:1.3 was claimed to be an

optimum for those nutrients to sustain plant growth (Stevenson, 1994).

Another key reason why organic matter can participate in a wide range of

chemical reactions in soils is due to the presence of oxygen-containing functional

groups (-CO2H, -OH, C=O). These functional groups are capable of enhancing

dissolution of soil minerals by complexing and dissolving metals, transporting


them throughout the soil solution, and making them available for plants and

microbes (Schnitzer, 2000). Interactions between soil organic matter and metal

ions include ion exchange, surface adsorption, chelation, peptisation, and

coagulation reactions (Schnitzer, 1978).

2.3.1.3 Peat as a source of organic matter and carbon Peat contains 0.551 x 1014 kg C globally (Brake et al., 1999), which is equal to

one quarter of the total organic-C reserve of world soils (Bohn, 1976). Peat

deposits are located mainly in cold and humid regions of Europe and North

America (Charman, 2002; Weiss et al., 2002), but they also exist over thousands

of km2 in equatorial zones, including Indonesia (Nichol, 1999; Page et al., 2002).

Compared with peat in Europe and America, tropical peat is less well studied

(Sieffermann et al., 1988).

Peat is widely used for a variety of purposes such as industrial or domestic fuel

(Korpilahti, 1998; McGovern et al., 2000). In agriculture, peat is useful as an

ingredient for compost mixtures. This mixture is able to retain not only water but

also oil up to seven times its weight (Viraraghavan et al., 1987). Peat can also be

used as an effluent treatment in industrial and municipal landfills to decrease the

content of pollutants and eutrifiers (Lyons and Reidy, 1997). For the purpose of

water treatment, peat can consume 35-50% biological oxygen demand (BOD)

and chemical oxygen demand (COD) after two-hour contact in septic tanks

(Viraraghavan et al., 1987).

Interest in the function of peat soils in global C turnover has also increased due to

the threat of global climate changes (Lal, 1999; Lal, 2001a; Reicosky, 2001;

Rosenzweig and Hillel, 2000), especially in term of the sources and sinks for CO2

and other gases (e.g. CH4)produced by peat land (Karjalainen et al., 1998;

Moore, 2002; Schilstra, 2001). Recently, different peat forming conditions such

as concave wetland and waterlogged organic deposit have been found to be

potential sources of CO2, CH4, and N2O to the atmosphere (Martikainen, 1996;

Weber et al., 2003).

The application of peat as an organic amendment has been studied to assess the

impact on soil physical (Cook and Baker, 1998; McCoy, 1998; Paul and Lee,

1976), chemical (Cook and Baker, 1998; Laverdiere and Kimpe, 1984), and

biological properties (Fernandez et al., 1999). Peat has also been used as a raw

material for some agricultural products. For example, organo-mineral fertilizers


containing humic substances such as ammoniated peat could reduce the

application of fertilizers. A slow-release fertilizer from the ammoniation of peat

can produce yields of field crops similar to those using mineral fertilizer (Abbes et

al., 1996; Abbes et al., 1995; Coca et al., 1984). Overall, peat could benefit soils

through effects related to (1) surface reaction with clay-size particles, (2)

chelation of polyvalent cations, and (3) biostimulation of micro-organisms, by

similar mechanisms to intrinsic soil organic matter (Johnston et al., 1997; Scagel,

2003; Torun et al., 2003). Table 2.3-1 summarises examples of individual effects

of peat on soil properties.

Table 2.3-1 Recent literature on extent to which addition of peat affects a range of physical, chemical and biological properties of soils

Soil property Effect Author Aggregate stability +

+ (Almendros, 1994) (Muggler et al., 1999)

Hydraulic conductivity + +

(Baker et al., 2000) (Nkongolo et al., 2000)

Physical

Porosity + +

(Baker et al., 2000) (Nkongolo et al., 2000)

Cation exchange capacity + (Almendros, 1994) (Balasoiu et al., 2001)

pH buffering capacity + (Balasoiu et al., 2001) Metal retention (Cu and Cr) + (Balasoiu et al., 2001) Water repellency + (Waniek et al., 2000) As adsorption - (Grafe et al., 2001)

Chemical

Al toxicity - (Pushparajah, 1998) 59Fe uptake + (Cesco et al., 2002) P uptake + (Cooper et al., 1998) Plant yield +

- (Almendros, 1994) (Nkongolo et al., 2000)

Plant growth + (Pushparajah, 1998) Dry weight + (Clapp et al., 1998) Microbial biomass carbon + (Willson et al., 2001)

(Manjaiah and Dhyan, 2001)

Biological

Bacteria and algae colonisation

+ (Beyer et al., 2001)

+ = increase - = decrease

2.3.1.4 Effect of organic matter addition on soil properties and crop responses

The usual dark colour of organic matter helps soil to retain the heat in temperate

regions. This colour can be used as an indirect factor for modelling organic

matter mineralisation in relation to soil moisture and soil temperature (Leiros et

al., 1999). In lateritic soils, organic matter increases aggregation of soil

associated with its Al and Fe content (Barthes et al., 1996; Muggler et al., 1999).


Macro-aggregates were formed within five months in an Oxisol having a high

content of exchangeable-Al (Barthes et al., 1996). Moreover, addition of organic

matter can increase water holding capacity of the soils (Das et al., 2002; Gispert

et al., 2000; Isaac and Nair, 2002) more than the weight of organic matter itself.

As shown in Figure 2.3-1, these physical effects of organic matter mainly occur at

the soil surface where litter, dead organisms, and their excretions are present.

The stable pool of organic matter which can reach the lower horizons is highly

humified materials, physically protected, and usually exists in micro-aggregates

(Monreal et al., 1997).

Functional groups in soil organic matter can participate in chelation reactions to

form stable complexes with trace element cations, resulting in more or less

availability of micronutrients for plant growth (Greenland, 1986; Pushparajah,

1998).


Litter, roots, dead organisms, excretion by organisms

Carbohydrates (50-60%)Proteins (1-3%)Lignins (10-30%)

CO2

Oxidativedegradation

Aromatic compounds

Amino acids Microbial biomassproteins, cell wall

Colloidal adsorption

BioavailableC, N, P, S

Humus

Pol

ymer

s

Mineralisation

Condensation

Synthesis

Syn

thes

is

Leaching

Chelates

Figure 2.3-1 Steps in carbon cycle and the formation of soil organic matter and humus in aerobic soils. Source: Modified from Coyne (1999) and Grunwald (2004).

In terms of chemical properties of soils, the most significant effects of soil organic

matter are cation exchange capacity, nutrient mineralisation, and pH buffering

capacity. These characteristics, together with pH, are the main factors in

capturing, holding, and providing nutrients for plants. In soils low in inherent

negative charge (lateritic soils), soil organic matter plays a considerable role in

increasing cation exchange capacity (Greenland, 1986; Pushparajah, 1998) since


soil organic matter has between eight to ten times the negative charge of

kaolinitic clays on a mass basis. Figure 2.3-1 describes all chemical reactions

which not only occur at the soil surface, but can also reach the lower part of soil

profile.

2.3.2 Organic matter decomposition and persistence In the first stage of decomposition, litter reaching the soil surface decreases in

volume due to the activity of earthworms and other soil animals (Lee, 1974;

Striganova and Bienkowski, 2000; Topoliantz et al., 2002). Following this stage,

microbial decomposition and intense leaching of solubilized organic substances

are mostly responsible for litter breakdown (Lee, 1974). The next stage is

decomposition of organic substances by microbial cells producing CO2, NH3, and

organic acids (Chen et al., 2001b; Haynes and Mokolobate, 2001). A wide range

of organic compounds are produced by micro-organisms. They include low

molecular organic acids or ligands such as malonic, fumaric, succinic acids

(Chen et al., 2001c; Cieslinski et al., 1997), and soluble humic molecules (Conte

and Piccolo, 2002; Negre et al., 2002; Piccolo et al., 2003). These breakdown

products can complex with phytotoxic monomeric Al to reduce its toxicity and

become adsorbed to Al and Fe oxides and aluminosilicates to block phosphate

from adsorption sites (Haynes and Mokolobate, 2001). The last stage is gradual

decomposition of more resistant substances such as lignin. Lignin decomposition

is a key factor influencing the rate of formation of humic substances. This is

partly due to the retarding effect of macronutrients in soil to the lignin

decomposition (Berg, 2000), even though it is not the case with N (Hobbie, 2000).

Either by the help of soil micro-organism, or by the formation of polyphenol, or by

condensation of sugar and amino acids, the end product of lignin decomposition

is humic substances. Lignin decomposition is one of four major pathways in the

formation of humic substances in soils (Stevenson, 1994). In this pathway, lignin

will be broken into products with low molecular weight. It can also be modified to

lose metoxyl (OCH3) groups in order to generate o-hydroxyphenol and to oxidise

aliphatic chains to form COOH groups. Finally, the modified material is converted

to humic acids and fulvic acids. The half life of lignin can reach more than 10

years (Ganjegunte et al., 2004). It can be assumed that the more lignin content

in organic materials the more resistant the materials from decomposition.


Accordingly, it can be assumed that modified lignin is major part of peat and

humus from poorly drained soils or from lake sediments.

Soil organic matter persistence is usually related to the age of the soil organic

matter itself, especially resistant soil organic matter which commonly occurs

within micro-aggregates (Monreal et al., 1997). Other persistent forms of soil

organic matter include ‘black carbon’ in Terra Preta soil in Amazon, characterized

by a large amount and stable soil organic matter pools and have high stocks of

nutrient N, P, and Ca (Glaser et al., 2001a; Sombroek et al., 1993; Zech et al.,

1990) or a product of incomplete combustion of vegetation (Schmidt et al., 2001).

Due to the polyaromatic component of the structure of black carbon, it is

chemically and microbiologically resistant in the environment over thousands

years (Glaser et al., 2001a; Glaser et al., 2001b). Therefore, charring of soil

organic matter may transform labile pools of soil organic matter to become stable.

Most studies of soil organic matter persistence have investigated the degradation

of organic amendments such as crop residues (Krauss and Deacon, 1994; Muhr

et al., 1999), composts (Jhorar et al., 1991), manures (Esse et al., 2001), and to

some extent, peat (Murayama and Bakar, 1996). The dominant factor

responsible for decomposition rates of such material is the C:N ratio (Jhorar et

al., 1991; Mishra et al., 2001). The higher the C:N ratio, the slower the

decomposition of the materials (Rahn et al., 2003) because the material contain

higher amounts of lignin, wax, and chitin which are more resistant to

decomposition.

2.3.2.1 The age of soil organic matter Soil organic matter is composed of materials ranging in age from days for plant

residues to greater than 1000 years for humic substances (Collins et al., 2000;

Trumbore et al., 1996; Wang and Chang, 2001) or charcoal (Schmidt et al.,

2001). Measurement of the age of soil organic matter is based on the mean

residence time which can be detected using radiocarbon (14C) dating (Desjardins

et al., 1996; Kalbitz et al., 2003). This procedure measures how long C in a

particular substance decreases to half its initial concentration. Carbon dating

utilises the naturally occurring 14C in soil organic matter to estimate mean

residence time. However, these measurements are not precise because it is very

difficult to monitor small changes in soil organic matter in the short term due to

the large background C concentrations and the natural variability of soils


(Sparling, 1992), including continued decomposition of old soil organic matter and

resynthesis of new organic matter by micro-organisms (Stevenson, 1994).

To compensate for difficulties in determining the age of soil organic matter, some

methods have been developed to study the age of soil organic matter more

accurately. Radiocarbon dating of inert organic matter in the interlayer space of

swelling clays has been used to determine the age of soil organic matter (Theng

et al., 1992). The combination of radiocarbon dating and aliphatic hydrocarbons

(biomarker analyses) in soil lipids was applied to access the age of peat soil (Bol

et al., 1996). For the young and active soil organic matter usually less than 10

years old, modelling based on decomposition rate (Janssen, 1984) or organic

matter compartment and separation can be used to predict the fate of organic

matter in soils (Andriulo et al., 1999; Balesdent, 1996). However, whilst the age

of soil organic matter has mostly been studied over longer time frames, shorter-

term decomposition of newly added organic matter could be more important in

regard to soil fertility because of a closer link to nutrient dynamics in soils.

Organic matter decomposition rates depend on C:N ratio of the source materials

(Jhorar et al., 1991; Mishra et al., 2001). The higher the C:N ratio, the slower the

decomposition of the materials (Rahn et al., 2003). In soils with very low organic

matter content such as lateritic soils, it is not known for how long recently added

organic matter with different C:N ratios can persist in the soil, and which C:N ratio

has an advantage in maintaining soil C storage in the long term. Many

experiments have been carried out on the mineralisation of organic matter in

soils, especially focusing on organic-C and N, to determined the appropriate rate

of organic matter application as materials such as manure or green manure

(Dunn and Beecher, 1994; Lupwayi and Haque, 1998), straw (Mueller et al.,

1998; Strong et al., 1987), and sewage sludge (Boucher et al., 1999; Iakimenko

et al., 1996) on various soils. However, the particular C:N ratio and the level of

application of biomass for Oxisols in Western Australia in relation to the

persistence of soil organic-C, and the availability of N and P has not been

studied. In order to determine what happens when different types of organic

matter are applied in these soil, it is important to know how and why various

sources of organic matter differ in C and N mineralisation, and their effect on

other soil properties.


2.3.2.2 Loss and persistence of newly added organic matter Soils play an effective role as storage for C, and as a source or loss of C to pools

outside the soil system. The balance between storage and loss depends on soil

management, as well as climate and soil properties. The losses of C from soil

are mainly from soil organic matter decomposition (respiration), erosion, and

leaching (Akala and Lal, 2000).

One of the problems in managing soil organic matter is its substantial loss due to

temperature and cultivation (Dalal and Chan, 2001). The first factor includes

rainfall and temperature of certain areas. Rainfall, especially mean annual

precipitation, affected initial organic-C content of undisturbed soil as well as

organic-C content of agricultural soil after 20 years of cultivation (Dalal and

Mayer, 1986). In general, temperature differences were associated with different

rates of loss of soil organic matter which were faster in tropical areas than in

subtropical or temperate regions (Bridge and Bell, 1994; Cogle et al., 1995). The

second factor is cultivation; loss of soil organic-C is enhanced by agricultural

activities or pasture establishment compared to losses under native vegetation.

This loss has been reported to be between 10-60% over 10-80 years of

cultivation (Chan, 1997; Russell and Williams, 1982; Standley et al., 1990).

Carbon losses from soil are mainly from soil organic matter decomposition

(respiration), erosion, and leaching (Akala and Lal, 2000). Leaching of C as

dissolved organic-C contributes between 6-46% of total-C loss in forest soils

(Cronan, 1985; Magill and Aber, 2000). Investigations of C balance have focused

on the effect of temperature on C mineralisation (Liechty et al., 1995; Tate et al.,

1993; Zogg et al., 1997), which is only important in surface soil (MacDonald et al.,

1999). For soils in the tropics, the balance between upward (respiration) and

downward (dissolved organic-C leaching) loss could be important, especially for

lateritic soils if the effectiveness of organic matter application is to be understood.

Heavy rainfall may be an additional factor in increasing organic-C loss from soil,

not only by increasing erosion but also by increasing infiltration of water through

the soil profile.

Soils at high latitudes had mean residence time of C between 10-50 years (Bird

et al., 1996). Soil microbial biomass-C was negatively correlated with elevation

and positively correlated with mean annual temperature (Piao et al., 2001). Soil

texture has also been known to affect the persistence of soil organic matter (Bird


et al., 2000). This could be because of the ability of organic matter to associate

with clays within aggregates or micropores, which protects it from microbial

decomposition. Furthermore, the sizes of aggregates are related to the age of

associated organic-C. Macro-aggregates >250 µm contained younger soil

organic-C with average turnover time of 14 years, while micro-aggregates <50

µm had older soil organic-C with turnover time of 275 years (Monreal et al.,

1997). From the age data and the space organic-C occupied within the

aggregates, it can be predicted that organic-C in micro-aggregates consisted of

highly humified materials and was physically protected, while organic-C in macro-

aggregates originated from molecules typically found in plant tissues and soil

micro-organisms such as lignin and amino acids.

Studies of newly added organic matter to soils with low pre-existing organic

matter content are limited to the study of soil organic matter formation after fire,

either forest fire or bush fire, and mine rehabilitation. To some extent, soil

organic matter formation has been observed in slash and burn farming systems.

The effects of fire on soil organic matter accumulation [Materechera, 1998

#47;Alcaniz, 1992 #48] and in nutrient status (Saharjo, 1999) have been

intensively studied. In contrast, the study of the effect of other organic

amendments (crop residues, composts, and manures) has been limited to short-

term effects in relation to crop productions. Apart from mine soil rehabilitation,

however, there is no information about the effect and persistence of newly added

organic matter after addition to soil with negligible organic matter content,

especially in the long-term. This information is potentially highly important in

regard to the reclamation of soils with low organic matter content such as lateritic

soils.

Moreover, the effect of recently added organic matter has to be differentiated

from the effect of inherent soil organic matter. The inherent and inert pools of soil

organic matter can be very old, ranging from hundreds to thousand years

(Eusterhues et al., 2003; Falloon and Smith, 1998; Hassink, 1997; Oades, 1995)

and their identity, as well as their physical and chemical properties, are not well

understood (Eusterhues et al., 2003; Hsieh, 1992; Ruhlmann, 1999; Theng et al.,

1992). In order to study the effect of adding organic matter to soil in terms of

phosphate adsorption, removing the stable pools of soil organic matter from soil


samples is necessary so that the effect of newly added organic matter does not

interact with that of pre-existing pools.

Soil organic matter plays an important role in phosphate adsorption (Brennan et

al., 1994; Erich et al., 2002; Leytem et al., 2002; Sinaj et al., 2002). As organic-C

is competitive for phosphate, this effect is expected to be greater for inert and

stable pools. As stable and inert pools of soil organic matter usually interact with

clay minerals and/or Fe-oxides (Baldock and Skjemstad, 2000; Eusterhues et al.,

2003), removing this pool from soil may affect phosphate desorption in soil with

newly add organic matter. The activity of phosphatase has also been shown to

be positively correlated with soil organic-C, especially in acid soils (Baligar et al.,

1999; Barrett et al., 1998; Canarutto et al., 1995; Dick et al., 1988). All of these

processes need to be assessed in order to identify the mechanisms involved in

organic matter-P interactions in lateritic soils amended with organic matter.

2.3.2.3 Priming effect The definition of a priming effect has been changed from time to time according

to the specific objectives of individual researchers’ work. The priming effect has

been defined as “an extra decomposition of organic-C after addition of easily

decomposable organic substances to soil” (Bingeman et al., 1953) or “extra soil N

which is taken up by plants after addition of N fertilisers, compared with non-N

treated plants” (Jenkinson et al., 1985) or even “strong short term changes in the

turnover of soil organic matter by comparatively moderate treatments of the soils”

(Kuzyakov et al., 2000). All definitions, therefore, describe similar effects. In this

thesis, the first definition is more related to the experiments. However,

contradictory results from some authors make it difficult to describe the effect of

C inputs from organic matter, especially if it is related to the final balance of C in

soils (Fontaine et al., 2004).

The term priming effect has also been subject to controversy since Löhnis (1926)

claimed that the effect played a critical role in the dynamics of soil organic matter.

Only after the invention of isotopic tracer techniques have researchers been able

to separate C from two different sources, i.e. C from fresh organic matter and C

from inherent soil organic matter (Bingeman et al., 1953; Broadbent and

Nakashima, 1974), to confront some issues in the claims. An additional

controversy emerged after Dalenberg and Jager (1989) found that the application

of glucose to soil samples induced no or little effect on soil organic matter


decomposition, compared with the effect of wheat straw. Wheat straw contains

less available energy than glucose. Therefore, it can be expected that wheat

straw might have a smaller priming effect than glucose. Hence, priming effect

mechanisms are complicated and remain poorly understood (Kuzyakov et al.,

2000).

A priming effect in the rhizosphere is easier to explain, since living plants change

their local environment (Kuzyakov, 2002). The mechanism is related to the

exudation of organic substances by roots; the increase of microbial activity in the

rhizosphere; and the subsequent microbial mobilization of nutrients from soil

organic matter. The last two processes lead to the idea that a priming effect in

soils only apparent when the role of soil organic matter mineralisation is not

dependent on fresh organic matter input. Therefore, any priming effect in soil

may be attributed to increases in soil enzyme activity, or to an increase in

microbial biomass induced by input of fresh organic matter. In order to refine the

mechanism, Fontaine (2003) proposed two conceptual mechanisms in which

priming effect occurs in soil: i.e. competition for energy and nutrient acquisition

between the micro-organisms specialised in the decomposition of fresh organic

matter and those feeding on polymerised soil organic matter. How far the

conceptual mechanisms can describe the priming effect in soil organic matter is

still questionable, as the priming effect itself only occurs in the short term

(Kuzyakov et al., 2000) compared with the formation of soil organic matter.

2.3.3 Pools of soil organic matter and its modelling There are several pools of soil organic matter based on their reactivity.

Conceptually, pools of soil organic matter can be differentiated qualitatively using

simulation programs (modelling), based on turnover time and C and N content.

Several popular soil organic matter models are ROTHC, DAISY, CANDY,

NCSOIL, DNDC, and CENTURY.

ROTHC is the Rothamsted C model which relies on C-turnover that is sensitive to

soil type, temperature, moisture, and plant cover (Jenkinson and Coleman, 1994;

Jenkinson et al., 1992). In ROTHC, the dynamics of N and C are not connected

each other, and organic matter input to soil is examined as well as inert organic

matter. Carbon dynamics is measured by radiocarbon dating and useful to gain

detailed additional information on soil organic matter turnover. ROTHC mainly


uses monthly input data and shares some of basic definitions with CENTURY

(Jenkinson and Coleman, 1994).

DAISY simulates the dynamics of N, soil water, and crop production in diverse

agricultural management systems (Mueller et al., 1996). A hydrological model

with a soil water submodel, a soil N model with a soil organic matter submodel,

and a crop model with an N uptake submodel are all used in combination as a

tool in field management as well as regional administrative purposes in a local

government. Soil microbial biomass sometimes is used as a dependent variable

for the model (Smith et al., 1997). This modelling has been applied to catchment

areas (Styczen and Storm, 1993) and farmlands (Jensen et al., 1994) to predict N

dynamics.

CANDY (Carbon-Nitrogen-Dynamics) modelling is based on a database from soil

management, weather, and measurement values from soil parameters. These

data are used to simulate N dynamics, temperature, and water in order to provide

information on N uptake by plants, leaching, and water quality (Franco et al.,

1996). This modelling can calculate litter decomposition and determine

‘biologically active time’ to compare one site of study to another. It also requires

inert organic matter in soil particles (<6 µm) as one of the parameters.

The model NCSOIL is based on four organic compartments (plant residues,

microbial biomass (pool I), humads (pool II), and stable organic matter (pool III))

to simulate N and C flow in soils (Molina, 1996; Nicolardot et al., 1994). In its

development, NCSOIL also simulates 14C and 15N as well as 12C and 14N and

incorporated into a deterministic model (NCSWAP) which simulates interaction of

C and N dynamics with crop growth and soil water (Lengnick and Fox, 1994).

Simplification was made to this model by collapsing microbial succession into one

microbial component and its dynamics determined by the rate of C flow through

populations (Molina, 1996).

The DNDC (Denitrification and Decomposition) model uses the denitrification and

decomposition which are influenced by the soil environment to predict CO2, N2O,

and N2 from agricultural soils (Li, 1996). DNDC has four submodels which

interact with one another, i.e. soil climate, decomposition, denitrification, and

plant growth. The plant growth submodel consists of fertilisation, irrigation,

tillage, crop rotation and manure addition to simulate soil organic matter turnover

(Li et al., 1994).


The most widely used model, CENTURY, was developed for grassland (Parton et

al., 1988), but has later been updated and enhanced to simulate organic matter

and nutrient dynamics for agricultural crops (Carter et al., 2003; Santen et al.,

2002). This model simulates the long-term dynamics of C, N, P, and S in the top

20 cm of soils on a monthly basis. The pools of soil organic matter included are:

active, slow, and passive; these have different turnover times. Figure 2.3-1

describes the overall conceptual model used in CENTURY, which consists of

several submodels including a soil organic matter submodel.

The active pool of soil organic matter represents soil micro-organisms and

microbial products. It has a turnover time of several months to a few years

depending on the environment. Soil texture influences the turnover rate of this

pool. Sandy soils have higher rates of turnover, whilst clay soils have slower

turnover due to higher stabilisation rate to convert active soil organic matter into

slow soil organic matter. The slow pool includes resistant plant material from the

structural pool and microbial products from active pools. It has a turnover time

from 20 to 50 years. The passive pool is the most resistant to decomposition and

includes physically and chemically stabilised soil organic matter and has a

turnover time of 400 to 2000 years (Brenner et al., 2001; Gijsman et al., 2002). In

this regards, peat is considered as a passive pool of soil organic matter as well

as inert soil organic matter that is present in soil micro- and macro-aggregates.


Climate Soil Management Topography

Residues

ActiveSOM

SlowSOM

PassiveSOM

SOMSubmodel

PlantGrowth

Submodel

WaterBalance

Submodel

CO2CO2

CO2

CO2

CO2

CO2

Figure 2.3-2 Soil organic matter pools according to the CENTURY model. The soil organic matter submodel is part of the overall model in which carbon cycling is affected by climate, soils, management, and topography. Source: (Brenner et al., 2001).

Further development of the CENTURY model has been made in order to meet

the requirements for farmers in developing countries (Gijsman et al., 2002).

Instead of relying on chemical fertilisers as in developed countries, low input

agricultural systems have been integrated into CENTURY using DSSAT

(Decision Support System for Agrotechnology Transfer) crop models. This new

development will make CENTURY more popular and useful for land reclamation

projects in developing countries.

Among the models discussed, CENTURY is known to have low errors for dataset

applied, to be low in bias, and able to simulate both low and high-N treatments

(Gijsman et al., 2002; Kelly et al., 1997; Smith et al., 1997), and is therefore

considered to be the superior model. However, CENTURY still does not include


soil pH as a factor affecting soil organic matter turnover (Smith et al., 1997) and

may overestimate soil organic-C in intensively managed agricultural soils (van

Santen et al., 2002). It has also resulted in failure to simulate final measured

data points in a deciduous woodland (Smith et al., 1997).

The C pool plays the main role in all models. For models like NCSOIL and

CENTURY, the C pool is even divided into several sub-pools according to C

stability in corporation with other organic compartments (plant residues, human

related-C, and stable soil organic matter), or in reactivity to other substances.

These pools are taken into account in order to follow their effects on the

dynamics of other elements. In most cases, soil C and its isotopes are at the

centre of the models indicating the dependency of other nutrient pools. Thus,

pools of C are followed by pools and sub-pools of N and P. The ability of C to

form many complex compounds is central to most nutrient cycles (Coyne, 1999)

making them dependent on C for their dynamics. However, in applying any

model, the behaviour of C may vary with type, source, and origin of organic

materials. Therefore, before use, each model needs to be precisely according to

the characteristics of the local area.

2.4 PHOSPHORUS CYCLING IN RELATION TO SOIL ORGANIC MATTER

Phosphorus is an essential nutrient for living organisms due to its vital role in life

processes including photosynthesis in green plants and transformation of energy

in all forms of life (Sanyal and De Datta, 1991). Compared with other essential

nutrients, P is by far the least mobile and least available to plants under most soil

conditions (Hinsinger, 2001). Therefore, P often becomes a major limiting factor

for plant growth.

In the soil solution, P usually occurs at fairly low concentration as orthophosphate

or organic phosphate, while a large proportion is more or less strongly held by

soil minerals (Frossard et al., 2000). Some phosphate ions can be adsorbed to

aluminosilicate clays and/or Fe and Al oxides. Phosphate ions can also form a

range of minerals in combination with metal cations such as Ca2+, Fe3+, and Al3+.

These sorption-desorption and precipitation-dissolution equilibria control

phosphate concentration and the same time both the chemical mobility and

bioavailability. According to Hinsinger (2001) the major factors that determine


those equilibria are: (1) soil pH, (2) the concentration of anions that compete with

phosphate ions for ligand exchange reactions. Including in these anions is

phosphate ion itself, and; (3) the concentration of metals (Ca, Fe, and Al) that can

precipitate with phosphate ions.

Other factors affecting phosphate availability are the amount and type of

adsorbing phases, such as dominant clay mineral and various oxides (Hue, 1991;

Wahba et al., 2002). Therefore, by considering these factors, the effect of

organic matter on phosphate ions must relate to the second factor due to organic

ligands such as carboxyl.

In general, the effect of soil organic matter on the P cycle is related to the effect

of biotic processes that control P release to soil solution (Frossard et al., 2000).

In this process, P turnover from organic matter plays an important role despite

the fact that organic matter also contributes via abiotic processes such as

adsorption-desorption and dissolution-precipitation. In biotic processes, soil

organic matter plays the central role in mineralisation and immobilisation.

Considering phosphate rocks as a non-renewable resource and the availability of

P is relatively low in soils, P supply to plant growth must be rationalised. This is

true especially for Oxisols and Ultisols that have Fe and Al-oxides which strongly

adsorb soluble phosphate from fertilisers. To improve the efficiency of P supply

in soils, it is imperative to maximise P recycling from crop residues or even from

organic and mineral fertilisers.

The P cycle is very dynamic and involves both geochemical and biochemical

reactions (Figure 2.4-1). The cycle of P is different from that of C, N, and S. This

is because P has no pathways to atmospheric pools. The overall cycle ranges

from solubilisation and fixation at clays and oxide surfaces in the soil solution to

mineralisation-immobilisation processes mediated by micro-organisms. The roles

of soil organic matter and soil micro-organisms are very significant in the P cycle.

Microbial activity is an agent that functions as a reversible sink for P, continuously

consuming and releasing P to the soil solution (Stewart, 1981).


Organicfertiliser

Mineralfertiliser

Plantaerial parts

Plant roots

Inorganic-P in primary minerals

Other forms of inorganic-P

Inorganic-P in the soil solution Microbial-

P

Labileorganic-P

Stableorganic-

P

Losses by erosion, run-off, and leaching

Soil/plant system

Exportation in plant and animal products

Figure 2.4-1 Soil phosphorus cycle, its components and measurable fractions (adapted from Stewart and Sharpley (1987)). Arrows represent fluxes between reservoirs.

In highly weathered soils, which are commonly acidic (Eswaran, 1993; West et

al., 1998), phosphate is usually adsorbed to Al and Fe oxides or precipitated as

insoluble Al- and Fe-phosphates (Iyamuremye and Dick, 1996; Lindsay et al.,

1989; Stevenson and Cole, 1999). Both forms are poor sources of P for plants

and P deficiency is common in soils rich in Fe and Al, such as Oxisols and

Ultisols of the tropics and sub tropics. The application of organic matter to those

soils may complex Al and Fe, in either ionic form or as oxides. Furthermore, in

many soils, the availability of P may depend more on the turnover of easily

decomposable soil organic matter than on the release of adsorbed phosphate.

2.4.1 Dissolution and precipitation The beginning of the P cycle involves parent materials, climate, and time as

factors affecting the existence and amount of P in soils. Dissolution of P from its

origin can usually be explained as dissolution from apatite [Ca10X2(PO4)6, where

27II. Literature Review

X = OH- or F-; Ca may also be substituted with Na or Mg, and PO4 with CO3]

which are the most common primary phosphate minerals.

Precipitation of phosphate with Ca carbonates and its adsorption on Al and Fe

hydrous oxides has been known since the mid-nineteenth century. Calcium

phosphate is formed following phosphate adsorption to calcite (Syers and Curtin,

1989). As phosphate is adsorbed to the surface of calcite, monocalcium

phosphate [Ca(H2PO4)2] precipitates and transforms to become dicalcium

phosphate dihydrate (CaHPO4.2H2O), to octocalcium [Ca8H2(PO4)6.5H2O] and

finally to hydroxyapatite [Ca10(PO4)6(OH)2]. Adsorption of phosphate with Al and

Fe oxides resulted in the formation of amorphous Al-phosphate and Fe-

phosphate which may later transform to variscite (AlPO4.2H2O) and strengite

(FePO4.2H2O) after prolonged aging (Lindsay et al., 1989). Phosphate

adsorption is not only attributed to the hydrous oxides of Al and Fe, but also to

1:1 lattice clay such as kaolinite, especially in acid tropical soils (Dubus and

Becquer, 2001; Sanyal et al., 1993; Uehara and Gillman, 1981).

Apatite dissolution requires a source of H+, which originates from plant roots,

micro-organisms, or from the soil itself (MacKay et al., 1986; Smillie et al., 1987).

Dissolution from precipitated P (with Ca, Al, and Fe) is also possible

(Iyamuremye and Dick, 1996), and can make P more available when organic

residues are added to soils. Dissolution of both groups of solid phases relies on

organic acids such as citrate and formic acid (Traina et al., 1986) as the source of

H+. Organic ligands such as citrate can form stable complexes with Al3+ or Fe 3+,

promoting dissolution of solid phases containing Fe or Al and phosphate.

2.4.2 Sorption and desorption The term sorption is used to describe the surface accumulation of phosphate on

soil components which can be accompanied by penetration of adsorbed P by

diffusion into the adsorbent body, resulting in further adsorption of the adsorbed

species (Sanyal and De Datta, 1991). These two processes take place

simultaneously. Desorption is defined as the adsorbed phosphate ions being

released as a reciprocal action of sorption.

The sorption of phosphate ions has been interpreted as a biphasic reaction

(Rhue and Harris, 1999), i.e. the initial and rapid sorption which is believed to last

in the order of minutes or hours. The second sorption is slow reaction lasting

weeks or months. There are two mechanisms responsible for the process:


1. Ion exchange, mechanism from the electrostatic attraction of phosphate

anions to positively charged sites exist on variable-charge surfaces below

the zero point charge, and

2. Ligand exchange, mechanism by which a phosphate anion replaces a

surface hydroxyl that is coordinated with a metal cation in a solid phase

(Rhue and Harris, 1999).

The last mechanism, ligand exchange, is also referred as specific adsorption and

is characterised by:

• Adsorption is accompanied by the release of OH-,

• Ligand exchange shows a high degree of specificity,

• The adsorption step often occurs much more rapidly than the

desorption step, leading to apparent hysteresis in the isotherm,

and

• Adsorption is accompanied by an increase in surface negative

charge (McBride, 1994).

The second sorption or the slow phase is thought to have two mechanisms, i.e.

diffusion (either into soil particles or to surface sites of limited accessibility), and

precipitation (either by direct heterogenous nucleation or following the dissolution

of the host solid, following an initial adsorption reaction) (Rhue and Harris, 1999).

Phosphorus sorption and desorption are important mechanisms controlling soil

phosphate partitioning between the sorbed and solution phases (Burkitt et al.,

2002) and have crucial implication for P management. This mechanism is

commonly referred to phosphate buffering capacity, which describes a soil’s

capacity to moderate changes in phosphate solution concentration when P is

added or removed from the soil (Ozanne, 1980).

In relation to soil organic matter, the release of phosphate by mineralisation may

be difficult to separate from the sorption mechanisms, especially in soils with high

sorbing capacity such as lateritic soils. This is not only because of the high

content of sesquioxides and 1:1 clay content in these soils, but also due to

negligible amounts of soil organic matter. Phosphate adsorbed to metal oxy-

hydroxides may be desorbed by ligand exchange with organic anions (Fox et al.,

1990). However, by observing the net release of extractable-P and determining


phosphate adsorption isotherms, the two processes can be separated as more

and less important in releasing phosphates to the soil solution. Furthermore, as

Afif et al. (1995) found that the effect of soluble organic matter on phosphate

release from Oxisols to be transient, this leads to the question of whether peat

would have a longer-term effect on P adsorption due to its resistance to

decomposition. At the same time peat can possibly slowly release soluble

organic ligands which compete for adsorption sites with phosphate. This also

could be determined by analysing phosphate adsorption isotherms.

2.4.3 Organic phosphorus In intensively-managed agricultural soils, most organic-P is considered a poor

nutrient source for agricultural production (Gressel and McColl, 1997). In

countries with fertiliser-based agriculture, research on organic-P has therefore

been limited (Magid et al., 1996). However, countries with low-input agricultural

systems rely more heavily on mineralisation of organic-P to supply P to crops.

Organic-P can also be important in P transfer, because organic-P constitutes a

large amount of total-P in the soil solution (Shand and Smith, 1997) or in the soil

leachate (Turner and Haygarth, 2000). Even though the process of transforming

of organic-P to inorganic-P remains poorly understood (Frossard et al., 2000)

research on this subject is challenging, especially in areas where P supply is very

limited due to fixation and lack of P natural resources.

Organic-P represents 15-80% of total-P in soils (Dalal, 1977; Stevenson and

Cole, 1999). The higher content would be in peat or Histosols and undisturbed

forest soils. Tropical soils, such as highly weathered lateritics, are known to have

higher proportions of organic-P than soils of temperate regions, especially in soils

in which P fertilisers are not utilized (Beck and Sanchez, 1994).

The relationship between soil organic matter decomposition and organic-P

mineralisation can be approached from different points of view. First, P can be

considered as part of soil organic matter quality, and therefore P content of

organic matter is a factor determining decomposition rate (Sinsabaugh and

Moorhead, 1994; Stevenson and Cole, 1999). Second, more inorganic-P release

to the soil can be considered as an independent function of decomposition of

organic residues (Gressel and McColl, 1997). Despite the fact that both

approaches only differ in the role of P, decomposition of soil organic matter itself


is strongly related to soil organic-C in the organic residues. Hence, the organic-P

mineralisation is expected to be strongly dependent on organic-C.

2.4.3.1 Organic phosphorus classification Three major groups of compounds have been identified so far in soil organic-P.

1. Inositol phosphates represent 50% of total organic-P in soils. The most

common inositol found in nature is myo-inositol (Stevenson and Cole, 1999).

Other isomers of this group also known to exist in soils are scyllo-inositol,

chiro-inositol, and neo-inositol (Cosgrove, 1980). Inositol phosphates in soil

exist in complex forms and are sometimes present in a complex containing

carbohydrate or protein. However, organic-P additions increased plant

growth in soil limited with available inorganic-P. In a pot experiment, the

application of inositol phosphate produced a greater amount of dry matter of

lupin (Lupinus albus and Lupinus angustifolius) (Adams and Pate, 1992).

The half life of this fraction of organic-P can exceed one year which suggests

that it is broken down slowly (Mueller-Harvey and Wild, 1986) (Table 2.4-1).

2. Phospholipids represent 0.5-7.0% of total organic-P in soils (Stevenson and

Cole, 1999). Phosphoglycerides contribute a major part of phospholipids,

although phosphoglycolipids, phosphodiollipids, phosphosphingolipids, and

phosphonolipids have also been found, the concentrations were much

smaller. Similar to inositol phosphate, phospholipids are derived from plant

debris, animal waste, and microbial biomass. This is in agreement with

phospholipids being the major fraction of total organic-P in plant tissue

(Anderson and Malcolm, 1974). The half life of phospholipids during

decomposition by microorganisms such as Streptomyces sp., thermophillic

Bacillus spp., and Mycobacterium spp. in soil was predicted to be about four

to six months, i.e. easier to decompose than inositol phosphate (Soliman and

Radwan, 1981) (Table 2.4-1).

3. Nucleic acids or their derivatives represent up to 3% of total organic-P in soils

(Stevenson and Cole, 1999). This form of organic-P can be rapidly

mineralised in soil and incorporated into micro-organisms (Dalal, 1977). This

fraction of organic-P has a short half life (Table 2.4-1), ranging from 4-40

hours (Brodl and Ho, 1992; DeRocher et al., 1991; Mason, 1986; Mueller-

Harvey and Wild, 1986).


The form of the rest of the organic-P in soil, about 40% of total organic-P, is

unknown. This fraction may play an important role in transformation of organic-P

to inorganic-P, especially where the amount of organic-P is high such as in

lateritic soils of the tropics.

Table 2.4-1 Half life comparison between three groups of organic phosphorus in soils

Organic phosphorus Item Half life Author

Inositol phosphates IP5+6 > 22 months (Mueller-Harvey and Wild, 1986)

Phospholipids - 4-6 months (Soliman and Radwan, 1981) Phosphatidylcho

line 92 hours (Moore, 1977)

Phosphatidylethanolamine

34-136 hours (Moore, 1977)

Nucleic acids RNA 12 hours (Mason, 1986) mRNA > 4 hours (Brodl and Ho, 1992) cDNA 37.7±8 hours (DeRocher et al., 1991)

2.4.3.2 Factors affecting organic phosphorus availability A contradiction in the scientific literature occurs in relation to the availability of

organic-P to plants. Under certain conditions, glycerol phosphate, sugar

phosphates, inositol hexaphosphate, and nucleic acids were found to be as

beneficial as sources of P for plant as was inorganic-P (Tarafdar and Claassen,

1988). Similarly, Firsching and Claassen (1996) found that Norway spruce can

take up about 75% organic-P out of total-P required for their growth. But other

authors observed that plants did not take up organic-P in the soil solution, even

though organic-P in the soil solution was almost completely absorbed

(Iyamuremye and Dick, 1996).

Despite the contradictory conclusions about the direct availability of organic-P to

plants, researchers agree that organic-P contributes to the P nutrition of plants

primarily after being mineralised into orthophosphate (Chapuis-Lardy et al., 1998;

Lopez-Hernandez et al., 1998; Perrott and Mansell, 1989). Factors affecting the

mineralisation process are:

1. Temperature. As the optimum temperature for micro-organisms to grow is

between 30° and 45° C, the optimum mineralisation of organic-P lies between

those temperatures. Temperature can also affect the production of

phosphate which in turn may affect the mineralisation. However, it can be


expected that in tropical soils organic-P may contribute more in P nutrition for

plant due to more rapid mineralisation of organic-P.

2. Water. Sufficient water content is needed for organic-P mineralisation. Soil

water content between 50-75% of water holding capacity is generally optimal.

Wetting and drying processes in soil also enhance organic-P mineralisation

(Chepkwony et al., 2001), even though the mechanism is not fully

understood.

3. Aeration. In general, if soil aeration is poorer, the rate of soil organic matter

decomposition is reduced due to the lower oxygen level in the soil. This

finally will effect the organic-P mineralisation through microbial community in

soils.

4. Soil pH. This factor will also influence microbial activity. Liming of acid soils

increased mineralisation of organic-P (Perrott and Mansell, 1989). The

explanation for this phenomenon was that as soil pH increased (from 4.0 to

7.5), microbiological activity markedly increased.

5. Inorganic-P supply. Conflicting results have been presented on the effect of

the addition of inorganic-P on mineralisation of organic-P. Fertilizer P can

either increase or decrease mineralisation of organic-P in soils (Dalal, 1977;

Zhang et al., 2004). When P supply (i.e. from fertilizer) is in abundance, other

biotic factor such as heterotrophic bacteria (Criquet et al., 2004) and

mycorrhiza (Koide and Kabir, 2000), or abiotic factors such as soil pH and

temperature (Criquet et al., 2004) were more responsible in controlling P

mineralisation.

6. Cultivation. Cultivated soils usually contain lower proportion of organic-P

than virgin soils (Vig et al., 1999). This could be due to the increase in soil

aeration that stimulates microbial activity and greater decomposition of soil

organic matter. Another possible effect could be from manipulation of soil

temperature and moisture regime in cultivated soils. Furthermore, it is also

known that plant roots produce phosphatase enzyme (Helal and Sauerbeck,

1991), capable of dephosphorylating organic-P. Hence monoculture practice

for many years would affect the ability of the soil to mineralised organic-P.

7. Soil micro-organisms. Organic-P mineralisation depends largely on the

population and the activity of soil micro-organisms. There are numbers of


micro-organisms capable in decomposing insoluble organic-P. Some of them

produce phosphatases that degrade glycerophosphates, nucleic acids, and

phytin (Iyamuremye and Dick, 1996).

Most of the above factors are related to soil micro-organisms which, especially in

nutrient cycles, are able to interact with plants to become the key functions in

every ecosystem (Schloter et al., 2003), as well as the P cycle. Conversely, all

aspects of soil micro-organisms such as their population size, activity rates, and

their community structures are able to influence mineralisation and transformation

of P. However, the effect of each single factor on the dynamic of P in soils is not

known.

2.4.4 Microbial biomass phosphorus The flux of P through microbial biomass is an important factor that determines the

availability of inorganic-P in the soil-plant system (Frossard et al., 2000; Magid et

al., 1996; Stewart and Tiessen, 1987). In Figure 2.4-1, microbial biomass-P is a

mediator between three pools of P (inorganic, labile, and stable). For this role,

microbial biomass-P may be as a major sink, source, and transformer of organic-

P in the soil (Kwabiah et al., 2003). Moreover, in studies with red soil (an Oxisol),

microbial biomass-P was applied as a potential biological index to the availability

of P (Chen et al., 2000) or could be applied as an indicator of P-supply (Chen et

al., 2001a). However, applying such findings to other soil types needs caution as

nutrient P distribution in various pools might be different from that of red soils.

In highly weathered soils, microbial biomass-P was correlated with organic-P and

Bray I extractable P, suggesting a dynamic balance between these two types of P

(Chen et al., 1999; Chen et al., 2000). As an outcome, for a very high-rate of

fixated inorganic-P in lateritic soils, promoting the growth of soil micro-organisms

will increase P supply. In an Oxisol from Colombia, the very fast turnover of

microbial biomass-P (within two weeks (Helal and Dressler, 1989)) indicated that

microbial biomass-P may play an important role in supplying P to plants growing

in this type of soil (Gijsman et al., 1997), especially if a steady-state P

concentration which is sufficient for plants can be maintained.

The importance of microbial biomass-P in the P cycle may be even more

pronounced as the residence time in the microbial biomass pool can protect P

from physico-chemical adsorption reactions, if the release of phosphate is

synchronised with the demand of plants and the subsequent generation of micro-


organisms (Magid et al., 1996). Immobilisation of inorganic-P by soil microbes

(measured by microbial biomass-P) could, therefore, contribute to maximising the

recycling of P from organic residues, especially in highly weathered soils where P

is deficient. Furthermore, it is recognised that microbial biomass may take up P

from organic matter added to soil (McLaughlin et al., 1988; Tiessen et al., 1994).

In line with this finding, Frossard et al. (2000) considered that this biotic

mechanism has an important role in increasing P release into soil solution.

Alternately, this biotic factor can be used to reduce P availability in soils with high

P status if combined with appropriate remediation techniques.

2.4.5 Phosphatase The role of phosphatase is an integral part of the role of microbial activity in soils.

In relation to P mineralisation from soil organic matter, the effect of soil microbial

activity becomes an integral part of microbial mineralisation, assimilation or

immobilisation in the P cycle (Figure 2.4-1). To a lesser extent, other soil

enzymes such as diesterases and phytase are also important in P transformation.

Phosphatases, either acid or alkaline phosphatases, are able to convert organic-

P to inorganic-P and release the inorganic-P. This transformation is even more

pronounced in soil with low P supply (Saker et al., 1999) or in roots of plants

deficient in nutrient P (Firsching and Claassen, 1996; Helal and Sauerbeck,

1991). Phosphatase activity is affected by some soil characteristics. Soil organic

matter content, P supply, microbial population, and soil pH being the most

influential factors (Baligar et al., 1999; Marinari et al., 2000; Tarafdar et al., 1989).

Under optimum conditions, phosphatase can have a half life of four weeks (Pettit

et al., 1977).

Some soils in Western Australia are lateritic, characterised by low amounts of

available P (Hingston et al., 1981) where labile organic matter and microbial P

fractions dominate the soil P in surface horizons. Therefore, mineralisation of

organic-P and turnover of dead biomass, as well as phosphatase, are likely to

control P availability (Adams, 1996; Turner and Lambert, 1986).

2.4.6 New and existing soil organic matter in phosphorus supply

Many studies have found that P availability to plants in soils is enhanced by

addition of organic matter through several mechanisms (Leytem et al., 2002;


Maroko et al., 1999; Pushparajah, 1998). Most of the mechanisms are related to

biochemical reactions in which soil micro-organisms are participating actively.

In several studies, the addition of organic matter to soils affected phosphate

adsorption (see Section 2.4.6.1 below) (Afif et al., 1995; Haynes and Mokolobate,

2001; Moody et al., 1997).

As organic residues decompose, soluble phosphate is released and this can

become adsorbed to oxide surfaces (Haynes and Mokolobate, 2001). In the

longer term, humic compounds from organic matter decomposition are able to

complex with Fe, Al, and to a lesser degree Ca to adsorb P (Bloom, 1981; Gerke

and Hermann, 1992) possibly by the formation of ternary compounds (humic

acids-metal-PO4) (Frossard et al., 1995). Complexation of Al by the newly-added

organic matter reduces the concentrations of exchangeable and soluble Al

present (Haynes and Mokolobate, 2001; Pushparajah, 1998; Schnitzer, 2000). In

highly P-sorbing soils, phosphate adsorption usually increases following the

addition of organic residues. But in low P-sorbing soils, the adsorption usually

decreases, depending on the type of residues applied. The phosphate sorbing

capacity increased with the duration of incubation and decreased with increasing

P content in the residue (Bumaya and Naylor, 1988).

2.4.6.1 Phosphate displacement Another process responsible for the release of P from organic matter added to

soil is the displacement of orthophosphate ion with organic ligands or anions.

These organic ligands are from plant residues having the important metabolites

di- and tri-carboxylic acids such as oxalic, oxalo-acetic, malic, fumaric, succinic,

α-cetoglutaric, isocitric, and citric acids (Hinsinger, 2001). These organic ligands

not only compete with phosphate for sorption sites (Hue, 1992; Violante and

Gianfreda, 1993), but also reduce the amount of Al and Fe-phosphates in soils

(Iyamuremye and Dick, 1996). Several organic ligands such as aromatic hydroxy

acids and aliphatic hydroxy acids were effective in preventing phosphates from

combining chemically with Al and Fe or in the displacement of phosphate

(Bhadoria et al., 2002; Iyamuremye and Dick, 1996).

In the displacement process, ligand exchange is also the mechanism. Surface

ligands in soils are not restricted to hydroxyls and water. Ligands such as

sulphate and silicate can also exchange with phosphate and influence phosphate

adsorption (Rhue and Harris, 1999). In soils dominated by amorphous Fe and Al


compounds, phosphate effectively replaced sulphate and silicate as well as

hydroxyl ions during the rapid sorption phase (Pardo and Guadalix, 1990).

2.4.7 Phosphorus leaching Phosphorus leaching is an important mechanism of P removal from soil after

phosphate adsorption and precipitation. As well as leaching removing P beyond

the soil-plant system (Figure 2.4-1), the effect of leaching may disturb the

balance among pools of P in the long term. As P is also usually the main limiting

nutrient in lateritic soils, P transportation from soil to water bodies through

leaching needs to be avoided, especially if P fertilizer is applied to the soils.

It is generally assumed that there is little or no vertical P movement or leaching in

soil, because of the high fixing capacity of many mineral soils including Ultisols

and Oxisols. Soil erosion caused by surface runoff has typically been considered

the primary mechanism of P loss from soil to water (Brye et al., 2001).

Phosphorus leaching was considered insignificant and unimportant from an

agronomic and environmental point of view (Hesketh and Brookes, 2000; Sims et

al., 1998). However, recent studies have indicated that concentrations of

subsurface leaching of P are higher than previously believed (Hooda et al., 1999;

Sims et al., 1998). It has been known that P leaching usually occurs due to the

combination of factors such as agricultural management practices, soil

properties, and climatic conditions (Sims et al., 1998). Therefore, P leaching can

occur in any soils, not only from heavily used agricultural areas (Sharpley et al.,

2001), but also in both organic and mineral soils (Cogger and Duxbury, 1984;

Miller, 1979).

With regard to lateritic soils, the studies of P leaching have been focused in

relation to phosphate adsorption due to Al and Fe hydroxides. This adsorption

can retard P leaching with the assumption that the adsorption itself has not

reached saturation (Sims et al., 1998). Cogger and Duxbury (1984) predicted for

the first time that sesquioxide (of Al and Fe) content can be used as the best

predictor of P leaching. In a more sophisticated way, models have been

developed with validated index to predict P loss based on phosphorus index, soil

test data, soil erosion and runoff potentials, and P fertilisers or organic waste

application rate, method, and timing (Lemunyon and Gilbert, 1993; Sharpley,

1995).


Concern is growing as P leaching accelerates freshwater eutrophication

(McDowell et al., 2001; Sharpley et al., 2001; Wen and Recknagel, 2002).

Nowadays, some efforts have been made to target critical source areas of P

transport (Kleinman et al., 2003), where high concentration of P are found in soils

due to surface runoff (Sharpley et al., 1994) or subsurface macropore flow (Sims

et al., 1998).

In detailed studies of P leaching, subsurface macropore flow or preferential flow

has been well documented through sandy soils (Sims et al., 1998), and in well-

structured and fine textured soils (Chardon and Faassen, 1999; Djodjic et al.,

2002). This leaching mechanism is dominated by preferential flow through soil

macropores (Simard et al., 2000). In urban areas this mechanism is more severe

due to artificial drainage, which provides lateral connections between subsurface

macropores and water bodies (Dils and Heathwaite, 1999).

In relation to soil organic matter, the topic of P leaching is more focused on

organic-P, phosphate buffering capacity, and phosphate retention index in

relation to the leaching of soil organic-C. According to Brennan et al. (1994),

phosphate buffering capacity and phosphate retention index was correlated with

organic-P in soils from Western Australia. Meanwhile, soil organic-C was

positively correlated with the activity of phosphatase, especially in acid soils in

other studies (Baligar et al., 1999; Barrett et al., 1998; Canarutto et al., 1995).

Therefore, soil organic-C plays an important role in affecting P dynamics in acidic

lateritic soils. If leaching of organic-C is prominent in these soils, and organic-P

affects P adsorption (Brennan et al., 1994), this leaching may also have an effect

on P mobility in the soil profile. This was supported by Chardon et al. (1997) who

found that 90 % of total-P in leachate was in the form of dissolved organic-P and

that leaching was positively correlated with dissolved organic-C in the leachate.

In a similar case, Rupp et al. (2002) suspected that decreasing redox potential

might be responsible for increasing P concentration in solution as well as

dissolved organic-C, and intensifying the leaching of both.

As the adsorption and the release of P is related to soil organic-C (Brennan et al.,

1994), whilst Al and Fe-hydroxides are factors in phosphate adsorption (Haynes

and Mokolobate, 2001), the mobility of soil organic-C in leaching may also affect

the sesquioxide concentrations through soil profile. In high rainfall tropical areas,


these mechanisms may transport P to the deeper profile of lateritic soils due to

higher leaching.

2.4.8 Summary points A large body of research literature on soil organic matter exists nowadays,

especially regarding the importance of soil organic matter in soil properties (i.e.

physical, chemical, and biological characteristics). During the past 50 years,

impressive progress has been achieved on the chemistry of C and N in soil

organic matter (Schnitzer, 2000). However, research on soil organic matter in

relation to P and other elements occurring in soil organic matter component is

lacking in some areas.

Organic-C is now considered a global context. According to Houghton et al.

(1996), interest in determining soil organic-C stocks on global, continental, and

regional scales has increased with progress in knowledge about climate change.

The 21st century concerns include (1) food security problems due to

overpopulation in some parts of the world, and (2) anthropogenic contributions to

green house gases (Lal, 2001b). The trend has also led to consideration of the

role of soil organic matter in soil quality and sustainability. Therefore, identifying

solutions to the problems must take into account factors such as reduction in C

loss from present sources, and creation and strengthening of C sinks through C

sequestration. This includes management of C pools (organic and inorganic) in

agricultural soils including degraded lands, and an attempt to rehabilitate

marginal soils so that they revert to natural ecosystems (Cole et al., 1997; Follet,

1993). This thesis addresses some aspects of these issues related to the

behaviour and cycling of P and C, especially in regard to marginal lateritic soils in

South Kalimantan where the author was born.

The limited information about organic-P in soils, especially in lateritic soils such

as Ultisols and Oxisols, hinders effective management of these soils. As food

security for developing countries is critically important, and the development or

reclamation of marginal lands becomes the most possible alternative, knowledge

about soil organic-P in such soils is required. In low P status environments such

as lateritic soils, soil organic-P contributes up to 80% of the total soil P (Dalal,

1977; Stevenson and Cole, 1999), especially in tropical regions where P

fertilizers are not extensively utilized (Beck and Sanchez, 1994).


The application of various types of organic matter is an important strategy for

managing Oxisols in farming systems (Dario et al., 2003; Thomas and Ayarza,

1999; Zinn et al., 2002). However, the persistence of recently added organic

matter or the extent to which the addition of organic matter has an advantage in

maintaining soil C storage in the long term is not well understood in these soils.

Moreover, the appropriate source and rate of organic matter for Oxisols in

Western Australia in relation to the persistence of soil organic-C, and the

availability of N and P has not been studied. In order to determine the kind of

organic matter to apply, it is important to know how and why various sources of

organic matter differ in C and N mineralisation and persistence.

Plant available phosphate (measured by bicarbonate extraction at pH 8.5) is

usually present only in very small amounts in Ultisols and Oxisols, partly because

of its adsorption by Al and Fe-oxides and kaolinite (Linquist et al., 1996; Zoysa et

al., 1999) or due to its precipitation as Al and Fe phosphates (Iyamuremye and

Dick, 1996). The concentration of organic-P, however, can reach more than 50%

of total-P in lateritic or intensely weathered soils (Beck and Sanchez, 1994;

Chepkwony et al., 2001) as highly charged monoester-P allows rapid adsorption

on soil minerals and extensive interaction with sesquioxides which protect inositol

phosphate from degradation (Turrion et al., 2001). Hence, any mechanism that

can ensure the continuity of transformation of organic-P to inorganic-P after fresh

organic matter addition may be important in lateritic soils. Moreover, the activity

of phosphatase has been shown to be positively correlated to soil organic-C,

especially in acid soils (Baligar et al., 1999; Barrett et al., 1998; Canarutto et al.,

1995; Dick et al., 1988). All of these processes need to be assessed in order to

determine mechanisms for P transformations when different sources of organic

matter are added to lateritic soils.

Phosphorus adsorption, measured by phosphate buffering capacity and

phosphate retention index, was correlated with organic-P in soils from Western

Australia (Brennan et al., 1994). Soil organic-C was positively correlated to the

activity of phosphatase, especially in acid soils in other studies (Baligar et al.,

1999; Barrett et al., 1998; Canarutto et al., 1995). Thus, soil organic-C plays an

important role in P transformation in acidic lateritic soils. If leaching of organic-C

is prominent in these soils, and organic-C affects P adsorption (Brennan et al.,

1994), this leaching will also have an effect on P mobility in the soil profile. For

soils in the tropics, the balance between upward (respiration) and downward


(dissolved organic-C leaching) loss could be important, especially for lateritic

soils if the effectiveness of organic matter application is to be understood. Heavy

rainfall may be an additional factor in increasing organic-C loss from soil, not only

by erosion but also in infiltration of water through the soil profile which transports

soluble organic matter. As the adsorption and release of P is related to soil

organic-C (Brennan et al., 1994), whilst Al and Fe hydroxides are factors in

phosphate adsorption (Haynes and Mokolobate, 2001), the mobility of soil

organic-C in leaching may affect the concentration and/or reactivity of

sesquioxides throughout the soil profile. In high rainfall tropical areas, these

mechanisms may occur in the deeper profile of lateritic soils due to higher

leaching.

Results from a preliminary experiment showed that phosphate extracted with 0.5

M NaHCO3 pH 8.5 (bicarbonate-P) was significantly higher in soil amended with

organic matter than in unamended soil, especially from green manure (lucerne

hay) followed by peat. Although the effect of organic matter is only transient in

such cases (Afif et al., 1995), the findings lead to the question of whether peat

would have a more persistent on P extractability due to its resistance to

decomposition, and this could be determined by analysing phosphate adsorption

isotherms from soil incubated with lucerne hay and peat for varying lengths of

time up to one year. Organic matter addition also releases P to the soil solution

(Iyamuremye and Dick, 1996; Samina et al., 2002; Tian and Kolawole, 1999), as

the result of mineralisation processes (Cobo et al., 2002; Kwabiah et al., 2003;

Lupwayi et al., 2003; Tate, 1984). These two mechanisms may be difficult to

separate in soils with high sorbing capacity of P such as lateritic soils. By

studying phosphate adsorption isotherms, the two possible processes (inorganic

effects on adsorption, and mineralisation of organic-P) can be separated as being

more and less important in increasing the extractable phosphate content of soils.

As lateritic soils contain considerable Al and Fe (Buol and Eswaran, 2000;

Eswaran, 1993; West et al., 1998), the changing of P dynamics either because of

adsorption or mineralisation would also affect the dynamics of Al and Fe. As

dissolved organic-C can control the mobility of Al (Baur and Feger, 1992; Dolfing

et al., 1999), the effect of organic matter addition may also have an impact on the

mobility both Al and Fe. The end product of these effects is likely to be very

important in predicting the general implications of organic matter addition to

lateritic soils.


The following hypotheses can be drawn and experiments can be conducted to

further examine ideas related to nutrient cycling in lateritic soils:

• To maintain organic matter in lateritic soils, it is expected that a resistant

source of organic matter such as peat will last longer than more rapidly

decomposable organic matter from agricultural sources. Therefore, net

mineralisation of nutrients such as C, N, P and K are expected to be greater

for a legume green manure than for wheat straw and least for peat.

Phosphorus availability is expected to gradually increase peat addition due to

slow release from inorganic fixation sites and/or mineralisation of organic

matter.

• At the same soil organic-C concentration in lateritic soils, newly added soil

organic matter will be more reactive than existing organic matter in minimising

phosphate adsorption as indicated by a greater release of bicarbonate-P to

the soil. With addition of recently added organic matter, soil micro-organisms

will become more active in mediating the transformation of organic-P to

inorganic-P, indicated by the increase in phosphatase activity. However, peat

is expected to have a lesser effect than lucerne hay and wheat straw.

• Rainfall will alter the ratio between leaching and respiration from organic

matter mineralisation and, to a certain extent, the persistence of the organic

matter in soil. In this context, the loss of C due to the downward movement of

water is expected to affect the mobility of organic-P, and at the same time,

changes in the content of Al and Fe oxides with depth are expected to be

closely related to mobility of organic-C in soil profile.

• Organic matter addition is expected to reduce phosphate adsorption, which

should be indicated by a greater release of bicarbonate-P to soil than from

soils which do not have added organic matter. Changes in adsorption

parameters of soil in relation to the addition of organic matter are expected.

Therefore, any reduction in phosphate adsorption will be temporary, and will

depend on the persistence and source of the organic matter added.

Consequently, peat is expected to reduce phosphate adsorption for a longer

time than lucerne hay.


2.5 CONCLUSION

There is no more important factor relating to long-term soil fertility and

productivity than soil organic matter. Thus, maintaining or increasing soil organic

matter would indirectly improve the prospect of the world food security, especially

when overpopulation problems still overshadow. This is commonly overcome by

developing or reclaiming marginal lateritic soils such as Ultisols and Oxisols.

Organic matter addition can be a solution to management of soil nutrient

dynamics and supply. However, the proper source and the persistence of

organic matter may be questionable as to its capacity contribute to crop nutrition

in the long term. Peat, despite its recalcitrant properties, may not be suitable due

to its minimal content of other nutrients. On the other hand, leguminous crops

which have high content of nutrients tend to decompose and disappear very

rapidly. Optimal proportions of both sources might be the solution for a good

nutritious source of organic matter which can exist longer in soils.

In terms of nutrient supply, soil organic matter plays an important role in P

dynamics, especially in lateritic soils, where P is in limited supply due to high

phosphate adsorption. The ability of soil organic matter to both minimise

phosphate adsorption and supply phosphate by mineralisation, and the high

proportion of organic-P in lateritic soils, lead to the idea that P can be managed

without other chemical inputs. Biochemical properties of the soil such as

phosphatase activity may also contribute to increasing bio-available P content.

High rainfall on lateritic soils in tropical regions leads to another form of soil

organic matter loss. Surface loss in the form of soil erosion, more or less, can be

managed with cover-crops and mulches. But loss of C due to leaching or soluble

organic matter still requires research, especially concerning its movement down

soil profiles. In addition, dissolved organic-C may influence other soil properties

such as P status and mobility of Al and Fe.

Alternative solutions need to be found for managing P status and transformation,

especially in countries where population increases need to be matched with

increases in food production.


C h a p t e r 3

TRANSFORMATION OF SOIL ORGANIC MATTER IN AN OXISOL IN RESPONSE TO

THE APPLICATION OF VARIOUS SOURCES OF ORGANIC CARBON

3.1 INTRODUCTION

Oxisols, together with Ultisols, are very often considered marginal for agricultural

production. They are very highly weathered soils, usually low in organic carbon

(C), cation exchange capacity, base saturation, and pH (Buol and Eswaran,

2000; West et al., 1998). These properties lead to the low availability of nitrogen

(N), phosphorus (P), and potassium (K) for plant growth. In making these highly

weathered soils more productive, inputs of soil amendments and proper

management practices are necessary.

Soil organic matter, a key to soil fertility (Schnitzer, 2000; Stevenson, 1994), is

involved in many reactions in creating favourable conditions for the availability of

nutrients due to its high capacity for nutrient adsorption, water retention, and pH

buffering (Baldock and Nelson, 1998; Schnitzer, 2000; Stevenson and Cole,

1999). It also promotes soil aggregation, and therefore, better physical and

biological characteristics of soils (Beyer et al., 2001; Manjaiah and Dhyan, 2001;

Marinari et al., 2000; Monreal et al., 1997; Willson et al., 2001). The application

of various types of organic matter is an important strategy for managing Oxisols

in farming systems (Dario et al., 2003; Thomas and Ayarza, 1999; Zinn et al.,

2002).

Organic matter content in lateritic soils is very low (Buol and Eswaran, 2000;

Pushparajah, 1998; West et al., 1998) due to warmer temperature and high

rainfall (West et al., 1998). The C:N ratio of materials is one of several factors

influencing decomposition rates of organic matter (Jhorar et al., 1991; Mishra et

al., 2001). The higher the C:N ratio, the slower the decomposition of the

materials (Rahn et al., 2003). In Oxisol with very low initial soil organic matter

content, it is not known for how long recently added organic matter of different

III. Transformation of Soil Organic Matter in An Oxisol... 44

types, with a range of C:N ratios, can persist in the soil, and which C:N ratio has

an advantage in maintaining soil C storage in the long term.

Many experiments have been carried out on the mineralisation of organic matter

in soils, especially focusing on organic-C and N. These have determined the

appropriate rate of organic matter application as materials such as manure or

green manure (Dunn and Beecher, 1994; Lupwayi and Haque, 1998), straw

(Mueller et al., 1998; Strong et al., 1987), and sewage sludge (Boucher et al.,

1999; Iakimenko et al., 1996) on various soils. However, the particular C:N ratio

and the rate of biomass for Oxisols in Western Australia in relation to the

persistence of soil organic-C, and the availability of N and P has not been

studied. In order to determine what happens when different types of organic

matter are applied, it is important to know how and why various sources of

organic matter differ in C and N mineralisation, and their effect on other soil

properties.

The hypotheses for this experiment were:

• to maintain organic matter in Oxisols, it is expected that a resistant source of

organic matter (high in C:N ratio such as peat) will last longer in soils compare

with lucerne hay and wheat straw (both low in C:N ratio). Therefore, net

mineralisation of C, N, P and K of the treatments will be greater for a legume

green manure than wheat straw and least for peat.

• P availability will gradually increase after addition of high C:N ratio organic

matter (i.e. peat) due to slow release from inorganic fixation sites and/or

mineralisation of such organic matter.

The objectives of this experiment were therefore to study the dynamics of various

forms of organic matter over time in an Oxisol, by measuring:

• the persistence of soil organic matter with different C:N ratio over time,

• how much effect the contribution of additional organic matter with different C:N

ration has, on the characteristics of soil, especially chemical and biological

properties such as soil pH, microbial biomass-C, and soil respiration, and

• to measure the long-term effect of different kinds of organic matter on the

availability of N, P and K, which are affected by C dynamics in the soil.


3.2 MATERIALS AND METHODS

3.2.1 Design of the experiment A pot experiment was set up as a factorial using completely randomised design.

The factors were:

1. Organic matter source (four treatments): control, peat, wheat straw (Triticum

aestivum L.), and lucerne hay (Medicago sativa L.).

2. Organic matter level (five treatments): no organic matter addition, 40, 80, 120,

and 160 ton ha-1 equivalent

3. Incubation time (four treatments): 3, 6, 9, and 12 months after addition.

Organic matter level was based on the fertilisation rate of triple superphosphate

(TSP) in lateritic soils in Indonesia (Suhadi, 2002), i.e. 100-150 kg TSP ha-1. The

level of 40 ton ha-1 peat which had 0.15% P (Table 3.3-2) equals to 64 kg P ha-1.

The amounts of materials applied were equal to 22, 44, 66, and 88 g dry

materials in each pot (1000 g soil). There were three replicates of each

treatment. Four pots were used for each treatment and each pot was designed

for one sampling. Pot size was 12 cm in diameter, with a height of 20 cm. Peat,

wheat straw, and lucerne hay requirements were calculated after preliminary

analysis of C and N content. These treatments were based on soil mass in pots.

Data were analysed statistically with GenStat (Payne et al., 1987). Analysis of

variance was followed by mean comparison (orthogonal polynomial contrasts) if

the treatment effect was significant. Correlation and regressions were carried out

using SPSS (Coakes, 2001) to determine any relationships between parameters.

3.2.2 Procedures Soil (Oxisol, Typic hapludox; “Balkuling”) was collected from a farmland near

York, Western Australia which had similar characteristics (such as cation

exchange capacity and aluminium (Al) and iron (Fe)-oxides content) to

Kalimantan Ultisols in Indonesia. Sampling depth was about 15-20 cm in order to

avoid excessive organic matter content and to obtain a material high in Al and

Fe-oxides. A commercially available peat (Richgro Garden Product, Canning

Vale, Western Australia) was air dried and passed through a ≤ 2 mm sieve. The

three farm products were purchased form City Farmers, Wembley Downs,

Western Australia. Lucerne hay as well as wheat straw was chopped with a


miller (RetschMühle) to pass a 0.5 cm sieve. For chemical characteristics, wheat

straw and lucerne hay were oven-dried (60° C) for two days, ground and sieved

with a 2 mm sieve.

One kg of soil was placed into each pot and the appropriate treatment was

applied by mixing the soil and respective organic material. Deionised-water was

added weekly during incubation (over 12 months) to reach a moisture content

equivalent to 60% of the maximum water holding capacity of the soil. Weighing

the pot while watering was carried out in order to reach the proper weight of

water to reach 60% water holding capacity. The procedure for measuring water

holding capacity is described in subheading 3.2.3.8.

3.2.2.1 Soil sampling All parameters were sampled every three months by taking one whole pot (four

pots for four sampling times, i.e. four incubation times). Part of the composite

sample (150 g) was kept fresh in 150 ml plastic (tight-capped) bottle for

ammonium, nitrate, and microbial biomass-C analyses the following day.

Samples for these analyses were kept overnight in cool storage. Three replicates

were applied for every experimental unit.

The rest of the sample was air dried and sieved (≤ 2 mm) and kept in sealed

plastic bags for other chemical analyses.

Methods of determination of all parameters are described below.

3.2.3 Measurements

3.2.3.1 Microbial biomass carbon Microbial biomass-C measurement was a modification the fumigation-extraction

methods of Vance et al. (1987) and Sparling (1991) from the original method by

Jenkinson (1966).

In brief, moist (60% water holding capacity) soil samples (20 g, equivalent of dry

weight) were fumigated in the presence of excess chloroform (CHCl3) in an

evacuated desiccator. These samples were extracted with 80 mL 0.5 M K2SO4.

Filtered extracts were kept in frozen (-18° C) until further analysis. The C content

in K2SO4 extracts of fumigated and unfumigated soils was measured using a

Shimadzu TOC-5000A instrument. Microbial biomass-C was calculated as the

difference of total extractable organic-C in extracts from fumigated and


unfumigated samples. Total extractable organic-C was calculated as the

difference between total extractable-C and total extractable inorganic-C

measured in the sample extract. And total extractable-C was the sum of total

extractable organic-C and total extractable inorganic-C.

3.2.3.2 Soil respiration Soil respiration was measured using the method of Anderson (1982). Soil (50 g

dry weight) was pre-incubated for one week at 40% water holding capacity

(3.2.3.8) before being incubated for another one week in sealed containers with a

CO2 trap (10 mL 0.5 M KOH). Titration of residual KOH with standardised HCl,

after CO32- was removed by precipitation as BaCO3, allowed calculation of

respired CO2 by difference after correction for titres from soil-free blanks.

3.2.3.3 Ammonium and nitrate Ammonium was measured by a modified Berthelot indophenol reaction that

utilizes the Griess-Ilosvay reaction (Searle, 1984). Ammonium is chlorinated to

monochloramine to form 5-aminosalicylate after reaction with salicylate. A green

colour is formed after oxidation and the absorbance is measured colorimetrically

at 660 nm.

Nitrate measurement was based on the hydrazinium reduction method. Nitrate is

reduced to become NO2. The amount of NO2 was measured by diazotising with

sulphanilamide and coupling with α-naphthylethylenediamine dihydrochloride to

form an azo dye measured at 540 nm. Using a dual-channel system, ammonium

and nitrate were measured using a Skalar SANplus Segmented Flow Analyzer.

3.2.3.4 Bicarbonate phosphorus Bicarbonate-P was extracted from soil using 0.5 M NaHCO3 at pH 8.5. The

method used was a modification from a method by Olsen et al. (1954) described

by Rayment and Higginson (1992). The manual colorimetric determination of

phosphate in extracts was based on the method of Murphy and Riley (1962).

3.2.3.5 Extractable potassium Extractable-K was determined in the same extract as bicarbonate-P (0.5 M

NaHCO3, pH 8.5) using atomic absorption spectrophotometry. Caesium chloride

(CsCl 1 %) was added to all samples and standards to avoid ionization effects.


3.2.3.6 Soil pH Soil pH measurement was carried out with a soil/water ratio of 1:5 at 25° C and a

shaking time of one hour. Five grams sub-samples of soil were weighed into 30

mL plastic vials and mixed with 25 mL DI water. Samples were shaken for one

hour and centrifuged to settle the soil. The pH measurement was done on the

supernatant without the electrode touching the soil sediment.

3.2.3.7 Water content Soil water content was measured exactly one week after pot watering. Twenty g

(more or less, recorded at two decimal places) soil was weighed into an

aluminium dish (∅ = 10 cm) and dried in an oven (105° C) overnight. Oven dry

soils were weighed within three minutes after being taken out of the oven to avoid

the adsorption of humidity back to the soil samples.

3.2.3.8 Water holding capacity Water holding capacity was also measured one week after pot watering on

sampling day. Twenty g (recorded at two decimal places) of moist soil was put

into a funnel with a filter paper (Whatman # 40). Water (DI) was squirted from

washing bottle to flood the soil at every two hours. Excessive water was drained

to a plastic container. The last flooding on the day was held by putting a rubber

plug at the end tip of the funnel. Soil samples were allowed to stand overnight.

The day after, the plug was removed to drain the water. Three hours later, soil

sample was removed from the filter paper in the funnel into an aluminium dish (∅

= 10 cm) and weighed. All samples were dried in an oven overnight at 105° C.

Weighing the oven-dry samples was conducted in a similar way to that of

moisture content.

3.3 RESULTS

Characteristics for the “Balkuling” soil are presented in Table 3.3-1, and C source

characteristics are presented in Table 3.3-2. The soil is sandy clay and is

slightly acid, with a total-C content of 3.12 %. Lucerne hay had a substantially

smaller C:N ratio than wheat straw or peat.


Table 3.3-1 Some physical and chemical characteristics of the Oxisol (Balkuling soil) from York, Western Australia

Characteristic 1Particle size (%): > 50 µm – 2 mm 20 – 50 µm 2 – 20 µm < 2 µm

61.4

4.0 13.7 20.9

pH (1:2.5) 5.46 2Total-C (%) 3.12 2Total-N (%) 0.03 3Bicarbonate-P (mg kg-1) 1.07 4Exch. cation (cmol+ kg-1) CEC 6.5 Ca 0.9 Mg 2.5 K 0.3 Na 0.6 1Pipette method, 2Leco C and N analyser, 3Bicarbonate extraction, 4Silver thiourea.

Table 3.3-2 Some chemical characteristics of organic matter sources for the three organic matter treatments

Source of carbon 1Total-N

(%) 1Total-C

(%) C/N ratio

2Total-P (%)

Peat 0.56 31.5 56 0.16 Wheat straw 0.76 43.4 57 0.20 Lucerne hay 2.69 41.3 15 0.25 1Leco C and N analyser, 2HNO3 and HClO4 digestion.

3.3.1 Carbon

3.3.1.1 Extractable organic carbon Extractable organic-C (0.5 M K2SO4) content of the soil decreased (p ≤ 0.01,

Table 3.3-3) with incubation time at all levels of organic matter. The decrease

ranged about 16-25% from the initial extractable organic-C. Incubation of organic

matter for three and six months led to the highest extractable organic-C content

in the soil. For the first two incubation times, the level of 120 and 160 ton ha-1 for

lucerne hay treatment showed decreasing extractable organic-C contents and

after that showed little change until the end of the experiment (Figure 3.3-1). The

trend was similar to wheat straw application with lower extractable organic-C

content. In contrast to lucerne hay and wheat straw, peat application showed no

effect on soil extractable organic-C with incubation time.


Organic matter application increased the extractable organic-C content in the soil

and the increase followed the order of control=peat<wheat straw<lucerne hay

(Figure 3.3-1). Among three different sources of organic matter, only wheat straw

and lucerne hay significantly increased (LSD 5%) extractable organic-C content.

The higher the level of organic matter applied, the more extractable organic-C

formed in soils, especially at three and six-month incubation. Peat treatment did

not affect extractable organic-C content and were not significant compared with

controls. The order of effect on extractable organic-C in all levels of organic

matter was control<peat<wheat straw<lucerne hay (LSD 5%).

Table 3.3-3 Analysis of variance for extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. Only main factors and their interactions are shown

Source of variation Degree

of freedom

Sum of squares

Mean square F P

Incubation 3 1026371 342124 8.97 < 0.001 Organic matter 3 29535481 9845160 258.13 < 0.001 Level of organic matter 3 10590147 3530049 92.55 < 0.001 Inc x OM 9 855714 95079 2.49 0.012 Inc x Level 9 389145 43238 1.13 0.344 OM x Level 9 17685078 1965009 51.52 < 0.001 Inc x OM x Level 27 639967 23702 0.62 0.924 Residual 128 4881989 38141 Total 191 65603893 Inc = incubation, OM = organic matter.


Figure 3.3-1 Comparative effect of organic matter sources, organic matter levels, and incubation times on extractable organic carbon (0.5 M K2SO4) in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

0

1000

2000

3000

0 3 6 9 12

0

1000

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3000

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month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

Ext

ract

able

org

anic

car

bon

(mg

kg-1

)


3.3.1.2 Microbial biomass carbon The main effect of organic matter application increased (p ≤ 0.01, Table 3.3-4)

soil microbial biomass-C over time. However, for wheat straw and lucerne hay,

the highest microbial biomass-C content was at six or nine months after

treatment (Figure 3.3-2). At 12 months after treatment, the level of microbial

biomass-C dropped and this occurred for all rates of wheat straw and lucerne hay

applications. Peat application, on the contrary, showed a steady increase in soil

microbial biomass-C over time as did controls. Peat even displayed higher

content of microbial biomass-C in soil at the level of 40 ton ha-1 compared with

wheat straw and lucerne hay application after 12 months’ incubation.

The organic matter content was related to soil microbial biomass-C (Figure

3.3-2). The higher the amount of organic matter applied the more microbial

biomass-C was present in the soils. Wheat straw and lucerne hay mostly

increased microbial biomass-C content more than peat applications. Lucerne

hay showed the greatest overall effect (LSD 5%) on soil microbial biomass-C

followed by wheat straw, then peat.

Table 3.3-4 Analysis of variance for microbial biomass carbon in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 1732398 577466 21.21 < 0.001 Organic matter 3 9680147 3226716 118.49 < 0.001 Level of organic matter 3 3960820 1320273 48.48 < 0.001 Inc x OM 9 2338144 259794 9.54 < 0.001 Inc x Level 9 552106 61345 2.25 0.023 OM x Level 9 3551483 394609 14.49 < 0.001 Inc x OM x Level 27 1728032 64001 2.35 < 0.001 Residual 128 3485715 27232 Total 191 27028844 Inc = incubation, OM = organic matter.


Figure 3.3-2 Comparative effect of organic matter sources, organic matter levels, and incubation times on microbial biomass carbon in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

0

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month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

Mic

robi

al b

iom

ass

carb

on (m

g kg

-1)


3.3.1.3 Soil respiration The incubation of soil with organic matter, in general, increased CO2 emission

from the soil at the beginning and this decreased as incubation time increased

(Figure 3.3-3). At the lower organic matter applications (40 and 80 ton ha-1), CO2

emissions were steady up to nine months after treatments. But at higher organic

matter additions (120 and 160 ton ha-1), CO2 emissions dropped after six months

incubation. The amount of CO2 emitted at twelve-month incubation time was

about half of the amount after three months incubation, especially for wheat straw

and lucerne hay at 120 and 160 ton ha-1.

Meanwhile, organic matter levels effected CO2 emission at regular pattern

(Figure 3.3-3). Although not significant at 40 ton ha-1 level, the higher the level of

OM applied the larger the amount of CO2 emission. Organic matter type also

affected CO2 emission from soil. At the level of 120 and 160 ton ha-1, lucerne hay

and wheat straw hay had significant differences (LSD 5%) from peat treatments,

especially at three, six, and nine-month incubation time, and the order was

control<peat<wheat straw<lucerne hay.

Table 3.3-5 Analysis of variance for soil respiration in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 91642.6 30547.5 43.09 < 0.001 Organic matter 3 668505.6 222835.2 314.34 < 0.001 Level of organic matter 3 346589.6 115529.9 162.97 < 0.001 Inc x OM 9 122749.7 13638.9 19.24 < 0.001 Inc x Level 9 48163.0 5351.4 7.55 < 0.001 OM x Level 9 310527.0 34503.0 48.67 < 0.001 Inc x OM x Level 27 73602.3 2726.0 3.85 < 0.001 Residual 128 90739.3 708.9 Total 191 1752519.1 Inc = incubation, OM = organic matter.


Figure 3.3-3 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil respiration in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

0

100

200

300

400

0 3 6 9 12

0

100

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300

400

0 3 6 9 12

0

100

200

300

400

0 3 6 9 12

0

100

200

300

400

0 3 6 9 12

Soi

l res

pira

tion

(mg

CO

2 kg-1

wee

k-1 )

56III. Transformation of Soil Organic Matter in An Oxisol...

3.3.1.4 The loss of carbon as carbon dioxide Cumulative respiration describing the amount of CO2 loss from all treatments is

presented in Figure 3.3-4.

Figure 3.3-4 Cumulative respiration representing total carbon loss following the application of organic matter during the experiment in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay.

The higher the levels of organic matter the more CO2 emission from soil, but the

trends decreased after three, six, and nine-month incubation. Carbon loss from

wheat straw and lucerne hay as CO2 was higher than peat.

Table 3.3-6 shows the percentage of C-loss as CO2 calculated from the initial

total-C in soil and each treatment. Carbon loss ranged from 0.4-5.5% from the

initial total-C in soil after treatment. Peat was the lowest and lucerne hay was the

highest in C loss. In addition, the higher the level of organic matter, the larger

proportional amount of C loss from corresponding organic matter treatment,

except controls.

40 ton ha-1

0

600

1200

0 12 24 36 48

80 ton ha-1

0

600

1200

0 12 24 36 48

120 ton ha-1

0

600

1200

0 12 24 36 48

160 ton ha-1

0

600

1200

0 12 24 36 48

mg

CO 2 k

g -1

wee

k-1

w eeks w eeks

mg

CO 2 k

g -1

wee

k-1


Table 3.3-6 Total carbon loss (%) during the experiment (48 weeks) in an Oxisol from York, Western Australia, calculated from the initial total carbon in soils

Level of organic matter (ton ha-1) Organic matter source 40 80 120 160 Control 0.5a 0.5a 0.5a 0.4a

Peat 0.6a 0.7a 0.7a 0.9a

Wheat straw 1.0ab 1.8abc 2.8c 3.1cd

Lucerne hay 1.1ab 2.5bc 4.4de 5.5e

Numbers followed by the same letters in superscript indicate differences according to LSD 5% (1.5).

3.3.2 Nitrogen

3.3.2.1 Ammonium Ammonium concentration decreased with increasing incubation time (Figure

3.3-5). The concentrations of ammonium were at the highest values at the first

two incubation times. For the first two incubation times, lucerne hay showed

significant differences (LSD 5%) in ammonium content compared with controls at

every level of organic matter addition, but these differences gradually declined as

the experiment progressed. The lower values of ammonium concentrations at

nine and twelve months incubation were still higher (LSD 5%) than the control.

Among the three different sources of organic matter, only lucerne hay had

significantly higher (p ≤ 0.01, Table 3.3-7) soil ammonium content. The higher

the rate of lucerne hay applied, the more ammonium formed in soils, especially at

three and six-months of incubation. The order of effect on ammonium was wheat

straw≤control≤peat<lucerne hay. However, there was no difference (LSD 5%) in

ammonium concentration in control soil samples or soil amended with peat or

wheat straw.


Table 3.3-7 Analysis of variance for ammonium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 879943.5 293314.5 336.26 < 0.001 Organic matter 3 5669619.4 1889873.1 2166.57 < 0.001 Level of organic matter 3 266073.9 88691.3 101.68 < 0.001 Inc x OM 9 2812928.6 312547.6 358.31 < 0.001 Inc x Level 9 232900.4 25877.8 29.67 < 0.001 OM x Level 9 788233.8 87581.5 100.40 < 0.001 Inc x OM x Level 27 683774.2 25325.0 29.03 < 0.001 Residual 128 111652.7 872.3 Total 191 11445126.5 Inc = incubation, OM = organic matter.

Table 3.3-8 Analysis of variance for nitrate in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 16226.77 5408.92 114.20 < 0.001 Organic matter 3 32882.43 10960.81 231.41 < 0.001 Level of organic matter 3 717.43 239.14 5.05 0.002 Inc x OM 9 34766.46 3862.94 81.56 < 0.001 Inc x Level 9 992.46 110.27 2.33 0.018 OM x Level 9 2843.80 315.98 6.67 < 0.001 Inc x OM x Level 27 2575.22 95.38 2.01 0.005 Residual 128 6062.67 47.36 Total 191 97067.24 Inc = incubation, OM = organic matter.


Figure 3.3-5 Interaction effect of organic matter sources, organic matter levels, and incubation times on ammonium (A) and nitrate (B) in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

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0 3 6 9 12

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80 ton ha-1

0

500

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1500

0 3 6 9 12month

0

40

80

120

0 3 6 9 12

month

Am

mon

ium

(mg

kg-1

)

Nitr

ate

(mg

kg-1

)

0

40

80

120

0 3 6 9 12

0

40

80

120

0 3 6 9 12

0

40

80

120

0 3 6 9 1

month

40 ton ha-1

120 ton ha-1

160 ton ha-1

A B

2


3.3.2.2 Nitrate Incubation time showed a significant effect (p ≤ 0.01, Table 3.3-8) on nitrate

concentration in soil. At three and twelve-month incubation, nitrate concentrations

were higher than that of six and nine-month incubation over the whole dataset.

This effect (p ≤ 0.01) occurred at all levels of organic matter, i.e. 40, 80, 120, and

180 ton ha-1 (Figure 3.3-5).

Among organic matter applications, peat application recorded a higher (LSD 5%)

nitrate content in soil compared with wheat straw, but lucerne hay was the

highest (LSD 5%), especially at the end of the experiment (twelve-month

incubation).

3.3.3 Phosphorus Incubation time had a decreasing effect (p ≤ 0.01, Table 3.3-9) on bicarbonate-P,

especially at 80, 120, and 160 ton ha-1 (Figure 3.3-6). This was despite the fact

that the increasing level of all organic matter treatments increased bicarbonate-P

contents in soil. The three-month incubation had the highest bicarbonate-P

concentration for peat, wheat straw, or lucerne hay, at 40, 80, 120, or 160 ton

ha-1. The exception was for the lucerne hay treatment (120 ton ha-1) where

maximum bicarbonate-P was at six months incubation.

Peat application showed greater effects (LSD 5%), compared with wheat straw,

on bicarbonate-P concentration in soil. Among all organic matter sources,

lucerne hay had the highest (LSD 5%) bicarbonate-P content in the soil. The

higher the level of lucerne hay applied the more bicarbonate-P content. The

overall order of bicarbonate-P contents among organic matter source was

control<wheat straw<peat<lucerne hay.


Table 3.3-9 Analysis of variance for bicarbonate phosphorus in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 1175.40 391.80 12.13 < 0.001 Organic matter 3 16127.90 5375.97 166.45 < 0.001 Level of organic matter 3 8695.19 2898.40 89.74 0.002 Inc x OM 9 982.19 109.13 3.38 < 0.001 Inc x Level 9 612.73 68.08 2.11 0.033 OM x Level 9 5803.90 644.88 19.97 < 0.001 Inc x OM x Level 27 1400.69 51.88 1.61 0.043 Residual 128 4134.00 32.30 Total 191 38931.98 Inc = incubation, OM = organic matter.


Figure 3.3-6 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil bicarbonate phosphorus in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

0

20

40

60

80

0 3 6 9 12

0

20

40

60

80

0 3 6 9 12

0

20

40

60

80

0 3 6 9 12

0

20

40

60

80

0 3 6 9 12

month

Bic

arbo

nate

pho

spho

rus

(mg

kg-1

)


3.3.4 Potassium As for bicarbonate-P, extractable-K decreased (p ≤ 0.01, Table 3.3-10) with

incubation time (Figure 3.3-7). At six and nine-month incubation, K contents

were at the highest points for wheat straw application at all levels and lucerne

hay at 40, 80, and 120 ton ha-1. The rest of the treatments had highest K content

after three months incubation.

Extractable-K in the soil increased as organic matter application rate increased

(p ≤ 0.01). The order of effect for organic matter source was control<peat<wheat

straw<lucerne hay.

Table 3.3-10 Analysis of variance for extractable potassium in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 319945 106648 11.92 < 0.001 Organic matter 3 28853900 9617967 1075.27 < 0.001 Level of organic matter 3 5222754 1740918 194.63 < 0.001 Inc x OM 9 593368 65930 7.37 < 0.001 Inc x Level 9 214673 23853. 2.67 0.007 OM x Level 9 5950499 661167 73.92 < 0.001 Inc x OM x Level 27 491309 18197 2.03 0.005 Residual 128 1144917 8945 Total 191 42791366 Inc = incubation, OM = organic matter.


Figure 3.3-7 Interaction effect of organic matter sources, organic matter, and incubation times on extractable potassium in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5 % for organic matter source in every graph.

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month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

Pot

assi

um (m

g kg

-1)


3.3.5 Other chemical properties

3.3.5.1 Soil pH Incubation with no organic matter treatment increased soil pH to 5.76 (total mean,

ANOVA). Peat tended to increase (LSD 5%) soil pH over time relative to

controls, as did wheat straw. Lucerne hay treatment decreased (LSD 5%) soil pH

over time (Figure 3.3-8).

The increasing level of organic matter demonstrated an increasing effect on soil

pH, especially for lucerne hay treatment, but a decreasing effect from peat. Both

were significant (LSD 5%).

Table 3.3-11 Analysis of variance for soil pH in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 0.18501 0.06167 3.70 0.014 Organic matter 3 35.94693 11.98231 718.38 < 0.001 Level of organic matter 3 0.32141 0.10714 6.42 < 0.001 Inc x OM 9 1.60576 0.17842 10.70 < 0.001 Inc x Level 9 0.24160 0.02684 1.61 0.119 OM x Level 9 5.31778 0.59086 35.42 < 0.001 Inc x OM x Level 27 0.41181 0.01525 0.91 0.591 Residual 128 2.13500 0.01668 Total 191 46.16529 Inc = incubation, OM = organic matter.


Figure 3.3-8 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil pH in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

5

6

7

8

0 3 6 9 12

5

6

7

8

0 3 6 9 12

5

6

7

8

0 3 6 9 12

5

6

7

8

0 3 6 9 12

month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

Soil

pH


3.3.5.2 Water content Water content of the soils (one week after watering to 60% water holding

capacity) increased (p ≤ 0.01, Table 3.3-12) with incubation time (Figure 3.3-9).

At the longest period (12 months), however, the water content decreased to a

point similar to that of the beginning of the experiment. This effect was significant

(p ≤ 0.01). At nine-month incubation, peat increased (p ≤ 0.01) water content

about 132% from the initial water content; whilst wheat straw increased (p ≤ 0.01)

water content about 77% and lucerne hay about 153%. Wheat straw and lucerne

hay showed their significant differences from control and peat on the last three

highest levels (80, 120, and 160 ton ha-1) on every incubation time.

The increased level of organic matter source raised the one-week water content

of the soil, especially wheat straw and lucerne hay at 80 and 120 ton ha-1. At the

highest level of organic matter (160 ton ha-1), the effect on water content was

about the same as 120 ton ha-1. The effect was about the same as of 40 ton ha-1,

especially at nine-month incubation. Peat levels had no significant effect on

water content and were not different from the control. The order of organic

matter sources in increasing soil water content was control<peat<wheat

straw<lucerne hay.

Table 3.3-12 Analysis of variance for soil water content in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 3054.946 1018.315 146.37 < 0.001 Organic matter 3 1883.195 627.732 90.23 < 0.001 Level of organic matter 3 556.260 185.420 26.65 < 0.001 Inc x OM 9 44.195 4.911 0.71 0.703 Inc x Level 9 41.503 4.611 0.66 0.741 OM x Level 9 380.362 42.262 6.07 < 0.001 Inc x OM x Level 27 105.812 3.919 0.56 0.958 Residual 128 890.538 6.957 Total 191 6956.812 Inc = incubation, OM = organic matter.


Figure 3.3-9 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water content in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

0

10

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0 3 6 9 12

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0 3 6 9 12

month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

Wat

er c

onte

nt (%

w/w

)


3.3.5.3 Water holding capacity As for soil moisture content, soil water holding capacity increased (p ≤ 0.01,

Table 3.3-13) with incubation time until nine months of incubation (Figure 3.3-10).

At twelve-month incubation, water holding capacity was returned to the value

near that of six-month incubation.

At the three, six, and nine-month incubation, wheat straw and lucerne hay

showed a significant effect to give more water holding capacity in soil compared

with peat application. But for longer times (twelve-month), peat addition

generated more water holding capacity in soils compared with wheat straw and

lucerne hay. This effect was not revealed by moisture content.

With regard to water content and water holding capacity, Figure 3.3-11 shows the

weekly records on the water remaining in pots after a week watering to 60%

water holding capacity in soils. Wheat straw and lucerne hay, as expected,

recorded more variation in creating the ability of soil to retain water. The higher

the level of organic matter applied, the more water can be held in soils. Between

organic matter sources, lucerne hay showed the strongest effect in increasing the

ability of soils in retaining water (Figure 3.3-12).

Daily temperatures affected water retention in the soil. The lower daily

temperature, the more water remained in the soil. Comparison between weekly

temperature and moisture content in Figure 3.3-12 illustrates the relationship. At

the end of experiment (twelve-month incubation), water content in soils tended to

return to the same as the beginning of the experiment, especially for control and

organic matter level of 40 ton ha-1. But for the higher levels (120 and 160

ton ha-1), the water content after a week watering was still higher than the initial

values after treatments.


Table 3.3-13 Analysis of variance for water holding capacity in an Oxisol from York, Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation 3 3908.89 1302.96 98.58 < 0.001 Organic matter 3 512.58 170.86 12.93 < 0.001 Level of organic matter 3 1241.29 413.76 31.31 < 0.001 Inc x OM 9 728.54 80.95 6.12 < 0.001 Inc x Level 9 172.99 19.22 1.45 0.172 OM x Level 9 655.37 72.82 5.51 < 0.001 Inc x OM x Level 27 449.62 16.65 1.26 0.197 Residual 128 1691.73 13.22 Total 191 9361.02 Inc = incubation, OM = organic matter.


Figure 3.3-10 Interaction effect of organic matter sources, organic matter levels, and incubation times on soil water holding capacity in an Oxisol from York, Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph indicates LSD 5% for organic matter source in every graph.

20

40

60

80

0 3 6 9 12

20

40

60

80

0 3 6 9 12

20

40

60

80

0 3 6 9 12

20

40

60

80

0 3 6 9 12

month

40 ton ha-1

80 ton ha-1

120 ton ha-1

160 ton ha-1

Wat

er h

oldi

ng c

apac

ity (%

w/w

)


Figure 3.3-11 The effects of organic matter source on water content measured one week after watering in an Oxisol from York, Western Australia. C = control, P = peat, W = wheat straw, and L = lucerne hay. Solid line is mean of C1-C4. 1 = 40 ton ha-1, 2 = 80 ton ha-1, 3 = 120 ton ha-1, and 4 = 160 ton ha-1. Bar in each graph indicates LSD 5% for organic matter source in every graph.

0

10

20

30

ec-99 Mar-00 Jun-00 Sep-00 Dec-00D

C1 C2 C3 C4 Series5

0

10

20

30

Dec-99 Mar-00 Jun-00 Sep-00 Dec-00

W1 W2 W3 W4 Mean C

0

10

20

30


P1 P2 P3 P4 Mean C

0

10

20

30


L1 L2 L3 L4 Mean C

Wat

er c

onte

nt (%

w/w

)



Figure 3.3-12 The effects of the highest organic matter level (160 ton ha-1) on water content in an Oxisol from York, Western Australia, measured one week after watering and maximum weekly temperature for Perth (Source: Aufdemkampe (2001)). = control, = peat, = wheat straw, and = lucerne hay. Solid line is maximum weekly temperature. Bar indicates LSD 5% for organic matter source.

-12 The effects of the highest organic matter level (160 ton ha-1) on water content in an Oxisol from York, Western Australia, measured one week after watering and maximum weekly temperature for Perth (Source: Aufdemkampe (2001)). = control, = peat, = wheat straw, and = lucerne hay. Solid line is maximum weekly temperature. Bar indicates LSD 5% for organic matter source.

0

10

20

30


28

40

3.3.6 Correlation matrices on all parameters

Table 3.3-14 Correlation matrices on all soil parameters, presented according to incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01)

A. Three-month incubation Parameter

Parameter TEC EOC CO2 MBC NH4-N NO3-N BPa Ka WC WHC

EOC 0.73** CO2 0.79** 0.93** MBC 0.73** 0.68** 0.77** NH4-N 0.46** 0.86** 0.71** 0.33* NO3-N -0.45** -0.46** 0.34*BP 0.72** 0.74** 0.61** 0.37* 0.73**K 0.64** 0.98** 0.87** 0.59** 0.91** 0.73**WC 0.81** 0.92** 0.92** 0.77** 0.71** 0.64** 0.88**WHC 0.58** 0.58** 0.54** 0.38** 0.42** 0.60** 0.51** 0.59**pH 0.36* 0.86** 0.74** 0.45** 0.92** 0.50** 0.90** 0.74**

TEC = Total Extractable-C, EOC = Extractable Organic-C, MBC = Microbial Biomass-C, BP = Bicarbonate-P, WC = Water Content, and

WHC = Water Holding Capacity.

a = in bicarbonate extract.


Table 3.3-4 (Continued)

B. Six-month incubation Parameter

Parameters TEC EOC CO2 MBC NH4-N NO3-N BPa Ka WC WHC

EOC 0.63** CO2 0.84** 0.85** MBC 0.70** 0.61** 0.80**NH4-N 0.44** 0.80** 0.70** 0.71** NO3-N 0.38** 0.39** 0.55**BP 0.67** 0.68** 0.72** 0.66** 0.78** 0.55**K 0.71** 0.89** 0.91** 0.86** 0.89** 0.51** 0.79**WC 0.73** 0.65** 0.81** 0.86** 0.60** 0.34* 0.53** 0.85**WHC 0.74** 0.65** 0.78** 0.72** 0.57** 0.29* 0.68** 0.76** 0.80**pH 0.32* 0.74** 0.66** 0.71** 0.91** 0.48** 0.54** 0.85** 0.67** 0.48**





Table 3.3-4 (Continued) C. Nine-month incubation

Parameter Parameter TEC EOC CO2 MBC NH4-N NO3-N BPa Ka WC WHC

EOC 0.63** CO2 0.70** 0.92** MBC 0.78** 0.87** 0.88** NH4-N 0.33* 0.73** 0.73** 0.58**NO3-N 0.49** 0.49** 0.58**BP 0.65** 0.65** 0.64** 0.60** 0.53** 0.32*K 0.72** 0.89** 0.91** 0.85** 0.81** 0.61** 0.60**WC 0.78** 0.66** 0.76** 0.78** 0.49** 0.30* 0.44** 0.81**WHC 0.72** 0.33* 0.47** 0.64** 0.33* 0.42** 0.68**pH 0.30* 0.80** 0.77** 0.68** 0.87** 0.52** 0.35* 0.84** 0.59**





Table 3.3-4 (Continued) D. Twelve-month incubation

Parameter Parameter

TEC EOC CO2 MBC NH4-N NO3-N BPa Ka WC WHC EOC 0.60** CO2 0.80** 0.64** MBC 0.67** 0.78** 0.60** NH4-N 0.33* 0.62** 0.47**NO3-N 0.61** 0.46** 0.95**BP 0.69** 0.63** 0.53** 0.66** 0.52** 0.42**K 0.73** 0.76** 0.64** 0.68** 0.81** 0.74** 0.62**WC 0.70** 0.48** 0.62** 0.46** 0.44** 0.37* 0.41** 0.72**WHC 0.33* 0.30*pH 0.33* 0.43** 0.34* 0.29* 0.74** 0.61** 0.77** 0.52**





Table 3.3-15 Correlation matrices on all soil parameters overall incubation times. Only significant correlations are presented. * represents significant (p ≤ 0.05), ** represents highly significant (p ≤ 0.01)

Parameter Parameter

TEC EOC CO2 MBC NH4-N NO3-N BPa Ka WC WHC EOC 0.64** CO2 0.73** 0.82** MBC 0.69** 0.65** 0.69** NH4-N 0.34** 0.67** 0.66** 0.33**NO3-N 0.35** 0.22**BP 0.66** 0.68** 0.61** 0.47** 0.65** 0.28**K 0.69** 0.87** 0.84** 0.73** 0.73** 0.35** 0.68**WC 0.55** 0.44** 0.54** 0.63** 0.27** 0.28** 0.61**WHC 0.45** 0.27** 0.38** 0.52** 0.17* -0.17* 0.31** 0.39** 0.74**pH 0.33** 0.72** 0.66** 0.55** 0.74** 0.32** 0.42** 0.85** 0.48** 0.19**

Transformation of Soil Organic Matter in An Oxisol... 79




III.

3.4 DISCUSSION

3.4.1 The persistence of organic matter When organic matter is added to soil, it is colonised by micro-organisms that

need C as their source of energy. This is supported by the increase in microbial

biomass-C content in the soil following organic matter addition (Figure 3.3-2).

The application of peat to the Oxisol steadily increased soil microbial biomass-C

throughout the year. This prolonged increase could be due to the considerable

resistance of peat C to microbial decomposition (Brake et al., 1999; Chesson,

1997; Handayanto et al., 1997), making it more slowly available to micro-

organisms than the other forms of organic matter.

Soil respiration declined with incubation time (Figure 3.3-3). The decomposition

of organic matter from wheat straw and lucerne hay was greatest during the first

three and six months of application. For lucerne hay, the most active period of

decomposition might have taken place before the first measurement was taken in

this experiment. In other studies with green manure (rice straw) and legumes,

the highest peaks in respiration were reached after four to six weeks after

application (Villegas-Pangga et al., 2000; Zaharah and Bah, 1999). In order to

estimate the persistence of organic matter in soil, the total soil respiration was

compared to the total-C at the beginning of the experiment. The higher the

amount of organic matter applied, the more CO2 produced from soils, with

lucerne hay giving the highest respiration rates (Table 3.3-6). Higher proportion

of respiration in lucerne hay compared with wheat straw, especially in higher level

of application (Figure 3.3-3), might be an indication of positive priming effect

(Fontaine et al., 2003; Kuzyakov et al., 2000), but the contributions to respiration

from added lucerne hay and existing soil organic matter are indistinguishable.

Data showed that lucerne hay respiration was twice than that of wheat straw at

nine months incubation time (Figure 3.3-3), despite almost similar total-C content

in both (Table 3.3-1). As expected, lucerne hay was mineralised at the fastest

rate, followed by wheat straw and peat. Peat persisted longer than lucerne hay

and wheat straw. The higher C and N content of green manure such as lucerne

hay, and its narrow C:N ratio, allowed a faster rate of decomposition (Bremer et

al., 1991; Curtin et al., 2000; Mapa, 1996).


The highest CO2 evolution for higher level of organic matter occurred either three

or six months after organic matter was added. The same pattern was observed

on soil microbial biomass-C. For lucerne hay, the decrease in release of CO2

was steep (Figure 3.3-3) where CO2 emission estimated to be similar to the

control several months after the end of the experiment.

Organic matter source, without doubt, increased soil respiration in every sample.

Lucerne hay and wheat straw, having more C in their tissues (Table 3.3-6) and

decomposing earlier, produced more CO2 and microbial biomass-C in soils. This

is supported, to some extent, that at all incubation times, either CO2 emission and

microbial biomass-C had very strong correlations with total extractable-C in the

soil (Table 3.3-14).

3.4.2 Phosphorus availability Contribution of total-P content in the biomass (Table 3.3-2) of organic matter

source was apparent, especially at three-month incubation (Figure 3.3-6). Even

though peat had a lower content of total-P in its biomass compared with wheat

straw, peat contribution to bicarbonate-P in soils was higher than that of wheat

straw. The difference, therefore, must be due to factors other than initial P

content of peat, since wheat straw also decomposed more rapidly (Figure 3.3-3).

It is possible that the higher microbial biomass-C observed when wheat straw

was added (Figure 3.3-2) also represented immobilisation of release phosphate.

Soil acidity (pH) due to application (Figure 3.3-8) might not affect the release of

P, contrary to literature (Fernandes and Coutinho, 1999; Hue, 1992; Perrott and

Mansell, 1989). The decreasing pH values with the increasing levels of peat and

the increasing pH values with the increasing levels of lucerne hay were not

consistent to support the effect of pH on P release. Actually, the point is that

lucerne hay release more bicarbonate-P than peat and increased the pH, so the

difference might be due to the higher pH in lucerne hay treatment desorbing

phosphate (Tang et al., 1999). Therefore, apart from direct contribution of

biomass P content, the difference in humification stage between peat, wheat

straw, and lucerne hay might have been influencing bicarbonate-P

concentrations. Peat is generally accepted to contain stabilised or humified

forms of organic matter formed by pedogenetic processes. This humified organic

matter will decompose more slowly than the bio-molecules present in crop

residues.


A gradual increase in P availability, as in the initial hypothesis, was not totally

supported during the experiment, especially for lucerne hay addition. However,

bicarbonate-P showed increasing concentrations after decreasing in the middle of

the experiment (i.e., the six- and nine-month incubations). Wheat straw and peat

showed an increasing trend on extractable-P (Figure 3.3-6) at the end of the

experiment, compared with lucerne hay. A possible explanation for this higher

phosphate release might be an effective chelating process of Al and Fe in soils

from organic matter functional groups (Alvarez-Fernandez et al., 1997; Jonge et

al., 1996) suppressing precipitation of Al- and Fe-phosphates, or just

mineralisation of organic-P from organic matter addition (Haynes and

Mokolobate, 2001; Tiessen et al., 1998). It can also be expected from the trend

that the release of phosphate might occur only after six months after organic

matter addition such as peat and wheat straw.

Based on the forward multiple regression analysis below, P extractability at all

incubation times was mostly dependent on total extractable-C, total extractable

organic-C, ammonium, and soil pH.

At the three-month incubation:

BP = 275.19 + 0.03 EOC - 0.05 CO2 + 0.07 NH4-N – 47.40 pH

R2 = 0.90

At the six-month incubation:

BP = 203.66 – 0.01 EOC + 0.05 NH4-N + 0.04 K – 0.91 WC – 32.58 pH

R2 = 0.88

At the nine-month incubation:

BP = 114.70 + 0.30 TEC + 0.02 EOC + 0.16 NH4-N - 0.024 WC - 20.06 pH

R2 = 0.69

At the twelve-month incubation:

BP = 8.27 + 0.71 TEC + 0.13 NH4-N - 0.70 WC + 0.77 WHC - 6.74 pH

R2 = 0.72

Over all incubation times (3-12 months), bicarbonate-P was dependent on almost

all soil parameters measured, as the following:


BP = 42.26 + 0.54 TEC + 0.01 EOC + 0.03 NH4-N + 0.04 NO3-N + 0.01 K - 0.63

WC + 0.37 WHC - 9.93 pH

R2 = 0.73

where BP = bicarbonate-P, TEC = total extractable-C, EOC = extractable

organic-C, NH4-N = ammonium, NO3-N = nitrate, WC = water content, and WHC

= water holding capacity.

Relationships in these regression equations are confirming the correlations as of

Table 3.3-14 and Table 3.3-15. The relationship between bicarbonate-P with

total extractable-C and extractable organic-C might be related to organic ligand

exchange (Bhadoria et al., 2002; Hinsinger, 2001; Iyamuremye and Dick, 1996).

The relationships between P and organic-C were also observed by Brennan et al.

in a range of soils of Western Australia (1994) and Condron et al. in cultivated

and uncultivated soils (1990). Moreover, as in this work, soil organic-C has been

shown previously to be positively correlated to the activity of phosphatase,

especially in acid soils (Baligar et al., 1999; Barrett et al., 1998; Canarutto et al.,

1995; Dick et al., 1988).

The relationship between ammonium, nitrate, and to some extent, potassium with

bicarbonate-P occurred perhaps due to residual effect and the increase of

electrical conductivity and soil pH from organic matter application (Eghball et al.,

2004), to make their solubility increased. Moreover, interaction effect between N

and P availability in soils (Abreu et al., 2002; Jha et al., 1993; Teng and Timmer,

1994) might enhance the relationship, especially in this type of soil where

nitrification developed without hindrance (Jha et al., 1993) to make N more

available.

3.4.3 Mineralisation of nitrogen and release of potassium Ammonium was formed in large amounts within the first three months after

organic matter addition, especially with lucerne hay application (Figure 3.3-5).

The higher N content of lucerne hay (Table 3.3-2) undoubtedly contributed to

higher concentration of ammonium in soil. The greatest ammonification rate, for

lucerne hay organic matter with low C:N ratio, may have been initiated as soon

as organic matter was applied to the soil and continued until six months. This is

in accordance to Azam et al. (1993) who found that from 13 to 41% of the applied

N from N-labelled plant residues were mineralised during a four-week incubation


(25° C) in Mollisols, the proportions decreasing according to their total N content.

The ammonification of organic-N may have contributed to the pH increases

observed for lucerne hay addition (Figure 3.3-8). This is despite the substantial

nitrification which occurred, a transformation known to produce acidity (Bolan and

Hedley, 2003; Sakala et al., 2004).

As OM decomposition enhances nutrient availability (Hartemink and O'Sullivan,

2001; Lupwayi and Haque, 1999; Maharudrappa et al., 2000; Tian, 1998;

Villegas-Pangga et al., 2000; Zaharah and Bah, 1999), this process is beneficial

for marginal soils such as Oxisols. Mineralisation of P and K showed the same

trend for all organic matter applications. Extractable amounts of both P and K

were generally greatest at the three-month sampling time (Figure 3.3-6 and

Figure 3.3-7), and subsequently declined. Thus, the first three months of

incubation was the most active decomposition period for P and K as well as for

ammonium, especially at the higher levels of organic matter applied. There was

little evidence of net nitrification until 12 months, indicating likely immobilisation of

N into the microbial biomass.

The order of mineralisation was control<peat<wheat straw<lucerne hay for nitrate

and K, whilst for ammonium and P the order was control<wheat

straw<peat<lucerne hay. Peat was more effective than wheat straw at releasing

ammonium, perhaps because peat contains more amine functional groups

(Landgraf et al., 1998). In addition, the C:N ratio of peat was slightly lower than

that of wheat straw (Table 3.3-2). Overall, a legume green manure, in this case

lucerne hay, produced greater net mineralisation for C, N, P, and K as

hypothesised, even though this effect was only observed within the first three

months after organic matter application.

3.5 CONCLUSION

Organic matter persistence in an Oxisol of Western Australia was dependent on

its C:N ratio. The smaller C:N ratio (i.e. lucerne hay), the faster the

decomposition processes.

The P-availability, which is expressed as net release of bicarbonate-P, increased

due to organic matter application, especially from lucerne hay and peat. Peat

persisted longer and can be expected to sustain bicarbonate-P availability over a

longer term than the other amendments. Bicarbonate-P related to total


extractable-C, extractable organic-C, NH4-N, and soil pH through out the

experiment. Thus, if organic amendments were the option to resolve limited

supply of P in lateritic soils, peat application is a reasonable choice for longer

term effect.

Organic matter mineralisation enhanced nutrient availability, especially

ammonium, nitrate, bicarbonate-P, and extractable-K. This process occurred

soon after organic matter applied to the soil, especially for lucerne hay. The

maximum mineralisation took place within the first three months.


C h a p t e r 4

EXISTING VERSUS ADDED SOIL ORGANIC MATTER IN RELATION TO PHOSPHORUS

AVAILABILITY ON LATERITIC SOILS

4.1 INTRODUCTION

Organic matter is involved indirectly in many reactions and processes in soils,

creating favourable conditions for the availability of nutrients due to its high

capacity for cation adsorption, water retention, and pH buffering capacity

(Schnitzer, 2000; Stevenson and Cole, 1999). Organic matter also promotes soil

aggregation and improves soil structure (Marinari et al., 2000; Monreal et al.,

1997) and influences soil biological characteristics of soils such as microbial

population and activities as well as soil enzymes also improve (Abbott and

Murphy, 2003; Beyer et al., 2001; Manjaiah and Dhyan, 2001). The

decomposition of soil organic matter will produce a pool of organic carbon (C)

which is easily mineralised and another pool which is slowly degradable

(Eusterhues et al., 2003). This pool is decomposed slowly due to: (i) chemical

recalcitrance, i.e. stabilisation attributable to the structural properties of the

organic matter, (ii) inclusion of organic matter into aggregates or micropores,

leading to physical protection from microbial attack (Strong et al., 1999), and (iii)

interaction of carbon compounds with soil minerals (Baldock and Skjemstad,

2000; Sollins et al., 1996).

Plant available phosphate (measured by bicarbonate extraction at pH 8.5) is

usually present only in very small amount in Ultisols and Oxisols, partly because

of its strong adsorption by aluminium (Al) and iron (Fe)-oxides (Linquist et al.,

1996; Zoysa et al., 1999) or by its precipitation with Al and Fe (Iyamuremye and

Dick, 1996). However, the concentration of organic phosphorus (P) can reach

more that 50 % of total-P in lateritic or intensely weathered soils (Beck and

Sanchez, 1994; Chepkwony et al., 2001) as highly charged monoester-P allows

rapid adsorption on soil minerals and extensive interaction with sesquioxides

which protect inositol phosphate from degradation (Turrion et al., 2001). Hence,

any mechanism that can ensure the continuity of organic-P to inorganic-P

IV. Existing versus Added Organic Matter … 86

transformation after fresh organic matter addition may become important in

lateritic soils.

The effect of recently added organic matter needs to be distinguished from the

effect of inherent soil organic matter. The stable and inert pools of organic matter

in soil can be very old, ranging from hundred to thousand years (Eusterhues et

al., 2003; Falloon and Smith, 1998; Hassink, 1997; Monreal et al., 1997; Oades,

1995) and their identity, as well as their physical and chemical properties, are not

well understood (Eusterhues et al., 2003; Hsieh, 1992; Ruhlmann, 1999; Theng

et al., 1992). In order to study the effect of adding organic matter to soil in terms

of phosphate adsorption, it would be useful to remove the stable pools of soil

organic matter from soil samples so that the effect of newly added organic matter

does not interact with that of stable pools.

Soil organic matter plays an important role in phosphate adsorption (Brennan et

al., 1994; Erich et al., 2002; Leytem et al., 2002; Sinaj et al., 2002). Newly added

organic matter has abundance of organic-C. With the assumption that at least

some organic compounds added in organic amendments are those competitive

for phosphate adsorption, any effects of freshly added organic matter on

phosphate adsorption must be greater than for inert and stable C pools. This

could be related to the formation of organic ligands adsorbing to sesquioxides,

and consequent blocking of phosphate adsorption sites (Haynes and Mokolobate,

2001; Syers et al., 1971). As stable and inert pools of soil organic matter usually

interact or are associated with clay minerals and/or Fe-oxides (Baldock and

Skjemstad, 2000; Eusterhues et al., 2003), removing this pool from soil may

disturb phosphate desorption in soil with newly added organic matter. Moreover,

soil organic-C has been shown to be positively correlated to the activity of

phosphatase, especially in acid soils (Baligar et al., 1999; Barrett et al., 1998;

Canarutto et al., 1995; Dick et al., 1988). All of these processes need to be

assessed in order to decide the most crucial mechanism in affecting P dynamics

if organic matter were added to lateritic soils.

The hypotheses drawn from this brief review are:

• at the same soil organic-C concentration, newly added soil organic matter will

be more reactive compared with pre-existing soil organic matter in minimising

phosphate adsorption indicated by more release of bicarbonate-P to the soil,


• with addition of fresh organic matter, soil micro-organisms will become more

active in mediating the transformation of organic-P to inorganic-P, indicated

by an increase in phosphatase activity.

The objectives of this study were therefore:

• to study the contribution of newly applied organic matter on different forms of

P in soil, such as bicarbonate-P, total-P, non-extractable-P, and microbial

biomass-P.

• to measure if microbial biomass-P and/or soil soluble organic-C generate

more P in low P environment indicated by their positive relationship with

bicarbonate-P, and

• to measure whether phosphatase becomes more active in mediating the

transformation of organic-P to inorganic-P in totally newly added organic

matter indicated by positive correlation between bicarbonate-P and

phosphatase activity.


4.2.1 Design of the experiment This incubation experiment was set up in a factorial design using completely

randomised block design. The treatments are presented in Table 4.2-2 with

treatment combination in Table 4.2-3.

Table 4.2-1 Description of soil used and locations in Western Australia where soils were collected

Soil Location Description Subsurface regolith ‘soil’ Boddington Subsurface soil from mining site; no soil organic

matter, high in extractable-Al and Fe Ultisol (Typic kandiudult) Jarrahdale Lower in P and higher in extractable-Al and Fe

Oxisol (Plinthic eutrudox) Bunbury Lower in P and higher in extractable-Al and Fe


Table 4.2-2 Organic matter sources (peat, wheat straw, and lucerne hay) used as treatments in the experiment

Type of soil organic matter Source

• Peat (organic-C in 80 ton ha-1 peat + organic-C in existing soil organic matter)

• Wheat straw (organic-C in 80 ton ha-1 wheat straw + organic-C in existing soil organic matter) Existing

• Lucerne hay (organic-C in 80 ton ha-1 lucerne hay + organic-C in existing soil organic matter)

• Peat (organic-C in 80 ton ha-1 peat + organic-C in additional peat equal to that of pre-existing soil organic matter)

• Wheat straw (organic-C in 80 ton ha-1 wheat straw + organic-C in additional wheat straw equal to that of pre-existing soil organic matter)

New

• Lucerne hay (organic-C in 80 ton ha-1 lucerne hay + organic-C in additional lucerne hay equal to that of pre-existing soil organic matter)

Table 4.2-3 Treatment combinations and abbreviations used from three factors of the experiment. Soil (subsurface soil, Ultisol, and Oxisol), source of organic matter (control, peat, wheat straw, and lucerne hay), and type of soil organic matter (existing and new)

Soil organic matter Existing New Soil

Control Wheat straw

Lucerne hay Peat Control Wheat

straw Lucerne

hay Peat

Subsurface soil SEC SEW SEL SEP SNC SNW SNL SNP

Ultisol UEC UEW UEL UEP UNC UNW UNL UNP Oxisol OEC OEW OEL OEP ONC ONW ONL ONP

The treatments with new soil organic matter were based on the organic-C

concentration in peat, wheat straw, and lucerne hay plus the organic-C in the

existing soil organic matter in the original soil samples. In the experimental

design, for any given organic matter source (new and/or existing), the

concentrations of organic-C was similar. The C, nitrogen (N), and P content of

the respective organic matter sources are given in Table 3.3-2 of Chapter 3.

4.2.2 Procedures An Ultisol (Typic kandiudult) was collected near Jarrahdale (Latitude 33°19’40” S;

Longitude 115°45’55” E) and an Oxisol (Plinthic eutrodox) near Bunbury (Latitude

32°19’00” S; Longitude 116°11’00” E) in Western Australia. Both soils were

selected due to their high content of extractable-Al and Fe (McArthur, 1991) and


in order to see their effects on phosphate adsorption and P mobility in the soils.

The third soil (subsurface soil) was a deep regolith material collected at

Boddington Gold mine in Western Australia where the soils have considerable

extractable-Al and Fe but had very low soil organic matter. This soil is developed

from deep chemical weathering of Archaean granites and migmatites. The

subsurface soil was selected to isolate the effect of combustion on soil physical

and chemical characteristics, since the amount of soil organic matter was

negligible (Table 4.2-4). Addition of soil starter (one gram of original soil similar

to the soils used for incubation) after combustion assumed that living soil

microbes (if any) would return to normal condition as before the combustion.

Soils (including the Ultisol from Jarrahdale) in this area are classified as Darling

Range laterites, which have very low concentrations of nutrients (Gilkes et al.,

1973; McArthur, 1991). Topsoils commonly have a coarse texture with 50-82%

gravel (3-8 mm) and slightly acidic pH (5.5-6.5) (McArthur, 1991). Selected

characteristics of the three soils are presented in Table 4.2-4.

Table 4.2-4 Characteristics of the three soils (subsurface soil, Ultisol, and Oxisol) used in the glasshouse incubation.

Soil Soil characteristic Subsurface soil

regolith ‘soil’ Ultisol

(Typic kandiudult) Oxisol

(Plinthic eutrodox) 1Texture Sand (%) 48 89 60 Silt (%) 45 5 9 Clay (%) 7 6 31 2DOC (mg kg-1) 3±0.8 30±3.0 13±3.1 3Org-C (%) 0.035 5.8 2.8 3Total-N (%) 0.029 0.20 0.15 4Total-P (mg kg-1) 237±5.5 57±3.2 83±6.7 5Bicarb. P (mg kg-1) 26±2.1 7±0.9 8±1.2 5Ext-K (mg kg-1) 37 174 37 6Exch. cat. (cmol+ kg-1) CEC 2.1 2.8 7.2 Ca 0.8 1.2 1.8 Mg 1.2 0.8 3.0 K 0.1 0.1 0.3 Na 1.1 0.9 1.0 pH (1:5 soil H2O) 6.8±0.06 6.2±0.12 6.2±0.20 7Oxalate-Al (g kg-1) 1.0±0.19 11±1 11±2 7Oxalate-Fe (g kg-1) 18±1 73±3 274±7 1Pipette method, 2TOC in water extract, 3Leco C and N analyser, 4Persulfate digestion, 5Bicarbonate extraction, 6Silver thiourea, 7Ammonium oxalate extraction. Values after ± are standard errors.


Peat, wheat straw, and lucerne hay were oven-dried (60 °C) for two days, and

chopped with a grinding mill (RetschMühle). They were then finely ground with

another mill (C and N Junior, ≤1 mm sieve) for more rapid decomposition.

Soil samples from the field were air-dried and sieved with a ≥2 mm sieve. For the

newly added organic matter treatments, soils were combusted at 450° C for 12

hours using a pottery kiln (Kiln West, model 6191Z). Soil samples were put into

three separate steel containers having dimension of 35x35x15 cm each. Soil

samples were poured into the container to about 3.5 cm depth to ensure a

homogenous heat distribution. Each container had a loosely fitting lid, allowing

fumes from combustion of samples to be released easily. Soil was combusted

until the colours become lighter than the original and the organic-C content about

0.00%. A temperature of 450° C is hot enough to deplete organic-C content, but

has been found in some previous works to have a minimal effect on other

physical and chemical properties of soils such as porosity, textural class, and soil

acidity (Giovannini et al., 1988; Kang and Sajjapongse, 1980).

Table 4.2-5 Some characteristics of soil (subsurface soil, Ultisol, and Oxisol) samples before and after combustion. Values after ± are standard errors.

Soil Subsurface soil

‘regolith’ soil Ultisol

(Typic kandiudult) Oxisol

(Plinthic eutrodox) Characteristic

Before After Before After Before After Org-C (%) 0.04 0.01 5.81 0.02 2.85 0.00

Total-N (%) 0.03 0.01 0.20 0.01 0.15 0.00

Total-P1 (mg kg-1) 237±6 244±4 57±3 69±6 83±7 89±2 Ext-Al (mg kg-1) 1±0.2 10±1.4 11±1.3 28±3.5 11±1.8 43±5.1 Ext-Fe (mg kg-1) 18±0.9 47±1.8 73±2.8 182±4.6 274±7.0 260±7.36 pH 6.8±0.06 6.5±0.16 6.2±0.12 7.1±0.13 6.2±0.20 6.0±0.04 1 = Persulfate digest method.

Treatments were applied together with a ‘soil starter’ as an inoculant to introduce

new decomposer micro-organisms. The ‘soil starter’ was one gram of original

soil similar to the soils used for incubation. Soil (0.5 kg) was weighed for each

pot and organic matter treatments were applied by homogeneously mixing the

soil and the respective treatments with mechanical mixer (100 times, end over

end). Soils were incubated for three months to allow decomposition before the

first sampling. Water was added weekly to reach a moisture content equivalent

to 60 % of the maximum water holding capacity of the soil, determined by

weighing. Soil characteristics before treatment (physical, chemical, and


biological) were analysed for comparison with the same parameters during the

course of incubation. Monocalcium phosphate was added (45.8 mg kg-1) to

equilibrate in term of precipitation reactions with exchangeable-Al and Fe, and

chemisorption to clay and sesquioxide minerals, in soils for a one-month period

prior to organic matter treatments. The amount of monocalcium phosphate was

equal to 100 kg ha-1 triple superphosphate (TSP), recommended P fertiliser for

lateritic soil in Indonesia (Suhadi, 2002). This additional P was added to ensure

that P content adsorbed to soil particles and Al- and Fe-oxide was high enough to

see the effect of organic matter addition on phosphate release.

The statistical significance of treatment and interaction effects was determined

using analysis of variance with GenStat (Payne et al., 1987). Correlation and

regressions were carried out using SPSS (Coakes, 2001) to determine any

relationships between parameters.

4.2.2.1 Soil sampling Parameters were measured at three sampling times, i.e. three, six, and nine

months after organic matter treatments. Part of the sample was kept fresh in a

150 mL plastic (tight-capped) bottle for ammonium and nitrate analyses the

following day. Samples for these analyses were kept overnight in cool storage.

The rest of the samples were air dried and sieved (≤ 2 mm) and kept in sealed

plastic bags for other chemical analyses.

4.2.3 Measurements

4.2.3.1 Dissolved organic carbon The principle of dissolved organic-C measurement was from the method by

Wagai and Sollins (2002). Dissolved organic-C was determined as the difference

between total-C and inorganic-C in solution based on platinum-catalysed

combustion/non dispersive infrared gas analysis.

Dissolved organic-C was extracted from 10 g air-dry soil by 25 mL deionised

(MilliQ) water in a centrifuge tube. Samples were shaken for 30 minutes on an

end-over-end shaker, and then centrifuged at 10,000 rpm for 10 minutes. The

supernatant was filtered through a 0.2 µm filter (25 mm, supor membrane, non

sterile, Pall Gelman Laboratory) using a syringe. Dissolved organic-C was

measured by a total organic-C analyser (Shimadzu TOC-5000A) as the

difference between total-C and inorganic-C in the solution.


4.2.3.2 Total phosphorus Total-P was measured in acid-persulfate (Nelson, 1987). Soil sample digestion

was carried out using autoclave.

In brief, total-P was measured as the following. A mixture of 0.5 g soil, 1.0 mL

deionised water, 1.0 mL 5.5 M H2SO4, and 0.400 g K2S2O4 in screwcapped Pyrex

containers (100 mL) was autoclaved for one hour at 130° C. Dilution to 50 mL

was carried out with deionised water. Samples were mixed and stood overnight

to allow particulate materials to settle. Total converted phosphate in extract was

measured colorimetrically as described by Rayment and Higginson (1992).

4.2.3.3 Bicarbonate phosphorus Bicarbonate-P was extracted using 0.5 M NaHCO3 pH 8.5. This is a modification

from a method by Olsen et al. (1954) described by Rayment and Higginson

(1992). The manual colorimetric method for determination of phosphate in the

extract was based on that of Murphy and Riley (1962).

4.2.3.4 Non-extractable phosphorus Measurement of non-extractable-P was conducted by measuring total-P with

acid-persulfate digestion by Nelson (1987) and then subtracting the bicarbonate-

P content of the same soil measured beforehand.

4.2.3.5 Phosphatase Acid phosphatase (phosphomonoesterase EC 3.1.3.2) activity was determined

on a one gram soil sample according to the method of Tabatabai (1994). Briefly,

soil samples were incubated with the substrate p-nitrophenyl phosphate (p-NPP)

for one hour. The reaction was terminated with 0.5 M CaCl2 and 0.5 M NaOH

and residual absorbance of p-NPP determined spectrophotometrically at 410 nm.

Controls were processed with each soil analysed to measure the absorbance not

derived from p-nitrophenol released by phosphatase. These were processed

with the addition of p-NPP solution after the additions of 0.5 M CaCl2 and 0.5 M

NaOH immediately before the filtration of the samples. Enzyme activities are

expressed as µmol p-NPP g(soil)-1 hour-1.

4.2.3.6 Microbial biomass phosphorus Microbial biomass-P was determined by the method by Wu et al., (2000),

following the fumigation and extraction method of Powlson and Jenkinson (1976).


After fumigation, soil P was extracted with 0.5 M NaHCO pH 8.5 as described by

Rayment and Higginson (1992). The manual colorimetric determination of

phosphate at the end of the method was based on the method of Murphy and

Riley (1962).

3

4.2.3.7 Extractable aluminium and iron These two oxides were calculated from acid oxalate extraction as explained by

Rayment and Higginson (1992), modified to use 0.250 mg of soil in 25 mL

reagent. The extraction was conducted in darkness to avoid photo-reduction of

Fe (Schwertmann and Taylor, 1989). A black plastic sheet was also used to

cover sample batches during transportation from shaker to centrifuges.

Aluminium and Fe concentrations in extracts were determined by atomic

absorption spectrophotometry. Caesium chloride solution (0.01 M CsCl) was

used for dilution (1+4) to reduce ionisation effects.

4.2.3.8 Water content and water holding capacity These parameters were measured using standard procedures described in

Chapter 3.

4.3 RESULTS

4.3.1 Carbon

4.3.1.1 Dissolved organic carbon Increasing incubation time decreased the content of dissolved organic-C in all

soils (Table 4.3-1 and Figure 4.3-1). At the end of the experiment, dissolved

organic-C in soil was lower than of the three-month incubation. This decrease

occurred, to a greater extent, in treatments with higher dissolved organic-C

content than the others, such as subsurface soil and Ultisol receiving new organic

matter.

The type of soil also had an effect on dissolved organic-C concentration. The

Ultisol had the highest dissolved organic-C content followed by subsurface soil

and the Oxisol.

New soil organic matter applications significantly (p ≤ 0.01, Table 4.3-1)

increased dissolved organic-C contents in all soils at three months following

addition compared with existing organic matter. In the Oxisol and Ultisol, the


increases in dissolved organic-C contents were more than 100%. In subsurface

soil the increase was only about 11%.

The source of organic matter addition also increased dissolved organic-C in all

three soils. Lucerne hay had the highest effect followed by wheat straw and peat.

Table 4.3-1 Analysis of variance for dissolved organic carbon in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 85020.1 42510.1 44.00 < 0.001 Soil 2 143665.1 71832.5 74.35 < 0.001 Organic matter 3 1747817.3 582605.8 603.05 < 0.001 Type of SOM 1 116236.6 116236.6 120.32 < 0.001 Inc x soil 4 27812.4 6953.1 7.20 < 0.001 Inc x TSOM 2 47807.3 23903.7 24.74 < 0.001 Soil x TSOM 2 51276.1 25638.0 26.54 < 0.001 Inc x OM 6 141104.4 23517.4 24.34 < 0.001 Soil x OM 6 229846.9 38307.8 39.65 < 0.001 TSOM x OM 3 173780.3 57926.8 59.96 < 0.001 Inc x soil x TSOM 4 3879.1 969.8 1.00 0.408 Inc x soil x OM 12 80466.5 6705.5 6.94 < 0.001 Inc x TSOM x OM 6 68470.9 11411.8 11.81 < 0.001 Soil x TSOM x OM 6 163640.0 27273.3 28.23 < 0.001 Inc x soil x TSOM x OM 12 52706.5 4392.2 4.55 < 0.001 Residual 144 139118.0 966.1 Total 215 3272647.8 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


Existing New

0

350

700

0 3 6 90

350

700

0 3 6 9

0

350

700

0 3 6 9

0

350

700

0 3 6 9

0

350

700

0 3 6 90

350

700

0 3 6 9

monthmonth

Sub

Ult

Oxi

Dis

solv

ed o

rgan

ic c

arbo

n (m

g kg

-1)

Sub

Ult

Oxi

Figure 4.3-1 Interaction effect of organic matter sources, type of organic matter, and incubation on dissolved organic carbon in three soils from Western Australia.

= control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.2 Phosphorus

4.3.2.1 Bicarbonate extractable phosphate

There was no main effect of incubation time on bicarbonate-P (Table 4.3-2). However, interaction effect between incubation and soil type was significant (p ≤ 0.01). Bicarbonate-P in the Oxisol and subsurface soil decreased over time, but increased in the Ultisol (

Figure 4.3-2).

Bicarbonate-extractable P differed between soil types at the first sampling with

the existing soil organic matter. The highest concentration on bicarbonate-P was

observed in the subsurface soil, followed by the Oxisol and then the Ultisol.

Treatments with new organic matter had more bicarbonate-P than those with

existing soil organic matter (p ≤ 0.01, Table 4.3-2). This effect was more

pronounced in the Oxisol and Ultisol than in the subsurface soil. The addition of

new organic matter increased the bicarbonate-P concentration by about 130%

above the existing soil organic matter in the Oxisol and 148% in the Ultisol. In

contrast, in the subsurface soil, the new soil organic matter decreased (p ≤ 0.01)

bicarbonate-P by about 20%.

As in the first experiment, organic matter addition, especially lucerne hay,

increased bicarbonate-P in soils, followed by peat and wheat straw. Only lucerne

hay showed a trend for bicarbonate-P to increase throughout the incubation,

especially in the Ultisol.


Table 4.3-2 Analysis of variance for bicarbonate phosphorus in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 81.86 40.93 1.77 0.174 Soil 2 12495.53 6247.76 270.07 < 0.001 Organic matter 3 71344.98 23781.04 1027.98 < 0.001 Type of SOM 1 7526.04 7526.04 325.32 < 0.001 Inc x soil 4 655.44 163.86 7.08 < 0.001 Inc x TSOM 2 232.69 116.35 5.03 0.008 Soil x TSOM 2 12454.19 6227.10 269.17 < 0.001 Inc x OM 6 3077.73 512.96 22.17 < 0.001 Soil x OM 6 2597.40 432.90 18.71 < 0.001 TSOM x OM 3 5305.98 1768.66 76.45 < 0.001 Inc x soil x TSOM 4 470.78 117.69 5.09 < 0.001 Inc x soil x OM 12 1589.52 132.46 5.73 < 0.001 Inc x TSOM x OM 6 902.90 150.48 6.50 < 0.001 Soil x TSOM x OM 6 7165.40 1194.23 51.62 < 0.001 Inc x soil x TSOM x OM 12 520.19 43.35 1.87 0.042 Residual 144 3331.33 23.13 Total 215 129751.96 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

0

40

80

120

0 3 6 90

40

80

120

0 3 6 9

0

40

80

120

0 3 6 90

40

80

120

0 3 6 9

0

40

80

120

0 3 6 90

40

80

120

0 3 6 9monthmonth

Sub

Ult

Oxi

Bic

arbo

nate

pho

spho

rus

(mg

kg-1

)

Existing

Sub

Ult

Oxi

Figure 4.3-2 Interaction effect of organic matter sources, type of organic matter, and incubation on bicarbonate phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.2.2 Total phosphorus Incubation time had a significant main effect (p ≤ 0.01) on total-P (Table 4.3-3).

Total-P concentrations increased at the second sampling (six-month incubation)

and decreased again at the end of the experiment (Figure 4.3-3).

The type of soil organic matter had a very significant effect (p ≤ 0.01, Table 4.3-3)

on total-P concentrations. New organic matter treatments had about 27% higher

total-P than the existing soil organic matter treatments for all soil types. For

Ultisol, the increase was even more pronounced (64%) than in the Oxisol (50%).

Total-P in soil was affected by the source of organic matter applied. Soil treated

with lucerne hay had the highest total-P contents followed by peat and wheat

straw treatments. Lucerne hay raised total-P concentration about 100% from the

control, while the increase for peat was about 46% and for wheat straw about

32%.

Table 4.3-3 Analysis of variance for total phosphorus in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 6980.0 3490.0 10.46 < 0.001 Soil 2 968976.6 484488.3 1451.55 < 0.001 Organic matter 3 548346.9 182782.3 547.62 < 0.001 Type of SOM 1 106577.8 106577.8 319.31 < 0.001 Inc x soil 4 2164.4 541.1 1.62 0.172 Inc x TSOM 2 515.1 257.6 0.77 0.464 Soil x TSOM 2 36838.2 18419.1 55.18 < 0.001 Inc x OM 6 1597.6 266.3 0.80 0.573 Soil x OM 6 15989.2 2664.9 7.98 < 0.001 TSOM x OM 3 35546.5 11848.8 35.50 < 0.001 Inc x soil x TSOM 4 644.1 161.0 0.48 0.749 Inc x soil x OM 12 2230.2 185.9 0.56 0.873 Inc x TSOM x OM 6 1440.5 240.1 0.72 0.635 Soil x TSOM x OM 6 21755.3 3625.9 10.86 < 0.001 Inc x soil x TSOM x OM 12 1698.0 141.5 0.42 0.952 Residual 144 48063.3 333.8 Total 215 1799363.9 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

0

200

400

0 3 6 90

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 90

200

400

0 3 6

monthmonth

Sub

Ult

Oxi

Tota

l pho

spho

rus

(mg

kg-1

)

Existing

Sub

Ult

Oxi

9

Figure 4.3-3 Interaction effect of organic matter sources, type of organic matter, and incubation on total phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.2.3 Non-extractable phosphorus Incubation time also significantly (p ≤ 0.01) decreased non-extractable-P content

in soil samples, though non-extractable-P also slightly increased at the second

sampling (Figure 4.3-4).

The main effect of soil types was the same as for in bicarbonate-P and total-P.

Subsurface soil had higher non-extractable-P concentration, followed by the

Oxisol and then the Ultisol.

The new organic matter significantly (p ≤ 0.01, Table 4.3-4) increased non-

extractable-P content in all soil samples, with the mean increased of about 23%.

This differed according to the type of soil; the increase was about 7% for

subsurface soil, 38% for the Oxisol, and 48% for the Ultisol.

Non-extractable-P in soil was also affected by the type of organic matter applied.

Lucerne hay increased non-extractable-P about 76% from control, while with peat

the increase was about 33% and with wheat straw about 27%.

Table 4.3-4 Analysis of variance for non-extractable phosphorus in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 5569.8 2784.9 7.69 < 0.001 Soil 2 769915.1 384957.6 1063.54 < 0.001 Organic matter 3 227023.0 75674.3 209.07 < 0.001 Type of SOM 1 57428.2 57428.2 158.66 < 0.001 Inc x soil 4 1031.7 257.9 0.71 0.585 Inc x TSOM 2 113.2 56.6 0.16 0.855 Soil x TSOM 2 6404.1 3202.1 8.85 < 0.001 Inc x OM 6 1502.1 250.4 0.69 0.657 Soil x OM 6 20395.7 3399.3 9.39 < 0.001 TSOM x OM 3 13498.0 4499.3 12.43 < 0.001 Inc x soil x TSOM 4 994.0 248.5 0.69 0.602 Inc x soil x OM 12 1580.2 131.7 0.36 0.974 Inc x TSOM x OM 6 627.2 104.5 0.29 0.941 Soil x TSOM x OM 6 5227.0 871.2 2.41 0.030 Inc x soil x TSOM x OM 12 2250.3 187.5 0.52 0.900 Residual 144 52122.0 362.0 Total 215 1165681.7 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

0

100

200

300

0 3 6 90

100

200

300

0 3 6 9

0

100

200

300

0 3 6 90

100

200

300

0 3 6 9

0

100

200

300

0 3 6 90

100

200

300

0 3 6 9

monthmonth

Sub

Ult

Oxi

Non

-ext

ract

able

pho

spho

rus

(mg

kg-1

)

Existing

Sub

Ult

Oxi

Figure 4.3-4 Interaction effect of organic matter sources, type of organic matter, and incubation on non-extractable phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.2.4 Phosphatase Phosphatase activities increased with increasing incubation time in all soils,

especially in the Ultisol and the Oxisol (Figure 4.3-5). By the end of experiment,

the mean activities had almost doubled compared with the initial phosphatase

activities in all soils.

Soil types were different in terms of changes in phosphatase activity. Subsurface

soil recorded lower phosphatase activities than those in the Ultisol and the

Oxisol. The Ultisol and Oxisol showed higher activities, especially the Oxisol,

which had about four times (main effect of soil type) higher phosphatase activity

than subsurface soil, and about 135% higher than the Ultisol.

The new soil organic matter treatments had significantly lower (p ≤ 0.01, Table

4.3-5) initial phosphatase activity in soils than existing soil organic matter

treatments.

The source of organic matter applied had different effects. Lucerne hay gave the

highest phosphatase activities, especially for the new soil organic matter

application. Peat showed a greater effect than wheat straw in the Oxisol. In the

existing soil organic matter treatments, the phosphatase activities were almost at

the same magnitude regardless of the type of organic matter applied, especially

on the Ultisol and Oxisol.


Table 4.3-5 Analysis of variance for phosphatase activity in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 1318.911 659.456 599.10 < 0.001 Soil 2 2210.482 1105.241 1004.09 < 0.001 Organic matter 3 627.752 209.251 190.10 < 0.001 Type of SOM 1 950.461 950.461 863.47 < 0.001 Inc x soil 4 723.057 187.764 164.22 < 0.001 Inc x TSOM 2 101.641 50.821 46.17 < 0.001 Soil x TSOM 2 393.373 196.687 178.69 < 0.001 Inc x OM 6 222.014 37.002 33.62 < 0.001 Soil x OM 6 52.426 8.738 7.94 < 0.001 TSOM x OM 3 332.842 110.947 100.79 < 0.001 Inc x soil x TSOM 4 156.398 39.099 35.52 < 0.001 Inc x soil x OM 12 79.187 6.598 5.99 < 0.001 Inc x TSOM x OM 6 79.214 13.202 11.99 < 0.001 Soil x TSOM x OM 6 264.462 44.077 40.04 < 0.001 Inc x soil x TSOM x OM 12 79.867 6.656 6.05 < 0.001 Residual 144 158.507 1.101 Total 215 7750.584 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


0

10

20

0 3 6 9

0

10

20

0 3 6 9

0

10

20

0 3 6 9

New

0

10

20

0 3 6 9

0

10

20

0 3 6 9

0

10

20

0 3 6

monthmonth

Sub

Ult

Oxi

Pho

spha

tase

act

ivity

(mg

kg-1

)

Existing

Sub

Ult

Oxi

9

Figure 4.3-5 Interaction effect of organic matter sources, type of organic matter, and incubation on soil phosphatase activity in three soils from Western Australia.

= control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.2.5 Microbial biomass phosphorus Microbial biomass-P concentration in soils tended to decrease with increasing

incubation time, especially for soil with new organic matter treatments (Figure

4.3-6). For these treatments, very high microbial biomass-P values were

observed at the beginning of incubations, but at the end microbial biomass-P

dropped to near zero.

Soil types had no significant effect (p = 0.11, Table 4.3-6) on microbial biomass-P

concentration.

In the same way as for phosphatase activity (Figure 4.3-5), new organic matter

had significantly lower microbial biomass-P in all soils. The lower microbial

biomass-P content for these treatments was observed until the end of the

experiment.

The type of organic matter applied created various effects on microbial biomass-

P, with lucerne hay causing the largest initial increase in microbial biomass-P (at

three months incubation) followed by wheat straw and peat (Figure 4.3-6).

Table 4.3-6 Analysis of variance for microbial biomass phosphorus in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 3841.78 1920.89 64.57 < 0.001 Soil 2 133.33 66.67 2.24 0.110 Organic matter 3 3802.53 1267.51 42.61 < 0.001 Type of SOM 1 5.04 5.04 0.17 0.681 Inc x soil 4 151.97 37.99 1.28 0.282 Inc x TSOM 2 536.33 268.17 9.01 < 0.001 Soil x TSOM 2 5.78 2.89 0.10 0.908 Inc x OM 6 2866.59 477.77 16.06 < 0.001 Soil x OM 6 256.37 42.73 1.44 0.205 TSOM x OM 3 761.13 253.71 8.53 < 0.001 Inc x soil x TSOM 4 62.81 15.70 0.53 0.715 Inc x soil x OM 12 999.55 83.30 2.80 0.002 Inc x TSOM x OM 6 712.56 118.76 3.99 < 0.001 Soil x TSOM x OM 6 214.00 35.67 1.20 0.310 Inc x soil x TSOM x OM 12 274.19 22.85 0.77 0.682 Residual 144 4284.00 29.75 Total 215 18907.96 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

-5

20

45

0 3 6 9 -5

20

45

0 3 6 9

-5

20

45

0 3 6 9 -5

20

45

0 3 6 9

-5

20

45

0 3 6 9 -5

20

45

0 3 6 9

monthmonth

Sub

Ult

Oxi

Mic

robi

al b

iom

ass

pho

spho

rus

(mg

kg-1

)

Existing

Sub

Ult

Oxi

Figure 4.3-6 Interaction effect of organic matter sources, type of organic matter, and incubation on soil microbial biomass phosphorus in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.3 Concentrations of different forms of phosphorus

4.3.3.1 Subsurface soil Concentrations of different forms of P in subsurface soil showed a steady

concentration throughout the incubation time (Figure 4.3-7). Meanwhile, organic

matter application, either by peat, wheat straw, or lucerne hay, increased the

concentration of P (microbial biomass-P, bicarbonate-P, non-extractable-P, and

total-P) from control. Among three sources of organic matter, lucerne hay

increased concentrations of the different forms of P more than peat and wheat

straw.

At the same source of organic matter, the proportioning between microbial

biomass-P, bicarbonate-P, non-extractable-P, and total-P in subsurface soil

remained relatively constant from the existing to the new organic matter

treatments. Microbial biomass-P was about 1% (of total-P) higher in the existing

than in the new organic matter treatments. Bicarbonate-P was 1-6% (of total-P)

higher in the existing than in the new organic matter treatments. Non-

extractable-P was from 1-6% (of total-P) higher in the new than in the existing

organic matter treatments.

4.3.3.2 Ultisol As in the subsurface soil, the Ultisol showed relatively steady concentrations of

all forms of P throughout the incubation time (Figure 4.3-8). New soil organic

matter application, as either peat, wheat straw, or lucerne hay, increased the

concentration of P (microbial biomass-P, bicarbonate-P, non-extractable-P, and

total-P) more than the control or existing soil organic matter treatments. Lucerne

hay, as in subsurface soil, increased P more than did peat or wheat straw.


biomass-P, bicarbonate-P, non-extractable-P, and total-P in the Ultisol remained

approximately constant from the existing to the new organic matter treatments.

Microbial biomass-P was 3-7% (of total-P) higher in the existing than in the new

organic matter treatments. Bicarbonate-P was 6-12% (of total-P) higher in the

new than in the existing organic matter treatments. Non-extractable-P was 6-8%

(of total-P) higher in the existing than in the new organic matter treatments.


4.3.3.3 Oxisol As in the subsurface soil and the Ultisol, all forms of P in the Oxisol showed

steady concentrations throughout the incubation time (Figure 4.3-9). New soil

organic matter application, as either peat, wheat straw, or lucerne hay, increased

the concentration of P (microbial biomass-P, bicarbonate-P, non-extractable-P,

and total-P) more than the control or existing soil organic matter treatments.

Lucerne hay, as in the subsurface soil and the Ultisol, increased P more than

peat and wheat straw.


biomass-P, bicarbonate-P, non-extractable-P, and total-P in the Ultisol remained

approximately constant. Microbial biomass-P was 1-5% (of total-P) higher in the

existing than in the new organic matter treatments. Bicarbonate-P was 7-10% (of

total-P) higher in the new than in the existing organic matter treatments. Non-

extractable-P was from 7-9% (of total-P) higher in the existing than in the new

organic matter treatments.

To summarise, the application of new soil organic matter to all soils increased

non-extractable-P and decreased microbial biomass-P and bicarbonate-P,

proportion to total-P. Application of lucerne hay resulted in the largest increases

in the proportion of non-extractable-P followed by peat and wheat straw. In

addition, lucerne hay tended to reduce microbial biomass-P concentration in all

soils. Among three soils, subsurface soil had higher concentrations of various

forms of P followed by the Oxisol and then the Ultisol.


0

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 9

New

0

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 9

C

P

W

L

monthmonth

mg kg-1 mg kg-1

Existing

C

P

W

L

Figure 4.3-7 Phosphorus status comparison between existing and new organic matter treatments in subsurface soil from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus.


New

month

-100

150

400

0 3 6 9-100

150

400

0 3 6 9

-100

150

400

0 3 6 9-100

150

400

0 3 6 9

-100

150

400

0 3 6 9-100

150

400

0 3 6 9

-100

150

400

0 3 6 9 -100

150

400

0 3 6 9

month month

mg kg-1 mg kg-1

Existing

C

P

W

L

C

P

W

L

Figure 4.3-8 Phosphorus status comparison between existing and new organic matter treatments in an Ultisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus.


month month

New

-100

150

400

0 3 6 9-100

150

400

0 3 6 9

-100

150

400

0 3 6 9-100

150

400

0 3 6 9

-100

150

400

0 3 6 9-100

150

400

0 3 6 9

-100

150

400

0 3 6 9-100

150

400

0 3 6

mg kg-1 mg kg-1

month month

Existing

C

P

W

L

C

P

W

L

9

Figure 4.3-9 Phosphorus status comparison between existing and new organic matter treatments in an Oxisol from Western Australia. (C = control, P = peat, W = wheat straw, and L = lucerne hay). = total phosphorus, = non-extractable phosphorus, = bicarbonate phosphorus, and = microbial biomass phosphorus.


4.3.4 Other parameters

4.3.4.1 Soil pH Soil pH increased with increasing incubation time (p = 0.01, Table 4.3-7).

The main effect of soil types was different among three soils. Subsurface soil

gave higher pH followed by Ultisol and Oxisol (Figure 4.3-10).

Addition of new soil organic matter significantly changed pH of soils (p ≤ 0.01),

but the direction of this change differed according to the type of soil. The new

soil organic matter application increased the soil pH in subsurface soil and the

Ultisol. In the Oxisol, addition of new soil organic matter decreased the soil pH.

Soil pH was also affected by the source of organic matter applied. Whilst only

lucerne hay increased soil pH, peat and wheat straw made the soil pH lower than

control, regardless of the soil type. However, only peat consistently decreased

soil pH in every type of soil. Wheat straw had a similar trend to peat, except in

the Ultisol with the new soil organic matter (Figure 4.3-10).

Table 4.3-7 Analysis of variance for pH in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 0.10562 0.05281 4.74 0.010 Soil 2 10.98550 5.49275 492.48 < 0.001 Organic matter 3 34.15630 11.38543 1020.82 < 0.001 Type of SOM 1 3.52667 3.52667 316.20 < 0.001 Inc x soil 4 6.59131 1.64783 147.74 < 0.001 Inc x TSOM 2 0.42391 0.21195 19.00 < 0.001 Soil x TSOM 2 5.97785 2.98893 267.99 < 0.001 Inc x OM 6 19.88841 3.31474 297.20 < 0.001 Soil x OM 6 1.89261 0.31544 28.28 < 0.001 TSOM x OM 3 0.30416 0.10139 9.09 < 0.001 Inc x soil x TSOM 4 14.47416 3.61854 324.44 < 0.001 Inc x soil x OM 12 3.47214 0.28935 25.94 < 0.001 Inc x TSOM x OM 6 2.44707 0.40785 36.57 < 0.001 Soil x TSOM x OM 6 3.46983 0.57830 51.85 < 0.001 Inc x soil x TSOM x OM 12 3.19575 0.26631 28.33 < 0.001 Residual 144 1.60607 0.01115 Total 215 112.51737 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

3.5

5.0

6.5

8.0

0 3 6 93.5

5.0

6.5

8.0

0 3 6 9

3.5

5.0

6.5

8.0

0 3 6 93.5

5.0

6.5

8.0

0 3 6 9

3.5

5.0

6.5

8.0

0 3 6 93.5

5.0

6.5

8.0

0 3 6 9

monthmonth

Sub

Ult

Oxi

Soi

l pH

Existing

Sub

Ult

Oxi

Figure 4.3-10 Interaction effect of organic matter sources, type of organic matter, and incubation on soil pH in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.4.2 Extractable iron Incubation time had no effect on soil oxalate extractable-Fe (Figure 4.3-11).

The oxalate extractable-Fe content in the soil was significantly different for each

type of soil (p ≤ 0.01, Table 4.3-8). The Oxisol had the highest content of

extractable-Fe (six-fold higher that subsurface soil), followed by the Ultisol (three-

fold higher than subsurface).

The new organic matter treatments had significantly increased extractable-Fe

contents (p ≤ 0.01) compared with the existing organic matter treatments,

especially the peat treatment. When soil types were considered separately, only

in subsurface soil and Ultisol did the new soil organic matter increase oxalate

extractable-Fe, whilst in the Oxisol the new soil organic matter decreased oxalate

extractable-Fe content in the soil.

When different sources of organic matter were considered separately, peat

increased oxalate extractable-Fe significantly (p ≤ 0.01) with concentrations

about 36% higher than the control. Meanwhile, oxalate extractable-Fe content

was significantly lower (p ≤ 0.01) following wheat straw (9% lower than control)

and lucerne hay (6% lower than control) applications.


Table 4.3-8 Analysis of variance for extractable iron in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 1032.7 516.4 1.14 0.322 Soil 2 2091701.9 1045851.0 2312.65 < 0.001 Organic matter 3 140920.1 46973.4 103.87 < 0.001 Type of SOM 1 30364.4 30364.4 67.14 < 0.001 Inc x soil 4 3369.2 842.3 1.86 0.120 Inc x TSOM 2 1031.8 515.9 1.14 0.322 Soil x TSOM 2 203174.4 101587.2 224.64 < 0.001 Inc x OM 6 6073.0 1012.2 2.24 0.043 Soil x OM 6 26038.5 4339.7 9.60 < 0.001 TSOM x OM 3 36088.6 12029.5 26.60 < 0.001 Inc x soil x TSOM 4 1424.4 356.1 0.79 0.535 Inc x soil x OM 12 4403.4 366.9 0.81 0.638 Inc x TSOM x OM 6 2516.3 419.4 0.93 0.477 Soil x TSOM x OM 6 24893.8 4149.0 9.17 < 0.001 Inc x soil x TSOM x OM 12 11346.2 945.5 2.09 0.021 Residual 144 65121.2 452.2 Total 215 2649500.1 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

0

200

400

0 3 6 90

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 9

0

200

400

0 3 6 90

200

400

0 3 6

monthmonth

Sub

Ult

Oxi

Ext

ract

able

iron

(g k

g-1)

Existing

Sub

Ult

Oxi

9

Figure 4.3-11 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable iron in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.


4.3.4.3 Extractable aluminium Oxalate extractable-Al in soil increased as incubation time increased (Figure

4.3-12). Although not significant, there was a decrease in oxalate extractable-Al

at the six-month incubation time. At the end of the experiment, oxalate

extractable-Al was almost 50 % higher compared with the initial content in soil.

The different soils had significantly different oxalate extractable-Al contents (p ≤

0.01). The Oxisol had the highest content of oxalate extractable-Al, followed by

Ultisol and subsurface soil. Compared with the subsurface soil, oxalate

extractable-Al contents were six times higher in the Oxisol and three times higher

in the Ultisol.

The new soil organic matter treatments had significantly higher oxalate

extractable-Al content (p ≤ 0.01, Table 4.3-9) than that of the existing organic

matter treatments. The average increase was about three-fold, and occurred for

all soil types and sources of organic matter.

Considering the main effect of source of organic matter, lucerne hay decreased

extractable-Al significantly (p ≤ 0.01) followed by wheat straw and peat. The

decreases from control were about 23% for lucerne hay, 18% for wheat straw,

and 7% for peat.


Table 4.3-9 Analysis of variance for extractable aluminium in three soils from Western Australia. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Incubation time 2 7269.069 3634.534 1271.10 < 0.001 Soil 2 14102.642 7051.321 2466.06 < 0.001 Organic matter 3 527.847 175.949 61.53 < 0.001 Type of SOM 1 13203.914 13203.914 4617.80 < 0.001 Inc x soil 4 1673.980 418.495 146.36 < 0.001 Inc x TSOM 2 1574.181 787.091 275.27 < 0.001 Soil x TSOM 2 3853.143 1926.572 673.78 < 0.001 Inc x OM 6 119.256 19.876 6.95 < 0.001 Soil x OM 6 178.955 29.826 10.43 < 0.001 TSOM x OM 3 418.940 139.647 48.84 < 0.001 Inc x soil x TSOM 4 227.600 56.900 19.90 < 0.001 Inc x soil x OM 12 73.824 6.152 2.15 0.017 Inc x TSOM x OM 6 89.724 14.954 5.23 < 0.001 Soil x TSOM x OM 6 187.080 31.180 10.90 < 0.001 Inc x soil x TSOM x OM 12 54.530 4.544 1.59 0.101 Residual 144 411.747 2.859 Total 215 43966.433 Inc = incubation time, OM = organic matter, TSOM = Type (existing or new) of soil organic matter


New

0

35

70

0 3 6 90

35

70

0 3 6 9

0

35

70

0 3 6 90

35

70

0 3 6 9

0

35

70

0 3 6 9

0

35

70

0 3 6 9

monthmonth

Sub

Ult

Oxi

Ext

ract

able

alu

min

um (g

kg

-1)

Existing

Sub

Ult

Oxi

Figure 4.3-12 Interaction effect of organic matter sources, type of organic matter, and incubation on extractable aluminium in three soils from Western Australia. = control, = peat, = wheat straw, and = lucerne hay. Sub = Subsurface soil, Ult = Ultisol, and Oxi = Oxisol. Bar in each graph indicates LSD 5% for organic matter source in every graph.



4.3.5 Correlations between all parameters Correlation analysis revealed that bicarbonate-P was positively correlated

(p ≤ 0.01) with dissolved organic-C throughout the experiment, and the coefficient

correlations (r) were increasing (0.47, 0.61, and 0.70) with increasing incubation

time, indicating stronger relationships. Bicarbonate-P was negatively correlated

(p ≤ 0.05) with oxalate extractable-Fe at the beginning of the experiment until six-

month incubation. Bicarbonate-P was also positively correlated with soil pH from

six-month incubation until the end of the experiment.

1

Table 4.3-10 Correlation matrices on all soil parameters. Presented according to incubation times. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

A. Three-month incubation Parameter

Parameter TP NP BP P’ase MBP DOC Al Fe pH

TP NP 0.98** BP 0.76** 0.63**P’ase -0.36** -0.37** MBP 0.38** 0.37** 0.31**DOC 0.42** 0.37** 0.47** 0.73**Al -0.35** -0.39** -0.32**Fe -0.48** -0.50** -0.25* -0.36** 0.54**pH

2 3 4 5 6 7 8 9

TP = Total Phosphorus, NP = Non-extractable Phosphorus, BP = Bicarbonate Phosphorus, P’ase = Phosphatase, MBP = Microbial Biomass Phosphorus, DOC = Dissolved Organic Carbon.


1 Table 4.3-10 (Continued) B. Six-month incubation

Parameter Parameter

TP NP BP P’ase MBP DOC Al Fe pH TP NP 0.98** BP 0.84** 0.73**P’ase 0.23* -0.28*MBP 0.38** 0.35** 0.40**DOC 0.51** 0.44** 0.61** 0.44**Al -0.30** -0.34** Fe -0.50** -0.53** -0.28* 0.67** 0.37** 0.58**pH 0.25* 0.31** 0.56**

2 3 4 5 6 7 8 9

10 11 12 13



1 2

Table 4.3-10 (Continued) C. Nine-month incubation

Parameter Parameter

TP NP BP P’ase MBP DOC Al Fe pH TP NP 0.97** BP 0.79** 0.62**P’ase -0.31** -0.43**MBP -0.50**DOC 0.47** 0.32** 0.70** 0.26** -0.41**Al -0.27** -0.34** -0.33**Fe -0.40** -0.46** 0.44** 0.26* 0.63**pH 0.26* 0.44** -0.39** 0.64** -0.39**

3 4

5 6



1

Table 4.3-11 Correlation matrices between phosphorus, carbon, iron, and soil pH for different incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

Organic matter Existing New Incubation

time Parameter NP BP DOC Fe NP BP DOC Fe

NP

BP 0.74** 0.48**DOC 0.39* 0.54** 0.35** 0.49**Fe -0.48** -0.44** -0.64**

Three months

pH -0.33* 0.52** -0.59**NP BP 0.79** 0.57**DOC 0.53** 0.73** 0.42** 0.50**Fe -0.52** -0.45** -0.41* -0.66** -0.46**

Six months

pH 0.50** 0.52** 0.75** -0.42* -0.39*NP BP 0.81** 0.45**DOC 0.38* 0.66** 0.72**Fe -0.47** -0.40* -0.37* -0.57**

Nine months

pH 0.45** 0.52** 0.64** -0.55** 0.34* 0.65**

NP = Non-extractable Phosphorus, BP = Bicarbonate Phosphorus, and DOC = Dissolved Organic Carbon. 2


1

Table 4.3-12 Correlation matrices between phosphorus, carbon, iron, and soil pH over all incubation times, separated into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

Organic matter Existing New Parameter

NP BP DOC Fe NP BP DOC Fe NP BP 0.78** 0.47**DOC 0.44** 0.64** 0.34** 0.47**Fe -0.48** -0.43** -0.35** -0.61** -0.26**pH 0.26** 0.30** 0.34** -0.27** -0.24*



ded Organic Matter … 128

1

Table 4.3-13 Correlation matrices between phosphorus, carbon, iron, and soil pH, separated into soil types and into ‘existing’ or ‘new’ soil organic matter. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

Organic matter Existing New Soil Parameter

NP BP DOC Fe NP BP DOC Fe NP

BP 0.73** 0.78**DOC 0.70** 0.74** 0.50**Fe -0.40* -0.46**

Subsurface soil

pH -0.73**NP BP 0.70** 0.73**DOC 0.84** 0.56** 0.67** 0.59**Fe -0.58**

Ultisol

pH 0.40* -0.62** -0.38*NP BP 0.71** 0.88**DOC 0.79** 0.59** 0.59**Fe 0.40* -0.42*

Oxisol

pH 0.50** -0.65**


IV. Existing versus Ad

4.4 DISCUSSION

The new organic matter treatment had significantly increased bicarbonate-P

concentrations compared with existing organic matter treatments, especially in

the Ultisol (ca. 150% increase) and the Oxisol (ca. 130% increase). In the case

of the subsurface soil, P immobilisation is likely to have been taking place after

lucerne hay addition, indicated by the higher microbial biomass-P in the first six

months of incubation (Figure 4.3-6). The higher bicarbonate-P content in

subsurface soil (Table 4.2-4) has probably allowed the population of soil micro-

organisms to increase faster than Ultisol and Oxisol, even in combusted soil

samples, causing immobilisation of P. Although P immobilisation can persist

longer due to green manure addition (da Costa, 2000), the results in this

experiment showed that at the end of the incubation, this P immobilisation had

declined indicated by smaller microbial biomass-P (Figure 4.3-6).

The increase of total-P (Figure 4.3-3) allows two interpretations. First, additional

organic matter to replace the existing soil organic matter contributed to the

increase. The increase in the Ultisol treated with lucerne hay, however, was too

high (90%) compared with the P content calculated from the P content of lucerne

hay (0.25% P; 41.3% C; Table 3.3-2, Chapter 3). Second, the possibility of

mineralisation or solubilisation of occluded P and stabilised source of P in the

existing soil organic matter or in inorganic forms pre-existing in the soil (Giardina

et al., 2000; Serrasolsas and Khanna, 1995) which can not be extracted with

total-P procedures by Nelson (1987). This occluded and stabilised source of P

might also be derived from monocalcium phosphate added one month before the

experimental treatments. Higher total-P in subsurface soil where organic-C

content was negligible (Table 4.2-5) and lower total-P in soils with higher organic-

C (Ultisol and Oxisol) were the indication of this pool of P. However, as soon as

the existing soil organic matter was removed by combustion, P was more readily

extracted. Alternately, the increase in soil pH (Figure 4.3-10), extractable-Al

(Figure 4.3-12), and extractable-Fe (Figure 4.3-11) in the Ultisol might have been

from the precipitation of Al and Fe-phosphate, or even Ca-phosphate (Giardina et

al., 2000; Olsen and Sommers, 1982) after combustion and/or new soil organic

matter treatment. In total-P extraction process, Al and Fe-phosphate are soluble

in alkaline extractants such as NaHCO3 and Ca-phosphate is soluble in acid


extractants such as H2SO4-K2S2O8 (Bhadoria et al., 2002) to increase the

concentration of extractable Al and Fe. Both acid extractants were used in total-

P analysis.

As total-P content increased with the new organic matter treatment,

consequently, non-extractable-P content in soils increased. The fact that peat

produced more non-extractable-P in soils than wheat straw treatment was

unexpected, since the total-P content in wheat straw biomass was higher than

that of peat (Table 3.3-2, Chapter 3). Different types of non-extractable-P as

products of decomposition of peat and wheat straw could be responsible for the

result, as peat has more phosphate diesters and other forms of P, including

inorganic-P (Bedrock et al., 1994).

Closer inspection of results obtained for subsurface soil indicated that the lower

concentration of bicarbonate-P in new soil organic matter treatment can be

assumed to be due to the heating process imposed on soil samples. The original

soil organic matter content in subsurface soil was negligible at 0.035% (Table

4.2-4) and the combustion of the soil increased extractable-Al and Fe content

(Table 4.2-1). The increased reactive Al and Fe would be expected to increase

phosphate adsorption capacity (Iyamuremye and Dick, 1996; Zoysa et al., 1999)

which may indicated by the lower bicarbonate-P in samples with new soil organic

matter treatment (Figure 4.3-2). However, for the other soils (the Ultisol and the

Oxisol), a decrease in bicarbonate-P was not observed, even though it might be

reasonable to assume a similar increase in phosphate adsorption capacity. The

increases in extractable-Al and Fe from the burning process occurred together

with increased bicarbonate-P concentration in the Ultisol and the Oxisol. This

could be due to process related to mineralisation of organic matter or desorption

process but not related to heating process as that of subsurface soil. Therefore,

the experiment could not distinguish mineralisation from desorption, but the effect

of combustion on phosphate adsorption can probably be neglected. In line with

this finding, increased concentration of extractable-P have also observed by

several authors, if soils were treated by combustion (Condron et al., 1990; Oniani

et al., 1973; Williams et al., 1970), especially in highly weathered soils. This

phenomenon was probably due to incomplete oxidation of organic-P during

combustion and the changes in acid solubility of soil inorganic-P as a result of

combustion (Anderson, 1960; Condron et al., 1990; Williams et al., 1970).

Condron et al. (1990) even concluded that these potential errors have to be


avoided if dealing with organic-P in strongly weathered soil samples such as

those used in this experiment. However, since subsurface soil did not show the

increase in either total-P or non-extractable-P (Figure 4.3-3 and Figure 4.3-4) due

to new organic matter applications (combustion procedure as well), it can be

assumed that two mechanisms might not have occurred. Unfortunately, there

was no data to support such claim in this experiment.

The Ultisol was the most responsive to organic matter applications (Figure 4.3-4)

by giving a greater percentage increase in non-extractable-P due to newly-added

organic matter, compared with increases in the subsurface soil and the Oxisol.

Higher O2 supply due to the sandy texture of the soil (Table 4.2-4) possibly

enhanced mineralisation processes in the Ultisol. However, the nature of the

increase in non-extractable-P content was not in the form of microbial biomass-P

as predicted beforehand. The positive correlation between non-extractable-P

and microbial biomass-P at three- and six-month incubation was not observed at

nine-month incubation (Table 4.3-10 and Table 4.3-11). Besides, new organic

matter application decreased microbial biomass-P in the soil. These

observations suggest that the increase of non-extractable-P may develop from

processes other than direct addition of organic-P from organic matter biomass

such as mineralisation or some other unknown process. All newly applied

organic matter decreased non-extractable-P relative to total-P, but increased the

proportion of bicarbonate-P relative to total-P. In other words, newly applied

organic matter not only increased non-extractable-P but might also have released

more bicarbonate-P from organic-P to inorganic-P transformations, despite the

observation that the treatment itself had created more bicarbonate-P.

In accordance with the increase in non-extractable-P content, a negative

correlation between non-extractable-P and sesquioxides of Al and Fe (oxalate-

extractable) was observed. The higher the non-extractable-P content, the lower

the Al and Fe concentration. As organic matter could increase non-extractable-P

due to direct effect of organic matter addition and mineralisation, the treatments

could precipitate Al- and Fe-oxides (Hue, 1992; Iyamuremye and Dick, 1996).

Moreover, the formation of complexes between Al and Fe and humic substances

(Chen et al., 1994; McCracken et al., 2002; Yang et al., 2001) may have taken

place during the experiment, making Al and Fe concentrations decline in soil

solution. Since acid ammonium oxalate also effectively extracts Al and Fe from


organic matter (McKeague and Schuppli, 1985; Rayment and Higginson, 1992),

therefore, the precipitation of Al and Fe must have occurred in the experiment.

If we follow the incubation process, bicarbonate-P content was positively

correlated with dissolved organic-C content (Table 4.3-10) and the relationship

was stronger at higher incubation times, indicated by increasing r values from

three- to nine-month incubation. At the same time, bicarbonate-P was negatively

correlated with extractable-Fe from three to six months of incubation. These

observations lead to the deduction that dissolved organic-C played a positive role

in releasing bicarbonate-P to the soil solution. Moreover, as the positive

correlation between dissolved organic-C and non-extractable-P was steady over

time and so was non-extractable-P and bicarbonate-P (Table 4.3-2), suggested

that the increase of bicarbonate-P content in soil solution was more likely to have

been a contribution from dissolved organic-C than from these other factors.

Three months after organic matter was applied into the soil, mineralisation

processes were occurring, indicated by the increased concentrations of dissolved

organic-C (derived either from soil organic matter and/or derived from added

organic materials), bicarbonate-P, and higher phosphatase activity compared

with control soils. At the same time, organic complexes were likely to have been

formed with Fe3+ and precipitated as insoluble Fe(OH)3 (Hue, 1992; Iyamuremye

and Dick, 1996; Schwertmann and Taylor, 1989). Another possible mechanism

is from soluble humic molecules and low molecular weight aliphatic organic acids

from added organic matter, which can adsorb to the surface of Al and Fe-oxides

and block phosphate adsorption sites, thus increasing soluble phosphate

concentration (Haynes and Mokolobate, 2001). A similar process of phosphate

adsorption sites blocking was also observed as higher C content organic matter

such as lucerne hay and wheat straw (Table 3.3-2, Chapter 3) significantly (p ≤

0.001) decreased extractable-Al (Table 4.3-9). The same applications, lucerne

hay and wheat straw treatments, significantly (p ≤ 0.001) increased bicarbonate-

P (Table 4.3-2). However, no correlation between extractable-Al with

bicarbonate-P (Table 4.3-10) might lead to estimation that the process occurred

and equilibrated before the first sampling due to lower initial content of soil

extractable-Al than extractable-Fe (Table 4.2-4).

The correlation of extractable phosphate with soil pH was explained by assuming

that a pH increase due to organic matter addition (data not presented) would also


favour dissolution of soil organic-C (Beck et al., 1999), and thus increase

dissolved organic-C in soils (Figure 4.3-1). The increase in pH was probably

caused by the oxidation of organic acid anions (e.g. oxalate, malate, and citrate)

from organic residues that consumed H+ and released OH- (Noble et al., 1996;

Tang et al., 1999), or from decarboxylation of those anions which consumed

protons, released CO2, and led to a liming effect (Haynes and Mokolobate, 2001;

Tang et al., 1999; Yan et al., 1996).

In the case of phosphatase, despite the application of organic matter, as either

peat, wheat straw, or lucerne hay increased phosphatase activity, there was no

correlation between phosphatase and bicarbonate-P during the experiment

(Table 4.3-10). Phosphatase may have been at steady-state, whilst bicarbonate-

P could have been continuously released or reach nearly steady-state

concentrations as well. Such a situation would not lead to a correlation even

though a causative effect existed. It is also possible that phosphatase may pre-

exist in soil and persist for a long time. In addition, phosphatase activity is

subject to end-product suppression by high levels of extractable-P (Allison and

Vitousek, 2005; Feng and Xiong, 2002) and phosphatase is not the only enzyme

involved in soil P transformation. At least in this experiment, phosphatase activity

was not a reliable parameter to estimate P extractability. Therefore, the last

hypothesis is only partly supported by the experiment. According to McCallister

et al. (2002) even though phosphatase was positively correlated with

bicarbonate-P, it could not be used to predict P supply to plants nor could it be

used as an estimate for the differences of P concentration in plant tissues (de

Andrade et al., 2001).

Despite the fresh supply of organic-C from new organic matter, micro-organisms

producing the phosphatase enzyme were not active enough to generate similar

phosphatase activity to that observed for the soils with the existing soil organic

matter. The trend of increasing phosphatase activity in samples treated with new

organic matter was similar to samples with existing soil organic matter. The

magnitude of increase for new organic matter was around half to that of samples

with existing soil organic matter only, especially in the Ultisol (Figure 4.3-6).

Therefore, it is not high enough to transform non-extractable-P to bicarbonate-P

as indicated by the lack of correlation between phosphatase activity and

bicarbonate-P (Table 4.3-11), or there was insufficient accessibility of organic-P

to phosphatases. Another explanation for this was the effect of soil combustion


on phosphatase. Although phosphatase is resistant to microwave irradiation

(Speir et al., 1986), its activity is heat-sensitive (Gosewinkel and Broadbent,

1986). It would be very unlikely if phosphatase or microbial biomass would have

survived the high temperature during the combustion period. In addition, the soil

starter may have had insufficient microbial population to produce significant

amounts of phosphatase.

4.5 CONCLUSIONS

Bicarbonate-extractable P increased in soil with applied organic matter (peat,

wheat straw, and lucerne hay) in which the existing soil organic matter had been

removed to a greater extent than in soils with existing soil organic matter. The

Ultisol had a larger increase in bicarbonate-P than the Oxisol. The addition of

organic matter increased non-extractable-P concentrations up to six-month

incubation. Transformation of non-extractable-P to extractable phosphate must

have been at its maximum around this time indicated by non-extractable-P

decreases as well as increases in bicarbonate extractable-phosphate.

Stronger correlation over time between dissolved soil organic-C and bicarbonate-

P indicated the importance of dissolved organic-C in generating bicarbonate-P

that could be from organic-P hydrolysis, transformation, or displacement of

adsorbed bicarbonate-P by soluble organic compounds. Contrary to the initial

hypothesis, there was only weak evidence that microbial biomass-P played a role

in providing more bicarbonate-P content, as well as phosphatase. However, data

showed that microbial biomass-P had greater effect in bicarbonate-P content

compared with phosphatase. Other mechanisms, either chemical or biological,

might have been involved in the process.

To sum, newly applied organic amendment was more effective in releasing

available P compared with the pre-existing soil organic matter. However, in order

to utilise other pools of P, the increased proportions of non-extractable-P to total-

P need to be resolved or analysed further, especially in lateritic soils.


C h a p t e r 5

SOIL ORGANIC CARBON LOSSES: THE BALANCE BETWEEN RESPIRATION AND

LEACHING, AND PHOSPHORUS MOBILITY IN LATERITIC SOILS

5.1 INTRODUCTION

Soils play an effective role as a sink or storage for carbon (C), and as a source or

loss of C to pools outside the soil system. The balance between storage and loss

depends on soil management, as well as climate and soil properties. The losses

of C from soil are mainly from soil organic matter decomposition (respiration),

erosion, and leaching (Akala and Lal, 2000). According to Cronan (1985) and

Magill and Aber (2000), C leaching contributes between 6-46% of total-C loss as

dissolved organic-C in forest soils. Investigations of C balance have focused on

the effect of temperature on C mineralisation (Liechty et al., 1995; Tate et al.,

1993; Zogg et al., 1997), which is only effective in surface soil (MacDonald et al.,

1999). The leaching of dissolved organic-C from soil may return to the

atmosphere as CO2 loss from streams, lakes, or oceans (Kling et al., 1991). For

soils in the tropics the balance between upward (respiration) and downward

(dissolved organic-C leaching) loss could be important, especially for lateritic

soils if the effectiveness of organic matter application is to be understood. Heavy

rainfall may be an additional factor in increasing organic-C loss from soil, not only

in erosion but also in infiltration of water through the soil profile.

Phosphorus adsorption, measured by phosphate buffering capacity and

phosphate retention index, was correlated with organic phosphorus (P) in soils

from Western Australia (Brennan et al., 1994). Phosphate buffering capacity was

also correlated with soil organic-C, Al, and Fe contents in Inceptisols in India

(Patiram et al., 1990). In other studies, soil organic-C was positively correlated

with the activity of phosphatase, especially in acid soils (Baligar et al., 1999;

Barrett et al., 1998; Canarutto et al., 1995). The increases of soil organic-P

content generally associated with increases in soil organic-C (Iyamuremye and

Dick, 1996; Reddy et al., 2000). Thus, if leaching of organic-C is prominent in

V. Soil Organic Carbon Losses: The Balance... 135

these soils, and organic-P affects P adsorption (Brennan et al., 1994), this

leaching will also have an effect on P mobility in the soil profile. Working with soil

columns, Chardon et al. (1997) found that 90% of total-P in leachate was in the

form of dissolved organic-P and that leaching was related to dissolved organic-C.

Rupp et al. (2002) suspected that decreasing redox potential might be

responsible for the increasing P solubility as well as dissolved organic-C, and

intensifying the leaching of both.

The decomposition of soil organic matter is associated with the release of P from

adsorption sites (Dalal, 1979; Gressel et al., 1996; Maroko et al., 1999). In this

process, mechanism such as organic ligand exchange (Hinsinger, 2001;

Iyamuremye and Dick, 1996) is responsible as part of dissolution and desorption

of phosphate (Burkitt et al., 2002; MacKay et al., 1986; McBride, 1994; Rhue and

Harris, 1999). For temperate forest soils, changes in soil organic-P are closely

related to changes in soil organic matter composition (Gressel et al., 1996),

although this study was limited to topsoil. Information on the relationship

between soil organic-C and soil organic-P in the profile of lateritic soils is limited,

especially in the tropics. Such information may be of special importance in the

tropics where soil organic matter decomposition is very rapid and may affect P

mobility. Additionally, P dynamics would be related to the activity of soil micro

organisms as indicated by the increase in phosphatase activity (Joner and

Jakobsen, 1995) and microbial biomass-P (Oberson et al., 1995) where organic

matter is applied to the soil.

As the adsorption and the release of P is related to soil organic-C (Brennan et al.,

1994), whilst aluminium (Al) and iron (Fe)-hydroxides are factors in phosphate

adsorption (Haynes and Mokolobate, 2001), the mobility of soil organic-C in

leaching may affect the sesquioxides concentrations through soil profile. In high

rainfall tropical areas, these mechanisms may occur to the deeper profile of

lateritic soils due to higher leaching.

In summary, organic-P has an impact on P cycles in soils. To a greater extent,

dissolved organic-P is the predominant form of P in subsoil of leaching columns

(Chardon et al., 1997). The turnover of dissolved organic-P, together with

dissolved organic-C and dissolved organic-N, is a major pathway in nutrient

cycling and may have more impact than previously assumed (Kalbitz et al.,

2000). In lateritic soils, these impacts could be more complicated due to the role


of Al and Fe-oxides in soils which may also change with leaching of organic-C.

These issues need to be explained in order to understand the mechanisms

involved in P mobility in lateritic soils.

The brief reviews leads to the following hypotheses:

• rainfall will alter the ratio between leaching and respiration from organic

matter mineralisation and to a certain extent the persistence of organic matter

itself;

• the loss of soluble-C due to the downward movement of water affects the

mobility of extractable-P, and;

• changes in the content of Al and Fe oxides with depth are closely related to

mobility of organic-C in soil profile.

The objectives of this experiment were: to compare fluxes of soil organic-C and

soil organic-P in two different lateritic soils, by:

• studying the effect of rainfall regime relevant to tropical and subtropical

environments on the loss of C from a soil profile, and

• measuring the effect of leaching on the change of soil organic-C, soil

extractable-P, and extractable-Al and Fe with depth in the soil.


5.2.1 Design of the experiment This leaching column (PVC plastic, 90x5.7 cm) experiment was set up in factorial

using a completely randomised block design. The treatments were:

• Two soils: Ultisol (Typic kandiudult) and Oxisol (Plinthic eutrodox).

• Two rainfall simulations: tropical (3000 mm year-1 equivalent) and subtropical

(900 mm year-1 equivalent). These are about the same as in South

Kalimantan, Indonesia and Perth metro area, Western Australia, and

• Four organic matter sources: control, peat, wheat straw, and lucerne hay.

The organic matter rate was 80 ton ha-1, similar to the application rate for the

second experiment.

Table 5.2-1 shows the whole treatment combinations between soils, rainfall

simulations, and organic matter sources.


Table 5.2-1 Treatment combinations and their abbreviation from two types of soils (Ultisol and Oxisol), two rainfall simulations (tropical and subtropical), and four sources of organic matter (control, peat, wheat straw, and lucerne hay)

Rainfall Tropical Subtropical Soil

Control Peat Wheat straw

Lucerne hay Control Peat Wheat

straw Lucerne

hay Ultisol UTC UTP UTW UTL USC USP USW USL Oxisol OTC OTP OTW OTL OSC OSP OSW OSL U = Ultisol, O = Oxisol, T = tropical, S = subtropical, C = control, P = peat, W = wheat straw, and L = lucerne hay.

Data were analysed statistically with GenStat (Payne et al., 1987). Analysis of

variance was followed by mean comparison (orthogonal polynomial contrasts) if

the treatment was significant. Correlation and regressions were carried out using

SPSS (Coakes, 2001) to determine any relationships between parameters.

5.2.2 Procedures An Ultisol from Western Australia was collected near Jarrah (Eucalyptus

marginata) forest, near Jarrahdale, and an Oxisol from near Bunbury. Both soils

were selected due to their high content of extractable-Al and Fe (McArthur,

1991), in order to see their effects on phosphate adsorption and P mobility

throughout the leaching columns. Some soil characteristics are presented in

Table 4.2-4 in Chapter 4. Samples were taken from lower horizon in the profile

(20-25 cm) where the amount of soil organic matter was less but the amount of

extractable-Al and Fe were expected to be higher.

In order to simulate a combination of leaching process and wet-dry conditions,

water equivalent to one year’s rainfall was added to each column; the equivalent

annual rainfall was distributed in equal increments applied weekly. Gradual

watering was applied using drippers made from plastic bottles. With the bottom

open, the bottles were positioned upside down on the top of leaching columns.

A 0.2 µm millipore filter (25 mm, supor membrane, non sterile, Pall Gelman

Laboratory) was fixed inside the lid of every bottle to ensure a realistic rate of

water delivery. After 24 hours of water application, a suction mechanism (10 cm

water) was applied to all leachate containers for another 24 hours to avoid water

saturation and reducing conditions at the bottom of every column.


Soil in each column was divided into seven layers. The first layer (0-10 cm) was

the mixture of treatment and soil sample (Figure 5.2-1). Soil packing was

performed with soil bulk density as an indicator where Ultisol was 1.32 g cm3 and

for Oxisol was 1.13 g cm3. The internal diameter of a leaching column was 5.7

cm; total volume of water for the tropics was 7655 mL and for the subtropics was

2297 mL. Before the first application of water, soils were wet to 60% water

holding capacity. The edge-flow effect due to plastic column was reduced by

using funnel o-rings (0.6 cm thick) made from styrofoam on every layer of soil.

The o-rings fitted tightly inside the column. Soil mesh was applied on every layer

on the top of o-ring. To evenly distribute water dripping, a filter paper was placed

on the top of the organic matter layer. The filter paper needed to be changed

every two weeks to avoid decomposition.

Fresh, dry, and chopped (≤ 0.5 cm) organic matter (12 g of peat, wheat straw,

and/or lucerne hay for Ultisol; 10 g for Oxisol) were mixed with soils (270 g Ultisol

or 230 g Oxisol) as the top layer (0-10 cm) of soil in the column. With the

replicate of three, the overall experimental unit was 2x2x4x3 = 48 leaching

columns.

5.2.2.1 Soil sampling To follow the mobility of dissolved organic-C and other factors affecting it at the

end of the experiment, sampling was done on six layers of the leaching columns

(0-10, 10-20, 20-30, 30-40, 40-50, and 50-60 cm). With 48 leaching columns,

there were 288 soil samples. Leachate was collected for further analysis of

dissolved organic-C, dissolved reactive-P, dissolved-Al and Fe, and soil pH

according to the methods described in the following section. There were 48

leachate containers collected.

Soil samples were air-dried and sieved (≤ 2 mm) and kept in sealed plastic bags

for other chemical analyses. Methods of determination or chemical analyses of

all parameters are described below.


0-10 cm

Soil mesh (pore ∅ = 1 mm)

PV

C, ∅

= 5

.7 c

m

Silica sand

Sealed leachate container

Soil + OM

Dripper

Vacuum air

O-ring 0.6 cm thick

10 cm water suction

Filter paper

Figure 5.2-1 Leaching column with six layers of soil and one layer of soil + organic matter. Suction apparatus (right part of the graph) was set at 10 cm water.

5.2.3 Measurements

5.2.3.1 Bicarbonate phosphorus in soil Bicarbonate-P was extracted from soil using 0.5 M NaHCO3 at pH 8.5. This is a

modification from a method by Olsen et al. (1954) described by Rayment and

Higginson (1992). The manual colorimetric method for determination of

phosphate in the extract was based on that of Murphy and Riley (1962).

5.2.3.2 Non-extractable phosphorus Measurement of non-extractable-P was conducted by measuring total-P with

acid-persulfate digestion (Nelson, 1987) and then subtracting the results of the

bicarbonate-P content of the same soil calculated beforehand.


5.2.3.3 Dissolved reactive phosphorus in leachate In leachate, dissolved reactive-P was measured directly in leachate after filtration

with 0.2 µm millipore filter (25 mm, supor membrane, non sterile, Pall Gelman

Laboratory) using the method of Rayment and Higginson (1992).

5.2.3.4 Soil respiration The method used to measure soil respiration in this experiment was similar to

method used in Chapter 3. The only modification was made in the time period

that the alkaline trap (0.5 M KOH) was exposed to soil inside the headspace of

the leaching column. Twenty and 30 mL (at the beginning of the experiment) of

0.5 M KOH were exposed for only 24 hours. Plastic containers (50 mL) were

used and put inside the columns sitting on the top of soil surface. The top of the

columns was sealed with parafilm (American National Can) and rubber bands.

Sampling was performed in every other two-week period for six months.

Total respiration during the experiment was calculated as total area under the

curve using the equation:

i

n

i

ii xyy

∆+∑

−

=

+1

1

1

2.................................................................................... 5.2-1

where ∆ xi = time interval for measuring respiration rate, yi = magnitude of

respiration (mg CO2 kg-1 day-1) at the ith sampling time, and n = number of

sampling times.

5.2.3.5 Total phosphorus in soil Total-P was measured in layers of each leaching column using the persulfate

digestion method for total-P in soils which was explained in non-extractable-P

procedure in Chapter 4. The method is based on procedures presented by

Nelson (1987).

5.2.3.6 Total phosphorus in leachate Total-P measurement in leachate was based on procedures outlined by Rayment

and Higginson (1992). The principle of this method is to convert organic-P and

other P fractions (e.g. particulate) in leachate by digestion (HNO3 and H2SO4) into

soluble phosphate. Phosphate content was measured using the

spectrophotometric method of Murphy and Riley (1962).


5.2.3.7 Dissolved organic carbon in soil Dissolved organic-C in soil was measured using a method similar to that

described in Chapter 4, which is based on the procedures by Wagai and Solin

(2002).

Dissolved organic-C in soil was extracted from 10 g air-dry soil with 25 mL

deionised (MilliQ) water in a centrifuge tube. Samples were shaken for 30

minutes on an end-over-end shaker, and then centrifuged at 10,000 rpm for 10

minutes. The supernatant was filtered through a 0.2 µm filter (25 mm, supor

membrane, non sterile, Pall Gelman Laboratory) using a syringe. Soluble

organic-C was measured by a TOC analyser (Shimadzu TOC-5000A) as the

difference between total-C and inorganic-C in the solution.

5.2.3.8 Dissolved organic carbon in leachate Dissolved organic-C in leachate was determined using filtered leachate (0.2 µm

filter; 25 mm, supor membrane, non sterile, Pall Gelman Laboratory). A

Shimadzu TOC-5000A instrument was used to measure organic-C as non-

purgeable organic-C. This fraction of C is also known as non-volatile organic-C,

which can be obtained by sparging the sample with high purity air before total-C

measurement.

5.2.3.9 Extractable and dissolved aluminium and iron The procedures for measuring extractable-Al and Fe were the same as those

described in Chapter 4. The extract solution was 0.2 M acid ammonium oxalate

pH 3.0 (16.2 g ammonium oxalate and 10.8 oxalic acid in 1 L deionised water)

with soil:extractant ratio of 1:100. With minimal room light the extraction was

conducted during the night to avoid photo-reduction of Fe (Schwertmann and

Taylor, 1989). A black plastic sheet was used to cover samples batch during

transportation from shaker to centrifuges. Aluminium and Fe concentrations in

extracts were determined by atomic absorption spectrophotometry using CsCl

solution 0.01 M (1+4) for dilution to reduce ionisation effect.

Dissolved-Al and Fe in leachate were measured directly with AAS in leachate

using standard stock solution for extractable-Al and Fe. Dilution up to 100 times

compared with standards for extractable-Al and Fe was applied with MilliQ water.


5.2.3.10 Leachate pH Leachate pH was measured directly in the leachate using a combined pH-

reference electrode.

5.3 RESULTS

5.3.1 Organic carbon mobility

5.3.1.1 Leachate carbon Organic matter addition significantly (p ≤ 0.001, Table 5.3-1) affected the amount

of dissolved organic-C in leachate (Figure 5.3-1). Type of soil was also differing

as Ultisol had higher C in leachate compared with Oxisol. Wheat straw

application produced the highest dissolved organic-C content in leachate followed

by lucerne hay, control, and peat. Peat application, in other words, had lower C

loss due to leaching, especially with tropical rainfall. This effect of peat on

dissolved organic-C leaching occurred in both soils. The amount of rainfall

affected the amount of C loss by leaching. The higher rate of rainfall (tropical)

had higher dissolved organic-C concentrations in the leachate.

Table 5.3-1 Analysis of variance for dissolved organic carbon in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Organic matter 3 396.29 132.10 8.76 < 0.001 Rainfall 1 182.91 182.91 12.13 0.001 Soil 1 2026.70 2026.70 134.44 < 0.001 OM x rainfall 3 118.56 39.52 2.62 0.068 OM x soil 3 196.00 65.33 4.33 0.011 Rainfall x soil 1 270.28 270.28 17.93 < 0.001 OM x rainfall x soil 3 155.22 51.74 3.43 0.028 Residual 32 482.41 15.08 Total 47 38328.37 OM = organic matter.

Soil types differed in producing dissolved organic-C in their leachate. The Ultisol

produced more C leaching over all organic matter treatments compared with the

Oxisol.


0.0

20.0

40.0

Control Peat Wheatstraw

Lucernehay

Oxisol Ultisol

0.0

20.0

40.0


Lucernehay

Dis

sove

d or

gani

c ca

rbon

(mg

kg-1

)Tropical rainfall

(4000 mm year-1)

Subtropical rainfall(900 mm year-1)

Figure 5.3-1 Cumulative dissolved organic carbon in leachate expressed per mass of soil after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.

The percentage of C in leachate compared with the total-C in soil and organic

matter sources was calculated and is presented in Table 5.3-2.

Table 5.3-2 Cumulative carbon loss (%) from leaching following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical)

Rainfall Tropical Subtropical OM

Source Ultisol Oxisol Ultisol Oxisol Control 0.13c 0.15c 0.04a 0.05a

Peat 0.10b 0.10b 0.03a 0.04a

Wheat straw 0.22d 0.14c 0.04a 0.05a

Lucerne hay 0.16c 0.10b 0.04a 0.04a

Numbers followed by the same letters in superscript indicate differences according to LSD 5% (0.032).


5.3.1.2 Leachate pH Rainfall affected (p ≤ 0.001, Table 5.3-3) leachate pH and the effect was highly

significant. Tropical rainfall caused the increase of leachate pH with the about

0.6 point. In Ultisol the effect even more pronounced with the increase about 1.0

point. Even though the soil pH of both soils were about the same (Table 4.2-4,

Chapter 4), the amount of rainfall can make the leachate from those differ from

each other.

Organic matter source only had significant effect on leachate pH. Peat tended to

decrease pH of leachate as of pH of soils in previous experiment (I and II), whilst

wheat straw showed the opposite. Lucerne hay treatment showed inconsistency

within the two soils and rates of rainfall applied. Under tropical rainfall, lucerne

hay in both the Ultisol and Oxisol decreased leachate pH compared with control.

With subtropical rainfall, however, lucerne hay increased leachate pH in the

Ultisol and decreased the pH in the Oxisol.

Table 5.3-3 Analysis of variance for leachate pH after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment


of freedom

Sum of squares

Mean square F P

Organic matter 3 1.8905 0.6302 3.97 0.016 Rainfall 1 3.5698 3.5698 22.50 < 0.001 Soil 1 0.5188 0.5188 3.27 0.080 OM x rainfall 3 1.7479 0.5826 3.67 0.022 OM x soil 3 0.3391 0.1130 0.71 0.552 Rainfall x soil 1 2.2925 2.2925 14.45 < 0.001 OM x rainfall x soil 3 2.4870 0.8290 5.22 0.005 Residual 32 5.0777 0.1587 Total 47 17.9232 OM = organic matter.


5.50

7.00

8.50


Lucernehay

5.50

7.00

8.50


Lucernehay

Oxisol UltisolLeac

hate

pH

Tropical rainfall(4000 mm year-1)


Figure 5.3-2 Cumulative pH on leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.

5.3.1.3 Carbon throughout the leaching column Throughout the leaching column, dissolved organic-C concentration in soil

increased approximately linearly with depth (Figure 5.3-3). This linear trend was

calculated from the main effect of column layers with orthogonal polynomial

contrast (p ≤ 0.01). Test statistics with t-test0.01 also showed that dissolved

organic-C concentrations in column layers were different from the control. The

Oxisol had higher soil dissolved organic-C concentrations compared with the

Ultisol.


Table 5.3-4 Analysis of variance for dissolved organic carbon throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Soil 1 3279.47 3279.47 121.21 < 0.001 Layers 5 426.06 85.21 3.15 0.025 Soil x layers 5 136.63 27.33 1.01 0.453 Residual 24 649.32 27.06 Total 35 4491.48

0

10

20

30

40

50

60

50 70 90

Dissolved organic carbon (mg kg-1)

Col

umn

laye

rs (c

m)

R2 = 0.77

R2 = 0.29

Figure 5.3-3 Dissolved organic carbon concentration throughout the leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.

5.3.1.4 Soil respiration There was no significant difference in soil respiration between tropical and

subtropical rainfall treatments. For both rates of rainfall, soil respiration from


wheat straw and lucerne hay treatments rapidly declined within the first four

weeks after a very high CO2 evolution at the beginning of measurement.

Table 5.3-5 Analysis of variance for soil respiration after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment


of freedom

Sum of squares

Mean square F P

Organic matter 3 2.551283 0.850428 334.05 < 0.001 Rainfall 1 0.005633 0.005633 2.21 0.147 Soil 1 0.997633 0.997633 391.87 < 0.001 OM x rainfall 3 0.007117 0.002372 0.93 0.437 OM x soil 3 0.236083 0.078694 30.91 < 0.001 Rainfall x soil 1 0.000033 0.000033 0.01 0.910 OM x rainfall x soil 3 0.008217 0.002739 1.08 0.373 Residual 32 0.081467 0.002546 Total 47 3.887467 OM = organic matter.


0

10

20

30

2 8 14 20 26

0

10

20

30

2 8 14 20 26

Ultisol Oxisol

week

-1)

Figure 5.3-4 Soil respiration throughout the experiment with tropical rainfall. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph represent

LSD 5% for organic matter.

Figure 5.3-5 Soil respiration throughout the experiment with subtropical rainfall. = control, = peat, = wheat straw, and = lucerne hay. Bar in each graph represent LSD 5% for organic matter.

The results are presented in Table 5.3-6 as follow:

Table 5.3-6 Respiration estimates (mg CO2) during the experiment for every source of organic matter (control, peat, wheat straw, and lucerne) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical)

Rainfall Tropical Subtropical Organic matter

source Ultisol Oxisol Ultisol Oxisol Control 378±48 303±18 398±68 307±24 Peat 662±19 492±11 531±18 508±5 Wheat straw 1254±158 1169±51 1301±50 1115±31 Lucerne hay 1186±63 1044±29 1026±53 985±5

Res

pira

tion

(mg

CO 2

k

g-1w

eek

0

10

20

30

2 8 14 20 260

10

20

30

2 8 14 20 26

OxisolUltisol

week

)w

eek-1

Res

pira

tion

(mg

CO 2

k

g-1


As for the first experiment, the amount of C loss from the total-C in soils can be

calculated from the above table, and the results are in Table 5.3-7.

Table 5.3-7 Cumulative carbon loss (%) from soil respiration following application of different organic matter (control, peat, wheat straw, and lucerne hay) at 80 ton ha-1 after 24 weeks in two soils (Ultisol and Oxisol) and two rainfall simulations (tropical and subtropical)

Rainfall Tropical Subtropical

Organic matter source Ultisol Oxisol Ultisol Oxisol

Control 0.18a 0.29b 0.19a 0.30b

Peat 0.30b 0.45c 0.24ab 0.46cd

Wheat straw 0.57e 1.05h 0.59e 1.00gh

Lucerne hay 0.54de 0.94fg 0.46cd 0.88f Numbers followed by the same letters in superscript indicate differences according to LSD 5% (0.084).

5.3.2 Balance on carbon losses between respiration and leaching

Table 5.3-8 shows the balance in percentage between respiration (Table 5.3-7)

and leaching (Table 5.3-2) on dissolved organic-C. The lowest value was 1.4 in

the control Ultisol receiving tropical rainfall, meaning that C loss from respiration

was 1.4 times higher than that of leaching for the particular treatment. Carbon

loss was 21.1 times higher in respiration than that of leaching for Ultisol receiving

subtropical rainfall and wheat straw treatments.

Table 5.3-8 Balance between respiration and leaching on every treatment, calculated from loss (%) due to respiration divided by loss (%) due to leaching

Rainfall Tropical Subtropical

Organic matter source Ultisol Oxisol Ultisol Oxisol

Control 1.4±0.13 2.0±0.20 4.8±0.48 6.9±1.21 Peat 3.2±0.32 4.8±0.46 9.4±1.30 11.4±1.96 Wheat straw 2.7±0.48 8.1±1.25 16.7±2.96 18.9±0.46 Lucerne hay 3.4±0.26 9.7±0.46 13.7±2.94 21.1±0.86

5.3.3 Effect of leaching on phosphorus concentrations

5.3.3.1 Phosphorus concentration in columns and leachate As the purpose of this experiment was to follow the mobility of bicarbonate

extractable-P throughout the leaching columns, the analysis of bicarbonate-P as


of experiment I and II was carried out in the same method. Bicarbonate-P was

determined in columns that had high concentration of dissolved organic-C in their

leachate, which were columns with wheat straw treatments. They were columns

with Oxisol treated with tropical rainfall and wheat straw (OTW) and Ultisol

treated with tropical rainfall and wheat straw (UTW) (Table 5.2-1).

There was a significant (p ≤ 0.01, Table 5.3-9 ) effect of tropical rainfall on the

distribution of bicarbonate-P concentration throughout the leaching column

(Figure 5.3-6).

Table 5.3-9 Analysis of variance for bicarbonate phosphorus throughout the leaching column treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Soil 1 5.09 5.09 0.24 0.629 Layers 5 758.84 151.77 7.15 < 0.001 Soil x layers 5 40.82 8.16 0.38 0.854 Residual 24 509.38 21.22 Total 35 1314.13


0

10

20

30

40

50

60

0.0 12.5 25.0

Col

umn

laye

rs (c

m)

Bicarbonate phosphorus (mg kg-1)

Figure 5.3-6 Bicarbonate phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.

Table 5.3-10 Analysis of variance for dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment


of freedom

Sum of squares

Mean square F P

Organic matter 3 0.0013676 0.0004559 2.11 0.119 Rainfall 1 0.0000081 0.0000081 0.04 0.848 Soil 1 0.0000536 0.0000536 0.25 0.622 OM x rainfall 3 0.0000691 0.0000230 0.11 0.956 OM x soil 3 0.0000471 0.0000157 0.07 0.974 Rainfall x soil 1 0.0000263 0.0000263 0.12 0.730 OM x rainfall x soil 3 0.0000725 0.0000242 0.11 0.953 Residual 32 0.0069187 0.0002162 Total 47 0.0085628 OM = organic matter.


0.0000

0.0175

0.0350


Lucernehay

Oxisol Ultisol

0.0000

0.0175

0.0350


Lucernehay

Dis

solv

ed re

activ

e p

hosp

horu

s (m

g kg

-1)



Figure 5.3-7 Cumulative dissolved reactive phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.

In leachate, there was no effect (p ≥ 0.05, Table 5.3-10) of soil and rainfall on

dissolved reactive-P concentrations, although the trend was about the same as of

the dissolved organic-C in leachate (Figure 5.3-7), especially in tropical rainfall.

As with dissolved organic-C, dissolved reactive-P concentration in leachate was

higher from columns treated with wheat straw, followed by lucerne hay and peat.

In case of total-P, the Oxisol had higher concentration than that of the Ultisol and

the difference was highly significant (p ≤ 0.001, Table 5.3-11). This could be due

to the initial total-P concentration (Table 4.2-4, Chapter 4). Rainfall also affected

total-P concentrations in every layer (p = 0.01, Table 5.3-11) of the leaching

column (Figure 5.3-8). As of total-P, non-extractable-P in the Oxisol was higher

than in the Ultisol, but rainfall had no effect on non-extractable-P concentrations

in either soil.


Table 5.3-11 Analysis of variance for total phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P


No amount of total-P concentration was detected in the leachate (Table 5.3-12) to

indicate the leaching of total-P from the columns.

Table 5.3-12 Analysis of variance for total phosphorus in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment


of freedom

Sum of squares

Mean square F P

Organic matter 3 0.21142 0.07047 1.31 0.289 Rainfall 1 0.00227 0.00227 0.04 0.839 Soil 1 0.00110 0.00110 0.02 0.887 OM x rainfall 3 0.00064 0.00021 0.00 1.000 OM x soil 3 0.00157 0.00052 0.01 0.999 Rainfall x soil 1 0.00152 0.00152 0.03 0.868 OM x rainfall x soil 3 0.00072 0.00024 0.00 1.00 Residual 32 1.72513 0.05391 Total 47 1.94438 OM = organic matter.


0

10

20

30

40

50

60

0 50 100

Total phosphorus (mg kg-1)

Col

umn

laye

rs (c

m)

Figure 5.3-8 Total phosphorus concentration throughout the leaching column treated with wheat straw and tropical rainfall. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.

Table 5.3-13 Analysis of variance for non-extractable phosphorus throughout the leaching column after treated with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P


There was a significant concentrations of non-extractable-P throughout the

leaching columns in every type of soil (p ≤ 0.05, Table 5.3-13). The trend was

linear (p ≤ 0.01, orthogonal polynomial contrast) as shown in Figure 5.3-9.


0

10

20

30

40

50

60

0 50 100

Col

umn

laye

rs (c

m)

Non-extractable phosphorus (mg kg-1)

R2 = 0.56

R2 = 0.70

Figure 5.3-9 Non-extractable phosphorus distribution in leaching column with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.

5.3.4 Factors affecting phosphorus transformation in the leaching columns

5.3.4.1 Extractable aluminium In the soil column treated with wheat straw and tropical rainfall, there was an

interaction effect (p ≤ 0.05, Table 5.3-14) of soil and column layer on oxalate

extractable-Al (Figure 5.3-12). In the Ultisol, Al showed a declining concentration

(p ≤ 0.01) to the lower layers of the column. On the contrary, in the Oxisol, Al

recorded an increasing concentration (p ≤ 0.01). Both in the Ultisol and the

Oxisol, these linear trends were significant according to t-test0.01. Oxisol had a

higher content of extractable-Al (206 mg kg-1) than the Ultisol (194 mg kg-1). The

content of extractable-Al in leaching column was between 194-217 mg kg-1.


Table 5.3-14 Analysis of variance for extractable aluminium throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Soil 1 1409.5 1409.5 5.37 0.029 Layers 5 2887.8 577.6 2.20 0.088 Soil x layers 5 3953.0 790.6 3.01 0.030 Residual 24 6300.3 262.5 Total 35 14550.6

In leachate, a fraction of dissolved-Al was recorded and showed a significant (p ≤

0.01, Table 5.3-15) response due to soils, sources or organic matter, and rainfall

simulations (Figure 5.3-10).

Table 5.3-15 Analysis of variance for dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment


of freedom

Sum of squares

Mean square F P

Organic matter 3 0.202717 0.067572 12.79 < 0.001 Rainfall 1 0.086700 0.086700 16.41 < 0.001 Soil 1 0.294533 0.294533 55.75 < 0.001 OM x rainfall 3 0.057117 0.019039 3.60 0.024 OM x soil 3 0.219617 0.073206 13.86 < 0.001 Rainfall x soil 1 0.100833 0.100833 19.09 < 0.001 OM x rainfall x soil 3 0.052471 0.017472 3.31 0.032 Residual 32 0.169067 0.005283 Total 47 1.183000 OM = organic matter.


0.0

0.4

0.8

Control Peat Wheat Lucerne

Oxisol Ultisol

0.0

0.4

0.8


Dis

solve

d al

umin

ium

(mg

kg-1

)



Figure 5.3-10 Cumulative dissolved aluminium in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.

5.3.4.2 Extractable iron As for Al, there was a significant interaction effect between soil type and colum

layer (p ≤ 0.05, Table 5.3-16) on oxalate extractable-Fe (Figure 5.3-12). The

trends of Fe concentration throughout the leaching column were linear (p ≤ 0.01).

In the Ultisol, Fe showed a declining concentration to the lower layers of the

column. On the contrary, in the Oxisol, Fe recorded an increasing concentration.

Similar to Al, these linear trends were different from the control according to t-

test0.01. The content of extractable-Fe ranged between 1534-1708 mg kg-1.

There was a significant difference (p ≤ 0.01) in Fe content between Ultisol and

Oxisol which were 902 and 2228 mg kg-1.


Table 5.3-16 Analysis of variance for extractable iron throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Soil 1 15813878 15813878 152.58 < 0.001 Layers 5 356450 71290 0.69 0.637 Soil x layers 5 1612456 322491 3.11 0.026 Residual 24 2487367 103640 Total 35 20270150

In leachate, a fraction of dissolved-Fe was measured and showed a significant (p

≤ 0.01, Table 5.3-17) response due to soil type, sources or organic matter, and

rainfall simulations (Figure 5.3-10).

Table 5.3-17 Analysis of variance for dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment


of freedom

Sum of squares

Mean square F P

Organic matter 3 1.71389 0.57130 18.92 < 0.001 Rainfall 1 0.43510 0.43510 14.41 < 0.001 Soil 1 3.80250 3.80250 125.90 < 0.001 OM x rainfall 3 0.41427 0.13890 4.57 0.009 OM x soil 3 1.74874 0.58291 19.30 < 0.001 Rainfall x soil 1 0.57860 0.57860 19.16 < 0.001 OM x rainfall x soil 3 0.41397 0.13799 4.57 0.009 Residual 32 0.96647 0.03020 Total 47 10.07355 OM = organic matter.


0.0

1.0

2.0


Oxisol Ultisol

0.0

1.0

2.0


Dis

solve

d iro

n (m

g kg

-1)



Figure 5.3-11 Cumulative dissolved iron in leachate after treatment with organic matter (control, peat, wheat straw, and lucerne hay) in two soils (Ultisol and Oxisol) and in two rainfall simulations (tropical and subtropical) at the end of the experiment. Error bars above each bar indicate the standard errors.


0

10

20

30

40

50

60

150 200 250

Aluminium (mg kg-1)

0

10

20

30

40

50

60

0 1500 3000

Iron (mg kg-1)

Col

umn

laye

rs (c

m)

R2 = 0.53

R2 = 0.36

R2 = 0.73

R2 = 0.82

Figure 5.3-12 Extractable aluminium and iron distribution in leaching column treated with wheat straw and tropical rainfall. R2 = coefficient of determination from linear trend line and data plot. --- --- = Ultisol, and ⎯ ⎯ = Oxisol. Bar in every observed point is the standard error.

5.3.4.3 Soil pH There was no difference (Table 5.3-18) on soil pH throughout the leaching

column. Water movement from rainfall treatments had no effect on the balance

between OH- and H+ in leaching columns.

Table 5.3-18 Analysis of variance for soil pH throughout the leaching column after treatment with wheat straw and tropical rainfall in two soils (Ultisol and Oxisol) at the end of the experiment. Only main factors and their interactions are shown


of freedom

Sum of squares

Mean square F P

Soil 1 0.0831 0.0831 0.36 0.554 Layers 5 1.2127 0.2425 1.05 0.411 Soil x layers 5 1.1310 0.2262 0.98 0.450 Residual 24 5.5355 0.2306 Total 35 7.9623


However, the amount of water from subtropical rainfall made the leachate more

acid than that of tropical rainfall (Table 5.3-3), either in the Ultisol or Oxisol.

5.3.4.4 Correlations between all parameters in the leaching column

Table 5.3-19 Correlation among all parameters in the leaching column. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

TP BP NP Al Fe pH DOC TP BP 0.47** NP 0.97** Al Fe 0.54** 0.59** 0.42* pH DOC 0.49** 0.56** 0.76** TP = Total Phosphorus, BP = Bicarbonate Phosphorus, NP = Non-extractable Phosphorus, and DOC = Dissolved Organic Carbon.

5.4 DISCUSSION

The high leaching of dissolved organic-C in the Ultisol treated with tropical rainfall

following application of wheat straw may have been related to the higher C

content of wheat straw residue (Table 3.3-2, Chapter 3). Recalcitrant soluble

organic compounds from peat are likely to remain in soil for longer period of time,

in contrast to the easily-decomposable, simple organic acids from lucerne hay or

wheat straw. However, C loss from leaching was not as much as C loss from

respiration. Even though the respiration decreased over time (Figure 5.3-5), the

overall C loss was much higher than that of leaching process (Table 5.3-2 and

Table 5.3-7). Subtropical rainfall showed a higher respiration rate in both soils

compared to tropical rainfall. The magnitude of C loss from respiration with

tropical rainfall ranged between 1 to 10 times higher in control and in soil with

wheat straw treatment than that of leaching. In columns receiving subtropical

rainfall, C loss from respiration ranged between 7 to 36 times higher in control

and in soil with wheat straw treatment (Table 5.3-4). Columns receiving tropical

rainfall also had higher C loss due to leaching, but columns receiving subtropical

rainfall had more C loss due to respiration.

Not only did dissolved organic-C leach from the columns, but soluble organic

matter concentrations increased in the deeper layers of soil in columns (Figure

5.3-3). This effect was also observed by Rumple et al. (2002) and Brye et al.


(2001), and may represent a mechanism for podzolisation (Egli et al., 2003;

Ugolini et al., 1987). Podzolisation involves the loss of some cations (Al3+ and

Fe2+/Fe3+) from surface soil horizons (Egli et al., 2003), and this experiment also

showed this effect on extractable-Al and Fe content in the leachate, especially in

the Oxisol (Figure 5.3-10 and Figure 5.3-11). In the leaching columns, there was

an interaction (p ≤ 0.01) between type of soil and column layers on extractable-Al

and Fe contents (Figure 5.3-12) indicating the mobility of those cations.

Consequently, longer exposure to a high rate of rainfall (more than the time of the

experiment) may also have leached extractable-Al and Fe through the Ultisol

columns.

In spite of a significant effect on dissolved organic-C concentration in leachate

due to organic matter treatment, rainfall, and type of soil, there was no significant

effect on dissolved reactive-P content in leachate (Table 5.3-10). However, the

non-significant trend in Figure 5.3-7 was similar to the significant trend in Figure

5.3-1 of dissolved organic-C, where wheat straw showed the greatest effect

followed by lucerne hay and peat. This trend in dissolved reactive-P could be the

explanation why this P pool contributes to the problem of water body, i.e.

eutrophication (Liikanen et al., 2004; McDowell and Sharpley, 2004). Moreover,

there was a significant difference with depth (p ≤ 0.01) in bicarbonate-P

concentrations throughout the leaching column. There was an increase of

concentration at the top layer of the column. This occurred in columns treated

with wheat straw with tropical rainfall (Figure 5.3-6). A similar trend was also

observed for total-P (Figure 5.3-8). Observations by Kleinman et al. (2003) who

also recorded that there was an increase in P concentrations (oxalate and

Mehlich 3-extractable) in the deeper layers of leaching columns, and there was

poor correlation of soil P fractions with leachate P. Kleinmann et al. (2003)

suggested that translocation of P due to preferential flow of P via macropores

(bypass leaching) might have taken place.

A closer inspection of the distribution pattern of bicarbonate-P along the leaching

column reveals that there was an initial deposition of bicarbonate-P at the first 10

cm from the soil surface. Working with soil samples from 0-5 cm depth,

Koopmans et al. (2003) found an accumulation of inorganic-P (extracted with

EDTA) after manure applications. This could be similar to the process of

bicarbonate-P deposition in this experiment, but with intermittent occurrence with


deposition. According to Kleinman et al. (2003), P leaching losses often occur in

discrete events or pulses that can be easily missed and to monitor this effect

continuously requires expensive equipment. The fluctuation of P might have

been due to the process of mobilisation and resorption at the wall of macropores

(Jensen et al., 1999). Working with glucose as a source of C in soil column,

Jensen et al. (1999) predicted the role of high C content (1000 mg glucose-C L-1),

Al and Fe(III)(hydr)oxide, and redox condition in creating the fluctuation. This

mechanism might have taken place in our experiment, even though no easily

metabolisable-C source was present.

An interesting result was observed for non-extractable-P throughout the soil in

leaching column. Non-extractable-P was correlated with dissolved organic-C in

Ultisol leaching column (Table 5.3-19) and the distribution of non-extractable-P

(Figure 5.3-9) was similar to that of dissolved organic-C (Figure 5.3-3). Both

showed linear trends with depth (p ≤ 0.05, orthogonal polynomial contrast). More

non-extractable-P and dissolved organic-C were recorded at greater depth in the

columns indicating leaching and deposition processes. In the Oxisol, the

deposition of non-extractable-P took place at 20-30 cm depth in the column. The

high content of extractable-Fe in the Oxisol (Table 4.2-4, Chapter 4) might

represent amorphous Fe sesquioxide which can adsorb this pool of P by ligand

exchange with surface hydroxyls, especially organic-P from inositol (Turrion et

al., 2001). Considering the effect of mobilisation and resorption on bicarbonate-P

and total-P in this experiment, non-extractable-P on the other hand, was not

affected by such mechanisms, but transported more freely to the deeper layers of

the column.

As for non-extractable-P in leaching columns, there were significant linear trends

(p ≤ 0.01) in Al and Fe distribution throughout the leaching column. In the Oxisol,

probably due to higher content of extractable-Al and Fe (Table 4.2-4, Chapter 4),

both Al and Fe were more mobile. Soil pH might also responsible for their

mobility. Even though soil pH was not affected, soil organic matter

decomposition created more acid leachate, especially at a subtropical rainfall

rate. This acid leachate was probably more easily to transport extractable-Al and

Fe throughout the leaching columns. As a result, extractable-Al and Fe

deposited at the deeper layers or even leached away (Figure 5.3-10 and Figure

5.3-11). This mechanism confirms the hypothesis that organic matter addition


will affect not only soil organic-C content of the soil, but also non-extractable-P,

extractable-Al, extractable-Fe, and pH (Chapter 4).

Figure 5.4-1 summarises average C mobility and loss in Ultisol columns treated

with tropical rainfall and 80 ton ha-1 wheat straw. Respiration was the highest C

flux, followed by leaching and deposition. The total C loss from the original pools

(soil organic C + wheat straw treatment at 80 ton ha-1) was about 0.8%.

Before After

Tropicalrainfall

(4000 mm year-1)

Ultisol = Wheat = 113.5 g C114.4 g C

252 mg C

66 mg C

646 mg C Respiration

Residual

Translocationwithin column

Leaching

Figure 5.4-1 Carbon losses (respiration and leaching) and deposition due to tropical rainfall (4000 mm year-1) in soil (Ultisol) column treated with wheat straw (80 ton ha-1).

5.5 CONCLUSION

Respiration contributed to C loss ranging from 0.2-1.00% of total-C in soils,

regardless of the rainfall rate. A small percentage of C loss occurred via leaching

(0.03-0.05% of total-C for subtropical and 0.10-0.22% of total-C for tropical

rainfall regimes). Respiration therefore contributed 1-36 times higher C loss than

leaching. However, at a tropical rate of rainfall, the leaching process was as

important as respiration as a C loss mechanism, and could reduce the

persistence of added organic matter.


There was no evidence of bicarbonate-P and non-extractable-P leaching from the

columns, but there was an inconsistent distribution of bicarbonate-P and linear

distribution of non-extractable-P within the leaching columns.

Added organic matter had an impact on the distribution of soil bicarbonate-P,

non-extractable-P and dissolved organic-C as well as sesquioxides down the

leaching columns. Dissolved organic-C was the most readily leached from within

the six month period. There was an indication that bicarbonate-P and non-

extractable-P will be leached away with longer period of time, especially in sandy

soil with higher rainfall.

In these scenarios, if organic matter amendments were to be applied to lateritic

soils, climatic factors, i.e. rainfall, need to be taken into account in order to make

amendments more effective and efficient. Moreover, to test the effectiveness of

organic matter amendments, in situ research is likely to be the most

advantageous approach to be employed.


C h a p t e r 6

THE RELATIONSHIP BETWEEN PHOSPHATE ADSORPTION AND SOIL ORGANIC CARBON

FROM ORGANIC MATTER ADDITION

6.1 INTRODUCTION

The addition of organic matter to soils may increase phosphate availability by

decomposition and mineralisation of organic phosphorus, or by abiotic processes

such as ligand-exchange effects on phosphate adsorption. It is difficult to

separate the effects of biotic and abiotic processes on the release of phosphate.

This chapter describes experimental work which investigated the direct effect of

organic matter addition on adsorption of phosphate in four types of lateritic soils.

The organic matter content of highly weathered soils has been shown to be

negatively correlated with phosphate adsorption capacity. For example, Singh

and Gilkes (1991) found a negative correlation between phosphate sorption by

soil (quantified by the Freundlich KF parameter) and soil organic matter content

for 97 mainly highly weathered soils of Western Australia. Based on Freundlich

KF values, Dubus and Bacquer (2001) also found a significant negative

correlation between phosphate sorption and soil organic matter content.

Three abiotic mechanisms are proposed to explain how organic matter additions

increase P availability in soils (Iyamuremye and Dick, 1996). First, soluble

organic molecules may specifically adsorb to soil minerals by ligand exchange in

competition with phosphate (Erich et al., 2002; Ohno and Crannel, 1996; Singh

and Jones, 1976). Second, the soluble organic matter may react with bound Al3+

or Fe3+ at the surface of mineral phases to form soluble complexes of these

elements and release phosphate which was previously sorbed or which was

present as insoluble Al and Fe-phosphate (Haynes and Mokolobate, 2001).

Third, organic matter may be sorbed to soil particles at non-specific adsorption

sites, resulting in higher negative charge of the particles (Haynes and Naidu,

1998; Naidu et al., 1997). This process reduces the electrostatic attraction of

phosphate and decreases phosphate anion activity at the reactive surface. All

VI. The Relationship between Phosphate Adsorption … 167

three abiotic mechanisms may occur simultaneously, and in combination with

biotic release of phosphate by mineralisation.

The experiment described in Chapter 3 showed that phosphate extracted with 0.5

M NaHCO3 at pH 8.5 (bicarbonate-P) was significantly higher in soil amended

with organic matter than in unamended soil, especially with green manure

amendment (lucerne hay), followed by peat. The effect of soluble organic matter

on phosphate release from Oxisols has been found to be transient (Afif et al.,

1995). Moreover, fresh organic matter, which produces soluble organic

compounds, has more active (particulate) pools prior to decomposition (Brenner

et al., 2001; Gijsman et al., 2002). The findings in this work therefore lead to the

question of whether peat (the passive pool) would have a longer abiotic effect

due to its resistance to decomposition, and possible slow release of soluble

organic ligands which can compete for adsorption sites with phosphate. This

issue is poorly understood. The abiotic effects of organic matter additions could

be determined by analysing phosphate adsorption isotherms for soil incubated

with lucerne hay and peat for varying lengths of time up to one year.

Organic matter addition also releases phosphate to the soil solution as a result of

biotic mineralisation processes (Cobo et al., 2002; Kwabiah et al., 2003; Lupwayi

et al., 2003). These two mechanisms may be particularly difficult to separate in

soils with high sorbing capacity for phosphate, such as lateritic soils with high

sesquioxide or 1:1 clay content. By (i) observing net release of extractable

phosphate (Chapter 3 and 4) and (ii) determining phosphate adsorption

isotherms, the two processes (inorganic effects on adsorption, and mineralisation

of organic-P) can be separated as being more and less important for releasing

soluble phosphate into soils.

In summary, it is expected that organic matter addition will reduce phosphate

adsorption in soils, and this hypothesis will be addressed by determining

phosphate adsorption at a range of organic matter application rates. This work

will also test the novel hypotheses that (i) any reduction in phosphate adsorption

will be temporary depending on the persistence of organic matter added (for

example, peat is expected to reduce phosphate adsorption for longer than

lucerne hay) and (ii) newly added organic matter will decrease phosphate

adsorption, in the short-term, more than pre-existing soil organic matter.

The objectives of this experiment were therefore:


• to determine the effect of organic matter application rate on phosphate

adsorption to soil;

• to determine the interactive effect of (i) application of different types of

organic matter (varying in persistence following application) and (ii) organic

amendment–soil incubation time on phosphate adsorption to soil;

• to determine the effect of pre-existing soil organic matter and newly added

organic amendments on PO4-P adsorption in soils;

• to study the effect of phosphate adsorption and P mineralisation after

organic matter addition on the process of bicarbonate-P release;

• to study the changes in dissolved organic-C after organic matter treatment

on closely related factors of phosphate adsorption in soil, e.g. Al, Fe, and

pH.


6.2.1 Design of the experiments Soil samples for the experiments in this chapter were from four lateritic soils (two

Oxisols, an Ultisol, and a deep regolith material or subsurface soil). The

experiments were divided into three subsets which had been subjected to the

following treatments:

1. Soil incubation with peat, wheat straw, and lucerne hay treatment at 80

ton ha-1 for different times (3, 6, 9, and 12 months).

2. Different organic matter levels (40, 80, 120, and 160 ton ha-1) for peat,

wheat straw, and lucerne hay, incubated with soil for six months.

3. Type of soil organic matter (existing and new). The existing refers to

inherent soil organic matter plus additional fresh organic matter as in

point 1 above. ‘New’ refers to equal (in organic-C concentration)

replacement of soil organic matter plus additional fresh organic matter to

soil in which the intrinsic organic matter has been removed by

combustion.

The first subset of this experiment was in a factorial design experiment with two

factors: incubation time (3, 6, 9, and 12 months), and source of organic matter

(nil (control), peat, wheat straw, and lucerne hay). The second subset was also


factorial with two factors: source of organic matter (nil (control), peat, wheat

straw, and lucerne hay), and organic matter levels (40, 80, 120, and 160 ton

ha-1). The third subset was a factorial design with three factors: soils (an Ultisol,

an Oxisol, and a subsurface soil), type of soil organic matter (existing and new),

and organic matter source (peat and lucerne hay).

Statistical analyses were performed either with GenStat (Payne et al., 1987) for

analyses of variance, or SPSS (Coakes, 2001) for regression analyses.

6.2.2 Procedures The preparations for the first and the second subset of the experiment are

described in Chapter 3. In brief, soil (Oxisol, Typic hapludox; “Balkuling”) was

collected from a farmland near York, Western Australia. One kg of soil was

placed into each pot and the appropriate treatment was applied by mixing the soil

and respective organic material. Deionised water was added weekly during

incubation (12 months) to reach a moisture equivalent of 60% of the maximum

water holding capacity of the soil (42%).

The preparations for the third subset are described in Chapter 4. In brief, soil

samples (Ultisol from Jarrahdale, Oxisol from Bunbury, and subsurface soil —

deep regolith material— from Boddington Gold Mine, all in Western Australia)

were air-dried and sieved to ≥ 2 mm. To generate soils having only newly-added

organic matter, soils were combusted at 450° C for 12 hours using a pottery kiln

(Kiln West, model 6191Z) as described in Chapter 4. A temperature of 450° C is

hot enough to deplete organic-C content, but has a minimal effect on other

physical and chemical properties of soils such as porosity, textural class, and soil

acidity (Giovannini et al., 1988; Kang and Sajjapongse, 1980).

6.2.3 Measurements

6.2.3.1 Phosphate adsorption isotherms Phosphate adsorption isotherms were measured using methods of Morel et al.

(1996) and Erich et al. (2002). To determine phosphate adsorption isotherms, six

amounts of added P (30, 40, 50, 60, 80, 100 mg P kg-1) were applied as KH2PO4

to one g of soil in 25 mL of 0.01 M CaCl2. These concentrations had been

established in a preliminary experiment to check for the appropriate concentration

range, given the high phosphate sorbing capacity of the soils. Suspensions were


shaken in 50 mL centrifuge tubes at 25° C for 17 hours. Tubes were centrifuged

at 3000 rpm for five minutes and the supernatants were filtered (Whatman #42)

prior to analysis of phosphate.

Phosphate remaining in solution after equilibrium process was measured using

the methods of Murphy and Riley (1962) as described in Rayment and Higginson

(1992).

6.2.3.2 Numerical methods Two common adsorption isotherm equations, Langmuir and Freundlich, were

used to fit the phosphate adsorption data. The Langmuir equation, which is

normally written as (Barrow, 1978):

)1()(

cKcxK

xL

mL

+= .................................................................................................. 6.2-1

becomes a linear equation when rearranged:

cxxKx

c

mmL

11+= ............................................................................................ 6.2-2

where c = concentration of P in equilibrium solution (µg P mL-1), x = amount of P

sorbed (µg P g-1 soil), xm = adsorption maximum (µg P g-1 soil), and KL =

coefficient related to bonding energy. A plot of x/c versus c therefore has a slope

of 1/xm and intercept of 1/ KL xm.

The Freundlich equation, which is normally written as (Barrow, 1978):

bF cKx = ......................................................................................................... 6.2-3

can be log-transformed to give simple linear equations:

cbKx F logloglog += ................................................................................... 6.2-4

where c = P concentration in equilibrium solution (µg P mL-1), x = amount of P

sorbed (µg P g-1 soil), KF and b = constants with KF being a measure of

adsorption surface and b relating to the energy of adsorption (Barrow, 1978). A

plot of log x versus log c has a slope b and intercept log KF.

Due to their simplicity, i.e. only two adjustable parameters, the Langmuir and

Freundlich equations do not always fit experimental data well. The Brunauer-

Emmett-Teller (BET) equation was applied to take into account the plateaus,


points of inflection, and maxima observed in some data (Giles et al., 1974; Hinz,

2001). Unlike the Langmuir and Freundlich equations, which are classified as

high affinity adsorption equations, the Brunauer-Emmett-Teller is an adsorption

equation that can explain sigmoidal isotherms, as observed for some data from

this experiment.

The equation is usually written (Burau and Zasoski, 2002):

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−+−

=

sBETs

mBET

ccKcc

cxKx

)1(1)(..................................................................... 6.2-5

Like the other two equations, the Brunauer-Emmett-Teller equation has a linear

form after algebraic rearrangement (Appendix A):

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛ −+=⋅

− smBET

BET

mBETs cc

xKK

xKxccc )1(11

........................................................ 6.2-6

where c = P concentration in equilibrium solution (µg P mL-1), cs = the

concentration of the solute (i.e. soil), xm = adsorption maximum (µg P g-1 soil),

and KBET = a constant for energy of interaction with soil particles (Burau and

Zasoski, 2002). A plot of c/(cs-c)(1/x) versus c/(cs-c) therefore has a slope

(KBET-1)/(KBETxm) and intercept 1/(KBETxm).

In order to determine the statistical significance of differences between

treatments, linear regression analysis with grouped data was performed on data

transformed to give linear Langmuir or Freundlich relationships, as described

above (Section 6.2.3.1). This procedure allowed statistical significance to be

assessed without replication of points on adsorption isotherms. Grouped

regressions were performed using GenStat (Payne et al., 1987).

6.2.3.3 Dissolved organic carbon Dissolved organic-C in soil samples was measured using the method by Wagai

and Solin (2002). Briefly, dissolved organic-C in soil was extracted from 10 g air-

dry soil with 25 mL deionised (MilliQ), shaken for 30 minutes on an end-over-end

shaker, and then centrifuged at 10,000 rpm for 10 minutes. Filtered supernatant

(0.2 µm filter, 25 mm, supor membrane, non sterile, Pall Gelman Laboratory) was

used to measured soluble organic-C using a TOC (Shimadzu TOC-5000A)

machine as the difference between total-C and inorganic-C in the solution.


6.2.3.4 Total phosphorus Total-P was measured by persulfate digestion as described by Nelson (1987). A

mixture of 0.5 g soil, 1.0 mL deionised water, 1.0 mL 5.5 M H2SO4, and 0.400 g

K2S2O4 in screwcapped Pyrex tubes (100 mL) was autoclaved for one hour at

130° C. Samples were diluted to 50 mL with deionised water, mixed, and

allowed to stand overnight to settle particulate materials. Total phosphate in

extracts was measured colorimetrically as described by Rayment and Higginson

(1992).

6.2.3.5 Bicarbonate phosphorus Bicarbonate-P was measured following extraction of soil with 0.5 M NaHCO pH

8.5. The method used was a modification of a method by Olsen et al. (1954)

described by Rayment and Higginson (1992). The manual colorimetric

determination of phosphate at the end of the method was based on the method of

Murphy and Riley (1962).

3

6.2.3.6 Non-extractable phosphorus Non-extractable-P was measured by assessing total-P with acid-persulfate

digestion as described by Nelson (1987) (see above) and then subtracting the

bicarbonate-P content.

6.2.3.7 Extractable aluminium and iron Oxalate extractable-Al and Fe, considered to represent amorphous Al and Fe

oxyhydroxides, were measured using an acid oxalate extraction as explained by

Rayment and Higginson (1992), modified to use 0.250 mg of soil in 25 mL

reagent. The extraction was conducted in the dark with minimal room light to

avoid photo-reduction of Fe (Schwertmann and Taylor, 1989). A black plastic

sheet was also used to cover sample batch during transportation from shaker to

centrifuges. Aluminium and Fe concentrations in extracts were determined by

atomic absorption spectrophotometry. Caesium chloride solution 0.01 M (1+4)

was used for dilution to reduce ionisation effects.

6.3 RESULTS

The Langmuir equation fitted experimental data better than the Freundlich

equation in describing phosphate adsorption (Table 6.3-1 and Table 6.3-2). This


conclusion was based on the R2 values from the Langmuir equation that tended

to be higher than that of the Freundlich equation. These were applied to the first

two sampling (subset 1 and 2, Section 6.2.1). For the last subset (3), the

Brunauer-Emmett-Teller equation provided a better fit to the experimental data,

due to the sigmoidal shape of the adsorption relationship.

6.3.1 The effect of incubation time The effect of prior incubation time with organic amendments on phosphate

adsorption varied, depending on the source of organic matter. Figure 6.3-1

shows the trends in adsorption isotherms at different incubation times.

Increasing incubation time had a tendency to reduce the affinity of adsorption up

to six or nine months after organic matter application. One-year after application,

organic amendment was less or not effective in reducing phosphate adsorption in

the soils. After this period of time, adsorption capacity seemed return to the initial

position or was similar to adsorption at three months after organic matter

addition.

These effects correspond with the trends KL and KF values presented in Table

6.3-1. Values of KL and KF decreased from three to nine months incubation, and

approached the initial values (three-month incubation) at the end of incubation.

Soil amended with peat recorded its lowest KL (0.42) and KF (338) values after

nine months of incubation. Similar trends were also recorded for lucerne hay.

Therefore, for peat and lucerne hay applications, the nine-month incubation

effectively reduced phosphate adsorption (p ≤ 0.01 on linear regression with

grouped data) more than three-month incubation (Figure 6.3-2).


0

500

1000

0.0 10.0 20.0

0

500

1000

0.0 10.0 20.0

0

500

1000

0.0 10.0 20.0

0

500

1000

0.0 10.0 20.0

Peat

Control

Wheat

Lucerne

P in solution (mg L-1)

P so

rbed

(mg

kg-1)

Figure 6.3-1 The effect of incubation and organic matter sources on phosphate adsorption at 80 ton ha-1 organic matter in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Lines are a logarithmic trend of observations and a visual guide only.


0.00

0.01

0.02

-1 7 14

c

2.4

2.8

3.1

-0.5 0.4 1.3

log c

log

x

BAc/

x

Figure 6.3-2 Linearised Langmuir (A) and Freundlich (B) plots of incubation effect on phosphate adsorption for peat treatment (80 ton ha-1) in “Balkuling” Oxisol. = three months, = six months, = nine months, and = twelve months incubation. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).

6.3.2 The effect of organic matter sources The effectiveness of organic matter source in reducing phosphate adsorption was

in the order of lucerne hay>wheat straw>peat>control (p ≤ 0.01). This order

occurred at almost every level of organic matter applied (Figure 6.3-3).

For wheat straw alone, phosphate adsorption was lower than the control until six

months after organic matter treatment (Table 6.3-1). After that, phosphate

adsorption returned to the initial condition for all sources of organic matter.

These conclusions are supported the magnitude of the Langmuir’s bonding

energy (KL) and KF at Freundlich. Both Langmuir KL and Freundlich KF

parameters were lower for soil samples having organic matter applications

compared with controls (Table 6.3-1).


0

500

1000

0.0 9.0 18.0

0

500

1000

0.0 9.0 18.0

0

500

1000

0.0 9.0 18.0

0

500

1000

0.0 9.0 18.0


P so

rbed

(mg

kg-1) 80 ton ha-1

40 ton ha-1

120 ton ha-1

160 ton ha-1

Figure 6.3-3 The effect of organic matter sources and their levels on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Lines are a logarithmic trend of observations and are a visual guide only.



0.00

0.01

0.02

-3 8 18

c

2.4

2.7

3.0

-0.6 0.5 1.5

log c

log

x

BAc/

x

Figure 6.3-4 Linearised Langmuir (A) and Freundlich (B) plots of organic matter sources (120 ton ha-1) on phosphate adsorption at six months incubation time in “Balkuling” Oxisol. = control, = peat, = wheat straw, and = lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).

6.3.3 The effect of organic matter levels Figure 6.3-3 shows that the higher the amount of organic matter applied the more

soluble phosphate were released in the soils. This could be an indication that the

higher the organic matter application, the lesser the phosphate adsorption, as all

adsorption parameter showed decreasing values, especially KL and KF (Table

6.3-2). Lucerne hay, for example, had KL = 0.63 at 40 ton ha-1 application and

the value of this parameter decreased regularly until KL = 0.41 at 160 ton ha-1.

Similar trends were recorded for the Freundlich equation parameter for

adsorption after addition of peat and wheat straw.

Increasing amounts of peat application also decreased (p ≤ 0.01) phosphate

adsorption (Figure 6.3-5). Increasing the level of peat from 40-120 ton ha-1

decreased phosphate adsorption more regularly compared with wheat straw and

lucerne hay. The graphs from linear regression with group were similar either

from linearised Langmuir or Freundlich equation.

0.000

0.007

0.013

-2 5 12

c

2.40

2.70

3.00

-1.0 0.3 1.5

log c

log

x

BAc/

x

Figure 6.3-5 Linearised Langmuir (A) and Freundlich (B) plots of peat levels at six months incubation time on phosphate adsorption. = 40, = 80, = 120, and = 160 ton ha-1 in “Balkuling” Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).

Table 6.3-1 The effect of organic matter sources (80 ton ha-1) and incubation time on phosphate adsorption parameters based on Langmuir and Freundlich equations

Langmuir Freundlich Organic matter source KL xm R2 b KF R2

Three months Control 0.7934 1094.1 0.997 0.41196 457.9 0.912 Peat 0.6068 1150.8 0.993 0.44838 424.4 0.949 Wheat straw 0.5894 1018.3 0.997 0.37781 387.2 0.967 Lucerne hay 0.4343 1065.0 0.996 0.42039 347.2 0.957 Six months Control 0.6869 1017.3 0.994 0.36010 414.7 0.985 Peat 0.6875 1021.5 0.996 0.36998 412.9 0.970 Wheat straw 0.4476 979.4 0.993 0.36612 342.1 0.990 Lucerne hay 0.4087 940.7 0.995 0.35026 325.5 0.991 Nine months Control 0.5981 1035.2 0.984 0.35554 406.5 0.912 Peat 0.4077 995.0 0.992 0.37298 338.5 0.952 Wheat straw 0.3761 1078.8 0.980 0.42485 331.3 0.974 Lucerne hay 0.2233 962.5 0.992 0.40546 246.4 0.975 Twelve months Control 0.6751 1074.1 0.995 0.40017 423.8 0.983 Peat 0.5086 1124.9 0.994 0.44033 385.7 0.986 Wheat straw 0.5359 1038.4 0.996 0.39345 376.3 0.976 Lucerne hay 0.4524 1012.2 0.993 0.38131 349.1 0.991


Table 6.3-2 The effect of organic matter sources and levels on phosphate adsorption parameters at six months incubation time based on Langmuir and Freundlich equations

Langmuir Freundlich Organic matter source KL xm R2 b KF R2

40 ton ha-1

Control 0.9764 1005.2 0.989 0.33493 467.8 0.972 Peat 0.7811 1049.3 0.992 0.36795 445.9 0.990 Wheat straw 0.7513 1000.0 0.990 0.34062 426.8 0.989 Lucerne hay 0.6391 956.9 0.989 0.30622 402.5 0.991 80 ton ha-1



Control 0.8603 1027.8 0.993 0.35592 454.0 0.973 Peat 0.7704 990.1 0.993 0.33136 428.5 0.990 Wheat straw 0.4651 956.9 0.986 0.34909 346.4 0.981 Lucerne hay 0.4023 925.1 0.979 0.30390 343.5 0.983

6.3.4 The effect of existing and new soil organic matter The effect of existing and new soil organic matter depended on both the source

of organic matter and incubation time. Neither the Langmuir nor Freundlich

equations provided a good fit in describing phosphate adsorption from the

treatments, as the adsorption relationships were sigmoidal. The Brunauer-

Emmett-Teller equation described this type of data better. The new soil organic

matter treatments of peat and lucerne hay resulted in an increase in phosphate

adsorption in soils compared with the existing soil organic matter (Figure 6.3-6).

This effects occurred in all soils (subsurface, Ultisol, and Oxisol), no matter how

high the initial bicarbonate-P content of any particular soil. New soil organic

matter treatment increased phosphate adsorption intensity, despite more

bicarbonate-P release in soils. The KBET parameter (Table 6.3-3) for new soil

organic matter treatments had higher values compared with existing soil organic

matter treatments.

For subsurface soil, due to negligible pre-existing soil organic matter content

(Table 4.2-5, Chapter 4), the effect of soil organic matter type (existing and new)



can be assumed to represent the effect of heating soil. The difference between

phosphate measured in solution from new and existing soil organic matter can be

assumed to represent the increase in adsorption capacity. The increase was

significant (p ≤ 0.05 on linear regression with grouped data) when using

Brunauer-Emmett-Teller equation to parameterise the adsorption data (Figure

6.3-8).

Figure 6.3-6 The effect of soils, existing and new soil organic matter on phosphate adsorption in peat and lucerne hay treatments at 80 ton ha-1 and six-month incubation time. = subsurface soil, = Ultisol, and = Oxisol. Lines are polynomial trend for observations and are a visual guide only.

ween Phosphate Adsorption … 182

0

450

900

0.0 40.0 80.00

450

900

0.0 40.0 80.0

0

450

900

0.0 40.0 80.00

450

900

0.0 40.0 80.0

P so

rbed

(mg

kg-1)

Existing New


Peat

Lucerne

VI. The Relationship bet

Table 6.3-3 The effect of soil organic matter types (existing and new) and soil types (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption parameters based on fitting data to the Brunauer-Emmett-Teller equation

Existing soil organic matter New soil organic matter Organic matter source xm KBET R2 xm KBET R2

Subsurface soil Peat -70 -8.4 0.543 408 49.0 0.885 Lucerne hay -132 -7.6 0.467 1111 4.5 0.047 Ultisol Peat 1351 18.5 0.666 767 34.3 0.986 Lucerne hay -625 -16.0 0.524 991 12.1 0.983 Oxisol Peat 820 61.0 0.858 998 501.0 0.995 Lucerne hay 893 56.0 0.739 903 158.1 0.959

6.3.5 The phosphate adsorption capacity of the three soils Phosphate adsorption capacity was different among the three soils. The Oxisol

had the highest adsorption capacity, followed by the Ultisol and subsurface soil

(Figure 6.3-6). This conclusion was supported by the higher magnitude of KBET

for the Oxisol compared with the other two soils (Table 6.3-3). Regression

analysis with grouped data also indicated that the Oxisol had the highest

phosphate adsorption capacity compared with Ultisol and subsurface soil (p ≤

0.01) (Figure 6.3-7).

VI. The Relationship between Phosphate Adsorption with… 183

0.0000

0.0001

0.0002

-0.01 0.03 0.07

c/cs

c/(c

s-c)

.1/x

Subsurface soil

Ultisol

Oxisol

Figure 6.3-7 Linearised Brunauer-Emmett-Teller equation of soil type (subsurface soil, Ultisol, and Oxisol) on phosphate adsorption in new soil organic matter + peat treatment. = subsurface soil, = Ultisol, and = Oxisol. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.01).

In subsurface soil, new soil organic matter treatments, of either peat (p ≤ 0.01) or

lucerne hay (p ≤ 0.05) increased phosphate adsorption capacity (Figure 6.3-8).

In the Ultisol and Oxisol, the same increase was recorded for new soil organic

matter + peat (p ≤ 0.01) and new soil organic matter + lucerne hay (p ≤ 0.01)

(Figure 6.3-9 and Figure 6.3-10).


0.0000

0.0007

0.0013

0.00 0.04 0.08

c/cs

c/(c

s-c)

.1/x

Existing SOM + peat

New SOM + peat

Existing SOM + lucerne

New SOM + lucerne hay

Figure 6.3-8 Linearised Brunauer-Emmett-Teller equation of soil organic matter type (existing and new) on phosphate adsorption in subsurface soil. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).

0.00000

0.00007

0.00014

0.000 0.021 0.041

c/cs

c/(c

s-c)

.1/x

Existing SOM + peat



New SOM + peat

Figure 6.3-9 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Ultisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).


0.00000

0.00003

0.00006

0.000 0.017 0.034

c/cs

c/(c

s-c)

.1/x


Existing SOM + peat


New SOM + peat

Figure 6.3-10 Linearised Brunauer-Emmett-Teller equation of organic matter type (existing and new) on phosphate adsorption in Oxisol. = existing soil organic matter + peat, = new soil organic matter + peat, = existing soil organic matter + lucerne hay, and = new soil organic matter + lucerne hay. Grouping regressions to allow different y-intercepts and slopes provided significantly improved regressions (p ≤ 0.05).

6.3.6 Correlation of phosphate adsorption parameters with dissolved organic carbon, extractable aluminium, and extractable iron

In order to see the effect of peat and lucerne hay on phosphate adsorption

capacity of the soil in the third subset of the experiment, we assessed the data

from uncombusted soil samples. Bicarbonate-P was positively correlated with

dissolved organic-C in soil treated with existing soil organic matter + lucerne hay

only. For existing soil organic matter + peat and existing soil organic matter +

lucerne hay treatments, bicarbonate-P was negatively correlated with extractable-

Al and Fe (Table 6.3-4). Only in existing soil organic matter + lucerne hay

application did bicarbonate-P show a positive correlation with dissolved organic-

C.


Table 6.3-4 Correlation between inorganic phosphorus, dissolved organic carbon, extractable aluminium, and extractable iron in uncombusted soils treated with peat and lucerne hay treatments. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

Existing SOM + peat Existing SOM + lucerne hay BP DOC Al BP DOC Al

DOC 0.63** Al -0.83** -0.72** -0.69** Fe -0.64** -0.44* 0.44* -0.61** -0.82** 0.56** SOM = soil organic matter, DOC = dissolved organic-C, BP = bicarbonate-P.

Further correlation analysis of adsorption parameters with soil chemical

properties in the first subset of the experiment showed that dissolved organic-C

and pH had negative correlations (Table 6.3-5) with KL and xm for the Langmuir

equation; and KF for the Freundlich equation. Even though KL represents the

bonding energy and KF relates to adsorption capacity, the coefficient correlations

for both on dissolved organic-C and soil pH were almost similar.

Table 6.3-5 Correlation between phosphorus adsorption parameters and soil chemical properties in “Balkuling” Oxisol. Only significant correlations are presented. * represents a significant (p ≤ 0.05) and ** a highly significant (p ≤ 0.01) correlation

KL xm b KF DOC xm 0.45** b -0.64* KF 0.96** 0.38* -0.59** DOC -0.44* -0.67** -0.42* pH -0.50** -0.52** -0.52** 0.75** DOC = dissolved organic-C

6.4 DISCUSSION

Based on the values of adsorption isotherm parameters for the Langmuir and

Freundlich equations (Table 6.3-1 and Table 6.3-2), the data presented support

the hypothesis that organic amendments decreased the soils’ affinity (Langmuir

KL) and capacity (Freundlich KF) for phosphate adsorption. These results are in

agreement with Iyamuremye and Dick (1996) and Hu et al. (1994) that addition of

organic amendments can significantly decrease phosphate adsorption capacity of

soil. Although wheat straw application reduced phosphate adsorption as also

observed by Reddy et al. (2001), peat and lucerne hay applications reduced


phosphate adsorption for longer (up to nine months after treatments). For peat

and lucerne hay applications the reduction in phosphate adsorption at nine-month

incubation was significantly greater (p ≤ 0.01) than at three-month incubation.

Peat persisted longer than lucerne hay in soil (Figure 3.3-3, Chapter 3), but

lucerne hay produced more dissolved organic carbon than other sources (i.e.

peat and wheat straw, Figure 4.3-1, Chapter 4) which would be expected to

decrease phosphate sorption (Erich et al., 2002). In addition, the nature of

soluble organic matter from peat is likely to be relatively recalcitrant and to have a

more long-lasting effect in blocking phosphate adsorption sites, whilst the

abundant simple organic acids originating from plant materials are easily

degradable. It is therefore assumed that peat application decreased phosphate

sorption, at least partly, by a mechanism other than competitive ligand exchange

with dissolved organic-C.

The reduction of phosphate adsorption by the organic amendments persisted for

at least 12 months following application, shown by the continued difference

between treatments and controls (e.g. Figure 6.3-1). This effect was observed

despite the organic amendments becoming less effective at reducing phosphate

adsorption with increasing time of incubation in soil. These conclusions are

supported by the magnitude of the xm parameter of the Langmuir equation and KF

of the Freundlich equation for every organic matter source throughout the

incubation time (Table 6.3-1). The xm parameter was decreasing in value across

incubations and increased again after nine-month incubation.

The hypothesis that newly added organic matter would decrease phosphate

adsorption, in the short-term, more than pre-existing soil organic matter was not

supported by the data in this chapter. A number of factors interacted to affect

phosphate adsorption by soils in this highly complex experiment, with no clear

effect of organic matter addition, as described below.

Contradictory results were obtained regarding the effect of new soil organic

matter compared with existing soil organic matter. Instead of dissolved

phosphate concentrations increasing due to addition of new soil organic matter

as predicted; these concentrations decreased indicating an increase in

phosphate adsorption (Figure 6.3-6). The Langmuir equation, which fit the

adsorption data from the first two subsets of data (Experiment I and II), was not

suitable. A more complex equation (the Brunauer-Emmett-Teller isotherm


equation) was needed in order to explain the sigmoidal curvature observed in

phosphate adsorption for soil subjected to the treatments in this experimental

subset (Hinz, 2001).

Combustion of soil samples prior to ‘new’ soil organic matter application might

have altered some physical and chemical characteristics of the soils which affect

adsorption of phosphate. Even though Kang and Sajjapongse (1980) and

Giovannini et al. (1988) found that a 450° C temperature had a minimal effect on

other physical and chemical properties of soils such as porosity, textural class,

and soil acidity, however, a temperature up to 160° C will almost certainly change

the microbial community structure (Pietikainen et al., 2000). Moreover, heating

soil to around 300° C will reduce the amount of soil organic matter, as well as

cation exchange capacity and exchangeable bivalent cations like Ca and Mg

(Badia and Marti, 2003; Brais et al., 2000; Forgeard and Frenot, 1996). High

temperature can change physical and chemical properties of the clays and

oxides. According to Kang and Sajjapongse (1980), high temperature up to 500°

C altered some soil chemical and physical properties. Giovannini et al. (1988)

found, however, that temperatures of 460° C or more need to be attained to

change the physical properties of the soil, such as particle size distribution,

plasticity, and aggregate stability. In this experiment, heating soil to 450° C

changed some parameters related to phosphate adsorption, such as oxalate

extractable-Al and Fe contents (Table 4.2-5, Chapter 4). Subsurface (regolith)

soil and the Ultisol appeared to change phosphate adsorption capacity more than

the Oxisol (Figure 6.3-6). This may be because the increases in extractable-Al

and Fe due to soil combustion were more pronounced in the subsurface soil (Alox

increased ~10×, Feox increased ~2.5×) and the Ultisol (Alox and Feox increased

~2.5×) than in the Oxisol (Alox increased ~4×, Feox did not increase). This

phenomenon was also observed by Gao et al. (2002) as they partially removed

soil organic matter from paddy soils. In line with these results, Su et al. (2001)

and Singh and Gilkes (Singh and Gilkes, 1991) also found a significant positive

correlation between amorphous Fe-oxides and phosphate adsorption in soils. In

soils with 1:1 lattice clays, phosphate adsorption is mainly attributed to the

hydrous oxides of Al and Fe, as well as occurring on the clay itself (Dubus and

Becquer, 2001; Sanyal et al., 1993).


The Brunauer-Emmett-Teller equation could explain the sigmoidal adsorption

isotherms due to the above combustion. However, adsorption parameters in the

original curvatures of adsorption (the Langmuir and Freundlich equation)

appeared in negative slopes in the Brunauer-Emmett-Teller equation (Figure

6.3-6) to make numerical comparison impossible. Nevertheless, the decreasing

trends of adsorption can be seen in Figure 6.3-6.

It is well known that the application of manure and organic materials reduces

phosphate adsorption capacity in soils (Berton and Pratt, 1997; Hundal et al.,

1988). In order to see the importance of phosphate adsorption in releasing

inorganic-P into soil solution I reviewed some bicarbonate-extractable phosphate

data from the second experiment (Chapter 4). New soil organic matter increased

bicarbonate-P content, especially peat and lucerne hay in the Ultisol and the

Oxisol (Figure 4.3-2, Chapter 4), despite increasing phosphate adsorption

capacity due to the combustion of the soils (Figure 6.3-6) as also indicated by the

increase of KBET in Table 6.3-3. In other words, changes in phosphate adsorption

appear to be less important as a bicarbonate-P release mechanism when soils

are treated with organic matter. According to Osiname et al. (Osiname et al.,

2000) a sizable proportion of P released from organic-P mineralisation might be

used to satisfy phosphate adsorption capacity of the soils. In applied practices,

however, P mineralisation from organic matter should be accounted for when the

adsorption isotherm technique is applied to soils that have been amended with

organic materials (Berton and Pratt, 1997). In our experiment, although

combustion plus treatment of soils with new soil organic matter increased

phosphate adsorption capacity, the bicarbonate-P release to soil solution showed

no sign of decreasing. Contributions from incomplete oxidation of organic-P

during combustion and the changes in acid solubility of soil inorganic-P

(Anderson, 1960; Condron et al., 1990; Williams et al., 1970) as a result of

combustion might also have occurred in soil samples.

Increasing the level of organic matter in all treatments (i.e. peat, wheat straw, and

lucerne hay) did not show the same trend in phosphate adsorption for all organic

matter sources. Wheat straw and lucerne hay, according to their values of KL in

Table 6.3-2, consistently decreased phosphate sorption as the level of their

applications increased. Peat, however, showed almost similar KL values as the

level of application increased. In other words, increasing level up to 180 ton ha-1


was ineffective for the purpose of decreasing phosphate adsorption in an Oxisol,

when peat was applied.

Decreasing phosphate adsorption due to the increasing level of organic matter

addition may also be explained as a consequence of P content in their biomass.

When phosphate is released during decomposition of organic matter, it is then

rapidly adsorbed onto sorption sites. Therefore, more adsorbed P is present

before the equilibration by CaCl2 solution (Haynes and Mokolobate, 2001;

Iyamuremye and Dick, 1996; Li et al., 1990). As a result, there is less phosphate

adsorption capacity of the soil with respect to subsequently added phosphate.

Different types of organic matter had different effects on phosphate adsorption.

Lucerne hay decreased phosphate adsorption more than wheat straw and wheat

straw decreased phosphate adsorption more than peat (Figure 6.3-3 and Table

6.3-1). Comparable values in Table 6.3-1 (row six months) and Table 6.3-2 (row

80 ton ha-1) appeared to be slightly different due to different sub samplings.

However, declining trends in parameter KL, xm, b and KF were similar to the effect

of different organic matter sources, indicating similar trend of declining phosphate

adsorption capacity. At the same time, increasing the amount of organic matter

application also reduced phosphate adsorption more. Even at 180 ton ha-1, the

order of effect was peat<wheat straw<lucerne hay. The fact that peat increased

bicarbonate-P content in soils (Chapter 3 and Chapter 4) higher than wheat straw

application again suggests that phosphate adsorption was not the dominant

factor affecting bicarbonate-P release after peat addition. Though peat has lower

P content than wheat straw (Table 3.3-2, Chapter 3), it could provide more

bicarbonate-P if it is applied to soil. Peat probably could facilitate organic-P

transformation to inorganic-P better than wheat straw by other mechanisms,

except decreasing phosphate adsorption.

6.5 CONCLUSION

Organic matter addition could reduce phosphate adsorption and the effect could

last up to nine months after application. Lucerne hay was more effective than

peat and wheat straw addition in reducing phosphate adsorption capacity.

Langmuir equation fitted better than Freundlich in describing phosphate

adsorption in this experiment.


Soil combustion in new soil organic matter addition treatments increased

phosphate adsorption capacity, especially in the Oxisol and Ultisol. At the same

time, only the Brunauer-Emmett-Teller equation could describe the sigmoidal

effect of treatments on phosphate adsorption. Consequently, comparing the

effect of existing and new soil organic matter treatment by comparing equation

parameters was impossible.

The new soil organic matter addition released more bicarbonate-P into soil

solution. The experiment revealed the reduction of phosphate adsorption

occurred during the first year of organic matter applications and long term release

of bicarbonate-P in soil contributed to mineralisation, P hydrolysis, or P

transformation.


C h a p t e r 7

GENERAL DISCUSSION AND CONCLUSION

7.1 BRIEF SUMMARY OF THE MAJOR FINDINGS AND THEIR INTERPRETATIONS

Lateritic soils, which have been generally considered marginal for food

production, occupy large areas in tropical regions. With rapidly increasing

populations in developing countries, these soils are under increasing pressure to

become agricultural land. Soil fertility management of these soils is required to

overcome problems such as low in cation exchange capacity, low and declining

soil pH, and low nutrient availability. Organic matter addition is identified to be

effective in reducing phosphate adsorption in soils high in aluminium (Al) and iron

(Fe) oxides. This study was therefore conducted to investigate the changes

chemical and biological properties of examples of lateritic soils when amended

with fresh organic materials. The emphasis on soil phosphorus (P) was essential

as most lateritic soils have abundant hydroxides which can intensively fix this

nutrient in the form of phosphate ions.

The major findings in the research were:

• Application of organic matter enhanced the availability of nutrients, especially

P in the lateritic soils studied. Its persistence, to some extent, depended on

the C:N ratio of the material applied. Peat, in spite of its low concentrations of

C and P, released more P compared with wheat straw. For the longer term

(more than a year), peat would release more P than lucerne hay.

• The stronger correlation between bicarbonate-P and organic-C over time (12

months) indicated the importance of organic-P hydrolysis, transformation, or

displacement of adsorbed bicarbonate-P by soluble organic compounds.

Therefore, respiration and leaching which were the keys to C loss from soil,

contributed to the P dynamics, especially in tropical environment.

• Despite the ability of organic amendment (especially lucerne hay) to decrease

phosphate adsorption up to nine months after treatment, the application of

lucerne hay to soil with no organic matter increased phosphate adsorption. At

VII. General Discussion and Conclusion 193


the same time, the increase in P mineralisation was greater than the increase

in phosphate adsorption, releasing more P into soil solution.

• Even though addition of organic material such as wheat straw and lucerne hay

can reduce phosphate adsorption in lateritic soils, exceptions may be revealed

in very specific circumstances. On one hand, as such organic materials

increased phosphate adsorption in soils with very low content of organic

matter, studies need to be done in order to understand the circumstances in

which organic matter addition is effective in reducing, and increasing,

phosphate adsorption. Or on the other hand, the artefact of combustion needs

to be scrutinised further, especially in soil without added organic matter.

Further major findings and reasoning for the whole experiments in this thesis

presents in Table 7.1-1

Table 7.1-1 Summary of findings in all experiments in this thesis

Chapter Major finding Comment • Despite a low content of P, peat increased bicarbonate-P

more and for longer than did lucerne hay with higher P content.

• Peat decomposition created conditions (either chemically or biologically) which enabled P to be released from sources other than the peat itself. 3

• Bicarbonate extractable phosphate was positively correlated with total extractable-C, extractable organic-C, NH4-N, and soil pH.

• This highlights a role of organic-C in phosphate release.

• New organic matter increased bicarbonate-P more than the pre-existing organic matter, especially in Ultisol.

• Freshly added organic matter may produce specific type of organic-C which was more reactive in releasing phosphate than that of the existing soil organic matter.

• The stronger positive correlation between bicarbonate-P and soil organic-C over time indicated the importance of organic-P hydrolysis, transformation, or displacement of adsorbed bicarbonate-P by soluble organic compounds.

• Even though correlations might indicate the formation of sesquioxides precipitates with organic compounds, there was no indication that precipitates could adsorb more bicarbonate-P. The two processes (organic-P hydrolysis and displacement) perhaps involved in releasing more P. However, the experiment could not single out one specific process.

4

• Contrary to the hypothesis, microbial biomass-P and phosphatase were not correlated with bicarbonate-P.

• The two parameters were not reliable in predicting P dynamics in lateritic soils with organic amendment, suggesting that other factors contributed to observed increases in extractable phosphate.


usion 196

Table 7.1-1 Continued Chapter Major finding Comment

• Respiration contributed between 1-36 times more C losses than leaching.

• The numbers are precise estimation for lateritic soil (Ultisol and Oxisol) from Western Australia.

• For simulated tropical rainfall, leaching was as important as respiration in reducing the persistence of soil organic matter.

• In the aspect of soil organic matter persistence in top soil, leaching might be more pronounced in tropical lateritics than that of sub-tropical.

• Leaching of dissolved organic-C influenced the mobility of bicarbonate-P and sesquioxides in soil leaching columns.

• More research needs to be done, especially on erratic total- and bicarbonate-P concentrations due to P mobilisation/resorption process proposed by other researchers for an Alfisol.

5

• There was a tendency of bicarbonate-P leaching, especially in sandy lateritic soil with longer expose of higher rainfall.

• Limited time and the amount of rainfall simulation was the cause why there was no dissolved reactive phosphorus in the leachate. Hence the results were not contradicted with P leaching in other experiments.

• Organic matter addition could reduce phosphate adsorption for up to nine months. The Langmuir adsorption isotherm equation fitted better than the Freundlich.

• The effect of organic matter additions on desorption of phosphate is transient, making reapplication necessary at intervals depending on organic matter source.

• The soil combustion procedure for removing the pre-existing soil organic matter might have altered physical and chemical properties of the soil and increased phosphate adsorption capacity. The Brunauer-Emmett-Teller equation described data showing sigmoidal adsorption isotherms.

• Under very limited soil organic matter, phosphate sorption isotherm could not be explained with the previous equations as the Brunauer-Emmett-Teller equation did. However, comparing the latest with the previous two was impossible as they were in different samplings.

6

• Recalcitrant sources of C such as peat, and low C:N ratio source of organic matter such as lucerne hay, were probably a suitable combination in reducing phosphate adsorption and its release from fixation sites through mineralisation.

• Suggestion for future research in order to optimise organic amendments in lateritic soils, especially in tropical environment.

VII. General Discussion and Concl

In the preliminary experiment (Chapter 3), as soon as fresh organic matter was

incorporated into soil (Typic hapludox, “Balkuling”; from Western Australia),

decomposition process starts immediately. Cumulative CO2 emission reached

1600 mg CO2 kg-1 year-1, representing about 6% of total organic-C in the soil after

organic matter addition. This was one of the causes why soil organic matter

disappears from soils and reduces its persistence (Curtin et al., 2000; Mapa,

1996). Surprisingly, peat application showed a greater effect in releasing more

bicarbonate-P to soil than wheat straw application, despite the fact that wheat

straw biomass had higher P content that peat. Peat persisted longer to make P

more available and related to total extractable-C, total extractable organic-C, and

soil pH through out the experiment. Furthermore, organic matter mineralisation

enhanced nutrient availability, especially ammonium, nitrate, P, and potassium

(K). This process occurred soon after organic matter applied to the soil. The

maximum mineralisation took place within the first three months and there was

indication of positive priming effect, especially C mineralisation (measured with

soil respiration) from soil samples treated with lucerne hay. The results

supported all hypotheses with most of them in agreement with previous

experiments by many authors (Hartemink and O'Sullivan, 2001; Haynes and

Mokolobate, 2001). As a preliminary experiment, these findings were expected

to develop the following ideas in conducting more advanced research, especially

in P dynamics of lateritic soils.

In accordance with the first experiment, in the second experiment (Chapter 4),

bicarbonate-P also increased in soil with applied organic matter (peat, wheat

straw, and lucerne hay), especially in treatments without pre-existing soil organic

matter. The Ultisol (Typic kandiudult) showed a larger increase in bicarbonate-P

than the Oxisol (Plinthic eutrodox). In the case of the subsurface soil, P

immobilisation might have been taking place after lucerne hay addition. The

higher inherent bicarbonate-P content in subsurface soil may have increased the

population of soil micro-organisms more than in the Ultisol and Oxisol (even in

combusted soil samples) causing P immobilisation. However, the results showed

that, by the end of the experiment, P immobilisation had declined indicated by

smaller microbial biomass-P. Similar to bicarbonate-P, total-P content in the soils

showed an increase after organic matter was applied to soil from which pre-


existing soil organic matter had been removed, especially in the Ultisol and the

Oxisol. This increase leads to the speculation that the combustion process for

removing existing organic matter solubilised the occluded P and P which was

stabilised in the existing soil organic matter (Giardina et al., 2000; Serrasolsas

and Khanna, 1995). Before combustion, this fraction of P in soil samples was not

extractable. As soon as the existing soil organic matter was removed by

combustion, P was more readily extracted. The increase in total-P consequently

increased non-extractable-P content in soils. This fraction of soil P, which was

greater than bicarbonate-P, is presumed to be beneficial in P dynamics since

some of this fraction is organic-P. A higher content of organic-P in the soil will

have a greater chance to be transformed to inorganic-P which is then available

for plants. Peat, as in the first experiment, increased non-extractable-P more

than wheat straw, despite the fact that wheat straw had higher P content in its

biomass. At the beginning of the experiment, there was an indication that non-

extractable-P (expected to be mostly organic-P) was correlated with microbial

biomass-P as hypothesised earlier in the proposal. Nevertheless, the correlation

was not observed at the end of the experiment.

As for the first experiment, the increasing C content due to organic matter

application was also accompanied by an increase in bicarbonate-P. This was

followed by the changes in Al, Fe, and soil pH. These reactions were similar to

the previous experiment and could be very common in lateritic soils when

amended with fresh organic matter. Contrary to the initial hypothesis; there was

only weak evidence that microbial biomass-P played a role in providing more

bicarbonate-P content. Meanwhile, phosphatase activity was not related to

bicarbonate-P release in soil for this experiment. The enzyme may have been in

steady state, or pre existing in soil, but in this experiment phosphatase was not a

reliable parameter to estimate phosphate release. However, other mechanisms,

either chemical or biological might have been involved in the release of

bicarbonate-P, but the experiment was not able to reveal these. One thing in

common from both experiments was that the increasing level of organic matter

increased dissolved organic-C as well as bicarbonate-P. For this, we can

conclude that stronger correlation between dissolved organic-C and bicarbonate-

P towards the end of the experiment could be from organic-P hydrolysis or

displacement of adsorbed bicarbonate-P by soluble organic compounds

produced by decomposition of added organic matter.


In the leaching columns in the third experiment (Chapter 5), a small percentage

of C loss (1.4%) occurred by leaching from soil. Simulated tropical rainfall,

indeed, resulted in higher leaching in C. Leaching contributed about one tenth of

the C loss due to respiration but, no doubt, leaching process reduced the

persistence of added organic matter. Therefore, most C was lost by respiration

on this situation, especially in lateritic soils treated with subtropical rainfall.

Moreover, organic-C was also deposited in the deeper layers of the column

indicating a podzolisation or similar process (Egli et al., 2003; Rumpel et al.,

2002).

There was no evidence of bicarbonate-P and non-extractable-P leaching from the

columns. Furthermore, the decomposition of added organic matter had an

impact on the distribution of soil bicarbonate-P, non-extractable-P and dissolved

organic-C as well as sesquioxides in the leaching columns. This was in

accordance with the previous experiments on the relationship between

bicarbonate-P and dissolved organic-C. Soil dissolved organic-C was the most

readily leached within the six months period. There was an indication that

bicarbonate-P and possibly non-extractable-P will be leached away with longer

period of time, especially in sandy soil with higher rainfall.

Another interesting record on this experiment was the apparent intermittent

leaching of bicarbonate-P and total-P. This rare finding was observed by few

authors (Jensen et al., 1999; Kleinman et al., 2003) and is still hard to

understand. The proposed mechanisms which involve mobilisation, resorption on

the wall of macropores (Jensen et al., 1999), redox condition, and the role of Al

and Fe need to be scrutinised separately in the future research of P dynamics,

especially in lateritic soils of the tropics.

In the fourth experiment (Chapter 6), organic matter addition could reduce

phosphate adsorption and the effect could last up to nine months after

application. Lucerne hay was more effective than peat and wheat straw addition

in reducing phosphate adsorption capacity. Langmuir equation fitted better than

Freundlich in describing phosphate adsorption. However, both equations could

not describe phosphate adsorption in samples where inherent or existing soil

organic matter was negligible due to combustion process. The temperature of

450° C probably has altered physical and chemical properties of the minerals in

soils. Besides, inherent high P content in subsoil may restrict their adsorption to


soil particles. As a consequence, the Brunauer-Emmett-Teller equation was

applied to fit the data where lower concentration of absorbate and its precipitation

with other soil component may have been occurred. This equation can also

explain sigmoidal isotherms (Hinz, 2001), as the data for some treatments in this

experiment revealed.

Bicarbonate-P release mechanism through mineralisation was still apparent

despite the fact that combustion of the soil samples has increased phosphate

adsorption capacity. Therefore, for soils with very low organic matter content, it

was possible that applying fresh organic matter could increase phosphate

adsorption capacity of lateritic soils. Peat treatment affected more bicarbonate-P

in soil and factors related to its release such as dissolved organic-C, Al, and Fe,

compared with lucerne hay treatment. This mechanism is supporting the idea

that organic matter addition will bring about the formation of metal ion complexes

with soluble organic components.

Some general soil mechanisms were noticed from the beginning of organic

matter application to the availability of P for plant in lateritic soils. Mineralisation

of P from added organic matter might be classified as the initial process followed

by immobilisation and adsorption. In these two processes, biological and

physical component of the soil were involved. The next processes were the

release of phosphate followed by the possible uptake by plants. While organic

matter persists in soil, these processes can perform their roles in turn or in unity,

indicating soil dynamics. The whole systems, of course, were dictated by

environmental factors such as water supply and temperature which in turn can be

increase or decrease the availability of P. But from all the experiments, selected

sources of organic matter can manage the whole system and become the

solution for lateritic soils.

In summary, addition of organic matter on lateritic soils was investigated as an

alternative solution to managing these marginal soils. High phosphate adsorption

capacity due to clay type and Al and Fe hydroxides was manageable by fresh

organic matter amendment. In this case, mineralisation of organic matter

addition not only released more bicarbonate-P and non-extractable-P to soil but

was also capable of satisfying phosphate adsorption, either with phosphate or

with adsorbed organic anions. Moreover, the problem related to C loss and

persistence, especially in tropical areas, can be overcome with the application of


resistant organic matter such as peat or a mixture of peat and lucerne hay.

However, this possibility needs to be assessed further; either from a more

detailed soil chemistry study or from economical and environmental point of

views. Excavation and removal of large quantities of peat may create additional

environmental problems, such as changes in hydrology or accelerated

acidification.

7.2 LIMITATIONS AND FUTURE RESEARCH NEEDS

Several limitations have been considered throughout the experiments and need

to be addressed in the future by related research.

Greater effects of peat treatment in releasing bicarbonate-P in these experiment

initiated a further thought that peat may beneficial in applied to lateritic than any

other soils. Complex reactions, either in releasing more P pools, its persistence

in soils, and in reducing phosphate adsorption; led to the prediction that peat has

better chemical properties in dealing with poor characteristics of lateritic soils.

More research on peat, not only its composition, but also its characteristics and

distribution in the world, may help considerably in making lateritic soils more

productive.

The apparent intermittent effect of leaching on bicarbonate-P and total-P in

macropores through preferential pathways of both Ultisol and Oxisol needs to be

investigated more carefully. Due to redox condition, phosphate sorption, and the

role of Al and Fe (hydr)oxides, this unusual process might be of importance in the

P dynamics of tropical lateritic soils where leaching is significant.

The validity of combustion of soils in order to eliminate the inherent soil organic

matter has to be assessed case by case. However, using other methods such as

hydrogen peroxide may only produce limited amount of soil for experiment and

may also change soil chemical properties related to organic matter cycling.

Hence, combustion method is not generally recommended to study physical and

chemical characteristic of soils, but may provide useful information in carefully

designed experiments. We need to pay more attention on the potential errors in

inorganic-P extraction from combusted samples (Condron et al., 1990; Oniani et

al., 1973; Williams et al., 1970), especially when highly weathered soils are the


subject of the experiment. Nevertheless, the specific design of the experiments

described in this thesis allowed the effects of combustion to be assessed

independently using a soil material with negligible organic content. The results of

this thesis are therefore interpretable by careful analysis of all data, taking the

independently-measured effects of combustion into account.

In terms of a more specific part of the system, the role of non-extractable-P, the

pool containing the biggest proportion of P in tropical lateritic soils, is still not well

understood. Even though the effect of fresh organic matter on P availability was

beneficial, but in terms of the proportion it was not only a small quantity. More

research therefore needs to be done in order to understand more about the

importance of non-extractable-P, especially in environment with a limited supply

of P. Any pathway that leads to the conversion of organic-P prior to uptake by

plants has to be resolved to maximise the benefit of the whole P pools. By doing

this, dependency on inorganic P fertilisers can be minimised.

In terms of the bigger scope of C dynamics in the tropics, due to continuous high

temperatures, losses as well as gains of C occur more rapidly. Understanding

the mechanisms, especially in relation to P, will help to realise the importance

and potential of lateritic soils in the tropics. Balanced mixture between green

manure and peat, which is abundant in Indonesia, maybe useful as a starting

point in managing P supply in this area. However, to be more accurate in

predicting these processes, in situ experiments needed to be carried out under

comparable condition to minimise variability due to environmental factors.


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.

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Appendix A. Linearisation of Brunauer-Emmett-Teller equation

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−+−

=

sBETs

mBET

ccKcc

cxKx

11)(..................................................................... 6.2-5

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−+

×−

=

sBET

mBET

s

ccK

xKcc

cx11

( )( )

mBET

sBET

s xKccK

xcc

c⋅−+

×=−

11

( )smBET

BET

mBETs cc

xKKx

xKx

ccc

⋅−

+=−

1

( )⎟⎟⎠

⎞⎜⎜⎝

⎛−

⋅−

+=− cc

cxK

KxK

xcc

c

smBET

BET

mBETs

11

( )smBET

BET

mBETs cc

xKK

xKxccc

⋅−

+=⋅−

111............................................................. 6.2-6

If y = a + bx, mBET xK

a 1= ,

( )mBET

BET

xKKb 1−

= , and sc

cx =

( )mBET

BET xKKb 11 ⋅−=

( ) aKb BET ⋅−= 1

abK BET =−1

abK BET +=∴ 1

mBET xKa 1=

VIII. Appendices 243

aKx

BETm

1=

aab

xm

⎟⎠⎞

⎜⎝⎛ +

=1

1

( )baxm +

=∴1


Appendix B Article for 17th World Congress of Soil Science, Bangkok 14-21 August 2002

TRANSFORMATION OF ORGANIC MATTER IN AN OXISOL

Fadly H. Yusran1,2, Andrew W. Rate2, and Lynette K. Abbott2.

1Soil Department, Faculty of Agriculture, Lambung Mangkurat University, Banjarbaru 70714, Indonesia

2School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia

Abstract The application of various types of organic matter (OM) could assist in managing Oxisols. However, key questions relate to how long recently added OM persists in the soil and whether the addition of organic-C (OC) has any advantage in the long term. The main objectives of this study were to determine: (1) the persistence of soil organic matter in an Oxisol, (2) the effect of additional OC on the soil characteristics, especially chemical and biological aspects, and (3) the long-term effect of additional OC to the extractability of nutrients, especially N, P, and K, which may be affected by C dynamics in the soil. An Oxisol from Western Australia was treated with peat, wheat straw (Triticum aestivum), and lucerne hay (Medicago sativa), and incubated wet and dry (weekly watering, up to 60% water holding capacity) for 3, 6, 9, and 12 months in a greenhouse. The rates of OC sources were 40, 80, 120, and 160 ton ha-1. Soil microbial biomass C increased over time suggesting an increase in the population of soil micro-organisms, while soil respiration increased then showed a decrease after six months. The loss of OC as CO2 ranged from 0.4-5.5% of total-OC during a 12-month incubation time, and the higher OC applied, the more C loss as CO2. Net mineralisation of N decreased over time. There was also a decrease in P but not K mineralisation. Above all, the maximum mineralisation of N, P, and K occurred within the first three months of incubation. Almost all variables measured increased in the order control<peat<wheat<lucerne, except for P, for which wheat was less than peat. The effect of increasing the amount of OC increased the content of all variables measured in the soil. There was a delay in the increase in NO3-N during experiment and this was probably due to immobilisation by soil micro-organisms. Each source of OC added to the soil persisted for at least twelve months irrespective of the amount applied, and the addition of OC affected N, P, and K dynamics.

Key words: Oxisols, soil organic matter, soil organic carbon, mineralisation, and

persistence.


Introduction

Oxisols, together with Ultisols, which are considered marginal for agricultural production, are very highly weathered soils, low in CEC, base saturation, and pH (Buol and Eswaran, 2000; West et al., 1998) These lead to the low availability of N, P, and K for plant growth. In making this soil more productive, inputs and proper management practices should be applied.

Soil organic matter, the key to soil fertility, is involved in many reactions in creating favourable conditions for the availability of nutrients due to its capacity to retain nutrients, water retention, and buffering capacity. It also promotes soil aggregation and improves biological characteristics of soils. The application of various sources of OM could be an important solution for managing Oxisols in farming systems. However, different sources of OM vary in their persistence in soil, so the addition some forms of OM could have an advantage in maintaining C-storage in the long term in Oxisols.

Many experiments have been carried out on the mineralization of OM in soil, especially with organic-C and N. These have determined the appropriate rate of the OM application such as manure, green manure (Dunn and Beecher, 1994; Lupwayi and Haque, 1998), straw (Mueller et al., 1998; Strong et al., 1987), and sewage sludge (Boucher et al., 1999; Iakimenko et al., 1996) on various soils. The appropriate source and rate of application of OM to Oxisols in Western Australia in relation to the its persistence and the availability of N and P has not been studied.

The objectives of this experiment were (1) to measure the persistence of OM added to soil, (2) to measure the contribution of additional OM on the characteristics of soil, especially chemical and biological aspects, and (3) to measure the long-term effect of additional OM to the availability of nutrient, especially N and P which affected by C dynamics in the soil in an Oxisol.

Materials and methods Soil samples (Typic hapludox; Balkuling) were taken from farmland near York. Peat Richgro Garden Products, Canning Vale, WA was air dried and passed through a sieve (≤ 2 mm). Lucerne as well as wheat straw was chopped with a miller (RetschMühle) and passed through a sieve (0.5 cm). For chemical characteristics, green manure and wheat straw were oven-dried (60° C) for two days, ground and passed through a 2 mm sieve.

One kg of soil was placed in each undrained pot and the appropriate treatment was applied by mixing the soil and the respective treatment. Deionized-water was added weekly during incubation (12 months) to restore a moisture equivalent of 60% of the maximum water holding capacity of the soil (42%).

Soil sample characteristics are presented in Table 1, and C source characteristics are presented in Table 2. The sandy and slightly acid soil had total-C content of 3%. Lucerne had a smaller C:N ratio than wheat straw and peat.


All parameters were measured 3, 6, 9, 12 months after treatment application. Methods of determination of all parameters were as follows.

Microbial biomass carbon Microbial biomass carbon (MBC) was measured using a modification of the fumigation-extraction methods of Vance et al. (1987) and Sparling (1991). Fumigation was done by adding chloroform (CHCl2) as proposed by Jenkinson (1966).

Twenty g (equivalent dry weight) of soil was weighed into a 50mL beaker glass. The soil was watered up to 60% water holding. The water holding capacity of soil in each treatment was different due to differences in the form of OM added. The beakers were placed in a desiccator with one empty beaker in the middle. Twenty five mL CHCl2 was poured into the empty beakers together with some anti-bumping granules. A lining of damp paper towel was placed between the beakers to control the moisture level inside the desiccator. Vaseline was applied around dessicator lid, and the dessicator was vacuumed until CHCl2 was boiling. The dessicator was covered with black plastic around and allowed stand for exactly one week.

Fumigated soils were extracted with 80 mL 0.5 M K2SO4. The extraction was done by transferring 20 g of soil into 120 mL plastic container. The rinsing process was done by dispensing 40 mL extractant twice. The mixture was shaken for 30 minutes and filtered with Whatman #40 into a 30 mL plastic container and kept in freezer until further analysis.

Six-fold dilution of the extract was used to measure C content using Shimadzu TOC-5000A. Total organic carbon (TOC) was calculated by subtracting IC (inorganic carbon) from TC (total carbon). Maximum number of injection used was six with two minutes sparging time for TC and no sparging time for IC. The volume of extract injected to the machine was 26 µL for TC and 33 µL for IC. 0, 50, and 100ppm working standards were applied to construct a calibration curve. Stock solution for TC was made from 2.125 g potassium hydrogen phthalate in one L MilliQ water, while stock solution for IC was made from 3.50 g NaHCO3 and 4.41 NaCO3 in one L MilliQ water. NaCO3 was heated at 285°C for one hour and cooled in desiccator before weighting.

Soil respiration The method used for measuring soil respiration was based on Anderson (1982) with a slight modification. The method used alkali trapping, followed by an acid-base titration for remaining OH from CO3

2- formation. This product is formed when the CO2-laden gas contacts an alkali absorber. The amount of CO3 was measured by titrimetric using a known concentration of HCl.

Fifty g (dry weight) of soil was placed in a 120 mL plastic container. Deionised water was added to make moisture content exactly 40%. The container was placed in a moist plastic bag. The open end was wrapped with a tissue paper and


a rubber band and left for one week at room temperature. The respiration jar (one L volume, air-tight sealed jar) containing five mL of DI water was prepared for incubating the soil samples. Ten mL 0.5 M KOH in a smaller plastic container was also put into the jar as a CO2 trap. Samples in the 120 mL container was arranged together with KOH container inside the jar and closed tightly. Incubation time was one week at room temperature. Blanks were also prepared for CO2 calculation.

After one week incubation, the KOH containers were removed from the jar and tightly closed with the lid. Four mL of the KOH solution was transferred to an erlenmeyer flask and mixed with 10mL BaCl2 and three drops of phenolphthalein indicator. Titration was employed against 0.1 M HCl until the pink colour disappeared. Total CO2 was calculated from the difference of HCl volume to neutralize the samples and the blanks.

Ammonium and nitrate Ammonium ions were measured by a modified Berthelot indophenols reaction that utilizes the Griess-Ilosvay reaction (Searle, 1984). Ammonia was chlorinated to monochloramine to form 5-aminosalicylate after reaction with salicylate. The absorbance of a green colour formed after oxidation was measured colorimetrically at 660 nm.

The NO3 measurement was based on the hydrazinium reduction method. The amount of NO2 was measured by diazotiting with sulphanilamide and coupling with α-naphthylethylenediamine dihydrochloride to form an azo dye measured at 540 nm.

The extract used was 2 M KCl. Five g moist soil was shaken with 50 mL extract in a 125 mL plastic container for one hour. Whatman #40 filter paper was used for filtration and the filtrates were kept at 4o C for up to four months prior to further analysis. The moisture content of the soils was assessed before the extraction process to determine the weight of soil samples. NH4-N and NO3-N were measured by a Skalar SANplus Segmented Flow Analyzer using a dual-channel system.

Inorganic phosphorus Inorganic-P was measured using 0.5 M NaHCO3 at pH 8.5. This is a modification from a method by Olsen et al. (1954) described by Rayment and Higginson (1992). The manual colorimetric procedure at the end of the method was based on that described by Murphy and Riley (1962).

Fifty mg dry soil and 50 mL 0.5 M NaHCO3 in plastic centrifuge containers were shaken end-over-end for 16 hours. Centrifusion and filtration with Whatman #42 filter paper was employed to ensure the mixture free from particulate OM. 2.5 mL aliquots of the filtrate were diluted to 10 mL with 5 mL DI water and 0.2 mL 1 M H2SO4. The mixture was leave for one hour to cease the effervescence.


Another 0.5 mL 0.5 M NaHCO3 was added to the system and mixed well. The samples were left to stand overnight to complete the removal of excess CO2.

A day later, 0.8 mL of colour reagent (ascorbic acid in Reagent A (ammonium molybdate-sulphuric acid-potassium antimony tartrate)) was added and after 30 minutes the absorbance of the solution was measured with a spectrophotometer at 882 nm.

Extractable potassium Extractable K was assessed from the same extract as inorganic-P (0.5 M NaHCO3 pH 8.5) using atomic absorption spectrophotometer. A solution of 1% CsCl was used in all samples and standards to avoid ionisation effects.

Results and discussion Effect of time on organic carbon When OM is added to soil, it is colonised by micro-organisms that need C as their source of energy. This is supported by the increase in MBC content in the soil (Figure 1). The application of peat, but not wheat and lucerne, to the Oxisol increased soil MBC throughout the year. This prolonged increase could be due to the considerable resistance of peat C to microbial decomposition (Brake et al., 1999; Chesson, 1997; Handayanto et al., 1997), making it more slowly available to microorganisms than the other forms of OM.

Soil respiration declined with incubation time (Figure 2). The decomposition of OM from wheat and lucerne was greatest during the first three and six months of application. For lucerne, the most active period of decomposition might have taken place before the first measurement was taken in this experiment. In other studies with green manure (rice straw) and legumes, the highest peaks in respiration were reached after four to six weeks after application (Villegas-Pangga et al., 2000; Zaharah and Bah, 1999).

In order to estimate the persistence of OM in soil, the total soil respiration was compared to the total-C at the beginning of the experiment. The higher the amount of OM applied, the more CO2 produced from soils, with lucerne being the most productive (Table 3).

As hypothesised, lucerne, which had the smallest C:N ratio, disappeared at the fastest rate, followed by wheat and peat. The higher and balanced C and N content of green manure such as lucerne has a narrow C:N ratio and a faster rate of decomposition (Bremer et al., 1991; Curtin et al., 2000; Mapa, 1996). Both nutrients are required for the growth of soil microorganisms.

The highest CO2 evolution for higher level of OM occurred either three or six months after OM was added. The same pattern was observed on soil MBC (r = 0.80 and 0.88 for three and six months, respectively). For lucerne, the decrease in release of CO2 was steep (Figure 2) and C content estimated to be similar to the control several months after the end of the experiment.


Extractability of N, P, and K Ammonium was formed within the first three months after OM adding, especially with lucerne application (Figure 3). The greatest ammonification process for OM low in C:N ratio may had been initiated as soon as OM was applied to the soil and continued until six months.

As OM decomposition enhances nutrient availability (Hartemink and O'Sullivan, 2001; Lupwayi and Haque, 1999; Maharudrappa et al., 2000; Tian, 1998; Villegas-Pangga et al., 2000; Zaharah and Bah, 1999), this process is beneficial for marginal soils such as Oxisols. Mineralisation of P and K showed the same trend for all OM applications. The extracts of both P and K were greatest at the three-month sampling time, and subsequently declined. Thus, the first three months of incubation time was the most active decomposition process for P and K as well as for NH4, especially for the higher levels of OM applied. There was little evidence of nitrification until 12 months, indicating that immobilisation of N in the microbial biomass was an important pool for storage of N.

The order of mineralisation was control<peat<wheat<lucerne for NO3-N and K, whilst for NH4-N and P the order was control<wheat<peat<lucerne. Peat was more effective than wheat in releasing NH4-N maybe because peat produced more amine functional groups (Landgraf et al., 1998). A possible explanation higher P release might be an effective chelating process of Al-P and Fe-P in soils from peat functional groups (Alvarez-Fernandez et al., 1997; Jonge et al., 1996).

Conclusions

Organic matter persistence in an Oxisol from Western Australia depended on its C:N ratio. Peat was expected to persist longer than lucerne and wheat straw. Organic matter application to this Oxisol needs to be made on a regular basis, with one- or two- yearly application for lucerne green manure and less regularly for peat.

Acknowledgement This research was supported by Australian Development Scholarship ADS-AusAID for Indonesia.

References Alvarez-Fernandez, A., A. Garate, and J.J. Lucena. 1997. Interaction of iron

chelates with several soil materials and with a soil standard. Journal of Plant Nutrition 20:559-572.

Anderson, J.P.E. 1982. Soil respiration, In A. L. Page, et al., eds. Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. ASA-SSSA, Madison.


Boucher, V.D., J.C. Revel, M. Guiresse, M. Kaemmerer, and J.R. Bailly. 1999. Reducing ammonia losses by adding FeCl3 during composting of sewage sludge. Water, Air, & Soil Pollution 112:229-239.

Buol, S. W, H. Eswaran. 2000. Oxisols, p. 151-195, In D. L. Sparks, ed. Advances in Agronomy, Vol. 68. Academic Press, San Diego.

Brake, M., H. Hoper, and R.G. Joergensen. 1999. Land use-induced changes in activity and biomass of microorganisms in raised bog peats at different depths. Soil Biology & Biochemistry 31:1489-1497.

Bremer, E., W.v. Houtum, and C.v. Kessel. 1991. Carbon dioxide evolution from wheat and lentil residues as affected by grinding, added nitrogen, and the absence of soil. Biology & Fertility of Soils 11:221-227.

Chesson, A. 1997. Plant degradation by ruminants: parallels with litter decomposition in soils, p. 47-66, In G. Cadisch and K. E. Giller, eds. Driven by Nature: Plant Litter Quality and Decomposition. CAB International, Wallingford, UK.

Curtin, D., H. Wang, F. Selles, R.P. Zentner, V.O. Biederbeck, and C.A. Campbell. 2000. Legume green manure as partial fallow replacement in semiarid Saskatchewan: effect on carbon fluxes. Canadian Journal of Soil Science 80:499-505.

Dunn, B.W., and H.G. Beecher. 1994. Green manuring legume pasture for aerial-sown rice. Australian Journal of Experimental Agriculture 34:967-975.

Handayanto, E., G. Cadisch, and K.E. Giller. 1997. Regulating N mineralization from plant residues by manipulation of quality, p. 175-185, In G. Cadisch and K. E. Giller, eds. Driven by Nature: Plant Litter Quality and Decomposition. CAB International, Wallingford.

Hartemink, A.E., and J.N. O'Sullivan. 2001. Leaf litter decomposition of Piper aduncum, Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea. Plant & Soil 230:115-124.

Iakimenko, O., E. Otabbong, L. Sadovnikova, J. Persson, I. Nilsson, D. Orlov, and Y. Ammosova. 1996. Dynamic transformation of sewage sludge and farmyard manure components. 1. Content of humic substances and mineralisation of organic carbon and nitrogen in incubated soils. Agriculture Ecosystems & Environment 58:121-126.

Jenkinson, D.S. 1966. Studies on the decomposition of plant material in soil. II. Partial sterilization on soil and the soil biomass. Journal of Soil Science 17:280-302.

Jonge, R.J.d., A.M. Breure, and J.G.v. Andel. 1996. Bioregeneration of powdered activated carbon (PAC) loaded with aromatic compounds. Water Research 30:875-882.

Landgraf, M.D., C.S.d. Silva, and M.O.O.d. Rezende. 1998. Mechanism of metribuzin herbicide sorption by humic acid samples from peat and vermicompost. Analytica Chimica Acta 368:155-164.

Lupwayi, N.Z., and I. Haque. 1998. Mineralization of N, P, K, Ca, and Mg from Sesbania and Leucaena leaves varying in chemical composition. Soil Biology & Biochemistry 30:337-343.

Lupwayi, N.Z., and I. Haque. 1999. Leucaena hedgerow intercropping and cattle manure application in the Ethiopian highlands. I. Decomposition and nutrient release. Biology & Fertility of Soils 28:182-195.


Maharudrappa, A., C.A. Srinivasamurthy, M.S. Nagaraja, R. Siddaramappa, and H.S. Anand. 2000. Decomposition rates of litter and nutrient release pattern in a tropical soil. Journal of the Indian Society of Soil Science 48:92-97.

Mapa, R.B. 1996. Coconut fibre: a biodegradable soil erosion control. Biological Agriculture & Horticulture 13:149-160.

Mueller, T., J. Magid, L.S. Jensen, H. Svendsen, and N.E. Nielsen. 1998. Soil C and N turnover after incorporation of chopped maize, barley straw and blue grass in the field: evaluation of the DAISY soil-organic-matter submodel. Ecological Modelling 111:1-15.

Murphy, J., and J.P. Riley. 1962. A modified single-solution method for the determination of phosphate in natural water. Analytica Chimica Acta 27:31-36.

Olsen, S.R., C.V. Cole, F.S. Watanabe, and L.A. Dean. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate Circular 939. US Department of Agriculture.

Rayment, G.E., and F.R. Higginson. 1992. Australian Laboratory Handbook of Soil and Water Chemical Methods Inkata Press, Melbourne.

Searle, P.L. 1984. The Berthelot or indophenol reaction and its use in analytical chemistry of nitrogen. A review. Analyst 109.

Sparling, G.P. 1991. Organic-matter and soil microbial biomass C as indicators of sustainable land use, p. 563-580, In J. Dumanski, et al., eds. Evaluation for sustainable land management in the developing world. International Board for Soil Research and Management Inc. (IBSRAM), Bangkok.

Strong, W.M., R.C. Dalal, J.E. Cooper, and P.G. Saffigna. 1987. Availability of residual fertilizer nitrogen in a Darling Downs black earth in the presence and absence of wheat straw. Australian Journal of Experimental Agriculture 27:295-302.

Tian, G. 1998. Effect of soil degradation on leaf decomposition and nutrient release under humid tropical conditions. Soil Science 163:897-906.

Vance, E.D., P.C. Brookes, and D.S. Jenkinson. 1987. An extraction method for measuring soil microbial biomass C. Soil Biology & Biochemistry 19:703-707.

Villegas-Pangga, G., G. Blair, and R. Lefroy. 2000. Measurement of decomposition and associated nutrient release from straw (Oryza sativa L.) of different rice varieties using a perfusion system. Plant & Soil 223:1-11.

West, L.T., F.H. Beinroth, M.E. Sumner, and B.T. Kang. 1998. Ultisols: Characteristics and Impacts on society, p. 179-236, In D. L. Sparks, ed. Advances in Agronomy, Vol. 63. Academic Press, San Diego.

Zaharah, A.R., and A.R. Bah. 1999. Patterns of decomposition and nutrient release by fresh Gliricidia (Gliricidia sepium) leaves in an Ultisol. Nutrient Cycling in Agroecosystems 55:269-277.


Tables and Figures

Table 1 Some physical and chemical characteristics of soil samples from York

Characteristic Particle size (%): > 50µm – 2mm 20 – 50µm 2 – 20µm < 2µm

61.42 3.96

13.74 20.87

pH (1:2.5) 5.46 Total-C (%) 3.12 Total-N (%) 0.03 Extractable-P (mg kg-1) 1.07

Table 2 Some chemical characteristics of organic matter sources for treatments

Source of carbon Total-N %

Total-C % C:N

Wheat straw 0.76 43.38 57:1 Lucerne 2.69 41.38 15:1 Peat 0.56 31.48 56:1

Table 3 Carbon loss (%) calculated from the initial total organic carbon in soils

Levels of OM (ton ha-1) OM Sources 40 80 120 160

Control 0.5 0.5 0.5 0.4 Peat 0.6 0.7 0.7 0.9 Wheat 1.0 1.8 2.8 3.1 Lucerne 1.1 2.5 4.4 5.5


0 500

1000

1500 2000

3 6 9 12

= LSD 5%

month

mg kg -1

Figure 1 Soil microbial biomass carbon with OM addition at 160 ton ha-1. = control, = peat, = wheat, and = lucerne.

0

100

200

300

400

500

3 6 9 1

month

mg kg -1

= LSD 5%

2

Figure 2 Soil respiration with OM addition at 160 ton ha-1. = control, = peat, = wheat, and = lucerne.


0

250

500

750

3 6 9 12

mg kg-1

months

Figure 3 Concentrations of NH4-N (bars) and NO3-N (line) for lucerne application (80 ton ha-1).

255VIII. Appendices

Appendix C Pictures from some experiments

Figure 8-1 Benches and pots in arrangement for the first experiment.

Figure 8-2 Pottery kiln to combust (450° C) inherent soil organic matter in the second experiment.


Figure 8-3 Half-burned soil samples.

Figure 8-4 Completely burned soil samples.


Figure 8-5 Pot arrangement for the second experiment.


Figure 8-6 Control for Oxisol (ONC) and Oxisol with peat (ONP) treatments for combusted soil samples.

Figure 8-7 Leachate containers under the leaching columns for the third experiment.


Figure 8-8 The bench and the overall view of leaching column. Wood pole clear plastic tube on the right was the indicator for suction mechanism (10 cm water).


Figure 8-9 The water dripper. The blue filter at the bottom was a 45 µm millipore filter (25 mm, supor membrane, non sterile, Pall Gelman Laboratory).


Figure 8-10 Suction apparatus for the columns. Each plastic tube represents one leachate container under one leaching column.


soil organic matter decomposition: effects of … · al qur’an 55: 10-13 . abstract relationships...

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