plant soil 2010 332 233 246 recovery nitrogen fertilizer traditional improved rice cultivars

8/13/2019 Plant Soil 2010 332 233 246 Recovery Nitrogen Fertilizer Traditional Improved Rice Cultivars.

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REGULAR ARTICLE

Recovery of nitrogen fertilizer by traditional and improved

rice cultivars in the Bhutan HighlandsBhim Bahadur Ghaley &Henning Hgh-Jensen &

Jrgen Lindskrog Christiansen

Received: 13 July 2009 /Accepted: 11 January 2010 /Published online: 13 February 2010# Springer Science+Business Media B.V. 2010

Abstract The recovery of soil derived nitrogen (NDFS)

and fertilizer N (NDFF) was investigated in highland

rice (Oryza sativa L.) fields in Bhutan, characterized

by high inputs of farmyard manure (FYM). The effect

of 60 kg N ha1 (60 N) applied in two splits to a

traditional and an improved cultivar, popular among

the farmers, was investigated using the 15N isotope

dilution technique. No differences were found between

cultivars with respect to the uptake of NDFSand NDFF,

but the improved cultivar yielded 27% more (P0.05)

grain compared with the traditional cultivar. This was

largely due to its greater harvest index (HI). The mean

percentage recovery of fertilizer N (REN) applied at

45 days after transplanting (DAT) was 34% compared

to 22% at 7 DAT, resulting in 56% greater uptake of

NDFF at 45 DAT. The overall REN for both the

improved and the traditional cultivars were 25.7% and

30% respectively, with no difference between cultivars,

but REN decreased with increasing FYM inputs.Fertilizer N recommendations that allow for previous

FYM inputs combined with applications timed to

coincide with maximum crop demand (45 DAT), and

the use of improved cultivars, could enhance N

fertilizer recoveries (REN) and increase rice yields in

the Bhutan Highlands.

Keywords Added nitrogen interaction .

Oryza sativa L. . 15N isotope dilution . Farmyard

manure . Environmental index .Nitrogen fertilizer

Introduction

Asian irrigated rice (Oryza sativa L.) constitutes 70%

of global rice production and feeds almost half of the

worlds population (Bouman et al. 2007). Given the

growth in world population and rising food demand, rice

yields need to be increased (Jing et al. 2008). Across

Asia, rice yields vary from 215 Mg ha1 depending

Plant Soil (2010) 332:233246

DOI 10.1007/s11104-010-0288-3

Responsible Editor: Elizabeth (Liz) A. Stockdale.

B. B. Ghaley

Renewable Natural Resources Research centre, Yusipang,

Council for Renewable Natural Resources Research

of Bhutan, Ministry of Agriculture,

Thimphu, Bhutan

J. L. Christiansen

Department of Agriculture and Ecology,

Faculty of Life Sciences, University of Copenhagen,

Hjbakkegrd All 13,

2630 Taastrup, Denmark

H. Hgh-Jensen

Department of Policy Analysis, National EnvironmentalResearch Institute, Aarhus University,

Frederiksborgvej 399,

4000 Roskilde, Denmark

Present Address:

B. B. Ghaley (*)

Department of Agriculture and Ecology,

Faculty of Life Sciences, University of Copenhagen,

Hjbakkegrd All 30,

2630 Taastrup, Denmark

e-mail: [email protected]


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on soil fertility, genotype, environment and manage-

ment practice (Romyen et al.1998; Cooper et al.1999;

Dobermann et al.2003; Whitbread et al.2003; Tang et

al.2007). In Asia, recovery of applied nitrogen (N) is

only 3050% (Cassman et al. 1993; Cassman et al.

1996b; Dobermann and Cassman2002; Dobermann et

al.2003) due to losses from volatilization, leaching andimmobilization in combination with sub-optimal appli-

cation procedures in terms of timing and placement of

N. Hence, improving N recovery efficiency may

increase crop yield, reduce production costs and limit

negative downstream environmental effects. The com-

mon method of N application is topdressing over the

standing crop, which leads to significant losses (Peng

and Cassman 1998). Hence, there is a need to

understand the demand-supply dynamics of N to make

the best use of the available N. On-farm studies have

demonstrated that 60% of the variation in crops

Nuptake from the soil (NDFS) is due to spatial variation

within single fields and temporal variation over seasons

(Cassman et al. 1996a). However, such variations are

not taken into account when making blanket N

recommendations, which may lead to over-fertilization

in fertile soils and under-fertilization in poor soils.

Rice is the staple food grain in Bhutan and rice is

cultivated in terraces by smallholder farmers primarily

to meet the food needs of the family (Ghaley and

Christiansen 2009). In the highlands of Western

Bhutan, farmyard manure (FYM), consisting of com-posted bedding material from cowsheds and leaf litter

from forest, is the main source of N input to the

terraces, supplemented with 3580 kg N ha1 urea

topdressing applied, at or shortly after transplanting the

rice seedlings. However, there is a lack of knowledge

regarding the optimal timing and dose of topdressed

urea and the inherent soil N supply in such highland rice

production systems. Availability of such information

may help to match the inherent N supply of the soil with

the corresponding requirement for additional N input,

based on local yield levels. Improved cultivars arereleased by the Bhutanese National Agricultural Re-

search System, based mainly on grain yield performance

and constitute 35% of total rice area. However,

knowledge of cultivar differences in N uptake and

utilization efficiency are lacking in both on-farm and on-

station cultivar evaluation.

Based on the hypothesis that cultivars have different

N utilization potentials and the farmersfields differ in

N supply potentials due to N management and soil

fertility, the objective of the current study was to

quantify N recovery efficiency in a traditional and an

improved rice cultivar under field conditions using 15N

labelling techniques.

Materials and methods

Site characterization

The trials were conducted in 2007 in the fields of ten

rice (Oryza sativa L.) farmers in Paro and Thimphu,

two of the main rice-growing districts in Western

Bhutan and representative of the region. The trial fields

were located within an altitude range of 2,2002,300 m

asl in Thimphu and 2,3002,500 m asl in Paro,

distributed over four different village clusters within

two districts in order to capture the variation in the ricecropping environments. The trial sites represented the

target population of environments with regard to differ-

ences in input access, irrigation facilities, rainfall, slope,

altitude and management factors. More than 90% of the

farmers at the trial sites grow rice for subsistence and

both traditional and improved cultivars are grown.

Improved cultivars are grown due to their high yield

whereas traditional cultivars are grown for their aroma,

good taste and for ritual offerings according to tradi-

tional cultural norms.

Rice is grown in terraced bunds along sloping hillswith irrigation water drawn from the nearby rivers

and supplemented with seasonal monsoon rainfall.

Soil fertility is traditionally maintained by annual

applications of 520 Mg ha1 of FYM, consisting of

composted bedding material from cowsheds and

forest leaf litter. The dominant cropping system is a

single rice crop from May to October and vegetables

like chilli, peas, potatoes and tomatoes in the spring

(FebruaryMay) for household consumption as well

as for sale. From November to May, farmers cultivate

winter wheat and barley in the paddy terraces as themain winter food crops.

Trial layout

All the participating farmers were supplied with seeds

of Janam, a traditional cultivar, and Khangma Maap,

an improved cultivar and established their own

nurseries. The terraces for transplanting were pre-

pared by puddling, levelling and removal of weeds,

234 Plant Soil (2010) 332:233246


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after which standing water was maintained in the

terraces. Transplanting took place from mid-May to

the third week of June, varying from village to

village. The seedling nurseries were watered a few

days before transplanting to ease removal of the

seedlings. The seedlings were packed in small

bundles and transported to the fields and 1

3 seed-lings were transplanted in a single hill.

Field trials were laid out in two parallel split plot

designs at each site. The first split plot set (cultivar

N supply) consisted of two N fertilizer levels as main

plots and two genotypes as subplots. The two main

plots were two paddy terraces supplied with 0 kg N

ha1 (0 N) and 60 kg N ha1 (60 N) and the subplots

consisted of two genotypes, Janam, and Khangma

Maap. Urea was the N source, and the traditional

cultivar with 0 N was treated as the farmers practice.

There were four treatments in the first set of splitplots. The second split plot set (cultivar topdress-

ing) was superimposed on 60 N plots with two

cultivars as the main plots and two 15N microplots

in each cultivar. The two microplots in each cultivar

consisted of two topdressing treatments at 7 and

45 days after transplanting (DAT), respectively. There

were four treatments in the second set of split plots.

Hence, at each site, there were eight treatments with

one replication per site. The first split plot set was

necessary to obtain a higher degree of precision in

varietal differences in N uptake under both fertilizedand non-fertilized conditions and the second split plot

set was designed to determine a higher precision in

the efficiency of topdressing timing relative to the

varietal differences. The traditional cultivar (Janam) is

a tall cultivar that has been cultivated for generations

and is valued for its aroma, taste and high straw yield.

Khangma Maap is a semi-dwarf, early-maturing

cultivar released by the National Agricultural Re-

search System of Bhutan in 1999.

In the first split plot (cultivar N supply), the main

plots were 4560 m2

, varying with the size of thepaddy terrace available in the farmers field. The

subplots were 1520 m2 (54 m2) in size and two

genotypes were randomly placed within each main

plot. 60 N was split into two equal doses and applied at

7 DAT and 45 DAT to meet the N requirement in the

early establishment phase following transplanting and

at the active tillering stage. In the second split plot set

(cultivar topdressing), the main plots were 1520 m2

in size and two microplots were inserted in each of the

two cultivars. To each microplot, 60 N was applied in

two splits of 30 kg N ha1 (30 N) each at 7 DAT and

45 DAT. Of two splits in each microplot, one split was

labelled with 3 atom % 15N whereas the other split was

applied as unlabelled 14N urea. In the 7 DAT split, one

of the two microplots in each cultivar was labelled with15

N while the other microplot was supplied withunlabelled 14N urea, and vice versa in the 45 DAT

split. Each microplot was 0.5 m2 and consisted of

rectangular iron frames (18 mm thick) measuring 50

100 cm with a height of 50 cm. After transplanting of

the rice seedlings, the frames were driven 20 cm into

the soil to avoid exchange of water with the bordering

rice plants outside the microplots. The 15N microplots

were placed 1 m apart in the field to eliminate cross

contamination. The microplots were treated in precise-

ly the same way as the surrounding 60N plots.

Fertilizer application, weeding and irrigation in themicroplots followed the same timing and frequency as

in the 60N plots. This trial design enabled a direct

estimation of the partial fertilizer use efficiency of 30N

applied at each timing and the total fertilizer use

efficiency of two-split application of 60N.

Management practices

All other management practices were treated as non-

experimental variables and was left to the farmers in

order to enable evaluation of the treatments underrealistic farming conditions. There was no major pest

or disease pressure during the crop cycle, so no control

measures were taken. No potassium or phosphorus

fertilizers were applied in the terraces as the traditional

practice of 7 t ha1 FYM provided 76 kg N, 38 kg P

and 138 kg K ha1, which is sufficient to produce rice

yields in the range of 46 Mg ha1 (Chettri et al.2003).

Due to surface application of FYM 1 to 2 months

before land preparation, a considerable quantity of N is

liable to be lost (via volatilization, leaching and surface

run-off) and consequently N was expected to be alimiting factor for the crops.

Field data collection and sampling

During the entire crop cycle, data was recorded on the

different crop management practices of each farmer.

This included recording dates of different cultural

practices, such as nursery sowing, transplanting, top-

dressing, weeding and harvesting. In the first split plot, a

Plant Soil (2010) 332:233246 235


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harvest area of 6 m2 (32 m) was demarcated in each

subplot (15 m2) after excluding the border rows. Within

the harvest area, the crop was cut at 2 cm above ground

level and the fresh biomass was recorded. Threshed

grains were cleaned and grain weights were recorded

along with the moisture content of the grain samples, so

that all grain weights could be standardized to 14%moisture content. A subsample of 500 g grain and straw

was weighed and dried at 70C for 2 days to constant

weight to determine total dry matter (TDM).

In the second split plot, all plants within the micro-

plots were harvested and the fresh weights were

recorded. The samples were separated into straw and

grain, weighed, and dried at 70C for 2 days to constant

weight to determine TDM. The samples were powdered

(mess 0.2 mm) and 15N and N contents were analysed

using an ANCA-SL elemental analyser coupled to a

20

20 Tracermass mass spectrometer (SerCon Ltd.,Crewe, UK) using the Dumas combustion method.

One soil sample from each trial site (i.e. from each

farmer) was collected 3040 days before transplanting

of the rice seedlings. Each soil sample consisted of 56

subsamples taken from a depth of 020 cm topsoil and

bulked into one composite sample. The soil samples

were then air-dried, sieved and analysed for total soil N

(Kjeldhal), pH (H20), and cation exchange capacity

(CEC) (Table 1). The trial sites were characterized by

heavy soils, ranging from sandy loam to loamy with

mean sand, silt and clay proportions of 52.83.1, 32.52.8 and 14.61.7, respectively. Soil analysis showed

significant differences in soil fertility characteristics

among the sites (Table1). Total soil N content ranged

from 0.100.22%. Significant differences (P0.001) in

total soil N content were recorded among the sites. The

C/N ratio differed (P0.05) among the farms, ranging

from 13.1 to 19.1 with a mean value of 16.1. pH values

ranged from 5.06.0 with significant differences

(P0.05) between the farms, and lowest and highest

CEC range varied by more than two-fold between thefarms, ranging from 7.0 to 15.3 me 100 g1 soil.

Calculations and statistics

Agronomic parameters (Bandyopadhyay and Sarkar2005;

Ghaley et al.2005) were calculated as given below:

% N derived from fertilizer% N DFF

15N excess in rice

15N excess in applied fertilizer 100% 1

Nitrogen derived from fertilizer NDFF inkgNha1

TDMN %NDFF

2

Nitrogen derived from soil NDFS inkgNha1

TDMN NDFF 3

Agronomic efficiency in N use AEN

in kg grain kg1N applied GYFGY0NF

4

District Site Village

clusters

Total soil

N (%)

C/N ratio pH CEC

(me 100 g1 soil)

Paro 1 Doteng 0.22 13.4 5.0 10.8

2 Doteng 0.11 19.1 5.1 7.0

3 Doteng 0.16 16.4 5.1 11.5

4 Shari 0.13 16.6 6.0 9.4

5 Chento 0.14 14.1 5.1 9.3

6 Chento 0.10 13.1 5.2 7.8

Thimphu 7 Khasa 0.13 15.2 5.5 14.5

8 Khasa 0.13 17.6 5.5 11.8

9 Khasa 0.14 18.4 5.7 10.6

10 Khasa 0.17 17.0 6.0 15.3

Mean 0.14 16.1 5.4 10.8

LSD 0.05 0.02 3.0 0.5 3.8

Table 1 Soil total nitrogen,

C/N ratio, pH and cation

exchange capacity (CEC) of

020 cm topsoil of ten

farmers fields (sites) in

Western Bhutan

236 Plant Soil (2010) 332:233246


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Recovery efficiency of applied N REN in %

NDFF

NF 100% 5

Physiological efficiency of N PEN

in kg grain kg1N uptake GYF GY0

NDFF

6

Partial factor productivity PFPN in kg grain kg1 soil

applied N GYF

NF7

Nitrogen harvest index NHI

GN

TDMN 8

where TDMN is total dry matter N, GY0 is grain yield

without fertilizer N application, GYFis grain yield with

fertilizer N application, NF is fertilizer N rate applied

(kg N ha1) and GN is grain N. Based on NDFSaccumulation, equivalent FYM quantity was calculated

based on 1.6% N content at 68% dry matter of FYM,

as reported in a similar field study in rice in Bhutan

(Chettri et al.2003). FYM content of P and K nutrients

are taken as 0.8% P and 2.9% K, respectively.

As there was only one replicate per site (per farmer),an analysis of variance over sites could not be carried

out to separate the effects of site on treatments. In such

situations, adaptability analysisprovides a good option

to determine site effects on the treatments (Hildebrand

and Russell 1996; IITA 1991; Mutsaers et al. 1997;

Mutsaers and Walker1990). Crop response in terms of

grain yield of a particular treatment in a given farmers

field is compared with the environmental index (EI),

which is the average of grain yields under all treatments

at a given site. The EI is thus an integrated index for

overall growing conditions in the field due to agronomic,biophysical and socioeconomic factors affecting crop

productivity.

Analysis of variance (ANOVA) was carried out for

combined data from ten sites with a nested design for

the main plots and the subplots. Separate ANOVA

for cultivar N supply (first split plot set) and

cultivar topdressing (second split plot set) was

carried out to determine the significance of treatment

effects on the respective dependent variables. EI-by-

treatment and EI-by-soil property interactions were

tested with multiple regressions to assess the signif-

icance of the relationship. Cultivar, N supply, top-

dressing timing as well as two-way interactions

between these factors were treated as fixed effects,

whereas sites and their interactions with treatment

factors were treated as random effects. All three-wayinteractions were treated as residuals and were

considered non-significant. Differences were consid-

ered significant if P0.05. Levels of significance

are denoted as follows: *** significant at P0.001,

** significant atP0.01, * significant atP0.05, ns =

not significant. Data were analysed with the Genstat

software package (Genstat 8.1 2005).

Results

Accumulation of dry matter and N in grain and straw

Grain and straw yield, harvest index (HI), grain and

straw N accumulation, TDMN, NDFS, and quantities

of estimated FYM are provided in Table2. Rice grain

yield (14% moisture content) was significantly affect-

ed by site (P0.001), cultivar (P0.001) and N

supply (P0.001), and there were interactions be-

tween N supply EI (P0.02) and cultivar EI (P

0.04). Among the farmers, Thimphu farmers 9 and 10

harvested significantly higher grain yields (6.07 and6.35 Mg ha1 respectively) compared with the other

farmers (3.365.85 Mg ha1). The difference between

the lowest and the highest average grain yield was as

high as 81% of the lowest average grain yield. Such

wide differences in average grain yields demonstrated

wide fertility differences between the farmers, a

measure of EI variability at the trial sites. Averaged

across sites and cultivars, the supply of 60 N

increased grain yields by 15% (P0.05) compared

to 0 N (Table 2). Averaged across site and fertilizer

treatments, farmers using the improved cultivarobtained 27% higher (P0.05) grain yields compared

with those using the traditional cultivar. This yield

improvement was mainly due to 27% higher HI of the

improved cultivar compared to the traditional cultivar.

Straw dry matter accumulation was significantly

affected by site (P0.001), cultivar (P0.001), N

supply (P0.01) and cultivar N supply (P0.04).

Averaged across sites and cultivars, 60 N increased

straw yields (12%) significantly compared to 0 N

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(Table 2). Averaged across site and fertilizer treat-

ments, the traditional cultivar produced 17% morestraw yields compared to the improved cultivar. HI

was significantly affected by site (P0.001), cultivar

(P0.001), and cultivar N supply interaction (P

0.02). HI differed widely among the farmers with a

difference of 34% between the highest and the lowest

HI (0.32 and 0.43). However, the N supply level had

no effect on HI. Accumulation of N in the grain was

significantly affected by site (P0.01), N supply (P

0.001) and cultivar (P0.001). Application of 60 N

increased grain N accumulation by 41% over 0 N, and

the improved cultivar accumulated 21% higher grainN compared with the traditional cultivar. Accumula-

tion of N in the straw fraction was affected by site

(P0.003) and N supply (P0.001). Farmer-to-

farmer differences in straw N accumulation were

recorded, with Thimphu farmer 9 harvesting the

highest quantity of straw N compared with other

farmers. 60 N increased straw N accumulation by

39% and there was no difference in straw N

accumulation between the two cultivars.

N accumulation in crop, soil N supply and FYM

application

Total accumulation of N in the whole crop (TDMN)

was affected by site (P0.01) and N supply (P

0.001). TDMN ranged from 82.7143.4 kg ha1

among the farms (Table 2) and 60 N supply resulted

in 40% increase (P


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by 100% and the improved cultivar had 7% higher

FYM application effect than the traditional cultivar.

Urea N uptake

Accumulation of N from the applied urea fertilizer

(NDFF) (kg ha1

) in grain + straw differed by site (P0.05), urea topdressing timing (P0.01), cultivar EI

(P0.05) and topdressing EI (P0.05) (Table 3).

Averaged over cultivars, NDFF (kg ha1) quantifica-

tion by 15N dilution in grain + straw was significantly

higher (56%) with top dressing at 45 DAT than at 7

DAT. Of 30 kg N ha1 top-dressed each at 7 and 45

DAT, the mean NDFF in grain + straw, averaged over

cultivars, were 10.2 kg N ha1 with 45 DAT

topdressing and 6.5 kg N ha1 with 7 DAT.

Correspondingly, the mean REN at 45 DAT was

34% compared to 22% at 7 DAT. Over two top-dressings, the mean REN in grain + straw in the

traditional cultivar and the improved cultivars were

30.0% and 25.7% respectively with no significant

difference between the cultivars. Compared to the 15N

dilution method, the mean REN based on N differ-

ence method (Table 3), were significantly higher in

both the traditional (56 .2%) and the improved

cultivars (46.2%) with no significant difference

between the cultivars.

N utilization efficiency

N utilization efficiency varied according to farm and

cultivar (Table 4). Agronomic efficiency in N use

(AEN) (kg grain kg1 N applied) varied among the

farmers (P0.01) from as low as 1.2 up to 22.9 kg

grain kg1 N applied (Table4). Across the farms, the

improved cultivar exhibited 66% higher AEN (P

0.001) than the traditional cultivar. Similarly, physi-

ological efficiency in N (PEN) (kg grain kg1 N

uptake) deviation was significant (P0.01) across the

farms and the PEN of the improved cultivar was

almost 100% more (P0.001) than the traditionalcultivar. Although, there was no cultivar differences

in fertilizer N uptake, higher PEN in the improved

cultivar resulted in higher AEN.

Partial factor productivity in N (PFPN) (kg grain

kg1 soil + applied N) differed significantly between

Table 3 N derived from fertilizer (NDFF in kg N ha1) and recovery of the fertilizer N (REN %) in grain + straw, from two

30 kg N ha1 splits applied at 7 and 45 days after transplanting (15N dilution) and REN% with N difference method in the traditional

and the improved cultivar in ten farmers fields (sites) in Western Bhutan

NDFF(kg N ha1)

by 15N dilution

REN by 15N

dilution (%)

REN by 15N dilution (%) REN by N

difference (%)

Traditional Improved Traditional Improved Mean

Traditional

Mean

Improved

Traditional Improved

Site (7) (45) (7) (45) (7) (45) (7) (45) (7+45) (7+45)

1 8.3 12.2 6.2 10.8 27.6 40.7 20.8 36.1 34.2 28.5 72.1 58.8

2 6.6 12.5 5.0 9.9 21.8 41.6 16.6 33.1 31.7 24.9 39.3 30.7

3 6.6 9.2 5.9 8.6 21.9 30.7 19.8 28.5 26.3 24.2 68.0 65.1

4 10.0 11.9 6.2 9.0 33.3 39.6 20.6 29.9 36.5 25.3 53.3 56.0

5 9.4 13.0 8.1 9.0 31.2 43.3 27.1 30.1 37.3 28.6 44.6 34.8

6 6.8 9.6 7.7 8.5 22.6 31.9 25.5 28.3 27.3 26.9 67.9 37.6

7 5.9 10.2 5.6 9.9 19.5 34.0 18.6 33.0 26.8 25.8 52.4 52.2

8 8.1 11.6 6.3 10.6 27.0 38.5 20.9 35.5 32.8 28.2 59.3 30.9

9 4.4 9.3 4.3 8.9 14.5 30.9 14.2 29.8 22.7 22.0 63.2 56.3

10 5.1 9.9 4.4 9.4 17.0 32.9 14.7 31.5 25.0 23.1 42.1 39.5

Mean (topdress) 7.1 10.9 6.0 9.5 23.6 36.4 19.9 31.6

LSD0.05 (topdress effect) 1.7 2.3 5.8 7.7

Mean (cultivar) 9.0 7.7 30.0 25.7 56.2 46.2

LSD0.05 (cultivar effect) 1.5 5.1 11.0

Table 3 N derived from fertilizer (NDFF in kg N ha1) and

recovery of the fertilizer N (REN %) in grain + straw, from two

30 kg N ha1 splits applied at 7 and 45 days after transplanting

(15N dilution) and REN% with N difference method in the

traditional and the improved cultivar in ten farmersfields (sites)

in Western Bhutan

Plant Soil (2010) 332:233246 239


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the farms (P0.05), with the improved cultivar

producing over 20 kg more grain kg1 N (P0.001)

applied than the traditional cultivar. Nitrogen harvestindex (NHI) (proportion of grain N to total TDMN)

varied across the farms (P0.05) according to farm

environments, and the improved cultivar showed 15%

(P0.001) higher allocation of N to the grain than the

traditional cultivar.

Adaptability analysis

Grain yields of the traditional and the improved

cultivar differed with EI with significant cultivar

EI interactions (P0.04) (Fig. 1a), demonstratinggenotypic differences in grain yield performance

across the environments. The improved cultivar

produced higher treatment grain yields across the

environments compared to the traditional cultivar.

Across the farms, grain yield of 60 N plots were

consistently higher than in 0 N plots (N supply EI)

across the EI (Fig. 1b). Grain yield differences

between the 0 and 60 N plots were higher in low-

yielding environments than in high-yielding environ-

ments, and grain yields differed significantly (P

0.02) between the N treatments.

Discussion

The current study tested the hypothesis that cultivarshave different N utilization potentials and that the

cultivars manifest such inherent potentials across a

wide range of environmental conditions. The study, to

our knowledge, is the first to combine the rigour of

the 15N dilution technique with the flexibility of field-

oriented adaptability analysis, based on large envi-

ronmental variations and under conditions prevailing

for highland rice farmers in the Himalayan region.

On-farm data with single replication per site is suited

to adaptability analysis, and EI-by-treatment interaction

analysis can be consistent over a number of years ifcertain requirements are met in a single years trial

(Hildebrand and Russell 1996; Meertens et al. 2003;

Fukai et al. 1999; Kajiru et al.1998; Sall et al. 1998).

Firstly, for linear and quadratic response estimation of

EI-by-treatment interaction, ten environments are rec-

ommended with four treatments in each environment

(Hildebrand and Russell 1996), and the trial set-up in

the current study contained ten environments with

eight treatments in each environment (site). Secondly,

EI values should represent distribution of the target

population of environments at the trial site forimproved data quality and reliability. In the present

study, EI values reflected a normal distribution with

most environments clustered around the average

environments, representative of actual farming envi-

ronments in the trial sites (Fig. 1). Thirdly, crop yield

levels should represent average yields in the area,

typical of a normal cropping season. According to the

national agriculture production census in Bhutan,

average yields reported for rice at the trial sites were

in the 3.84.2 Mg ha1 range (MOA2001,2004), and

yield data from the current study conforms to theselevels. Our dataset therefore meets the requirements for

carrying out adaptability analysis.

Cultivars differ in response to environmental

conditions

Cultivar EI (P0.05) values indicate that NDFFvaried

across the range of EI depending on the field

conditions. Differences in field characteristics among

Table 4 Agronomic efficiency in N (AEN) (kg grain kg1 N

applied), physiological efficiency in N use (PEN) (kg grain

kg1 uptake), partial factor productivity in N use (PFPN) (kg

grain kg1

soil + applied N) and nitrogen harvest index (NHI)

in the traditional and the improved cultivar in ten farmers

fields (sites) in Western Bhutan

Site AEN PEN PFPN NHI

1 22.9 73.3 59.6 0.48

2 6.6 24.2 66.9 0.48

3 19.0 75.9 71.0 0.44

4 11.5 38.6 86.4 0.48

5 1.2 2.6 68.5 0.59

6 7.7 28.4 68.9 0.54

7 15.7 60.2 86.0 0.55

8 7.6 27.1 87.6 0.55

9 9.7 43.9 95.8 0.45

10 13.9 58.1 93.9 0.56

Mean 11.6 43.2 78.5 0.51

Cultivar

Traditional 8.7 29.1 68.3 0.48

Improved 14.5 57.4 88.6 0.55

LSD0.05 (site main effect) 7.8 32.7 18.2 0.08

LSD0.05 (cultivar main effect) 2.8 10.3 3.1 0.03

240 Plant Soil (2010) 332:233246


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10/14

phase of the seedlings (Zhang et al. 2007; Peng and

Cassman1998). A similar study on transplanted rice in

China found that the indigenous soil N supply fulfilled

the N requirement at the early rice seedling stage (Xue

and Yang2008).

Environmental variation and soil fertility

The measured soil properties were found to have

varying degrees of effects on EI (Fig. 2a, b, c, d).

Total soil N % and EI were not correlated (P0.24)

(Fig. 2a), which conforms to the results in other on-

farm studies of rice-wheat and ricerice systems

(Adhikari et al. 1999; Cassman et al. 1996b). A

possible reason for the non-correlation is that the

warm monsoonal climate during paddy growth is

conducive for sufficient mineralization of N, masking

the soil N status measured before transplanting(Adhikari et al. 1999). Soil C/N ratio, however, was

positively correlated with EI (Fig. 2b), indicative of a

better proxy for potential soil N mineralization than

total soil N content. Similarly, soil pH was positively

correlated with EI (Fig. 2c), indicating that the soils

with neutral pH (5.56) had higher yield levels

compared to the slightly acidic soils (55.5). CEC

was not found to be correlated (P0.31) to EI(Fig. 2d), in contrast to other studies (Ruth and

Lennartz 2008; Yanai et al. 2000, 2001). However,

the high fertility of the soils and the tradition of

consistently applying FYM to the fields may elimi-

nate or reduce the importance of CEC effects.

Environmental variation and management

FYM application and EI were not correlated (R2=

0.25), but the direct role of FYM in supplying N to

the crops (Fig. 3a) was significant in line with thefindings from other studies, including a study in

y = 1.44x + 8.68

R2= 0.41*

0

10

20

0 2 4 6 8

Environmental Index (Mg ha-1)

SoilC/Nratio

(b)y = -0.01x + 0.22

R2

= 0.17

0.00

0.10

0.20

0.30

0 2 4 6 8

Environmental Index (Mg ha-1

)

0 2 4 6 8


)

0 2 4 6 8


)

(a)

y = 0.33x + 3.74

R2 = 0.67**

0

2

4

6

8

SoilpH

(c) y = 1.02x + 5.59

R2= 0.13

0

5

10

15

CEC(me100g-1s

oil)

(d)

TotalsoilN%

Fig. 2 Relationship

between environmental

index and (a) total soil N %

(b) soil C/N ratio (c) soil pH

and (d) cation exchange

capacity (me 100 g1 soil )

in ten farmers fields in

Western Bhutan

242 Plant Soil (2010) 332:233246


11/14

Bhutan (Ghosh and Sharma1999; Chettri et al.2003;

Satyanarayana et al. 2002). FYM may thus have a

priming effect or added nitrogen interaction (ANI)

(Azam 1990; Ghosh and Sharma 1999; Jenkinson et

al.1985). Consequently, NDFS and EI were correlated

(Fig. 3b). However, there was a negative correlation

bet ween FYM quant ity and REN (Fi g. 3c) atrelatively high FYM input levels, found in some sites

(Table 2, site 3, 10) compared with others (Table 2).

Correspondingly, REN values were low (Table3, site

3,10) in high fertility sites, and FYM input therefore

needs to taken into account when formulating

fertilizer N recommendation for farmers. The tenden-

cy of decreasing REN from fertilizer with increasing

availability of N from indigenous sources has been

reported in similar rice field studies in the Philippines

and Bangladesh (Hossain et al. 2005; Cassman et al.

1996c).

Soil N supplying capacity

Nitrogen derived from soil constituted 7791% of

TDMN (Table2), corresponding with the results from

a very large study covering 179 sites over 2 years in

eight key rice domains in Asia (Dobermann et al.

2002). In flooded rice paddies, NDFS constitutes Ncontribution from the soil organic matter fraction

(FYM), atmospheric deposits, irrigation water, stubble

residues from preceding crops and N2 fixation. N

contribution from atmospheric deposits and irrigation

water are in the range of 13 kg ha1 per rice crop and

considered negligible (Cassman et al. 1996b). Nitro-

gen contributions from stubble residue and N2fixation are interrelated as availability of crop residue

enhances N2 fixation due to increased carbon sub-

strate for microbial activity (Roper and Ladha1995).

Nitrogen fixation is reported in the range of 28

y = 11.04x + 11.83

R2= 0.74

y = 15.30x + 10.29

R2= 0.88***

0

40

80

120

0 5 10

TraditionalImprovedLinear (Traditional)Linear (Improved)

(d)

y = 12.80x + 0.66

R2

= 0.95***

0

40

80

120

0 3 6 9 12

Farmyard manure (Mg ha-1)

Farmyard manure (Mg ha-1)

Nitrogenderivedfromsoil(kgNha)

(a)y = 13.36x + 21.51

R2

= 0.44*

0

40

80

120

0 2 4 6 8

Nitrogenderivedfromsoil(kgha-

1) (b)



y = -0.02x + 0.40

R2

= 0.49*

0

20

40

0 4 8 12

(c)

Recoveryefficiencyof

Nin%(REN)

PFPN(kggrainkg-1so

il+appliedN)

-1Fig. 3 Relationship

between nitrogen derived

from soil (NDFS) and (a)

farmyard manure (FYM)

and (b) environmental index

(EI); (c) farmyard manure

(FYM) and recovery

efficiency of N (REN %)

and (d) environmental index

(EI) and partial factor

productivity in N use

(PFPN) in ten farmersfields in Western Bhutan

Plant Soil (2010) 332:233246 243


12/14

51 kg Nha1 per crop cycle with sufficient crop

residue and phosphorus availability, and N2 fixation

might have contributed significantly due to the

abundance of FYM as substrate for microbial activity.

After N2 fixation is accounted for, the remaining

organic soil N must have come from mineralization of

FYM, constituting the major bulk of soil N, whichcorresponds with the strong correlation (R2=0.95***)

between the NDFS and quantity of FYM applied

(Fig. 3a).

Quantification of REN

Quantification of REN can vary according to the

method used. The N difference method may in some

cases give a higher estimate of REN than the 15N

tracer method in transplanted rice (Bronson et al.

2000). In concurrence with other studies (Rao et al.1991; Bronson et al. 2000), REN estimated by the

difference method in our study was 2026% greater

than that obtained by the 15N dilution method. This

positive Added Nitrogen Interaction (ANI) may be

real or apparent. A real ANI may arise due to greater

accumulation of soil N in plots receiving fertilizer N

compared with unfertilized plots, while an apparent

ANI may arise as a result of pool substitution. A real

positive ANI may arise due to enhanced soil organic

matter mineralization following the application of N

fertilizer. It may also occur as a result of greater soilexploration by plant roots and enhanced uptake of

unlabelled N in fertilized plots. Both of these

mechanisms may have operated in this study, but

uptake of N mineralized from previous inputs of FYM

may also have enhanced the potential for pool

substitution. Hence, REN estimation by 15N dilution

method may have been affected by both real and

apparent ANI.

Conclusions

The wide variability in rice grain yields across the

environments are related to differences in soil

properties, FYM application and cultivar response to

fertilizer N. Agronomic efficiency of N use was

relatively low across the environments, with wide

differences between farmers, highlighting opportuni-

ties for the transfer of knowledge on better N

management to the farmers. The improved cultivar

yielded significantly more grain compared with the

traditional rice variety given the same amount of

fertilizer N. Consequently, the wider use of improved

cultivars may enhance rice yields without the need for

greater N inputs. The decreasing REN with increasing

FYM application is a matter of concern as N lost from

fields may contribute to eutrophication of rivers andstreams, as well as increase the cost of rice production

due to the decreasing marginal rate of return at higher

FYM input. Nitrogen application at 45 DAT resulted

in 56% greater uptake of NDFF than at 7 DAT,

indicating that the first topdressing could be delayed

until 45 DAT (maximum tillering) to coincide with

the maximum crop N demand and enhance the

opportunity for N uptake. To make the best use of

both fertilizer and soil-derived N, the fertilizer N

recommendations for rice farmers in the Bhutan

highlands should to take account of previous inputsof FYM and the timing of maximum crop N demand.

Acknowledgements We gratefully acknowledge the financial

assistance by the Consultative Research Committee for Devel-

opment Research/Danish International Development Agency,

under the Ministry of Foreign Affairs in Denmark. The study

would not have been possible without the help of Dr. Lungten

Norbu, Mr. Padam Lal Giri, Hema Devi Nirola and Kalpana

Rai, who provided full assistance in terms of planning and

implementing the field activities with the farmers in Thimphu

and Paro districts in Western Bhutan. Thanks are due to Mrs.

Marie Bcker Pedersen for language editing and feedback on

the paper and to the anonymous reviewer for the significantcontributions in improving the scientific content of the

manuscript.

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