research article assessing sediment-nutrient export rate...

Hindawi Publishing CorporationISRN Soil ScienceVolume 2013 Article ID 748561 10 pageshttpdxdoiorg1011552013748561

Research ArticleAssessing Sediment-Nutrient Export Rate and Soil Degradationin Mai-Negus Catchment Northern Ethiopia

Gebreyesus Brhane Tesfahunegn12 and Paul L G Vlek2

1 College of Agriculture Aksum University Shire Campus PO Box 314 Shire Ethiopia2 Center for Development Research University of Bonn Walter-Flex-Straszlige 3 53113 Bonn Germany

Correspondence should be addressed to Gebreyesus Brhane Tesfahunegn gebre33gmailcom

Received 13 April 2013 Accepted 12 May 2013

Academic Editors T J Cutright J A Entry M Goss C Martius and W R Roy

Copyright copy 2013 G B Tesfahunegn and P L G Vlek This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Even though soil degradation challenges sustainable development the use of degradation indicators such as nutrient export (NE)and nutrient replacement cost is not well documented at landform level This study is aimed to investigate the extent of soildegradation NE rates and their replacement cost across landforms in the Mai-Negus catchment northern Ethiopia Differenterosion-status sites (aggrading stable and eroded) in the landforms were identified and soil samples were randomly collectedand analysed Nutrient export replacement cost and soil degradation were calculated following standard procedures This studyshowed that soil degradation in the eroded sites ranged from 30 to 80 compared to the corresponding stable site soils but thehighest was recorded in the mountainous and central ridge landforms Average NE of 95 68 91 32 25 and 007 kg haminus1 yminus1 forsoil calcium carbon nitrogen potassium magnesium and phosphorus respectively was found from the landforms Significantlystrong relationships between NE and sediment yield in the landforms were observed Annual nutrient replacement costs variedamong the landforms though the highest was in the reservoir (C9204 inMay 2010)This study thus suggests that while introducingantierosionmeasures priority should be given to erosion sources to the reservoir such asmountainous and central ridge landforms

1 Introduction

Soil erosion is a challenge for sustainable agricultural devel-opment in many developing countries [1 2] The problemis more serious in the Ethiopian highlands such as theTigray region [3ndash5] Inappropriate agricultural practiceshigh population pressure from human and livestock higherrainfall intensity and rugged topography have been reportedas the main facilitators for having severe erosion [6 7]

In the Tigray region average soil loss by erosion oncultivated land is more than 49 t haminus1 yminus1 [8] which exceedsthe average soil loss of 42 t haminus1 yminus1 for Ethiopia as a whole[3] Such soil loss through water erosion is almost alwaysaccompanied by losses of essential soil nutrients Erosionis selective for fine soil particles which are relatively richerin soil nutrients [9 10] In line with this Stoorvogel andSmaling [11] and UNDP [12] reports showed that comparedto rates in sub-Saharan Africa Ethiopia has the highest soil

nutrient outflow rates of 60 kg haminus1 (30 kg haminus1 nitrogen and15ndash20 kg haminus1 phosphorous) while inflows from fertilizers arevery low (lt10 kg haminus1) In the long term such soil nutrientlosses by erosion adversely affect soil productivity of thesource areas and are expected to lead to an increase innutrients in the deposition areas [9 10]

Nutrient export rates can well describe the level of soilnutrient degradation in the source soils and the enrichmentsin the deposition sites [13] Despite such utility the rateof soil nutrient export as a means of assessing degradationfromdifferent landforms in a catchment considering differenterosion-status sites has not yet received research attentionHowever such research is pertinent to the development ofsite-specific strategies targeting the source sites

Other studies have shown that erosion and depositionprocesses can significantly contribute to the variability offine soil particles and the associated nutrient exports fromcatchment topography [14 15] Soil erosion and sediment

2 ISRN Soil Science

Table 1 Biophysical description of the landforms in the Mai-Negus catchment northern Ethiopia

Landform Areae () Land-use cover ()a Lithology bSlope degc Elevation md

Arable Grazing Bush and Woodf Othersg BM LP STRolling hills 100 80 10 4 6 100 na na 3ndash16 2150ndash2240Mountainous 145 36 34 26 4 35 65 na 4ndash79 2350ndash2650Central ridge 255 70 13 5 12 5 95 na 3ndash25 2230ndash2450Valley 199 65 31 2 2 77 23 na 0ndash6 2070ndash2100Plateau 98 47 16 30 7 58 42 9 3ndash10 2500ndash2550Escarpment 191 47 31 14 5 84 7 na 3ndash30 2270ndash2540Reservoir 12 na na na na 100 na na 0 2060ndash2080BM basic metavolcanics LP lava pyroclastic ST sandstone na not applicableaThe proportion of the land covers was derived from a Landsat image of November 2007 overlaid by the landform mapbThis was derived by overlaying the landform map with the geology map of EthiopiacDeveloped from digital elevation model (DEM)dDerived from DEMeTotal catchment area is 1240 km2 and reservoir area is 015 km2fIncludes closed area plantations and natural vegetationgIncludes settlements rock-out crop and marginalized areas

delivery processes which are responsible for high sedimenttransport and the associated export of sediment-boundnutrients to deposition areas in a catchment are influencedby landscape characteristics [13] In line with this studieson nutrient balances in Ethiopia indicate that the balance atfarm level is more positive than plot level [11 16 17] whilelosses from some fields may be of benefit to other fields Thismay be attributed to the effect of nutrient redistribution byerosiondeposition processes or to active nutrient transfers byfarmers

Studies of erosion-induced changes in soil properties atthe field scale have constantly shown that soil texture surfacesoil organic carbon nitrogen and phosphorus concentra-tions are higher in areas of soil deposition compared to areasof soil removal by erosion [18] Assessing the effect of erosionon soil properties variability as a result of nutrient exportfrom catchment landforms can provide more meaningfulresults This study aims to (1) assess soil nutrient depletionand soil physical degradation comparing the eroded andstable-sites soil (2) examine the rate of soil nutrient exportconsidering the aggrading and eroded erosion-status sitesin the landforms of the Mai-Negus catchment northernEthiopia and (3) quantify the costs of soil degradationassociated with nutrient losses through water erosion atlandform and catchment scale and extrapolate the findingsto the Tigray region and highlands of the country

2 Materials and Methods

21 Study Site This study was conducted in the Mai-Negus catchment in the Tigray region of northern Ethiopia(Figure 1)The catchment has an area of 1240 ha and altitudesranging from 2060 to 2650m asl It has a mean annualtemperature of 22∘C and precipitation of 700mm Most ofthe rainfall (gt70) occurs in July and August Land useis predominantly arable with teff (Eragrostis tef ) being themajor crop along with different-sized areas of pasture landand scattered patches of trees bushes and shrubs The major

rock types are lava pyroclastic and metavolcanic Soils aremainly leptosols on very steep positions cambisols onmiddleto steep slopes and vertisols on flat areas

22 Terrain Assessment In this study field reconnaissancesurveys were carried out to gain an overall image of thecatchment characteristics (Table 1) Data were collected fromJune to December 2009 The landforms in the catchment(Figure 1) were developed in ArcGIS software using fieldsurvey-based data in combination with the information fromthe topographic map Considering elevation slope geologyand geomorphologic characters (surface flows alluvial andcolluvial deposition) the catchment topography is classifiedinto six main landforms (Figure 1) namely valley (19 of thecatchment area) plateau (8) rolling hills (9) central ridge(27) escarpments (29) and mountainous (6)

The reservoir was considered a separate landform in thecatchment and is located at the toeslope of the catchmentSediment deposition in the reservoir was used to estimatethe level of soil degradation nutrient export rate and theassociated replacement costs from the entire catchmentDeposition sites in the other landforms of the catchment wereused to assess soils deposited on the way to the reservoirafter being transported from the original place towardsthe outlets of the landforms Generally during the fieldsurvey three sites namely depositionaggrading stable anderoded erosion-status sites in the six landforms of the studycatchment were identified using standard geomorphologicalprocedures of erosion-deposition indicators

23 Erosion-Status Site Selection and Soil Sampling PointsRepresentative erosion-status sites (aggrading stable anderoded) and the corresponding soil sampling points (Figure 1)were selected in four steps A reconnaissance survey onland use lithology soil types slopes and elevation rangesof the landforms (Table 1) followed by informal discussionswith farmers and development agents to gain insight onland use history as well as land- and crop-management

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Tigray

Southern

Easternzone

zone

zone

zone

Central

North-western

Western zone

Mekelle

(b) Tigray

N

N

(a) Ethiopia

Projection Universal TransverseMercator (UTM)

Zone 37NDatum AdindanSpheroid Clarke 1880

Erosion-status sitesAggrading in reservoirAggrading in catchmentStableEroded

Land forms

Reservoir

Rolling hillsPlateauValleyCentral ridgeEscarpmentMountainous

(c) Mai-negus catchment

N

45∘09984000998400998400E40

∘09984000998400998400E35

∘09984000998400998400E

45∘09984000998400998400E40

∘09984000998400998400E35

∘09984000998400998400E

5∘09984000998400998400N

10∘09984000998400998400N

5∘09984000998400998400N

10∘09984000998400998400N

Figure 1 Study areas Ethiopia (a) Tigray (b) and Mai-Negus catchment (c) (landforms) with representative soil sampling points foraggrading (deposition) stable and eroded erosion-status sites

practices was carried out Subsequently erosion-status siteswere selected using soil morphology such as thickness ofalluvialcolluvial deposits and degree of truncation of thetop soil horizon (soil profile) and erosion indicators (rillsgullies surface sheet wash exposure of roots and stonesand depositions) In the stable sites slopes are flat to gentleand there is little evidence of soil truncation or depositionindicating that soil loss and gain is more or less balancedTheeroded sites were identified based on a combination of erosionindicators and features associated with soil profile truncationWhen aggrading sites were selected depression areas thatreceived sediment from upper slopes and erosion channelswere considered Finally representative soilsediment sam-pling points of the erosion-status sites were located andgeoreferenced Composite samples were randomly collectedfrom sampling grid areas ranging from 150 to 300m2 foreach soil sampling point in the erosion-status site Each sitehad two soil sampling points and 5ndash8 composite sampleswere collected at 0ndash20 cm soil depth The composite samples

(weighted 500 gm each) were pooled and mixed thoroughlyin a basket manually A subsample of 500 g was taken foranalysis after being air dried and sieved to pass through 2mmsieve A total of 36 soil samples (6 landforms times 3 erosion-status sites in each landfrom times 2 sampling points) and 6sediment samples from the reservoir resulted in a grand totalof 42 samples for analysis

The sampling point selection in the reservoir took intoaccount the sediment depth variability flow source out-flow and length of time where water was in the reservoirThere was no water at the selected sampling points inthe reservoir on the second week of June 2009 For thesampling points identified in the reservoir 6 representativepits with depths ranging from 10 to 25m were dug downto reach the interface with the original soil Then threecomposite samples were collected from the entire depth ofeach pit The samples from each pit were pooled and mixedthoroughly in a basket and a subsample of 500 gwas taken foranalysis

4 ISRN Soil Science

24 SoilSediment Analysis Soilsediment samples were de-termined for texture using the Bouyoucos hydrometermethod [19] bulk density (BD) by core method [20] organiccarbon by Walkley-Black method [21] available phosphorus(Pav) byOlsenmethod [22] and total nitrogen (TN) and totalphosphorus (TP) by Kjeldahl Digestion method [23] Cationexchange capacity (CEC) is determined by ammoniumacetate extraction buffered at pH 7 [24] Exchangeable bases(calcium Ca magnesium Mg potassium K) were analyzedafter extraction using 1M ammonium acetate at pH 70 Iron(Fe) and zinc (Zn) were determined using 0005Mdiethylenetriamine pentaacetic acid extraction by themethod describedin Baruah and Barthakur [25]

25 Estimation of the Extent of Soil Degradation Soil nutrientand soil physical degradation were assessed based on thedifferences in the content of the soil nutrients and fine soilparticles in the stable and eroded sites The stable site soilswere used as a reference The magnitude of degradation wassimilarly calculated as the ratio of the difference in the contentof the soil properties between the stable and eroded sites to thereference soils

26 Quantification of Nutrient Export Rate Associated withWater Erosion The nutrient export (NE) to the reservoirassociated with runoff and sediment and to the hypotheticaldeposition areas on the way to the reservoir at the outletsof the landforms was calculated using equations defined inVerstraeten and Poesen [26] as

NE119903

=

SM lowast NC119860 lowast 119884 lowast NTE lowast 10

SM = SV lowast dBD

NE119897

= SSY( NCNTE lowast 10

)

SSY = SM119860 lowast 119884

(1)

where NE119903

is the nutrient export (kg haminus1 yminus1) from theentire catchment to the reservoir NE

119897

is the nutrient export(kg haminus1 yminus1) to the outlet of each landform in the catchmentSM is total sediment mass (ton) NC is average nutrientcontent (ton of nutrient per kg of sediment) 119860 is drainagearea (ha) 119884 is duration (age) of sediment accumulationwhich was 16 years for the reservoir SV is total sedimentvolume (m3) dBD is dry bulk density (tonmminus3) and SSY isarea-specific sediment yield (t kmminus2 yminus1) The nutrient trapefficiency (NTE) of the reservoir or the depositional areasin the landforms is assumed to be equal to sediment trapefficiency (STE in)The parameters used for the calculationof nutrient export rates are presented in Table 2

An empirical model was used for the verification of theinformation gathered through discussions with the farm-ers on the condition of outflow from the reservoir for

N

0 200(km)

1560500 1560600 1560700 1560800

1560500 1560600 1560700 1560800

463000

463100

463200

463300

463400

463000

463100

463200

463300

463400

Figure 2 Spatial distribution of sediment depths measured (bluedots) and polygons (areas) influenced by the dots in the reservoir ofthe Mai-Negus catchment northern Ethiopia

determining STE The STE was calculated on the basis of theformula given in Verstraeten and Poesen [26] as

STE = 100 (1 minus 1

1 + 00021119863 (119862119860)

) (2)

where119862 is reservoir storage capacity (m3) and119860 is catchmentarea (km2) 119863 has constant values ranging from 0046 to 1with amean value of 01 In this study considering the scarcityof local information for defining a value of119863 a119863 value of 01was used

The nutrient trap efficiency was considered to be equal tothe sediment trap efficiency (STE) as (1) no spillage occurredexcept in one season in 2007 (2) the stored water is usedfor irrigation during the dry period once the sedimentsand nutrients have settled in the reservoir and (3) soilnutrients are mainly transported bound to sediment as soilnutrient solubility is low in the soils of the region [13] Suchassumptions imply that both nutrients and sediments areassumed to have the same chance of being trapped in thereservoir Similar assumptions were also used in past studieswhile estimating the STE of different reservoirs in the Tigrayregion catchments [13]

The area-specific sediment yield (SSY) of the reservoirwas determined based on the field survey of sedimentdepth measured Spatially distributed sediment depths (bolddots) were located within the reservoir (Figure 2) basedon visual observation of sediment size pattern and depthAugeringwas also conducted to assess the sediment thickness(pit depth) Differentiation between bottom in situ materialand newly deposited sediment was easy due to sedimentstratificationThe sampling pits were georeferenced and theninterpolated using Thiessen polygonsin ArcGIS to calculate

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Table 2 Sediment yield and other related parameters of the different landforms in the Mai-Negus catchment northern Ethiopia

Landform SV (m3) SM (t) A (Km2) dBD (tmminus3) STE () SSY (t Kmminus2 yminus1)Rolling hills 1024 1566 125 153 63 1254Mountainous 3579 5762 182 161 55 3165Central ridge 4761 7568 320 159 57 2366Valley 4327 5884 252 136 66 2335Plateau 965 1408 121 146 62 1164Escarpment 3212 4786 240 149 69 1994Reservoira 343776 388467 1240 113 99 1958Average 51663 59349 354 145 67 2034Standard deviation 128818 145143 307 017 1480 690SV annual sediment volume SM annual sediment mass A drainage area dBD dry bulk density STE sediment trap efficiency SSY area specific sedimentyieldaSediment in volume andmass is total amount of sediment accumulated in the reservoir over 16 years (1994ndash2009) SSY coming from the whole catchment area(1240 km2) was calculated after the depth of the spatially distributed pits opened in the reservoir sediment wasmeasured georeferenced and then interpolatedthroughThiessen polygons in a GIS environment

SSY for each polygon using the equation defined in Tamene[8] However the SSY of the landforms in the study catch-ment was estimated from the Soil and Water Analysis Tool(SWAT) model simulation result of the subbasinsThemodeltakes into account transfers of SSY using routing procedurebetween landforms for example from mountain to plainlandTheSWATmodelwas calibrated validated and assessedfor uncertainty using Nash-Sutcliffe coefficient (gt05) andcoefficient of determination (gt06) Such model efficiencyvalues indicated that the model is adequate for predictionin the study catchment conditions The details of the modelevaluation result can be found in Tesfahunegn [27]

Distributed sediment delivery ratio (SDR) estimation isnecessary for different landscape positions (landforms) Asimple approach to estimate SDR is defined by Tamene [8]as

SDR119894

= (

HD119894

SL119894

) 10and

(3)

where SDR119894

= SDR of a land cell (landform) HD119894

= ele-vation difference between a land cell at a given point andthe associated main stream outlet cell in the landform SL

119894

= length of the flow path between the inlet and the mainchannel outlet of a landform To estimate the distributedSDR values for each landform using (3) the study catchmentwas first subdivided into different landforms The STE ofthe landforms was found after the SDR was estimated andassumed to be equivalent to the sediments not delivered intothe outlets of the landforms

Annual sediment mass deposition (SM ton) was calcu-lated by multiplying SSY (t kmminus2 yminus1) by the area of eachlandform There was no sediment trapping structure in thesix landforms during the study period In the presence ofimpoundments the trap efficiency can be higher than theassumption used in this study Generally the procedure forestimating sediment trap efficiency at the outlets of thelandforms was considered to be similar to the sediment notdelivered and comparison of SM among the landforms forpriority setting is thus scientifically unbiased In addition

in the absence of other reliable alternative procedures fordetermining STE in landforms within catchments and sub-catchments without reservoirs and ungauged measurementsthe use of the present approach and assumption can supportfor site-specific decision-making processes

The measured sediment volumes were converted to sed-iment masses using the mean value of the bulk densitydetermined for each landform To determine the mean bulkdensity duplicate undisturbed sediment core samples werecollected from the six pits at the dry-bed reservoir For theother landforms mean bulk density was determined usingcore samples collected from the deposition sites at 0ndash20 cmdepth

27 Costs of Nutrient Loss through Erosion To assess thecosts of sediment-associated nutrient losses a ldquoreplacementcostrdquo approach has already been applied to compare amongdifferent catchments [13 28] However there is still an infor-mation gap with regard to the application of such approach tolandforms within a catchment with and without reservoir Inthis study the loss of nutrients associated with water erosionwas calculated using the equivalent price of commercialfertilizers for example urea and di-ammonium phosphate(DAP) First the nitrogen (N) and phosphorous (P) exportrate from each landform and the entire catchment to thereservoir was assessed Second the P value was converted toan equivalent P

2

O5

value and then to DAP fertilizer Thirdthe amount of N for DAPwas estimated from theN exportedFourth the remaining N exported was converted to ureafertilizer Finally the cost of nutrient export was estimatedin terms of the money required for replacing the nutrientlosses as urea and DAP fertilizer The local price in May2010 for urea and DAP was used to estimate the replacementcost of the exported nutrients The price of 100 kg of DAPwas 360 Ethiopian Birr currency (ETB) (C2118) and that of100 kg urea was 320 ETB (C1882) The replacement cost inthe catchment was projected to the scale of the Tigray regionand Ethiopian highlands

6 ISRN Soil Science

0

10

20

30

40

50

60

70

80

90

100

Rolling hills Mountainous Central ridge Valley Plateau Escarpment

Soil

attr

ibut

es d

egra

datio

n (

)

LandformSiltClayExK

ExCaExMgCEC

OCTNPav

TPFeZn

Figure 3 Extent of soil degradation using selected soil properties () in the landforms of Mai-Negus catchment northern Ethiopia Y Errorbars are standard deviation of samples Note ExK exchangeable potassium ExCa exchangeable calcium ExMg exchangeable magnesiumCEC cation exchange capacity OC organic carbon TN total nitrogen Pav available phosphorus TP total phosphorus Fe iron Zn zinc

28 Data Analysis Data were subjected to one-way analysisof variance (ANOVA) using SPSS 180 release software [29]The landform was considered as a group variable Normalityand homogeneity assumptions of ANOVA were checkedMeans were separated using the least significant difference(LSD) and tested by all-pairwise comparisons at probabilitylevel 119875 le 005 Both SPSS and Excel sheets were also used fordescriptive statistics and regression analysis

3 Results and Discussion

31 Extent of Soil Degradation across the Landforms Toassess the extent of soil degradation in the landforms soilparameters in the eroded sites were compared with that of thestable site soils (reference soils) Soil parameters values belowthe level of the stable site soils indicated that the extent of soildegradation is higher than aggradation Accordingly a highlevel of soil degradation was observed across the landformsfor all soil parameters but the highest was observed in themountainous and central ridge landforms (Figure 3) Thelevel of soil degradation for most soil indicators in the erodedsites of the central ridge and mountainous landforms rangedfrom 50 to 80 compared to the corresponding stable sitesoils The soil nutrients of OC TN and Pav were amongthe most degraded parameters across the landforms Inagreement to this study the finding in Haileslassie et al [30]indicated that soil erosion accounted for losses of TN by 70and Pav by 80 while assessing nutrient balance at a regionalscale in Ethiopia

The landforms with less degraded soils were the plateaufollowed by the valley and escarpment where soil nutrientsdegraded in the range of 30ndash40 as compared to the stablesite soils (Figure 3) The magnitude of physical degradationas silt was higher than that of clay and ranged from 19in the plateau to 69 in the mountainous and centralridge landforms The reason could be attributed to highersusceptibility of silt to erosion processes However this studyshowed that soil nutrient degradation is higher than soilphysical degradation in the study catchment landformsThusappropriate interventions that improve the soil nutrientsshould be introduced in the landforms

32 Evaluation of Nutrient Export Rate Associated with Sedi-ment Yield This study showed that soil nutrient export rateswere significantly (119875 le 005) varied among most landforms(Table 3) On average nutrient export rates (kg haminus1 yminus1) of 95for Ca 68 for OC 91 for TN 32 for K 25 for Mg and 007for Pav were observed in the landforms But the export ratefrom the entire catchment to the reservoir was significantlyhigher than the average of all landforms and even the valuesof the individual landforms (Table 3)This could be attributedto the fact that the reservoir is the largest sink for sediment-attached soil nutrients exported from the hotspot erosionsources in the entire catchment This was followed by thevalley which received sediment coming from the upstreamlandforms Sediment from all other landforms is routedthrough the valley and then finally to the reservoir Generallythis study indicated that from the erosion source areas large

ISRN Soil Science 7

Table 3 Nutrient export rate associated with sediment yield across the landforms in Mai-Negus catchment northern Ethiopia

Landform Rate of nutrient export (kg haminus1 yminus1)K Ca Mg OC TN Pav

Rolling hills 136e 3543e 121d 2376e 423d 0032deMountainous area 381b 1268c 291b 8649b 1036b 0087cCentral ridge 373b 1157c 282b 5773c 866bc 0096bValley 391b 1359b 298b 9811b 1156b 0102bPlateau 152d 2587e 079e 2939e 392d 0022eEscarpment 197c 5727d 22c 4237d 711c 0041dReservoir 614a 1668a 479a 1357a 1774a 0135aAverage 321 948 253 676 908 0074Standard deviation 171 548 132 409 478 0042K potassium Ca calcium Mg magnesium OC organic carbon TN total nitrogen Pav available phosphorus

amounts of soil nutrients are exported to the deposition sitesin the same or other landforms Such erosion processes areaggravated by terrain characteristics (eg slope) high gullynetworks poor soil cover and conservation measures whichhave been reported as themain controlling factors for the rateof sediment yield variability in the Tigray catchments [8 31]

The nutrient export rate from the valley was higherthan the remaining landforms except the reservoir eventhough the valley is located at a place where losses can becompensated by imports from the upstream landforms (egmountainous and central ridge) The higher nutrient exportfrom the valley seems to be inconsistent with the point thatthis landform showed the largest sediment deposits (dueto flat slope) next to the reservoir But the fact for beinghigher nutrient export by erosion is that the valley receivedsediment coming from all the other landforms by sedimentrouting procedure and such amount of sediment is beyondthe capacity of this landform to deposit Thus introducingappropriate interventions to the upstream landforms (sourcesof erosion) that reduce sediment due to water erosion can bethe remedy for decreasing the higher nutrient export from thevalley landform

The nutrient export rates from the other landformsshowed a well-defined pattern for example the mountain-ous followed by the central ridge landform exported largeamounts of nutrients to the valley Most rates of soil nutrientsexported to the aggrading sites in the mountainous areacentral ridge and valley landforms were not significantly(119875 gt 005) different (Table 3) This indicated that themountainous and central ridge could be the major sourcesof the sediment-attached nutrients exported to the valleylandform and thereby to the reservoir The nutrient exportrate to the reservoir associated with sediment is higher inthe present study catchment when compared to the meanof 13 other catchments in the Tigray region reported byHaregeweyn et al [13] However the nutrient export rate inthis study is lower than the findings reported in other sub-Saharan Africa such as Ghana by Amegashie [32]

A significant (119875 lt 005) and high coefficient ofdetermination (1198772) that is 087 085 083 078 073 and068 for Ca Pav K TN OC and Mg respectively wasobserved between the nutrients exported and area-specific

sediment yield (SSY) This indicated that the SSY accountedfor between 68 and 87 of the variability in the nutrientsexported across the landforms The significant and strongrelationships between nutrient export and SSY illustrated thaterosion could be the main responsible factor for sediment-attached nutrient export variability among the landformsin the catchment regardless of the inherent variability ofthe original soils Generally hydrological processes suchas erosion can be affected by variability in soil propertiestopography land usecover and human-induced changesand these in turn affect the soil nutrient distribution inlandforms [8 27]

The 1198772 (068) for the relationship between Mg andSSY in this study is higher than the 1198772 (029) reportedin the same study region but in different catchments inHaregeweyn et al [13] Some among the 13 catchmentsshowed lower nutrient export rates than those estimatedfor the landforms of this study and others showed highernutrient export rates Despite such differences the nutrientexport rates calculated in the previous and present studiesdeviated by lt15 which indicates that the present resultshardly under- or overestimate the nutrient export associ-ated with sediment yield Hence scaling-up of the presentapproach to catchments or subcatchments without reservoirsto assess sediment-attached nutrient export is applicable formanagement planning

33 Cost of Soil Nutrient Degradation Caused by ErosionThis study presented the replacement costs related to N andP exported by water erosion from the different landformsof the catchment (Table 4) Taking the average value of alllandforms may underestimate the replacement cost for theentire catchment as the nutrient transported to the aggradingsites in the landformswas calculated based on the assumptionofminimum sediment deliveryThe sumof all landformsmayalso overestimate the replacement cost because part of theexported nutrients from the upper landforms can be routedto the reservoir and the remaining deposited on theway to thereservoir We therefore restricted the calculation to nutrientsexported to the reservoir from the entire catchment whileextrapolating the result of this study

8 ISRN Soil Science

Table 4 Replacement costs for erosion-associated nutrients exported as N and P from the landforms and the entire catchment

LandformN

exporteda(kg haminus1 yminus1)

Convertedto urea

(kg haminus1 yminus1)

Replacementcost as urea

(ETB haminus1 yminus1)

P exported(kg haminus1 yminus1)

Convertedto DAP


Replacementcost as DAP(ETBhaminus1 yminus1)

TotalRCbC


Total RC perlandformd

(ETB yminus1)Rolling hills 423 920 2952 003 016 057 3009 3761Mountainous 1036 2252 7232 008 043 155 7386 13443Central ridge 866 1883 6052 009 047 171 6222 19910Valley 1156 2513 8071 010 050 181 8252 20795Plateau 392 852 2733 002 011 039 2772 3354Escarpment 711 1546 4958 004 020 073 5031 12074Reservoir 1774 3857 12379 014 067 240 12619 156476Average 908 1975 6339 007 036 131 6470 32830DAP diammonium phosphate ETB currency Ethiopian Birr 17 ETBsim C 1 in May 2010 on averageaPart of the N exported is converted to the N in DAPbRC replacement cost for both N and P exported as urea and DAPcThe price of each 100 kg of urea and DAP in 2010 crop season was 320 and 360 ETB respectivelydColumn 8 multiplied by the area of each landform which is found in Table 2

The total replacement cost as urea and DAP forthe soil nutrients exported (N and P) from the erosionsource areas of the entire study catchment to the reser-voir was 126 ETBhaminus1 yminus1 (C740 in May 2010) This valuewas followed by the valley landform where the cost was8250 ETBhaminus1 yminus1 (C500)The lowest total replacement costfor the same nutrients exported to the outlet of the plateaulandform was 2770 ETBhaminus1 yminus1 (C160) followed by therolling hills of 3000 ETB haminus1 yminus1 (C180) (Table 4) Theaverage replacement cost for all landforms including thereservoir was 6470 ETBhaminus1 yminus1 (C380) which was lowerthan the replacement cost for the reservoir

The replacement costs estimated for rolling hills plateauand escarpment landforms in this study are consistent withthe costs estimated by FAO [33] for cropped areas as theyare within the range of 29 to 44 ETBhaminus1 yminus1 (C170ndash260in May 2010) But when compared to the replacement costsquantified for the other landforms the report by FAO [33]underestimated the cost that replaces the nutrients exported(N and P) as a result of erosion Alternatively the replacementcost in the remaining landforms including the reservoirwas within the range of the replacement costs estimatedfor the 13 reservoir catchments in the Tigray region byHaregeweyn et al [13] regardless of the differences in theforeign currency exchange rate across years

The total nutrient replacement cost for the entirestudy catchment (Mai-Negus) was calculated using the costrequired to replace the nutrient exported to the reservoirmultiplied by the catchment area of 1240 ha that is perannum 156476 of ETB (C9204 in May 2010) Extrapolationthe nutrient replacement cost to the Tigray regional statewith a total area of 53000 km2 ranged from 632 million ETB(C372 million) to 700 million ETB (C412 million) and forthe Ethiopian highlands that covers 490000 km2 is estimatedfrom 62 billion ETB (C364 million) to 70 billion ETB (C412million) which are quite impressive figures for researchersplanners and decision and policy makers to come up with

suitable erosion controlling measures The replacement costfor the Ethiopian highlands accounts for about 8 of theEthiopian national annual budget in 201011 which is almosttwice the Tigray region (northern Ethiopia) annual budgetThe above-mentioned replacement costs will become higherif other nutrient losses (eg OC cations) and the losses inphysical soil attributes are included In general the results ofthis study can be considered as an indication of the need toimplement well-defined and appropriate land use redesignand soil and water management practices in order to reducecosts related to nutrient replacement while maintainingsustainable soil productivity in the landforms The use ofinorganic fertilizer as part of the soil management practicesis indispensable to replace such nutrients losses Howeverthis has to be complemented with organic amendments asorganic matter sources are locally available It is also nowwidely recognized among experts and policy makers that theincreasing application of fertilizer at the current prices is notaffordable by most farmers and possibly by the governmentand requires foreign exchange to import fertilizers

4 Conclusions

This study showed that the level of soil degradation rangedfrom 30 to 80 in the source soils (eroded site) whencompared to the stable sites by the effect of soil erosion Butthe severity of degradation was higher in the central ridgeand mountainous landforms which are the main sources ofsediment to downstream landformsThenutrient export ratesof OC TN Pav K Ca and Mg showed variability amongthe landforms even though significantly higher values wereexported into the reservoir A strong and positive relationshipbetween nutrient export rates and sediment yields alsoindicated that erosion plays the prime role for nutrient exportinto the aggrading sites in the landforms In addition thisstudy indicated that an annual nutrient replacement costvaried across the landforms with the highest in the reservoir

ISRN Soil Science 9

followed by the valley mountainous and central ridge land-forms This study thus supports in identifying and prioritiz-ing landforms which are sources of higher nutrient exportrates and thereby higher soil degradation while introducingremedial technologiesThe technologies should be targeted toimproving soil resistance to erosion and retaining the soil inplace just after detachment by raindrops and before transportby runoff from the erosion source areas in the landforms ofthe study catchment

Acknowledgments

The authors gratefully acknowledge the financial support byDAADGTZ (Germany) through the Center for Develop-mentResearch (ZEF)University of Bonn (Germany) and thesupport of Aksum University (Ethiopia) for the first authorrsquosfield work The authors also greatly appreciate the assistanceoffered by the local farmers and extension agents during thefield study

References

[1] UNEP (United Nations Environment Program) amp UNESCO(United Nations Educational Scientific and Cultural Organi-zation) Provision Map of Present Land Degradation Rate andPresent State of Soil FAO Rome Italy 1980

[2] H Eswaran R Lal and P F Reich ldquoLand degradation anoverviewrdquo in Response to Land Degradation E M Bridges I DHannam L R Oldeman F W T Penning de Vries J S Scherrand S Sombatpanit Eds pp 20ndash35 Science Publisher EnfieldNH USA 2001

[3] H Hurni ldquoLand degradation famine and land resource sce-narios in Ethiopiardquo in World Soil Erosion and Conservation DPimentel Ed pp 27ndash62 Cambridge University Press 1993

[4] W Mekuria E Veldkamp M Haile J Nyssen B Muys andK Gebrehiwot ldquoEffectiveness of exclosures to restore degradedsoils as a result of overgrazing in Tigray Ethiopiardquo Journal ofArid Environments vol 69 no 2 pp 270ndash284 2007

[5] P L G Vlek Q B Le and L Tamene ldquoAssessment of landdegradation its possible causes and threat to food security inSub-Saharan Africardquo in Advances in Soil Sciences Food Securityand Soil Quality R Lal and B A Stewart Eds pp 57ndash86 CRCPress Boca Raton Fla USA 2010

[6] P Dubale ldquoSoil and water resources and degradation factorsaffecting productivity in Ethiopian highland agro-ecosystemsrdquoNortheast African Studies no 8 pp 27ndash52 2001

[7] S Damene L Tamene and P L G Vlek ldquoPerformance ofexclosure in restoring soil fertility a case of Gubalafto districtin North Wello Zone northern highlands of Ethiopiardquo Catenavol 101 pp 136ndash142 2013

[8] L Tamene Reservoir siltation in the drylands of northernEthiopia causes source areas and management options [PhDthesis] Center for Development Research University BonnBonn Germany 2005

[9] W D Ellison ldquoFertility ErosionrdquoThe Land vol 9 p 487 1950[10] G M Hashim K J Coughlan and J K Syers ldquoOn site nutrient

depletion an effect and a cause of soil erosionrdquo in Soil Erosionat Multiple Scales Principles and Methods For Assessing Causesand Impacts FW T Penning de Vries F Agus and J Kerr Edspp 207ndash221 CAB International 1998

[11] J J Stoorvogel and E M A Smaling ldquoAssessment of soilnutrient depletion in Sub-Saharan Africa 1983ndash2000rdquo Report28 DLO Winand Staring Center for Integrated Land Soil andWater Research Wageningen The Netherlands 1990

[12] UNDP (United Nations Development Programme) TechnicalNote on the Environment DCGE Addis Ababa Ethiopia 2002

[13] N Haregeweyn J Poesen J Deckers et al ldquoSediment-boundnutrient export from micro-dam catchments in NorthernEthiopiardquo Land Degradation and Development vol 19 no 2 pp136ndash152 2008

[14] J R Stone JWGilliamDKCassel R BDaniels L ANelsonandH J Kleiss ldquoEffect of erosion and landscape position on theproductivity of Piedmont soilsrdquo Soil Science Society of AmericaJournal vol 49 no 4 pp 987ndash991 1985

[15] W R Kreznor K R Olson W L Banwart and D L JohnsonldquoSoil landscape and erosion relationships in a northwestIllinois watershedrdquo Soil Science Society of America Journal vol53 no 6 pp 1763ndash1771 1989

[16] E Elias S Morse and D G R Belshaw ldquoNitrogen and phos-phorus balances of Kindo Koisha farms in southern EthiopiardquoAgriculture Ecosystems and Environment vol 71 no 1ndash3 pp93ndash113 1998

[17] I Scoones Ed Dynamics and Diversity Soil Fertility andFarming Livelihoods in Africa Case Studies From Ethiopia Maliand Zimbabwe Earthscan Publications Ltd London UK 2001

[18] S K Papiernik T E Schumacher D A Lobb et al ldquoSoilproperties and productivity as affected by topsoil movementwithin an eroded landformrdquo Soil and Tillage Research vol 102no 1 pp 67ndash77 2009

[19] G W Gee and J W Bauder ldquoParticle-size analysisrdquo inMethodsof Soil Analysis Part 1 A Klute Ed pp 383ndash411 AmericanSociety ofAgronomy Soil Science Society ofAmericaMadisonWis USA 2nd edition 1986

[20] G R Blake and K H Hartge ldquoBulk Densityrdquo inMethods of SoilAnalysis Part 1 A Klute Ed vol 9 of Agronomy Monographpp 363ndash375 American Society of Agronomy Madison WisUSA 2nd edition 1986

[21] J M Bremmer and C S Mulvaney ldquoNitrogen totalrdquo inMethodof Soil Analysis Part 2 Chemical andMicrobiological PropertiesA L Page Ed vol 9 of Agronomy Monograph pp 595ndash624American Society of Agronomy Madison Wis USA 1982

[22] S R Olsen and L E Sommers ldquoPhosphorusrdquo in Method ofSoil Analysis Part 2 Chemical and Microbiological PropertiesA L Page Ed vol 9 of Agronomy Monograph pp 403ndash430American Society of Agronomy Madison Wis USA 1982

[23] J M Anderson and J S I Ingram Tropical Soil Biologyand Fertility A Handbook of Methods CAB InternationalWallingford UK 1993

[24] J D Rhoades ldquoCation exchange capacityrdquo in Methods of SoilAnalysis Part 2 A L Page R H Miller and D R Keeney Edsvol 9 of Agronomy Monograph pp 149ndash157 American Societyof Agronomy Madison Wis USA 1982

[25] T C Baruah and H P Barthakur A Text Book of Soil AnalysisVikas Publishing House New Delhi India 1999

[26] G Verstraeten and J Poesen ldquoRegional scale variability in sedi-ment and nutrient delivery from small agricultural watershedsrdquoJournal of Environmental Quality vol 31 no 3 pp 870ndash8792002

[27] G B Tesfahunegn ldquoSoil erosion modeling and soil qualityevaluation for catchment management strategies in northernEthiopiardquo Ecology and Development Series No 83 Center for

10 ISRN Soil Science

Development Research University of Bonn Bonn Germany2011

[28] J Bojo ldquoThe costs of land degradation in Sub-Saharan AfricardquoEcological Economics vol 16 no 2 pp 161ndash173 1996

[29] SPSS (Statistical Package for Social Sciences) Statistical PackageFor Social Sciences Release 180 SPSS Inc 2011

[30] A Haileslassie J Priess E Veldkamp D Teketay and JP Lesschen ldquoAssessment of soil nutrient depletion and itsspatial variability on smallholdersrsquo mixed farming systems inEthiopia using partial versus full nutrient balancesrdquoAgricultureEcosystems and Environment vol 108 no 1 pp 1ndash16 2005

[31] J Nyssen J Poesen J Moeyersons M Haile and J DeckersldquoDynamics of soil erosion rates and controlling factors in theNorthern Ethiopian highlandsmdashtowards a sediment budgetrdquoEarth Surface Processes and Landforms vol 33 no 5 pp 695ndash711 2008

[32] B K Amegashie Assessment of catchment erosion sedimenta-tion and nutrient export into small reservoirs from their catch-ments in the upper east region of Ghana [MS thesis] KwameNkrumah University of Science and Technology KumasiGhana 2009

[33] FAO (Food and Agriculture Organization of the UnitedNations) ldquoEthiopian highland reclamation studyrdquo Final ReportFAO Rome Italy 1986

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental and Public Health

Journal of



EcosystemsJournal of


MeteorologyAdvances in

EcologyInternational Journal of


Marine BiologyJournal of


Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Advances in


Environmental Chemistry

Atmospheric SciencesInternational Journal of



Waste ManagementJournal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics


Geological ResearchJournal of

EarthquakesJournal of


BiodiversityInternational Journal of


ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


ClimatologyJournal of

2 ISRN Soil Science

Table 1 Biophysical description of the landforms in the Mai-Negus catchment northern Ethiopia

Landform Areae () Land-use cover ()a Lithology bSlope degc Elevation md

Arable Grazing Bush and Woodf Othersg BM LP STRolling hills 100 80 10 4 6 100 na na 3ndash16 2150ndash2240Mountainous 145 36 34 26 4 35 65 na 4ndash79 2350ndash2650Central ridge 255 70 13 5 12 5 95 na 3ndash25 2230ndash2450Valley 199 65 31 2 2 77 23 na 0ndash6 2070ndash2100Plateau 98 47 16 30 7 58 42 9 3ndash10 2500ndash2550Escarpment 191 47 31 14 5 84 7 na 3ndash30 2270ndash2540Reservoir 12 na na na na 100 na na 0 2060ndash2080BM basic metavolcanics LP lava pyroclastic ST sandstone na not applicableaThe proportion of the land covers was derived from a Landsat image of November 2007 overlaid by the landform mapbThis was derived by overlaying the landform map with the geology map of EthiopiacDeveloped from digital elevation model (DEM)dDerived from DEMeTotal catchment area is 1240 km2 and reservoir area is 015 km2fIncludes closed area plantations and natural vegetationgIncludes settlements rock-out crop and marginalized areas

delivery processes which are responsible for high sedimenttransport and the associated export of sediment-boundnutrients to deposition areas in a catchment are influencedby landscape characteristics [13] In line with this studieson nutrient balances in Ethiopia indicate that the balance atfarm level is more positive than plot level [11 16 17] whilelosses from some fields may be of benefit to other fields Thismay be attributed to the effect of nutrient redistribution byerosiondeposition processes or to active nutrient transfers byfarmers

Studies of erosion-induced changes in soil properties atthe field scale have constantly shown that soil texture surfacesoil organic carbon nitrogen and phosphorus concentra-tions are higher in areas of soil deposition compared to areasof soil removal by erosion [18] Assessing the effect of erosionon soil properties variability as a result of nutrient exportfrom catchment landforms can provide more meaningfulresults This study aims to (1) assess soil nutrient depletionand soil physical degradation comparing the eroded andstable-sites soil (2) examine the rate of soil nutrient exportconsidering the aggrading and eroded erosion-status sitesin the landforms of the Mai-Negus catchment northernEthiopia and (3) quantify the costs of soil degradationassociated with nutrient losses through water erosion atlandform and catchment scale and extrapolate the findingsto the Tigray region and highlands of the country

2 Materials and Methods

21 Study Site This study was conducted in the Mai-Negus catchment in the Tigray region of northern Ethiopia(Figure 1)The catchment has an area of 1240 ha and altitudesranging from 2060 to 2650m asl It has a mean annualtemperature of 22∘C and precipitation of 700mm Most ofthe rainfall (gt70) occurs in July and August Land useis predominantly arable with teff (Eragrostis tef ) being themajor crop along with different-sized areas of pasture landand scattered patches of trees bushes and shrubs The major

rock types are lava pyroclastic and metavolcanic Soils aremainly leptosols on very steep positions cambisols onmiddleto steep slopes and vertisols on flat areas

22 Terrain Assessment In this study field reconnaissancesurveys were carried out to gain an overall image of thecatchment characteristics (Table 1) Data were collected fromJune to December 2009 The landforms in the catchment(Figure 1) were developed in ArcGIS software using fieldsurvey-based data in combination with the information fromthe topographic map Considering elevation slope geologyand geomorphologic characters (surface flows alluvial andcolluvial deposition) the catchment topography is classifiedinto six main landforms (Figure 1) namely valley (19 of thecatchment area) plateau (8) rolling hills (9) central ridge(27) escarpments (29) and mountainous (6)

The reservoir was considered a separate landform in thecatchment and is located at the toeslope of the catchmentSediment deposition in the reservoir was used to estimatethe level of soil degradation nutrient export rate and theassociated replacement costs from the entire catchmentDeposition sites in the other landforms of the catchment wereused to assess soils deposited on the way to the reservoirafter being transported from the original place towardsthe outlets of the landforms Generally during the fieldsurvey three sites namely depositionaggrading stable anderoded erosion-status sites in the six landforms of the studycatchment were identified using standard geomorphologicalprocedures of erosion-deposition indicators

23 Erosion-Status Site Selection and Soil Sampling PointsRepresentative erosion-status sites (aggrading stable anderoded) and the corresponding soil sampling points (Figure 1)were selected in four steps A reconnaissance survey onland use lithology soil types slopes and elevation rangesof the landforms (Table 1) followed by informal discussionswith farmers and development agents to gain insight onland use history as well as land- and crop-management

ISRN Soil Science 3

Tigray

Southern

Easternzone

zone

zone

zone

Central

North-western

Western zone

Mekelle

(b) Tigray

N

N

(a) Ethiopia




Land forms

Reservoir



N

45∘09984000998400998400E40

∘09984000998400998400E35

∘09984000998400998400E

45∘09984000998400998400E40

∘09984000998400998400E35

∘09984000998400998400E

5∘09984000998400998400N

10∘09984000998400998400N

5∘09984000998400998400N

10∘09984000998400998400N





4 ISRN Soil Science




NE119903

=


SM = SV lowast dBD

NE119897


)

SSY = SM119860 lowast 119884

(1)

where NE119903


119897



N

0 200(km)

1560500 1560600 1560700 1560800

1560500 1560600 1560700 1560800

463000

463100

463200

463300

463400

463000

463100

463200

463300

463400




1 + 00021119863 (119862119860)

) (2)




ISRN Soil Science 5





SDR119894

= (

HD119894

SL119894

) 10and

(3)

where SDR119894



119894






2

O5


6 ISRN Soil Science

0

10

20

30

40

50

60

70

80

90

100


Soil

attr

ibut

es d

egra

datio

n (

)

LandformSiltClayExK

ExCaExMgCEC

OCTNPav

TPFeZn







ISRN Soil Science 7











8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














ISRN Soil Science 3

Tigray

Southern

Easternzone

zone

zone

zone

Central

North-western

Western zone

Mekelle

(b) Tigray

N

N

(a) Ethiopia




Land forms

Reservoir



N

45∘09984000998400998400E40

∘09984000998400998400E35

∘09984000998400998400E

45∘09984000998400998400E40

∘09984000998400998400E35

∘09984000998400998400E

5∘09984000998400998400N

10∘09984000998400998400N

5∘09984000998400998400N

10∘09984000998400998400N





4 ISRN Soil Science




NE119903

=


SM = SV lowast dBD

NE119897


)

SSY = SM119860 lowast 119884

(1)

where NE119903


119897



N

0 200(km)

1560500 1560600 1560700 1560800

1560500 1560600 1560700 1560800

463000

463100

463200

463300

463400

463000

463100

463200

463300

463400




1 + 00021119863 (119862119860)

) (2)




ISRN Soil Science 5





SDR119894

= (

HD119894

SL119894

) 10and

(3)

where SDR119894



119894






2

O5


6 ISRN Soil Science

0

10

20

30

40

50

60

70

80

90

100


Soil

attr

ibut

es d

egra

datio

n (

)

LandformSiltClayExK

ExCaExMgCEC

OCTNPav

TPFeZn







ISRN Soil Science 7











8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














4 ISRN Soil Science




NE119903

=


SM = SV lowast dBD

NE119897


)

SSY = SM119860 lowast 119884

(1)

where NE119903


119897



N

0 200(km)

1560500 1560600 1560700 1560800

1560500 1560600 1560700 1560800

463000

463100

463200

463300

463400

463000

463100

463200

463300

463400




1 + 00021119863 (119862119860)

) (2)




ISRN Soil Science 5





SDR119894

= (

HD119894

SL119894

) 10and

(3)

where SDR119894



119894






2

O5


6 ISRN Soil Science

0

10

20

30

40

50

60

70

80

90

100


Soil

attr

ibut

es d

egra

datio

n (

)

LandformSiltClayExK

ExCaExMgCEC

OCTNPav

TPFeZn







ISRN Soil Science 7











8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














ISRN Soil Science 5





SDR119894

= (

HD119894

SL119894

) 10and

(3)

where SDR119894



119894






2

O5


6 ISRN Soil Science

0

10

20

30

40

50

60

70

80

90

100


Soil

attr

ibut

es d

egra

datio

n (

)

LandformSiltClayExK

ExCaExMgCEC

OCTNPav

TPFeZn







ISRN Soil Science 7











8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














6 ISRN Soil Science

0

10

20

30

40

50

60

70

80

90

100


Soil

attr

ibut

es d

egra

datio

n (

)

LandformSiltClayExK

ExCaExMgCEC

OCTNPav

TPFeZn







ISRN Soil Science 7











8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














ISRN Soil Science 7











8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














8 ISRN Soil Science


LandformN


Convertedto urea





Convertedto DAP



TotalRCbC








4 Conclusions


ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics














ISRN Soil Science 9


Acknowledgments


References








































Journal of












Volume 2014

Advances in









Geophysics


























Journal of












Volume 2014

Advances in









Geophysics














research article assessing sediment-nutrient export rate...

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