soil organic carbon stocks in soil aggregates under different land use systems in nepal

Soil organic carbon stocks in soil aggregates under different land usesystems in Nepal

B.M. Shrestha1,*, B.K. Sitaula2, B.R. Singh1 and R.M. Bajracharya3

1Department of Plant and Environmental Sciences, Agricultural University of Norway, P.O. Box 5003, N-1432Ås, Norway; 2Centre for International Environment and Development Studies (Noragric), AgriculturalUniversity of Norway, P.O. Box 5001, N-1432 Ås, Norway; 3Department of Biological and EnvironmentalSciences, Kathmandu University, P.O. Box 6250, Kathmandu, Nepal; *Author for correspondence (e-mail:[email protected]; fax: +47-6494-8211)

Key words: Land use, Mid-hill watershed, Nepal, Soil aggregates, Soil organic carbon, SOC stock

Abstract

We investigated the soil organic carbon �SOC� associated with various aggregate size fractions in soil profilesunder different land uses. Bulk soil samples were collected from incremental soil depths �0–10, 10–20, 20–40,40–60, 60–80 and 80–100 cm� from sites with the four dominant land use types �forest, grazing land, irrigatedrice in level terraces �Khet� and upland maize-millet in sloping terraces �Bari�� of the Mardi watershed �area 144km2�, Nepal. The bulk soil was separated into five aggregate size fractions and the associated SOC contents weredetermined. Soil physical parameters necessary for estimating the soil SOC stock such as bulk density, stone andgravel content, and SOC content, were also measured for each soil depth. The SOC stock �mean � SE, kg Cm–2� in the topsoil �0–10 cm� was higher in grazing land soil �3.4 � 0.1� compared to forest soil �1.4 � 0.2�and cultivated soil �Bari �2.0 � 0.2� and Khet �1.2 � 0.2��. Forest and grazing lands had similar SOC contents,but the higher content of gravel and stone in forest soil resulted in a lower estimate of the SOC stock per unitarea. The total SOC stock in the soil profile �to 1 m depth� over the entire watershed was estimated to be 721470TC �tonnes of carbon�. Its distribution was 52, 30, 11 and 7% in forestland, Bari, grazing land and Khet, respec-tively. The estimated depth wise distribution of SOC stock for 1 m soil depth in the entire watershed was 28, 22,28 and 22% in the 0–10, 10–20, 20–40, and � 40 cm soil depths, respectively. There was a net loss of SOCstock �0–40 cm soil depth� of 29%, due to internal trading of land uses in the period from 1978 to 1996. Macroaggregates � � 1 mm� were found to be the dominant size in Bari and grazing land, whereas in forest and Khetsoil micro aggregates � � 1 mm� dominated. Micro aggregates of size � 0.25 mm had a higher SOC concen-tration than aggregates of 0.25–0.5 mm, regardless of the depth or land uses and they may therefore contribute tosoil C sequestration.

Introduction

Soil organic carbon �SOC� content exhibits consider-able spatial variability both according to land use andsoil depth. The SOC generally diminishes with depthregardless of vegetation, soil texture, and clay sizefraction �Trujilo et al. 1997�. Soils of the world arepotentially viable sinks for atmospheric carbon �C�and may significantly contribute to mitigation of glo-bal climate change �Lal et al. 1995, 1998; Bajracharya

et al. 1998a; Singh and Lal 2004�. However, the as-sessment of potential C sequestration in soil requiresthe estimation of C pools under existing land uses,and of the distribution of C in the soil profile.

A number of factors and processes operating in thewatershed influence the C pools and fluxes, the mostsignificant being land uses, land use changes, soilerosion and deforestation. Removal of trees from theforest displaces a large amount of sequestered carbon�IPCC 2000� and consequently reduces the SOC held

Nutrient Cycling in Agroecosystems 70: 201–213, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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in soil profiles �Glaser et al. 2000�. The impact of de-forestation on SOC decrease is more pronounced inthe upper soil layer �Sombroek et al. 1993�. Gradualconversion of forest and grassland to cropland has re-sulted in historically significant losses of soil carbonworldwide �Lal 2002�. Minimizing the disturbance oftopsoil increases C accumulation, while a high inten-sity and frequency of cultivation leads to lower SOC�Bajracharya 2001�. Martin et al. �1990� reported a33–45% decrease in SOC after 5 years of continuouscropping while Hao et al. �2001� found that SOC inthe upper 30 cm depth was significantly affected bytillage. Both total soil nitrogen and SOC were greaterunder minimum tillage than under conventional till-age.

Land use and management affects the SOC stockin the soil profile. For example, a study in grazingland by Franzluebber et al. �2001� showed that totalC �down to 60 cm depth� was significantly higher insoils with heavy grazing than with light or no graz-ing.

Several global studies and models have been pro-posed to evaluate SOC storage and dynamics, in aneffort to provide better guidelines for soil manage-ment. In the global change context, soil carbonsequestration through aggregation is an important as-pect of better soil management. The SOC in microaggregates is believed to be protected from degrada-tion and hence relevant for C sequestration �Bajra-charya et al. 1998b�. Better understanding of theeffect of soil management on the distribution andforms of SOC in different soil aggregate size-fractions and at different depths within the soil pro-file could contribute to improved understanding andmodeling of C dynamics and sequestration.

There are few, if any, studies with direct focus onestimating the SOC stocks in the entire soil profileunder different land use categories in the Himalayanregion. This study was conducted with the aim ofquantifying the amount and distribution of SOCwithin the soil profile under dominant land use cat-egories in a mid-hill watershed of western Nepal. Thespecific objectives were to �i� estimate SOC stocks inthree dominant land uses, and �ii� to quantify therelative distribution of SOC in different aggregatesize fractions.

Materials and methods

Study area

The Mardi watershed �83°50� E to 83°56� E and28°19� N to 28°29� N; area 144 km2; Figure 1� repre-sents the mid-hill watershed of western Nepal. Theelevation ranges from 915 m to 5588 m above meansea level. The climate varies with altitude from warmand humid subtropical in the valley floor to cool anddry alpine on the mountain peaks. The mean annualtemperature in the valley floor ranges from a maxi-mum of 26 ºC in July to a minimum of 13 ºC inJanuary. The mean annual temperature on the ridge is16 ºC with a maximum of 20.2 ºC in August and aminimum of 9 ºC in January. Rainfall is monsoonalwith an annual average of 4300 mm. Major soil typesfound in the area are Luvisol, Cambisol, Rigosols,and Fluvisols while Entisols are the predominant typeof soils in the valleys �Awasthi et al. 2002; Thapa1996�.

Land uses of the watershed include forest �62%�,cultivation �22%� and grazing �5%�. The vegetationtype ranges with altitude from Schima-Castanopsismixed forest type to Daphiniphyllum, Quercus andRhododendron mixed type. Dominant agriculturalpractices are irrigated rice �Khet� and upland maize-millet �Bari� cultivation. Agricultural rotation is usu-ally maize-millet-fallow in Bari, and paddy-wheat-fallow or paddy-fallow-maize in Khet �Gurung 2000�.Farmyard manure and compost made from forestproducts collected for animal bedding are the tradi-tional nutrient supply to crops. In addition chemicalfertilizers have been in use since the 1980s �Poudeland Thapa 2001�. In common to other mid-hillregions �Bajracharya 2001� land is prepared for cul-tivation manually or by using a wooden plow withoxen.

Sampling site, soil sampling, processing andanalysis

Four sites were chosen to represent each of the domi-nant land use types. Each site comprised four plots�10 m2 each�. Soil sampling pits were excavated atrepresentative points within each plot �Table 1�. Soilsamples from each plot were collected from an areaof 0.5 m � 0.5 m for each incremental depth. Thedepth increment was 10 cm for the upper 20 cm, and20 cm for the remaining of the profile, down to 1 mor bedrock. About 1.5–2.0 kg of fresh soil was col-

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lected from each depth, kept in polythene packs andtransported to the laboratory for further processing.

To prepare for bulk soil SOC determination,air-dried sub-samples were passed through a 2 mmmesh. A further sub-sample of 200 g was separatedby sieving into five different size classes of diameter2–5, 1–2, 0.5–1, 0.25–0.5 and � 0.25 mm. Processedsamples were transported to the soil lab for furtherchemical analysis. Basic soil parameters such as soilorganic carbon �SOC�, bulk density �BD�, gravimet-ric moisture content �GMC� and soil reaction �pH�

were determined �Table 7�. The SOC was determinedby the colorimetric method �Anderson and Ingram1993�, BD by the core ring method �Blake and Hartge1986�, GMC by oven drying, and pH was determinedin a 1:2.5 soil/water ratio �McLean 1982�.

Calculation and analysis

The soil carbon stock was calculated using the fol-lowing equation �De Wit and Kvindesland 1999�;

Figure 1. Location map of the study area �Source: Awasthi et al. 2002�.

Table 1. Site description.

SN Land use Coordinates Altitude �m� Vegetation/crop rotation

1 Khet 28º19.7� N, 83º53.4� E 1120 Paddy–wheat–fallow2 Forest 28º19.4� N, 83º52.4� E 1219 Schima-Castanopsis forest3 Bari 28º19.2� N, 83º52.3� E 1550 Maize–millet–fallow4 Grazing 28º20.1� N, 83º51.6� E 1966 Grass and shrubs

Khet = lowland irrigated rice field, Bari � upland rainfed maize-millet field.

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Carbon stock = d × BD × SOC-content × CFst

where Carbon stock �kg/m2�, d: depth of horizon �m�,BD: bulk density �kg/m3�, SOC-content �g/g� andCFst: correction factor for stone and gravel content:

CFst= 1 � �%stone +%gravel�/100

The Carbon stock in each depth-layer was esti-mated for each of the dominant land use types of thewatershed by multiplying the SOC stock in each unitarea �kg C m–2� by the area covered. The sum of theSOC stock in each horizon gave the SOC stock ineach land use type in the watershed. The effect of landuse change on the SOC stock was estimated by tak-ing account of the net change in area under differentland uses during an 18-year period from 1978 to 1996�Awasthi et al. 2002� in the same watershed. Thechange in average SOC stock per unit area �0–40 cmsoil depth� found in this study was multiplied by thenet change in area �taking account of internal tradingbetween land uses� to estimate the net change in SOCstock due to land use changes. The total SOC in eachaggregate size per unit area was calculated by usingits relative presence and its % SOC for each depth ofthe soil.

The data were analyzed using SAS software �SASSoftware Inc. 2000�. The effect of land use and depth

on different soil parameters was analyzed by the gen-eral linear model procedures. Multiple comparison ofmeans for each class variable �among land uses, soildepth, aggregate size classes� was carried out usingthe Student-Newman-Keuls �SNK� test at � � 0.05.The variability in measured data was evaluated interms of range and coefficient of variation �CV�.

Results and discussion

Soil organic carbon stocks: effect of land use andsoil depth

Both land use �P � 0.0001� and soil depth �P �0.01� had a significant effect on the SOC stocks in thesoil. Multiple comparison of means �SNK, � � 0.05�revealed that the SOC stock in Bari land �2.5 � 0.2kg C m–2� was higher than in other land use types�Figure 2�. This could be attributed to high inputs offarmyard manure and agricultural residues in the Bariland. The Bari lands are well managed by farmers, interms of the soil organic matter �SOM� supply beingcloser to the farm houses than in Khet lands �Pilbeamet al. 2000; Neupane and Thapa 2001�. In the surfacesoil, grazing land had the highest SOC stock,followed by Bari, forest and Khet, while in depthsbelow 20 cm Bari lands had the highest SOC stock.

Figure 2. Soil organic carbon stock in different depths under various land use types. Khet = lowland irrigated rice field, Bari � uplandrainfed maize-millet field.

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In spite of the depth-wise decreasing trend of the SOCstock, Bari soils have somewhat more stock in the60–80 cm soil layer than in the 40–60 cm layer �Table3�. This may be due to translocation of C in the formof dissolved organic carbon �DOC�, soil faunal activ-ity �especially earthworms�, and/or the effects ofdeep-rooting crops. In Khet soil, however, the SOCstock was significantly higher in the 0–10 cm soillayer compared to lower depths. This could be attrib-uted to waterlogged conditions, leading to shallowrooting and confinement of biological activity to theupper soil layer. The decrease in the SOC stock withsoil depth was more pronounced in grazing land �P� 0.0005� compared to other land use systems. Asignificantly higher SOC stock was observed in thetopsoil than in other depths within the profile of thesame land use �Figure 2�.

Storage profile

In general, the estimated SOC stock was higher in thetopsoil �0–10 cm� compared to lower depths in thevarious land use types �Table 3�. In Khet soil 43% ofthe total SOC stock was in the 0–10 cm layer whilevalues of 34% in forest, 31% in grazing land and 13%in Bari were observed. In most cases, the SOC stockdecreased down to a depth of 20 cm, but increasedagain slightly in the third layer �20–40 cm�. Such a

trend was likely to be due to mixing �either by culti-vation or by soil fauna under natural vegetation� ofthe upper soil layer and leaching and illuviation in theB-horizon, which usually occurred at a depth of20–30 cm. A similar result was reported by Manjaiahet al. �2000� in a maize-wheat cropping managementsystem in India. Soil management activities that ma-nipulate soil fauna affect SOM dynamics �Frenandeset al. 1997�. Earthworm activity can increase soilstructure stability and the storage of soil C and N insoil �Ketterings et al. 1997�.

The SOC stock in forest soil was found to be nearlyuniform throughout the profile, while for grazing landa marked decrease in SOC stock by 35% from thefirst to the second layer was noted �Table 3�.Non-uniform C distributions within the profile ofgrazing soil �as seen by 20% increase from second tothird layers, and the 41% decrease from third tofourth layers� is likely to reflect the interaction ofcomplex biophysical and chemical processes involv-ing rooting, microbial and faunal activities. Soil faunainfluence long-term soil carbon balances by affectinghumus and litter decomposition rates �Andrén et al.2001�.

Stones and gravels are predominant in the hill soilsand have an important implication in SOC stock es-timation. It is often neglected, but important, to takeinto account the stone and gravel content to estimate

Table 2. Soil organic carbon stock �kg C m–2� in different depths of various land use types.

Soil depth �cm� Land use type Mean pH SOC stock �kg C m–2�

Min Max Mean SD CV �%�

0–10 Forest 4.7 1.0 1.8 1.4 0.3 23Grazing 4.5 3.3 3.5 3.4 0.1 4Khet 6.4 0.6 1.5 1.2 0.4 34Bari 5.1 1.5 2.4 2.0 0.4 18



40–60 Grazing 5.0 0.4 2.4 1.5 0.8 57Khet 6.8 0.2 0.9 0.5 0.5 88Bari 5.2 1.6 4.2 2.9 1.3 45


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actual SOC stock in the soil. For example, if stoneand gravel content is neglected, estimated SOC stockper unit area is 41% more for Bari and 100% morefor forest soil than we reported in this paper.

Estimation of SOC stock in the watershed

The total SOC stock in the soil profile �1 m depth� inthe entire watershed was estimated to be 721 470 TC.The SOC stock was estimated to include 379 764 TC�52% of the total� in forest land �total area � 9042ha�, 77 687 TC �11%� in grazing land �722 ha�,215 873 TC �30%� in Bari �1345 ha� and 48 146 TC�7%� in Khet �1810 ha� �Table 3�. The data indicatethat a considerable portion of land was occupied bynatural vegetation with a reserve of 63% of the totalSOC stock. Cultivated land made up 37% of the landarea of the watershed, of which Bari soil contained82% and Khet soil 18% of the SOC stocks. Withinthe natural vegetation area, forest comprised 83% oftotal SOC stocks and grazing land 17%. Thus, theMardi watershed can be considered to have fairly lowanthropogenic influences compared to the nearbyPokhara valley �Gurung 2000�.

The estimated SOC stock for the watershed wasfound to be 28, 22, 28 and 22% in the 0–10, 10–20,

20–40 and � 40 cm soil depths, respectively. This isconsistent with the observed decreasing trend in SOCstock with soil depth for all the land uses. Forestshares the higher SOC stock compared to other landuse types in each depth. This is due to both the SOCcontent and the area covered by the forest.

Effects of land use change on SOC stock

The conversion of forestland into Khet land may re-sult in 49% losses of SOC compared to SOC level inthe forest. On an area basis, the SOC losses were 2kg C m–2 by converting forest to Khet. Studies con-ducted in parts of the tropics and sub-tropics found a20 to 50% loss of carbon in the topsoil after forestclearance and conversion to farmland �Sombroek etal. 1993�. The conversion of forest to Bari and graz-ing land may result in a net gain of 2.6 and 4 kg Cm–2 �assuming all other factors remain the same�. Theestimated effects of other internal land use changesare shown in Table 4. Internal trading of land usesduring the 18-year period from 1978 to1996 indicatednet loss of SOC stock by 29% in the Mardi water-shed. This value, however, is based entirely on theconversion of land use areas, assuming similar SOClevels as found in this study for given land use cat-

Table 3. Carbon stock in different soil profile depths of various land use types.

Land use system and area covered Soil depth �cm� Mean C pool �kg C m–2� C stock �TC�

Khet �1810 ha� 0–10 1.16 20 99610–20 0.42 7 60220–40 0.53 9 59340–60 0.55 9 955

Bari �1345 ha� 0–10 2.02 27 16910–20 1.67 22 46220–40 2.97 39 94640–60 2.91 39 14060–80 3.80 51 11080–100 2.68 36 046

Forest �9042 ha� 0–10 1.44 130 20510–20 1.24 112 12120–40 1.52 137 438

Grazing �722 ha� 0–10 3.35 24 18710–20 2.17 15 66720–40 2.61 18 84440–60 1.46 10 54160–80 0.68 4 91080–100 0.49 3 538

Total C stock 721 470


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egories for the entire watershed. The conversion offorest, grazing and Bari to Khet and the conversionof grazing land to other land uses may all attribute toSOC losses from the watershed over a period of time.Due to the fact that several relevant factors and pro-cesses �topographic, climatic and socio-economiclimitations� are ignored in such empirical estimates,the implication of this for the effects of future landuse changes should be interpreted with caution. Nev-ertheless, historical land use changes in the Mardiwatershed may have had a negative impact on theSOC stock. Converting other land uses to Khet mayresult in loss of SOC. The lower input of soil organicmatter �SOM� to Khet by farmers may be due to thefact that no crop residue is left behind after harvestand the land is mostly located rather far from thesource of farmyard manure �Poudel and Thapa 2001�.The Bari lands, having close proximity to the farm-yard, are well supplied with SOM by farmers. There-fore the conversion of forestland to Bari may notnecessarily decrease the SOC stock as compared tothe conversion of other land uses to the Khet land.

SOC in bulk soil and aggregate fractions: effect ofland use and depth

Bulk soilThe forest soil had the highest mean bulk soil SOCcontent �3%�, followed by Bari �2.5%�, grazing land�1.7%� and Khet �0.5%�. This result agrees with thatof Islam and Weil �2000�, who reported that the SOCin the soil of cultivated land was lower than that offorest and grassland. In Nepal, Burton et al. �1989�reported SOC contents of 1.9% in natural forest and2.0% in grazed forest, while in agricultural land SOCcontent ranged from 1.5 to 1.7% for different crop-ping systems. The lower SOC content in Khet soilprobably reflects continuous cultivation with minimalinputs of SOM �Poudel and Thapa 2001� and thehighly sandy texture of the alluvial soil, which has atendency to have a lower associated SOC content.

Farmers typically use more compost in Bari com-pared to Khet �Pilbeam et al. 2000; Neupane andThapa 2001� and, consequently, a higher SOC con-tent was observed in the former. The finding thathigher SOC in Bari land reflects the farmer’s agricul-tural practices of farmyard manure supplement fornutrient input corroborated with other studies �Kan-chikerimath and Singh 2001; Lal 2002�. Kanchikeri-math and Singh �2001� reported increased soilmicrobial biomass C from 122 �in unfertilized treat-

Table 4. Effect of land use changes in SOC stock �0-40 cm� in Mardi watershed.

Land use change Change in area �ha�a Change in SOC stock �0–40 cm�

From – to per unit area �kg C m–2�b watershed level �TC�c % change

Forest – Bari 26.9 2.6 699.4 63.4Forest � Khet 67 � 2 � 1340 � 48.8Forest – Grazing 8.6 4.1 352.6 100.0

Bari – Forest 128 � 2.6 � 3328 � 38.8Bari � Khet 19.4 � 4.6 � 892.4 � 68.7Bari – Grazing 25.7 1.5 385.5 22.4

Khet – Forest 63 2 1260 95.2Khet – Bari 4.6 4.6 211.6 219.0Khet – Grazing 7.7 6.1 469.7 290.5

Grazing – Forest 118 � 4.1 � 4838 � 50.0Grazing – Bari 4.3 � 1.5 � 64.5 � 18.3Grazing – Khet 26.6 � 6.1 � 1622.6 � 74.4

Total � 8706.7 � 29.4

aLand area change as estimated by Awasthi et al. �2002�; bEstimated difference in SOC stock due to the change in per unit area; cChange inSOC stock due to internal trading of area between the land uses for the entire watershed; Khet = lowland irrigated rice field, Bari � uplandrainfed maize-millet field.

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ments� to 331 mg/kg �in plots amended with 100%NPK and manure� in 26 years of a maize-wheat-cow-pea cropping experiment in India.

Comparison of the SOC in natural and cultivatedland gives some insight into the present managementeffect on SOC content. In the study area, people usemuch less organic manure in Khet compared to Bariland and usually leave bare fallow after harvesting ofrice, the main crop �Poudel and Thapa 2001�. Theycollect all the above-ground straw for animal feed

from the Khet, and hence the organic matter turnoverrate is lowered. The comparatively higher SOC con-tent in grazing land may be due to a lower decompo-sition rate �due to location at a higher altitude thanthe other sites�, low soil disturbance, greater root bio-mass and returns of vegetative residue as well ascattle dung and manure �Lal 2002�.

Table 5. Correlation between measured parameters.

Deptha BDb pHc GMCd SOC SOC1 SOC2 SOC3 SOC4

BD 0.34*PH � 0.007 0.59*GMC 0.03 � 0.36** � 0.39**SOCe � 0.53* � 0.84* � 0.64* 0.49*SOC1 � 0.56* � 0.77* � 0.55* 0.48* 0.94*SOC2 � 0.52* � 0.75* � 0.58* 0.55* 0.94* 0.96*SOC3 � 0.55* � 0.84* � 0.59* 0.48* 0.97* 0.96* 0.96*SOC4 � 0.53* � 0.86* � 0.61* 0.44** 0.96* 0.94* 0.94* 0.98*SOC5 � 0.53* � 0.85* � 0.92* 0.44** 0.96* 0.94* 0.93* 0.98* 0.98*

aSoil depth; bBulk density; cSoil pH; dGravimetric moisture content; eSoil organic carbon �SOC� content in bulk soil; 1, 2, 3, 4, 5 SOC contentin aggregate size classes of 2–5, 1–2, 0.5–1.0, 0.25–5.0 and � 0.25 mm, respectively; *, ** Significant at p � 0.0001, p � 0.001; Khet =lowland irrigated rice field, Bari � upland rainfed maize-millet field.

Table 6. Aggregate fractionation �%� in soil profile of different land uses.

Land use Soil depth �cm� Aggregate size class �mm�

2–5 1–2 0.5–1.0 0.25–0.5 � 0.25 Stone/gravel

Forest 10 19 10 12 21 6 3220 19 6 7 14 9 4540 15 7 8 20 4 46

Grazing 10 21 16 18 28 7 1020 20 20 19 17 17 740 29 16 17 19 12 760 14 21 22 32 6 580 4 14 29 32 12 9

100 4 7 18 42 23 6

Khet 10 6 10 29 30 24 120 4 10 28 34 21 340 1 8 30 40 19 260 1 3 27 45 16 8

Bari 10 20 18 12 14 6 3020 36 12 10 12 5 2540 33 13 12 8 8 2660 25 13 14 15 6 2780 66 6 6 11 6 5

100 59 9 9 13 4 6


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Figure 3. Soil organic carbon �mean � SE� distribution among soil aggregates in different soil depths. Khet = lowland irrigated rice field,Bari � upland rainfed maize-millet field.

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Aggregate size fractionsMacro aggregates �1–2 and 2–5 mm� were found tobe the dominant aggregates in Bari and grazing land,whereas forest and Khet soil had a dominance of mi-cro aggregates � � 0.25, 0.25–0.5 and 0.5–1 mm��Table 6�. Distribution of the SOC contents in aggre-gates at different depths within the profile under vari-ous land use types indicated a significant effect ofland use �P � 0.0001� and soil depth �P � 0.0001�on the SOC content in different aggregate sizeclasses. The mean SOC contents in 2–5 mm aggre-gates of forest and Bari soils were similar to eachother but significantly different from the SOC in thesame size aggregates in grazing land and Khet soils.This is possibily due to the lower input of SOM. TheSOC plays a vital role in soil aggregation by provid-ing binding force �Lal 2000� among soil particles.Thus, the presence of a higher macro aggregate frac-tion in Bari and forest soils might be attributed to theassociated SOC in those soils. Interference by humanactivity reduces the aggregation as well as the SOClevel �Post and Kown 2000�. This was shown by thehigher SOC content in forest soil in all aggregate sizefractions compared to other soils. Moreover, the SOCin micro aggregates �0.5–1.0, 0.25–0.5 and � 0.25mm� was found to be higher in forest soil comparedto other land uses.

The decrease in SOC with soil depth was morepronounced in grazing land compared to forest soil inall aggregate size-fractions �Figure 3�. The compara-tively higher SOC in forest soil aggregates than thoseof soils from other land uses, for all soil depths, con-firms the role of biomass input in SOC accumulation.

A higher SOC was found to be associated with ag-gregate fractions of � 0.25 mm than with aggregatesof 0.25–0.50 mm, regardless of the soil depth in eachland use type, except for Khet �Figure 3�. The higherSOC in these aggregate size fractions may beassigned to their higher content of silt and clay par-ticles. It is widely accepted that the clay size fractionprovides protection for C against microbial and enzy-matic degradation �Trujilo et al. 1997�. Hence, theseaggregates appear to have an important role in C se-questration �Bajracharya et al. 1998b�.

Relationship between measured parameters

The SOC content in bulk soil and aggregatesize-classes were negatively correlated with BD, soilpH and soil depth, and positively correlated withgravimetric moisture content �GMC� �Table 5�. TheSOC contents in different aggregate size classes of thesoil samples were positively correlated with eachother and with the SOC content of bulk soils. Thisindicates that soils having a higher SOC content inthe macro size-fractions had also a high SOC contentin micro aggregates. This is because large soil aggre-gates are composed of fine aggregates held togetherby organic matter as the binding agent �Tisdal andOades 1982; Unger 1997�.

The ratio of CO2-C emission rate to SOC stock perunit area in topsoil was similar for forest, Bari andKhet lands but significantly lower for the grazing land�data not presented�. This indicates that there may beslower microbial activity or a lower microbial popu-lation. A more probable explanation is the differentquality of C-input, with that to the grazing land being

Table 7. Important soil properties in soil profiles under different land uses.

Variables Soil depth �cm�

10 20 40 60 10 20 40 60 80 100

----------------------Khet---------------------- --------------------------------------Bari------------------------------------BDa �g cm–3� 1.2 1.3 1.3 1.3 0.9 1.0 1.0 0.9 0.9 0.9GMCb �%� 6.7 5.5 6.8 4.2 25.1 21.9 15.5 19.3 33.6 31.1SOCc �%� 1.0 0.3 0.2 0.2 3.3 2.7 2.2 2.2 2.1 1.5pH �1:1.25 water� 6.4 6.6 6.9 6.8 5.1 5.1 5.2 5.2 5.1 5.1

----------------------Forest--------------------- ----------------------------------Grazing land------------------------------BDa �g cm–3� 0.7 0.7 0.7 1.0 1.0 1.0 1.2 1.3 1.3GMCb �%� 11.4 4.1 4.6 25.1 20.5 14.6 8.8 5.5 3.0SOCc �%� 3.4 2.8 2.6 3.7 2.4 1.5 0.8 0.4 0.2pH �1:1.25 water� 4.7 4.8 4.9 4.5 4.6 4.7 5.0 5.1 5.0

aBulk density; bGravimetric moisture content; cSoil organic carbon content. Khet = lowland irrigated rice field, Bari � upland rainfedmaize-millet field.

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more recalcitrant to soil respiration. This may lead toa higher SOC accumulation in grazing land soil, withthe possibility to sequester more C. Lal �2002�reported similar findings where soils under grazingland have a higher potential to sequester C than thecultivated soil.

Soil bulk density generally increased with depthunder all land use systems �Figure 4�. However, thistrend was more pronounced in grazing land and Bariland than in other land uses. This may be partly at-tributed to the decrease in SOC content with depth;however, other factors, such as textural changes, soilcompaction and rooting characteristics may also be

involved. As mentioned earlier, dissolved organiccarbon and deep rooting crops in Bari soil may havecontributed to the higher SOC content in the lowerdepths.

Variability in data

The SOC stock �kg C m–2� in the topsoil �0–10 cm�varied between 1–1.8 in forest, 3.3–3.5 in grazingland, 1.5–2.4 in Bari and 0.6–1.5 in Khet soil �Table2�. In the topsoil, Khet soil showed higher �CV 34%�and grazing land lower �CV 4%� variability comparedto other land use types. For the second layer �10–20

Figure 4. Soil organic carbon content and BD in different depths of various land uses. Khet = lowland irrigated rice field, Bari � uplandrainfed maize-millet field.

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cm� the highest variability occurred in forest soil �CV60%� followed by Khet soil �CV 41%�. Increasingvariability �CV 4–58%� was observed with soil depthin grazing land. However, it was less in Bari soil be-low 20 cm depth, reflecting homogeneity due to top-soil mixing through tillage.

The SOC content in the bulk soil �0–10 cm� wasfound to vary least in Bari �CV � 6%� and most inKhet �CV � 46%� �Table 2�. Among macroaggregates, SOC in 2–5 mm sized aggregates showedthe highest variability in Khet �CV 32%� and 1–2 mmsized aggregates showed the highest variability inforest �CV 32%�. Notably the SOC content in the mi-cro aggregates of size � 0.25 mm showed the leastvariability in Bari �CV 2%� and the most in Khet �CV� 33%�. In general, the greater the depth the higherthe variability in SOC content for bulk soil and forall aggregate fractions except the finest � � 0.25mm�. The SOC content in the finest aggregatesshowed decreasing variability at greater soil depths.

Sources of error

There might be error in our estimation due to thegeneralization of various factors prevailing in the wa-tershed. It is a mountain watershed with variable al-titude, which has a significant effect on the microcli-mate. Variation in SOC content might be due tospatial variability in location of the site or due to landuse. Thus, any generalization of these results at a re-gional level should be made with caution. A greaternumber of sampling sites and replication can elimi-nate such error in future studies.

Conclusions

The results of this study indicate that land use has asignificant effect on the SOC content in the soil pro-file, as well as in the different soil aggregatesize-classes. Soils under natural vegetation had ahigher SOC content in soil compared to cultivatedsoil. Within the cultivated soils, Bari soils had a sig-nificantly higher SOC content than Khet soils, whichcan be attributed to the organic input to Bari soils bythe farmers. A decrease in SOC content with depthwas observed, regardless of land use type. Macro ag-gregates �size classes 1–2 and 2–5 mm� were foundto be the dominant aggregates in Bari and grazingland, whereas in forest and Khet soils micro aggre-gates dominated �size classes � 0.25, 0.25–0.5,

0.5–1 mm�. Soil aggregates of size � 0.25 mm hada higher SOC content than other micro aggregates re-gardless of depth and land use �except Khet). TheSOC associated with micro aggregates may have beenphysically protected from microbial degradation, andhence may contribute to soil C sequestration. Forestsoil has a higher SOC stock than other land use sys-tems, and this is attributed to its higher SOC contentand coverage. There was a net loss of SOC stock by29% from the 0–40 cm soil layer in the 18-year pe-riod from 1978 to 1996, due to internal trading of landuse change. A large part of that loss is due to conver-sion into Khet, implying a need for a change in farm-ing practice towards more sustainable land use byimproving SOC content. The ratio of CO2-C and SOCstock in the topsoil of grazing land is minimal, indi-cating the possibility to sequester more soil carbon inthis land use system.

Acknowledgements

We wish to thank Prof. M.K. Balla and AssociateProf. K.D. Awasthi, Institute of Forestry, TribhuvanUniversity, Pokhara, Nepal for their kind cooperationin field work of this research. A sincere thanks toanonymous referees whose comments and sugges-tions improved the scientific quality of this paper. Thefinancial support from the Norwegian ResearchCouncil �NFR� funded project �No. 141343/730� andthe Norwegian Agency for International Development�NORAD� is gratefully acknowledged. Logistic facil-ities provided by Institute of Forestry, Pokhara, Ne-pal and Dept. of Soil and Water Sciences, AgriculturalUniversity of Norway are highly acknowledged.

References

Anderson J.M. and Ingram J.S.I. �eds� 1993. Total organic carbonin soil: colorimetric method. In: Tropical Soil Biology and Fer-tility: A Handbook of Methods, 2nd edition. CAB International,Wallingford, UK, pp. 62–64.

Andrén O., Kätterer T. and Hyvönen R. 2001. Projecting soil faunainfluence on long-term soil carbon balances from faunal exclu-sion experiments. Appl. Soil Ecol. 18: 177–186.

Awasthi K.D., Sitaula B.K., Singh B.R. and Bajracharya R.M.2002. Land use changes and morphometric analysis using GISfor two mountain watersheds of western Nepal. Land Degrad.Develop. 13: 1–19.

Bajracharya R.M. 2001. Land preparation: an integral part offarming system in the mid-hills of Nepal. Nepal J. Sci. Technol.3: 15–24.

212

Bajracharya R.M., Lal R. and Kimble J.M. 1998a. Long-term till-age effect on soil organic carbon distribution in aggregates andprimary particle fractions of two Ohio soils. In: Lal R., KimbleJ.M., Follett R.F. and Stewart B.A. �eds�, Management of Car-bon Sequestration in Soil. CRC Press, Boca Raton, Florida,USA, pp. 113–123.

Bajracharya R.M., Lal R. and Kimble J.M. 1998b. Soil organiccarbon distribution in aggregates and primary particle fractionsas influenced by erosion phases and landscape position. In: LalR., Kimble J.M., Follett R.F. and Stewart B.A. �eds�, Soil Pro-cesses and the Carbon Cycle. CRC Press, Boca Raton, Florida,USA, pp. 353–367.

Blake G.R. and Harte K.H. 1986. Bulk density. In: Methods of SoilAnalysis part 1. Physical and Mineralogical Methods-AgronomyMonograph �2nd edition�. American Society of Agronomy – SoilScience Society of America, Madison, Wisconsin, USA, pp.425–442.

Burton S., Shah P.B. and Schreier H. 1989. Soil degradation fromconverting forest land into agriculture in the Chitawan district ofNepal. Mount Res. Develop. 9: 393–404.

De Wit H.A. and Kvindesland S. 1999. Carbon stock in Norwe-gian forest soils and effects of forest management on carbonstorage. Report fra skogforskingen-suppelement 14: 1–52.

Fernandes E.C.M., Motavalli P.P., Castilla C. and Mukurumbira L.1997. Management control of soil organic matter dynamics intropical land-use systems. Geoderma 79: 49–67.

Franzluebber A.J., Haney R.L., Honeycutt C.W., Arshad M.A.,Schomberg H.H. and Hons F.M. 2001. Climatic influence on ac-tive fractions of soil organic matter. Soil Biol. Biochem. 33:1103–1111.

Glaser B., Turrion M.B., Solomon D., Ni A. and Zech W. 2000.Soil organic matter quantity and quality in mountain soils of theAlay Range, Kyrgyzia, affected by land use change. Biol. Fertil.Soils 31: 407–413.

Gurung N.R. 2000. Forest degradation/regeneration in the hills ofNepal, a study at watershed level. M.Sc. Thesis. Noragric, Ag-ricultural University of Norway, Ås, Norway.

Hao X., Chang C. and Lindewall C.W. 2001. Tillage and crop se-quence effect on organic carbon and total nitrogen content in anirrigated Alberta soil. Soil Till. Res. 62: 167–169.

IPCC 2000. Land Use, Land-Use Change and Forestry. IPCC Spe-cial Report on Land Use, Land-Use Change And Forestry. Inter-governmental Panel on Climate Change. http://www.grida.no/climate/ipcc/land_use/ �Date: 01.07.2001�

Islam K.R. and Well R.R. 2000. Land use effect on soil quality ina tropical forest ecosystem of Bangladesh. Agric. Ecosyst. En-viron. 79: 9–16.

Kanchikerimath M. and Singh D. 2001. Soil organic matter andbiological properties after 26 years of maize-wheat-cowpeacropping as affected by manure and fertilization in a Cambisolin semiarid region of India. Agric. Ecosyst. Environ. 86: 155–162.

Ketterings Q.M., Blair J.M. and Marinissen J.C.Y. 1997. Effects ofearthworms on soil aggregate stability and carbon and nitrogenstorage in a legume cover crop agro ecosystem. Soil Biol. Bio-chem. 29: 401–408.

Lal R. 2002. Soil carbon dynamics in cropland and rangeland. En-viron. Pollut. 116: 353–362.

Lal R., Kimble J. and Follett R. 1998. Land use and C pools interrestrial ecosystems. In: Lal R., Kimble J.M., Follett R.F. andStewart B.A. �eds�, Management of Carbon Sequestration inSoil. CRC Press, Boca Raton, Florida, USA, pp. 1–10.

Lal R., Kimble J. and Stewart B.A. 1995. World soils as a sourceor sink for radiatively-active gases. In: Lal R. and Stewart B.A.�eds�, Soil Management and Greenhouse Effect. Lewis Publish-ers, Boca Raton, Florida, USA, pp. 1–8.

Manjaiah K.M., Voroney R.P. and Sen U. 2000. Soil organic car-bon stocks, storage profile and microbial biomass under differ-ent crop management systems in a tropical agriculturalecosystem. Biol. Fertil. Soils 31: 273–278.

Martin A., Mariotti A., Balesdent J., Lavelle P. and Vuattoux R.1990. Estimates of the organic matter turnover rate in a savannaby the 13C natural abudance. Soil. Biol. Biochem. 22: 517–523.

McLean E.O. 1982. Soil pH and lime requirement. In: Page A.L.�ed.�, Methods of Soil Analysis Part 2. Chemical and Microbio-logical Properties, 2nd Ed. American Society of AgronomyMonograph No. 9. ASA-SSSA Inc., Madison, Wisconsin, USA,pp. 199–224.

Neupane R.P. and Thapa G.B. 2001. Impact of agroforestry inter-vention on soil fertility and farm income under the subsistencefarming system of the middle hills, Nepal. Agric. Ecosyst. En-viron. 84: 157–167.

Pilbeam C.J., Tripathi B.P., Sherchan D.P., Gregory P.J. and GauntJ. 2000. Nitrogen balances for households in the mid-hills ofNepal. Agric. Ecosyst. Environ. 79: 61–72.

Post W.M. and Kwon K.C. 2000. Soil carbon sequestration andland use change: processes and potential. Global Change Biol.6: 317–327.

Poudel G.S. and Thapa G.B. 2001. Changing farmer’s land man-agement practices in the hills of Nepal. Environ. Manage. 28:789–803.

Lal R. 2000. Mulching effects on soil physical quality of an alfisolin western Nigeria. Land Degrad. Dev. 11: 383–392.

SAS Institute 2000. SAS/Procedures Guide, Release 8 edition.SAS, Cary, North Carolina, USA.

Singh B.R. and Lal R. 2004. The potential of soil carbon seques-tration through improved management practices in Norway. En-viron. Develop. Sustainabil. �in press�

Sombroek W., Nachtergaele F. and Hebel A. 1993. Amounts, dy-namics and sequestering of carbon in tropical and subtropicalsoils. Ambio 22: 417–426.

Thapa G.B. 1996. Land use an land management and environmentin a subsistence mountain economy in Nepal. Agric. Ecosyst.Environ. 57: 57–71.

Tisdall J.M. and Oades J.M. 1982. Organic matter and water stableaggregates in soils. J. Soil Sci. 33: 141–163.

Trujilo W., Amezquita E., Fisher M.J. and Lal R. 1997. Soil or-ganic carbon dynamics and land use in the Colombian SavannasI. Aggregate size distribution. In: Lal R., Kimble J.M., FollettR.F. and Stewart B.A. �eds�, Soil Processes and the CarbonCycle. CRC Press, Boca Raton, Florida, USA, pp. 267–280.

Unger P.W. 1997. Aggregate and organic carbon concentration in-terrelationships of a Torrertic Paleustioll. Soil Till. Res. 42:95–113.

213

soil organic carbon stocks in soil aggregates under different land use systems in nepal

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