Carbon storage and nitrogen cycling in silvopastoral systems on asodic soil in northwestern India

B. Kaur1, S. R. Gupta1, * and G. Singh2

1

Department of Botany, Kurukshetra University, Kurukshetra-136119, India; 2 Central Soil SalinityResearch Institute, Karnal, India (*Author for correspondence; E-mail: sgupta2002@yahoo.com)

Received 17 September 1998; accepted in revised form 15 April 2001

Key words: carbon flux, fine roots, herbaceous vegetation, litterfall, nitrogen uptake

Abstract

Tree-based land-use systems could sequester carbon in soil and vegetation and improve nutrient cyclingwithin the systems. The present investigation was aimed at analyzing the role of tree and grass specieson biomass productivity, carbon sequestration and nitrogen cycling in silvopastoral systems in a highlysodic soil. The silvopastoral systems (located at Saraswati Reserved Forest, Kurukshetra, 29°4

′ to30°15′ N and 75°15′ to 77°16′ E) consisted of about six-year-old-tree species of Acacia nilotica, Dalbergiasissoo and Prosopis juliflora in the mainplots of a split-plot experiment with two species of grasses,Desmostachya bipinnata and Sporobolus marginatus, in the subplots. The total carbon storage in thetrees + grass systems was 1.18 to 18.55 Mg C ha–1 and carbon input in net primary production variedbetween 0.98 to 6.50 Mg C ha–1 yr–1. Carbon flux in net primary productivity increased significantly dueto integration of Prosopis and Dalbergia with grasses. Compared to ‘grass-only’ systems, soil organicmatter, biological productivity and carbon storage were greater in the silvopastoral systems. Of the totalnitrogen uptake by the plants, 4 to 21 per cent was retained in the perennial tree components. Nitrogencycling in the soil-plant system was found to be efficient. Thus, It is suggested that the silvopastoralsystems, integrating trees and grasses hold promise as a strategy for improving highly sodic soils.

Agroforestry Systems 54: 21–29, 2002. 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Introduction

Saline and sodic soils are of widespread occur-rence in the arid and semiarid regions of northernIndia, limiting the productivity of more than 2.5million ha of otherwise arable lands in theIndo-Gangetic plains (Abrol and Bhumbla, 1971).These soils are characterised by high pHthroughout the soil profile, high exchangeablesodium and low soil organic matter content (Guptaet al., 1984) and a sparse cover of natural vegeta-tion. Afforestation and reclamation agroforestrysystems have been reported to improve thebiological production of sodic soils (Singh, 1996;Singh and Singh, 1997). Acacia nilotica and

Prosopis juliflora, growing on sodic soils, havebeen reported to increase soil organic mattercontent and bioavailability of inorganic nitrogen(Singh, 1995; Bhojvaid et al., 1996). In Prosopis-Leptochloa agroforestry systems, soil organiccarbon and available nitrogen showed a markedincrease after six years (Singh, 1995).

Trees are known to maintain soil organic matterand nutrient cycling through the addition of litterand root residues into the soil. There is a largepotential of sequestering carbon in soil andvegetation by adopting suitable agroforestrysystems on salt affected soils (Singh and Singh,1997). However, only a few studies have focussedon productivity and nutrient cycling in natural

vegetation and agroforestry systems of saltaffected soils (Gupta et al., 1990; Singh, 1995;Singh and Singh, 1997). The present investigationwas aimed at quantifying the role of tree and grassspecies on biomass productivity, carbon seques-tration in the plant biomass and nitrogen cyclingin the silvopastoral systems on a highly sodic soilof northwestern India.

Materials and methods

Site description

The study was carried out in natural vegetationand silvopastoral systems on a sodic soil atBichian, Saraswati Reserved Forest, Kurukshetra(29°4′ to 30°15′ N and 75°15′ to 77°16′ E) innorthwestern India. The study site represented alarge expanse of highly sodic soil with sparsecover of salt tolerant plants. The climate of thearea is semiarid and monsoonic, characterised byhot dry summers and cold winters. The meanannual rainfall of the area is about 600 mm, nearly80% of which is received during June toSeptember. The pan evaporation exceeds precipi-tation throughout the year except the rainyseason.The soil of the study area is highly alkalinehaving a pH of more than 10.2 upto a depth of2 m. The electrical conductivity (EC) of thesurface 15-cm layer varied from 2.0 to 6.4 dS m–1.The most peculiar feature of the soil profile wasthe presence of precipitated CaCO3 layer atvarious depths of soil resulting in low soil per-meability and impeded drainage. The CaCO3

content varied from negligible in the surface layerof soil but as high as 20 percent at about one metersoil depth. Two sites, stand I and II, were selectedfor the study. Desmostachya bipinnata (Stand I,soil pH 8.6 to 9.8) and Sporobolus marginatus(Stand II, soil pH 9.6 to 10.2) dominated thenatural vegetation of the two sites. On stand I, theplants having a moderate density (4 to 30 tillerm–2) were represented by Cynodon dactylon,Cyperus rotundus, Dichathium annulatum,Erigeron linifolius, Leptochloa panacea, Rumexdentatus and Sporobolus marginatus. On the otherhand, stand II was represented by a mixture ofgrasses such as Sporobolus marginatus (142tiller m–2), Cynodon dactylon (10 tiller m–2),

Desmostachya bipinnata (9 tiller m–2),Dichanthium annulatum (6 tiller m–2) andLeptochloa panacea (10 tiller m–2).

Experimental design

A three times replicated split-plot experiment withthree tree species in the main plot and twointerplanted grass treatments in the subplots wasinitiated during August and September, 1991. Thetree species were Acacia nilotica (L.) Willd. exDel. sub sp. indica (Benth) Brenan, Dalbergiasissoo Roxb. and Prosopis juliflora Linn. Thesubplot treatments comprised of Desmostachyabipinnata and Sporobolus marginatus. The speciesare referred to as An (Acacia nilotica), Ds(Dalbergia sissoo), Pj (Prosopis juliflora), Sm(Sporobolus marginatus) and Db (Desmostachyabipinnata). They were planted in experimentalplots of 12 × 32 m. The tree saplings were plantedin the augerholes (20 to 25 cm dia and 120 to140 cm deep). The augerholes were filled withthe original soil mixed with gypsum (3 kg) andfarmyard manure (8 kg). The distance between therows of trees was kept 4 m and between trees ina row was 2 m to give a stand density of 1250trees ha–1. The experimental plot, having naturallygrowing grasses of Db and Sm were treated ascontrol. Initially, the soil carbon content forvarious treatments varied from 0.10 to 0.16% andthe soil pH from 10.0 to 10.2.

Litterfall, ground floor litter and fine rootbiomass

The litterfall was collected at monthly intervalsduring March 1997 to March 1998 using the littertraps (five in each experimental plot). Thestanding crop of forest floor litter was determinedseasonally using the quadrat method by placingfive quadrats (1 × 1 m) during March 1997, June1997, October 1997 and March 1998. The littersamples were made free from adhering soil parti-cles and separated into foliage (leaf and twig litter)and mixed litter. All samples were oven dried at65 °C for determining their dry weights.

The soil core method was used for excavatingfine roots to a depth of 0 to 15 cm. The soil coreswere soaked in water for 24 h and washed undera fine jet of water using a sieve shaker fitted with

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2 mm to 0.5 mm mesh screens successively. Thewashed materials were soaked dry within folds ofblotting paper and oven dried at 65 °C.

Biomass and productivity of trees

Biomass and productivity of trees were estimatedby dimension analysis of sample trees usingallometric regression equations between the cir-cumferences at breast height (cbh) and weight ofvarious tree components. All trees within the treat-ment plots were marked and their circumferencesmeasured at 1.35 m height from the ground duringMarch 1997 and March 1998.

The regression equations showing the relation-ship between growth and weight of trees werecomputed to determine the biomass of tree com-ponents (Table 1). Total biomass of trees wasestimated on the basis of total stand density of1250 trees ha–1. Aboveground net primary pro-ductivity (ANP) of trees was calculated on thebasis of sum of increment in the biomass of non-photosynthetic parts over a time period of one yearand litter production during the same period oftime.

Growth of Acacia nilotica was poor whengrown along with Sporobolus marginatus as itcould not withstand frost and waterlogging. On theSporobolus stand, the Dalbergia trees were

marked in the field and felled during March 1997and March 1998 to determine plant biomass andthe increment in their growth during one year.

Biomass of herbaceous vegetation

Seasonal variations in the biomass of grasses indifferent treatments were studied by the harvestmethod using 50 × 50-cm harvest plots. Fivequadrats were harvested randomly from within theexperimental plots at an interval of two to threemonths during April 1997 to March 1998. Rootbiomass (up to 15 cm depth) was determined byexcavating soil cores from the harvest plots. Theroots were separated from soil by soaking in waterand washing under a fine jet of water. The shootand root biomass was determined on oven dryweight basis. The ANP of grasses was computedusing the trough peak analysis of data on live anddead shoots. The trough peak analysis of timeseries data is based on summation of all positivechanges in live biomass and concurrent positivechanges in the standing dead biomass.Belowground net production (BNP) was calculatedby summing all positive changes in belowgroundbiomass (up to 15 cm-soil depth) as recordedduring different seasons. The values of ANP andBNP were summed to give the total net primaryproductivity (TNP).

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Table 1. Regression of log dry weight of different tree components (Y) and log cbh (X) for Acacia nilotica, Dalbergia sissooand Prosopis juliflora in silvopastoral systems on a sodic soil in northwestern India.

Tree species/ Corr. Intercept Slope SE of SE ofPlant component coeff. (r) a b intercept b

Acacia nilotica Bole 0.74 –1.149 1.277 0.043 0.123(N = 8) Branch 0.74 –0.850 0.999 0.124 0.357

Root 0.94 –0.420 0.615 0.029 0.084 Total 0.96 –0.173 0.007 0.038 0.110

Dalbergia sissoo Bole 0.89 –1.037 1.226 0.117 0.225(N = 8) Branch 0.72 –0.037 0.508 0.096 0.183

Root 0.98 –0.583 0.857 0.032 0.061 Total 0.92 –0.023 0.852 0.070 0.134

Prosopis juliflora Bole 0.86 –0.239 0.709 0.051 0.156(N = 9) Branch 0.88 –1.886 1.945 0.122 0.369

Root 0.95 –0.142 0.569 0.020 0.060 Total 0.92 –0.728 1.388 0.072 0.218

N = No. of trees felled; cbh = Circumference at breast height.log10 Y = a + b log10 X, where X = cbh (cm) and Y = Tree biomass (kg).

Carbon and nitrogen analysis of plant samples

The subsamples of various tree components andherbaceous vegetation were analyzed for carbonand nitrogen. Oven dried plant samples wereground in a Wiley mill equipped with 2 mm sieve.Organic carbon in plant samples was determinedusing powdered ground samples by dichromateoxidation method. Total nitrogen concentrationwas estimated using semi-microkjeldahl method.

Results and discussion

Biomass and productivity of herbaceous vegetation

The monsoon rains triggered an active growth ofherbaceous vegetation and biomass attained a peakvalue in the rainy season (June to September). Asecond and limited spurt of growth occurred insummer (March to May) following rains duringJanuary and February. The differences in speciescomposition of grasses had a significant effect onthe biomass in primary producer compartments.Plant biomass was greater in Db-dominated veg-etation as compared to that of Sm stand (Table 2).The root: shoot ratio was high and varied from0.88 to 1.25. The development of high below-ground biomass in natural ecosystems is anadaptive strategy for the plants to survive undermoisture, oxygen, salinity or solonetzity stressconditions (Bazilevich and Titlyanova, 1980).

Net primary productivity of herbaceous vege-tation on Desmostachya stand (Mg ha–1 yr–1) was:ANP = 6.50, BNP = 3.87, TNP = 10.37 (Table 2).On the Sporobolus stand, net primary productivity(Mg ha–1 yr–1) were: ANP = 1.87, BNP = 1.33,TNP = 3.20. The differences in productivity of thetwo systems could be attributed to the markeddifferences in soil conditions and phenology ofdominant plant species as reported by Gupta andSingh (1981) from a tropical successional grass-land. For the Db- and Sm-dominated communitiesof sodic soils, the total net primary productivityvaried from 14.7 to 24.7 Mg ha–1 yr–1 (Gupta etal., 1990). The present study also showed thatsodic soils are potentially productive under theadaptive natural vegetation.

The carbon concentration in different primary

producer compartments of grasses were: 33.6 to35.4% live shoots, 36.9 to 37.5% dead shoots, 37.5to 38.0% litter and 35.1 to 36.2% roots. Nitrogenconcentration in plant components was: live shoots1.19 to 1.12%, dead shoots 0.89 to 0.95%, litter0.81 to 0.68%, roots 0.98 to 1.01%. Nitrogenconcentration in the live shoots was high inrainy season and it declined during winter. Theshoots had higher amount of carbon and nitrogencompared to that of roots in terms of carbon andnitrogen content of herbaceous vegetation(Table 2). Of the total carbon input into thesystem, 38 to 42% was associated with below-ground and 62 to 58% with aboveground produc-tion. The annual uptake of nitrogen was 0.115and 0.034 Mg N ha–1 yr–1 in Desmostachya andSporobolus dominated vegetation, respectively.

Tree biomass, litterfall, groundfloor litter andfine root biomass

Tree biomass in Acacia, Dalbergia and Prosopis+ Desmostachya systems (15.43 to 45.43 Mg ha–1)

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Table 2. Average plant biomass, productivity, carbon andnitrogen content and uptake in shoots and roots ofDesmostachya bipinnata and Sporobolus marginatus in‘grass-only’ treament on a sodic soil in Kurukshetra, north-western India.

Plant species/ Average biomass/ Productivity/components C and N contents C and N uptake

(Mg ha–1) (Mg ha–1 yr–1)

Desmostachya bipinnata

ShootsDry matter 6.50 6.50Carbon 2.23 2.18Nitrogen 0.165 0.077

RootsDry matter 2.91 3.87Carbon 1.02 1.36Nitrogen 0.028 0.038

Sporobolous marginatus

ShootsDry matter 3.05 1.87Carbon 1.13 0.66Nitrogen 0.031 0.021

RootsDry matter 1.21 1.33Carbon 0.35 0.48Nitrogen 0.010 0.013

was greater as compared to the trees + Sporobolussystems (1.13 to 29.54 Mg ha–1), Table 3. On theaverage, 63 to 87% of tree biomass was accumu-lated in perennial structures (20 to 33% bole, 22to 46% branches, 16 to 22% coarse roots). In thestudied systems, coarse and fine roots accountedfor 21 to 27% of total tree biomass, which iscomparable to the value of 20% of coarse and fineroots of total tree biomass in a dry tropical forest(Singh and Singh, 1991).

Biomass of grasses growing along with treesvaried from 0.42 to 3.92 Mg ha–1. The biomass ofgrasses decreased due to integration of trees insilvopastoral systems. For example, in the case ofsilvopastoral system of Pj+Db, the herbaceousbiomass was significantly lower (0.42 to 0.72 Mgha–1) compared to An+Db and Ds+Db systems(1.41 to 3.92 Mg ha–1). Analysis of varianceindicated significant differences for abovegroundtree biomass (P < 0.05), belowground biomass(P < 0.05) and total plant biomass (P < 0.05)across the treatments in different silvopastiralsystems (Table 3).

The relative contribution of litterfall varied dueto presence of tree species and climatic season-ality. Total litterfall from trees ranged from 0.34to 5.21 Mg ha–1 yr–1, the values being maximumin the case of Pj+Db silvopastoral system (5.21Mg ha–1 yr–1). The mixed litterfall was greater inthe rainy season (June-September), whereas insummer (March-May) and winter (October-February) the proportion of leaves was higher. Thelitter accumulation on the groundfloor was higherin the case of Prosopis juliflora (2.13 to 3.40 Mg

ha–1) followed by Dalbergia sissoo (0.29 to 2.05Mg ha–1) and lowest in the case of Acacia nilotica(1.81 Mg ha–1). Addition of organic matter inlitterfall and fine roots had a direct effect on thestatus of soil organic matter in the silvopastoralsystems. In similar studies, reclamation agro-forestry systems have been reported to increasesoil organic matter content of sodic soils due tolitter input and ground floor litter accumulation(Singh, 1995; Singh, 1996).

The fine root biomass varied significantly indifferent tree and grass species. It is also evidentfrom Table 3 that fine roots formed an appreciableproportion of the net primary production and alarge proportion of carbon and nitrogen wasallocated to belowground parts. The organic mattercontent in fine roots (0.72 to 2.32 Mg ha–1) was33 to 44% to that of litterfall (2.18 to 5.21 Mgha–1) in the trees + Desmostachya silvopastoralsystems. Fine root biomass was found to becomparatively lower in the case of trees +Sporobolus systems, which could be attributed topoor performance of trees at high soil pH on theSporobolus site. A greater amount of fine rootbiomass in the trees + Desmostachya systemscould improve nutrient and water absorption, asfine roots remained in constant flux over the timeand their turnover rate was high.

Net primary productivity

Net primary productivity of the tree + grasssystems showed significant differences due to treespecies (Table 4). Total net productivity of the

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Table 3. Plant biomass, litterfall, and standing crop of ground floor litter in silvopastoral systems of Acacia nilotica (An), Dalbergiasissoo (Ds) and Prosopis juliflora (Pj) alongwith Desmostachya bipinnata (Db) and Sporobolous marginatus (Sm) on a sodic soilin Kurukshetra, northwestern India.

Plant An+Db An+Sm Ds+Db Ds+Sm Pj+Db Pj+Sm LSDP < 0.05

components ————————————————— (Mg ha–1) ——————————————————

TreesFoliage* 2.18 – 3.95 0.34 05.21 2.76 0.88Branch 4.16 – 6.29 0.25 20.82 11.55 2.21Bole 5.04 – 4.62 0.23 09.78 7.90 0.56Coarse roots 3.33 – 4.32 0.23 07.30 6.15 0.37Fine roots 0.72 – 1.29 0.08 02.32 1.18 0.26Ground floor litter 1.81 – 2.05 0.29 03.40 2.13 2.10

Grasses 1.41 3.35 3.92 2.94 00.42 0.72 0.28

* Calculated from litterfall during March 1997 to March 1998.

trees + Desmostachya systems was greater (4.69to 15.67 Mg ha–1 yr–1) than the trees + Sporobolussystems (0.62 to 7.65 Mg ha–1 yr–1). Total netproduction in different silvopastoral systemsfollowed the order: Prosopis juliflora > Dalbergiasissoo > Acacia nilotica. Aboveground net pro-duction attained appreciably high values (3.69 to12.74 Mg ha–1 yr–1) in the case of trees +Desmostachya systems, while it was only 0.49 to6.08 Mg ha–1 yr–1 in the tree + Sporobolus systemsdue to poor growth of trees. For various silvo-pastoral systems, foliage production (as calculatedfrom the annual litterfall) accounted for 45 to 69%of aboveground tree net production. The contri-bution of bole + branches was 31 to 59%, whereascoarse and fine roots accounted for 21 to 28% ofthe total net production.

Total net production of grasses ranged from0.79 to 6.91 Mg ha–1 yr–1 in the case of trees +Desmostachya based systems. Total net productionof Desmostachya when grown with treesaccounted for 35%, 47% and 5% of the total net

production of An+Db, Ds+Db and Pj+Db basedsilvopastoral systems, respectively. In the case ofthe trees + Sporobolus systems, total net produc-tion of grasses varied from 0.64 to 3.10 Mg ha–1

yr–1, accounting for 8 to 77% of total productivity.The primary productivity of grasses in Prosopisbased silvopastrol systems was reduced due toincrease in the tree biomass. Analysis of varianceindicated significant differences for ANP (P <0.05) and BNP (P < 0.05) due to tree and grassspecies in various treatments.

Fertility of soil, water availability and vegeta-tion structure determine to a large extent thefraction of net primary productivity being allo-cated belowground. The belowground allocationof total NPP has been found to be greater in grassesthan woody plants which could increase withnutrient and water stress (Scholes and Hall, 1996).This study also showed that the belowgroundallocation of net productivity was more in grassonly systems (1.33 to 3.87 Mg ha–1 yr–1) than inthe tree + grass systems (1.00 to 2.93 Mg ha–1 yr–1).

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Table 4. Net primary productivity of Acacia nilotica, Dalbergia sissoo and Prosopis juliflora silvopastoral systems on asodicsoil in Kurukshetra, northwestern India during March 1997 to March 1998.

Plant NPP (Mg ha–1 yr–1)components

Acacia Dalbergia Prosopis LSDP < 0.05

Tree + Desmostachya systems

TreesFoliage 2.18 –3.95 –5.21 1.28Branch 0.59 –0.62 –6.51 1.31Bole 0.92 –1.17 –1.02 0.46Coarse roots 0.28 –0.74 –0.61 0.18Fine roots 0.72 –1.29 –2.32 0.32Above-ground 3.69 –5.74 12.74 1.92Below-ground 1.00 –2.03 –2.93 0.35

Grasses 2.57 –6.91 –0.79 0.31Total 7.26 14.68 16.46 1.38

Tree + Sporobolus systems

TreesFoliage – –0.34 –2.76 0.63Branch – –0.05 –2.70 1.06Bole – –0.10 –0.62 0.17Coarse roots – –0.05 –0.39 0.11Fine roots – –0.08 –1.18 0.25Above-ground – –0.49 –6.08 1.08Below-ground – –0.13 –1.57 0.31

Grasses 3.10 –2.17 –0.64 0.24Total 3.10 –2.79 –8.29 1.21

Carbon and nitrogen content in plant biomass

In the tree-based systems, aboveground carbonpool was greater in branches and boles ascompared to foliage (Table 5). The extent ofstorage of carbon in aboveground parts of the tree+ Desmostachya systems (Mg ha–1) were: 4.95An+Db, 6.03 Ds+Db, 14.80 Pj+Db, accounting for66 to 80% of total carbon content of the vegeta-tion. Similarly, 83% of vegetation carbon has beenreported in the above ground parts, while the netprimary productivity contributed 72% of totalcarbon input into the soil in a dry tropical forest(Singh and Singh, 1991). The total carbon storagein the trees + Desmostachya systems ranged from6.80 to 18.55 Mg C ha–1 and in roots it was 1.48to 3.66 Mg C ha–1 across the treatments. Carboncontent in total plant biomass was 1.44 Mg C ha–1

and 12.32 Mg C ha–1 in the case of Ds+Sm andPj+Sm treatments, respectively. It may be pointedout that Acacia nilotica could not survive alongwith Sporobolus under sodic conditions of the soildue to the adverse effect of water logging andfrost.

Nitrogen concentration of the plant componentsand the extent of tree growth influenced the trendof nitrogen storage (Table 5). Nitrogen content(Mg N ha–1) in the case of trees + Desmostachyasystems were: 0.044 to 0.128 branches, 0.008 to0.044 bole, 0.052 to 0.092 coarse roots. Comparedto the trees + Desmostachya systems, nitrogenstorage was lower in the trees + Sporobolussystems ranging from 0.038 to 0.298 Mg N ha–1.

Carbon flux and nitrogen cycling

Carbon flux refers to the input of carbon thoughnet primary productivity into the system and itssubsequent transfer to the soil through litter androot turnover. The amount of total carbon inputthrough net primary production in the trees +Desmostachya systems (Mg ha–1 yr–1) was: 2.81An+Db, 5.37 Ds+Db, and 6.50 Pj+Db (Table 6).The wood (bole + branches) and coarse rootsaccounted for 33 to 54% of total carbon input inthe tree layer, the remaining being channelised forthe formation of foliage (31% to 51%) and fineroots (14 to 16%). Organic carbon input in the

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Table 5. Carbon and nitrogen content of Acacia nilotica (An), Dalbergia sissoo (Ds) and Prosopis juliflora (Pj) alongwithDesmostachya bipinnata (Db) and Sporobolous marginatus (Sm) in silvopastoral systems during March 1997 on a sodic soil inKurukshetra, northwestern India.

Plant components An+Db An+Sm Ds+Db Ds+Sm Pj+Db Pj+Sm LSDP < 0.05

—————————————————— (Mg ha–1) ———————————————————

Trees

FoliageCarbon 0.90 – 1.54 0.13 1.96 1.04 0.44Nitrogen 0.063 – 0.116 0.010 0.141 0.075 0.039

BranchCarbon 1.78 – 2.55 0.10 8.68 4.86 0.88Nitrogen 0.044 – 0.046 0.002 0.128 0.074 0.024

BoleCarbon 2.27 – 1.94 0.10 4.16 3.38 0.43Nitrogen 0.008 – 0.022 0.001 0.044 0.035 0.009

Coarse rootsCarbon 1.20 – 1.57 0.08 2.77 2.35 0.29Nitrogen 0.052 – 0.046 0.003 0.092 0.078 0.039

Fine rootsCarbon 0.28 – 0.49 0.03 0.89 0.45 0.14Nitrogen 0.009 – 0.019 0.001 0.037 0.019 0.012

Grasses

Carbon 0.37 1.18 1.01 1.00 0.09 0.24 0.18Nitrogen 0.024 0.038 0.070 0.035 0.012 0.017 0.001

form of foliage and fine roots into the soil variedfrom 1.17 to 2.84 Mg C ha–1 yr–1 in the trees +Desmostachya systems. The carbon input throughnet primary productivity in the case of Ds+Sm andPj+Sm treatments was 0.98 and 3.24 Mg Cha–1 yr–1, respectively. The coarse and fine rootsformed about 20% of total net production in thetrees + Sporobolus systems. The addition ofcarbon into the soil through fine roots and above-ground litter amounted to 0.16 to 1.49 Mg Cha–1 yr–1 in the trees + Sporobolus systems. Thecarbon fixed by the plants is the primary sourceof organic matter input into the soil, which providesubstrate for microbial processes and accumula-tion of soil organic matter. Thus, belowgroundallocation of photosynthates is an important factorfor improving soil carbon content in the silvo-pastoral systems.

The flux of carbon in net primary productivityincreased in the Ds+Db and Pj+Db and Pj+Smsystems. Improvement in the flux of carbon inAn+Db (Acacia + Desmostachya) and An+ Sm(Acacia + Sporobolus) systems was not so evident.It may be stated that silvopastoral systems are theaccumulating systems in terms of carbon storagein the perennial tree components. The results arealso in conformity to the earlier reports on carbon

storage in vegetation and carbon input in netprimary production in a tropical forest, savannasand grassland ecosystems (Singh and Singh, 1991;Scholes and Hall, 1996).

The annual uptake, return and retention ofnitrogen in silvopastoral systems are given inTable 6. Total uptake of N for the tree +Desmostachya systems (Mg N ha–1 yr–1) was inorder: 0.231 (Pj+Db) > 0.156 (Ds+Db) > 0.083(An+Db), while it amounted to only 0.038 to0.135 Mg N ha–1 in the case of trees + Sporobolustreatments. Nitrogen uptake by grasses growing inassociation with the trees varied from 0.027 to0.082 Mg N ha–1 yr–1. Addition of nitrogen throughlitterfall and herbaceous litter was 0.092 to 0.167Mg N ha–1 yr–1 and 0.036 to 0.091 Mg N ha–1 yr–1

in the trees + Desmostachya and trees +Sporobolus systems, respectively. Assuming aturnover rate of one year for the fine roots, thenitrogen return through fine roots would equal toits uptake. Nitrogen return through fine roots(0.008 to 0.037 Mg N ha–1) was appreciable in thetrees + Desmostachya based systems. About 87%(An+Db), 89% (Ds+Db) and 77% (Pj+Db) of thenet annual N uptake by the vegetation wasreturned to the soil through litterfall and turnoverof fine roots. Trees retained 4 to 21% of the total

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Table 6. Carbon input and nitrogen uptake in silvopastoral systems of Acacia nilotica, Dalbergia sissoo and Prosopis julifloraduring March 1997 to March 1998 on a sodic soil in Kurukshetra, northwestern India.

Plant components Carbon input (Mg C ha–1 yr–1) N-uptake (Mg N ha–1 yr–1)

Acacia Dalbergia Prosopis Acacia Dalbergia Prosopis

Tree + Desmostachya systems

Trees Foliage 0.90 1.54 1.95 0.063 0.12 0.14Branch + bole 0.65 0.73 3.17 0.007 0.009 0.046Coarse roots 0.10 0.27 0.23 0.004 0.008 0.008Fine roots 0.27 0.49 0.89 0.008 0.019 0.037

Grasses 0.89 2.34 0.26 0.029 0.082 0.027 Total 2.81 5.37 6.50 0.111 0.238 0.258

Tree + Sporobolus systems

TreesFoliage – 0.13 1.04 – 0.010 0.075Branch + bole – 0.064 1.39 – 0.0009 0.02Coarse roots – 0.02 0.15 – 0.0005 0.005Fine roots – 0.03 0.45 – 0.001 0.019

Grasses 1.06 0.74 0.21 0.35 0.026 0.016Total 1.06 0.98 3.24 0.35 0.038 0.135

N uptake in bole, branches and coarse roots inDalbergia and Prosopis based systems. Thus,there was rapid circulation of nitrogen betweensoil and vegetation compartments with morereturns than retentions. On the basis of storage ofcarbon and nitrogen in the perennial tree compo-nents as well as improvement of soil organicmatter status, the silvopastoral systems were foundto be accumulating systems that could effectivelyincrease biological production.

Conclusions

Protection of salt tolerant natural vegetation wasfound to be effective for improving the fertilityof highly sodic soil. The association of trees withthe grasses in the silvopastoral systems revealedthat an increased input of plant residues into thesoil played a significant role to improve nutrientcycling and biological productivity in the treebased systems. On the basis of increased biolog-ical production, greater carbon storage in thevegetation and the soil along with an enhancednitrogen uptake by the plants, the tree basedsystems were found to be promising for the highlysodic soil.

Acknowledgements

Financial assistance in the form of SRF by theCSIR to one of us (BK) is gratefully acknowl-edged. We thank Mr S. K. Jain and Miss Neerajfor their help in statistical analysis of data andDr C. B. Singh for reading the manuscript.Anonymous referees provided helpful commentson our manuscript.

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