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International Journal of Ecology and Environmental Sciences 36 (1): 75-86, 2010© NATIONAL INSTITUTE OF ECOLOGY, NEW DELHI

Carbon Sequestration in the Grevillea robusta Plantation on a Reclaimed

Sodic Soil at Karnal in Northern India

REKHA JANGRA, S.R. GUPTA*, RAVI KUMAR AND G. SINGH1

Department of Botany, Kurukshetra University, Kurukshetra 136 119 (Haryana), India1Central Soil Salinity Research Institute, Zarifa Farm, Karnal 132 001 (Haryana), India* Corresponding Author: Email: [email protected]

ABSTRACT

This study estimates carbon sequestration, and soil carbon stability in a 25 year old plantation of Grevillea robusta,at the Central Soil Salinity Research Institute, Karnal (290 59' N, 760 51' E; 250 m above sea level). The climateof the area is semiarid and monsoonic and characterized by hot dry summers and cold winters. The soil organiccarbon varied from 0.96-0.12% in 0-100 cm soil depths. The organic matter input to the soil in litterfall was 3.458Mg C ha-1. The fine root biomass varied from 2.279 to 8.732 Mg ha-1 in different seasons. The biomassaccumulation in different tree components (Mg ha-1) was: 216.943 bole > 41.380 branches > 7.590 foliage. Rootbiomass accounted for 14.59% of total tree biomass. Total aboveground net production was 17.389 Mg ha-1 yr-1.The carbon flux through total net primary productivity was 11.322 Mg C ha-1 yr-1. The organic and inorganic carbonstock up to 1-m soil depth was 48.058 Mg C ha-1 and 28.698Mg IC ha-1, respectively. The soil microbial biomass,being an active pool of carbon, formed 1.91% of soil organic carbon up to 30 cm soil depth (0.571 Mg C ha-1). Themicroaggregates (250 µm, 53 µm and <53 µm) formed a large fraction of soil aggregates and protected most of soilorganic carbon in the soil. Montmorillonite, chlorite, illite, kaolinite and vermiculite were found to be the main clayminerals. The plantations of Grevillea, by increasing plant biomass production and soil carbon pool, can play animportant role in carbon sequestration on marginal lands. The soil microbial biomass was found to be a goodindicator of improved soil conditions.

Key Words: Plant Biomass, Productivity, Carbon Pool and Fluxes, Clay, X-ray Diffraction, Organic carbon,Inorganic carbon

INTRODUCTION

Soil salinity and sodicity are serious land degradationissues in arid and semiarid regions of the world. InIndia, 9.38 million hectare (Mha) land is salt affected,out of which 5.5 Mha land is saline and 3.88 Mha isalkaline/sodic soils (IAB 2000). The studies on soil andvegetation carbon pools and soil biological processescontrolling soil carbon dynamics have implications fornatural resource management and soil carbon seques-tration. Improvement of soil carbon is critical for themaintenance of soil fertility and productivity of saltaffected soils. Only a few studies have analyzed theeffect of soil sodicity on carbon dynamics and soilbiological processes (Kaur et al. 2002a and 2002 b,

Tripathi and Singh 2005, Wong et al. 2008).Revegetation of salt wastelands has been found toameliorate soil conditions and improve soil biologicalactivity (Tripathi and Singh 2005). The soil carbondynamics in reclaimed sodic soils have been scantlyinvestigated (Singh 1995, Singh et al. 1997, Bhojvaidet al. 1996, Kaur et al. 2000). The soil inorganic carbonstock of soil has the potential to improve soil physicalproperties, and help in the establishment of vegetationas well as sequestering organic carbon in the soils (Palet al. 2000, Bhattacharya et al. 2004). Thus it isimportant to understand the role of soil organic carbonand inorganic carbon in soil carbon sequestration.

Soil organic carbon associated with aggregates isan important reservoir of carbon, protected from

Jangra et al.: Carbon Sequestration in Grevillea Plantation Int. J. Ecol. Environ. Sci.76

mineralization and enzymatic degradation (Trujillo etal. 1997). The active fraction of soil organic matterconsists mainly of microbial biomass and its meta-bolites (Paul and Voroney 1980). The soil microbialbiomass forms a labile pool of organic matter compri-sing 1-3% of total soil organic matter (Jenkinson andLadd 1981).

Clay minerals strongly influence the major physicaland chemical properties of soil as well as soil organicmatter and its chemical nature. Clay mineralogy variesspatially as a function of climate and parent material,and temporally as a function of soil development (Jenny1941, Torn et al. 1997). Clay mineralogy plays animportant role in the stabilization of soil organic carbon(Laird 2001, Ramson et al. 1998). The largest changesin the quantity and turnover of soil organic carbonacross landscapes and over long time scales may be dueto variation in passive (mineral-stabilized) carbon inthe soil (Torn et al. 1997). Clay mineralogy isimportant in determining the quantity of organiccarbon stored in soil, its turnover time, and atmo-sphere-ecosystem carbon fluxes during long-term soildevelopment (Torn et al. 1997, Baldock and Skjemsted2000, Laird 2001). However, studies on clay minera-logy of salt-affected soils need more attention toanalyze the effect of inter-particle interactions of thesoil minerals and soil carbon stability.

Keeping in view the importance of tree plantationsin carbon sequestration, the aim of this study was toanalyze plant carbon pools and fluxes, soil organic andinorganic carbon pools, soil aggregate carbon andmicrobial biomass carbon in the Grevillea robusta A.Cunn. ex R.Br. plantation on a reclaimed sodic soil. Itwas also aimed at analyzing the role of clay mineralogyin soil carbon stability.

STUDY SITE

The study was carried out in a 25-year old tree planta-tion of Grevillea robusta on a reclaimed sodic soil at theCentral Soil Salinity Research Institute, Karnal (29° 59'N, 76° 51' E; 250 m above sea level) in north-westernIndia. Saplings of Grevillea robusta were planted during1982 in augerholes filled with original soil mixed withgypsum (3 kg) and farmyard manure (8 kg). Thedistance between the rows of trees was 6.0 m and 3.0 mbetween trees in a row. The density of trees was 550trees ha-1, and basal area was 34.21 m2 ha-1. Theplantation has been managed by ploughing once after

the rainy season every year and pollarding the treesafter rainy season every two years.

Climate

The climate of the study area is semiarid and mon-soonic, characterized by hot dry summers and coldwinters. The annual rainfall varied from 341-1020 mmduring the study period from January 2006 toDecember 2007. The mean maximum temperaturevaried from 19.62 to 38.0 °C and mean minimumtemperature ranged from 5.6 to 29.9 °C during January2006 to December 2007(Figure 1).

Figure 1. Monthly variations in mean maximum, temperaturemean minimum temperature and rainfall during thestudy period from January 2006 to December 2007 atCentral Soil Salinity Research Institute (CSSRI),Karnal.

Soil

The soil of the study area is Haplic Salonetz, sandyloam in texture in the surface 0-5 cm layer and clay

36: 75-86 Jangra et al.: Carbon Sequestration in Grevillea Plantation 77

loam at lower depth. Initially, the soils of the study sitewere highly sodic (pH 9.26-9.34) with a sparse coverof salt tolerant grasses, and low soil organic carboncontent (0.22 to 0.28%; Kaur et al. 2000).

In the 25-yr old Grevillea robusta tree plantation onreclaimed sodic soil, the pH was 7.11 to 8.65 at 0 cm to1 m soil depth (Table 1). There was an increase in soilpH and a marked decrease in soil carbon with increasein soil depth (Table 1). Soil organic carbon in the treeplantation ranged from 0.96% (0-15cm soil depth) to0.12% (>60 cm soil depth; Table 1).

METHODS

Estimation of Plant Biomass and Net PrimaryProductivity

Three 20 x 20 m experimental plots were demarcatedwithin the Grevillea robusta plantation for recording theobservations on growth parameters of trees during July2006 and July 2007. Biomass and productivity of treeswere estimated by dimension analysis of sample treesusing allometric regression equations between circum-ference at breast height (cbh) and dry weight of varioustree components. The regression equations giving therelationship between cbh, and the weight of bole,branches and roots are as:

Bole log10 Y = -0.2055 + 1.221 log10 XBranches log10 Y = -1.9583 + 1.9585 log10 XRoots log10 Y = -0.5337 + 1.2607 log10 X

where Y = biomass (kg) of tree components, and X = cbh (cm), N= 30 (number of trees felled)

Total biomass of trees (kg ha-1) was estimated fromthe circumference of trees in the experimental plots andtotal density of trees per hectare.

The litter was collected at monthly intervals fromMarch 2006 to April 2007 in the plantation using thelitter trap method. Biomass of fine roots was deter-mined by using soil core method up to 0-60 cm soildepth, and were separated into two diameter classes i.e.1-2 mm and <1mm and weighed. The fine roots couldnot be recovered from soil cores beyond 60 cm soildepth. Therefore, the root biomass was sampled only to60 cm soil depth across the seasons.

Aboveground net primary productivity (ANP) oftrees was calculated as the sum of increment in biomassof non-photosynthetic parts over a time of one year andthe annual leaf litter production (Olson 1975). Thebelowground net primary productivity of trees wascalculated as the sum of increment in biomass of rootsover a time of one year and the annual fine rootproduction.

Carbon pool in various tree components wascalculated using the average dry weight of biomassduring July 2006 and July 2007 and their mean carbonconcentrations.

Analysis of Soil Aggregates, Microbial biomass andClay Mineralogy

Water stable aggregates into three size classes (2mm-250 µm, 250-53 µm and <53 µm) were determinedusing the wet-sieving procedure (Elliott 1986). The soilsamples from different soil depths (0-100 cm) werecollected from within the three experimental plotsduring October, 2006. The microbial biomass carbon insoil and aggregate fractions was determined by thefumigation extraction method (Vance et al. 1987).

Table 1. Some characteristics of soil in the Grevillea robusta tree plantation on a reclaimed sodic soil at CSSRI,Karnal

Soil Depth Soil pH Soil Texture Bulk Density Organic Carbon Inorganic Carbon (cm) (1:2) Sand (%) Silt (%) Clay (%) (g cm-3) (%) (%)

0- 15 7.11± 0.03 56.0± 2.48 27.0± 2.34 17.00± 1.68 1.14± 0.037 0.96± 0.04 Nil15- 30 7.88± 0.03 47.0± 1.29 32.0± 0.91 21.00± 1.29 1.27± 0.01 0.52± 0.03 Nil30-45 8.48± 0.04 46.33± 1.16 30.1± 1.75 23.16± 1.23 1.44± 0.02 0.37± 0.01 0.065± 0.0145-60 8.52± 0.01 43.83± 0.90 30.9± 1.74 25.00± 0.98 1.51± 0.01 0.19± 0.01 0.25± 0.0260-100 8.65± 0.05 46.23± 0.85 26.8± 1.4 27.50± 1.25 1.69± 0.03 0.12± 0.01 0.32± 0.02


The <2 µm clay fraction that was wholly separatedfrom the soil was used to examine the clay minera-logical composition by the X-ray diffraction (XRD)method. Oriented clay samples were prepared todetermine the clay mineral constituents. Two pre-treatments were made for each, glycolated and glyco-lated-heated (at 550o C for 4 hr) treatments. The XRDanalysis was performed using XPERT-PRO modeldiffractometer with Cu as anode material using CuK"

radiations at 45KV and 40mA and at a scanning speedof 0.017 in a continuous scanning mode over a range of4o 2› to 40o 2› (untreated samples) and 4o 2› to 60o

2› position (glycolated samples). Relative mineralcontents in clay fractions were semi-quantitativelyestimated on the basis of XRD peak intensities byassuming the relative proportion of the minerals ofsamples normalized to 100% and the same propor-tionality between the peak intensity and the content foreach mineral.

Analysis of Soil Organic and Inorganic Carbon

Organic carbon in plant samples was analyzed followingthe method of Kalembasa and Jenkinson (1973).Sub-samples of air-dried soil and separated soil aggre-gates were analyzed for organic carbon by dichromateoxidation method (Kalembasa and Jenkinson 1973).The amount of organic and inorganic carbon in soil wasestimated from the bulk density, soil depth, and organicand inorganic carbon concentration in soil of therespective soil depth.

RESULTS

Plant Biomass and Productivity

Litterfall from trees played an important role in organicmatter addition to the soil. The litter accumulation onthe ground floor was 6.378 Mg ha-1 during 2006 to2007. The litter deposition on ground floor was highduring winter and summer months.

Monthly variations in litter fall in the treeplantation are shown in Figure 2. The total annual litterfall was 7.590 Mg ha-1; leaf litter contributed 93% oftotal litter. The carbon concentration in leaf litter was41.28% and fiber content in the leaves was 59%.

The fine root biomass was greatest in July (rainyseason) coinciding with the production of high foliagebiomass production (Table 2). Most of the fine rootswere concentrated in the top 0-15cm soil during all the

three seasons and contained 64% of total fine rootbiomass. The fine root biomass during the two years ofstudy varied from 10.575 to 11.390 Mg ha-1. The fineroots were grouped into two diameter classes, viz.,1-2mm and <1mm. The contribution of 1-2mmdiameter roots was 21.55% to total fine root biomass.The biomass in different tree components of Grevillearobusta (Mg ha-1) was: 216.943 bole > 41.380 branches> 7.590 foliage (Table 3). Root biomass, including fineroots, accounted for 17.97% of total tree biomass. Thepercentage contribution of different tree components tothe total aboveground biomass was: bole = 66.91%;branches = 12.76% foliage = 2.34%.

Figure 2. Monthly variations in litterfall in Grevillea robustaplantation during March 2006 to February 2007.

Productivity estimates for different components oftrees are given in Table 3. Total aboveground netproduction was 17.389 Mg ha-1 yr-1. Foliage productionaccounted for 31.07% of total net primary productivity.The belowground net primary production, includingcoarse and fine roots, was 7.035 Mg ha-1 yr-1.

Carbon Partitioning and Accumulation in trees

In general, the bole and branches had the higherconcentration of carbon followed by leaves and roots.The carbon concentration in different tree componentswas bole= 49.50%; branches= 48.46%, leaves=45.57%, roots (coarse) = 42.18%, and fine roots=43.52%.


Table 2. Fine root biomass (Mg ha-1) averaged over seasons in tree plantation of Grevillea robusta during June 2006to July 2007 at CSSRI, Karnal. Values after ± represent Standard Errors

Soil depth Fine Root Size Class cm 2006 2007

1-2mm <1mm Total 1-2mm <1mm Total

0-15 1.319± 0.0548 5.498± 0.0508 6.817 1.446± 0.0115 5.618± 0.0162 7.06415-30 0.575± 0.0561 1.697± 0.0299 2.272 0.686± 0.0102 1.812± 0.0050 2.49830-45 0.264± 0.0396 0.773± 0.0184 1.037 0.312± 0.0073 0.886± 0.0087 1.19845-60 0.121± 0.0064 0.328± 0.0118 0.449 0.214± 0.0057 0.416± 0.0065 0.630

Table 3. Total biomass (Mg ha-1) and net primary produc-tivity (NPP, Mg ha-1 yr-1) of different components ofGrevillea robusta plantation at CSSRI, Karnal.

Components Biomass NPP

Bole 216.943 7.506Branch 41.380 2.293Foliage* 7.590 7.590Roots 47.302 1.690Fine roots 10.983 5.345Total 324.198 24.424

*From litterfall data

Table 4. Carbon content (Mg C ha-1) and carbon flux(Mg C ha-1 yr-1) of different tree components ofGrevillea robusta plantation at CSSRI, Karnal.

Tree components Carbon content Carbon flux June 2006 July 2007 (2006-2007)

Bole 105.529 109.244 3.715Branches 19.497 20.608 1.111Foliage 3.458 3.458 3.458Total Aboveground 128.485 133.312 8.284

Coarse Roots 19.595 21.921 0.712Fine roots 3.275 3.636 2.326*Total Belowground 22.871 25.557 3.038

Total 151.356 158.869 11.322

* Calculated on the basis of carbon inputs in NPP of fine roots.

Total aboveground carbon pool was 128.48 to133.31 Mg C ha-1 during 2006 and 2007, respectively.The carbon pool in the coarse roots varied from 19.595to 21.921 Mg C ha-1 and that in fine roots from 3.275to 3.636 Mg C ha-1 during the two years of study(Table 4).

The carbon flux in the system through net primaryproductivity was 11.322 Mg C ha-1 yr-1 (Table 4). Theaboveground carbon input through net primaryproduction was 73.17% of the total carbon flux. Therelative contribution of different plant components tototal aboveground carbon flux was in the order: bole>foliage> fine roots>branches> coarse roots (Table 4).

Soil Organic and Inorganic Carbon Pool

In the Grevillea robusta tree plantation, the soil organiccarbon concentration was 0.96% at 0-15 cm soil depth.There was decrease in concentration of soil carbon withincrease in soil depth. The soil carbon (Mg C ha-1) atdifferent soil depths was: 17.093 (0-15 cm), 9.906(15-30 cm), 8.008 (30-45 cm), 4.416 (45-60 cm) and8.635 (60-100 cm) (Figure 3). The soil organic carbonpool at 0-30 cm soil depth accounted for 56.18% of thetotal organic carbon pool up to 100 cm soil depth.

In contrast to soil organic carbon distribution,most of soil inorganic carbon was found concentratedfrom 30 to 100 cm soil depth (Figure 3). There weresignificant variations in quantity and vertical distri-bution of SIC. The soil inorganic carbon stock atdifferent soil depths in the tree plantation was (Mg ICha-1): 1.404 (30-45cm), 5.662 (45-60 cm) and 21.632(60-100 cm). The total soil organic and inorganiccarbon stock up to 1 m depth was 76.756 Mg C ha-1.


Figure 3. Soil organic carbon pool (Mg C ha-1) and soilinorganic pool (Mg IC ha-1) up to 1 m soil depth inGrevillea robusta plantation.

Soil Aggregates and Microbial Biomass Carbon

The aggregate size fractions in the soil from differentsoil depths are given in Table 5. The amount of macro-aggregates (2 mm-250 µm) varied from 1.84 to 6.90%.The microaggregates (250 µm-53 µm) ranged from17.23 to 32.42%. The silt and clay associated soilfractions ranged from 62.28 to 77.44%. Organic carbonin soil aggregates varied significantly among the sizefractions and the soil depth (p<0.01). There wassignificant increase in carbon concentration (%) withincreasing aggregate size class. The total carbon concen-tration was greater in microaggregates (250 µm-53 µm)

as compared to silt and clay associated to soil fractions(<53 µm) (Table 6). The concentration of carbon insoil aggregate fractions followed the order: macro-aggregates > microaggregates > silt and clay fractions.The carbon concentration in different size fractions ofsoil ranged from 0.07 to 1.35% across soil depths. Thecarbon content of silt and clay associated fractionsvaried from 0.07 to 0.60% (Table 6). The differences inorganic carbon storage in soil aggregates could beattributed to organic matter inputs into the soil, relativedecomposition rates of litter and fine roots, and clayand silt content of soil. It is interesting to note that clayand silt content of soils showed increase with increasein soil depth. Soil organic carbon showed significantrelationship with soil aggregate carbon.

Table 5. Soil weight (%) distribution in aggregate sizeclasses at different depths in the Grevillea robustaplantation at CSSRI, Karnal. (± Standard Error)

Soil depth Percent Weight in Soil Aggregate Fractions cm 2 mm -250 µm 250 µm-53 µm <53 µm

0 - 5 5.28± 0.69 17.23± 1.11 77.44± 1.05 - 15 3.96± 0.46 20.01± 0.73 76.03± 0.2815 - 30 1.84± 0.12 24.42± 0.99 73.74± 1.0930 - 45 3.50± 0.44 28.72± 1.71 67.78± 1.6945 - 60 3.49± 0.35 32.42± 1.45 64.09± 1.5960 - 100 6.90± 0.48 30.82± 1.97 62.28± 1.70CV (%) 22.10 10.92 3.78LSD 1.25 4.15 3.94(p<0.05)

Table 7. Seasonal variation in microbial biomass carbon(µg C g-1 soil) in soils of Grevillea robusta plantationat CSSRI, Karnal. (± Standard Error)

Depth Microbial Biomass Carbon cm July 2007 Nov. 2007 Feb. 2008

0- 5 202.42± 3.26 238.18± 2.02 277.72± 5.46 5-15 156.94± 5.01 183.68± 2.90 198.06± 2.7615-30 76.98± 1.74 94.15± 1.85 104.12± 2.11


Table 6. Organic carbon (%) distribution in soil and aggregate size classes at different soil depths in tree plantationof Grevillea robusta at CSSRI, Karnal. Values after ± are Standard Errors.

Soil depth Aggregate Size Classescm Soil 2 mm -250 µm 250 µm-53 µm <53 µm

0 - 5 1.06± 0.04 1.35± 0.02 0.89± 0.04 0.60± 0.035 - 15 0.86± 0.03 1.06± 0.09 0.63± 0.09 0.57± 0.0515 - 30 0.52± 0.03 0.74± 0.02 0.42± 0.01 0.36± 0.0130 - 45 0.37± 0.01 0.51± 0.02 0.29± 0.01 0.20± 0.0145 - 60 0.19± 0.01 0.29± 0.01 0.19± 0.01 0.14± 0.0160 - 100 0.12± 0.01 0.17± 0.01 0.11± 0.01 0.07± 0.01CV (%) 10.00 12.92 20.89 16.38LSD (p<0.05) 0.05 0.13 0.13 0.08

Due to rapid turnover of soil microbial biomassand increasing demand of nutrients by the plants,microbial biomass showed a decrease in rainy seasonduring the month of July, 2007 as compared to that inFebruary and November (Table 7). In the tree planta-tion, microbial biomass carbon (µg C g-1 soil) across soildepths was: 76.98 to 202.42 (rainy season); 94.15 to238.18 (winter season); 104.12 to 277.72 (summerseason). Averaged across the sampling dates, microbialbiomass carbon varied from 91.75 to 239.44 µg C g-1

soil across soil depths.

Clay Mineralogy

The XRD pattern of the oriented, glycolated andglycolated-heated samples of clay fractions of soil isshown in Figures 4 and 5. X-ray diffraction is the mostcommon technique used to study the characteristics ofcrystalline structure and to determine the mineralogy offiner grained sediments, especially clays. Angles ofdiffraction, as affected by differentiating sampletreatments, are distinctive for a particular mineral andhelp to identify the mineral. Intensities of diffractionmaxima are related to the number of correspondingdiffraction planes in a sample and provide a basis forthe estimation of concentrations of the mineral speciespresent.

The clay fraction was dominated by Illite-micacomplex and was identified by the presence of 3.342 Åpeak along with its higher order reflection at 9.977 Åand 4.987 Å ( Figure 4). The presence of Kaolinite wasascertained by the relative sharp peak at 7.11 Å only.Further, the presence of Kaolinite was confirmed from

Figure 4. X-ray diffraction pattern of untreated clay fractionof soil from Grevillea robusta tree plantation at CSSRI,Karnal (I = Illite; M = Montmorillonite; Mc = Mica; Q= Quartz; Kt = Kaolinite).

heated-glycolated samples where corresponding peakdisappeared. Montmorillonite was identified by thepeaks at 4.48 Å was confirmed by increase in intensityafter glycolation. The glycolated and heated sample alsoshowed the presence of Montmorillonite peak. Inglycolated samples, the clay minerals belonging toDickite, Chlorite, and Metahallocite have been identi-fied at 4.266 Å, 2.001 Å, and 2.509 Å, respectively. Inglycolated and heated samples, Dickite, Chlorite, andMetahallocite persisted at 4.266 Å, 2.001 Å and 2.509Å, respectively (Figure 5).


Figure 5. (a) X-ray diffraction pattern of glycolated clayfraction from Grevillea robusta tree plantation (I = Illite;Kt = Kaolinite; M = Montmorillonite; Mc = Mica; Ch= Chlorite; De = Dickite).(b) X-ray diffraction pattern of glycolated heated clayfraction from Grevillea robusta tree plantation (I = Illite;M = Montmorillonite; Mc = Mica).

Carbon Budget of the System

On the basis of data on tree biomass, litter fall, fineroot biomass, microbial biomass carbon, soil organiccarbon and soil inorganic carbon, the carbon budget of25 year-old Grevillea robusta tree plantation is shown inFigure 6. There was appreciable pool of carbon inabove-ground biomass of trees (130.898 Mg C ha-1).The allocation of carbon to belowground tree compo-nents amounted to 23.408 Mg C ha-1. The inputorganic matter to the soil in litter fall was 3.458 Mg Cha-1. The input of carbon in fine roots amounted to3.455 Mg C ha-1 assuming that fine roots have

complete turnover over the annual cycle. The total fluxof carbon in net primary productivity in the treeplantation was 11.322 Mg C ha-1 yr-1. The organic andinorganic carbon stock up to 1-m soil depth was 48.058Mg C ha-1 and 28.698 Mg IC ha-1, respectively (Figure6). The soil microbial biomass being an active pool ofcarbon formed 1.91% of soil organic carbon up to 30cm soil depth (0.517 Mg C ha-1).

DISCUSSION

The carbon fixed by plants is the primary source oforganic matter inputs into the soil both from above-ground and belowground parts of plants. A largefraction of the terrestrial aboveground net primaryproduction finds its way to the soil surface in the formof litter fall. The tree plantations and silvopastoralagroforestry systems raised on sodic soils have beenfound to improve soil carbon and microbial activitythrough input of organic matter from aboveground andbelowground parts of the plants (Kaur et al. 2002a and2002 b, Mishra and Sharma 2003).

The fine and coarse roots add an appreciableamount of organic matter into the soil (Raich andNadelhoffer 1989). The fine roots (1-2 mm and <1mm diameter) constitute dynamic and active compo-nents of the root system, and their fast turnover playsa key role in soil carbon dynamics. There was appre-ciable soil carbon at 60-100 cm depth (0.12%) and therole of fine roots could be important. However, in thisstudy, the fine roots were sampled only up to 60 cmsoil depth. Fine roots production forms a large propor-tion of total net production, and account for about 33%of the global annual net primary productivity (Gill andJackson 2000). In this study, the fine root productionaccounted for 21.88% of total net production.

The potential role of forest tree plantations insequestering carbon to reduce the buildup of CO2 in theatmosphere has been recognized (Houghton et al.2001). According to FAO report, the total carbon stockin Indian forests amount to 10.01 Gt C, the forest soilaccounts for 50% of the total soil carbon (FAO 2006).On the basis of Comprehensive Mitigation AnalysisProcess (COMAP) model, Ravindranath et al. (2008)have shown a large stock of carbon in forest soils ofIndia. Tree based systems accumulate large amount ofbiomass and sequester substantial amount of carbon inperennial tree components. Approximately 88% of thetotal tree biomass in plantation and agroforestrysystems is stored mostly in tree trunks as aboveground


Figure 6. Carbon budget of a 25 year old Grevillea robusta plantation on a reclaimed sodic soil. The values in compartmentsrepresent the carbon stock (Mg C ha-1). The values on the arrows are the flows (Mg C ha-1 yr-1).

biomass, and the remaining as belowground (Sharrowand Ismail 2004). In the 25 year old Grevillea robustatree plantation, the aboveground net primary produc-tion was 17.389 Mg dry matter ha-1 yr-1. For variousmultipurpose trees used in agroforestry system and inplantations, above-ground net primary productivity hasbeen reported to range from 23.700 to 38.200 Mg drymatter ha-1 yr-1 (Young 1989).

Jobbagy and Jackson (2000) have reported verticaldistribution of soil carbon from various studies. Thisstudy on carbon stocks in the Grevillea robustaplantation showed that soil inorganic carbon (SIC) atincreasing soil depth provided greater potential forcarbon sequestration, whereas SOC sequestered innearer surface soil less than 60 cm depth. Carbonateformation in soil solution is usually controlled by


equilibrium reactions in the solid phase carbonateminerals and gaseous phase CO2 (Robbins 1986). TheCO2 derived from the decomposition of organic matterand plant respiration dissolves in the soil solution toform carbonate. These in turn can precipitate with thecalcium ions often supplied by rain or irrigation waterwhen the conditions are favorable (Robbins 1986).

The inorganic carbon is found mostly in the formof CaCO3 in the soils of the study area (Bhumbla et al.1970). In contrast to spatial distribution of soil organiccarbon, most of soil inorganic carbon was concentratedat 60 to 100cm soil depth. The dissolution of pre-existing carbonates in the upper soil layer may gettranslocated vertically and its precipitation in thesubsoil. Formation of soluble calcium bicarbonateshelps in restoring soluble and exchangeable calciumlevels in the soils as well as improving soil physicalproperties by decreasing exchangeable sodium (Sahra-wat 2003, Bhattacharyya et al. 2004). Thus, inorganiccarbon provides a mechanism for carbon transferbetween organic and inorganic form for carbon seques-tration and maintaining soil quality (Sahrawat 2003,Bhattacharyya et al. 2004).

In the studied tree plantation, it was found thatorganic carbon concentration decreased from macro-aggregates to microaggregates at various soil depths.Organic matter associated with macroaggregates is morereadily mineralized than that associated with micro-aggregates (Beare et al. 1994, Gupta and Germida1988). Golchin et al. (1994) suggested that macro-aggregates are stabilized mainly by recently depositedresidues and carbohydrate rich root or plant debrisoccluded within aggregates. In the soil of studiedplantation, microaggregates (250 µm-53 µm and <53µm) formed a large fraction of soil aggregates andprotected most of soil organic carbon in the soil. Therapid turnover of macroaggregates reduces the forma-tion of microaggregates within the macro-aggregatesand favors the stabilization of carbon within micro-aggregates (Six et al. 1998). On the basis of severalreports on soil organic carbon stabilization, Six et al.(2002) reported that silt plus clay fraction in soil playsa key role in the protection of soil organic matter.

The stabilization of organic material by soil matrixis a function of chemical nature of mineral fraction andits surfaces capable of adsorbing the organic material(Baldock and Skjemsted 2000). In arid and semi-aridregions, smectite, chlorite, illite, kaolinite and vermi-culite are the dominant clay minerals (Gonzalez andLaird 2003). The transformation of plant residues intostabilized clay humic complex is the main process

which regulates soil quality and determines soils to bea net sink and source of carbon (Gonzalez and Laird2003).

Illite is the main precursor mineral for theformation of smectite in salt - affected soils. In thisstudy, the illite and montmorillonite (a member of thesmectite family, 2:1 clay) were predominant in the soil.In the surface layer of soils in Alberta, Canada, Kohutand Dudas (1994) reported highly diffuse smectitediffraction maxima because of mineralogical inter-actions with organic matter. Gonzalez and Laird (2003)while studying the distribution of newly formed humicmaterials into minerologically distinct clay size fractionson a silt loam soil showed that new humic materials arepreferentially accumulated on smectite surfaces. In thisstudy, the predominance of illite and montmorillonitein the clay could play an important role in soil carbonstability. The association of organic matter in soil withminerals is a controlling factor of C storage in soil(Baldock and Skjemsted 2000). The soil organic matterprotection through intimate association with clayparticles can provide long term stability for carbonsequestration (Qualls 2004, Ratnayake et al. 2008).

Healthy soil provides a wide range of ecosystemgoods and services that play a crucial role in sustainingbiological productivity on marginal lands. There is largepotential for carbon sequestration in Grevillea robustatree plantations, amounting to 157.257 Mg C ha-1 overa period of 25-years. Soil inorganic carbon (SIC) at soildepth greater than 45 cm provided greater potential forcarbon sequestration. The soil microbial biomass wasfound to be a good indicator of improved soil condi-tions. There was stabilization of carbon within micro-aggregates, and the predominance of illite and mont-morillonite in the clay could relate to soil organiccarbon stability.

ACKNOWLEDGEMENT

The financial assistance to Rekha Jangra fromKurukshetra University, Kurukshetra in the form ofUniversity Research Scholarship is gratefullyacknowledged.

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