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Effect of fertilisation on carbon sequestration in soybean--wheat rotation under two contrasting soils and management practices in the Indian Himalayas.

Introduction

Attempts are needed to enrich global soil organic carbon (SOC) stocks through sequestration of atmospheric C to curb global warming. Optimum levels of SOC can be managed through adoption of an appropriate crop rotation (Wright and Hons 2005), fertility maintenance including use of inorganic fertilisers and organic manures (Reeder et al. 1998; Rice 2000; Schuman et al. 2002), tillage methods (Bhattacharyya et al. 2008), and other cropping system components (Huggins et al. 1995; Janzen et aL 1998). In general, the use of organic manures and compost enhances the SOC pool more than application of the same amount of nutrients as inorganic fertilisers (Gregorich et al. 2001).

Average SOC concentrations in the Indian Himalayan region ranged from 24.3 g/kg in cultivated soils to 34.5 g/kg in native or undisturbed soils (Lal 2004). Lower levels of SOC in cultivated soils are attributed to excessive tillage, imbalance in fertiliser use, little or no crop residue returned to the soil, and severe soil erosion (Lal, 2004). The productivity of rainfed crops is very low, which is mainly attributed to high soil erosion, poor soil physical (coarse-textured soils with high proportion of stones) and chemical (poor nutrient status) conditions, minimal fertiliser use, and low soil moisture. Applying manure in coarse-textured soils under rainfed conditions can improve soil moisture (Bhattacharyya et al. 2009) and SOC storage (Whalen et al. 2003; Kundu et al. 2007). Manure addition also improves SOC storage under wet-temperate climatic conditions of north-west Himalayas (Sharma et al. 1995). The effects of moisture availability on SOC decomposition (and hence, long-term SOC storage) vary significantly; in the original Century formulation (Patton et al. 1987), soil decomposition is fastest when precipitation equals potential evapotranspiration, while soil moisture is assumed optimum when the degree of saturation is midway between permanent wilting point and complete saturation (Cox 2001), and when precipitation/potential evapotranspiration is 1.70 (lse and Moorcroft 2006). Thus, soil moisture is an important determinant for C sequestration, and water limitation in ecosystems may affect C sequestration in both a positive and negative way and, hence, merits spatial measurements.

Many studies have indicated a strong positive relationship between the amounts of C incorporated into soil, either from crop residues or from external sources such as manure, and total SOC content (Havlin et al. 1990; Paustian et al. 1992; Kundu et al. 2001,2007). Changes in SOC were linearly related to gross C input to the soil (Rasmussen and Collins 1991; Buyanovsky and Wagner 1998). Annual C input was 22% of the harvestable above-ground biomass of soybean in a Vertisol in Central India (Kundu et al. 2001). Although around 13% of the total soybean biomass (roots and shoots) has been found in roots alone (Kemper et al. 1998), there are no data on quantifying rhizodeposition of the soybean crop.

The net change in SOC depends not only on the current management practices but also on the management history of the soil. Long-term experiments are usually the only source of information for determining agricultural sustainability (Barnett et al. 1995) and to define land use effect on SOC (Paul et al. 1997). For getting meaningful estimates of the rate of added biomass incorporation into SOM, it is desirable to have experiments with full records of gross C inputs into the soil through various sources. Therefore, quantification of SOC in relation to various soil management practices that add differential C inputs under the rainfed or irrigated systems is of value in identifying the pathways of C sequestration in contrasting soils.

Although crop productivity is sustained mainly through the application of organic manures in the Indian Himalayas, limited information is available on the effects of long-term manure addition along with mineral fertilisers on C sequestration and the contribution of total C input towards SOC storage under irrigated conditions. Bhattacharyya et al. (2007) reported soil properties (total SOC, bulk density, aggregate stability, hydraulic conductivity, aggregate-associated SOC) under an irrigated wheat-soybean rotation after 8 years of experimentation. Although C sequestration and relationships between C addition and storage in a rainfed experiment after 30 years were reported (Kundu et al. 2007), the same information under irrigated condition is limited. Moreover, the knowledge of SOC stabilisation and losses of applied C under both rainfed and irrigated conditions in contrasting soils is scarce.

Hence, we hypothesised that balanced fertilisation increased C sequestration under both rainfed and irrigated systems, and estimated that added C humified faster under an irrigated soybean wheat system in a silty clay loam soil than a rainfed system in a sandy loam soil, due to faster oxidative losses in the later system. As both rainfed and irrigated systems were not maintained on each soil type, the soils' contribution to C sequestration under rainfed and/or irrigated systems could not be determined. Thus, the objectives of this study were: (i) to quantify the effects of fertilisation on C sequestration and to establish a relationship between annual C addition and storage; and (ii) to measure the role of soybean wheat system in stabilising added C through FYM and crop residues under 2 distinct management practices (soybean and wheat crops were grown under residual fertility in the irrigated and rainfed soybean-wheat cropping systems, respectively) in 2 contrasting soil types (silty clay loam and sandy loam, respectively) under the subtemperate climate of the Indian Himalayas.

Materials and methods

Experimental site

Both long-term field experiments were initiated at the experimental farm of Vivekananda Institute of Hill Agriculture, located in the Indian Himalayan region at Hawalbagh (29[degrees]36'N, 79[degrees]40'E; 1250 m above mean sea level), in the state of Uttarakhand, India. The climatic properties during the experimental period (1973-2005) are shown in Fig. 1. The fields were newly reclaimed for cultivation in 1973 and in 1995. Prior to that, the fields were native grasslands that were yearly cut and grazed by goats and cattle.

[FIGURE 1 OMITTED]

Experimental details

The rainfed experiment included 2 crops per year, soybean (June-October) and wheat (October-April), with 6 treatment combinations (nutrients in kg/ha): no fertiliser and no manure (unfertilised control); 20 N + 35 P (NP); 20 N + 33 K (NK); 20 N + 35 P + 33 K (NPK); 20 N + FYM at 10 Mg/ha (N + FYM, commonly used by the local farmers); and NPK + FYM at l0 Mg/ha (NPK + FYM). The experiment started in June 1973 with sowing of soybean. Treatments were distributed in a randomised block design with 6 replications. The net plot size was 5.4 by 2.0 m. Based on the chemical analysis of every fifth year, FYM had 370 g moisture/kg and contained 7.3 ([+ or -]0.2) g N/kg, 2.25 ([+ or -]0.25) g P/kg, and 5.55 ([+ or -]0.25) g K/kg on oven-dry weight basis, and C : N ratio of the applied FYM was 29.8. Farmyard manure (10 Mg/ha on a fresh weight basis) and mineral fertilisers were applied before soybean sowing. Soybean was harvested manually just above the ground in the first week of October using sickles, and above-ground biomass was removed from the field. Soybean grain yield was adjusted to 90g moisture/kg. Wheat straw (0.05 m above the soil surface) was removed from the plots in late April and straw dry weights were recorded. Wheat grain yield was adjusted to 120g moisture/kg. Wheat stubble (straw below 0.05 m) was incorporated into the soil during land preparation for soybean. Detailed management practices of both crops are given elsewhere (Bhattacharyya et al. 2006). Wheat and soybean were grown in residual fertility under rainfed and irrigated systems, respectively.

The irrigated experiment also included 2 crops per year, wheat and soybean, with 6 treatments arranged in a randomised complete block design with 4 replications. The experiment started in early November 1995 with sowing of winter wheat. The treatment combinations (nutrients in kg/ha) were: no fertiliser and no manure (unfertilised control); 120 N (N); FYM at 10 Mg/ha (FYM); 120 N + 26 P + 33 K (NPK); 120 N + FYM at 10 Mg/ha (N + FYM); and NPK + FYM at 10 Mg/ha (NPK + FYM). The individual plot size was 17.5[m.sup.2]. In all plots, pre-sowing irrigation (except in 1997) was given in the first week of November to facilitate field preparation and crop (wheat) establishment (Table 1). Three days after irrigation, FYM was applied in the plots under respective treatments and incorporated in the soil during field preparation. Wheat cultivar VL-616 was sown (100kg seed/ha) in rows 20cm apart at a depth of 50-60 mm manually. Soybean (cv. VLS 2, 80 kg seed/ha) was sown in the second week of June each year in rows 0.4 m apart. The climatic pattern of this region (especially the rainfall) has necessitated irrigation, as only 4 months (June-September) in the year get surplus rainfall. Moreover, due to soil compaction (Bhattacharyya et al. 2007), the wheat crop requires water in drier months for better seedling emergence and root proliferation. Hence, irrigations (0.05m each) were applied through lift irrigation in all treatments uniformly to both crops at 80% pan evaporation as detailed in Table 1. The farmers of this region mostly use the lift irrigation system. Disease and weed management details are reported elsewhere (Bhattacharyya et al. 2007).

Soil sampling and analysis

Soil analysis was done from archive soil samples of 1973 and 1995 as well as after the harvest of wheat in 2005 and soybean in 2004 of the rainfed and irrigated experiments, respectively. Initial soil characteristics of both rainfed and irrigated experiments are given in Table 2. Depth-wise (0-0.15, 0.15-0.3, 0.3-0.45m) triplicate soil samples were collected from all plots, combined, ground, dried (65[degrees]C), passed through 0.2-mm sieve, and total C contents were measured. Soil was analysed for pH in 1:2.5 soil:water suspension (Jackson 1973), available P following the method of Olsen et al. (1954), and available K by 1 N N[H.sub.4]OAc using a flame photometer (Jackson 1973). Total C and N of soil samples, manure, and all plant inputs (leaf-fall, nodule, root, and rhizodeposition for soybean and stubble, root, and rhizodeposition for wheat) were measured with a CHN analyser. Core sampler was used for soil bulk density determination. Soil texture was determined by Bouyoucos hydrometer (Bouyoucos 1927). Soil organic C concentrations were converted from g C/kg to Mg C/ha using measured soil bulk density and soil depth (m). Mean soil bulk density, SOC concentration, and contents in the 0-0.45 m were determined from the corresponding values of 3 soil depths (0-0.15, 0.15-0.3 and 0.3-0.45 m).

Calculation of SOC stabilisation

Carbon sequestration potential (CSP; Mg/ha.year) of a particular treatment was calculated by the following relationship:

CSP = ([SOC.sub.treatment] - [SOC.sub.initial])/n (1)

where [SOC.sub.treatment] and [SOC.sub.initial] represent SOC content (Mg C/ha) in the treated and initial plots, respectively, and 'n' indicates years of experimentation. The amount of SOC remained and stabilised in the entire 0-0.45 m depth was estimated as:

SOC stabilisation (%) = CSP/ECI x 100 (2)

where ECI is the estimated amount of C (Mg C/ha.year) input through crop residues and applied manure (FYM). Soil organic C budgeting for the studied systems was done as:

SOC buildup rate (Mg C/ha.year) = ([SOC.sub.NPK+FYM] or [SOC.sub.NPK] - [SOC.sub.cont])/n (3)

where [SOC.sub.cont] represents SOC content (Mg C/ha) in the unfertilised control treatment.

Carbon inputs through plant and manure

Using biomass yield of soybean and wheat, annual C inputs to the soil through leaf-fall, roots, rhizodeposition, and nodules for soybean and stubble, roots and rhizodeposition for wheat were computed. Leaf-fall from all treatments was manually collected during 2002-04 of both experiments from 45 days after sowing until harvest, dried, and dry weight recorded. Under the rainfed condition, soybean leaf-fall biomass constituted 9.8, 16.1, 7.5, 21.0, 19.8, and 19.1% of the harvestable above-ground biomass in the plots under unfertilised control, NP, NK, NPK, N + FYM, and NPK + FYM, respectively. Whereas, under irrigated conditions, the leaf-fall biomass comprised 12, 11, 14, 17, 16, and 16% of the harvestable above-ground biomass in the plots under the unfertilized control, N, FYM, NPK, N + FYM, and NPK + FYM treated plots, respectively.

Root and nodule biomass of soybean was calculated using the root/shoot and nodule/root biomass ratios recorded from the pot experiments (Kundu et al. 2001). The pot experiments were conducted simultaneously using 10-kg capacity earthen pots filled with soil collected during 2002-04 from the field experiments. The treatment-wise soil samples collected from respective field plots were put in the pots and equivalent amounts of fertiliser and manure were applied. After harvest of above-ground biomass (shoot), soil was removed gently by placing the pot on a sieve and washed slowly using a water jet. Root biomass with entangled nodules was recovered, dried, and dry weights of root and nodules recorded. It was estimated that under the rainfed condition, root biomass represented 40.5, 42, 39.9, 37.2, 31.4, and 34.1% of the harvestable above-ground biomass in the pots with unfertilised control, NP, NK, NPK, N + FYM, and NPK + FYM, respectively. However, under the irrigated conditions, estimated root biomass in the unfertiliszed control, N, NPK, FYM, N + FYM, and NPK + FYM treated pots was 31,29, 28, 26, 26, and 24% of the harvestable above-ground biomass, respectively. Again, under the rainfed system in the pots under unfertilised control, NP, NK, NPK, N + FYM, and NPK + FYM treatments, nodule biomass comprised 11.3, 11.5, 11.1, 15.3, 15.0, and 13.9% of the soybean root biomass, respectively. In contrast, under the irrigated system in the pots with unfertilized control, N, NPK, FYM, N + FYM, and NPK + FYM treatments, nodule biomass constituted 9, 11, 12, 12, 13, and 13% of the soybean root biomass, respectively.

During 2002-04, samples of soybean leaves were collected after senescence, nodules were excavated on 65 days after sowing, and roots were excavated at 85 days after sowing from all replications in each treatment and analysed for total C content. Rhizodeposition of C from root turnover and exudates was assumed to be 10% of the harvestable above-ground biomass of soybean (Shamoot et al. 1968). Organic C of FYM was 22% on an oven-dry weight basis. We assumed that the entire amount of soybean biomass through leaf-fall and FYM-C, and 90% of the biomass contributed by soybean roots, nodule, and rhizodeposition, remained within 0-0.45 m soil depth under both rainfed and irrigated regimes.

An unmeasured biomass input by the wheat crop to soil was through stubble, roots, and rhizodeposition. We estimated stubble biomass of wheat crop (from our sample survey during 2001-02 to 2003-04) and found that under the rainfed condition the stubble biomass constituted 3.8, 3.4, 4.0, 3.1, 3.2, and 3.1% of the wheat straw yield in the unfertilised control, NP, NK, NPK, N + FYM, and NPK + FYM treated plots, respectively. Different treatments under the irrigated condition also had almost similar proportion. Root biomass of wheat under both rainfed and irrigated conditions was calculated using the root/shoot biomass ratios recorded from simultaneous pot experiments (Kundu et al. 2001). We estimated that in the unfertilised control, NP, NK, NPK, N + FYM, and NPK + FYM treated plots, the root biomass constituted 28.7, 28, 27.2, 26.1, 25.6, and 24.9% of the straw yield of wheat, respectively, under the rainfed system and almost similar proportions under the irrigated system. Wheat stubble and root samples were collected after harvest (2002-04) from all field plots and C inputs through stubble and roots were calculated after C measurements. Carbon contribution through wheat rhizodeposition was estimated by multiplying the values of total root C inputs with a factor of 1.4 (in both irrigated and rainfed conditions) as observed by Regmi (1994). We assumed that entire amount of biomass through stubble, FYM-C and 80% of the biomass contributed by roots and exudates remained within 0-0.45 m soil depth under both rainfed and irrigated conditions. During growth of soybean and wheat, weeds were either removed or killed by herbicides. Hence, C inputs from roots and rhizodeposition by the weeds were not considered.

Statistical analysis

Statistical analyses were done using standard analysis of variance (Gomez and Gomez 1984). Treatment means were compared at P=0.05 using least significant difference (1.s.d.). The interaction effects between treatments and growing conditions on SOC stabilisation were calculated using a 2-way ANOVA and the data were presented as boxplots using Minitab 15.

Results and discussion

Crop residue C inputs to soil

The roots and rhizodeposition were major components of estimated C inputs from soybean under both rainfed (Table 3) and irrigated (Table 4) conditions. Without any external fertiliser input, total C input from soybean was 362 and 620 kg/ha.year under the rainfed and irrigated conditions, respectively. Under both rainfed and irrigated systems, the highest mean harvestable above-ground biomass of soybean (Table 3) and wheat (Table 4) was observed in the plots under NPK + FYM and the lowest under the unfertilised control. Mean (across treatments) annual total estimated C input from crop residues was only ~7% higher in plots under irrigated soybean than rainfed soybean (~1034kgC/ha) even though nutrients were applied before the rainfed soybean (calculated from Tables 3 and 4). Apart from FYM's own contribution (1587kg C/ha.year), an additional gain of 1397 kg C/ha.year occurred with NPK + FYM compared with NPK only in the rainfed system. Under the irrigated wheat-soybean system, additional C input was ~501 kgC/ha.year in the plots under NPK + FYM compared with NPK. The higher above-ground biomass in the FYM-amended soils was probably associated with other benefits apart from N, P, and K supply, such as improvements in microbial activity (Ved Prakash et al. 2007) and soil physical conditions (Bhattacharyya et al. 2008). Kundu et al. (1997) also estimated soybean root biomass added the highest amount (31% of the above-ground biomass) to soil.

As with soybean, roots and rhizodeposition from wheat also contributed the highest amount of C input to soil under both rainfed (Table 3) and irrigated conditions (Table 4). The residual effect of FYM resulted in an additional gain of 177 kg C/ha.year in wheat compared with that under NPK only. Wheat above-ground biomass yield varied due to dissimilarity in residual fertility from different fertiliser treatments applied to soybean. However, mean (across treatments) annual total C input from crop residues was ~54% higher in plots under irrigated wheat than rainfed wheat (~734kg C/ha) (Table 4). Mean total C input values by crop residues under the rainfed and irrigated soybean wheat systems were 1.77 and 2.24 Mg C/ha.year (Tables 3 and 4). The higher C input under the irrigated experimental condition than under the rainfed condition was attributed to higher aboveground biomass under the former condition. Higher biomass under the irrigated regime was mainly due to the effect of soil type. In New Zealand, Tate and Ross (1997) observed that future C storage could be favoured in soils of moderate-to-high clay content and low (or moderately high) soil moisture status.

Soil bulk density

Soil bulk density was lower with FYM than with mineral fertilisation only and in the unfertilised control under the irrigated condition (Table 5). Highest soil bulk density in the 0-0.45m soil layer under the rainfed condition was in the unfertilised control plots and the lowest (1.25 Mg/[m.sup.3]) in the NPK + FYM treated plots. Likewise, application of NPK + FYM showed significantly lower soil bulk density (1.33 Mg/[m.sup.3]) than the unfertilised control (1.39 Mg/[m.sup.3]) plots in the 0-0.45 m soil layer (Table 5) under irrigated condition. Soil bulk density decreased with FYM application due to higher SOC and increased root biomass (Halvorson et al. 1999), which resulted in better soil aeration and aggregation.

Soil organic C stock and its build-up

Total SOC stock in the long term (32 years) rainfed experiment increased over the initial value in the unfertilised control plots (Table 5). However, under the irrigated system, SOC content decreased from the start of the experiment in the unfertilised control plots (Table 5). Decrease in SOC content is due to oxidation of SOC stock as a result of land preparation (Mandal et al. 2008). Such SOC loss on cultivation is a global phenomenon but its magnitude mainly varies with the types of crops/cropping systems being employed, geographical locations, inherent soil properties, cropping history of the land, and the duration of fallow period (Mandal et al. 2008). It is known that clay colloids have the capacity to build the SOC, and thus, clay particles protect it from undergoing oxidation (Denef and Six 2005). Furthermore, higher moisture availability might have increased the SOC losses under the irrigated condition. There was substantial annual C addition through the roots and crop residues in both systems, even in the unfertilised control plots (Kundu et al. 2007). The duration of the experimental period also played a role in stabilising the added C to SOC stock. Double cropping along with C addition was carried out for 32 years under the rainfed study, compared with only 9 years under the irrigated condition.

Under the rainfed experimental set-up, there was a net buildup of total SOC in the plots under NPK + FYM and NPK, the mean magnitude being ~24% and 78%, respectively, over the unfertilised control. This was mainly due to addition of a greater amount of residues and biomass C to the NPK + FYM treated plots than the unfertilised control (9.64 v. 1.92 Mg C/ha.year), and in the NPK treated plots than the unfertilised control (4.58 v. 1.92 Mg C/ha.year). There was nearly 603 kg C/ha.year increase in SOC stocks in the plots under NPK + FYM compared with NPK. Carbon sequestration potential (CSP), defined as the rate of increase in SOC content over the initial soil in the 0-0.45 m soil depth, ranged from 0.08 Mg C/ha.year in the unfertilised control plots to 0.95 Mg C//ha.year in the plots with NPK + FYM under rainfed condition (Fig. 1a). CSP was negative in the unfertilised control plots and increased significantly with NPK and NPK + FYM application under irrigated condition (Fig. 1b). Balanced fertilisation (NPK) significantly increased SOC stock in the 0-0.45 m soil layer over the imbalanced (NP/NK) and no application of nutrients under the rainfed system.

Carbon sequestration

in the irrigated study, NPK application also showed ~15% higher SOC than the unfertilised control (37.66 Mg C/ha) in the 0-0.45 m soil layer. There was a significant improvement in SOC in the FYM-treated plots (43.34 Mg C/ha) over N alone (40.31 Mg C/ha) in the same soil depth. At the end of 9 years, soils in the NPK + FYM plots contained higher SOC by ~10 and 15% in the 0-0.45 m soil layer than the NPK (43.34 Mg C/ha) and FYM (41.76 Mg C/ha) treated plots (Table 5). Carbon sequestration potential in the 0-0.45m soil depth ranged from 0.3 Mg C/ha.year in the plots with N to 1.19 Mg C/ha. year in the plots with NPK + FYM. Thus, cultivation along with N application only under the irrigated condition increased SOC stock in the 0-0.45 m layer, and manure addition significantly increased SOC sequestration. Fronning et al. (2008) also observed that total SOC in the 0-0.25m profile increased by 41 and 25% for the compost and manure treatments, respectively, at Michigan, USA. Higher C retention in manure amended plots was probably because manure is already partly decomposed and contains a larger proportion of chemically recalcitrant organic compounds (Paustian et al. 1992).

Carbon input through FYM and SOC enrichment and stabilisation

Under the rainfed system, 37.8% of added C could be accounted for in the form of total SOC (Table 6). In contrast, under the irrigated condition, only 29% of the added C through FYM was accounted for in the form of total SOC in the NPK + FYM treated plots (Table 6). Although this result is comparable with C stabilisation, it is not comparable under these 2 systems as found from the significant (P < 0.001) linear relationships between the gross C inputs and the changes in SOC. Thus, apart from FYM combined treatments, other treatments might be more efficient in sequestering SOC under the silty clay loam soil of the irrigated wheat-soybean rotation. It was observed that mean C stabilisation of the rainfed system (~16%) was notably higher than that under the irrigated system (~13%) (Fig. 2). Thus, under the rainfed condition, the added C was better converted to add SOC stock. Conversion of the added crop residue C to SOC was similar in the plots under NPK and NPK + FYM under the rainfed condition. Thus, with balanced fertilisation (NPK), the added amount of crop residue C helped to enrich the native SOC stock by stabilising a part of it. The observed conversion efficiencies (~13-16%) of the added C to total SOC in the Indian Himalayas was in the range of that reported earlier from temperate and Mediterranean climatic conditions (Rasmussen and Smiley 1997; Kong et al. 2005).

If the additional C added to the soil through crop residues in the plots under NPK + FYM was due to increased crop yield over the NPK, the total loss of applied C would be ~20% (Table 6) under the rainfed system and ~22% under the irrigated system (Table 6). it is interesting to note that the estimated proportion of the applied C lost from FYM + crop residues was higher in the irrigated than the rainfed system. Thus, a greater stabilisation of applied C under the rainfed condition was mainly due to a smaller loss of added FYM-C over the years. Although the plots under NPK + FYM had greater SOC build-up than NPK, C stabilisation was similar with the NPK + FYM and NPK treated plots under both rainfed and irrigated conditions. Thus, C stabilisation was found to be a better indicator for assessing SOC sequestration.

Rate constant (h) of added biomass C incorporation into soil organic matter

The annual rate of change in SOC was positively correlated (P<0.001) with the gross C inputs, under both rainfed and irrigated conditions (Fig. 3a, b). The slopes of these relationships represent the conversion rate of added C to total SOC. Numerically greater slope in the irrigated system might be due to the soil type and higher weed root biomass than the rainfed system. SOC retention was perhaps higher in a silty clay loam soil than a sandy loam soil, yielding a higher humification rate constant under the irrigated condition. Thus, after 9 years of continuous C additions under the irrigated condition, the soils of the experimental site are still unsaturated in their capacity for storing C and, therefore, have potential for further C sequestration. Similarly, after 32 years of continuous C additions, the sandy loam soil has the potential for further C sequestration. As proposed by Six et al. (2002), however, such capacity and/or storage rate cannot continue indefinitely. Each treatment (with different C loading) might lead to a new steady-state of SOC over time (Mondal et al. 2007). The significant positive linear relationships between changes in total SOC and the total cumulative C inputs to the soils over the years corroborate the findings of other researchers in temperate climatic conditions of Canada (Rasmussen and Collins 1991). The intercept (-1.94 and -2.35 Mg C/ha under the rainfed and irrigated conditions, respectively) of the equations (Fig. 3a, b) would represent the C loss from native SOM in 32 and 9 years, respectively.

Minimum C input to maintain SOC

Decay rate constant (k) of native SOC was calculated using the following equation (Jenkinson 1988):

d[C.sub.s]/dt = hA k[C.sub.s] (4)

where [dC.sub.s]/dt is the change in native SOC over time (years), h is humification rate constant, and A is the cumulative C input in an experiment. Carbon inputs required to maintain initial SOC ([C.sub.o]) was estimated as:

AE = k[C.sub.0]/h (5)

where AE is the amount of C input values required to be incorporated to maintain SOM content at equilibrium. The AE values were ~0.29 Mg/ha.year in the rainfed soybean wheat system and ~1.08 Mg/ha.year in the irrigated system. Annual C loss from native SOM in the Pacific North-West of the USA was 0.2-2.0 Mg/ha.year (Rasmussen and Albrecht 1998), and in a subtropical Central India 0.89Mg/ha.year (Kundu et al. 2001). Our relatively low value at 0.29 Mg/ha.year under the rainfed condition suggests a relatively slow decomposition rate of the native SOM.

Under the unfertilised condition in the rainfed system, average productivity (grain + straw yield) of soybean (1.31 Mg/ha) and wheat (2.12 Mg/ha) contributed C input of 0.37 and 0.52 Mg/ha.year (Table 3), respectively. The total annual C input with the unfertilised control plots was nearly 3 times more than the amount of C input required (0.29 Mg/ha. year) to maintain SOM at equilibrium under the rainfed condition. So, even removing the above-ground biomass of both crops, these production levels under the rainfed condition would supply enough C input to soil (provided soil erosion be checked) so that SOC could be maintained. It is also estimated that the SOC content in the 0-0.45 m layer would reduce to half ([T.sub.1/2] = 0.693/k) in ~385 and 99 years under the rainfed and irrigated conditions, respectively.

[FIGURE 3 OMITTED]

The estimated value at 1.08 Mg/ha.year suggests a reasonable decomposition rate of the native SOM in this heavy-textured soil with the irrigated wheat soybean rotation. In the plots under N only, average productivity (grain + straw yield) of wheat (5.00 Mg/ha) and soybean (3.43 Mg/ha) contributed C input of 0.97 and 0.78 Mg/ha.year, respectively (Table 4). The total annual C input with the N-treated plots was nearly 1.6 times more than the C input required (1.08 Mg/ha.year) to maintain SOM at equilibrium. Hence, under the irrigated condition of a silty clay loam soil, some amount of nutrients is required to be added (so that it may offset the removal of above-ground biomass of both wheat and soybean) to maintain SOM.

Conclusions

Under both rainfed and irrigated soybean-wheat systems, NPK application had a significant effect on C sequestration, which was further increased with annual NPK + FYM application. About 20 and 25% of the gross C input contributed to SOC accumulation under the rainfed and irrigated conditions, respectively. Annual C loss from native SOM averaged ~61 and 261 kg C/ha under the rainfed and irrigated conditions, respectively, and ~0.32 and 1.08 Mg C/ha was estimated to be needed to maintain SOM at equilibrium (at zero change) under the rainfed and irrigated soybean-wheat systems, respectively. Estimated C input indicates that the rainfed soybean-wheat cropping can sequester SOC even with no fertiliser or manure addition. However, under the irrigated condition, annual recommended fertiliser N input to wheat is required to sequester SOC.

Mean C stabilisation was notably higher under the rainfed condition than irrigated, indicating that duration of experiment might have played a major role in SOC stabilisation. Although the plots under NPK + FYM had higher SOC build-up (and, hence, carbon sequestration potential) than NPK, SOC stabilisation was similar in the NPK + FYM and NPK treated plots under the rainfed condition. The same was true under the irrigated condition. Thus, C stabilisation that takes into account estimated annual C input is a better indicator to judge soil C sequestration and merits wide attention.

Acknowledgments

The authors thank Mr Ramesh, Mr L. D. Malkani, and Mr Narayan Ram for their technical support in conducting field and laboratory investigations.

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Ranjan Bhattacharyya (A,C), Ved Prakash (A), S. Kundu (B), S. C. Pandey (A), A. K. Srivastva (A), and H. S. Gupta (A)

(A) Vivekananda Institute of Hill Agriculture, Almora--263 601, Uttarakhand, India.

(B) Current address: Indian Institute of Soil Science, Bhopal, India.

(C) Corresponding author. Current address: School of Applied Sciences, University of Wolverhampton, Wulfruna Street, West Midlands, WV1 1LY, UK. Email: ranjan_vpkas@yahoo.com
Table l. Monthwise rainfall (mm) received and number of irrigations
applied during the experimental period in a long-term irrigated
wheat-soybean rotation

Data in parentheses indicate number of irrigations (of 5 cm each)
applied

Month 1995-96 1996-97 1997-98 1998-99 1999-00

 Wheat growing season

Nov. 0 (1) 0 (1) 52 5 (1) 0 (1)
Dec. 1 (1) 0 (1) 70 0 (1) 0 (1)
Jan. 83 66 8 (1) 49 105
Feb. 64 1 (1) 46 11 (1) 56
Mar. 33 (1) 31 (1) 80 0 (2) 30 (1)
April 38 (1) 110 63 (1) 0 (2) 57 (1)

 Soybean growing season

June 141 129 120 74 303
July 171 223 213 187 194
Aug. 197 107 257 50 268
Sept. 73 108 170 198 106

Month 2000-01 2001-02 2002-03 2003-04

Wheat growing season

Nov. 10 (1) 0 (1) 0 (1) 3 (1)
Dec. 3 (1) 3 (2) 25 (1) 17 (1)
Jan. 23 (1) 50 60 65
Feb. 34 (1) 115 117 19 (1)
Mar. 30 (1) 14 (2) 57 (1) 52 (1)
April 32 (1) 56 (1) 47 (1) 70

Soybean growing season

June 123 131 54 (1) 119
July 199 196 271 115
Aug. 134 111 287 185
Sept. 9 (1) 150 198 138

Table 2. Soil properties of initial conditions NA, Not analysed

Property Rainfed experiment (start 1973)

Soil depth (m): 0-0.15 0.15-0.3 0.3-0.45

pH (soil : water, 1:2.5) 6.2 6.1 NA
EC (dS/m) 0.08 0.07 NA
CEC [cmol(+)/kg soil] 8.7 NA NA
Bulk density (Mg/[m.sup.3]) 1.32 1.33 1.36
Total organic C (g/kg soil) 5.8 5.6 5.3
Available N (kg/ha) 127 119 88
Available P (kg/ha) 12 10 7
Available K (kg/ha) 65 59 48
Texture Sandy loam Sandy loam NA

Property Irrigated experiment (start 1995)

Soil depth (m): 0-0.15 0.15.3 0.3-0.45

pH (soil : water, 1:2.5) 6.4 6.3 NA
EC (dS/m) 0.12 0.11 NA
CEC [cmol(+)/kg soil] 13.5 11.9 NA
Bulk density (Mg/[m.sup.3]) 1.36 1.37 1.40
Total organic C (g/kg soil) 7.31 6.09 5.09
Available N (kg/ha) 295 278 205
Available P (kg/ha) 9.2 9.1 8.3
Available K (kg/ha) 164 142 109
Texture Silty clay Silty clay NA
 loam loam

Table 3. Mean (1973-2005) annual C input to soil (0-0.45 m depth)
from rainfed soybean, wheat, and soybean-wheat system under
different fertiliser treatments

HY, Harvestable above-ground biomass (shoot) yield; RD,
rhizodeposition; TCI, total C input by the system

 Soybean

 HY Leaf- Root Nodule RD
Treatment (kg/ha) fall C input (kg/ha) FYM Total

Control 1301 48 167 17 130 -- 362
NP 2146 126 246 28 215 -- 645
NK 1605 47 191 20 161 -- 419
NPK 3410 276 400 57 341 -- 1074
N + FYM 5809 426 567 75 581 1587 3236
NPK + FYM 7170 515 729 95 717 1587 3644
Mean 3574 240 383 49 358 -- 1563
1.s.d. 296 19 37 5 33 -- --
 (P=0.05)

 Wheat

 HY Stubble Root TCI
Treatment (kg/ha) C input (kg/ha) RD Total (kg/ha)

Control 2150 35 196 274 505 867
NP 2533 38 227 318 583 1228
NK 2396 41 214 300 555 974
NPK 3248 41 267 374 682 1756
N + FYM 4638 62 384 538 984 4220
NPK + FYM 5504 73 426 597 1096 4740
Mean 3412 48 286 400 734 2297
1.s.d. 281 4 28 42 75 --
 (P=0.05)

Table 4. Mean (1990-2004) annual C input to soil (0-0.45 m depth) from
irrigated soybean, wheat, and soybean-wheat system under different
fertilizer treatments

HY, Harvestable above-ground biomass (shoot) yield;
RD, rhizodeposition; TCI, total C input by the system

 Soybean

Treatment HY (kg/ha) Leaf-fall Root Nodule RD Total

Control 2566 118 251 20 231 620
N 3453 137 306 30 311 784
FYM 3997 220 338 38 360 956
NPK 5568 365 448 48 501 1362
N + FYM 5711 349 456 54 117 1373
NPK + FYM 6722 399 489 60 605 1553
Mean 4670 265 381 42 354 1108
1.s.d. 356 28 39 4 51 90
 (P=0.05)

 Wheat

Treatment HY (kg/ha) Stubble Root RD FYM Total

Control 2145 22 195 218 -- 436
N 5005 45 436 489 -- 970
FYM 4048 44 399 417 1587 2247
NPK 7967 67 656 734 -- 1457
N + FYM 8131 68 674 755 1587 3084
NPK + FYM 10258 87 791 886 1587 3351
Mean 6250 56 525 583 -- 1924
1.s.d. 452 8 54 66 --
 (P=0.05)

 TCI
Treatment (kg/ha)

Control 1056
N 1754
FYM 3203
NPK 2819
N + FYM 4457
NPK + FYM 4907
Mean 3032
1.s.d. --
 (P=0.05)

Table 5. Soil bulk density and soil organic C in the 0-0.45 m
depth as affected by fertiliser application in soybean-wheat
rotations

 Rainfed soybean-wheat cropping

 Soil bulk Soil organic C
 density Concentration Amount
Treatment (Mg/[m.sup.3]) (g/kg soil) (Mg/ha)

Initial soil 1.34 5.54 33.31
Control 1.32 6.01 35.73
NP 1.28 6.83 39.34
NK 1.31 6.48 38.17
NPK 1.28 7.70 44.36
N + FYM 1.26 10.05 56.96
NPK + FYM 1.25 11.32 63.66
1.s.d. (P=0.05) 0.02 0.09 0.82

 Irrigated wheat-soybean cropping

 Soil bulk Soil organic C
 density Concentration Amount
Treatment (Mg/[m.sup.3]) (g/kg soil) (Mg/ha)

Initial soil 1.38 6.16 38.11
Control 1.39 6.05 37.66
N 1.37 6.56 40.31
NPK 1.35 7.13 43.34
FYM 1.36 6.85 41.76
N + FYM 1.35 7.36 44.54
NPK + FYM 1.33 7.92 47.44
1.s.d. (P=0.05) 0.02 0.16 1.26

Table 6. Effects of balanced fertilisation (NPK and NPK + FYM)
on SOC build-up and estimated amount of SOC lost from applied
FYM and crop residues in the 0-0.45 m soil depth under studied
cropping systems

TECR, Total estimated C applied through crop residues: residue
C applied = residue C in (NPK + FYM - NPK)

 SOC build-up rate

Cropping system NPK NPK + FYM via FYM (A)

 (Mg C/ha.year)

Rainfed soybean wheat 0.27 0.87 0.60
Irrigated wheat-soybean 0.63 1.09 0.46

 SOC build-up Total C applied
Cropping system via FYM (B) via FYM TECR

 (Mg C/ha)

Rainfed soybean wheat 19.20 50.78 44.70
Irrigated wheat-soybean 4.14 14.28 4.48

 % applied C lost from:

 FYM + crop
Cropping system FYM residues

Rainfed soybean wheat 62.2 79.9
Irrigated wheat-soybean 71.0 77.9

(A) (NPK + FYM - NPK).

(B) Also indicates total SOC loft over in soil from FYM.

Fig. 2. Conversion of added C into soil organic C (SOC) in the 0-0.45m
soil layer of long-term rainfed (R) and irrigated (I) soybean-wheat
cropping systems under different management practices. Boxes are
interquartile range (Q1-Q3); [??], mean; --. Median. Two-way ANOVA:
effects of treatments and management regimes on SOC stablisation (%):

Source F P

Treatments 202.3 <0.001
Management regime 28.1 <0.001
Interaction 32.7 <0.001
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Author:Bhattacharyya, Ranjan; Prakash, Ved; Kundu, S.; Pandey, S.C.; Srivastva, A.K.; Gupta, H.S.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:9INDI
Date:Sep 1, 2009
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