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Published online 2 December 2005
Published in Soil Sci Soc Am J 70:121-129 (2006)
DOI: 10.2136/sssaj2005.0180
© 2005 Soil Science Society of America
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Soil & Water Management & Conservation

Soil Organic Matter in a West Bengal Inceptisol after 30 Years of Multiple Cropping and Fertilization

M. C. Mannaa, A. Swarupb,*, R. H. Wanjaria, Y. V. Singha, P. K. Ghosha, K. N. Singha, A. K. Tripathia and M. N. Sahac

a Indian Institute of Soil Science, Bhopal-462 038, Madhya Pradesh, India
b Central Soil Salinity Research Institute, Karnal, Haryana, India
c Central Research Institute-132001 for Jute and Allied Fibers, Barrackpore-743 101, West Bengal, India

* Corresponding author (aswarup{at}cssri.ernet.in)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rice-based multiple cropping systems are predominant in the Indo-Gangetic Plains of Indian subcontinent. A decline in yield of such systems has been observed and ascribed to quantitative and qualitative variations of soil organic matter (SOM). We evaluated the impact of the annual rotation: rice (Oryza sativa L.), wheat (Triticum aestivum L.), jute (Corchorus olitorius L.), with and without fertilizer treatments (control, N, N–P, N–P–K, and N–P–K plus farmyard manure [FYM]) on SOM and aggregate properties. At 0- to 15-cm soil depth, microbial biomass C and N, hot water–soluble C and N and hydrolyzable carbohydrates, and particulate organic matter C (POMC) and N (POMN) were found in the order N–P–K plus FYM > N–P–K > N–P > N > control. Over the course of the experiment, application of N alone decreased total organic C (TOC) by 20.4%, whereas N–P–K with or without FYM addition either maintained or enhanced compared to initial. Total soil N and mineralizable N declined in all the treatments except N–P–K plus FYM. Irrespective of treatments, microaggregates (53–250 µm) dominated with 43.9 to 51.3% of total soil aggregate size distribution, followed by macroaggregates (250–2000 µm with 34.6 to 40.1%). The C and N mineralization rate was greater in macroaggregates than in microaggregates, and correlated significantly with POMC (r = 0.67, P ≤ 0.01) and POMN (r = 0.88, P ≤ 0.01). Nitrogen–phosphorus–potassium plus FYM also improved overall soil aggregation as compared to other treatments. Therefore, the results suggest that the gradual depletion of nutrients and structural degradation may have collectively contributed to the crop yield declines in the rice–wheat–jute rotation and that the integrated use of N–P–K and FYM is an important nutrient management option for sustaining this cropping system.

Abbreviations: FA, fulvic acid • FYM, farmyard manure • HA, humic acid • LFC, light fraction of C • LFN, light fraction of N • MBC, microbial biomass C • MBN, microbial biomass N • POM, particulate organic matter • POMC, particulate organic matter C • POMN, particulate organic matter N • SOM, soil organic matter • TN, total soil N • TOC, total organic C • WSC, water-soluble C • WSN, water-soluble N


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ANNUAL TRIPLE-CROPPED rice-based systems predominate in the lowland irrigated Indo-Gangetic Plains of the Indian subcontinent. In South Asia, the rice-based cropping system occupies about 13.5 million ha in the Indo-Gangetic Plains and provides food for 400 million people (Ladha et al., 2000). These systems are perhaps the most intensive cereal-based crop production in the world. Under such intensive cultivation, several reports indicated declining or stagnating trends in grain output (Swarup, 1998; Dawe et al., 2000; Yadav et al., 2000; Bhandari et al., 2002). Ladha et al. (2003) emphasized that a decrease in soil C, N, and K is one of the causes of yield decline in long-term rice–wheat experiments in Asia. However, in long-term experiments of India, decline in SOM is the major cause of yield decline particularly in plots receiving only N (Swarup et al., 2000), irrespective of cropping system and soil type. This eventually leads to deterioration of soil quality. Losses and gains of SOM are influenced by land management practices such as cropping frequency (Campbell et al., 1995), reduced tillage (Reicosky et al., 1995), fertilizer application (Gregorich et al., 1996), manure application (Sommerfeldt et al., 1988), and also by cultivation of perennial legumes and grasses (Campbell et al., 1991). We hypothesized that a yield decline could be due to qualitative and quantitative changes in SOM and its impact on soil aggregates and nutrient supply. However, increasing levels of SOM does not necessarily maintain or increase yield (Ladha et al., 2003). Bronson et al. (1998) emphasized that it is the size of active fraction that is directly involved in nutrient availability. Thus, there is a great need to fully quantify the role of SOM fractions particularly C and N pools in relation to crop productivity and nutrient availability in rice–wheat system.

Our objectives were: (i) to assess changes in yield decline and trends of SOM as a result of 30 yr of continuous cropping and fertilizer use under the rice–wheat–jute system, (ii) to quantify the changes in soil aggregation and various soil C and N pools under this system, and (iii) to assess changes in their interrelationships with the influence of manure, fertilizer, and cropping sequences.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Methods
The site is located at the experimental station of the Central Research Institute for Jute and Allied Fiber, Barrackpore, West Bengal, India (22°45' N, 88°26' E) at an altitude of 9 m above mean sea level. Since 1971, the experiment has been conducted as part of the All India Coordinated Research Project on Long-Term Fertilizer Experiments, under the Indian Council of Agricultural Research. It is located in a hot, moist, subhumid, subtropical climate, with an annual average rainfall of 1698 mm, of which 75 to 80% occurs between June and September. In January mean monthly temperature falls to a minimum of 13°C; the maximum is 38°C in May. Initial soil characteristics of the experimental site are shown in Table 1. The experimental site and fallow plots had never been cultivated before 1968 and were previously covered with natural forest vegetation such as perennial weeds and grasses. Rice was the first crop sown in 1969 followed by jute in 1970.


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  		Table 1. Initial soil characteristics of experimental field.

 
Field plots (20 by 10 m) were established after annual crop residue removal. Five fertility treatments consisted of a control (no fertilizer), N, N–P, N–P–K, and N–P–K plus cattle FYM. The N–P–K applications were equivalent to 120, 26, and 50 kg ha–1 for rice and wheat and 60, 13, and 50 kg ha–1 for jute, respectively. Nutrient sources included urea, superphosphate, KCl, and FYM. The cattle FYM contained 6.5 N, 2.5 P, and 4.5 K g kg–1 and was applied at 10 Mg ha–1. Details of the experiment have been reported elsewhere (Saha et al., 2000). After harvest, the crop residues of all the three crops were collected from each treatment annually and removed from the plots.

The soil was sampled from experimental plots after wheat harvest in April 2001. An uncropped and unfertilized fallow plot under undisturbed vegetation was also sampled after collection of residue, if any, of the natural vegetation in April 2001. Four soil cores were taken in each field plots to a 0.45-m depth and sectioned into 0.15-m depth increments. The samples were subdivided into two subsamples. One was processed, sieved through a 2-mm mesh, and kept in polyethylene bags at 4°C for biochemical analyses. The other subsample was air dried and kept for chemical analyses and aggregate separation.

Laboratory Analysis
Aggregate Size Fractions and Associated C and N Concentrations
Samples of the 0- to 15-cm depth (>2 mm) were used for aggregate separation by wet sieving method (Elliott, 1986; Camberdella and Elliott, 1992). Two 100-g subsamples (capillary-rewetted) were wet sieved by double stage Yodder's apparatus through a series of three sieves to obtain four size fractions: >2000 µm, 250 to 2000 µm, 53- to 250-µm, and <53 µm. Aggregate fractions retained on each sieve were transferred into a container, dried at 65°C in a vacuum oven, and ground to pass 150-µm size before chemical analysis. Sand-free C and N concentrations in each aggregate-size class were calculated following dispersion and determinations of their sand content (Elliott, 1986; Six et al., 2000).

Soil Total and Mineralizable C and N Pools
Total organic C content of whole soil samples or aggregate size fractions was determined by the dichromate oxidation method (Nelson and Sommers, 1975). Subsamples (1 g) were digested at 150°C for 30 min and the C content was calculated from the unreacted dichromate. Total N was estimated in whole soil by automated analyzer following a wet digestion with concentrated H2SO4 and H2O2 at 360°C temperature (Thomas et al., 1967). Available P was determined following extraction with 0.5 M NaHCO3 and measured colorimetrically according to the phosphomolybdate–ascorbic acid method (Watanabe and Olsen, 1965). Steam distillation (Bremner, 1965) in the presence of MgO and Devarda's alloy was used to determine 2 M KCl–extractable N (NH4–N and NO3–N) from whole soil samples.

Anthrone-reactive C was determined according to the method described by Brink et al. (1960). Subsamples of whole soil (5 g) were equilibrated with 50 mL of 1.5 M H2SO4 for 24 h at 85°C. Aliquots of the extracts were reacted with 0.2% anthrone reagent and measured colorimetrically at 625 nm. In addition, hot water–soluble C was determined in whole soil subsamples (10 g) by shaking with 100-mL deionized water for 1 h followed by centrifugation at 1000 rpm for 30 min, and the supernatant was filtered through a 0.45-µm membrane (Campbell et al., 1999). Total Kjeldahl N content of the water extract was determined following digestion according to the semi-micro Kjeldahl method (Bremner, 1965).

Microbial biomass C (MBC) and N (MBN) were determined in moist soil samples after 3 d of pre-incubation period at 25°C to attain basal respiration conditions (Srivastava and Singh, 1989). The MBC in subsamples of the pre-incubated soils (12 g dry weight equivalent) were determined by the fumigation–incubation method (Jenkinson and Powlson, 1976) using a kc = 0.45 conversion factor (Jenkinson and Ladd, 1981). For MBN, the fumigated-incubated sample at 10 d was extracted with 50 mL (1:10 soil/solution ratio) of 2 M KCl. The mineral N (NH4+ + NO3–) from fumigated and unfumigated samples was determined by steam distillation and MBN was calculated by a conversion factor of kn = 0.54 (Brookes et al., 1985).

Aggregate Size Fraction Associated Mineralizable C and N Pools
Carbon mineralization was measured from the soil (10 g dry weight equivalent), retained in 250- to 2000-µm, 53- to 250-µm, and <53-µm sieve size classes at 1, 2, 4, 6, 8, 10, 12, 14, and 16 wk after initiation of incubation. The CO2 evolution was determined by 15 mL 0.5 M NaOH following the titration of unreacted NaOH using 0.25 M HCl.

The N mineralization was measured as per the method described by Standford and Smith (1972). For this determination, 15-g oven-dried soil samples for each treatment were used from whole soil as well as from soil retained on 250- to 2000-µm, 53- to 250-µm, and <53-µm sieve size classes in a separate incubation. All samples were leached at 1, 2, 4, 8, and 16 wk after initiation of incubation and the extractable N were analyzed for N analysis (Bremner, 1965).

Light Fraction and Heavy Fraction Particulate Organic Matter
Seventy-gram subsamples of whole soil (>2000 µm) were dispersed in 175-mL NaI solution (2 g cm–3) with low energy (5.57 J s–1 for 60 s) ultrasonic treatment allowed for removal of floatable free light fraction of particulate organic matter (POM). After a setting period of 24 h, the suspended material (i.e., light fraction) was centrifuged and floated material was washed with distilled water and collected in a filter (0.42 µm). The light fractions of C (LFC) and N (LFN) were analyzed for total C and N contents using the method described by Janzen et al. (1992). After floating off, the soil was dispersed with 175 mL of distilled water using high-energy (12 500 J for 13 minute) ultrasonic treatment for determination of heavy fractions. The material retained on the sieve (<53 µm) was dried at 65°C, and ground for analysis of C and N concentration. Particulate organic matter C and POMN of the samples were calculated as the C or N difference in soil aggregates with <53-µm and whole soils sample, expressed on dry weight equivalent.

Humic C and N Fractions
The principal extraction procedure (Stevenson, 1994) was followed by separation after extracting with freshly prepared 0.5 M NaOH at pH 13.0 with acid wash (0.1 M HCl). An exactly 10-g oven-dried sample (0–15 cm) was taken in a polypropelyne centrifuge bottle and 200 mL of 0.5 M NaOH was added to it. The humic acid (HA) solution was precipitated with 3 M HCl at pH 1.5 and allowed to stand for 16 h. The HA-precipitate was treated (three times) with HCl–HF (0.1 M HCl: 3 M HF = 1:1, HA: HCl–HF = 1:10) by shaking for 24 h and dialysed with distilled water until Cl– was no longer detected with AgNO3 and vacuum oven dried at 40°C to get powder of HA sample. Similarly, the fulvic acid (FA) fraction was estimated and calculated as percentage of FA of organic matter.

Yield Trends and Statistical Analysis
A linear regression analysis of grain yields over the years was done to determine a time trends (slope) variable for all treatments to state the hypothesis that yield trends throughout the experimentation period are not significantly different from zero. Analysis of variance (Gomez and Gomez, 1984) across years was done to determine the effects of treatments.

The experiment was conducted in a randomized complete block design with four field replications for each treatment. The least significant difference (LSD) test for means comparison was used to determine significant differences between measured properties. Duncan's multiple range test was used to compare treatment means. Test of significance for the treatments differences was done on the basis of t test. The statistical significance was evaluated at P ≤ 0.05 and P ≤ 0.01.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Yield Trends and Soil Organic C
The yield trends and TOC over 30 yr of multiple cropping systems are presented in Fig. 1 and Table 2. Linear regression analysis showed negative yield trends ranging from 0.02 to 0.09, 0.01 to 0.04, and 0.015 to 0.04 Mg ha–1 yr–1, of rice, wheat, and jute, respectively. For rice and jute, however, significant (P ≤ 0.01) negative yield trends were observed in N, N–P, and N–P–K plots (Fig. 1). The declining trends are also observed in TOC content in control, N, and N–P plots compared to the initial value (Table 1) and were quantified using linear models shown in Fig. 1.



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  		Fig. 1. Yield trends of (A) rice, (B) wheat, and (C) jute in the long-term experiment at Barrackpore, West Bengal, India; * and ** represent significance at P ≤ 0.05 and P ≤ 0.01, respectively.

 

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  		Table 2. Long-term effect of manure and fertilizer application on active pools of C and N under rice–wheat–jute system.{dagger}, {ddagger}

 
Multiple Cropping Effect on Soil C and N Concentrations
Total organic C and N (TN) concentrations in bulk soil samples of the unfertilized control plots lost approximately one-third of its original TOC and two-thirds of its initial TN concentration (7.12 g TOC kg–1 and 960 mg N kg–1 soil at 0–15 cm depth in 1971) (Table 2). In the treatment receiving 100% N or N–P, the magnitude of decrease was less compared to control whereas 100% N–P–K or N–P–K plus FYM treatment either maintained or enhanced TOC and TN concentrations as compared to unfertilized (control) treatment. There were higher concentrations of C and N pools in surface layer (0–15 cm) than in the deeper layers (15–30 and 30–45 cm) (Table 2). The MBC and MBN in the treatment receiving FYM with fertilizer N–P–K were about 48.6 to 200% and 32.8 to 88.8% more than in N, N–P, and N–P–K treatments and approximately 1.5 and 1.3 times higher than fallow soils in surface layer (0–15 cm). The results showed that MBC ranged from 2.8 to 6.1% of TOC and MBN from 1.6 to 2.7% of total N in the surface layer whereas in lower depths these values were relatively higher in all the treatments. On an average the water-soluble C (WSC) and water-soluble N (WSN) accounted for 0.2 to 1.4% of TOC and 1.0 to 2.3% of TN, respectively, whereas hydrolyzable carbohydrates accounted for 9.2 to 12.6% TOC at the top surface layer (0–15 cm depth). In lower depths (15–30 and 30–45 cm), both WSC and WSN followed an almost similar trend to that of surface soil.

C and N Distribution in Organic Matter Fractions
Analysis of the LFC and LFN for different treatments showed that N, N–P, or N–P–K had no temporal build up of LFC and LFN in 0- to 15-cm depth under rice–wheat–jute system (Table 3). The percentages of soil C present in the light fraction and the heavy fractions of POM in the top 0- to 15-cm depth of cultivated soils were 1.2 to 2.7% and 10.6 to 28.0% of TOC, respectively. The corresponding POMN comprised 5.8 to 21.1% of TN. The mineral associated organic C and N increased with N, N–P, and N–P–K application. Therefore, 69.5 to 82.4% of TOC was present in the mineral associated organic matter in these treatments. It was observed that application of N, N–P, or N–P–K for 30 yr, the heavy fractions of POMC and POMN reduced by 5 to 38.6% and 5.2 to 28.5% compared to fallow soil. These patterns imply that decomposition and fraction of newly humified C in POM, derived from roots, leaf fall, and stubble biomass was possibly similar in these inorganic fertilizers plots.


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  		Table 3. Effect of manure and fertilizer on light fraction C (LFC) and N (LFN), particulate organic matter C (POMC) and N (POMN), and mineral-associated C and N (0–15 cm depth).

 
Aggregate Size Distribution and Associated C and N
Since no significant differences between 15- to 30-cm and 30- to 45-cm soil depths were noticed, discussion of various C and N fractions of slow pools are confined to only 0- to 15-cm depth (Fig. 2 ). In all the treatments rewetted aggregate size distributions were dominated by 53- to 250-µm microaggregates that accounted for 43.9 to 51.3% of dry soil weight. However, 250- to 2000-µm macroaggregates accounted for 34.6 to 40.1% of the dry soil weight (Fig. 2A). The relative weight of soil increased with decrease in aggregate size classes, and significant (P ≤ 0.05) differences due to fertilizer treatments were observed in all aggregate size classes (> 2000, 250–2000, and 53–250 µm) except in the silt–clay fractions (<53 µm). The recommended dose of N–P–K plus FYM improved macroaggregates in the cultivated soil by 10% over fallow soil. Continuous application of N, N–P, and N–P–K significantly reduced larger macroaggregates (>2000 µm), and it varied from 27 to 53.2%. Overall, the C and N concentrations were greater in microaggregates compared to small macroaggregates (250–2000 µm). Irrespective of aggregate size classes, the concentration of C and N was higher in the N–P–K plus FYM treatment followed by N–P–K and fallow soils (Fig. 2B, 2C). No significant variation in aggregate size classes between N and N–P treatment were observed. The least C and N were present in the <53-µm size class for all the treatments.



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  		Fig. 2. (A) Aggregate-size distribution, (B) aggregate C, and (C) N concentrations in the surface layer (0–15 cm) of long-term fertilizer experimental site after 30 yr under rice–wheat–jute system. Bars labeled with different letters are significantly different between treatments at P ≤ 0.05 by the Duncan's multiple range test.

 
C and N Mineralization in Soil Aggregates
The C and N mineralization rate constants (k) often increased in the order of macroaggregates > microaggregates > mineral-associated organic matter (Table 4). The k of C and N in microaggregates (53–250 µm) varied significantly (P ≤ 0.01) among the treatments. Higher amounts of C and N mineralization were observed in N–P–K with FYM treatments compared to N, N–P, and N–P–K treatments. However, there were no significant differences in k of N mineralization in 250- to 2000-µm macroaggregates and the <53-µm mineral-associated SOM. The maximum k value was observed in the FYM plus N–P–K treatment. Irrespective of treatments, the N mineralization rate was relatively higher than C mineralization in 250- to 2000-µm macroaggregates.


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  		Table 4. Long-term effect of manure and fertilizer application on C- and N-mineralization rate (k) under different aggregates from the top-soil layer (0–15 cm).{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
A significant yield decline along with decline of TOC under imbalanced fertilizer application was observed in this experiment (Fig. 1). It is also evident that yield declined in N–P–K plots despite the fact that TOC was maintained in this treatment. This study clearly indicates that TOC content is not necessarily related to yield decline. For instance, Bronson et al. (1998) and Yadav et al. (2000) observed yield decline of rice and wheat in the long-term experiments though SOM content increases. A similar result was observed by Regmi et al. (2002) using continuous addition of FYM for 20 yr under a rice–wheat system.

Thirty years of continuous cropping without application of adequate quantity of nutrients in balanced doses and/or without addition of organics led to loss of TOC, TN, and a gradual decline in active C and N pools (Table 2), which resulted in unsustainability of crop production. Total organic C generally increased in the 0- to 15-cm depth of all fertilized plots and was improved by 4% in the N–P–K and 11% in the N–P–K plus FYM treatment (Table 2). A similar observation was reported by Kundu and Ladha (1995) in a rice–wheat system. Though the rate of decomposition was higher in N–P–K with FYM than inorganic fertilizer plots (Table 4), the higher value of C input in the latter one was because of relatively greater amount of organic inputs in the system. Further, many studies reported that materials with a higher lignin content (FYM, sawdust) result in a greater accumulation per unit of C input than that of low-lignin residue amendments (Paustian et al., 1992; Stevenson, 1982).

The water-soluble fractions (WSC and WSN) are considered the most active component of SOM. Though it is a small fraction of SOM, it acts as buffering agent in replenishment mechanisms like desorption from soil colloids, dissolution from litter, and exudation from plant roots (McGill et al., 1986; Groot and Houba, 1995; Campbell et al., 1999; Curtin and Wen, 1999; Six et al., 2000). It appears to be the immediate substrate for the soil organisms. MBC was positively correlated with TOC (r = 0.87, P ≤ 0.01), TN (r = 0.83, P ≤ 0.01), and 2 M KCl extractable N (r = 0.93, P ≤ 0.01). There was a significant and positive correlation between soluble carbohydrates and MBC (r = 0.88, P ≤ 0.01) and between WSC and MBC (r = 0.89, P ≤ 0.01). The contribution of water-soluble fractions of SOM in the inorganic fertilizer treatment was less because aboveground biomass did not return to the soil in any form. There was a significant relationship (r = 0.87, P ≤ 0.01) between 2 M KCl extractable N and soluble carbohydrates, suggesting that KCl–N may be selective for a water-soluble fraction of SOM. Gianello and Bremner (1986) also made a similar observation. The reduced amount of active pools of C and N after long-term cultivation of soil leads to depletion of soil fertility through reduction of labile sources of nutrients, faster decomposition, and lower bioavailability of nutrients. Furthermore, continuous cultivation with cereal-based cropping system reduced total amount of nutrients as well as soil microbial biomass, which could lead to degradation of soil biological functions. Likewise, removal of aboveground biomass significantly reduced not only total amount of nutrients but also the active pools of C and N resulting in decline of crop yields. It is often difficult to maintain or enhance the organic matter and N in cultivated soil unless a cover or legume crop is included in the rotation or a heavy application of manures and crop residues is made (Stevenson, 1982). Therefore, balanced plant nutrition (fertilizer in combination with manure) every year may contribute more labile C substrates to sustain the mineralization process (Curtin and Wen, 1999).

Alternate wetting and drying conditions resulted after continuous intensive conventional tillage operations and removal of aboveground residues induced a rapid mineralization of aggregate-associated SOM which decreased by 13.6 to 53.2% of the >2000-µm aggregates compared to fallow treatment (Fig. 2A). The correlation between reduction in aggregates and loss of SOM with cultivation has been used to explain aggregate hierarchy theory by many authors (Tisdall and Oades, 1982; Camberdella and Elliott, 1992). Increasing cultivation intensity with repeated application of inorganic fertilizers (N, N–P, and N–P–K) caused reduction of macroaggregates. It may be due to the lack of a significant release of WSC and hydrolyzable carbohydrates (which acted as a binding agents) from belowground biomass decomposition on microbial action. This may have resulted in loss of soil aggregates (Jastrow, 1996; Six et al., 1998). Cassman et al. (1995) also observed that puddling operation creates a slurried soil by physical destruction of macropores and aggregates and results in a lower bulk density than for plowed or dry soil under upland condition.

As cultivation continued with N, N–P, and N–P–K fertilization, there was an extensive depletion of organic matter associated with POM, LFC, and LFN (Table 3). The slow pools of C and N particularly free LFC and LFN did not show any significant changes with N, N–P, and N–P–K treatments except FYM with N–P–K treatment. In our study, LFC and LFN originating from stubble biomass and root biomass in cultivated soil were mostly affected by both residue input and soil microclimatic conditions. Light fractions are more labile organic matter slow pools but constitute partially decomposed organic matter (Greenland and Ford, 1964). Belowground root-residue input was significantly correlated with LFC (r = 0.86, P ≤ 0.05) (data not presented). The LFC and LFN seem to be the POM fraction that is especially affected by tillage and residue input, whereas other fractions are affected by aggregation and aggregate mineralization (Six et al., 1998; Chan, 2001).

In general under laboratory incubation, the N-mineralization rate constant was higher than C-mineralization rate in 250- to 2000-µm macroaggregates, while in 53- to 250-µm microaggregates and the < 53-µm mineral-associated SOM C-mineralization was greater (Table 4). Such treatment differences in C- and N-mineralization rate in each aggregate size classes might be due to initial nonsoluble C/N ratio and changes in microbial species owing to changes in substrate quality and quantity such as LFC, LFN, and heavy fractions of POM. However, more research is required to explain whether significant fraction of labile materials are transformed to stable fraction during different land use management system that may eventually effect nutrient supply to plants. The practice of residue incorporation during brief transition period between two crops is difficult. For example, after harvest of rice the transitional gap is only 20 to 25 d before wheat sowing. Further investigations needed to explain as to how decomposition of residue could be managed in such a short period without adversely affecting the subsequent crop. Thus, regular addition of residue along with balance fertilizer could maintain active and slow pools of C and N under high intensive cropping system over time.

The humus fraction or nonextractable humus was thought to include POM entrapped within mineral aggregates (Goh et al., 1976; Anderson, 1979). The percentage of mineral-associated organic C to TOC was relatively higher in N, N–P, and N–P–K compared to N–P–K with FYM (Table 5). The acid hydrolyzable N was higher in FA fraction than HA fraction. The results showed that the passive fractions of C and N pools did not change significantly even by the application of manure and fertilizer thereby indicating that more time frame was required to effect a change in passive pools.


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  		Table 5. Humic acid (HA) and fulvic acid (FA) fractions of soil organic matter (SOM) as influenced by different treatments in the top layer (0–15 cm) after 30 yr of cultivation.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
After 30 yr of continuous cropping, the gradual depletion of one or more nutrients may have collectively contributed to the yield decline and stagnation in the rice–wheat–jute system in the Indo-Gangetic Plains of the Indian subcontinent. We hypothesized that depletion of organic pools is most likely to be the major concern. Active pools of C declined remarkably in the treatment with N and N–P. The slow pools of POMC and POMN decreased significantly with concomitant decrease of C and N mineralization rate in the aggregates in N- and N–P-treated plots which leads to lower nutrient supplying capacity of the soils. Application of balanced fertilizer N–P–K, either alone or in combination with FYM maintained active and slow pools of C and N at the surface (0–15 cm depth). This indicated that the organic pools of C-associated nutrients particularly N may be maintained in rhizosphere zone and thereby sustaining soil quality and productivity. Improvement of WSC and carbohydrates helped in improving soil nutrient dynamics and transformation through biological means. Passive pools of C viz., HA and FA fractions remained unchanged. The pronounced effect of integrated plant nutrient supply system on the distribution of organic matter among labile and slow pools is an indication of greater impact on soil fertility improvement. Thus, it is clear from the study that a more efficient and integrated nutrient supply strategy is necessary to sustain the long-term productivity and soil quality.


   ACKNOWLEDGMENTS
 
The authors are grateful to Dr. J.S Kanwar (Formaer DDG) ICRISAT, Hyderabad, and Dr. N. N. Goswami (Former Vice Chancellor, CSAUAT, Kanpur) for their untiring guidance and constructive criticism in overall improvement of this manuscript. Special appreciations are due to Indian Council of Agricultural Research, New Delhi for financial support to this programme through National Agricultural Technology Project. The technical assistance of Mrs. Seema Sahu, Mr. Hukum Singh, Mr. Bhoilal Uikey and Mr. A.K. Mishra is duly acknowledged.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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