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An understanding of the dynamics of carbon (C) stock in soils, as impacted by management strategies, is necessary to identify the pathways of C sequestration in soils and for maintaining soil organic C (SOC) at a level critical for upkeeping soil health and also for restraining global warming. This is more important in tropical and subtropical region where soils are inherently low in organic C content and the production system is fragile. We evaluated the long-term role of crop residue C inputs to soil in SOC sequestration and also the critical value of C inputs for maintenance of SOC across five different ricebased cropping systems and four soil management practices including a fallow (no cultivation since initiation of the experiments) using five long-term (7-36 years) fertility experiments in subtropical India. Cropping per se always caused a net depletion of SOC. Such depletion was inversely proportional to the amount of crop residue C incorporated into the soils (r 5À0.92, P 5 0.001). Balanced fertilization with NPK, however, caused an enrichment (9.3-51.8% over the control) of SOC, its extent being influenced by the cropping systems. Long-term application of organic amendments (5-10 Mg ha À1 yr À1 ) through farmyard manure (FYM) or compost could increase SOC hardly by 10.7% constituting only 18% of the applied C, the rest getting lost through oxidation. The total quantity of soil C sequestered varied from À11.5 to 14.5 Mg C ha À1 and was linearly related (r 2 5 0.40, P 5 0.005) with cumulative crop residue C inputs to the soils. On an average, the rate of its conversion to SOC came out to be 6.4%. This was more in presence of added organics (6.9%) than in its absence (4.2%). For sustenance of SOC level (zero change due to cropping) we found that a minimum quantity of 2.9 Mg C is required to be added per hectare per annum as inputs. The cropping systems and the management practices that could provide C input higher than the above critical level are likely to sustain the SOC level and maintain good soil health in the subtropical regions of the Indian subcontinent.

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.

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