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Abstract 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 rice‐based 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 =−0.92, P =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 −1 yr −1 ) 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 −11.5 to 14.5 Mg C ha −1 and was linearly related ( r 2 =0.40, P =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.
Mandal et al. (Wed,) studied this question.