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Potential of double‐cropped rice ecology to conserve organic carbon under subtropical climate

Understanding the processes of soil organic carbon (SOC) accumulation or depletion under different management strategies is vital for maintaining soil health and curbing global warming. Using a 36‐year‐old fertility experiment under subtropical climate, we investigated the impact of long‐term intensive rice–rice cropping system with different managements on the SOC stock. The mechanistic pathway of stabilization of the SOC into different pools, with a tentative C budgeting was also established. Biochemical composition of the organic residues involved, SOC pools of different oxidizability and methane (CH4) emission were estimated for the experiment conducted using organic and inorganic sources of nutrients. Cultivation over the years caused a net decrease in SOC stocks but with balanced fertilization it increased. With increasing depth, the stock decreased on average, to the extent of 50%, 26% and 24% of the total at 0–0.2, 0.2–0.4 and 0.4–0.6 m, respectively. About 4.0% of the crop residues C incorporated into the soil were stabilized into SOC. This was further enhanced (1.6 times) by the application of compost. Carbon loss through CH4 emission was very low (2.6% of the total). ‘Summer fallow’ had a positive significant influence on C loss from the system. As much as 29% of the compost C added to the soil was stabilized into SOC mostly in the less‐labile or nonlabile recalcitrant pools preferentially in the surface layer of the soil. Large polyphenol and lignin contents of crop residues including compost, and the long period of soil submergence under rice cultivation might have conferred recalcitrant character to the SOC leading to its stabilization in nonlabile pools. This would result into an enrichment of the SOC stock and restriction to the gaseous C loading into the atmosphere.