In order to utilize agricultural soils as an option to offset atmospheric carbon, it is essential to ascertain the degree of stability of the accrued carbon. A two step acid hydrolysis technique was used to separate labile and recalcitrant carbon pools in soil upto 30cm depth to analyze their responses to different tillage managements after eight years of continuous practice in a sub-humid tropical rice-wheat system of Indo-Gangetic plains. There were four tillage practices; tillage before sowing/transplantation of every crop (RCT-WCT), tillage before transplanting of rice but no tillage before sowing the succeeding wheat crop (RCT-WNT), tillage before sowing of wheat but no tillage before sowing of rice (RNT-WCT), and no tillage before sowing of rice as well as wheat (RNT-WNT). It was observed that reduction in tillage frequency enhanced the total and recalcitrant carbon contents in soil with the maximum rate of sequestration recorded under RNT-WNT (0.59tCha-1yr-1). The fraction of carbon translated into recalcitrant pool was constant under all the tillage combinations indicating that carbon stabilization was dependent predominantly on organic matter input in the rice-wheat system. Conventional tillage on the other hand caused loss of carbon from the soil as observed by reduction in total soil carbon content under RCT-WCT. Reduction in recalcitrant carbon content under RCT-WCT further indicated that acid hydrolysis might not represent long term carbon accumulation reliably. Concentration of phenolics in labile pool increased under RNT-WNT, RCT-WNT and RNT-WCT practices which also had higher total and recalcitrant carbon pools. This indicated towards contribution of phenolics in carbon stabilization in the soil. Results of the present study further suggested that adoption of no till agriculture in the region offers significant carbon sequestration opportunity under proper agricultural management.
