Long-term effect of rice-based cropping systems on pools of soil organic carbon in farmer's field in hilly agroecosystem of Manipur, India.

A comprehensive study on various pools of soil organic carbon (SOC) under different rice-based cropping systems is necessary for predicting their effect on soil quality through carbon build-up in soil and their impact on global climate change. The present investigation was undertaken to study the long-term effect of six different rice-based cropping systems (continuously followed by farmers > 10 years) on various SOC pools viz., total organic carbon (TOC), oxidizable organic carbon (Coc) and its different fractions [Cfrac1 (very labile), Cfrac2 (labile), Cfrac3 (less labile) and Cfrac4 (non-labile)], soil microbial biomass carbon (SMBC) and lability index (LI) and SOC stock at the farmer's field of Kakching district under hilly ecosystems of Manipur, India. In every cropping system, all the fractions of Coc were significantly decreased with increasing soil depth. Among all the fractions, Cfrac4 (non-labile) constituted the largest percentage of TOC for both surface (0-20 cm) and sub-surface (20-40 cm) soil varying from 47.95-58.45% and 55.76-64.83% with average values of 51.87 and 59.73% respectively. Results also revealed that the Cfrac1 (very labile) of Coc constituted highest (42.79%) percentage of Coc and that of Cfract4 constituted highest percentage (55.80%) of TOC. In both soil depths, rice-pea cropping system recorded highest TOC, Coc and SMBC followed by rice-French bean and rice-potato. In surface soil, the lowest TOC, Coc and SMBC were recorded in rice-mustard which was statistically at par with rice-cabbage. The SOC stock of both soil layers was also recorded highest in rice-pea. The highest LI of surface soil was recorded in rice-potato which was statistically equal with rice-pea and rice-French bean. Significant correlations among different pools/fractions of C and with available nutrients indicate their importance in improving soil quality. Long-term combination of rice with the leguminous crops and/or potato enhanced Coc, TOC, SMBC, LI and active pools (Cfrac1 + Cfrac2) of rapid turnover rate that may influence the quality and productivity of soil. Long-term cultivation of rice-French bean with high passive C along with good active C and LI is proved to be a good cropping system for sustaining soil and environment by enhancing quality and C reserve of degraded soils of hilly agroecosystem.