Conservation agriculture effects on soil organic carbon accumulation and crop productivity under a rice-wheat cropping system in the western Indo-Gangetic Plains

Retention of carbon (C) in arable soils has been considered as a potential mechanism to mitigate soil degradation and to sustain crop productivity. Hence, we appraised the 3-year effect of different conservation agriculture (CA) practices on grain yield, biomass productivity and soil organic C (SOC) accumulation rates under a tropical rice (Oryza sativa L.)-wheat (Triticum aestivum L.) and rice-wheat-green gram (mung-bean in Hindi; Vigna radiata) cropping systems. Results indicate mean (of three years) rice grain yield under mungbean residue + direct seeded rice (DSR) followed by zero tilled wheat (ZTW) with rice residue (RR) retention and zero tilled relay summer mungbean (MBR + DSR-ZTW + RR-ZTMB) plots was similar to farmers' practice [transplanted rice (TPR)- conventionally tilled wheat (CTW)], despite TPR-CTW plots had similar to 18% higher rice yield than MBR + DSR-ZTW + RR-ZTMB plots in the first year. The MBR + DSR-ZTW + RR-ZTMB treated plots had about 15 and 10% higher mean wheat grain yield and mean system productivity (sum of grain yields of all crops) than TPR-CTW plots, respectively. The plots under DSR+ brown manuring (BM)-ZTW+ RR plots had comparable mean rice and wheat yields to MBR + DSR- ZTW + RR-ZTMB plots. Harvestable aboveground biomass productivity of MBR+ DSR-ZTW + RR-ZTMB treated plots was similar to 2.89 Mg ha(-1) yr(-1) higher than TPR-CTW. Total estimated C input (similar to 12.1 Mg C ha(-1) in three years) under MBR + DSR-ZTW + RR-ZTMB treated plots was similar to 117 and 127% higher than DSR-ZTW and TPR-CTW treatments, respectively. All CA plots had significantly higher gain (over initial value) in total SOC than that in TPR-CIW and TPR-ZTW treatments in the 0-15 cm layer and the gain in total SOC in the plots under MBR + DSR- ZTW+ RR-ZTMB was significantly higher than all CA plots, despite having similar total SOC stocks. Again, plots under MBR + DSR-ZTW + RR-ZTMB had similar to 24% larger labile C pools than that of TPR-CTW (3.1 g kg(-1)) treated plots in the topsoil. Soil bulk density under MBR + DSR- ZTW + RR-ZTMB and DSR + BM-ZTW + RR treated plots significantly decreased in the 5-15 cm layer compared to TPR-CTW plots. Thus, the MBR + DSR-ZTW + RR-ZTMB treatment (a novel CA practice), has considerable potential to retain C in surface soil, decrease soil compaction and increase system (rice-wheat-green gram) productivity and hence its adoption is recommended. (C) 2015 Elsevier B.V. All rights reserved.