Impact of Conservation Agriculture on Soil Carbon Sequestration under a Rice (Oryza sativa)-Wheat (Triticum aestivum)-Mungbean (Vigna radiata) Cropping System in the Indo-Gangetic Plains

One of the potential methods for reducing soil deterioration and maintaining crop yield in arable soils is to retain carbon (C). Despite many studies reporting the impact of conservation agriculture (CA) practices on soil organic carbon (SOC) pool, the impact of CA on tropical rice (Oryza sativa L.)-wheat (Triticum aestivum L.) and rice–wheat–mung bean (green gram; Vigna radiata) cropping systems with diverse combinations of tillage and residue retention viz. rice, wheat and mungbean are rarely reported. Therefore, this study reported the impact of CA on SOC sequestration rates in the 0–30 and 30–60 cm soil depths under a rice-wheat system in the Indo-Gangetic Plains (IGP). Results indicated that MBR+ZTDSR-RR+ZTW-WR+ZTMB (CA₃), WR+ZTDSR+BM-RR+ZTW (CA₂) and WR+ZTDSR-RR+ZTW (CA₁) plots had ~26.5%, 23.8%, and 22% more total SOC stock than farmers’ practice (TPR-CTW, denoted as CT) plots (24.14 Mg C ha^-1), respectively, in the 0-30 cm soil layer. In the 0-5 cm layer, the plots under CA₃ had 27.9% higher labile C than CT plots (4.55 g kg^-1) after nine years of rice-wheat cropping system. Residue retention resulted in higher labile C in this depth than residue removal plots. Compared to CT plots, CA practices improved the microbial biomass carbon (MBC) of the topsoil by ~49 to 58.7%. The CT plots considerably decreased the permanganate-oxidizable carbon (POXC) concentration. The lower POXC content in CT plots indicates the high carbon loss due to greater disruption of the soil. The result revealed that the CA₃ was the best management practice in the rice-wheat-mungbean cropping system to increase the labile carbon and carbon sequestration in the soil. Thus, adopting zero-tilled direct seeded rice followed by zero-tilled wheat practices (CA₃) that includes three types of residue retention (rice, wheat and mungbean residue) has potential to retain C in surface soil and results in carbon sequestration in deeper layers, which is considered to be a key mechanism for the long-term sequestration of SOC in rice-wheat cropping system of IGP.