Reversing negative nitrogen balances in intensive rice-based cropping with Conservation Agriculture

Abstract The extent to which Conservation Agriculture (CA) practices alter nitrogen (N) balance in intensive rice-based cropping systems of the Eastern Indo-Gangetic Plain was examined, focusing on a legume dominated-system (LDS) and a cereal dominated-system (CDS) in north-west Bangladesh. Three crop establishment methods were imposed—strip planting (SP) and bed planting (BP) for non rice crops with non-puddled rice establishment; compared with conventional tillage (CT) for non rice crops along with puddled transplanting of rice. Two levels of crop residue retention were superimposed—high (HR) and low (LR - conventional farm practice) crop residue retention. The CA practices (SP and non-puddled rice with HR) increased total soil N concentrations, and the soil N-stocks at 0–0.075 m over CT and LR in both CDS and LDS sites after 2.5 years. At 0–0.15 m soil depth, total soil N concentrations increased over time under SP and BP, but decreased with CT. In LDS, annual soil N increase (65 kg N ha −1 ) occurred with SP while negligible N losses were observed under CT at both levels of residue retention at 0–0.15 m soil depth. The N accumulation rate under HR was 24.5 kg N ha −1 higher than LR. The N balance calculation over 2.5 years indicated an estimated soil N gain (8–11%) in SPHR and SPLR but a loss in CT. In CDS, neither treatments accumulated soil N but N losses were greater in CT. The effect of crop establishment methods on soil NH₄-N and NO₃-N at the early growth stage of cool dry season cropping was small and inconsistent. However, SP with HR maintained higher overall crop and system N uptake compared to CT and LR. Thus, CA practices altered the N balance which slowed the decline in soil N-stocks in the cereal-dominated rotation while resulting in a positive N balance in the legume-dominated rotation.