Effects of biochar co-application with urea on nitrogen losses, rice yield, and nitrogen agronomic efficiency in rice production system

Balancing nitrogen (N) productivity with environmental sustainability is critical in flooded rice systems, where N losses are substantial. Rice straw, a major agricultural waste, can be converted into biochar to improve nutrient retention and yield. This study compared four treatments in a continuously flooded tropical paddy field: an unfertilized control, chemical fertilizer (CF), rice straw with CF (RS + CF), and rice straw-derived biochar with CF (BC + CF). We quantified major N-loss pathways, agronomic efficiency of applied N (NAE), and N gas-related global warming potential (GWP N ). The results indicate that BC + CF reduced total N loss to 33% of total N input, compared with 38% under RS + CF and 43% under CF. Compared with CF, BC + CF significantly reduced NH 4 + –N leaching, NH 3 volatilization, and N 2 O emissions through enhanced soil cation exchange capacity and mineral N retention. In contrast, RS + CF significantly increased NH 4 + –N leaching but significantly reduced NH 3 volatilization and N 2 O emissions. Both RS + CF and BC + CF significantly reduced GWP N relative to CF, while BC + CF achieved the highest grain yield and NAE. These findings demonstrate that the co-application of rice straw-derived biochar with chemical fertilizer is a promising strategy for mitigating N losses and improving agronomic efficiency in flooded rice systems.