Rice straw-derived biochar reduces nitrogen losses and greenhouse gas emissions while improving rice productivity and agronomic efficiency in a tropical paddy soil

Balancing nitrogen (N) productivity with environmental sustainability is critical in flooded rice systems, where N losses by leaching and gaseous emissions are substantial. This study conducted a system-level comparison of 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 minimized total N loss to 33% of total N input, compared to 38% under RS + CF and 43% under CF. Compared with CF, BC + CF significantly reduced NH ₃ volatilization and NH ₄ ⁺ -N leaching, driven by increased soil cation exchange capacity and mineral N retention. While RS + CF yielded the lowest N ₂ O emissions, it presented greater NH ₄ ⁺ -N leaching than BC + CF. Both organic amendments reduced GWP _N and greenhouse gas intensity (GHGI) compared to CF; however, BC + CF resulted in superior grain yield and NAE. Conversely, the unfertilized control produced the highest GHGI due to low productivity. These findings demonstrate that integrating rice straw-derived biochar with chemical fertilizer represents an optimal system-level strategy for mitigating N losses, enhancing agronomic efficiency, and lowering climate impacts of flooded rice production.