Nano-biochar amendment reduces reactive gaseous nitrogen losses and improves grain yield in alternate wetting and drying paddy fields

• Nano-biochar has better physicochemical properties than biochar. • Nano-biochar improves rice yield under alternate wetting and drying irrigation. • Nano-biochar reduces gaseous nitrogen losses by enhancing soil nitrogen retention. Biochar (BC) demonstrates considerable potential for reducing nitrogen emissions and improving crop yield. However, it frequently exhibits limited capacity and may increase ammonia (NH 3 ) volatilization. Nano-biochar (NBC) is attracting growing attention due to its higher surface energy, but there is a lack of information for rice production systems, especially under alternate wetting and drying (AWD). Therefore, a two-year field experiment was conducted in 2023 and 2024, involving six treatments: continuous flooding (CF) without BC (I CF B 0 ), AWD without BC (I AWD B 0 ), AWD with 20 t ha -1 BC (I AWD B 20 ), AWD with 5,10 and 20 t ha -1 NBC (I AWD NB 5 , I AWD NB 10 and I AWD NB 20 ). Their effects on reactive gaseous nitrogen losses (NH 3 and N 2 O; Nr), floodwater nitrogen, soil environment variables, nitrogen uptake, grain yield, and nitrogen-related global warming potential ( GWP N ) were evaluated. Results showed that there was no significant difference in NH 3 volatilization and grain yield between I AWD B 0 and I CF B 0 treatments, but AWD increased N 2 O emission by 41.71-53.25%. Compared with without BC addition, NBC application increased soil mineral nitrogen while decreasing floodwater NH 4 + -N, thereby reducing NH 3 volatilization, N 2 O emission, Nr and GWP N by 5.92-34.41%, 9.95-25.49%, 6.37-33.39%, 12.20-26.11%, respectively, in AWD paddy fields. Compared to I AWD B 20 , I AWD NB 20 reduced NH 3 volatilization, N 2 O emission, Nr losses and GWP N by 12.97-13.45%, 9.47-17.26%, 13.69%, 9.95-17.89%, respectively, and I AWD NB 10 showed no significant difference in 2023, but significantly reduced N 2 O emission, lowering GWP N by 7.81% in 2024. NBC also promoted aboveground dry matter and nitrogen accumulation in rice plants, ultimately increasing grain yield by 1.95-12.25%, and no significant difference was observed between I AWD NB 10 and I AWD B 20 . Therefore, even with the biochar application rate halved, NBC can still enhance soil nitrogen content, thereby mitigating Nr losses and GWP N , while simultaneously improving grain yield in AWD paddy fields.