Biochar incorporation increases grain yield, net ecosystem CO₂ exchange, and decreases CH₄ emissions in an alternate wetting and drying paddy ecosystem

Biochar is widely used for soil carbon sequestration and soil improvement. However, little information is available on its effects on net ecosystem CO2 exchange (NEE) and CH4 emissions in paddy rice systems, especially under alternate wetting and drying irrigation (IAWD). A two-year field experiment was conducted with two irrigation regimes (ICF: continuous flooding irrigation; IAWD) as main plots and 0 (B0) and 20 t ha-1 (B1) biochar as subplots. IAWD greatly decreased CH4 emissions by 81.1-87.6% and yield-scaled CH4 emissions by 81.3%-88.2% without grain yield penalty, but decreased NEE by 6.5-13.9%. The decreased NEE was mainly caused by increasing soil heterotrophic respiration (Rh) and ecosystem respiration (Re). Biochar increased grain yield by 8.1-11.3%, reduced CH4 emissions by 25.8-38.9%, and yield-scaled CH4 emissions by 30.4-44.6% under both irrigation regimes. In addition, biochar increased ecosystem carbon input (gross primary product, GPP) and output (Re), but a higher increase in GPP than Re, thus increasing NEE by 9.7-11.1% under both irrigation regimes. Biochar combined with IAWD can increase NEE and grain yield and further decrease CH4 emissions compared to IAWD without biochar, achieving a win-win situation of food-water-greenhouse gas emissions trade-off, which is beneficial to sustainable agricultural production.