Alternate wetting and drying irrigation (IAWD) is one types of water-saving technologies to repeatedly dry and re-flood the fields. However, the frequent alternate aerobic and anaerobic environment under IAWD has increased the reactive nitrogen gases emission in rice fields. Fortunately, biochar has been widely used to improve the land productivity (including the crop production, carbon sequestration, mitigation of GHG, and remediation of heavy metal pollution) in the agricultural soil amendment. However, little information is available on the effects of biochar on the NH3 volatilization, N2O emissions, and reactive nitrogen gas emissions in paddy rice systems, especially under IAWD. This study aims to explore the response mechanism of reactive nitrogen gas emissions to the biochar application under IAWD conditions. A two-year field split-plot experiment was conducted with the biochar application rates (0 and 20 t/hm²) under two irrigation regimes (continuously flooded irrigation, ICF and IAWD). Two irrigation regimes were used as the main plots, whereas, two biochar additions were the subplots. Each plot was in the size of 3 m (width) × 6 m (length) that separated by the PVC barrier at a depth of 30 cm. in order to avoid the lateral flow of nutrients and water. The results showed that the IAWD increased yields by 1.7% to 5.1% (P>0.05), NH3 volatilization 8.9%(only in 2020), and N2O emissions by 105.0% to 115.0%, compared with the ICF (P<0.05). Biochar addition at the rate of 20 t/hm² was significantly reduced the NH3 volatilization by 8.7% to 20.5% and N2O emissions by 21.6% to 24.2% (P<0.05). There was an increase in the yield by 0.2% to 12.5% and a reduction in the reactive nitrogen gas emissions by 6.1% to 11.7% in biochar combined with the IAWD, compared with the control (ICFB0). Furthermore, the inorganic nitrogen existed mainly in the form of NH4+-N in the soil environment of rice field, accounting for 88.6% to 94.7%. The IAWD had no significant some impact on the soil NH4+-N, compared with the ICF, whereas, the significant increase was found in the content of surface soil NO3−-N by 50.3% to 74.0% and the oxidation-reduction potential by 47.6% to 54.1% (P<0.05). Moreover, the biochar addition increased the soil NH4+-N by 25.9% to 52.41% (P<0.05), and pH by 0.6% to 1.0%, compared with the biochar free treatment. Principal component analysis showed a strong negative correlation between NH3 volatilization and soil NH4+-N content, yield, and aboveground nitrogen accumulation. It indicated that the biochar addition was significantly improved the soil adsorption capacity for the inorganic nitrogen, thereby reducing the NH3 volatilization losses and increasing aboveground nitrogen accumulation, ultimately leading to increase the yield. N2O emissions were strongly positively correlated with the oxidation-reduction potential and soil NO3−-N. Therefore, the frequent alternate aerobic and anaerobic environment under IAWD increased the concentration of soil NO3−-N and the oxidation-reduction potential, thereby increasing denitrification substrates and N2O emissions. Anyway, the biochar addition coupled with alternate wetting and drying can be expected to promote the rice production with water saving, carbon sequestration and emission reduction, while reduce the environmental costs associated with the reactive nitrogen emissions in the paddy field. [ABSTRACT FROM AUTHOR] more...