Effects of mild alternate wetting and drying irrigation and mid-season drainage on CH₄ and N₂O emissions in rice cultivation

Rice, one of the major sources of CH₄ and N₂O emissions, is also the largest consumer of water resources. Mild alternate wetting and drying (AWD) irrigation is widely adopted to save irrigation water resources and maintain rice production, but its effects on CH₄ and N₂O emissions are unclear. In addition, previous studies have revealed different effects of mid-season drainage on global warming potential (GWP), owing to the different criteria used. In this study, a pot experiment was conducted to investigate the effects of mild AWD irrigation and mid-season drainage (a specific soil moisture) on CH₄ and N₂O emissions during rice cultivation. Four water management systems were applied: AWD + D0 (mild AWD irrigation without mid-season drainage), AWD + D1 (mild AWD irrigation with mid-season drainage), CF + D0 (continuous flooding without mid-season drainage) and CF + D1 (continuous flooding with mid-season drainage); nitrogen was applied at two levels (N90 and N180) along with each treatment. The results showed that mild AWD irrigation reduced CH₄ cumulative emissions by an average of 87.1% but increased N₂O cumulative emissions by an average of 280% compared to the values observed with CF irrigation. Mid-season drainage did not affect N₂O emissions but interrupted CH₄ fluxes and significantly reduced CH₄ cumulative emissions. CH₄ and N₂O cumulative emissions were reduced by an average of 25.0% and 54.2%, respectively, with N90 application compared to values observed with N180 application. Unexpectedly, mild AWD irrigation did not reduce GWP and yield-scaled GWP unlike CF irrigation because a high N₂O emission peak occurred during mild AWD irrigation. Furthermore, we observed an obvious trade-off between CH₄ and N₂O. We suggest that maintaining flooding during nitrogen application but applying mild AWD irrigation for the remaining period may be helpful in reducing CH₄ and N₂O emissions and GWP.