Mitigation of environmental N pollution and greenhouse gas emission from double rice cropping system with a new alternate wetting and drying irrigation regime coupled with optimized N fertilization in South China

Reducing greenhouse gas (GHG) emission and nitrogen (N) pollution are essential for sustainable crop pro-duction. An alternate wetting and drying irrigation management ('safe' AWD) was invented and implemented to reduce water input and GHG emission in many Asian countries. The 'safe' AWD allows the soil to dry and re -irrigated when water level reaches 15 cm below soil surface and is called 'safe' as it will not cause yield decline in most cases. To further improve water productivity (WPT) and reduce GHG emission and N pollution, a modified AWD irrigation (MAWD) was developed in current study. Field experiment was carried out to evaluate the GHG emission and N losses under different irrigation and N management during 2017-2020. The treatments were: (i) zero N application with farmers' irrigation practice, (ii) farmers' N fertilization and irrigation practice (FP), (iii) farmer's irrigation practice with optimized N fertilization (OPTN), (iv) 'safe' AWD with optimized N fertilization (OPTN+AWD), and (v) MAWD with optimized N fertilization (OPTN+MAWD). Compared with FP, grain yield in OPTN, OPTN+AWD and OPTN+MAWD was increased by 12.1%, 13.6% and 14.4%, respectively. The OPTN, OPTN+AWD and OPTN+MAWD were comparable in plant N accumulation and grain yield, sug-gesting that irrigation did not have detectable effects on yield. Water input in OPTN+MAWD was 3.68-26.0% lower than OPTN+AWD. N losses loading was positively correlated with water input. Relative to OPTN, OPTN+AWD and OPTN+MAWD reduced N loss through leaching and surface runoff due to lower water input and enhanced rainwater storage capacity. N losses loading in OPTN+MAWD was 20.5% lower than OPTN. Greenhouse gas intensity and net GWP were lowest in OPTN+MAWD. CH4 emission in OPTN+MAWD was 16.2% lower than OPTN+AWD. MAWD irrigation increased the N2O emission, but the net GWP was 13.9% lower than OPTN+AWD due to reduced CH4 emission. Our results suggested that integrating MAWD with optimized N fertilization could synergistically improve grain yield and reduced GWP and N pollution in rice production of South China.