Water-use efficiency and greenhouse gas emissions of rice affected by water saving and nitrogen reduction

We investigated the effects of reduced irrigation and N application, using green manure and rice straw, on the water-use efficiency (WUE) and greenhouse gas (GHG) emissions of machine-transplanted rice (Oryza sativa L.) to provide theoretical and technical guidance for water and N management in double-cropping rice. Field trials were conducted from 2014 to 2016 using two irrigation methods (flood irrigation, W0; intermittent irrigation, W1) and three N application methods (no nitrogen application, N0; chemical nitrogen constant application, N1; chemical nitrogen reduction combined with green fertilizer and straw, N2). Methane (CH4) and nitrous oxide (N2O) emissions, water consumption, and grain yield were compared for the different treatment measures. Under the same N fertilizer management, intermittent irrigation reduced cumulative emissions of CH4 of early and late season rice by 14.54-37.37% and 16.70-52.25%, respectively, whereas N2O emissions increased by 12.50-35.29% and 23.08-34.09%, compared with flood irrigation. Under identical irrigation conditions, the cumulative CH4 and N2O emissions of early and late rice were N2 > N1 > N0. Compared to W0N1 treatment, W1N2 decreased the annual average water consumption by 4.40 and 5.53% for early and late rice, whereas yield and WUE increased by 4.27 and 3.98% and 9.54 and 9.71%, respectively. In addition, the annual global warming potential and greenhouse gas intensity of double-cropped rice increased by 47.62 and 41.77%, respectively. The incorporation of green manure and rice straw with reduced N fertilizer application using intermittent irrigation in double rice cropping systems reduced water consumption and increased grain yield but also increased GHG emissions.