Effects of Nitrogen Reduction and Optimized Fertilization Combined with Straw Return on Greenhouse Gas Emissions and Crop Yields of a Rice–Wheat Rotation System

Straw return and optimized nitrogen fertilizer application are common agronomic measures used in agricultural production, but the greenhouse effect these measures induce is often ignored. In this study, four treatments were employed: conventional fertilization (CF), optimized nitrogen fertilization (OF), conventional fertilization + straw return (CF + S), and optimized nitrogen fertilization + straw return (OF + S). A “closed static box” method was used to monitor methane (CH 4 ) and nitrous oxide (N 2 O) emissions, and the variations in the ground temperature at 5-cm depth, soil nitrogen content, and soil organic matter content were analyzed. The results showed that OF reduced the soil nitrogen content, which reduced soil N 2 O emissions. Straw return increased the ground temperature at 5-cm depth and increased the soil organic matter content, which increased CH 4 emissions. Compared with CF, the cumulative emissions of CH 4 from the paddy soils of OF and OF + S decreased by 10.75% and 18.48%, respectively, while that of CF + S increased by 13.83%. The cumulative N 2 O emissions from the paddy soils of OF, CF + S, and OF + S decreased by 18.36%, 24.64%, and 32.85%, respectively. The global warming potential (GWP) of OF and OF + S decreased significantly, but the GWP of CF + S increased, and all treatments improved rice and wheat yields. The trend in the greenhouse gas emission intensity (GHGI) was the same as that of GWP. In general, the application of straw provided additional nutrients to the soil, compensated for the reduction in soil nutrients caused by nitrogen reduction and fertilizer reduction, and ensured high rice yields.