Long-Term Nitrogen and Straw Application Improves Wheat Production and Soil Organic Carbon Sequestration

The interaction between the soil organic carbon (SOC) sequestration and the availability of soil nitrogen (N), as well as N application, remains unknown. We evaluated the response of SOC sequestration to exogenous C input and soil N availability in a 17-year field experiment of a winter wheat monoculture system with favorable precipitation. Four experimental treatments were included: control (CK), straw returning (S), nitrogen application (N), and the combination of straw returning and nitrogen application (S + N). N application (N, S + N) increased the wheat yield when the experimental period was increased. Under an equivalent input of N (N), the average yield of straw returning (S + N) in 17 years increased by 6.5%. The significant interactions of straw returning and N application increased the SOC content and stocks in the 0–20 cm soil ( p < 0.05). Compared with no N application (S), straw returning with sufficient N (S + N) not only contributed to the increment of the SOC stocks but also improved the dissolved organic carbon, easily oxidizable organic carbon, particulate organic carbon, and microbial biomass carbon by 10.2%, 49.2%, 48.3%, and 38.1%, respectively. In addition, even though it was not straw amended, N application significantly increased the SOC content and stocks by 7.1% and 6.4%, respectively. After the input of organic materials, maintaining sufficient N is beneficial to SOC sequestration. Furthermore, these findings indicate that the importance of soil N availability on crop yield and SOC sequestration should be considered when N reduction measures are implemented in agricultural practices.