Impacts of nitrogen management and organic matter application on nitrous oxide emissions and soil organic carbon from spring maize fields in the North China Plain

Soil carbon dioxide (CO2) and nitrous oxide (N2O) emissions and carbon (C) sequestration are important processes for determining net greenhouse gas (GHG) emissions from maize fields. Farming management practices (FMPs) substantially affect crop productivity, soil organic carbon (SOC), and N2O emissions. However, relative knowledge is not sufficient, and further research is required for sustainable agriculture development. A multiyear experiment was performed on a spring maize field in the North China Plain (NCP) to quantify changes in SOC stocks, N2O emissions, and maize yields. Seven treatments were involved in the experiment, including no N fertilization (CK), farmers' conventional N fertilization (FP), sole application of manure N (M), a combination of M and FP (MFP), FP combined with returned maize straw (SFP), fertilization with reduced synthetic N amount (RN), and fertilization with both reduced synthetic and manure N (RMN). The field measurements demonstrated that urea applications and returned maize straw increased N2O emission. The seasonal total N2O emissions under FP were significantly (P < 0.05) higher than those under CK by 176% to 222% and lower than those under SFP by 20% to 61% throughout the three maize growing seasons. The application of organic manure or maize straw enhanced SOC storage for the studied cropping system. The SOC stocks were relatively high in the plots that received applications of organic manure or maize straw, with the SOC stock of the M plot being significantly higher (P < 0.05) than that of the CK, FP, SFP, RN, and RMN plots in 2016 after eight years of manure amendments. The average net GHG emission from 2012 to 2014 was 203 kg CO2 equivalent ha(-1) year(-1) (i.e., a net GHG source) under FP. The other treatments converted the maize fields from net GHG sources to GHG sinks. By considering both the maize yields and net GHG emissions, RMN would be an optimal management option, although MFP, SFP, and RN also mitigated the GHG emissions and maintained the yields. Large variations were observed in the contributions of the SOC changes and N2O emissions to net GHG emissions among the treatments. The large variations demonstrate the need to consider both SOC changes and N2O emissions when evaluating the impacts of alternative FMPs on net GHG emissions.