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

•Maize fields could be GHG sources if both SOC change and N2O emissions are considered.•Urea application and maize straw return increased N2O emissions.•Adding manure or straw increased SOC and converted maize fields to GHG sinks.•Reducing urea rate by 40% maintained maize yields and mitigated GHG emissions.•Replacing part of synthetic N with manure was optimum for mitigating GHG emissions.