Net global warming potential and greenhouse gas intensity in a double-cropping cereal rotation as affected by nitrogen and straw management
The effects of nitrogen and straw management on global warming potential (GWP) and greenhouse gas intensity (GHGI) in a winter wheat–summer maize double-cropping system on the North China Plain were investigated. We measured nitrous oxide (N 2O) emissions and studied net GWP (NGWP) and GHGI by calculating the net exchange of CO 2 equivalent (CO 2-eq) from greenhouse gas emissions, agricultural inputs and management practices, as well as changes in soil organic carbon (SOC), based on a long-term field experiment established in 2006. The field experiment includes six treatments with three fertilizer N levels (zero N (control), optimum and conventional N) and straw removal (i.e. N 0, N opt and N con) or return (i.e. SN 0, SN opt and SN con). Optimum N management (N opt, SN opt) saved roughly half of the fertilizer N compared to conventional agricultural practice (N con, SN con), with no significant effect on grain yields. Annual mean N 2O emissions reached 3.90 kg N 2O-N ha −1 in N con and SN con, and N 2O emissions were reduced by 46.9% by optimizing N management of N opt and SN opt. Straw return increased annual mean N 2O emissions by 27.9%. Annual SOC sequestration was 0.40–1.44 Mg C ha −1 yr −1 in plots with N application and/or straw return. Compared to the conventional N treatments the optimum N treatments reduced NGWP by 51%, comprising 25% from decreasing N 2O emissions and 75% from reducing N fertilizer application rates. Straw return treatments reduced NGWP by 30% compared to no straw return because the GWP from increments of SOC offset the GWP from higher emissions of N 2O, N fertilizer and fuel after straw return. The GHGI trends from the different nitrogen and straw management practices were similar to the NGWP. In conclusion, optimum N and straw return significantly reduced NGWP and GHGI and concomitantly achieved relatively high grain yields in this important winter wheat–summer maize double-cropping system.