Maize stalk incorporation increases N2O emissions that offset the benefit of SOC sequestration in a winter wheat-summer maize field: A four-year measurement in long-term fertilizer experiments

Soil organic carbon (SOC) sequestration, nitrous oxide (N2O) emissions, and net global warming potential (GWP) are variable following crop residue retention in agroecosystems. We measured the SOC content (from 1990 to 2018) and N2O fluxes (from October 2014 to September 2018) in a long-term fertilizer experiment with winter wheat-summer maize rotation (since 1990) in the Guanzhong Plain. The primary aim was to quantify the effects of maize stalk incorporation on N2O emissions, SOC sequestration (SOCSR), and net GWP during the measurement period. We examined two fertilizer management practices, i.e., synthetic fertilizers in both crop seasons (NF) and synthetic fertilizers in both crop seasons with maize stalk incorporation in the winter wheat season (NFS), and an unfertilized control (CK). Annual N2O emissions ranged from 0.11 to 0.27 kg N ha–1 yr–1 in CK. Fertilization significantly increased N2O emissions (p < 0.001), with values ranging from 1.65 to 5.65 and 3.08 to 7.85 kg N ha–1 yr–1 in NF and NFS, respectively. The emissions from NFS were 27–253% higher than those from NF, with significant differences (p < 0.05) during three out of four experimental years. The SOC contents linearly increased with the experimental year (p < 0.001), giving SOCSRs of 0.13, 0.45 and 0.60 t C ha−1 yr−1 in CK, NF and NFS, respectively. The net GWP averaged − 0.38, − 0.36, and 0.01 t CO2 equivalent ha−1 yr−1 in the three treatments, denoting that NFS was a minor greenhouse gas source during the measurement period. This study provided evidence that maize stalk incorporation increased both N2O emissions and SOC sequestration, while the N2O emissions offset the benefit of SOC sequestration. We conclude that maize stalk incorporation does not appear to be an effective approach to decrease net greenhouse gas emissions from the winter wheat-summer maize rotation in the Guanzhong Plain. This issue can probably be resolved by avoiding fertilizer application to wet soil in the winter wheat season and/or returning winter wheat straw in the summer maize season.