δ 15 N–N 2 O signatures in response to N fertilization in a wheat–maize rotation

The responses of δ15N–N2O isotopic signatures to nitrogen (N) fertilization in in situ soils are not yet clear in agricultural field settings. We therefore conducted a 1-year field experiment to assess how N fertilization (0 and 400 kg N ha−1 year−1, N0 and N400) affects the δ15N–N2O isotopic signatures both at soil surface and in soil profile (0–3 m) in a wheat–maize rotation in North China. N fertilization increased N2O concentrations by 18–79% in the soil profile. It increased N2O fluxes by 9 times and 1–7 times at the soil surface and 0–0.9 m soil depth, respectively. In comparison, N fertilization reduced δ15N–N2O values by 10–74% in the soil profile. It reduced δ15N–N2O isotopic signatures at the soil surface and in the soil profile by 18% and 33%, respectively. The δ15N–N2O isotopic signature was dominated by substrate δ15N signature and N isotope fractionation during cold and warm seasons, respectively. Our results indicate that denitrification steps of NO3− → N2O dominated N2O production (over 99%) in the N0 treatment and the N400 treatment during N-fertilizer-free period. Following N fertilization, the NO3− → N2O dominated N2O production (over 99%) in the soil profile, N2O flux from the soil surface was mostly derived from NO3− → N2O and less from NH4+ → N2O. Overall, denitrification was found to dominate soil N2O transformation metabolism in the wheat–maize rotation cropland in North China.