The legacy effect of long-term nitrogen fertilization on nitrous oxide emissions.

The primary driver of increasing atmospheric concentrations of nitrous oxide (NO) is the use of organic and synthetic fertilizer to increase agricultural crop production. Current global estimates are based on IPCC NO emission factor (EF) calculations, although there are shortcomings as many of the NO EFs are derived from measurements during the cropping season. These neglect the fallow season, and do not adequately account for double or even triple cropping systems or legacy effects on soil NO emissions in the following year. In this study, we assessed the legacy effect of fertilization on soil NO fluxes using data from a long-term double-cropping field experiment with summer maize and winter wheat in rotation, in which no nitrogen (N; NN) and balanced manure with synthetic N (MN) fertilized treatments were switched to allow an assessment of legacy effects. Based on high-frequency measurements of NO and previous data, we calculated that the historical N fertilization, or legacy effect, explained 23 % of the annual flux of 0.81 kg N ha yr in the first season of observation. In the following three seasons, the legacy effect of the previous N fertilization regime decreased to a negligible level, with NO emissions mainly driven by in-season fertilization. Our data show that, on average, the seasonal EF for NO was about 0.11 % higher in response to the previous N fertilization. Our study indicates that the current NO EF may severely underestimate emissions because studies ignore legacy effects on NO emissions from zero N plots and only compare zero N with N fertilization treatments for a given season or year to derive seasonal or annual NO EF.