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

The primary driver of increasing atmospheric concentrations of nitrous oxide (N₂O) is the use of organic and synthetic fertilizer to increase agricultural crop production. Current global estimates are based on IPCC N₂O emission factor (EF) calculations, although there are shortcomings as many of the N₂O 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 N₂O emissions in the following year. In this study, we assessed the legacy effect of fertilization on soil N₂O 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 N₂O 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^-1 yr^-1 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 N₂O emissions mainly driven by in-season fertilization. Our data show that, on average, the seasonal EF for N₂O was about 0.11 % higher in response to the previous N fertilization. Our study indicates that the current N₂O EF may severely underestimate emissions because studies ignore legacy effects on N₂O 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 N₂O EF.