Distinct driving mechanisms of non-growing season N2O emissions call for spatial-specific mitigation strategies in the US Midwest

Agricultural N2O emission is a growing concern for climate change. Recent field evidence suggests that nongrowing seasons (NGS) may contribute one-third to half of the annual N2O emissions, but implications on management adaptations remain unclear. Here we used an advanced process-based model, ecosys, to investigate the magnitude and drivers of NGS N2O emissions from the US Midwest. Results showed that simulated NGS N2O emissions accounted for 6-60% of the annual fluxes under continuous corn systems, peaking in counties with NGS precipitation (PNGS) around 300 mm. Divergent patterns of spatial-temporal correlations between NGS N2O emissions and environmental variables were shown in the southeast (PNGS > 300 mm) and the northwest (PNGS < 300 mm) of the study area by simulations. Causal analysis indicates that more intensive freezing caused by decreased air temperature (Ta) is the dominant driver that leads to NGS N2O emissions increasing within the southeast of the study area, while increased PNGS and increased Ta cooperatively result in soil moisture decreasing at soil thaws that enhances NGS N2O production within the northwest of the study area. Scenario simulations suggest that annual N2O emissions in the US Midwest are likely to reduce under climate change primarily due to the reduction of NGS N2O emissions. Our estimates on monetized social benefits inform the necessity to implement spatial-specific mitigation strategies, i.e. determining fertilizer timing and use of nitrification inhibitors (NI). Spring fertilizer application is more beneficial than fall fertilizer application for most counties, however, the latter can bring extra benefits to some counties in the west of the study area. Introducing NI with either spring or fall applications can greatly increase social benefits by reducing N2O emissions and N leaching. This study addresses possibly effective adaptations by providing seasonal- and spatial-explicit mitigation potentials.