Midwestern N2O Emissions Linked at Regional Scales to Remotely Sensed Soil Moisture in a North American Inversion

Abstract

The interactions between soil moisture (SM), agricultural practices, and microbially driven N₂O emissions have been described in detail at the field scale. The relationships among those variables are investigated here at larger scales using Soil Moisture Active-Passive remote sensing data and a regional atmospheric inversion. In the atmospheric N₂O data set used in the inversion, 13 large pulse events were observed during the crop growing season from 2015 to 2021 in the U.S. Midwest, mainly in Iowa. These events were linked to rapid changes in SM, either increasing from dry to wet conditions, or vice versa, within a week preceding the N₂O pulse. However, no significant correlations were found between SM or soil temperature and posterior N₂O fluxes from the inversion integrated over Iowa across the peak emission months of May–June. Analysis over the full growing season suggested compensating emissions, for example, higher than normal N₂O fluxes in July following a dry June. These results suggest a relatively consistent ∼4% yield of N₂O from anthropogenic N inputs to croplands in Iowa regardless of short-term variability in soil conditions. Net growing season N₂O emissions in the DayCent biogeochemistry model were also not correlated to SM or temperature, although the model tended to underestimate interannual variability relative to the inversion. An expanded atmospheric observation network, together with an extended SM time series, would allow a better understanding of the relationship between variability in SM and N₂O emissions at regional scales.