Isotopic Constraints on Nitrous Oxide Emissions From the US Corn Belt

Abstract Agriculture is the dominant source of anthropogenic nitrous oxide (N 2 O) –a greenhouse gas and a stratospheric ozone depleting substance. The US Corn Belt is a large global N 2 O source, but there remain large uncertainties regarding its source attribution and biogeochemical pathways. Here, we interpret high frequency stable N 2 O isotope observations from a very tall tower to improve our understanding of regional source attribution. We detected significant seasonal variability in δ 15 N bulk (6.47–7.33‰) and the isotope site preference (δ 15 N SP = δ 15 N α –δ 15 N β , 18.22–25.19‰) indicating a predominance of denitrification during the growing period but of nitrification during the snowmelt period. Isotope mixing models and atmospheric inversions both indicate that indirect emissions contribute substantially (>35%) to total N 2 O emissions. Despite the relatively large uncertainties, the upper bound of bottom‐up indirect emission estimates are at the lower bound of the isotopic constraint, implying significant discrepancies that require further investigation.