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Abstract Agriculture is the dominant source of anthropogenic nitrous oxide (N₂O) –a greenhouse gas and a stratospheric ozone depleting substance. The US Corn Belt is a large global N₂O source, but there remain large uncertainties regarding its source attribution and biogeochemical pathways. Here, we interpret high frequency stable N₂O isotope observations from a very tall tower to improve our understanding of regional source attribution. We detected significant seasonal variability in δ¹⁵N_bulk(6.47–7.33‰) and the isotope site preference (δ¹⁵N^SP = δ¹⁵N^α–δ¹⁵N^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 N₂O 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.