Isotopic Constraints on Nitrous Oxide Emissions From the US Corn Belt.

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 δ15Nbulk (6.47–7.33‰) and the isotope site preference (δ15NSP = δ15Nα–δ15Nβ, 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. Plain Language Summary: Increasing use of synthetic nitrogen fertilizers for agricultural production is causing higher atmospheric nitrous oxide (N2O) concentrations. Nitrous oxide is a long‐lived greenhouse gas and degrades the protective stratospheric ozone layer. Using tall tower N2O isotope observations from within the US Corn Belt, we examine how different processes (denitrification vs. nitrification) and sources (corn fields vs. wetlands, rivers, and streams) contribute to variations in atmospheric N2O. The findings indicate that a substantial amount of nitrogen leakage from agricultural crops contributes to N2O emissions via indirect sources such as drainage networks. These findings can help inform mitigation strategies targeting nitrogen use and leakage pathways from agricultural systems. Key Points: Seasonality of N2O isotope fingerprint driven by snowmelt and fertilizer inputSnowmelt and growing season emissions dominated by nitrification versus denitrification pathwaysIsotope fingerprints were used to constrain direct and indirect N2O emissions [ABSTRACT FROM AUTHOR]