Stream Transport and Substrate Controls on Nitrous Oxide Yields From Hyporheic Zone Denitrification

Abstract Rivers and streams act as globally significant sources of nitrous oxide (N 2 O) to the atmosphere, in part through denitrification reactions that will increase in response to ongoing anthropogenic nitrogen loading. While many factors that contribute to the release of N 2 O relative to inert dinitrogen (N 2 ) are well described, the ability to predict N 2 O yields from streams remains a fundamental challenge. Here, I revisit results from the second Lotic Intersite Nitrogen eXperiments (LINX II) in the context of turbulent hyporheic exchange. Denitrification efficiency, or the fraction of nitrate delivered to the streambed by stream turbulence that is chemically reduced, emerges as the single best predictor of N 2 O yields and underpins the first statistically significant models of inter‐site N 2 O yields. This mechanistic connection is supported by reactive transport modeling of hyporheic zone denitrification representing advective flowpaths, flowpath mixing, and diffusion‐dominated anoxic microzones. Simulated N 2 O yields are inversely correlated with denitrification efficiency; however, advective models are unable to capture low LINX II N 2 O yields at low denitrification efficiencies. Hyporheic zone mixing exacerbates this inability to capture observed N 2 O yields via the promotion of N 2 O release from fast, oxic flowpaths. Instead, anoxic microzones are required to account for LINX II observations through consistently low N 2 O yields and the consumption of upstream‐produced N 2 O. Together, these results provide a framework for controls on stream N 2 O yields and suggest that stream corridor restoration designs aimed at increasing the capacity of hyporheic zones to remediate nitrate loading, as opposed to increasing hyporheic exchange, will also reduce proportional N 2 O emissions.