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 (N2O) 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 N2O relative to inert dinitrogen (N2) are well described, the ability to predict N2O 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 N2O yields and underpins the first statistically significant models of inter‐site N2O 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 N2O yields are inversely correlated with denitrification efficiency; however, advective models are unable to capture low LINX II N2O yields at low denitrification efficiencies. Hyporheic zone mixing exacerbates this inability to capture observed N2O yields via the promotion of N2O release from fast, oxic flowpaths. Instead, anoxic microzones are required to account for LINX II observations through consistently low N2O yields and the consumption of upstream‐produced N2O. Together, these results provide a framework for controls on stream N2O 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 N2O emissions.

Improving Predictions of Stream CO2 Concentrations and Fluxes Using a Stream Network Model: A Case Study in the East River Watershed, CO, USA

“…While short transit times, likely on the order of seconds to an hour, may limit the extent of chemical reactions (Harvey et al, 2019), fast aerobic respiration rates compounded by higher respiration rates within the shallowest benthic depths may act to increase their role in overall fluxes (Li et al, 2017). Turbulence-based models have shown promise in explaining benthic DO profiles (O'Connor & Hondzo, 2008), limits to hyporheic zone denitrification (Grant, Azizian, et al, 2018), conservative tracer tailing patterns (Roche et al, 2019), and N 2 O yields from hyporheic zone denitrification (Winnick, 2021). Further, models of turbulent exchange have the potential to be highly scalable (e.g., Grant, Gomez-Velez, et al, 2018;Packman et al, 2004) and involve relatively low data requirements (Grant, Azizian, et al, 2018).…”

Vertical Hydrologic Exchange Flows Control Methane Emissions from Riverbed Sediments

“…The role of surface water–groundwater interaction in regulating biogeochemical “hotspots” in hyporheic zones has been extensively studied over the last decade. − The underlying principle is that the mixing of groundwater and surface water with distinctly different properties facilitates coupled aerobic/anaerobic reactions in the hyporheic zone. For example, vertical hydrologic exchange flows (VHEFs) control the extent of denitrification and the flux of nitrous dioxide from the riverbed sediments. , River hydrological conditions (gaining/loosing) affect hyporheic respiration and the production of CO 2 and N 2 .…”

Impacts of Carbonate Buffering on Atmospheric Equilibration of CO2, δ13CDIC, and Δ14CDIC in Rivers and Streams

“…Briefly, the East River watershed is an 87 km 2 , high elevation, mountainous watershed, with annual flow dominated by spring snowmelt (Winnick et al., 2017). In this model, we consider CO 2 fluxes from (a) groundwater with a specified p CO 2 and inflow fluxes that scale with changes in upstream accumulating area and specified runoff values reflecting flow conditions from August 2019; (b) hyporheic exchange parameterized using a surface renewal‐theory mass‐transfer model (Grant et al., 2018; Winnick, 2021) and assuming a constant offset between hyporheic zone and stream CO 2 meant to reflect net respiration; (c) water column net respiration at a specified volume‐normalized rate; and (d) atmospheric exchange in which gas exchange velocities ( k 600 ) are parameterized based on slope and discharge via empirical correlations between energy dissipation rates and k 600 (Ulseth et al., 2019). The full details of model derivation and parameterization are presented in Saccardi and Winnick (2021), and the simulated map of stream p CO 2 values is shown in Figure 1.…”