Controls on stream corridor N2O yields during hyporheic zone denitrification

Abstract:

Stream corridors are recognized as an important global source of nitrous oxide (N₂O) emissions to the atmosphere, largely via denitrification reactions. However, global flux estimates vary by over an order of magnitude due to poor constraints on % N₂O yield during denitrification. To date, the largest systematic efforts to interrogate stream corridor nitrogen processing have been the Lotic Inter-site Nitrogen eXperiments (LINX I and II), and while these yielded fundamental insights into nitrogen fate and transport, no significant correlations were found between measured variables and % N₂O yields. Since the LINX II experiments, new theoretical advances in turbulent exchange have been used to explore limits on nitrogen processing rates. Here, we revisit LINX II % N₂O yield data in the context of surface renewal theory representations of hyporheic exchange. We find that denitrification efficiency, calculated as the ratio of denitrification uptake velocity to exchange velocity, is the single best predictor of % N₂O yield (log-log r² = 0.40, p < 10^-6). We further interrogate transport and stream chemistry controls on % N₂O yield using a CrunchFlow-based reactive transport model of hyporheic zone denitrification. In these experiments, model runs are initialized with LINX II stream geochemistry and include simulations of advective flowlines and sediment diffusion profiles representing anoxic micro-zones. Under advective transport, we find significant correlation between predicted and observed denitrification efficiency; however, modeled efficiency is typically 2-3 orders of magnitude lower than observations, suggesting a primary role for dissolved organic carbon (DOC) produced within the hyporheic zone in driving denitrification. Additionally, simulations of sediment diffusion profiles capture maximum observed % N₂O yields from sediment-water interface fluxes. Together, these results suggest that under stream DOC-limited conditions, bulk oxic hyporheic zones limit denitrification efficiency and shift denitrification to diffusion-dominated anoxic micro-zones, which causes elevated % N₂O yields.