Terrestrial-aquatic interfaces are widely recognized as active zones of organic matter metabolism, and hyporheic zones in particular are critical in determining the fate of organic matter globally. In particular, subsurface zones of groundwater-surface water mixing, known as hyporheic zones, can contribute up to 96% of whole-ecosystem respiration; yet, the mechanisms governing enhanced metabolism in hyporheic zones are poorly understood. Previous results suggest that hyporheic zone hotspots tend to occur in the presence of both thermodynamically-favorable C and organic N; and further that interactions between favorable C and organic N may be critical mechanisms underlying hotspots. To resolve metabolic controls on hyporheic function and enable process-based predictions of river corridor health, we will use controlled experiments to decipher thermodynamic and microbial mechanisms that couple carbon (C) and N cycles in biogeochemical hotspots. We hypothesize that (1) organic N availability constrains the spatiotemporal extent of hyporheic zone hotspots and (2) that metabolic processes in the hyporheic zone shift from the cycling of thermodynamically-unfavorable, mineral-associated C at low levels of respiration to thermodynamically-favorable reactions involving the gain or loss of organic N at higher respiration rates. We will use column experiments packed with sediments with different in situ activities to evaluate interactions between thermodynamically-favorable C and organic N in the development of hyporheic zone biogeochemical hotspots. By adding substrate of different thermodynamic favorability with and without organic N, we will evaluate interactions that stimulate respiration in sediments that have a broad range of in situ respiration rates.
Additionally, we will use process-based understanding generated in this experiment to extend the suite of biogeochemical reactions considered in reactive transport models to further improve model predictive capacity. While many reaction models incorporate knowledge on terminal electron acceptors, our research is improving models with by partitioning electron donors into multiple pools, and data generated in this proposal will inform the development of next-generation reactive transport models. Our integrated biogeochemical-microbial modelling platform is actively being coupled to broader scale hydrobiogeochemical models as part of the SBR-SFA and will ultimately be incorporated into watershed-scale predictions of ecosystem health in the Columbia River watershed. This research is essential for understanding mechanisms underlying ecosystem health and increasing the accuracy of predictive hydrobiogeochemical models.