The complex interactions among soil, vegetation, and site hydrologic conditions driven by precipitation and tidal cycles control the biogeochemical transformations and bi-directional exchange of carbon and nutrients across the terrestrial-aquatic interfaces (TAIs) in coastal regions. This study uses a highly mechanistic model, Advanced Terrestrial Simulator (ATS)-PFLOTRAN, to explore how these interactions affect exchanges of materials and carbon and nitrogen cycling. We used a transect in the Chesapeake Bay region that spans zones of open water, coastal wetland, transition, and upland forest. We designed several simulation scenarios to parse the effects of the individual controlling factors and the sensitivity of carbon cycling to reaction rate parameters derived from laboratory experiments. Our simulations reveal an active zone for carbon cycling under the transition zones between the wetland and the upland. Evapotranspiration is found to enhance the exchange fluxes between the surface and subsurface domains, resulting in a higher dissolved oxygen concentration in the TAIs. The transport of organic carbon derived from plant leaves and roots provide an additional source of organic carbon needed for the aerobic respiration and denitrification processes in the TAIs. The variability in reaction rate parameters associated with microbial activities is also found to play a dominant role in controlling the heterogeneity and dynamics of the simulated redox conditions. This modeling-focused exploratory study enabled us to better understand the complex interactions among soil, water and microbes that govern the hydro-biogeochemical processes at the TAIs, which is an important step toward representing coastal ecosystems in larger-scale Earth system models. The hydrological environment of vegetated coastal ecosystems is directly influenced by precipitation and seawater flooding, which mediates biogeochemical processes within these areas. However, the specific effects of dynamic precipitation and flooding on oxidation-reduction conditions in these complex terrestrial-aquatic interfaces (TAIs) are poorly understood, especially when considering the ecological processes of above-ground plants. To address this gap, this study used integrated process-based models, the Advanced Terrestrial Simulator (ATS) and PFLOTRAN, to examine the effects of hydrological and ecological controls on biogeochemical reactions and exchange fluxes across a TAIs transect spanning from a coastal upland forest and salt marsh to the open seawater. Our numerical experiments showed that the mixing of different waters within the TAIs significantly influenced the spatial and temporal variability in exchange fluxes across this interface along with the spatial extent of oxic subsurface zones. The interface between the oxic and anoxic zones shifts in response to periodic fluctuations in tidal elevations as higher tides drive more oxygenated water toward the TAIs. Meanwhile, vegetation evapotranspiration removes more water from the subsurface during warm summer months, leading to larger exchange fluxes across the TAIs. Reaction rate parameters that depend on the interactions between the soil and microbes have a large effect on carbon and oxygen consumption represented in our models. A higher aerobic respiration rate results in larger hypoxic and anoxic zones because the dissolved oxygen is consumed more quickly. Our modeling-based study provided insights into the mechanisms that control the exchange fluxes and cycling of carbon and nitrogen at coastal TAIs, which can be used to inform potential management strategies for mitigating the impacts of climate change on these ecosystems. Tidal elevations, precipitation, and evapotranspiration (ET) interact to control dynamic exchange fluxes across the coastal terrestrial aquatic interface Integrated hydrobiogeochemical modeling reveals variability in redox conditions along gradient of upland, transition, and wetland to ocean The high uncertainty in microbial-remediated aerobic respiration rate has significant impact on modeling carbon cycling in coastal regions
