The complex interactions among soil, vegetation, and site hydrologic conditions driven by precipitation and tidal cycles control biogeochemical transformations and bi-directional exchange of carbon and nutrients across the terrestrial-aquatic interfaces (TAIs) in the coastal regions. This study uses a highly mechanistic model, ATS-PFLOTRAN, to explore how these interactions impact the material exchanges and carbon and nitrogen cycling along a TAI transect in the Chesapeake Bay region that spans zones of open water, coastal wetland and upland forest. Several simulation scenarios are designed to parse the effects of the individual controlling factors and the sensitivity of carbon cycling to reaction constants derived from laboratory experiments. Our simulations revealed a hot zone for carbon cycling under the coastal wetland and 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 higher dissolved oxygen concentration in the TAI. The transport of organic carbon decomposed from leaves provides additional source of organic carbon for the aerobic respiration and denitrification processes in the TAI, while the variability in reaction rates mediated by microbial activities plays 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 of various system components at the TAIs that control the hydro-biogeochemical processes, which is an important step towards representing coastal ecosystems in larger-scale Earth system models.

How will coastal forests respond to rising sea levels, disturbances such as storms, drought, and climate change, which are combining to cause widespread impacts on ecosystems at the terrestrial-aquatic interface? One particular area of uncertainty is how these processes may mobilize soil organic carbon, resulting in changes in the dissolved or gaseous greenhouse gas (GHG) fluxes. We present results from the first year of a manipulative experiment looking at the effects of changes in salinity exposure and water availability on soil GHG fluxes. Large (40 cm diameter) soil cores were transplanted along natural salinity and elevation gradients in a Maryland, USA, coastal forest subject to rapid sea level rise. Comparative observations were also made in a western Washington, USA watershed with slow sea level rise but strong tidal fluctuations. Disturbed and undisturbed control cores allow us to distinguish transplant from salinity effects; soil respiration measurements were made every 7-10 days. Cores transplanted to lower-salinity sites, and to higher-elevation plots, exhibited elevated GHG fluxes relative to disturbance controls. These preliminary results suggest that changes in salinity exposure and water availability may exert significant effects on coastal forest GHG fluxes and the stability of soil carbon.