The carbon balance of coastal wetlands has substantial uncertainties due to the complex vegetation-soil biogeochemical processes and the compound influence of inland groundwater discharge and tidal changes. The relative roles of lateral dissolved carbon fluxes and surface-atmosphere exchanges are difficult to constrain and remain a major gap in coastal wetland carbon budgets. Most previous studies combined measurements of isotopes (e.g., Rn) and dissolved or gaseous carbon species (e.g., DIC, DOC, CO2, CH4) to estimate lateral carbon discharge and/or greenhouse gas emissions. However, lateral flux measurements are typically sparse in space and time, making it difficult to capture the hydrodynamic influence at the daily to seasonal time scale. To address this challenge, we used the Energy Exascale Earth System Model (E3SM) Land Model (ELM) to simulate the aboveground and belowground carbon processes of a salt marsh in the Louisiana delta. We coupled ELM to a subsurface multiphase reactive transport model, PFLOTRAN, to simulate aqueous carbon discharge to the ocean and CH4/CO2 emission from sediments. This model combination could reproduce the observed water level and porewater measurements. Our preliminary results suggest that lateral flow plays a major role for DIC export from land to ocean, and highlight the importance of groundwater upwelling for DOC exchange through the sediment-water interface. The magnitude of gaseous CO2/CH4 emission is strongly correlated with the inundation status, where full inundation facilitates CH4 production and emission. Our approach shows that mechanistically linking the carbon and hydrological processes of coastal wetlands could advance understanding of the impacts of climate change and human activity on the carbon balance of coastal wetlands.