The biogeochemical fate of terrigenousorganic carbon at land-ocean interfaces is a critical issue because it cancause significant impacts on coastal ecosystems. In Southeast Asia, a hotspotof terrigenous organic carbon export from peatlands into the ocean, ourmulti-year field observation survey showed that 60 - 70% of this organic carbonflux is rapidly remineralized within the coastal waters. Photochemicaldegradation is a quantitatively important driver as revealed by our incubationexperiments and model simulation. This extensive remineralization of organiccarbon is causing a seasonal ocean acidification of an up to 0.15-unit drop inpH and fueling strong sea-to-air CO2 emissions in the region. To better predictcoastal carbon emissions, we are currently building a model representation forsoil biogeochemistry associated with tidal cycles, seasonality of salinity, andplant-mediated gas transport for tidal wetlands based on observation data fromthe Parker River estuary in northeast Massachusetts. This work will connect thesubsurface carbon biogeochemistry with the ecosystem-scale carbon fluxes andenable us to predict how these carbon fluxes will respond to future climatechange scenarios.