Coastal wetlands are widely recognized as effective long-term carbon (C) sinks, largely due to high sediment burial rates. Yet, the magnitude of hydrologic C export relative to net ecosystem production (NEP) remains poorly constrained, as complete ecosystem-scale C budgets that explicitly quantify lateral losses are rare. By integrating high-resolution, year-round lateral C flux measurements, continuous eddy-covariance/chamber-based CO₂ and CH₄ exchange, and sediment C accumulation over a complete annual cycle, we construct a comprehensive net ecosystem carbon balance (NECB) for two coastal marshes situated along the salinity gradient in the globally significant Yangtze River Delta. R C export seasonally. Results show that lateral fluxes were substantial (232-300 g C m^-2 yr^-1), offsetting approximately half of NEP in these two coastal marshes. Dissolved inorganic carbon dominated these lateral losses (DIC; 87%), followed by dissolved organic carbon (DOC; 12%) and a minor contribution from particulate organic carbon (POC; 0.6%). Notably, while marsh lateral POC concentrations were comparable to DOC, its flux was an order of magnitude lower due to sedimentation and hydrodynamic regulation, with the turbid Yangtze River playing a key role in delivering and modulating particulate C export seasonally. These results demonstrate that, despite high burial efficiency, coastal wetlands redistribute a significant fraction of fixed C laterally, with lateral losses representing a disproportionately large component of the NECB. This study provides a mechanistic and scalable framework for resolving wetland NECB, highlighting that integrative, ecosystem-scale measurements are essential for assessing coastal wetlands’ climate mitigation potential at regional to global scales.