Abstract: Coastal wetlands are crucial “blue carbon” ecosystems. Previous studies primarily focus on vertical carbon sequestration in carbon sink assessments, often overlooking the key pathway of lateral carbon exchange. We systematically reviewed the biogeochemical transformation processes, key measurement methods, and driving mechanisms of key components in lateral carbon exchange (dissolved inorganic carbon, dissolved organic carbon, and particulate organic carbon) in coastal wetlands. Compared to vertical carbon burial rates, lateral carbon export rates in coastal wetlands are higher, with no differences among different types of coastal wetlands. The key processes include porewater exchange driven by tidal pumping and submarine groundwater discharge (SGD). Tidal processes drive periodic porewater exchange between sediments and overlying water bodies, promoting the release of dissolved substances from sediments. Meanwhile, multiple geophysical processes, including hydraulic gradients, tidal-induced seabed pressure variations, wave action, and density differences between seawater and groundwater collectively govern the occurrence and intensity of SGD. In terms of driving mechanisms, the tidal pumping effect, porewater exchange, and SGD are the core drivers of lateral carbon export, while extreme weather events (e.g., typhoons, heavy rainfall) dominate short-term carbon flux variations through triggering effects and hydrodynamic intensification. Carbon exported laterally to offshore areas achieves long-term sequestration via pathways such as the carbonate system buffering and sedimentation into deep-sea sediments, yet remains strongly perturbed by human activities and climate change. Future research should conduct fine-scale quantification of the physical-biogeochemical coupling processes in lateral carbon export, perform coordinated observations of multiple carbon components and isotopic tracing, and develop dynamic predictive models of carbon budgets under multiple environmental stressors, providing scientific support for precise global blue carbon accounting and adaptive coastal zone management.

Key words: coastal wetlands, lateral carbon exchange, carbon budget, biogeochemical cycle, driving mechanism