The importance of porewater exchange process on carbon lateral export from saltmarsh creek to coastal sea

Saltmarsh is one of the blue carbon ecosystems for the highest carbon burial efficiency. However, the buried carbon in saltmarsh may still be exported to coastal water through porewater exchange, a process that has often been overlooked in previous studies. A typical tidal creek of the Dongtan saltmarsh wetland in Chongming Island, Shanghai, China, was studied. The ²²⁴Ra and ²²³Ra activities were measured and the hydrological parameters such as water flow were determined, from which the porewater exchange rate in the tidal creek was estimated to be 1.78±1.73 cm/d. Meanwhile, the carbon concentrations in porewater were determined, based on which the fluxes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), CH₄, and CO₂ exported from porewater exchange were derived to be 60±17, 6.6±4.0, 0.082±0.079, and 16±11 mmol/(m²·d), respectively. In addition, analysis on different species of carbon in the creek water showed that, the fluxes of DIC, DOC, CH₄, and CO₂ exported laterally from tidal creek to coastal sea were 58±14, 7.6±2.3, 0.001 1±0.000 63, and 1.5±0.68 mmol/(m²·d), respectively, indicating that the porewater exchange-derived carbon fluxes accounted for a large portion of the lateral carbon outwelling, and even much higher than those in CH₄ and CO₂ fluxes. Furthermore, the carbon exported from porewater exchange accounted for 50% of the carbon burial in the tidal creek system, of which DIC accounted for 73% of the total carbon flux transported by porewater exchange. Therefore, this study indicated that the porewater exchange-derived carbon fluxes to the tidal creek water may cause an overestimation in the carbon sequestration capacity of saltmarsh wetlands, and revealed the importance of porewater exchange for the carbon cycle of tidal creek system of saltmarsh wetlands.