Salt marsh invasion reduces recalcitrant organic carbon pool while increases lateral export of dissolved inorganic carbon in a subtropical mangrove wetland

Mangrove wetlands are one of the most productive ecosystems and store large amounts of organic carbon (blue carbon). However, mangrove wetlands have been invaded by S. alterniflora-dominated salt marsh globally, the understanding of how this ecological invasion affects carbon cycling in mangroves remains limited. In this study, we conducted intensive investigations and measurements in a subtropical mangrove wetland (S.E. China) to evaluate the impacts of S. alterniflora invasion on sediment organic carbon (SOC) pools, sources, stability, and dissolved inorganic carbon (DIC) export. Results showed that S. alterniflora invasion increased sediment labile organic carbon (LOC) content and decreased the proportion of recalcitrant organic carbon (ROC) compared with other habitats (e.g., mudflat, mangrove, and marsh-mangrove transitional community). The δ13C values showed that mangroves can provide more SOC and ROC than S. alterniflora. The SOC provided by S. alterniflora mostly consisted of LOC, which was unstable and easily transformed, leading to the higher anaerobic mineralization rate and CO2 production. Different from the acidic environment of mangrove sediment, the alkaline environment of S. alterniflora sediment preserves more CO2 in the form of DIC in porewater, which could be transported during the movement of tides. Thus, S. alterniflora potentially exported more DIC to the estuary than mangroves. These findings suggested that S. alterniflora-dominated salt marsh invasion can substantially perturb the mangrove wetlands by reducing ROC pool and potentially increasing the lateral export of DIC.