Impacts of vegetation replacement on organic carbon burial in coastal wetlands of Sansha Bay, Southeast China

Coastal wetlands are essential yet vulnerable carbon sinks; however, the impacts of vegetation replacement on carbon dynamics remain less understood. This study investigates the sources, burial rates, and historical trends of sedimentary organic carbon (OC) across various wetland types, including mangroves, Spartina alterniflora (S. A.), and barren tidal flats in Sansha Bay, Fujian Province, Southeast China. We analyzed sediment cores for geochemical (TOC, TN, δ¹³C) and radionuclide (²¹⁰Pb, ¹³⁷Cs) proxies. Our results reveal that riverine inputs are the primary sources of OC, contributing 51–75 % of the total. The mangrove ecosystem exhibits the highest OC burial rate at163 ± 49 g/m²/yr, followed by the S. A. marshland at 115 ± 16 g/m²/yr, and barren flats at 69 ± 13 g/m²/yr. However, human interventions, particularly land reclamation and the invasion of S. A., have resulted in a 13.64 % decrease in the bay-wide OC burial rate from 1999 to 2018. While S. A. enhances local carbon sequestration, its invasion disrupts mangrove ecosystems and destabilizes existing carbon pools. This study underscores the dual impact of vegetation changes on coastal carbon dynamics: invasive species may temporarily enhance OC burial rates but ultimately undermine long-term ecosystem resilience. To maintain blue carbon functions, we advocate prioritizing mangrove conservation, restoring degraded wetlands, and curbing the spread of S. A. These findings provide valuable insights for balancing carbon mitigation efforts and biodiversity conservation in rapidly changing coastal environments.