Abstract: Coastal salt marshes are important blue carbon sinks. The photosynthetic carbon fixation capacity of salt marsh vegetation directly affects organic carbon content and carbon storage in sediments. In this study, we analyzed the interannual variation in the distribution area of major vegetation in the coastal salt marshes of Yancheng from 1975 to 2021 through literature review and data integration. Then, we examined the differences in PSII maximum photochemical efficiency (F_v/F_m) and maximum net photosynthetic rate (P_n) among Phragmites australis, Suaeda salsa, and Spartina alterniflora. Finally, we assessed the organic carbon content and carbon storage in the sediments of the Yancheng salt marsh. The results showed that the areas of P. australis and S. alterniflora in the core area of Yancheng National Rare Bird Nature Reserve increased year by year from 1983 to 2021, rising from 50 and 280.54 hm² to 6532.55 and 4281.86 hm² respectively, while the area of S. salsa decreased from 3293.2 to 1315.33 hm². From 1975 to 2019, the areas of P. australis and S. salsa outside the core area decreased significantly, dropping from approximately 60000 and 70000 hm² to approximately 8000 and 5000 hm², respectively. The average net photosynthetic rate of S. alterniflora was 14.77 μmol CO₂·m^-2·s^-1, higher than that of P. australis (11.45 μmol CO₂·m^-2·s^-1), indicating a stronger carbon fixation capacity for S. alterniflora. This resulted in significantly higher organic carbon content and carbon storage in the sediments of areas dominated by S. alterniflora than those in areas dominated by P. australis and S. salsa. The findings provide theoretical guidance for the biodiversity conservation of coastal wetlands and the study of carbon fixation in salt marshes in China.

Key words: coastal wetland, wetland vegetation, sediment, organic carbon, carbon storage