Abstract: Natural wetlands play an important role in global carbon cycle. Understanding the carbon cycling in coastal wetlands has great significance for estimating the carbon budget of coastal areas. To explore the carbon cycling of Yancheng coastal wetland in Jiangsu Province, we measured soil respiration in three types of wetlands respectively dominated by Spartina alterniflora, Phragmites australis, and Suaeda salsa using the static chamber technique. The root removal method was used to measure soil autotrophic and heterotrophic respiration. The results showed that soil respiration in the three wetland types exhibited significant seasonal variations, with higher values in summer and lower values in winter. Soil respiration varied significantly across vegetation types, with the highest value in the S. alterniflora wetland and the lowest value in the S. salsa wetland. The annual average soil respiration in the S. alterniflora, P. australis, and S. salsa wetlands were 210.7, 174.7, and 115.3 mg CO₂·m^-2·h^-1, respectively. The contributions of heterotrophic respiration to CO₂ emissions for the three vegetation types were 38.8%, 35.9%, and 52.0%, respectively. The contributions of autotrophic respiration were 61.2%, 64.1%, and 48.0%. There were significant differences in plant biomass, soil dissolved organic carbon (DOC) content, and microbial biomass carbon (MBC) among different vegetation types. The results of correlation analysis and stepwise regression showed that belowground biomass, soil DOC content and MBC were the main factors driving the variation of soil respiration across different vegetation types. The estimation of the wetlandatmosphere carbon balance showed that S. alterniflora, P. australis, and S. salsa wetlands were all sinks of atmospheric carbon, with carbon sequestration rates of 811.1, 760.0, and 145.9 g C·m^-2·a^-1, respectively. Among all the wetlands, the S. alterniflora wetland had the highest sequestration rate.

Key words: coastal wetland, Spartina alterniflora, soil respiration, heterotrophic respiration, carbon balance.