Plant community composition influences soil carbon (C) storage and stability in coastal wetlands, but such effects remain unclear in the non-growing season. In this study, the soil C content, density and stability were examined across five coastal plant communities—Spartina alterniflora, Suaeda salsa, Phragmites australis, mixed S. alterniflora–S. salsa communities and bare flat in the non-growing season in Yancheng, Jiangsu Province, China. The S. alterniflora community exhibited elevated soil organic and inorganic C contents, owing to its high biomass, strong C retention capacity and reduced bacterial abundances. The P. australis community showed higher dissolved organic C and microbial biomass C contents, driven by increased soil moisture and inorganic nitrogen (N) that promote microbial decomposition of plant residue. The S.salsa community had the lowest soil organic C density due to its low aboveground biomass, soil moisture and inorganic N and jointly microbial effects. However, the highest soil inorganic C density in bare flat was due to high soil moisture and microbially mediated stabilization of recalcitrant C. The lowest resistance index of C in P. australis community was associated with low electric conductivity, high C and N availability and bacterial effects. Soil C fractions, densities and resistance index of C decreased with soil depth, likely reflecting reduced water and N availability that constrain root and microbial activities. The results suggest that the S.alterniflora community enhances soil C accumulation, while P. australis community accumulate more labile C fractions, evoking low C stability due to interaction between soil physicochemical and microbial properties.