Vegetation composition and sediment texture jointly shape carbon density in China’s coastal salt marshes: implications for stratified monitoring, reporting and verification

Coastal salt marshes store substantial organic carbon, but strong heterogeneity in carbon density complicates upscaling for blue carbon accounting, national inventories, and restoration planning. Using a standardized monitoring dataset from 361 salt marsh sites across eight coastal provinces in China (2021–2024), we quantified sediment carbon density in the upper 0–1 m and biomass carbon density (aboveground and belowground), and evaluated plausible drivers. Carbon densities were highly skewed, with sediment carbon dominating the combined carbon density. Vegetation composition explained the strongest contrasts: Spartina spp.-dominated marshes exhibited higher sediment and combined carbon density than Phragmites spp. and Bulrush/Sedge (mixed Cyperaceae taxa; including Schoenoplectus spp., Bolboschoenoplectus spp., and Carex spp.) marshes, whereas Phragmites spp. marshes supported the highest total biomass carbon density. After adjustment for vegetation group, sampling year, and latitude, sediment carbon density showed a modest negative latitudinal trend. Sediment fine fraction had little marginal association with sediment carbon density, but emerged as a positive predictor of sediment and combined carbon density once geographic and compositional structure was accounted for, consistent with context-dependent texture effects. In contrast, total biomass carbon density showed limited covariate-adjusted association with either fine fraction or sediment carbon density, and biomass allocation metrics did not provide a direct proxy for sediment carbon density. These results support stratified monitoring, reporting and verification designs that use vegetation group as a first-order stratum and sediment texture as a secondary modifier to strengthen higher-tier, accounting-relevant reporting and restoration targeting.