Vertical stratification of microbial communities and predicted carbon-cycling potential in Suaeda salsa salt-marsh...

Suaeda salsa salt marshes are important blue carbon habitats in estuarine wetlands, but the vertical organization of microbial communities and their potential roles in carbon transformation remain insufficiently understood. In this study, sediments were collected from a S. salsa salt marsh in the Liao River estuary at three depth intervals: 0-10, 10-30, and 30-60 cm, with adjacent mudflat sediments used as a reference. Sediment physicochemical properties, carbon stocks, 16S rRNA gene sequences, community assembly processes, co-occurrence networks, and predicted functions were analyzed to assess the vertical organization of microbial communities and their potential roles in carbon cycling. Sediment depth significantly influenced microbial community structure, indicating clear vertical differentiation in salt-marsh sediments. Proteobacteria and Desulfobacterota dominated across samples. The sediment organic carbon (SOC) stocks within the 0-60 cm profile were similar between the S. salsa habitat and the adjacent mudflat, whereas higher dissolved organic carbon concentrations in the S. salsa habitat suggested greater labile carbon availability. Community assembly was predominantly classified as homogeneous selection across habitats and sediment depths. The S. salsa habitat also showed numerically higher network modularity, whereas depth-associated community variation was numerically greater in the mudflat. Functional prediction indicated that chemoheterotrophy and aerobic chemoheterotrophy were the dominant potential functions. Associations between SOC and predicted carbon-fixation marker genes were partly attributable to covariation along the sediment-depth gradient, highlighting the need for cautious interpretation of inferred functional relationships. Overall, the results support habitat-associated differences in labile carbon availability, microbial community organization, and predicted functional potential rather than evidence for enhanced sediment carbon storage in the S. salsa habitat.