Wetland succession reshapes microbial degradation of plant- and microbial-derived carbon

Plant- and microbial-derived organic carbon require distinct microbial enzymes, but how wetland succession regulates these substrate-specific degradation pathways in estuarine soils remains unclear. We collected 0-10 cm soils from four wetland types in the Liaohe River Estuary, China-tidal flat, restored wetland, Suaeda salsa wetland, and reed wetland-with three independent replicate sites per type. Shotgun metagenomic sequencing, CAZy annotation, taxonomic annotation, co-occurrence networks, and Mantel tests were used to examine CAZyme genes targeting plant-, fungal-, and bacterial-derived carbon. We identified 16,346,752 CAZyme-encoding sequences assigned to 749 families. Carbon-cycling gene composition differed significantly among wetland types (ANOSIM R = 0.37, p = 0.034). Gene diversity was higher in early to mid-successional stages, whereas the abundances of plant-, fungal-, and bacterial-derived carbon degradation genes increased along succession. Lignocellulose-degrading genes were most enriched in reed wetland, including AA3, CBM9, and CE1. Microbial hosts shifted markedly, with Bacteroidota increasing from 8.53% to 38.28% among plant-derived carbon degraders. Plant-derived carbon degrader networks were densest in tidal flat soils, suggesting a transition from stress-associated microbial associations to resource-specialized assemblages. Environmental controls were substrate-specific: plant-derived genes correlated only with nitrate, fungal-derived genes with moisture, nitrogen, salinity, and electrical conductivity, and bacterial-derived genes with none of the measured variables. These findings reveal substrate-specific microbial mechanisms linking wetland succession to carbon turnover and identify Bacteroidota, AA3, and nitrate availability as candidate indicators for restoration assessment and carbon-sequestration management.