Microbial Network Restructuring and Functional Partitioning of Carbon Fixation Along a Natural Salinity Gradient in the Yalu River Estuary Wetland

Estuarine wetlands are significant stocks of soil organic carbon (SOC) and play an essential role in carbon sequestration. However, how natural salinity gradients shape carbon-fixing microbial communities and functional genes remains poorly understood. Soil samples were collected from low-salinity (LS), medium-salinity (MS), and high-salinity (HS) zones of the Yalu River Estuary wetland. To assess the taxonomic composition and carbon-fixation potential of soil microbial communities under a natural salinity gradient metagenomic sequencing was used. Because analyses were based on metagenomic profiles, the results reflect potential functional capacity rather than direct measurements of in situ carbon-fixation activity. The carbon-fixing microbial community composition differed among LS, MS, and HS soils. Analysis of carbon-fixation pathways revealed that the reductive tricarboxylic acid (rTCA) pathway represented the largest proportion of carbon fixation-related genes across all salinity groups. Genes associated with the rTCA pathway accounted for 59.72–62.03% of all carbon-fixation-related genes. Specifically, the rTCA pathway-related genes korA and korB were significantly enriched in MS soils, whereas aclB and the Wood–Ljungdahl pathway-related genes acsA/cdhA exhibited higher abundances in both MS and HS soils. The abundance of carbon-fixation-related genes was highest in MS soils despite SOC concentrations not being maximal in this salinity zone, suggesting a decoupling between microbial carbon-fixation potential and SOC accumulation. Additionally, the co-occurrence networks of carbon-fixing microorganisms were more tightly connected in HS soils. This study provides a crucial scientific reference for further elucidating the ecological mechanisms underlying microbial carbon fixation in estuarine wetlands.