Combining soil microbial communities and greenhouse gas fluxes along a salinity gradient in temperate Mediterranean coastal wetlands

• Soil physicochemical traits and microbes characterized along a salinity gradient • Salinity, sulfur, and iron shape microbial structure across wetlands • Sulfur-reducing bacteria dominate highly saline soils • Salinization lowers CH 4 but increases CO 2 emissions Coastal wetlands play a critical role in carbon sequestration, biogeochemical cycling, and ecosystem stability. These habitats support diverse microbial communities that regulate organic matter decomposition and greenhouse gas fluxes, influencing climate-related feedback mechanisms. However, rising sea levels and saltwater intrusion may disrupt microbial processes, particularly those associated with the sulfur cycle and methane dynamics. Here, we characterize soil physicochemical properties and microbial communities along a salinity gradient in three temperate coastal wetlands to assess the impact of salinity on organic matter decomposition and greenhouse gas emissions. Using full-length Oxford Nanopore MinION 16S rRNA amplicon sequencing, we analyzed microbial communities across freshwater, brackish, and saline wetland soils. Our results indicate that sulfur-reducing bacteria dominate salinized sites, while brackish environments are characterized by obligate anaerobic taxa involved in sulfate reduction, fatty acid degradation, and denitrification. These microbial assemblages contribute to lower CH 4 emissions but increased CO 2 fluxes in the brackish areas, highlighting key microbial-mediated trade-offs in wetland carbon cycling. By integrating microbial diversity, and metabolic functions with soil geochemistry, this site-specific but ecologically meaningful case study improves our understanding of microbe-soil interactions in temperate wetland ecosystems facing increased salinization due to climate change.