Shifting flood regimes alter iron-sulfur metabolism and greenhouse gas associations

Coastal wetlands store carbon, but compound stressors, including saltwater intrusion, sea level rise, and precipitation extremes, threaten this benefit by altering microbial communities and influencing greenhouse gas emissions. Mediating soil gas exchange, nutrient and carbon availability, and soil moisture, wetland vegetation interacts with these compound stressors. However, the exclusion of plant-driven redox shifts from many mesocosm studies limits our understanding of their role in shaping microbial responses to hydrologic conditions. This study employed a soil mesocosm approach to investigate the impact of hydrology (wet, dry, and interim) and plant presence on microbial communities and greenhouse gas fluxes in coastal freshwater wetland soils with varying past hydrologic regimes (i.e., soil history) and salinity stress. We used shotgun metagenomic sequencing to characterize the functional potential of soil microbes, measured \textit{in situ} greenhouse gas fluxes, and characterized soil physicochemistry. Results showed that contemporary hydrology and soil history significantly impacted microbial gene composition related to sulfate and iron reduction. The compositions of genes for sulfate and iron reduction were correlated, and dissimilatory sulfate reduction genes influenced methane emissions. Findings highlight the roles of historical hydrology, potential saltwater exposure, and soil iron in shaping microbial responses to future changes in soil moisture, plant cover, and salinity. While plants did not significantly influence sulfur or iron metabolism, plant presence did impact greenhouse gas fluxes. We found a strong relationship between sulfate reduction and methanogenesis, which complements previous studies that have shown enhanced methanogenesis with seawater amendment. These results indicate that flooding without salinity is sufficient for coupled sulfate reduction and methanogenesis, provided that a legacy of saltwater intrusion has altered soil sulfate concentrations and sulfate-reducing microbial communities. Understanding microbial community metabolism in coastal wetlands is crucial for predicting their role in carbon sequestration and greenhouse gas emissions under future climate scenarios, developing effective management strategies to mitigate climate change impacts, and preserving these vital wetland ecosystems.