Microbial ecology and site characteristics underlie differences in salinity-methane relationships in coastal wetlands

Methane (CH ₄ ) is a potent greenhouse gas emitted by archaea in anaerobic environments such as wetland soils. Tidal freshwater wetlands are predicted to become increasingly saline as sea levels rise due to climate change. Previous work has shown that increases in salinity generally decrease CH ₄ emissions, but with considerable variation, including instances where salinization increased CH ₄ flux. We measured microbial community composition, biogeochemistry, and CH ₄ flux from field samples and lab experiments from four different sites across a wide geographic range. We sought to assess how site differences and microbial ecology affect how CH ₄ emissions are influenced by salinization. CH ₄ flux was generally, but not always, positively correlated with CO ₂ flux, soil carbon, ammonium, phosphate, and pH. Methanogen guilds were positively correlated with CH ₄ flux across all sites, while methanotroph guilds were both positively and negatively correlated with CH ₄ depending on site. There was mixed support for negative relationships between CH ₄ fluxes and concentrations of alternative electron acceptors and abundances of taxa that reduce them. CH ₄ /salinity relationships ranged from negative, to neutral, to positive and appeared to be influenced by site characteristics such as pH and plant composition, which also likely contributed to site differences in microbial communities. The activity of site-specific microbes that may respond differently to low-level salinity increases is likely an important driver of CH ₄ /salinity relationships. Our results suggest several factors that make it difficult to generalize CH ₄ /salinity relationships and highlight the need for paired microbial and flux measurements across a broader range of sites.