Microbial Ecology and Site Characteristics Underlie Differences in Salinity‐Methane Relationships in Coastal Wetlands

Abstract 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.
Plain Language Summary Sea level rise will lead to increases in salinity in coastal wetlands and estuaries. Salinity is a key variable that controls the amount of greenhouse gases emitted from coastal wetlands, but we do not know if increases in salinity will increase or decrease emissions of the potent greenhouse gas methane, which is produced by microorganisms in wetlands. We examined salinity‐methane relationships at four different sites in the United States, and assessed how environmental factors and microorganism communities affected those relationships. We found discrepancies in salinity‐methane relationships that are likely driven by site‐specific communities of microorganisms that may respond differently to increasing salinity. Relationships between methane emissions and environmental variables were not consistent across sites, highlighting the difficulty of making generalizations about methane emissions from coastal wetlands and how they will respond to seawater intrusion.
Key Points Salinity‐methane relationships are variable across sites Relationships between nutrients, alternative electron acceptors, and methane flux were not consistent across sites Coastal wetland microbial consortia are site‐specific and may respond differently to seawater intrusion, with implications for methane flux

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