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

Abstract

AbstractMethane (CH4) 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 CH4emissions, but with considerable variation, including instances where salinization increased CH4flux. We measured microbial community composition, biogeochemistry, and CH4flux 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 CH4emissions are influenced by salinization. CH4flux was generally, but not always, positively correlated with CO2flux, soil carbon, ammonium, phosphate, and pH. Methanogen guilds were positively correlated with CH4flux across all sites, while methanotroph guilds were both positively and negatively correlated with CH4depending on site. There was mixed support for negative relationships between CH4fluxes and concentrations of alternative electron acceptors and abundances of taxa that reduce them. CH4/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 CH4/salinity relationships. Our results suggest several factors that make it difficult to generalize CH4/salinity relationships and highlight the need for paired microbial and flux measurements across a broader range of sites.