Regulators of coastal wetland methane production and responses to simulated global change

Wetlands are the largest natural source of methane (CH4) to the atmosphere, but their emissions vary along salinity and productivity gradients. Global change has the potential to reshape these gradients and therefore alter future contributions of wetlands to the global CH4 budget. Our study examined CH4 production along a natural salinity gradient in coastal Alaska wetlands. In the laboratory, we incubated natural sediments to compare CH4 production rates between freshwater and intertidal wetlands, and quantified the abundances of methanogens and sulfate-reducing bacteria in these ecosystems. We also simulated sea-level rise and enhanced organic matter availability, which we predicted would have contrasting effects on coastal wetland CH4 production. Intertidal wetlands produced less CH4 than freshwater wetlands due to high sulfate availability and generally higher abundances of sulfate-reducing bacteria, whereas freshwater wetlands had significantly greater methanogen abundances. Simulated sea-level rise in freshwater sediments, however, did not reduce CH4 production, perhaps because the 14d incubation period was too short to elicit a shift in microbial communities. In contrast, increased organic matter generally enhanced CH4 production rates, but this response varied by the macrophyte species added. Our study suggests that CH4 production in coastal wetlands, and therefore their overall contribution to the global CH4 cycle, will be sensitive to increased organic matter availability and potentially sea-level rise. To better predict future wetland contributions to the global CH4 budget, future studies and modeling efforts should investigate how multiple global change mechanisms will interact to impact CH4 dynamics.