Methane Biogeochemistry and Microbial Communities in Natural and Restored Freshwater Depressional Wetlands
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Abstract
Wetlands are the largest natural source of methane (CH4), a potent greenhouse gas. Wetland CH4 emissions are dependent on rates of microbial CH4 production (methanogenesis) and consumption (methanotrophy). These processes vary spatially and temporally with environmental conditions, edaphic characteristics, and microbial community structure, making it difficult to predict wetland CH4 emissions. This high variability can be further pronounced in restored wetlands that have undergone environmental and edaphic disturbances. The following work aims to understand this variability by assessing patterns of methanogenesis and methanotrophy, and their associated microbial communities, across natural and restored freshwater depressional wetlands on the Delmarva Peninsula (USA). Sites addressed in this work were restored from agricultural land between 1986 and 2004 through multiple programs funded by the United States Department of Agriculture (USDA). In the first set of experiments, we identified a high abundance of active acetoclastic methanogens in intact core incubations from a restored wetland suggesting a higher potential for methanogenesis in situ compared to the natural wetland assessed. The co-occurrence of active methanogens and Fe-reducing bacteria in these restored wetland cores contradicted the hypothesis that loss of competition may allow methanogens to be the primary users of acetate. Following assessments across vegetative-hydrologic zones in a series of restored wetlands of varying ages, and their natural counterparts, highlighted vegetation type and extent as a driver of methanogen community abundance, composition, and activity. In turn, restored wetlands showed elevated potentials rates of methanogenesis compared to natural sites. Potential rates of methanotrophy (aerobic and anaerobic), however, were also elevated in restored wetlands, which could constrain CH4 emissions in situ. Variability of environmental conditions (ie. hydrology and vegetation) and edaphic measures (ie. soil organic matter (SOM)) across all sites sampled are reflected in distinct microbial community composition and CH4 biogeochemistry. Clear patterns of SOC accumulation and CH4 biogeochemistry with restoration age were not observed for these wetlands, and variability in environmental conditions and edaphic measures across the sites (restored and natural), emphasize the need for continued monitoring and maintenance of the wetlands. Our results suggest efforts to manage herbaceous vegetation extent and maintain regular seasonal hydrology in future restorations may help prevent high potentials for CH4 production, and thus emissions.