Wetlands store between 15 and 30% of the global soil carbon pool as dissolved and particulate organic matter (DOM and POM). Wetlands are also the largest natural source of atmospheric methane (CH4) and given the high warming potential of CH4, formation and loss processes of CH4 in wetlands have received considerable research interest. Yet, our understanding of methanogenesis in wetlands is incomplete. Recent studies have implicated DOM and POM in two processes that suppress methanogenesis in wetlands. The first is utilization of DOM and POM as terminal electron acceptors (TEAs) in anaerobic microbial respiration. Electron transfer to OM is thought to competitively suppress methanogenesis. The second process is the direct inhibition of methanogens by polyphenolic DOM constituents. The goal of the proposed research is to quantify the importance of these two processes by incubating natural wetland samples in the laboratory and selectively perturbing them to study the two processes. The importance of DOM and POM as TEAs will be assessed by quantifying changes in their redox states during anoxic incubations using analytical electrochemistry combined with monitoring the formation of CH4 and carbon dioxide. Incubation experiments with alternating anoxic and oxic conditions will be carried out to determine whether electron transfer to OM samples is reversible and, thus, whether suppression of methanogenesis via this process is sustained over repeated redox cycles in periodically anoxic wetlands. The potential inhibitory effects of polyphenolic DOM on methanogens will be assessed by measuring changes in methanogenesis in anoxic samples after addition of both natural and model polyphenols. The proposed research will derive a mechanistic understanding of the two studied processes. This understanding will allow transfer of the results from the lab-scale investigations to processes at the field scale and will help to improve wetland management policies.
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