The objective of this research is to understand the factors that control the production of the greenhouse gases carbon dioxide and methane in wetland soils under anaerobic (i.e., waterlogged with oxygen depletion) conditions in current and future climates. These two gases are the end products of a complicated, interlinked set of microbial processes during the anaerobic decomposition of soil organic matter, and the goal of this project is to understand the relative ratio at which these two gases are produced within the context of the larger anaerobic carbon cycle. Both field and laboratory experiments will be used to examine the chemical, environmental, and biological factors (including microbial community structure) that control the ratio at which these important greenhouse gases are produced in six wetlands in northern Michigan. These wetlands represent a broad range of conditions in terms of hydrology, chemistry, and plant community composition.
The proportion of soil carbon that ends up as carbon dioxide and methane during anaerobic decomposition varies by several orders of magnitude among different types of wetlands, and the factors that control this variation are not well understood. Because methane is a much more potent greenhouse gas than carbon dioxide, the ratio at which these two gases are produced can have a substantial impact on the Earth?s climate system. Also, methane emissions from wetlands have been sensitive to climate change in the past and may have strong feedbacks to future human-induced climate change.
Methane is a potent greenhouse gas, and wetlands are a significant natural source of methane. There is great concern that global change may enhance methane emissions from wetlands, and thus exacerbate direct human-caused climatic warming. Methane production is widely variable among different types of wetlands for reasons that are not fully understood. It is necessary to understand the basic processes that control methane production in the diversity of wetlands on the landscape to successfully predict how they will respond to global change. We examined a wide variety of biological and non-biological factors that control methane production in six peatlands (wetlands with thick deposits of soil organic matter) in northern Michigan that represent the regional diversity of wetlands in this region (and somewhat more broadly in the boreal zone). In a series of field and laboratory studies, we found that ‘bogs’ (peatlands with only precipitation inputs) are very inefficient at producing methane, whereas ‘fens’ (peatlands with groundwater and/or surface water inputs) are much more efficient at producing methane. Low pH in bogs was a partial reason for their inefficiency in methane production, but other factors are also important. Another reason for low methane production in some wetlands is due to their complicated interactions with other microbial groups. Methanogens (the microbial group that makes methane) can only exist under strict anaerobic conditions (i.e., without oxygen) that typically occur after prolonged waterlogged conditions in wetlands. Methanogens in freshwater ecosystems entirely depend on two simple substrates, acetate and H2, and they are dependent on other ‘upstream’ fermentative bacteria to produce these substrates in a syntrophic interaction. However, other microbial groups also compete with methanogens for the acetate and H2 formed during fermentation. The efficiency of methane production during anaerobic carbon decomposition is thus a delicate balance between these syntrophic and competitive microbial interactions. We explored these interactions in our six wetland sites by examining the rates of various microbial pathways that both produce and consume acetate and H2. Microbes that utilize chemically complex organic compounds termed humic substances were found to be particularly important competitors with methanogens, but this varied by wetland type. These humic substances, or some similar organic compounds, were also found to be highly directly inhibitory (i.e., not through competition) to methanogens in bogs and may explain the very low methane efficiency in these wetlands. Our findings may have importance consequences for the response of northern peatlands to climate change. Methane emissions from bogs may currently be relatively insensitive to warming because of inhibition from low pH and organic compounds. However, one class of potentially inhibitory organic compounds to methanogens are derived from Sphagnum mosses that are ubiquitous in bogs. Experiments have shown that drier conditions and/or increasing nutrient availability (e.g., through atmospheric nitrogen deposition) shift the bog communities to other plant species that lack these inhibitory compounds. If this were to occur due to global change, then methane emissions from bogs may be much more responsive to warmer temperatures. We are also using cutting-edge molecular techniques to determine the presence and activity of methanogens seasonally in the six experimental wetlands. The microbial community data will be correlated with the various rate processes described above to determine to what extent methanogen community composition predicts methane dynamics in northern peatlands. This project also significantly enhanced the education and professional development of many people. Four postdoctoral associates worked on this project. Two have permanent academic positions at this point and two are continuing in postdoctoral positions elsewhere. One Ph.D. student was worked extensively on this project at the interface between molecular microbial ecology and geochemistry, which will provide him with a solid interdisciplinary background for his future scientific career. Many undergraduates worked on this project at both the University of Oregon and Chapman University. They were provided valuable training in how to conduct research, and many are now in graduate or medical school. Most of these undergraduates are women, and several are from minority groups (Native American and Hispanic). A number of review papers and book chapters for both scientific and undergraduate/graduate audiences were written by the research team on anaerobic carbon cycling in wetlands during this grant.
