Understanding the dynamics of methane (CH4 ) emissions is of paramount importance because CH4 has 25 times the global warming potential of carbon dioxide (CO2 ) and is currently the second most important anthropogenic greenhouse gas. Wetlands are the single largest natural CH4 source with median emissions from published studies of 164 Tg yr(-1) , which is about a third of total global emissions. We provide a perspective on important new frontiers in obtaining a better understanding of CH4 dynamics in natural systems, with a focus on wetlands. One of the most exciting recent developments in this field is the attempt to integrate the different methodologies and spatial scales of biogeochemistry, molecular microbiology, and modeling, and thus this is a major focus of this review. Our specific objectives are to provide an up-to-date synthesis of estimates of global CH4 emissions from wetlands and other freshwater aquatic ecosystems, briefly summarize major biogeophysical controls over CH4 emissions from wetlands, suggest new frontiers in CH4 biogeochemistry, examine relationships between methanogen community structure and CH4 dynamics in situ, and to review the current generation of CH4 models. We highlight throughout some of the most pressing issues concerning global change and feedbacks on CH4 emissions from natural ecosystems. Major uncertainties in estimating current and future CH4 emissions from natural ecosystems include the following: (i) A number of important controls over CH4 production, consumption, and transport have not been, or are inadequately, incorporated into existing CH4 biogeochemistry models. (ii) Significant errors in regional and global emission estimates are derived from large spatial-scale extrapolations from highly heterogeneous and often poorly mapped wetland complexes. (iii) The limited number of observations of CH4 fluxes and their associated environmental variables loosely constrains the parameterization of process-based biogeochemistry models.

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Peatlands contain one-third of soil carbon (C), mostly buried in deep, saturated anoxic zones (catotelm). The response of catotelm C to climate forcing is uncertain, because prior experiments have focused on surface warming. We show that deep peat heating of a 2 m-thick peat column results in an exponential increase in CH4 emissions. However, this response is due solely to surface processes and not degradation of catotelm peat. Incubations show that only the top 20–30 cm of peat from experimental plots have higher CH4 production rates at elevated temperatures. Radiocarbon analyses demonstrate that CH4 and CO2 are produced primarily from decomposition of surface-derived modern photosynthate, not catotelm C. There are no differences in microbial abundances, dissolved organic matter concentrations or degradative enzyme activities among treatments. These results suggest that although surface peat will respond to increasing temperature, the large reservoir of catotelm C is stable under current anoxic conditions.

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Peatland soils represent globally significant stores of carbon, and understanding carbon cycling pathways in these ecosystems has important implications for global climate change. We measured aceticlastic and autotrophic methanogenesis, sulfate reduction, denitrification, and iron reduction in a bog, an intermediate fen, and a rich fen in the Upper Peninsula of Michigan for one growing season. In 3-d anaerobic incubations of slurried peat, denitrification and iron reduction were minor components of anaerobic carbon mineralization. Experiments using 14 C-labeled methanogenic substrates showed that methanogenesis in these peatlands was primarily through the aceticlastic pathway, except early in the growing season in more ombrotrophic peatlands, where the autotrophic pathway was dominant or codominant. Overall, methane production was responsible for 3-70% of anaerobic carbon mineralization. Sulfate reduction accounted for 0-26% of anaerobic carbon mineralization, suggesting a rapid turnover of a very small sulfate pool. A large percentage of anaerobic carbon mineralization (from 29% to 85%) was unexplained by any measured process, which could have resulted from fermentation or possibly from the use of organic molecules (e.g., humic acids) as alternative electron acceptors.