12.491 Biogeochemistry of Sulfur Fall 2007 - Student Paper

INTRODUCTION Methane is a highly efficient greenhouse gas, approximately 21 times more effective than CO2 at trapping infrared radiation from Earth (Seinfeld and Pandis, 2006). Although the importance of methane in the climate system is recognized, its global budget is poorly constrained. The atmospheric reservoir is easily estimated using measured methane concentrations; residence time and primary sinks are reasonably well known (Fig. 1). Source terms, however, are more ambiguous. The bulk of atmospheric methane is of biological origin, and its sources are influenced by natural climate shifts as well as human activities. The largest single source is wetland methanogenesis, which releases 115 Tg of methane to the atmosphere per year (Reeburgh, 2007). Consequently, understanding the wetland source term is critical in constructing an accurate global methane mass-balance. In any environment the production of methane is controlled in large part by the presence or absence of sulfate. Sulfate generally suppresses methanogenesis by stimulating sulfate-reducing bacteria (SRB) that outcompete methanogenic archaea for hydrogen and acetate, substrates common to both groups (Madigan et al., 2006). Low sulfate concentrations in terrestrial freshwater environments enable large-scale methane production in anoxic sediments and waters. Recently, Gauci et al. (2004) presented evidence that anthropogenic sulfur dosing through acid rain and dry deposition may reduce wetland methanogenesis by up to 15% by 2030. They recognize that methane production is only inhibited, not halted, by sulfur dosing; the extent of inhibition likely depends on a complex interplay of factors including wetland hydrology and temperature, degree of sulfate pollution, and distance of pollutant source from wetlands. The composition of wetland methanogen communities is likely to be another important control on their response to sulfate dosing, and is the focus of the research proposed here. In particular, understanding the viability of methanogenesis using non-competitive substrates (those for which methanogens and SRB are not in direct competition) is important in modeling the effect of sulfur pollution on the global wetland methane source term. Because the wetlands source term is so large, even small changes in its magnitude can be significant; even the margins of error associated with the estimates of suppression are greater than some sources.