TROPHIC STATUS AND THE PATH TO METHANE IN PEATLANDS: IMPLICATIONS FOR METHANOGENESIS IN A CHANGING WORLD

Typical methanogenic pathways include near-terminal C intermediates that turn over rapidly with small pool sizes. However, laboratory and field experiments demonstrated that these intermediates accumulate in many northern peatlands due to the lack of consumption by methanogens. When this inhibition occurs, acetate becomes the primary organic end product of decomposition, which can exceed CO₂ production. Other organic acids such as propionate and butyrate can also accumulate. These organics are readily converted to CO₂ when alternate electron acceptors are present, so C destined for CH₄ in typical freshwater habitats, is converted to CO₂, a weaker greenhouse gas, as acetate diffuses to the surface or to areas adjacent to plant roots. The extent to which methanogenesis is inhibited and organic compounds accumulate is connected to trophic status, with nutrient-poor, low pH regions with less vascular plants tending to favor organic acid accumulation and low rates of methanogenesis. Hence, increases in the cover of vascular plants in peatlands that are occurring as a result of climate warming should greatly increase the rate of methanogenesis as inhibition is relieved and acetate conversion to CH₄ increases. The connection to trophic status and vegetation distribution allows for the mapping of the changing path to CH₄ and may provide a predictive tool. Seasonal temperature regimes also appear to affect the relative importance of CH₄ or acetate as terminal decomposition products, so at higher latitudes, a greater extent of wetlands will accumulate acetate rather than produce CH₄. Similarly, high elevation peats accumulate acetate more readily than lower elevation peats of similar trophic status. A change in the pathway to CH_4, as vegetation cover responds to warming, should increase the production rate of CH₄ much more drastically than increases due to warming alone, and needs to be included in C models used to predict climate in a warming world.