Processes Regulating Methane Emissions from Wetlands and What Might We Expect with Climate Change.

This presentation will review the mechanisms of methane transport and emissions from wetlands, particularly focusing on the role of surface vegetation.  Wetland vegetation is directly related to methane emission through controls on primary production and the transport of methane from flooded soils.  Surface vegetation is also important in the control of methane production and fermentation of organic matter in two other manners:  the nature and reactivity of dissolved organic carbon (DOC), and the control of the relative importance of methane production pathways.  There appear to be qualitative differences in pore-water DOC related to the extent of sedge cover, suggesting that the DOC in sedge-dominated peatlands is more reactive than DOC in peatlands where Sphagnum-woody plants dominate.  The higher reactivity of DOC from sedge-dominated peatlands may be a function of either different source materials or environmental factors that are related to the abundance of sedges in peatlands.  Vegetation differences also appear to be related to ratios of end product formation indicative of differences in the pathway of C flow and methanogenesis.  The connection with vegetation suggests that climate change scenarios leading to increases in vascular plant cover in northern wetlands may shift methanogenic pathways toward increased acetotrophy and greatly increase methane formation, which is a positive feedback on warming.  Other effects of climate change on methane emissions include permafrost decomposition that results in lake or wetland formation and subsequent increases in methane emission.  Two mechanisms appear to drive increased methane emission from permafrost decomposition, one is the decomposition of ancient organic matter which has been frozen for extended periods of time, and the second is that permafrost decomposition provides conditions along the collapsing edge of the frozen peat plateau that are conducive to colonization by fen-like vegetation that stimulate acetate fermentation and increase methane production and emission rates.