Quantifying subsurface rates of methane production and oxidation in a New Hampshire wetland: Implications for the role of boundary-layer methane production and oxic processes

In this dissertation, a mechanistic understanding of what controls the rates of methane production and oxidation in terrestrial wetland soils is derived. The methodology adapts a geochemical approach, commonly used to study microbial processes in marine sediments, and applies it to terrestrial methane production and release. A time series of pore water profiles was collected from a New Hampshire poor fen from which we quantified the isotopic composition of the pore water dissolved inorganic carbon (DIC) and the concentrations of DIC and methane (CHO with depth. These profiles were used, in combination with a model that simulates reaction and transport, in order to quantify the microbial rates of methane production, methane oxidation, and respiration occurring with depth in the saturated peat. This represents the first characterization of subsurface methane production, oxidation and respiration rates in situ over a seasonal cycle.;The most important finding of this three-year time-series of data is that the isotope-constrained profiles of methane production are not consistent with the high summertime methane fluxes observed independently from this site. The depth-integrated rates of net methane production, calculated using the carbon isotope profiles, accounts for only between 6 and 35% of the observed methane emissions, depending on the sampling location. Constrained by multiple data profiles, we propose that very high rates of methane production must exist either in the unsaturated peat, or the very top centimeters of the saturated peat, during the summer months. The CO2 that results from this methane production equilibrates with the atmosphere rapidly, on the same timescale that the CH4 is released, such that the isotopic signal of this methane production never penetrates deeper into the pore water DIC and remains invisible to our methods. We explore the possibility that these high rates of methane production are the result of oxic processes providing a flux of degradable organic material across the oxic-anoxic interface to the anoxic microbial community that, at all other depths, is severely limited by substrate availability.