Modeling CH 4 and CO 2 cycling using porewater stable

17 Quantifying rates of microbial carbon transformation in peatlands is essential for gaining 18 mechanistic understanding of the factors that influence methane emissions from these systems, 19 and for predicting how emissions will respond to climate change and other disturbances. In this 20 study, we used porewater stable isotopes collected from both the edge and center of a 21 thermokarst bog in Interior Alaska to estimate in situ microbial reaction rates. We expected that 22 near the edge of the thaw feature, actively thawing permafrost and greater abundance of sedges 23 would increase carbon, oxygen and nutrient availability, enabling faster microbial rates relative 24 to the center of the thaw feature. We developed three different conceptual reaction networks that 25 explained the temporal change in porewater CO2, CH4, !C-CO2 and !C-CH4. All three 26 reaction-network models included methane production, methane oxidation and CO2 production, 27 and two of the models included homoacetogenesis — a reaction not previously included in 28 isotope-based porewater models. All three models fit the data equally well, but rates resulting 29 from the models differed. Most notably, inclusion of homoacetogenesis altered the modeled 30 pathways of methane production when the reaction was directly coupled to methanogenesis, and 31 it decreased gross methane production rates by up to a factor of five when it remained decoupled 32 from methanogenesis. The ability of all three conceptual reaction networks to successfully match 33 the measured data indicate that this technique for estimating in-situ reaction rates requires other 34 data and information from the site to confirm the considered set of microbial reactions. Despite 35 these differences, all models indicated that, as expected, rates were greater at the edge than in the 36 center of the thaw bog, that rates at the edge increased more during the growing season than did 37 rates in the center, and that the ratio of acetoclastic to hydrogenotrophic methanogenesis was 38 greater at the edge than in the center. In both locations, modeled rates (excluding methane 39 oxidation) increased with depth. A puzzling outcome from the effort was that none of the models 40 could fit the porewater dataset without generating “fugitive” carbon (i.e., methane or acetate 41 generated by the models but not detected at the field site), indicating that either our 42 conceptualization of the reactions occurring at the site remains incomplete or our site 43 measurements are missing important carbon transformations and/or carbon fluxes. This model– 44 data discrepancy will motivate and inform future research efforts focused on improving our 45 understanding of carbon cycling in permafrost wetlands. 46