Northern peatlands are an important long-term sink for atmospheric carbon dioxide (CO2) and a contemporary source of methane (CH4). Under contemporary climate and environmental change, including enhanced nutrient deposition through industrialization and commercial peat harvesting, the microbial environment in peat is altered. Microorganisms are responsible for the net production of greenhouse gases in these sites, although controls on microbial activity and microbial communities are poorly understood, limiting our ability to predict greenhouse gas emissions. The objective of this thesis was to determine the microbial role in peat decomposition and greenhouse gas fluxes in northern peatlands. Nutrient, carbon (C) substrate, and microbial-ecological controls on microbial activity under natural climate variability, increased nutrient deposition, and commercial harvesting and restoration were explored in detail. Environmental change effects were evaluated in relation to processes and temporal variability in pristine sites. / The natural temporal variability of decomposition, microbial biomass, and nitrogen (N) was characterized in the Mer Bleue bog near Ottawa, ON over two years. In a warmer, drier year, lower water table position corresponded to increased N availability, which was in turn linked to enhanced microbial CO2 production, consistent with patterns in ecosystem respiration measured at the site level. It was shown that microbial activity can play an important role in inter-annual climate driven ecosystem respiration and net ecosystem CO2 exchange. / Through field and laboratory nutrient fertilization experiments, it was shown that increased nitrogen (N) deposition altered the heterotrophic microbial community at Met Bleue and led to decreased decomposition rates after one year, despite increased total microbial biomass. After the second year of fertilization, however, decomposition rates were elevated, presumably a result of a concomitant shift in moss species and supply of more bioavailable plant material. Comparison of fertilizations in the presence and absence of vegetation indicated that in oligotrophic sites, vegetation mediated elevated nutrient effects on decomposition and that N cycling occurred largely in the organic forms. / Aerobic and anaerobic microbial activity, peat organic and nutrient chemistry, microbial biomass, and methanogen, CH4-oxidizing bacteria, bacteria, and archaea were characterized in two sets of pristine, actively harvested, harvested and abandoned, and harvested and restored peatlands in Quebec and New Brunswick.

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