With the exception of photosynthesis, carbon dioxide (CO2 ) emissions from soils exceed all other terrestrial-atmospheric carbon fluxes. Due to the magnitude of this soil-to-atmosphere CO2 flux, and the great mineralization capabilities associated with this large soil carbon pool, any increases in soil carbon fluxes have the potential to provide a large positive feedback to global warming. This dissertation examines the role of atmospheric sulfur deposition on anaerobic carbon fluxes from peatland soils, which contain one-third of the world's soil carbon pool. The first objective of this dissertation was to determine the contribution of sulfate reduction to anaerobic carbon mineralization in peatlands across a regional atmospheric sulfur deposition gradient. The second objective was to determine the fate of currently retained sulfur in peatlands under changing atmospheric sulfur depositional regimes, using stable sulfur isotopes as a tool.

We determined the relationship between rates of sulfate reduction, and CO2 and CH4 production in peatlands spanning a wide atmospheric sulfur gradient in central Alberta, Canada and Cervené Blato and Oceán bog, The Czech Republic. Results from this study suggest that although peatlands are important global sources of CH4 , methanogenesis is responsible for a small proportion of anaerobic carbon cycling in these ecosystems.

We further tested the hypothesis that in peatlands receiving low sulfate inputs, methane production will predominate during anaerobic carbon mineralization. We hypothesized that with sulfate amendments, anaerobic carbon mineralization at Bleak Lake Bog would be governed by sulfate reduction. In opposition to our hypotheses, sulfate amendments did not increase rates of sulfate reduction, increase CO 2 production, or decrease CH4 production.

We conducted a peat transplant experiment to evaluate the impact of atmospheric sulfur deposition on sulfur cycling processes in soil under alternate scenarios of increasing and decreasing atmospheric sulfur deposition. We found that a substantial fraction of the sulfur that enters a peatland via atmospheric deposition is retained in the peat, and this fraction increases with a concomitant increase in rates of atmospheric sulfur deposition. Implications for total sulfur pools, extrapolated to the long term, may be a gain of total sulfur under a scenario of both increasing and decreasing rates of sulfate reduction.