Response of anaerobic carbon mineralization rates to sulfate amendments in a boreal peatland

A small body of research suggests that dissimilatory sulfate reduction can affect the carbon balance of peatlands, yet this has not been tested widely, despite the fact that peatlands contain approximately one‐third of the global soil carbon pool. Here we evaluate the role of dissimilatory sulfate reduction in a site that receives low atmospheric sulfur deposition. We hypothesized that, in peatlands receiving low sulfate inputs, methane production should dominate anaerobic carbon mineralization. We further hypothesized that with sulfate amendments, anaerobic carbon mineralization could show an overall increase if terminal carbon mineralization in unamended peat is limited by an inadequate supply of electron acceptors. To test these hypotheses, we delineated six 1‐m² plots in an ombrotrophic, boreal peatland in central Alberta, Canada (Bleak Lake Bog), which receives <2 mmol S·m⁻²·yr⁻¹. Three of the plots were amended with sulfate (78 mmol S·m⁻²·yr⁻¹). We measured anaerobic rates of sulfate reduction, CH₄ production, and CO₂ production. In opposition to our hypotheses, sulfate amendments did not increase rates of sulfate reduction, increase CO₂ production, or decrease CH₄ production over a 2‐yr period, but did increase both sulfate pool sizes and residence times. Despite low rates of sulfate reduction compared to other freshwater wetlands, daily average sulfate reduction (1.7 mmol/m²) exceeded regional annual inputs of atmospheric sulfate deposition (1.6 mmol/m²). Between 77% and 99% of reduced sulfate was incorporated into the carbon‐bonded sulfur pool, which turns over slowly. The slow turnover rate and comparatively low sulfate reduction rates may be related to the low iron content of the bog peat. Additionally, sulfate reduction initially was sulfate limited in the control plots, but with sulfur amendments, the sulfate limitation was removed; sulfate reduction appeared to be limited by some other factor, possibly labile carbon. Dissimilatory sulfate reduction was more important to total anaerobic carbon mineralization than methane production, although neither process dominated overall anaerobic carbon mineralization (<2% of total). Fermentation appeared to be the dominant anaerobic carbon mineralization pathway at Bleak Lake Bog, yet the mechanisms of how this process affects the carbon balance of peatlands has never been evaluated. Overall, our results suggest that, in at least the short term, soil carbon turnover in peatlands will not be enhanced by increased atmospheric sulfur deposition. If we want to understand the controlling factors on soil carbon storage in sites like Bleak Lake Bog, we should begin to examine anaerobic carbon mineralization via fermentation pathways. Corresponding Editor: J. M. Melack