The effects of drying and re-wetting and increased temperature on sulphate release from upland and wetland material

In central Ontario, elevated SO₄ concentrations and export have been measured in both upland and wetland-draining catchments following summer droughts, although the source of excess SO₄ is unclear. The objective of this study was to determine the effects of drying and re-wetting and temperature, respectively, on the release of SO₄ from the primary S pools in wetlands (Sphagnum and peat) and uplands (forest floor and mineral soil), using material collected from the PC1 catchment in Haliburton County, and from catchment S50 in the Turkey Lakes Watershed. Peat exhibited the most marked response to drying of the four materials considered, and within 24 h of re-wetting dried peat from both catchments released 3-4 times more SO₄ (50-67 mg kg⁻¹ S-SO₄) than continuously moist peat (16 mg kg⁻¹ S-SO₄), although temperature had only a marginal effect on SO₄ concentrations. There was no immediate response of Sphagnum to either drying or temperature, although S-SO₄ concentrations in Sphagnum tended to increase over the 30-day (d) incubation. There was a small but immediate increase in S-SO₄ concentrations in forest floor material (LFH) from both catchments within the first 24 h of incubation, which was greatest in treatments that were dried and/or incubated at a higher temperature. In contrast, neither temperature nor drying appeared to affect SO₄ release from mineral soil collected from either site. Results of laboratory incubations suggest that increases in SO₄ concentration that have been reported in wetland-draining streams immediately following summer dry periods may be quantitatively explained by drying and re-wetting of peat rather than increased mineralization in Sphagnum. Similarly, the higher SO₄ concentrations that have been measured in upland streams following summer droughts may in part be due to enhanced SO₄ release from the forest floor following drying and re-wetting. In contrast, while the mineral soil constitutes a large pool of total S, it does not appear to be responsive to changes in moisture or temperature in the short-term (<30 d) and therefore likely does not contribute to reported climate-related temporal variations in stream SO₄.