Evidence of secondary sulfate production in the mineral soil of a temperate forested catchment in southern Québec, Canada

Abstract

The analysis of sulfate stable isotope ratios (δ18O-SO4 and δ34S-SO4) in different hydrological compartments of forested catchments has revealed the major role of the humus layer in recycling atmospherically derived sulfur (S). The contribution of the mineral soil to S recycling is still uncertain and may vary among forest types. Here, seasonal variations in SO4 concentration, δ18O-SO4 and δ34S-SO4 were investigated over a period of two and a half years in precipitation and at various depths in the soil solutions in a temperate forest catchment dominated by sugar maple in southern Québec, Canada. δ18O-SO4 declined from precipitation (11.8‰) to the humus solution (4.8‰) and to the soil solution beneath the upper B-horizon (1.2‰). No decline was observed below the upper B-horizon. This decline from precipitation to the humus layer reflected a production of secondary sulfate through chemical oxidation of SO2 in the canopy, while the decline in the soil resulted from microbial mineralization of organic S. In contrast with findings at other boreal and temperate forest sites, lower δ18O-SO4 in the upper mineral soil than in the humus layer was indicative of microbial transformations of S not only in the humus layer but also deeper in the upper part of the mineral soil. Significant seasonal variations were found for δ18O-SO4 in precipitation and in soil solutions beneath the humus and the top mineral horizon, reflecting the influence of both hydrological and microbiological factors. Higher δ18O-SO4 in the soil solution in fall than in spring and summer resulted from the release of primary sulfate from snow cover in early spring, which was subsequently recycled by soil microorganisms during the growth season, resulting in a higher proportion of secondary sulfate in fall than in spring and summer.