In recent decades, SO42- concentrations have increased in groundwater and surface water of freshwater wetlands. For many minerotrophic peatlands, S originating from SO42--polluted groundwater and surface water is a more significant source of SO42- than the actual atmospheric deposition of S compounds. Lowered groundwater tables in wetlands, as a result of either natural or anthropogenic desiccation, may cause acidification because of concomitant geochemical oxidation processes. The impact of the enhanced availability of reduced S compounds, due to preceding SO42- pollution, on these processes was tested in a mesocosm experiment, using soil cores including vegetation from a mesotrophic wet meadow. The soils had been maintained in waterlogged condition for seven months, using two environmentally relevant SO42- concentrations (2 and 4 mmol L-1). The groundwater table was reduced in two successive steps: 10 cm below soil surface, and complete desiccation. Control pretreated soils did not show a decrease in soil pH during desiccation, due to adequate buffering by bicarbonate. However, both SO42--pretreated groups showed a significant drop in pH (from 6.5 to 4.5) caused by additional sulfide oxidation, leading to high SO42- concentrations (10 and 16 mmol L-1, respectively). Cation exchange and acidification-related solubilization processes induced the mobilization of base cations and potentially phytotoxic metals like Al. Nutrient concentrations in soil moisture were influenced strongly by SO42- pretreatment, showing distinct patterns for P, N and K. Therefore, S polluted groundwater and surface water may severely increase the sensitivity of wetlands to desiccation. The results are discussed in relation to wetland management. References: 43
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