ACIDIFICATION OF FRESH-WATER WETLANDS - COMBINED EFFECTS OF NON-AIRBORNE SULFUR POLLUTION AND DESICCATION

In recent decades, SO42- concentrations have increased in groundwater and surface water of freshwater wetlands. For many minerotrophic peatl ands, S originating from SO42--polluted groundwater and surface water is a more significant source of SO42- than the actual atmospheric depo sition of S compounds. Lowered groundwater tables in wetlands, as a re sult of either natural or anthropogenic desiccation, may cause acidifi cation because of concomitant geochemical oxidation processes. The imp act of the enhanced availability of reduced S compounds, due to preced ing SO42- pollution, on these processes was tested in a mesocosm exper iment, using soil cores including vegetation from a mesotrophic wet me adow. The soils had been maintained in waterlogged condition for seven months, using two environmentally relevant SO42- concentrations (2 an d 4 mmol L-1). The groundwater table was reduced in two successive ste ps: 10 cm below soil surface, and complete desiccation. Control pretre ated soils did not show a decrease in soil pH during desiccation, due to adequate buffering by bicarbonate. However, both SO42--pretreated g roups showed a significant drop in pH (from 6.5 to 4.5) caused by addi tional 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 soi l moisture were influenced strongly by SO42- pretreatment, showing dis tinct patterns for P, N and K. Therefore, S polluted groundwater and s urface water may severely increase the sensitivity of wetlands to desi ccation. The results are discussed in relation to wetland management.