SULFATE-INDUCED ENTROPHICATION AND PHYTOTOXICITY IN FRESH-WATER WETLANDS

In recent decades, sulfate concentrations in many European freshwater wetlands have increased by 10-fold or more, due mainly to the use of s ulfate-polluted river water to compensate for water shortage in these areas. To test the effect of surf ate enrichment a mesocosm experiment was set up, using waterlogged soil cores, intact with vegetation, fro m a mesotrophic fen meadow. During sulfate addition at environmentally relevant levels (0, 2, and 4 mmol L-1), phosphate concentration and a lkalinity of the pore water rapidly rose due to increased sulfate redu ction rates. Free sulfide accumulated to levels toxic to several wetla nd plant species and biomass regrowth after harvesting was significant ly lower on treated soils, especially for Carer species. Eventually, t he concentrations of ammonium, phosphate, and potassium increased stro ngly in the treated soils due to reduced uptake by plants and extra mi neralization. Sulfate availability was rate limiting, until the supply of readily decomposable organic matter became limited. It is argued t hat the Significance of the observed changes in free sulfide concentra tions and in the rate of nutrient mobilization should be recognized, a nd that these effects can be as important as direct eutrophication cau sed by the import of nutrients. The reported changes may severely infl uence the plant species composition of freshwater wetlands.