Differential responses of freshwater wetland soils to sulphate pollution

Sulphate (SO42-) reduction rates are generally low in freshwater wetlands and are regulated by the scarce availability of the ion. Increased concentrations of this electron acceptor due to sulphur (S) pollution of groundwater and surface water may, however, lead to high SO42- reduction rates now regulated by the availability of appropriate electron donors. Due to variations in this availability, the response to S pollution (e.g. from surface water or groundwater) is expected to differ between soils. This hypothesis was tested in laboratory mesocosm experiments by comparing two wetland soil types with distinctly different humus profiles: a Hydromoder and a Rhizomull type. In the first type, expected to have a higher availability of degradable soil organic matter (SOM), SO42- availability appeared to be rate limiting for SO42- reduction. In the Rhizomull soils, in contrast, the electron acceptor did not limit SO42- reduction rates at higher concentrations. These differences in response could not, however, be attributed to differences in the various SOM fractions or in SOM densities. Eutrophication and free sulphide accumulation, two major biogeochemical problems caused by SO42- pollution, occurred in both types. The absolute extent of phosphorus mobilisation was determined by the concentration of this element in the soil (C/P ratio), while the level of sulphide accumulation was governed by the concentration of dissolved iron in the pore water. It was therefore concluded that neither the humus profile nor the concentrations of different SOM fractions in the soils are reliable indicators for the sensitivity of wetland types to S pollution.