Differential responses of freshwater wetland soils to sulphate pollution

Sulphate (SO42-) reduction rates are generally low in freshwater wetlands a nd are regulated by the scarce availability of the ion. Increased concentra tions of this electron acceptor due to sulphur (S) pollution of groundwater and surface water may, however, lead to high SO42- reduction rates now reg ulated by the availability of appropriate electron donors. Due to variation s in this availability, the response to S pollution (e.g. from surface wate r or groundwater) is expected to differ between soils. This hypothesis was tested in laboratory mesocosm experiments by comparing two wetland soil typ es with distinctly different humus profiles: a Hydromoder and a Rhizomull t ype. In the first type, expected to have a higher availability of degradabl e soil organic matter (SOM), SO42- availability appeared to be rate limitin g for SO42- reduction. In the Rhizomull soils, in contrast, the electron ac ceptor 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 sul phide accumulation, two major biogeochemical problems caused by SO42- pollu tion, occurred in both types. The absolute extent of phosphorus mobilisatio n was determined by the concentration of this element in the soil (C/P rati o), while the level of sulphide accumulation was governed by the concentrat ion of dissolved iron in the pore water. It was therefore concluded that ne ither the humus profile nor the concentrations of different SOM fractions i n the soils are reliable indicators for the sensitivity of wetland types to S pollution.