Alewell, C; Gehre, M: Patterns of stable S isotopes in a forested catchment as indicators for biological S turnover, Biogeochemistry, 47, 319-333 (1999)
Abstract: Despite intensive biogeochemical research during the last thirty years, the relative importance of biological S turnover for the overall SO2-4 budget of forested catchments remains uncertain. The objective of the present study was (i) to gain new insight into the S cycle of the Lehstenbach catchment (Northeastern Bavaria, Germany) through the analysis of stable isotopes of S and (ii) to differentiate between sites which are "hot spots" for SO2-4 reduction and sites where mineralization and adsorption/desorption processes are more important. The d34S values and SO2-4 concentrations of soil solutions, throughfall and groundwater at four different sites as well as runoff of the catchment were measured. The relatively low variability of d34S in throughfall and bulk precipitation was in contrast to the high temporal and spatial variability of d34S in the soil solution. Sulfate in the soil solution of upland sites was slightly depleted in d34S compared to input values. This was most likely due to S mineralization. Sulfate in the soil solution from wetland soils was clearly enriched in 34S, indicating dissimilatory SO2-4 reduction. The observed spatial and temporal patterns of 34S turnover and SO2-4 concentrations might explain the overall balanced S budget of the catchment. At a time of decreasing anthropogenic deposition SO2-4 is currently released from upland soils. Furthermore, mineralization of organic S may contribute to SO2-4 release. Wetland soils in the catchment represent a sink for SO2-4 due to dissimilatory SO2-4 reduction.