Patterns of stable S isotopes in a forested catchment as indicators for biological S turnover

Abstract

Despite intensive biogeochemical research during the last thirty years, the relative importance of biological S turnover for the overall SO ^2-₄ 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 SO ^2-₄ reduction and sites where mineralization and adsorption/desorption processes are more important. The δ³⁴S values and SO ^2-₄ 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 δ³⁴S in throughfall and bulk precipitation was in contrast to the high temporal and spatial variability of δ³⁴S in the soil solution. Sulfate in the soil solution of upland sites was slightly depleted in ³⁴S 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 SO ^2-₄ reduction. The observed spatial and temporal patterns of ³⁴S turnover and SO ^2-₄ concentrations might explain the overall balanced S budget of the catchment. At a time of decreasing anthropogenic deposition SO ^2-₄ is currently released from upland soils. Furthermore, mineralization of organic S may contribute to SO ^2-₄ release. Wetland soils in the catchment represent a sink for SO ^2-₄ due to dissimilatory SO ^2-₄ reduction.