INTERPRETATION OF SULFUR CYCLING IN 2 CATCHMENTS IN THE BLACK-FOREST (GERMANY) USING STABLE SULFUR AND OXYGEN-ISOTOPE DATA

The isotopic composition of SO42- in bulk precipitation, canopy throughfall, seepage water at three different soil depths, stream water, and groundwater was monitored in two forested catchments in the Black Forest (Germany) between November 1989 and February 1992. Isotope measurements on aqueous sulfate were complemented by delta(34)S-analyses on SO2 in the air, total sulfur and inorganic sulfate in the soil, and bedrock sulfur, in order to identify sources and biogeochemical processes affecting S cycling in catchments with base poor, siliceous bedrock. Stable S isotope data indicated that atmospheric deposition and not mineral weathering is the major source of S in both catchments since delta(34)S-values for sulfate in the soil, in seepage water, and in stream water were generally found to be similar to the mean delta(34)S-values of precipitation SO42- (+2.1 parts per thousand). However, delta(18)O-values of seepage water SO42- at 30 cm and especially at 80 cm depth were depleted by several per mil with respect to those of the atmospheric deposition (+7.5 to +13.5 parts per thousand). This indicates that in both catchments a considerable proportion of the seepage water SO42- is derived from mineralization of carbon-bonded soil S and must therefore have cycled through the organic soil. S pool. delta(34)S-values for different S compounds in the solid soil were found to differ markedly depending on S fraction and soil depth. Since atmospheric S deposition with rather constant delta(34)S-values was identified as the dominant S source in both catchments, this is interpreted as a result of in situ isotope fractionation rather than admixture of isotopically different S. The differences between the delta(34)S-values of seepage water and soil sulfate and those of organic soil S compounds are consistent with a model in which SO42- uptake by vegetation and soil microorganisms favours (SO42-)-S-34 Slightly, whereas during mineralization of organic soil S to aqueous SO42-, S-32 reacts preferentially. However, the data provide evidence for negligible isotope fractionation during physico-chemical S transformations such as adsorption/desorption in aerated forest soils.