Isotopic evidence for processes of sulfur retention/release in 13 forested catchments spanning a strong pollution gradient (Czech Republic, central Europe) -: art. no. GB4012

[1] Sulfur isotope systematics were studied in 13 small catchments in the Czech Republic, similar in topography (V-shaped valley) and vegetation ( Norway spruce). The sites differed in elevation, rainfall, bedrock, soil type and S pollution. Across the sites, delta S-34 values decreased in the order: bulk deposition > runoff > spruce throughfall > C-horizon soil > A/B-horizon soil > O-horizon soil > bedrock ( means of 5.5, 4.8, 4.7, 4.6, 4.2, 3.1 and 1.5 parts per thousand, respectively). Some of the sites had a net export of S, while others accumulated S. Sites exporting S were located in the polluted north where atmospheric S input started to decrease in 1987. Sites retaining S were located in the relatively unpolluted south. Sulfur isotope composition of runoff depended on whether the catchment accumulated or released S. Sites releasing S had runoff delta S-34 values lower than deposition. In contrast, sites retaining S had runoff delta S-34 values higher than deposition. Across the sites, the delta S-34 values of runoff were not correlated with delta S-34 values of bedrock, indicating that the contribution of bedrock to S in runoff was negligible. The delta S-34 values of runoff were strongly positively correlated with the delta S-34 values of soil. Sulfur present in the C-horizon of soils was mainly derived from atmospheric deposition, not bedrock. Sulfur isotope mass balances were constructed for each catchment, making it possible to quantify the difference between delta S-34 values of the within-catchment source/sink of S and runoff S. Sulfur isotope mass balances indicated that the sink for the retained S at unpolluted sites and the source of the released S at polluted sites were isotopically fractionated by the same amount relative to runoff S. Inorganic and organic processes were considered as possible causes for this observation. Biological S cycling involves a variety of reactions, some of which fractionate S isotopes. In contrast, adsorption/desorption of inorganic sulfate in soil and weathering of S-containing minerals do not fractionate S isotopes. Therefore the within-catchment source/sink of S must be largely a result of biological S cycling. Organic S cycling played an important role over a wide range of atmospheric S inputs from 13 to 130 kg S ha(-1) yr(-1).