In various parts of the Northern hemisphere air-borne S exhibits a seasonality, with isotopically light (i.e., 32S-rich) sulfur predominating in the warm summer months. Such seasonality has been reported from the United States, Canada, Japan, and China. Elevated biological emissions of isotopically light S in summer, a temperature-dependent isotope fractionation accompanying the oxidation of SO2, and heavy rains in winter bringing 34S-rich marine S have been suggested as the controlling mechanisms. In the atmosphere of Central Europe, one of the most severely polluted regions of the world, we have found an opposite seasonal trend: Isotopically light SO2-S predominates in the cold winter months, whereas isotopically heavy SO2-S is typical of the summer. The low delta34S values of air-borne SO2 in winter are influenced by low-delta34S emissions from local coal-burning power plants. The coal contains isotopically light S (mean delta34S of 1.6/1000). Higher demand for electricity during the heating season leads to higher anthropogenic S emission rates in winter. On a yearly basis, atmospheric sulfate S in Central Europe is isotopically heavier than atmospheric SO2-S by 4/1000. Atmospheric oxidation of SO2 is accompanied by an isotope fractionation resulting in 34S-enriched sulfate. In addition to the seasonality in air-borne delta34S(SO2), we report also an interannual trend of 1/1000 yr(-1) toward isotopically light sulfate S in atmospheric deposition. This interannual trend cannot be explained by a change in pollution sources accompanying the present massive environmental cleanup. To investigate the role of biological S emissions from the soil of heavily polluted ecosystems, we conducted a series of laboratory experiments using repacked soil columns and 34S-enriched precipitation under summer and winter temperatures. These experiments indicate that, under summer temperatures, the 34S-labeled precipitation is largely captured by the upper organic-rich soil horizons, a high proportion (53-74%) of S input is revolatilized, and the biologically reemitted S is isotopically light. Under winter temperatures more precipitation S is leached to the bottom of the soil columns. Our experiments have shown that biological emissions in Central Europe can be sizable. Yet, they cannot be singled out in the overall SO2 isotope pattern in the atmosphere. The main reason is continuous, variable (0-4/1000), open-system depletion in 34S in the residual SO2 during the isotopically selective SO2-to-SO4(2-) conversion.

[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, d 34 S 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%, 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 d 34 S values lower than deposition. In contrast, sites retaining S had runoff d 34 S values higher than deposition. Across the sites, the d 34 S values of runoff were not correlated with d 34 S values of bedrock, indicating that the contribution of bedrock to S in runoff was negligible. The d 34 S values of runoff were strongly positively correlated with the d 34 S 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 d 34 S 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 À1 yr À1 .

No abstract

Oxygen (O) and sulfur (S) isotope systematics in bog water sulfates were determined for six Sphagnum dominated wetlands located in the British Isles and the Czech Republic, Central Europe. Comparison of a polluted and unpolluted site showed that 4 times higher atmospheric S inputs led to 3 times higher bog water sulfate concentrations and substrate S concentrations, 3 times increased ranges of substrate S concentrations, and 3 times increased ranges of d 34 S values. Sites with elevated atmospheric S inputs exhibited greater geochemical variability in wetland S species. Sulfate O-S isotope composition of bog pore water at a depth of 40 cm below surface differed from that of surface bog water, indicating that dissimilatory bacterial sulfate reduction, a process known to discriminate against the heavier isotopes 18 O and 34 S, occurred in surface peat layers. While bacterial sulfate reduction remained to be one of the main isotope-selective processes for sulfate in peat, it could not fully explain the O-S isotope systematics of peat waters. The 'residual' sulfate was not simultaneously enriched in the heavier isotopes 18 O and 34 S. Mixing of residual sulfate following bacterial sulfate reduction with the product of S 2À reoxidation, cleavage of esters, and isotope exchange reactions may have contributed to the decoupling of the d 34 S SO4 and d 18 O SO4 values. Large within-site differences in d 18 O SO4 and d 34 S SO4 (up to 13 and 15&, respectively) indicated little communication between the 0 and 40 cm peat depth at some sites. Extremely high d 18 O SO4 and d 34 S SO4 values found in several peat bog water samples from Connemara (Ireland), Thorne Moors (England) and Ocean (Czech Republic) were not seen in streams draining the wetlands. Direct runoff of atmogenic sulfate constituted a significant portion of the bog outflow. At the wetland scale, zones of dissimilatory bacterial sulfate reduction form pockets whose lateral hydrological fluxes are small.

Sulfate aerosols are harmful as respirable particles. They also play a role as cloud condensation nuclei and have radiative effects on global climate. A combination of delta18O-SO4 data with catchment sulfur mass balances was used to constrain processes affecting S cycling in the atmosphere and spruce forests of the Czech Republic. Extremely high S fluxes via spruce throughfall and runoff were measured at Jezeri (49 and 80 kg S ha(-1) yr(-1), respectively). The second catchment, Na Lizu, was 10 times less polluted. In both catchments, delta18O-SO4 decreased in the following order: open-area precipitation > throughfall > runoff. The delta18O-SO4 values of throughfall exhibited a seasonal pattern at both sites, with maxima in summer and minima in winter. This seasonal pattern paralleled delta18O-H2O values, which were offset by -18 per thousand. Sulfate in throughfall was predominantly formed by heterogeneous (aqueous) oxidation of SO2. Wet-deposited sulfate in an open area did not show systematic delta18O-SO4 trends, suggesting formation by homogeneous (gaseous) oxidation and/or transport from large distances. The percentage of incoming S that is organically cycled in soil was similar under the high and the low pollution. High-temperature 18O-rich sulfate was not detected, which contrasts with North American industrial sites.