Sulfur isotope inventories of atmospheric deposition, spruce forest floor and living Sphagnum along a NW-SE transect across Europe

At five European sites, differing in atmospheric S inputs by a factor of 6, and differing in S isotope signatures of these inputs by up to 14 parts pe r thousand (CDT), we investigated the direction and magnitude of an assimil ation-related delta S-34 shift and the relationship between atmospheric dep osition and S retention in selected ecosystem compartments. Bulk precipitat ion and spruce throughfall were collected between 1994 and 1996 in the Isle of Mull (Scotland), Connemara (Ireland), Thorne Moors (England), Rybarensk a slat' and Ocean (both Czech Republic) and analyzed for sulfate concentrat ions and delta S-34 ratios. Eighteen replicate samples per site of living S phagnum collected in unforested peatlands and 18 samples of spruce forest f loor collected near each of the peatlands were also analyzed for S concentr ations and delta (3)4S ratios. Assimilation of S was associated with a nega tive delta S-34 shift. Plant tissues systematically preferred the light iso tope S-32, on average by 2 parts per thousand. There was a strong positive correlation between the non-marine portion of the atmospheric S input and t otal S concentration in forest floor and Sphagnum, respectively (R = 0.97 a nd R = 0.85). Elevated S inputs lead to higher S retention in these two org anic-rich compartments of the ecosystem. It follows that equal emphasis mus t be placed on organic S as on adsorption/desorption of inorganic sulfate w hen studying acidification reversal in ecosystems. The sea-shore sites had rainfall enriched in the heavy isotope S-34 due to an admixture of sea-spra y. The inland sites had low delta S-34 reflecting delta S-34 of sulfur emit ted from local coal-burning power stations. Sphagnum had always lower S con tents and higher delta S-34 ratios compared to forest floor. The within-sit e range of delta S-34 ratios of Sphagnum and forest floor was wide (up to 1 2 parts per thousand) suggesting that at least six replicate samples should be taken when using delta S-34 as a tracer.