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 per thousand (CDT), we investigated the direction and magnitude of an assimilation-related delta S-34 shift and the relationship between atmospheric deposition and S retention in selected ecosystem compartments. Bulk precipitation and spruce throughfall were collected between 1994 and 1996 in the Isle of Mull (Scotland), Connemara (Ireland), Thorne Moors (England), Rybarenska slat' and Ocean (both Czech Republic) and analyzed for sulfate concentrations and delta S-34 ratios. Eighteen replicate samples per site of living Sphagnum collected in unforested peatlands and 18 samples of spruce forest floor collected near each of the peatlands were also analyzed for S concentrations and delta (3)4S ratios. Assimilation of S was associated with a negative delta S-34 shift. Plant tissues systematically preferred the light isotope 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 total S concentration in forest floor and Sphagnum, respectively (R = 0.97 and R = 0.85). Elevated S inputs lead to higher S retention in these two organic-rich compartments of the ecosystem. It follows that equal emphasis must be placed on organic S as on adsorption/desorption of inorganic sulfate when studying acidification reversal in ecosystems. The sea-shore sites had rainfall enriched in the heavy isotope S-34 due to an admixture of sea-spray. The inland sites had low delta S-34 reflecting delta S-34 of sulfur emitted from local coal-burning power stations. Sphagnum had always lower S contents and higher delta S-34 ratios compared to forest floor. The within-site range of delta S-34 ratios of Sphagnum and forest floor was wide (up to 12 parts per thousand) suggesting that at least six replicate samples should be taken when using delta S-34 as a tracer.