Concentrations, sulfur isotopic compositions and origin of organosulfur compounds in pore waters of a highly polluted raised peatland

Sulfur cycling in peatlands plays a key role in their response to atmospheric deposition of acid sulfate: however, the role of dissolved organic sulfur species has received little attention. in this study, we assess the quantitative importance of dissolved organic sulfur species in a heavily polluted peatland and their role in sulfur cycling. Surface and pore waters from a raised peat bog have been analyzed for dissolved organic and inorganic species. Except in the surface water, organically bound sulfur dominates over sulfate sulfur and the organosulfur is predominantly in the humic fraction, which is not GC amenable. Two organosulfur compounds are present in the GC amenable fraction at all depths; a compound containing a benzene ring, sulfone and amide groups dominates while a compound closely similar to either benzothiazole or 1,2-benzisothiazole is present at 100-1000 times lower concentrations. Other organosulfur species were detected in some samples by GC-AED (gas chromatography-atomic emission detection) at similar or yet lower concentrations but not identifiable with GC-MS. In the shallowest parts of the peat, sulfur isotopic data on the humic and non-humic dissolved organic sulfur fractions show a pronounced difference, implying different sources for these organosulfur compounds. Humic organosulfur is (34)S enriched, indicating a source for this component from breakdown of plant materials, which have similar delta(34)S (0.1 parts per thousand and 4.2 parts per thousand). By contrast, the non-humic organosulfur is more (34)S depleted at shallow depths (-11.7 parts per thousand in surface water), indicating an origin from reaction of bacterially produced sulfide (-9.9 parts per thousand in surface water) with dissolved organic material. Deeper in the peat the non-humic organosulfur becomes more (34)S enriched (up to +6.8 parts per thousand), which may result from either: (i) reactions between organic material and more (34)S enriched sulfide; or (less likely) (ii) a shift to production of non-humic organosulfur by breakdown of humic compounds or plant material in the peat matrix. These data highlight the importance of organosulfur species relative to inorganic species in peatland groundwaters. Organosulfur species comprise the bulk of the dissolved sulfur budget and are involved in the bacterial cycling of sulfur in the peatland ecosystem. (C) 2009 Elsevier Ltd. All rights reserved.