Characterization Of Sulfur Forms And Isotopic Compositions In Wetland Soils

Sulfur is an important element in the biogeochemistry of wetlands as a nutrient element for biota, existing in many different oxidation states, some of which are highly reactive with metals, and a key reactant in microbial processes such as microbial SO42- reduction. Microbial SO42- reduction is a key biodegradation process in many wetland environments, important in the degradation of organic matter, the recycling of nutrient elements, and the speciation of many metals. For these reasons, understanding how S is transformed and cycled through wetlands is important to geochemists and biologists studying wetland ecosystems. This chapter discusses methods for characterizing the forms and isotopic composition of S in wetland soils and sediments. The emphasis on wetland soils and sediments reflects the fact that many of the important biogeochemical transformations and reactions involving S occur here under anoxic conditions and driven by microbial SO42- reduction. We discuss different approaches for determining the total S content of soils and detailed, step-by-step approaches for examining forms of S in wetland soils, including elemental S, monosulfides (acid volatile sulfides), disulfides (FeS2, etc.), sulfates, and bulk organic S. Sulfur isotope analysis is also discussed. Sulfur isotopes are useful for examining sources of S to wetland ecosystems and for following the biogeochemical cycling of S in wetland soils. Advances in analytical instrumentation have greatly facilitated analysis of the stable isotopic composition of S in solid-phase material and allow rapid S isotope analysis of solids such as wetlands soils at a sample mass of <1 mg.