Sulfur Chemistry of Louisiana Tidal Marsh Soils.

The amounts and profile distribution of various sulfur forms in Louisiana coastal marshes were investigated to understand sulfur cycling as related to the origin and type of tidal wetland marshes. Sampling was conducted quarterly from a Panicum hemitomon freshwater marsh, a Spartina patens brackish marsh, and a Spartina alterniflora salt marsh in Barataria Basin, Louisiana. Soil samples were fractionated into acid-volatile sulfides (AVS), elemental sulfur, HCl-soluble sulfur, pyrite sulfur, ester-sulfate sulfur, carbon-bonded sulfur, and total sulfur. Seasonal changes showed a greater effect on the distribution of S fractions in brackish and salt marsh than in the freshwater marsh. Season appeared to have a pronounced effect on the distribution of acid-volatile sulfides, elemental sulfur, and pyrite sulfur. The profile distribution of acid-volatile sulfides and elemental sulfur in all marshes was similar. These sulfur forms showed the greatest abundance in the oxidized zone (<20 cm) during spring, summer, and fall. Marsh soil oxidation processes occurring during the plant growing season and a falling water level over the marsh surface resulted in a low pyrite sulfur concentration in the surface. The maximum concentration of pyrite sulfur was observed below 20 cm in all marsh types and its concentration was relatively high during winter. The presence of HCl-soluble sulfur was contributed largely from pore water sulfate, which was greater in salt marsh than in brackish and freshwater marshes, respectively. Organic sulfur, in the forms of ester-sulfate sulfur and carbonbonded sulfur, was the major contributor to the total sulfur pool in all marsh types. Carbon-bonded sulfur was generally 2 to 3 times more abundant than ester-sulfate sulfur with less seasonal variation. Sulfur concentration (volume basis) and sulfur accumulation rates increased from freshwater marsh to salt marsh. Reduced sulfur species removed by marsh accretion or sedimentation process are likely significant in marsh energy flow. Plant activity, tidal fluctuation, and distance along a salinity gradient from the Louisiana coast seemed to have a strong influence on the distribution of sulfur. INTRODUCTION Sulfur (S) is an important element of both natural and anthropogenic processes (Landers et al., 1983). Sulfur plays two key roles in wetland biological systems; it is an essential element required by all plants and organisms, and acts as a significant electron carrier in anoxic and partially anoxic ecosystems (Howarth, 1984). Because S can be present in inorganic and organic forms with oxidation states varying from -2 to +6, several important transformations in the S cycle involve changes in these oxidation states (Trudinger, 1986). It is obvious that S can undergo several biogeochemical transforma­ tion in coastal wetlands, including sulfate reduction, iron sulfides and pyrite formations, metal-trace element cycling, energy and nutrient transport, and atmospheric S emission. Each of these processes is reviewed in Chapter I. The emphasis of this literature review is placed on the biogeochemical significance and dynamics of S in coastal marshes and estuarine systems. One important aspect of marsh composition that has received little attention is the nature and distribution of inorganic and organic forms of S as related to the origin and type of marsh material (Lowe and Bustin, 1985). Since Louisiana’s coastal zone is one of the most wetland-rich regions of the world (Mitsch and Gosselink, 1986), an under­ standing of the distribution and dynamics of S-containing components is very important in studies of marsh development and their ecological significance including energy flow. The distribution of inorganic and organic S forms has been reported for salt marsh soils of California (Kaplan et al., 1963) and Maryland (Haering et al., 1989), and Florida marine-freshwater peats (Casagrande et al., 1977; Altschuler et al., 1983). Only one previous study has documented the seasonal cycling between inorganic and organic S forms in salt marsh soil (Luther and Church, 1988). Moreover, no attempt has has been made to examine the seasonal distribution of inorganic and organic S forms in freshwater and brackish marsh soils. It is obvious that seasonal changes display a dramatic effect on the concentration, form, and profile distribution of S-containing compounds in several