Sulfate reduction in Louisiana marsh soils of varying salinities

Potential sulfate reduction and in situ hydrogen sulfide emission rates for three Louisiana marsh soils of varying salinities (salt, brackish, and freshwater) were used to evaluate the influence of soil physicochemical parameters on sulfur transformations in different seasons (summer, winter, and spring). Solid adsorbent preconcentration and emission flux chambers were used in field experiments to measure hydrogen sulfide emissions. Soil redox potential (Eh) was measured at depths between 5 and 50 cm. Duplicate soil cores were obtained and sectioned for laboratory analysis. The fresh and brackish marsh soils were composed predominantly of organic matter, while the salt marsh soils were composed of predominantly minerals. Fresh marsh soils were generally the most oxidized and salt marsh soils the most reduced. In situ hydrogen sulfide emissions, but not potential sulfate reduction rates, were highest for the brackish marsh. Potential sulfate reduction assays may have overestimated in-situ rates, particularly for the more oxidized fresh marsh. Sulfate turnover times were longer in the salt marsh (approximately 40 days) than in either the fresh or the brackish marshes (3-8 days). Tidal action replenishes sulfate less frequently in the fresh and brackish marshes, thus sulfate depletions are more likely. The lowest potential sulfate reduction rates for all marshes occurred in winter. This was attributed to lower temperature and decreased soil sulfate content.