Sulfur Cycling, Retention, and Mobility in Soils: A Review

Sulfur inputs to forest ecosystems originate from mineral weathering, atmospheric deposition, and organic matter decomposition. In the soil, sulfur occurs in organic and inorganic forms and is cycled within and between those forms via mobilization, immobilization, mineralization, oxidation, and reduction processes. Organic sulfur compounds are largely immobile. Inorganic sulfur compounds are more mobile, and sulfate is the most mobile. However, adsorption onto soil limits or delays sulfate ion transport. Nonspecifically adsorbed sulfate ions are held only by electrostatic charges in the double diffuse layer, so they are not held as tightly as specifically adsorbed ions that are bonded to metal oxides in the Helmholtz layer. Sulfate adsorption and desorption are controlled predominantly by pH, sulfate concentrations, concentrations and types of other cations and anions in solution, and the character of the colloidal surfaces. Watershed hydrology and subsurface flow paths play important roles in determining the fate of sulfate in soils. Theories and models of sulfate transport from and retention within watersheds focus on contact times between ions and soil materials, macropore, mesopore, and micropore flow contributions to streamflow, and overall soil moisture conditions. Retention also is affected by deposition levels. As sulfur deposition to a watershed decreases, retention decreases; however, rates of decrease depend on whether the initial adsorption was completely irreversible, partially reversible, or completely reversible. Published by: For additional copies: USDA FOREST SERVICE USDA Forest Service 5 RADNOR CORP CTR SUITE 200 Publications Distribution RADNOR PA 19087-4585 359 Main Road Delaware, OH 43015 September 1998 Fax: (740)368-0152 The Author PAMELA J. EDWARDS is a research hydrologist with the Northeastern Research Station’s Timber and Watershed Laboratory in Parsons, West Virginia. She earned B.S. and M.S. degrees in forest science and forest hydrology, respectively, from The Pennsylvania State University, and a Ph.D. in forest soils from North Carolina State University. She has conducted watershed management and forest hydrology research since 1981. Manuscript received for publication 8 April 1998