Sulfur Cycling, Retention, and Mobility in Soils: a Review
United States Department of Sulfur Cycling, Agriculture Forest Service Retention, and Mobility Northeastern in Soils: A Review Research Station General Technical Report NE-250 Pamela J. Edwards Abstract 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. The Author PAMELA J. EDWARDS is a research hydrologist with the Northeastern Research Station’s Timber and Watershed Laboratory in Parsons, West Virginia.
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