Sulfur inputs to forests 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. Adsorption onto soil limits or delays sulfate ion transport. Nonspecifically adsorbed sulfate ions are held by electrostatic charges, so they are not held as tightly as specifically adsorbed ions. 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. 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, overall soil moisture conditions and deposition levels.
Edwards, Pamela J. 1998. Sulfur cycling, retention, and mobility in soils: A review. Gen. Tech. Rep. NE-250. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Research Station. 18 p.