Models of sulfur dynamics in forest and grassland ecosystems with emphasis on soil processes

The major S constituents in terrestrial ecosystems include inorganic SO ₄ ^2– , C-bonded S and ester sulfate with the organic fractions constituting the major soil S pools. Conceptual models of S dynamics link inorganic SO ₄ ^2– flux to organic sulfur transformations and other elements such as N and C. Mass balance models have been useful in ascertaining whether a system is at steady-state with respect to adsorption processes and/or nutritional demands of vegetation for S. Chemical equilibrium/surface complexation models have been used to evaluate the effects of a complex of factors, including effects of pH on SO₄ ^– adsorption and precipitation; these models have not generally been integrated into ecosystem models of S dynamics. Models such as ILWAS, Birkenes, Storgama, Trickle-Down and MAGIC were developed to ascertain surface water acidification processes within watersheds; these models incorporated SO₄ ^2– adsorption in some formulation combined with hydrological considerations. None of these models explicitly treat organic S transformations and fluxes. In contrast, grassland ecosystem models detail organic S transformations, but give little attention to adsorption and hydrologic factors. More detailed simulation models of S transformations in forest and grassland soils have recently been developed, but these results have yet to be incorporated into ecosystem and watershed models.