A simulation model of sulfur transformations in forested Spodosols

Transformations of organic and inorganic S in two forested Spodosols from the Hubbard Brook Experimental Forest, New Hampshire and the Huntington Forest in the Adirondack Mts. of New York were investigated using laboratory³⁵SO₄^2- incorporation experiments. Sulfur transformations were modeled as a set of three reversible, first-order reactions in which soluble SO₄^2- is converted to adsorbed SO₄^2-, ester sulfate and carbon-bonded S. Reaction rate contants for³⁵SO₄^2- adsorption/desorption and immobilization reactions involving ester sulfate and carbon-bonded S were determined using a fifth order Runge-Kutta-Fehlberg integration routine combined with least squares fitting. Model simulations were able to account for over 93% of the variation in the distribution of³⁵S in S fractions. A hypothetical application of immobilization rate constants to field situations at the Hubbard Brook Experimental Forest suggests that large quantities of S cycle through organic forms in Northern Hardwood Forest Ecosystems.