Implementation of Sulfate Adsorption in the SAFE Model.

An SO₄²⁻ adsorption submodel has been implemented in the dynamic soil chemistry model SAFE. The submodel calculates pH-dependent SO₄²⁻ and H⁺ adsorption to the soil, as well as the net surface charge development due to uneven adsorption of SO₄²⁻ and H⁺, using the empirical equations derived from an electrostatic model (Extended Constant Capacitance Model, ECCM) of SO₄²⁻ adsorption. The resulting new SAFE model was applied on a roof experiment plot in the Norway spruce [Picea abies (L.) H. Karst.] stand at Solling, Germany, where atmospheric S and N deposition was artificially reduced by the roof construction. The model performance was compared with the previous versions that used a pH-independent Freudlich model of SO₄²⁻ adsorption or assumed no SO₄²⁻ adsorption. With the ECCM-based SO₄²⁻ adsorption submodel, SAFE simulated soil solution SO₄²⁻ concentration and base saturation better, in comparison with measured data, than with the previous SO₄²⁻ adsorption formulations. Through the model application, also, need of additional improvement was suggested, such as calibration of mass transfer coefficients. [ABSTRACT FROM AUTHOR]