Implementation of sulfate adsorption in the SAFE model

An SO₄^2- adsorption submodel has been implemented in the dynamic soil chemistry model SAFE. The submodel calculates pH-dependent SO₄^2- and H⁺ adsorption to the soil, as well as the net surface charge development due to uneven adsorption of SO₄^2- and H⁺, using the empirical equations derived from an electrostatic model (Extended Constant Capacitance Model, ECCM) of SO₄^2- 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 SO₄^2- adsorption or assumed no SO₄^2- adsorption. With the ECCM-based SO₄^2- adsorption submodel, SAFE simulated soil solution SO₄^2- concentration and base saturation better, in comparison with measured data, than with the previous SO₄^2- adsorption formulations. Through the model application, also, need of additional improvement was suggested, such as calibration of mass transfer coefficients.