Predicting Aluminum and Soil Organic Matter Solubility Using the Mechanistic Equilibrium Model WHAM

The mechanistic equilibrium model WHAM is used to describe solution–solid phase interactions in the forest floor with regard to Al and soil organic matter (SOM) solubility. WHAM takes into account specific and nonspecific ion‐binding to humic compounds. Experimental data from a forest soil that was manipulated with respect to its Al content were obtained from batch studies and a field manipulation experiment. WHAM was parameterized using observations of pH, concentrations of inorganic Al, and DOC obtained in batch. Model fits of pH, concentrations of inorganic Al (>5 × 10−6 mol L−1), and DOC were good to tolerable (1, 9, and 15% deviation, respectively). Values of optimized model parameters agreed reasonably with analytically determined quantities. Using the optimized parameters, WHAM simulated addition of AlCl3 to the same soil. Comparison between model predictions and batch observations showed a deviation for pH, Al (>5 × 10−6 mol L−1), and DOC of 3, 60, and 15%, respectively. We regard this as a reasonable model performance and support for the assumption of an organic complexation control of Al solubility in organic soils. Application of WHAM to predict effects of AlCl3 addition in the field resulted in qualitative agreement between simulations and observations from tension lysimeters in the forest floor, but in a failure regarding the observed ranges of H, Al, and DOC. The discrepancy between model simulations and field observations may be explained qualitatively by a lack of equilibrium due to the diffusion‐limited exchange of solutes between immobile water in micropores and mobile water in macropores.