THE ROLE OF ORGANIC-MATTER IN CONTROLLING ALUMINUM SOLUBILITY IN ACIDIC MINERAL SOIL HORIZONS

Despite the ecological importance of potentially phytotoxic Al, its so lubility control in acidic mineral soils remains unresolved. We examin ed the solubility of Al in mineral horizons of two acidic forest soils (Inceptisol and Spodosol) in southern Sweden using a series of batch experiments. Dissolution of Al was found to consist of a rapid solubil ization of reactive solid phase Al, which quickly reached an equilibri um state, superimposed on a slow dissolution of less reactive Al-conta ining phases (e.g., primary Al-silicates). Titration experiments in th e pH range 3.2-4.7 using an equilibration time of 5 days showed that a t pH < 4.1, all suspensions were undersaturated with respect to gibbsi te (Al(OH)(3); log K-so = 8.85 at 8 degrees C). Under such conditions , the Al solubility could be explained qualitatively by equilibrium co mplexation reactions with soil organic matter. Quantitatively, our res ults could be reproduced reasonably well using the mechanistic model W HAM, which describes the binding of Al by humic substances in organic soils. This suggests that the pool of organically bound soil Al contro ls the Al solubility in suspensions of strongly acidic soils. Due to t he kinetically constrained release of Al from primary and secondary mi nerals, the amount of organically bound Al, and therefore the Al solub ility in the suspensions, gradually increases with time. Consequently, a quantitative evaluation of Al solubility data from long-term batch experiments should consider both equilibrium and kinetic processes.