Aluminium solubility mechanisms in moderately acid Bs horizons of podzolized soils

The processes controlling the retention and release of aluminium in acid forest soils are still subject to controversy, and therefore a universal hypothesis as to what mechanisms are operating has not been firmly established. By studying the Bs horizons of Swedish and Swiss podzolized soils, and by analysing data in the literature, we have found that aluminium hydroxide, and in some cases also poorly ordered imogolite, may control Al solubility in moderately acid (pH>4.2-4.3) Bs horizons. The strongest evidence in support of the presence of a quickly reacting Al(OH)(3) pool came from the temperature dependence of Al solubility in a Bs horizon, which was consistent with the reaction enthalpy of an Al(OH)3 phase such as gibbsite, and from the observation that the eon activity product for Al(OH)3 was the same regardless of whether equilibrium was reached from over- or undersaturation. The pool of Al(OH)3 is commonly small and may be completely dissolved after large additions of acid. This may be explained by the continuing redissolution of reactive Al(OH)(3) to form less soluble imogolite-type phases. By using the same methods it was found that soil suspensions did not reach equilibrium with poorly ordered imogolite even after 17 days. Thus, imogolite probably does not control Al solubility in the short term in many soils despite the common occurrence of this mineral. This is due to the relatively slow kinetics of imogolite formation and dissolution, especially at low temperatures and at small solution H4SiO4 concentrations.