The role of soil organic matter and short-range ordered aluminosilicates in controlling the activity of aluminum in soil solutions of volcanic ash soils

Mechanisms controlling the activity of free aluminum (Al) in soil solutions and their implications for pedogenesis in two Japanese volcanic ash soils were investigated by means of acid-base titrations and kinetic studies of mineral dissolution. The A and AB horizons of both soils are rich in soil organic matter (SOM) and organically bound Al. In addition, short-range ordered aluminosilicates such as imogolite and proto-imogolite are abundant. Despite their abundance, the A horizons are undersaturated with respect to short-range ordered aluminosilicates, such as imogolite and proto-imogolite. This suggests low dissolution rate of these minerals, possibly due to alteration of the physico-chemical characteristics of their surfaces, caused by complexation with SOM. The AB horizons in both soils are close to equilibrium with short-range ordered aluminosilicates. The fast release of Al upon addition of strong acid to A horizons, combined with the reaction stoichiometry, observed in acid-base titrations suggests that the activity of Al3+ and H+ are controlled by complexation reactions with SOM. Furthermore, our data support previous studies indicating that SOM input in A horizon of volcanic ash soils dissolves short-range ordered aluminosilicates. Thus, simultaneous accumulation of SOM and depletion of short-range ordered aluminosilicates, due to leaching, are important long-term processes of the pedogenesis in soils developed from volcanic ash deposits.