SPECIATION OF AL IN SOIL SOLUTION FROM FOREST SOILS IN NORTHERN KYOTO WITH SPECIAL REFERENCE TO THEIR PEDOGENETIC PROCESS

Speciation of Al in the soil solution from brown forest soils and podzolic soils in the northern Kyoto region was carried out. In the soil solution, more than 60% of dissolved Al consisted of inorganic monomeric species, whereas only a minor portion consisted of organically-complexed or polymeric Al. The concentration of the organic species was correlated with that of dissolved organic carbon, whereas the concentration of inorganic monomeric Al was controlled by the pH of the soil solution. Based on the ion activities of the soil solutions along with thermodynamic data of the soil minerals, the soil solutions were gradually saturated with amorphous Al(OH)3 in the illuvial B horizons. A certain amount of Al was detected in a colloidal fraction without Si component especially in the soil solutions with high Al saturation, suggesting that polymeric Al hydroxides may be the major products of the hydrolytic reactions. These results suggest that Al translocation in the soil profile in this region occurs mainly in the form of inorganic Al species. The large amount of NO3 as well as organic acids may be responsible for the decrease of the pH of the soil solution in the surface horizons. Al is considered to be eluviated from the surfaces due to soil acidification, and precipitate as a result of a hydrolytic reaction associated with the increase of the soil solution pH in the subhorizons.