A natural process of soil acidification in the pedogenesis of the forest soils (i.e. brown forest soil and podzolic soil) in the northern Kyoto area was studied with special reference to the dynamics of amorphous and exchangeable Al in the profiles.
The very low base saturation with low pH of the sampled soils showed that all the profiles were highly acidified. The amount of exchangeable Al was largest at the surface in the brown forest soil profiles, whereas it was largest in the E to B horizons in the podzolic profiles. These findings suggested that soil acidification affected the deeper horizons in the podzolic profiles.
To analyze the mechanism involved in the pedogenetic acidification process, acid and alkali titrations of the sampled soils were performed. The titratable alkalinity was highly correlated with the amorphous Al content (Alo) among various soil properties, suggesting its importance in the in situ acid buffering by the solid phase of the forest soils. On the other hand, the titratable acidity was contributed by the contents of exchangeable Al and organic carbon, the latter forming complexes with amorphous Fe and/or Al. Pedogenetic acidification was considered to involve both an increase and a subsequent decrease of the titratable acidity; that is, an increase in the amount of organo-mineral complexes in the C to B horizons, an increase in the amount of exchangeable Al in the upper B horizons, and a decrease in the amount of both organo-mineral complexes and exchangeable Al in the E horizon of the podzolic profiles.
Based on the physico-chemical properties and the titration data of the soil solid phase and the soil solution composition, the pedogenetic process of the respective horizons in this area in relation to soil acidification is described as follows: the first stage of soil acidification is characterized by an accelerated increase in the amount of amorphous Al compounds caused by Al illuviation with weak podzolization and a secondary adsorption of dissolved organic matter onto them; the second stage of acidification is characterized by a successive monomerization of amorphous Al hydroxides due to the increase in the acid load; and the third stage of acidification is characterized by the decrease in the amount of amorphous Al and active eluviation of Al, and often of Fe, from that horizon.
Each profile may be described with a combination of the horizons characterized above, and the podzolic soil in this area can be considered to be a highly acidified brown forest soil.