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Abstract
Characteristics of humic substances extracted from soils, litter extracts, and soil solutions in forest soils in the northern Kyoto area were investigated, and the nature and dynamics of dissolved organic matter in the forest soil profiles were discussed with special reference to their pedogenetic processes.
Based on fractionation, the humic substances in the B horizons showed typical characteristics for the podzolic B horizon and no significant differences were detected among the soil or vegetation types. The contents of carboxyl and phenolic hydroxyl functional groups of the fulvic acids which corresponded to about 5 and 1 mmol g-1 organic matter, respectively, were lower than the reported values of soil fulvic acids in cool and temperate zones. According to 1H-NMR analysis, the fulvic acids from litter extracts and soil solutions were considered to consist of mixtures of polyphenols, aliphatic acids, and/or polysaccharides. In contrast, those from the soils consisted of polyphenols with a high degree of substitution by relatively short aliphatic or saccharide chains. Thus, it was considered that the fulvic acids in this area, especially those from soil solutions and litter extracts, exhibited a low metal-chelating capacity and played a certain role in pedogenesis through the supply of protons.
On the other hand, the content of dissociated acidic functional groups of dissolved organic matter in the soil solution was about 2.2 mmol(–) g-1 organic matter as estimated from the DOC concentration and anion deficit in the soil solution. It was considered that acidic functional groups were not fully saturated with metal ions. The dynamics of dissolved organic matter with certain negative charges in the profiles was considered to be controlled by the adsorbing capacity of amorphous compounds of Al and Fe in the soil solid phase, which had previously been translocated mainly as inorganic forms.
The amorphous compounds derived through these processes in the illuvial B horizons were often rich in compounds which did not form complexes with organic matter compared to the surface horizons, and the organo-mineral compounds showed a low molar ratio of acidic functional groups of humus/metal ions.