Ethanol-soluble and insoluble fractions of humic substances in soil fulvic acids

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Abstract
Humic substances in fulvic acids (FAs) obtained as polyvinylpyrrolidone (PVP)-adsorbed fractions were further fractionated based on their solubility in ethanol. Elementary composition, IR spectra, UV-Vis spectra, saccharide contents, molecular size distribution, and 13C-NMR spectra were clearly different between the ethanol-soluble and the insoluble fractions. The yield of the ethanol-soluble fractions exceeded 80%. These fractions showed higher aromatic-and carbonyl-C contents than the insoluble fractions. The differences in the signal intensities of phenol or aryl ether C1 and methoxyl groups in the 13C-NMR spectra (lowland paddy soil > brown forest soil > ando soil) as well as the aromatic-C contents (lowland paddy soil > ando soil > brown forest soil) of these fractions among the three soils suggested that the content and composition of the aromatic components varied depending on the soil types. Saccharides and nitrogen compounds were mainly distributed in the ethanol-insoluble fractions and it was suggested that these fractions were composed of humic substances combined with polysaccharides. The weight average molecular weights of these fractions which were 25,000–30,000, were about two times as large as those of the ethanol-soluble fractions. The lower Δlog K values of the insoluble fractions than those of the soluble fractions indicated the presence of structural differences in the aromatic components between the two fractions.

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