Properties of Soil Humic Substances in Fractions Obtained by Sequential Extraction with Pyrophosphate Solutions at Different pHs : III. FT-IR and ^1H NMR Spectra of Humic Acids (Soil Chemistry and Mineralogy)

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The FT-IR and ^1H NMR spectra of humic acids, which were obtained from four soils of different types (a Fibric Histosol, Dystric Cambisol, Umbric Andosol, and buried Umbric Andosol) by sequential extraction with pyrophosphate solutions at different pH values (i.e. at pH 3, 5, 7, 9, 11, and 13), were measured. The shape of the IR spectra for each humic acid depended on the differences in the soil origin rather than the pH values. The IR spectra of humic acids in the Histosol and Cambisol showed typical absorption associated with lignin. The humic acids in the Andosol and buried Andosol were characterized by the disappearance of the absorption assigned to aliphatic C-H and the prominence of the absorption attributed to aromatic C=C and COOH groups. The humic acids in the Histosol and Cambisol showed aliphatic properties compared with those in the Andosol and buried Andosol. As the pH values of the extractant increased, the absorption strength of the bands attributed to aliphatic and amide groups increased and those to carboxylic groups decreased. The XH NMR spectra were broadly divided into five regions, namely aromatic (H_<ar>), methoxyl (H_<C-O>), and aliphatic protons a (H_α, β(H_β), and γ(H_γ) to aromatic rings, while the proportion of each proton species was estimated. The proportion of H_<ar> in all the humic acids, except for the humic acids at pH 3 in the Histosol and the Andosol, decreased with the increase of the pH values. The increase of the ratio of H^ to Har with the increase of the pH values, was more conspicuous in the following order: Cambisol > Andosol > Histosol and buried Andosol. Based on the results obtained in this study and the results from repeated analysis of the particle size distribution by gel permeation chromatography conducted in Fujitake et al. (.Soil Sci. Plant Nutr., 44: 253-260, 1998), a model representation of the relationship between the proton composition and particle size in taking account of the differences in the properties of humic acids among soil types and extraction pH values, was illustrated.
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Properties of Soil Humic Substances in Fractions Obtained by Sequential Extraction with Pyrophosphate Solutions at Different pHs : III. FT-IR and ^1H NMR Spectra of Humic Acids (Soil Chemistry and Mineralogy)

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