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Organic components of an Alfisol and a Vertisol in adjacent locations in Indian semi-arid tropics were characterized using chemical analysis, photometric and high resolution solid-state 13C nuclear magnetic resonance (NMR) spectroscopies, and 14C dating. The organic carbon contents of the Alfisol (0.50-0.25%) were about half the ones of the Vertisol (0.69-0.48%). Among the organic components, fulvic acids were found to be the major constituents, about 80–90% in the Alfisol and 70% in the Vertisol. These figures suggested faster organic decomposition in the Alfisol. The color intensities (K600/C) of humic acids of the Alfisol increased from the surface through deeper layers, while those of the Vertisol remained constant in all the layers. The 13C NMR measurements on the humic acids of both soils suggested the presence of polymethylene-, methyl-, N-methyl-, O-methyl-, dioxygenated-, O-alkyl-, aromatic-, phenolic or quaternary-, and carboxyl carbons. The Alfisol humic acid samples were characterized by a higher content of unsaturated (110-0 ppm) carbons and a lower content of quaternary ( 145-120 ppm) carbons than those of the Vertisol. Radiocarbon dating showed that the humic acids of the Alfisol were of modem (10 cm) through 2520 yr BP (40 cm) years old, while those of the Vertisol were in a narrower range of 4650 yr BP (10 cm) through 5440 yr BP (40 cm). Close relationships among radiocarbon dating, photometric, and NMR spectrometric characteristics of humic acids were suggested. |
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