Characterization of organic matter in soils by thermochemolysis using tetramethylammonium hydroxide (TMAH)

Tetramethylammonium hydroxide (TMAH) thermochemolysis-gas chromatography/mass spectrometry (GC/MS) was employed to study the chemical structure of soil organic matter sampled from a soil plot in which corn (Zea mays L.) was farmed continuously for 15 yr. The chromatograms exhibited peaks related to compounds derived from lignin, fatty acid methyl esters (FAMEs), non-lignin aromatic structures, and heterocyclic N compounds. The dominant lignin-derived peaks in the TMAH thermochemolysis-GC/MS chromatograms were mainly derivatives of p-hydroxyphenyl and guaiacyl structures, suggesting a non-woody (grass) lignin type. With depth, the ratio of syringyl to guaiacyl compounds (S/G) decreased, suggesting a preferential degradation of the syringyl units by microorganisms. Fatty acid methyl esters of varying C-chain length (C₇ to C₂₇) were identified in the soil chromatograms. Both TMAH-GC/MS and ¹³C-NMR (nuclear magnetic resonance) data suggested a relative increase of long-chain fatty acids with soil depth (or degree of humification), suggesting a refractory nature for these compounds. The heterocyclic N compounds yielded from the TMAH thermochemolysis were mainly pyrroles, pyridines, and pyrazoles. In addition, low levels of methylated amino acids (phenylalanine, leucine, and valine) were detected. The presence of the amino acids in the bottom layer of the soil suggests a preservation mechanism. The changes identified in the chemical components provide clues as to the nature of the humification processes in the soil profile and also yield information on the nature of the sources of soil organic matter.