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

Tetramethylammonium hydroxide (TMAH) thermochemolysis gas chromatography/ma ss spectrometry (GC/MS) was employed to study the chemical structure of soi l organic matter sampled from a soil plot in which corn (Zea mays L.) was f armed 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-der ived peaks in the TMAH thermochemolysis-GC/MS chromatograms were mainly der ivatives of p-hydroxyphenyl and guaiacyl structures, suggesting a non-woody (grass) lignin type. With depth, the ratio of syringyl to guaiacyl compoun ds (SIG) decreased, suggesting a preferential degradation of the syringyl u nits by microorganisms. Fatty acid methyl esters of varying C-chain length (C-7 to C-27) were identified in the soil chromatograms, Both TMAH-GC/MS an d C-13-NMR (nuclear magnetic resonance) data suggested a relative increase of longchain fatty acids with soil depth (or degree of humification), sugge sting a refractory nature for these compounds. The heterocyclic N compounds yielded from the TMAH thermochemolysis were mainly pyrroles, pyridines, an d pyrazoles, In addition, low levels of methylated amino acids (phenylalani ne, leucine, and valine) were detected. The presence of the a,nino acids in the bottom layer of the soil suggests a preservation mechanism. The change s identified in the chemical components provide clues as to the nature of t he humification processes in the soil profile and also yield information on the nature of the sources of soil organic matter.