Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/1999
Publication Date: N/A
Interpretive Summary: Soil organic matter is the basis for judging good soil and for deciding best economic and environmental land use. This paper provides new techniques for identifying the nature of humus formation and also gives information on the sources of soil organic matter. The data show that over 50 identifiable organic compounds resulted from treatment of unextracted samples down to a depth of 18 inches in a continuous corn-soil experiment. The results of this research can be used by agricultural scientists to help unravel the mysteries of how soil organic matter provides the fiber and network to bind separate soil particles into an aggregated structure.
Technical Abstract: 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, which was farmed with continuous corn for 15 years. 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. FAMEs of varying C chain length, (C7 to C27) were identified in the soil chromatograms. Both TMAH GC/MS and 13C-NMR 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 in the chemical components identified 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.