NEW EVIDENCE FOR THE MOLECULAR COMPOSITION OF SOIL ORGANIC MATTER IN VERTISOLS

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Abstract
The distribution of soil organic matter (SOM) in particle-size fractions and the molecular composition of SOM were investigated for five FAO/UNESCO reference Vertisols. Throughout the soil profiles, more than 80% of SOM was associated with clay-size fractions. The remaining proportions were distributed in silt (mean: 15%) and sand (mean < 1%). The uniform depth distribution of organic carbon (Corg) and total nitrogen (Nt) in size fractions distinguished the Vertisols from other major soil groups. The composition of SOM in the surface horizons was studied by solid-state 13C nuclear magnetic resonance (13C-NMR) spectroscopy, analytical pyrolysis, and wet chemical analyses of organic N forms. The 13C-NMR spectra showed low signal-to-noise ratios and indicated the predominance of alkyl C in four of the samples. In contrast to the general low extractability of SOM in Vertisols, unexpectedly large proportions of C and N (61-95%) could be pyrolyzed and analyzed by field-ionization mass spectrometry (Py-FIMS) and Curie-point gas chromatography/mass spectrometry (Py-GC/MS). Signals of nitrogen-containing compounds were pronounced in the FI mass spectra, and carbohydrates also contributed significantly to the total ion intensity (TII). Quantitative evaluation showed that the large proportions of heterocyclic N-containing compounds and peptides (up to 20% of TII) distinguished these Vertisols from Regosols, Cambisols, Podzols, Chernozems, and Histosols which were analyzed by Py-FIMS. Derivatives of pyrrole and pyridine as well as aromatic nitriles were observed in the GC/mass spectra of all samples. Hydrolysis and fractionation of organic N forms yielded large proportions of hydrolyzable N (84-98% of total N) and NH3-N (32-53% of total N), adding further evidence to the importance of N-containing molecules in Vertisols. These results suggest that recent concepts regarding the molecular composition of SOM in Vertisols should be revised because they overemphasize long-chain aliphatics and neglect aromatic and aliphatic N-containing molecules, which were unequivocally identified by analytical pyrolysis.