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

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 (C org ) and total nitrogen (N t ) in size fractions distinguished the Vertisols from other major soil groups. The composition of SOM in the surface horizons was studied by solid-state 13 C nuclear magnetic resonance ( 13 C-NMR) spectroscopy, analytical pyrolysis, and wet chemical analyses of organic N forms. The 13 C-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 NH 3 -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.