New evidence for the molecular composition of soil organic matter in vertisols

The distribution of soil organic matter (SOM) in particle-size fractions an d the molecular composition of SOM were investigated for five FAO/UNESCO re ference Vertisols. Throughout the soil profiles, more than 80% of SOM was a ssociated with clay-size fractions. The remaining proportions were distribu ted in silt (mean: 15%) and sand (mean < 1%). The uniform depth distributio n of organic carbon (C-org) and total nitrogen (N-t) in size fractions dist inguished the Vertisols from other major soil groups. The composition of SO M in the surface horizons was studied by solid-state C-13 nuclear magnetic resonance (C-13-NMR) spectroscopy, analytical pyrolysis, and wet chemical a nalyses of organic N forms. The C-13-NMR spectra showed low signal-to-noise ratios and indicated the predominance of alkyl C in four of the samples. I n contrast to the general low extractability of SOM in Vertisols, unexpecte dly large proportions of C and N (61-95%) could be pyrolyzed and analyzed b y field-ionization mass spectrometry (Py-FIMS) and Curie-point gas chromato graphy/mass spectrometry (Py-GC/MS). Signals of nitrogen-containing compoun ds were pronounced in the FI mass spectra, and carbohydrates also contribut ed significantly to the total ion intensity (TII). Quantitative evaluation showed that the large proportions of heterocyclic N-containing compounds an d peptides (up to 20% of TII) distinguished these Vertisols from Regosols, Cambisols, Podzols, Chernozems, and Histosols which were analyzed by Py-FIM S. Derivatives of pyrrole and pyridine as well as aromatic nitriles were ob served in the GC/mass spectra of all samples. Hydrolysis and fractionation of organic N forms yielded large proportions of hydrolyzable N (84-98% of t otal N) and NH3-N (32-53% of total N), adding further evidence to the impor tance of N-containing molecules in Vertisols. These results suggest that re cent concepts regarding the molecular composition of SOM in Vertisols shoul d be revised because they overemphasize long-chain aliphatics and neglect a romatic and aliphatic N-containing molecules, which were unequivocally iden tified by analytical pyrolysis.