Aliphatic components of forest soil organic matter as determined by solid-state ¹³C NMR and analytical pyrolysis

Forest soil organic matter contains a refractory alkyl-carbon component of unknown structure and composition. Soil samples obtained from different types of forest humus and from litter-bag experiments were examined by the complementary techniques of solid-state ¹³C NMR and analytical pyrolysis to determine the structural composition of the unknown alkyl carbon. The NMR techniques of cross polarization magic angle spinning and dipolar dephasing provide quantitative data on the average structural composition, whereas the pyrolysis method provides detailed molecular information. The data suggest that the unknown alkyl-carbon structures in fresh forest litter are composed of the plant polyester cutin and another aliphatic biopolymer. These components correspond to a mobile and a more rigid fraction of aliphatic carbon as determined by dipolar dephasing ¹³C NMR spectroscopy. With increasing humification in the forest soils the percentage of rigid carbon increases. This could be indicative of a selective preservation of rigid carbon moieties derived from plants or soil microorganisms. The molecular structure of the aliphatic moieties was further investigated with Curie-point pyrolysis-gas chromatography (-mass spectrometry). The results indicate that aliphatic biomacromolecules of plant or microbial origin are present in very small quantities in these soil samples. They are not selectively preserved during decomposition. It is concluded that the rigid carbon moieties result from an increase in cross linking during the humification process.