Nature and distribution of alkyl carbon in forest soil profiles: implications for the origin and humification of aliphatic biomacromolecules

Resistant residues isolated from whole soils were subjected to analysis by solid-state CPMAS ¹³C NMR and Curie-point pyrolysis-gas chromatography-mass spectrometry to delineate the nature of the resistant aliphatic components. These components constitute a major fraction of soil organic matter and associated humic substances and appear to be concentrated during soil formation initially by a process of selective preservation. Changes in the nature of these biomacromolecules with increasing depth and decomposition in the soil profile indicate that they are being altered significantly. The NMR data show that the aliphatic structures in forest soil organic matter can be assigned to mobile and rigid carbon moieties. The mobile and rigid carbon types are possibly associated with different types of macromolecules. One type are the polyesters cutin and suberin from leaves, barks and roots, the other is a resistant non-saponifiable aliphatic biomacromolecule derived from leaves and barks or microorganisms. At depth in the soil profile the mobile components are degraded but the rigid aliphatic biomacromolecules appear to be selectively preserved. However, the pyrolysis data indicate that these non-saponifiable aliphatic macromolecules bear no resemblance to the resistant aliphatic biomacromolecules in fresh leaf cuticles or microbial cell walls. This lack of resemblance is probably due to the fact the selective preservation of resistant, non-saponifiable plant or microbial macromolecules is not the dominant process leading to the accumulation of alkyl carbon moieties in forest soil organic matter. Alternatively, it seems possible that the structural differences observed between the alkyl carbon moieties in forest litter and humified soil horizons result from an increase in cross-linking during humification.