ASSESSING THE EXTENT OF DECOMPOSITION OF NATURAL ORGANIC MATERIALS USING SOLID-STATE C-13 NMR-SPECTROSCOPY

Solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy has bec ome an important tool for examining the chemical structure of natural organic materials and the chemical changes associated with decompositi on. In this paper, solid-state C-13 NMR data pertaining to changes in the chemical composition of a diverse range of natural organic materia ls, including wood, peat, composts, forest litter layers, and organic materials in surface layers of mineral soils. were reviewed with the o bjective of deriving an index of the extent of decomposition of such o rganic materials based on changes in chemical composition. Chemical ch anges associated with the decomposition of wood varied considerably an d were dependent on a strong interaction between the species of wood e xamined and the species composition of the microbial decomposer commun ity, making the derivation of a single general index applicable to woo d decomposition unlikely. For the remaining forms of natural organic r esidues, decomposition was almost always associated with an increased content of alkyl C and a decreased content of O-alkyl C. The concomita nt increase and decrease in alkyl and O-alkyl C contents, respectively , suggested that the ratio of alkyl to O-alkyl carbon (A/O-A ratio) ma y provide a sensitive index of the extent of decomposition. Contrary t o the traditional view that humic substances with an aromatic core acc umulate as decomposition proceeds, changes in the aromatic region were variable and suggested a relationship with the activity of lignin-deg rading fungi. The A/O-A ratio did appear to provide a sensitive index of extent of decomposition provided that its use was restricted to sit uations where the organic materials were derived from a common startin g material. In addition, the potential for adsorption of highly decomp osable materials on mineral soil surfaces and the impacts which such a n adsorption may have on bioavailability required consideration when t he A/O-A ratio was used to assess the extent of decomposition of organ ic materials found in mineral soils.