Solid-state 13C nuclear magnetic resonance (NMR)
spectroscopy has become an important tool for examining the chemical structure
of natural organic materials and the chemical changes associated with
decomposition. In this paper, solid-state 13C NMR data
pertaining to changes in the chemical composition of a diverse range of
natural organic materials, including wood, peat, composts, forest litter
layers, and organic materials in surface layers of mineral soils, were
reviewed with the objective of deriving an index of the extent of
decomposition of such organic materials based on changes in chemical
composition. Chemical changes associated with the decomposition of wood varied
considerably and were dependent on a strong interaction between the species of
wood examined and the species composition of the microbial decomposer
community, making the derivation of a single general index applicable to wood
decomposition unlikely. For the remaining forms of natural organic residues,
decomposition was almost always associated with an increased content of alkyl
C and a decreased content of O-alkyl C. The concomitant 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) may provide a sensitive index of the
extent of decomposition. Contrary to the traditional view that humic
substances with an aromatic core accumulate as decomposition proceeds, changes
in the aromatic region were variable and suggested a relationship with the
activity of lignin-degrading fungi. The A/O-A ratio did appear to provide
a sensitive index of extent of decomposition provided that its use was
restricted to situations where the organic materials were derived from a
common starting material. In addition, the potential for adsorption of highly
decomposable materials on mineral soil surfaces and the impacts which such an
adsorption may have on bioavailability required consideration when the
A/O-A ratio was used to assess the extent of decomposition of organic
materials found in mineral soils.
Novel Problems in the Solid State Cathodoluminescence of Organic Materials