The <53-□m particle-size fractions of 5 different Australian soils
were subjected to high energy ultraviolet (UV) photo-oxidation for a period of
2 h in order to remove most of the physically unprotected organic material.
Solid-state 13C and 15N nuclear
magnetic resonance (NMR) spectroscopy was applied for characterising the
chemical nature of the remaining organic fraction. The
13C NMR spectroscopic comparison of the residues after
UV photo-oxidation and the untreated bulk soils revealed a considerable
increase in condensed aromatic structures in the residues for 4 of the 5
soils. This behaviour was recently shown to be typical for char-containing
soils. In the sample where no char was detectable by NMR spectroscopy, the
physically protected carbon consisted of functional groups similar to those
observed for the organic matter of the bulk sample, although their relative
proportions were altered. The solid-state 15N NMR
spectrum from this sample revealed that some peptide structures were able to
resist UV photo-oxidation, probably physically protected within the core of
microaggregates. Heterocyclic aromatic nitrogen was not detected in this
spectrum, but pyrrolic nitrogen was found to comprise a major fraction of the
residues after photo-oxidation of the <53-□m fractions of the char-
containing soils. Acid hydrolysis of these samples confirmed that some
peptide-like material was still present. The identification of a considerable
amount of aromatic carbon and nitrogen, assignable to charred material in 4 of
the 5 investigated soils, supports previous observations that char largely
comprises the inert or passive organic matter pool of many Australian soils.
The influence of such material on the carbon and nitrogen dynamics in such
soils, however, requires further research.
Solid-state 15N NMR spectroscopy of coal