Nature of organic carbon and nitrogen in physically protected organic matter of some Australian soils as revealed by solid-state 13C and 15N NMR spectroscopy

Abstract

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.