Nature of organic nitrogen in fine particle size separates of sandy soils of highly industrialized areas as revealed by NMR spectroscopy

The structure of nitrogen-containing compounds of the fine particle size fractions (<20 μm) of two Podzols obtained from the highly industrialized Ruhr area in Germany, were examined by means of solid-state 15N nuclear magnetic resonance (NMR) spectroscopy. In order to improve the signal-to-noise ratio of the spectra, the samples were treated with hydrofluoric acid (HF), prior to NMR analysis. Comparing the solid-state 15N NMR spectra of plant incubates obtained before and after HF treatment revealed no major alteration of the nitrogen fraction induced by HF. From 60 to 90% of the nitrogen detectable in the solid-state 15N NMR spectra of the soil particle size fractions were assigned to amides. A smaller signal derives from free amino groups, leading to the conclusion that most of the nitrogen was derived from peptide-like structures. The calculated high contribution of peptides to the total organic carbon and nitrogen of the samples confirms earlier studies demonstrating that peptide-like material plays a more important role in refractory soil organic matter formation than commonly thought. Major contributions of N-containing heterocyclic aromatic compounds, formed by recondensation reactions or deriving from the input of coal and soot particles from coal processing industries, were not identified. Obviously, in these fractions, contamination did not significantly alter the chemical composition of the organic nitrogen.