Nitrogen speciation in fine and coarse clay fractions of a Cryoboroll - new evidence from pyrolysis-mass spectrometry and nitrogen K-edge XANES

Abstract

Soil clay fractions are usually enriched in nitrogen (N), but the chemical identity of this N is largely unknown. Therefore, we investigated organic N in fine and coarse clay of a clay-rich Cryoboroll by Curie-point pyrolysis-gas chromatography/mass spectrometry (Cp Py-GC/MS), Pyrolysis-field ionization mass spectroscopy (Py-FIMS) and synchrotron-based nitrogen K-edge X-ray absorption near edge structure (N-XANES) spectroscopy. The Cp Py-GC-MS revealed 30 structurally different N-containing compounds, such as substituted pyridines, pyrroles; pyrazines, pyrazoles, imidazoles, quinolines, side-chain N-containing benzenes, and single compounds of substituted benzotriazole, purine and indole. These accounted for about 10% of peak area in the Py-GC chromatograms. The Py-FIMS and N-XANES spectra indicated interlayer-NH4+ and revealed pyridinic and nitrilic N compounds, but disagreed in the proportions of pyrroles. All three complementary methods confirmed to different extents previous wet-chemical data on N-fractions in these samples, and provided new evidence for about 30 to 40% non-proteinaceous N as major constituent of the so-called “unknown organic N” in soil. Key words: Soil clay, organic matter, pyrolysis-mass spectrometry, synchrotron, X-ray absorption, NEXAFS