N K-Edge XANES and Pyrolysis-Field Ionization Mass Spectrometry – Clues to Disclose the Chemistry of "known" Organic Nitrogen in Organic-Mineral Soil Clay Particles.
Peter Leinweber, Univ of Rostock, Justus-von-Liebig-Weg, Rostock, Germany, Fran L. Walley, Dept. of Soil Science, Univ of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada, Alexander Jokic, Department of Soil Science, Univ Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada, and Tom Regier, Canadian Light Source Inc., 101 Perimeter Road, Saskatoon, SK S7N 0X4, Canada.
About one third of soil organic nitrogen (N) is still unknown or poorly understood in terms of molecular structure and ecological functions. Throughout the past decade, there has been considerable controversy in the literature regarding whether or not heterocycles constitute an important compound class of the „unknown“ N. Since organic N is systematically enriched for clay particles with large specific surface (Schulten & Leinweber, 2000), we studied the fine (< 0.2 µm) and the coarse clay size fraction (0.2 - 2 µm) of a Chernozem using synchrotron-based soft x-ray spectroscopy and pyrolysis-field ionization mass spectrometry. N K-edge XANES spectra were recorded using the Spherical Grating Monochromator (SGM) Beamline at the Canadian Light Source, Saskatoon. The photon energy scale was calibrated using the 1sΠ* transition for N2 gas (Schwartzkopf et al, 1999). Measurements of both the fluorescence and electron yield for samples with a nitrogen content as low as 1 g kg-1 were made. Reference compounds included pyridines, pyrimidines, pyrroles, imidazoles and amides, each having characteristic peaks of normalized fluorescence yields (FY). The N XANES spectra of the clay samples showed a broad first feature with peak at 399.5 eV, and second less pronounced features in the range of 400.6 to 402.6 eV. The prominent peak at 399.5 eV exactly matched the peak position of pyrimidinic N in adenine, however, the shape of the feature also suggested the abundance of pyridinic and other heterocyclic N. The region in the XANES spectra that could be assigned to amide-N was slightly more pronounced in fine than in coarse clay, but generally was subordinate as compared to heterocyclic N compounds. These results were confirmed by temperature-resolved pyrolysis-field ionization mass spectra, which revealed a great variety of substitutes at the N heterocyclic rings. Moreover, thermograms recorded for single N-containing ions and classes of N compounds indicated strong bonds to clay surfaces which may explain the resistance of these compounds to microbial decomposition. In conclusion, the complementary non-destructive N XANES and (soft) field-ionization mass spectrometry provided unequivocal evidence for the stabilization of substantial amounts of heterocyclic N in organic-mineral soil clay fractions. The ongoing research is directed to disclose if these N compounds also contribute substantially to the organic N in other soil clay fractions of contrasting mineral composition.