Investigation of humic acid N with X-ray photoelectron spectroscopy: Effect of acid hydrolysis and comparison with N-15 cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy

Nitrogen in humic substances has an important function as a reservoir of plant nutrient. To elucidate the stability of humic acid N in soil with respect to chemical structure, X-ray photoelectron spectroscopy (XPS) and N-15 cross polarization/magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy were applied to humic acids with various degrees of humification. The composition of the non-hydrolyzable fraction of humic acid N remaining after step-wise acid hydrolysis was also investigated by XPS. The proportion of non-hydrolyzable N in aromatic N (including imine, heterocyclic C=N, and aromatic amine), 26.2 +/- 5.4%, was greater than that in peptide bond N or primary amine N, 4.2 - 21.5%. However, peptide bond N was the dominant species in the non-hydrolyzable humic acid N, 58.7 - 79.5%, and aromatic N and primary amine N accounted for 12.2 - 22.3% and 5.6 - 19.8%, respectively. The proportions of heterocyclic N, peptide/amide N, guanidine/aniline N, and free amino N in total N, estimated by N-15 CPMAS NMR, were 5 - 21%, 59 - 83%, 4 - 8%, and 3 - 12%. The relative concentration of peptide/amide N estimated by N-15 CPMAS NMR correlated negatively with the degree of humification that was estimated from two variables indicating the degree of darkening. In general, significant differences between the proportion of peptide/amide N (or peptide bond N) in total N estimated by N-15 CPMAS NMR and XPS were not detected. The proportion of heterocyclic N in total N estimated by N-15 CPMAS NMR correlated positively with the degree of humification. The absence of significant differences between the proportions of aromatic N (XPS) and heterocyclic N (N-15 CPMAS NMR) in total N suggested that aromatic amines are minor constituents in humic acid N. Since pyrrolic N is not included in XPS measurements of aromatic N, it was considered that the relative concentration of heterocyclic C=N increases with increasing degree of humification. (c) 2005 Elsevier Ltd. All rights reserved.