X-Ray Photoelectron Spectroscopy of Nitrogen Functional Groups in Soil Humic Acids

The composition of nitrogen (N) species in 11 humic acids obtained from a wide variety of soils was investigated by X-ray photoelectron spectroscopy (XPS). The N1s spectra of the humic acids were fitted by three Gaussian curves with binding energy at 399.0 ± 0.1, 400.4 ± 0.1, and 402.4 ± 0.2 eV, which were assigned to aromatic N including pyridine, imine, and aniline derivatives; peptide bond N including other amides, pyrrole, and secondary and tertiary amines; and primary amine N, including protonated amines. The aromatic, peptide bond, and primary amine N groups accounted for 3.0–18.7%, 66.7–90.1%, and 6.9–16.7%, respectively, of total humic acid N. The proportion of aromatic N in total N was correlated positively with a parameter indicating the degree of humification, A600/ C (absorbance at 600 nm in 0.1 M NaOH per mg C mL−1; r = 0.95***), whereas that of peptide bond N was correlated negatively with A600/ C (r = −0.78***). A significant positive correlation was also found between the proportion of aromatic N in total N and that of aromatic C in total C estimated by 13C CPMAS NMR spectroscopy (r = 0.94***). By contrast, a negative correlation was observed between the proportion of aromatic N in total N and total N content in humic acids (r = −0.89***). The results suggest that the aromatic N content is maintained selectively during the progress of humification.