Nature of humic substances of Mollisol and Luvisol in the Canadian Prairies

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The nature of humic substances (0–25 cm) of a Mollisol (Udic Haploboroll) and a Luvisol (Typic Cryoboralf) in the Canadian Prairies was investigated. The amount of humic acid (HA) was higher in the Mollisol than in the Luvisol. The opposite trend was observed for fulvic acid (FA). There was no great difference in elemental composition of HAs between the Mollisol and Luvisol. The contents of N and S of the FAs were higher in the Mollisol than in the Luvisol. Infrared spectra of the two HAs were essentially similar. The infrared data also indicated that compared with HAs, FAs contained more carboxyl groups and less aromatic C. 13C NMR spectra of the HAs and FAs showed the presence of the pronounced peaks at 20–35 ppm (aliphatic C in alkyl chains), 75 ppm (carbohydrates), 135 ppm (aromatic ring C), and 165–175 ppm (COOH groups). The peaks at 60 ppm (methoxy groups) and 150–155 ppm (O and N substituted aromatic C) were also present in 13C NMR spectra of both HAs. HAs in both Mollisol and Luvisol were substantially more aromatic than FAs. Aliphaticity of HAs was more pronounced in the Luvisol than in the Mollisol. The NMR evidence was in accord with the data obtained from the measurement of the extinction coefficient (E280) and differential thermal analysis. The HA and FA of the Mollisol appeared to contain a higher proportion of more condensed aromatic structures than those of the Luvisol. Both differential thermograms and X-ray diffractograms indicated that HAs and FAs in the Mollisol were better ordered than those in the Luvisol. The data indicated that differences in pedogenesis exerted the influence on the nature of humic substances in grassland and parkland in temperate region.