Comparisons of Humic Substances Extracted from Contiguous Alfisols and Mollisols of Southwestern Ohio

Humic and fulvic acids extracted from contiguous Alfisols and Mollisols were characterized to determine if any dissimilarities could be documented between humic substances extracted from soils formed under nearly identical environmental conditions, but belonging to different taxonomic classes. Comparisons of humic substances from two Alfisol‐Mollisol pairs, one from the Wisconsinan and one from the Illinoian till plain of southwestern Ohio, were based on wet chemical, infrared, and 13 C cross‐polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy studies. Using 0.1 M NaOH, more humic substances were extracted from the Mollisols than the Alfisols, with the majority being humic acids. Small amounts of fulvic acids were recovered from all pedons. Data from infrared and 13 C CPMAS NMR spectroscopy indicate that humic acids from both the Alfisols and Mollisols are dominated by alkyl‐substituted aromatic structures, followed by aliphatic, O‐substituted alkyl, carboxylic acid, and small amounts of carbonyl structures. Whereas 13 C NMR spectroscopy data suggest Mollisol humic acids contain higher aromatic and carboxylic acid contents than those of the Alfisols, Alfisol humic acid spectra show more evidence for phenolic‐OH groups and lignin and lignin‐like products than those of the Mollisols. Fulvic acids from Alfisols and Mollisols contained mostly aliphatic, O‐substituted alkyl, and carboxylic acid, followed by aromatic groups and small amounts of carbonyl groups. The proportion of nonaromatic and aromatic structures in the fulvic acids of the Alfisols and Mollisols was similar, but slight differences were noted in elemental ratios and 13 C NMR spectra. Although only minimal differences were noted among fulvic acids, differences were evident in the chemical and structural properties of humic acids from contiguous Alfisols and Mollisols in southwestern Ohio.