Aromaticity of Humic Substances in Soils

Abstract Humic acids from different climatic zones and a fulvic acid from an Armadale soil, Prince Edward Island, Canada, have been examined by cross polarization, magic‐angle spinning 13 C nuclear magnetic resonance (NMR). Aromaticities calculated from the NMR spectral data are generally high, ranging from 35 to 92%, in general agreement with values obtained from the results of chemical oxidation/gas chromatographic mass spectrometric studies. Most NMR spectra, however, indicate the presence of significant aliphatic structures that generally were not observed by chemical oxidation studies. Chemical techniques have overestimated the degree of aromaticity of humic substances. Phenolic carbons that have always been considered to be major “building blocks” of humic substances are not observed as intense signals by NMR; this fact suggests that these carbons may be minor rather than major components. Carboxyl contents calculated from NMR spectra are greater than those calculated by chemical techniques and approximate the concentrations of total acidic groups.