Abundant Nonprotonated Aromatic and Oxygen-Bonded Carbons Make Humic Substances Distinct from Biopolymers

Abstract

The nature of humic substances (HSs) extracted from soil organic matter and their distinction from degrading plant material has recently been called into question. Quantitative solid-state 13C nuclear magnetic resonance (NMR) analyses, in good agreement with elemental analysis, show that HSs are distinct from common biopolymers, refuting claims to the contrary made based on 1H-detected NMR spectra of the same standard peat humic acid as studied here. Aromatic carbons not bonded to hydrogen or oxygen, which are rare (≤10%) in common biomolecules, constitute a large fraction (28%–33%) of prairie soil and peat humic acids. Oxygen-bonded nonprotonated carbons such as aryl ketones, nonprotonated alkyl C–O, and abundant COOH groups are also characteristic of humic and fulvic acids but not observed in plant biopolymers. These distinctive structural differences between HSs and biopolymers challenge the recent proposal that HSs are unremarkable intermediates in a continuum of degrading biomolecules.