Chemical Structural Studies of Forest Soil Humic Acids: Aromatic Carbon Fraction

Abstract

AbstractForest soil organic matter contains about 20 to 30% aromatic carbon moieties. In order to follow the changes in the structural composition of the aromatic C during humification a combination of cross‐polarization magic‐angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy and CuO‐oxidation was used. Humic acids were extracted from the forest floor, A, and B horizons of a forested Typic Dystrochrept, an Entic Haplorthod, and a Lithic Udorthent. The materials collected differ in the degree of humification, from fresh litter material to humified mineral soil horizons, as determined by macromorphological observation. The humic acid fraction isolated from fresh litter of European beech (Fagus silvatica L.) and Norway spruce (Picea abies Karst) shows mainly peaks attributable to aromatic C derived from lignin and tannin structures, which are both partly extractable by alkaline solvents. The most prominent feature of the NMR spectra is the decrease of phenolic and methoxyl C with increasing degree of humification, accompanied by decreasing yields of lignin‐derived CuO‐oxidation products. Simultaneously, the signal intensity at 130 ppm in the 13C‐NMR spectra increased. Detailed structural assignments for this signal can be obtained by measuring the percentage of signal intensity from protonated and nonprotonated C by dipolar dephasing 13C‐NMR spectroscopy. The percentage of nonprotonated aromatic C, and more specifically the C‐substituted aromatic C fraction, increase when humification proceeds. Assuming that lignin is the primary precursor of the aromatic C components of humic acids in forest soils, then the lignin structure is altered considerably during humification, resulting in lignin‐derived aromatic structures with a high degree of C substitution and carboxyl functionality.