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Chemical Structural Studies of Forest Soil Humic Acids: Aromatic Carbon Fraction
Author(s) -
KögelKnabner Ingrid,
Zech Wolfgang,
Hatcher Patrick G.
Publication year - 1991
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1991.03615995005500010041x
Subject(s) - humus , lignin , chemistry , picea abies , magic angle spinning , humic acid , nuclear magnetic resonance spectroscopy , soil organic matter , organic matter , soil water , carbon 13 nmr , organic chemistry , botany , geology , soil science , fertilizer , biology
Forest 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) 13 C 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 13 C‐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 13 C‐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.