Chemical structural studies of forest soil humic acids: aromatic carbon fraction

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 3 forest soils. The humic acid fraction isolated from fresh litter of European beech and Norway spruce 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. The percentage of nonprotonated aromatic C, and more specifically the C-substituted aromatic C fraction, increase when humification proceeds. Lignin structure is altered considerably during humification, resulting in lignin-derived aromatic structures with a high degree of C substitution and carboxyl functionality. -from Authors