Structural units in humic acids from south-eastern Queensland soils as determined by 13nmr spectroscopy

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Abstract
Solution 13C Fourier transform nuclear magnetic resonance (FTNMR) spectra of six humic acids extracted from a range of soil types and a solid state, cross polarization/magic angle spinning (CP/ MAS) 13C NMR spectrum of a lignin are reported. Quantifying specific regions of the spectra reveals the presence of alkyl, carbohydrate, aromatic and carboxyl carbons. Alkyl carbon is in general the major contributor to these spectra, averaging 38% of the total carbon. Long-chain units comprise a significant proportion of this group, although highly branched systems are also in evidence in two samples. Carbohydrate carbon is a major contributor in two samples, representing 27% and 28% of the total carbon nuclei measured. Aromatic carbon content shows the greatest variability, ranging from 10 to 45%. Humic acids from soils developed under rainforest show the lowest aromaticity, and it is suggested that aromatic structures are not integral building blocks of terrestrial humic acids. Little evidence was found for the presence of hydroxy- and methoxy-substituted aromatic carbons even in humic acids high in aromaticity.