Structural differences between humic fractions from different soil types as determined by FT-IR and 13C-NMR studies

Abstract

Structural characteristics of fulvic and humic fractions from various Spanish soils are characterized by their 13 C-NMR spectra registered under quantitative acquisition conditions, and their resolution-improved FT-IR spectra obtained by substracting the original spectra from a positive multiple of their Laplacians. Although the humic ssmples analyzed had a very different origin, the spectral features do not reflect the influence of any specific soil-forming factor upon the spectral patterns of the humic fractions from a particular soil. A Correlation study between the most relevant peaks in the IR and NMR spectra shewed that both techniques lead to similar conclusions in terms of the different contributions of aromatic and aliphatic moieties, but to very independent results with regard to the oxygen-containing functional groups. The most significant differences between the two humic materials studied is that carbohydrate predominate in the fulvic acids (FA's), whereas in the humic acids (HA's) those of the aromatic carbons. The variations in the carboxyl/carbonyl and the aliphatic ranges are minor at best. The presence of a large fraction of carboxyl groups bound to aromatic carbons is suggested.