Assessment of the effects of environmental factors on humification processes by derivative infrared spectroscopy and discriminant analysis

The potential of infrared spectroscopy to establish differences in terms of the environmental factors on the humification processes has been assessed on Mediterranean forest soils from Central Spain developed under sclerophyllic and gymnosperm vegetation. Humic acids were isolated from 21 soil samples differing in terms of vegetation (pine, holm oak, oak), geological substrate (granite, limestone), soil type (Humic Dystrudepts, Humic Dystroxerepts, Humic Haploxerepts, Calcic Argixerolls), soil horizon (O, A1, A2), humus type (calcic mull, mesotrophic-acid mull, moder), physiographic unit (hillside, moorland, ramp), altitudinal step, epipedon (mollic or umbric) and moisture regimen (udic or xeric), which were used as environmental classification factors. Quantitative peak intensity data were obtained from the second derivative infrared spectra of the humic acids. Discriminant analysis with automatic variable backward selection was used to identify the infrared bands showing more systematic changes in their intensities depending on the above classification factors. The diagnostic infrared bands chosen were at 1720, 1260 and 1330 cm− 1, corresponding mainly to carboxyl and lignin-derived O-containing functional groups. The bands at 2920 and 1510 cm− 1, corresponding to aliphatic and aromatic structures of the carbon backbone, respectively, were not significantly responsive to the environmental variability in the studied samples. The results showed how, in the scenario under study, taking advantage of the relative intensities of specific peaks from the second derivative infrared spectra, it is possible to discriminate different soil Suborders and humus types, as well as between several physiographic and bioclimatic features.