Methods for the Assessment of Humic Substances Quality in Forest Soils

This paper is focused on soil organic matter quality assessment in acid forest soils. Soil samples were collected in the Jizera Mountains region. Vegetation cover of sampling sites was formed by spruce or beech monocultures. Humus quality was assessed by the ratio of absorbances of pyrophosphate soil extract at the wavelengths of 400 and 600 nm (A400/A600). Humus fractionation was performed on selected soil samples. DRIFT spectra of individual fractions were measured. Higher pH and lower C and N contents were found in beech forest than in spruce forest. A400/A600 well correlates with C and N contents (r = 0.510*** and 0.615***, respectively). C and N content increases as to humus quality decreases. DRIFT spectra of fulvic acids turned out to be unsuitable for describing differences in humus quality. DRIFT spectra of humic acids and humin were hence more suitable. The difference between spruce and beech forest was found in 1514.cm-1 (C=C bounds of benzene rings) and 1550 cm-1 (N-H bounds in monosubstituted amides) bands intensities. Humic acids and humin coming from the O horizons of beech forest are relatively enriched by nitrogen functional groups. Values of humic acids aromaticity index did not differ between beech and spruce forests. DRIFT spectroscopy was shown as a possible method for detailed humus quality studying.