Humic acids in the first generation of EUROSOILS

Abstract

Humic acids (HAs) were isolated from the first generation of the EUROSOILS, a reference set of five samples representing the most widespread soil units in the European Union (EU). The EUROSOIL (E)-HA were characterized for their compositional, structural, and functional properties by using chemical methods and spectroscopic techniques, including Fourier transform infrared (FT IR), nuclear magnetic resonance, fluorescence, and electron spin resonance. The E-HAs generally exhibit features typical of soil HAs of similar origin and nature, and differ in their composition, structure, and chemical properties on dependence on the parent soil type, vegetation cover, and land use. The E-HA from the Orthic Podzol under coniferous forest (E5-HA) exhibits the largest oxygen and acidic functional group contents, organic free radical concentration, aromatic polycondensation and humification degree, and the distinct presence of outer-sphere complexed Mn 2+ ions. The E-HAs from the Dystric Cambisol under grassland/pasture (E3-HA) and the Orthic Luvisol cultivated for wheat (E4-HA) are generally characterized by a great content of nitrogen-containing and methoxyl groups and polysaccharide structures, small contents of acidic functional groups and free radicals, relatively small aromaticity and humification degree, and the presence of both inner-sphere complexed Cu 2+ ions and outer-sphere complexed Mn 2+ ions. The E-HAs from the Vertic Cambisol under grassland/meadow (E1-HA) and the Rendzina under broad-leaved tree cover (E2-HA) are characterized by properties that are generally intermediate between those of E5-HA and E3- and E4-HAs, being E1-HA more similar to E5-HA and E2-HA to E3- and E4-HAs. The differences in chemical and physicochemical properties of these E-HAs are expected to affect differently their reactivity toward soil organic and inorganic contaminants.