ИСТИНА

Water stability of the aggregate structure is directly related to the surface properties of elementary soil particles (ESP) in the solid phase of the soil. In case of high hydrophilicity of ESP, the water flows through the capillaries in the dry aggregate and leads to an increase of water pressure in the aggregate and its destruction. When ESP are hydrophobic, the adhesion is reduced as there is no attraction between the water molecules and the non‐polar surface. Contact angle (CA) is used as a wettability characteristic value; it quantifies the physicochemical interactions at the liquid‐solid interface (Letey et al., 1962). The purpose of this study was to link the densitometric fractions surface wettability with the hydrophobic and hydrophilic components of humus substances (HS) contained in these fractions. The samples were taken from the two horizons (10‐15, 40‐45 cm) of chernozem within an oak wind‐proof forest strip without grass cover (Voronezh region, Russia). We used the procedure (Golchin et al., 1994) to obtain the density fractions of soil organic matter: occluded particulate organic matter (oPOM) with a density <1.6 g cm‐3 (oPOM<1.6); 1.6–1.8 g cm‐3 (oPOM1.6–1.8); 1.8– 2.2 g cm‐3 (oPOM1.8–2.2) and mineral associated OM with a density >2.2 g cm‐3. Total organic carbon (TOC) was obtained with CN analyzer (Vario EL, Elementar, Germany). Static CA were determined by means of the sessile drop method with the help of a digital goniometer (Drop Shape Analysis System, DSA100, Krüss, Germany). Using Octyl Sepharose CL‐4B HS, isolated from Densitometry fractions (extraction by solution NaOH + Na4P2O7), fractionated by hydrophobic interaction chromatography (HIC) at low pressure liquid chromatograph BioLogic LP (BioRad, USA). TOC content and C/N ratio of the fractions decreased along with the increase of their density (from 47‐30% to 0,6% and from 28‐25 to 8 respectively). At the same time the surface of the oPOM<1.6 was hydrophobic (CA 90°) while the rest were hydrophilic (CA 59‐50‐33°). HIC allows to physically divide a set of HS in extract into components (5 fractions), which differ in ability to participate in hydrophobic interactions with gel matrix. It was found that a decrease in CA of wetting of densitometric fractions was accompanied by an increase of the relative content of hydrophilic components of HS (fraction 1) and the reduction of hydrophobic compounds (fraction 4). The value of the CA was highly correlated with hydrophobic components (R2 = 0.93) content, but not with the hydrophilic HS. Hydrophobic components of soil HS are indigenous formations, spatially localized with humification products of organic material in situ within the aggregates (Shein & Milanovskii, 2003). Our results indicate the heterogeneity of sur