Elizaveta Ilyinichna Karavanova

The dynamics of mineralization of organic substances in soil solutions sampled by the tension lysimeters from podzolic soil has been studied. The stability of dissolved organic matter (DOM) varies with the horizon. The greatest content of resistant to biodegradation DOM (was mineralized less than 50%) was found in the upper organic horizons, down the profile it is decreased. The fraction of mineralized carbon is inversely proportional to the relative content of hydrophobic and phenolic fractions in DOM. The lowest percentage of mineralized carbon (23%) resides in the upper Elhi-horizon, where DOM is contained maximum of hydrophobic fraction (44%). In the course of oxidative transformation the degree of aromaticity of the DOM and the proportion of phenolic compounds increased (SUVA280 - by 2-12 times; phenols - 2 to 10 fold), while molecular masses decreased up to 5-5,5 kDa. These changes are manifested more intensively in solutions from the lower horizons, thus, the non-decomposable residue of this DOM is represented by substances most resistant to the decomposition.

The resistance to mineralization of water soluble organic matter (WSOM) extracted from a litter of Podzol is increased upon the interaction with the mineral horizons. Both the fraction of WSOM susceptible to mineralization and the average rate of decomposition decrease after the passage through E-horizon, by 2-3 times respectively, in comparison with the values obtained for the initial WSOM. Sorption in the BF horizon gives extra stability to the incoming organic matter. The relative content of mineralized carbon is reduced to 2% (by 8 times). In natural soils sorbed organic matter can be even more resistant to the mineralization, due to lower temperatures and less biological activity, so it can be considered as the basis for the formation of a humus storage. Taking into account the calculated rate of mineralization, the accumulation of the organic carbon in the BF horizon due to WSOM sorption, may range up to 13,5/100 g per year, but this value depends on the concentration of organic substances in soil solutions, hydrological regime and other factors.

The composition of water-soluble organic matter (WSOM) of coniferous-deciduous, coniferous and moss litter was studied. Type of decay affects the composition of WSOM. Sphagnum litter contains less N and dissolved organic carbon (DOC), but more carbohydrates, the WSOM is characterized the lowest extinction coefficients Е₂₅₄. WSOM from mixed litter with a predominance of birch litter has a maximum content of N and a small amount of soluble phenolic (SP) substances. WSOM from litter of pine needles differ from the rest with a lower content of carbohydrates, the maximum amount of SP and hydrophobic fraction. All investigated WSOM are characterized by high rates of decomposition: half-decomposition time of their stable fractions is 2-4 months. WSOM from pine litter at pH ~ 6 are absorbed by the BF horizon of Podzol, since the concentration of DOC of 100 mg/l. Substances from birch-spruce litter in similar conditions adsorbed slightly.

The migration of the water-soluble fraction of petroleum hydrocarbons in typical soils of Bashkortostan is studied. In a lysimetric model experiment, the possibility of water migration of petroleum hydrocarbons in Fluvic Phaeozems, Haplic Phaeozems and Amphiskeletic Pantocalcic Chernozems was revealed. Among the dissolved organic substances that pass into lysimetric waters from oil-contaminated soils, the components that are not extracted by hexane, but have fluorescence in water at the wavelengths typical for petroleum products are revealed. These substances differ from soluble organic matter of background soils by higher HIX humification indices and presumably represent oxidized products of transformation of petroleum hydrocarbons. Lysimetric waters from all oil-contaminated soils are characterized by biotoxicity, regardless of the presence of non-polar petroleum hydrocarbons. The toxicity T-index positively correlates with the content of dissolved organic carbon and chloride ions that accompany soils oil pollution.

Water-soluble organic substances (WSOM) of sod and humus horizons of Mollic Gleysol artificially saturated to full moisture capacity were isolated by sequential centrifugation from pores with a diameter of more than and less than 30 microns. The total concentration of carbon and phenolic compounds, the absorption and fluorescence spectra of the solutions do not differ significantly, but a strong variation in the parameters (the ratio of maximum and minimum values is 415 times) indicates the heterogeneity of the composition of the extracted substances. The concentration of simple phenolic acids in solutions of large pores of the humus horizon is 48 times higher than in smaller ones, salicylic, benzoic and cinnamic acids predominate (75% of the total amount). In small pores, p-hydroxybenzoic and benzoic acids predominate (67% of the total amount). Dissolved organic matter (DOM) of soil solution of the Podzolic soil (ELBTg horizon, moisture 22%), were obtained by tension lysimeters from pores with a diameter of more and less than 14.7 microns. DOM in solutions from large pores contain more C, N, phenolic compounds and, according to the absorption spectra, have a large molecular weight and a degree of aromaticity. Organic substances from small pores are more hydrophobic and are characterized by a greater C:N ratio; these features potentially promote DOM resistance to decomposition. Predominant localization of phenolic substances in large pores is favourable for the conservation in soil since transport with water flows assist their chemical stabilization in the lower horizons as organo-mineral compounds.