Andrey Anatolyevich Stepanov

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.