**3.7 Microbial utilization of oil hydrocarbons and SOM transformation**

As follows from Table 6, the oil hydrocarbons introduced into soil were mineralized to CO2 to the extent of about 4.59 (0.2) and 4.81 (0.15) mg C-CO2 g-1 DS or 16.7 and 17.5 percents of the initial crude oil quantities in the soil over the course of 67-day exposure in experiments 1 and 2, respectively.


\*The CO2 evaluation from SOM calculated as QCO2(SOM) = vCO2(SOM +SUB)·Δt·FSOM bR= (Q biomass + exometabolites from oil carbon) / (QSOM mineralized of SOM);

cParts (%) of the initial amount of SOM and crude oil mineralized to CO2 in soil dParts of the initial amount of crude oil (in percents) consumed by microorganisms producing CO2 and organic substances (biomass and exometabolites). Standard deviations are given in brackets.

Table 6. The quantities of SOM mineralization and crude oil consumption by microbiota during the 67-day exposure in soil.

Previously (Zyakun et al. 2003), it was shown that during the growth of microbial cells on hydrocarbons the ratio of biomass and CO2 carbon quantities was corresponding 1:1. In view of the above, we believe that the quantity of oil hydrocarbons taken up for the biosynthesis of cell biomass and organic exometabolites in soil during the 67-day exposure will be close to the carbon quantity of CO2 production and make no less then 16.7 and 17.5 percents of the oil introduced in experiments 1 and 2, respectively. By this is meant that the oil hydrocarbon consumption by microbial pool in soil amounts up 33.4 and 35 percent of total oil, respectively.

Extrapolation of the obtained data (Table 6) to a 6-month season, when the temperature conditions in the Krasnodar region provide for the metabolic activity of soil microbiota, shows that the uptake of crude oil hydrocarbons by native soil microbiota may reach no more than 92±2 % of the total oil hydrocarbon quantity in the oil. At a positive PE of oil hydrocarbons in soil, there is more intensive microbial degradation of SOM compared to the processes in native soil. On the other hand, oil hydrocarbons consumed by microorganisms are spent both for CO2 production and for the biosynthesis of biomass and organic exometabolites, which then are included in SOM and transform the structure of soil. The newly synthesized metabolites and microbial biomass components can be used by other biological systems (plants, macro- and microorganisms) that are incapable of direct utilization of oil hydrocarbons. The quantitative and isotopic data obtained in the experiments were used as a basis for estimation of the degree of replacement of part of SOM mineralized to CO2 by the newly synthesized products under microbial utilization of oil hydrocarbons. Table 6 shows the rates of microbial degradation and production of cell biomass and organic exometabolites in model experiments with microbial utilization of crude oil as a substrate. As a result of oil consumption both by native soil microbiota (Exp. 1) and introduced the bacterium strain *P. aureofaciens* BS1393(pBS216) (Exp. 2), the quantity of the newly synthesized organic products (carbon of cell biomass and exometabolites) nearly 1.6-fold exceeds the carbon quantity of SOM taken up for the CO2 mineralization (Table 6, R). It means that microbial transformation of oil hydrocarbons into products available as substrates for other living systems may be a peculiar source of organic fertilizers. In addition, there is more and more evidence that the bioremediation of oilpolluted soils is companied by plant growth stimulation.
