**3.2 Crude oil test-substrate**

The crude oil as hydrophobic compound was applied as follows: crude oil (4 ml of oil corresponding to 3200 mg) was added to 10 g of dried and dispersed soil and then 10 g of the soil was mixed with fresh moist soil equivalent to 100 g of dry material. The final substrate concentration was 27.43 mg C g-1 soil. Since the content of SOM in the initial dry soil sample was about 19.6 mg C/g DS, the share of oil hydrocarbons introduced into the soil exceeded 1.4-fold the quantity of SOM. Assuming that the major part of crude oil spilled over the soil is contained in the upper 10-cm layer, we find that the supposed degree of soil pollution will be about 32 tons per 1 ha.

The carbon isotope composition of the oil hydrocarbons used in these experiments was characterized by a 13C value of -28.4 0.1 ‰, the light and heavy oil hydrocarbon fractions having values -28.9 %o and -27.2 %o, respectively. The isotopic characteristics (13C) of the oil used in the experiments were found to be close to the samples of crude oil from oilfields of the Arabian region, where the 13C value was –27.5 0.5 ‰ for oil, -28 0.5 ‰ for alkane fraction, and -26.5 1.5 ‰ for the fraction containing mainly aromatic hydrocarbons, respectively (Belhaj et al. 2002).
