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Information on ecotoxicity of organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), in terrestrial environment is needed for establishing soil quality criteria and for risk assessment purposes. An ecotoxic effect of a model PAH compound (phenanthrene) toward soils microorganisms (nitrifying bacteria) was evaluated in 50 different soils. The soil samples were collected from agricultural land in four regions of Poland with varying levels of industrialization (Slaskie, Dolnoslaskie, Podlaskie, and Lubelskie voievodeships). Soils were characterized for basic physicochemical properties (texture, organic matter content, pH(KCl), total nitrogen content, total sorption capacity) and the content of contaminants including PAHs (73-800 microg kg(-1)), Pb (6-720 mg kg(-1)), and Zn (9-667 mg kg(-1)). Ecotoxicity of phenanthrene (applied at 10, 100, 500, and 1000 mg kg(-1)) to soils microorganisms was evaluated in laboratory studies in control conditions (incubation of soils for 7 d at 20 +/- 2 degrees C). Nitrification potential was used as the ecotoxicity measurements end point. The EC50 values (146-1670 mg kg(-1)) calculated from the square root-X linear regression model differed significantly in various soils, although it was difficult to establish a causative relationship between soil physicochemical characteristic and phenanthrene toxicity. A significant factor in the assessment of soils vulnerability to the effect of phenanthrene was level of soil contamination, particularly with PAHs. Soils with previous contamination were more susceptible (mean EC50, 325 mg kg(-1)) than soils from uncontaminated, rural areas (mean EC50, 603 mg kg(-1)).