The response of soil microflora to increasing concentration of three pesticide preparations containing diazinon (insecticide), linuron (herbicide) and mancozeb + dimethomorph (fungicide) as an active substances, was determined under laboratory conditions. All pesticides were applied at three different dosages, i.e. 7, 35 and 700 mg kg⁻¹ soil for the insecticide, 4, 20 i 400 mg kg⁻¹ soil for the herbicide and 15, 75 and 1500 mg kg⁻¹ soil for the fungicide mixture. The lowest dosages corresponded to the maximum predicted environmental concentration (PEC) of pesticides applied in field conditions, while the other dosages were the multiplicities of PEC. After 1, 14 and 28 days of experiment the numbers of heterotrophic bacteria, fungi and bacteria involved in nitrogen transformation (nitrifying, denitrifying and N₂ -fixing bacteria) was estimated. The numbers of colony forming units (CFU) of respective groups of microorganisms were determined on selective media by means of the serial dilution technique and the spread plate method. The plate-count data indicated that pesticide used affected the numbers of tested microbial groups. All chemicals stimulated the numbers of heterotrophic bacteria and fungi at the higher dosages. However, the reverse effect was detected at the beginning of the experiment. In turn, bacteria involved in nitrogen turnover, particularly N₂ - fixing and nitrifying bacteria, were the most sensitive to the tested pesticides and their numbers were significantly reduced on each time-point. This great susceptibility of these physiological groups of microorganisms predispose them to be warning biomarkers for the side-effects of environmental pollution caused by chemicals.
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