The aim of the study was to determine the abundance of soil microbiome and enzymatic activity in a control soil sample and in samples with post−mining sediment containing high concentrations of arsenic and some trace elements. The samples were collected under two−year−old seedlings of Norway maple (Acer plantanoides L.) and small−leaved lime (Tilia cordata Mill.), which came from the Pniewy Forest District (western Poland). Optical emission spectrometry with excitation in plasma induced by the Agilent 5110 ICP−OES spectrometer was applied to analyse the concen− tration of arsenic and other heavy metals in the substrates and plant material. Before analysis the samples were mineralised with concentrated (65%) nitric acid (V). The count of selected groups of soil microorganisms was measured with the serial dilution method by Koch. The microbiome was measured on selective media. The activity of dehydrogenases as well as acid and alkaline phosphatase was measured spectrophotometrically. The soil and sediment on which the seedlings grew were characterised by the following physicochemical parameters: soil texture (control – loamy sand, sediment – silt), pH (control – 5.85−5.92, sediment – 7.13−7.43), the content of phosphorus (control – 117−121 mg/kg, sediment – 38−46 mg/kg), potassium (control – 6.97−7.03 mg/kg, sediment – 57−61 mg/kg), nitrogen (control – 0.53−0.65 mg/kg, sediment – 1.7−2.1 mg/kg) and total carbon (control – 9.4−10.2 g/kg, sediment – 54.2−57.2 g/kg). The samples contained: control – 0.24 mg/kg, sediment – 282 mg/kg of Astotal, control – below the detection limit, sediment – 1,766 and 16,220 mg/kg of As(III) and (V) respectively. The counts of the microorganisms were lower (103− −105) than in typical arable soils (105−109). The counts of bacteria and actinobacteria under the lime seedlings in the control were greater than in the sediment, whereas the counts of moulds were roughly identical. The counts of bacteria and moulds in the sediments under the maple seedlings were greater than in the control, whereas the counts of actinobacteria were roughly identical. The sediments under both species exhibited higher alkaline and acid phosphatase activity than the control soil sample. Only dehydrogenases exhibited higher activity in the control soil under both tree seedlings.