One of the bioindicators most often applied to assess the quality of soil is its enzymatic activity. Undesirable changes in the activity of enzymes can imply excessive presence of substances which are harmful to soil environment, such as heavy metals. Being a heavy metal, zinc is also an element essential for maintaining proper functions of live organisms. The purpose of this study has been to determine the significance of changes occurring in moderately heavy soil under the influence of zinc. The experiment was carried out in three replicates under laboratory conditions. Sandy loams of pH 5.5 and 7.0 were used for the trials. The soils were contaminated with zinc according to the following design: control (natural content), raised content (70 mg Zn2+ kg–1), weakly polluted (200 mg Zn2+ kg–1 ), moderately polluted (500 mg Zn2+ kg–1), heavily polluted (1,500 mg Zn2+ kg–1) and very heavily polluted soil (5,000 mg and 10,000 mg Zn2+ kg–1). The soil samples prepared as above were brought to the moisture content of 50% maximum water capacity and incubated at 25oC for 120 days. On day 30, 60 and 120, the activity of dehydrogenases, β-glucosidase, urease, acid phosphatase and arylsulphatase was determined. Based on these determinations, the following indices were calculated: ED50, the index for resistance (RS) and the index for resilience (RL). The tests have demonstrated that as the rate of soil contamination with zinc increased, the activity of all the analyzed enzymes was significantly depressed. The negative influence of zinc contamination on the activity of particular enzymes, irrespective of the soil pH, persisted throughout the whole experiment. In respect of their sensitivity to zinc, the enzymes can be ordered as follows: arylsulphatase > dehydrogenases > acid phosphatase > urease > β-glucosidase. Zinc contamination caused lasting changes in the soil environment, but the return to the state of equilibrium was the quickest in the case of dehydrogenases (RL = 0.276), less rapid for arylsulphatase (RL = 0.173) and the slowest for acid phosphatase (RL = 0.064). In contrast, the activity of urease, instead of regenerating, was increasingly disturbed (RL = 0.350). Soil acidification was the factor that most evidently exacerbated the negative influence of zinc on the activity of β-glucosidase and arylsulphatase. Values of ED50 for the activity of particular enzymes were varied. In the soil of pH 7.0, they ranged from 3,324 mg Zn2+ kg–1 for β-glucosidase to 412 mg Zn2+ kg–1 for dehydrogenases, and in the soil of pH 5.5, they varied from 1,008 Zn2+ kg–1 for β-glucosidase to 280 mg Zn2+ kg–1 for arylsulphatase.
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