Effect of anthracene and pyrene on dehydrogenases activity in soils exposed and unexposed to PAHs

The effect of anthracene and pyrene on dehydrogenases activity was compared between freshly spiked soils and soils previously exposed to PAHs. Four different soils ( two soils from an unpolluted rural area and two soils from highly polluted industrial areas) were spiked with anthracene and pyrene at the levels of 1, 10, 100 and 400 mg Sigma 2PAH per kg of soil ( laboratory experiments). The soil samples were moisted to 60% of water holding capacity and incubated in the dark at 18-20C. Dehydrogenase activity was determined after 7 and 30 days. The results indicate that long-term exposure of soil from an industrial area to high level of PAHs (Sigma13PAH = 40 mg kg(-1)) caused the acclimatisation of soil microorganisms and lowered their sensitivity to these compounds. After the longer incubation period ( 30 days vs. 7 days) the soil microorganisms in the uncontaminated rural samples showed an increase in their tolerance to the freshly introduced PAH compounds; in the industrial soils the effect was less pronounced. However, this apparent adaptation of soil microorganisms may be also partly explained by the dissipation and/or ageing processes of anthracene and pyrene during incubation.