Populations of Cd-tolerant microorganisms in soils polluted with heavy metals

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Abstract
In order to estimate the effect of heavy metal pollution on soil microorganisms, field surveys were carried out in paddy fields, upland fields, and fallow paddy fields located in two areas with heavy metal pollution in Japan (Izuhara area, where the soil samples contained 0.2–15.0 mg Cd kg-1, 11–696 mg Zn kg-1, 0.8–16.2 mg Cu kg-1; Toyama area, where the soil samples contained 0.2–3.9 mg Cd kg-1, 4.7–146 mg Zn kg-1, 0.5–21 mg Cu kg-1). There was no significant correlation between the viable counts of microorganisms and heavy metal content of the soils. The dominance value of the bacteria tolerant to 10 mg kg-1 Cd (D 10 value; D c =log(VC c /VC0), where VC c is the viable count on the plates containing c mg L-1 of Cd, VC0 is the viable count on the plates without Cd) increased significantly with the increase of the heavy metal content in the paddy fields. However, there was no significant correlation between the D100 value of the bacteria or the D10 value of the fungi and the heavy metal content of the soils. The D100 value of the bacteria tended to be lower in paddy field soils than in upland or in fallow paddy field soils. In a polluted area, the D500 value of the fungi was higher in the fallow paddy field soils than in the paddy or upland field soils. These results suggest that the effects of heavy metal pollution on soil microorganisms vary depending on the land use.