COPPER TOLERANCE OF THE BACTERIAL COMMUNITY IN HEAVY METAL POLLUTED SOIL TREATED WITH A NEW ORGANIC COVER LAYER

COPPER TOLERANCE OF THE BACTERIAL COMMUNITY IN HEAVY METAL POLLUTED SOIL TREATED WITH A NEW ORGANIC COVER LAYER

Oili Kiikkilä, Taina Pennanen, Janna Pietikäinen & Hannu Fritze

Finnish Forest Research Institute, Vantaa Research Center, PO Box 10, Fin-01301 Vantaa

Background: The soil bacterial growth rate has been studied using radioactive thymidine incorporation into macromolecules of bacteria. The same method has been used to determine heavy metal tolerance of the soil bacterial population. For this assay the bacterial suspension obtained from soil homogenization-centrifugation is incubated with water (control) and a range of heavy metal concentrations resulting in no inhibition to complete inhibition of bacterial growth. The growth is estimated by measuring (³H)-thymidine incorporation into bacteria. Assuming the control sample to have 100 % growth, the IC₅₀ value, that is the metal concentration resulting in 50 % inhibition, is calculated. A higher IC₅₀ value indicates a more tolerant bacterial community (Fig. 1).

Hypothesis: Bacterial communities develop tolerance against heavy metals accumulated in soil. This developed tolerance is energy consuming and unnecessary and will decrease when the bacterial community is not exposed to the stress agent anymore. Thus a reduction in heavy metal tolerance can be thought as a measure of a recovery of the soil ecosystem.

Field trial: Study plots of a heavy metal polluted forest soil (humus layer Cu_tot content:

8 000 mg kg^-1) were left untreated (Control; n=18) or covered (Cover; n=18) with a new organic layer in spring 1996. The layer was a mixture of biocompost and wood chips and was spread onto the polluted soil to reach a 5 cm thickness. Field samples from the humus layer were collected twice in 1997 and investigated for the bacterial community Cu tolerance.

Figure 1.

Figure 2.

Conclusion: The second sampling in 1997 showed a reduction of the Cu IC₅₀ value of the soil treated with the cover (Fig. 2) indicating a recovery of the soil from Cu stress. Whether this decrease is due to a changed bacterial community and/or the immobilization of free Cu will be investigated.