ASSESSMENT OF FREE-LIVING NITROGEN-FIXATION ACTIVITY AS A BIOLOGICAL INDICATOR OF HEAVY-METAL TOXICITY IN SOIL

The suitability of free-living N2-fixing bacteria as biological indicators for heavy metal toxicity in soil was assessed. The sensitivity of potential N2-fixation activity by cyanobacteria and aerobic heterotrophic bacteria were assessed using the acetylene reduction assay (ARA) on contaminated soils containing various concentrations of Zn, Cu, Ni, Cd, Cr and Pb. Soils from three long-term experiments (Woburn, Lee Valley, Luddington sites) were used which had previously been treated with metal-contaminated sewage sludge. A gradient of increasing heavy metal concentrations was made up by mixing uncontaminated and metal-contaminated Woburn soils. On the Woburn soils, concentrations of heavy metals in soil close to or below the current U.K. permissible limits (except for Cd which was greater than the limit) led to 94% inhibition of heterotrophic and up to 98% reduction of cyanobacterial N2-fixation activity. Heterotrophic activity was also strongly inhibited in the Lee Valley soils. On the other soils, little or no N2-fixation activity could be measured even in uncontaminated soils possibly due to low soil pH, the presence of mineralisable N in the soil, or absence of the microorganisms. Our results indicate that both methods are only of limited suitability for widespread use as biological indicators of metal toxicity. Assessment of heterotrophic N2-fixation is more promising in this respect than measurement of cyanobacterial activity, as results are obtained more easily and quickly.