Influence of red mud, zeolite and lime on heavy metal immobilization, culturable heterotrophic microbial populations and enzyme activities in a contaminated soil

In this study we compared the efficiency of different amendments, notably red mud (a bauxite residue), natural zeolite and lime, to immobilize the heavy metals present in a polluted acidic soil [Pb (3266 mg kg− 1 d.m.), Cd (35.4 mg kg− 1 d.m.) and Zn (1495 mg kg− 1 d.m.)] and to influence several microbiological properties. The addition of all the amendments decreased significantly (P  0.15) by the different microbial communities (richness) was following the order: red mud-soil > lime-soil > zeolite-soil = control-soil. The same trend was observed for the urease activity but not for dehydrogenase and β-glucosidase. It is concluded that all the amendments considered influenced with a different extent the heavy metal mobility in soil as well as the structure and function of the resident culturable microbial communities. Red mud was the most effective at reducing the “mobile” form of the metals considered and at promoting bacterial abundance and soil enzyme activity. Nevertheless, red mud and lime addition caused a dramatic shift of the culturable bacterial population from Gram positive to Gram negative forms. The poor catabolic activity detected with the Biolog assay for zeolite and control-soil suggested the inability of Arthrobacter and Bacillus strains to oxidize the substrates in the Ecoplate rather than reflect the actual catabolic versatility of the microbial communities.