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 < 0.05) the solubility of Pb, Cd and Zn and the increase of soil pH was identified as a common mechanism of action for both red mud and lime. Despite reducing the heavy metal solubility, the addition of zeolite and lime did not affect the number of culturable fast-growing heterotrophic bacteria whereas the red mud caused a significant increase (P < 0.05) of this soil microbial component. All the amendments reduced the number of heterotrophic fungi (P < 0.05) while only the lime addition influenced negatively the number of free-living N2-fixing bacteria. Furthermore, the addition of red mud and lime caused a significant change of the dominant culturable bacterial community. Partial sequencing of the 16S rDNA gene of randomly selected bacterial isolates revealed that dominant strains from the lime and red mud-soils were mostly belonging to ?-Proteobacteria and Bacteroidetes/Chlorobi group whilst only Actinobacteria and Firmicutes were identified in control and zeolite soils. These latter were largely colonized by Arthrobacter and Bacillus while Ralstonia and Pedobacter prevailed in soils treated with lime and red mud. Moreover, the Biolog Ecoplate assay revealed that the number of substrates utilized (OD590 > 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 b-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.