Vicia-micronucleus test as a new tool to assess soil genotoxicity potential: Application to the evaluation of the effects of biochar in industrial contaminated soils

International audience; The risk assessment of industrial contaminated soils is an important challenge in remediation process. Although chemical analyses reveal the typology of pollution in a given matrix, they do not give information about the real ecotoxic potential of the matrix, which takes into account the bioavailability of pollutants. This information requires the development of biological tests, and especially with plants. Plants present indeed a particular interest in ecotoxicology due to their (i) immobility, (ii) important roots network, and (iii) fundamental role in ecosystems as primary producers (Hock and Elstner, 2005). Ma (1999) described higher plants as the most sensitive organisms for the detection of mutagens and genotoxic effects of environmental pollutants. Although ecologically relevant for soil toxicity assessment, plants are surprisingly not the most commonly used organisms for genotoxicity tests (White and Claxton, 2004). This is the reason why genotoxicity tests with higher plants have been promoted (IPCS – United Nations Environment Programme, 1999).Genotoxicity - simply defined as the toxicity on the genome - is an indicator of dysfunctions appearing at sub-lethal concentrations. An easy endpoint to observe is the formation of micronuclei, that are small nuclei appearing whenever a chromosome fragment or a complete chromosome is not incorporated into the nuclei during mitosis. It therefore reveals a break of genetic material (clastogenic effect) or a dysfunction of mitotic spindles (aneugenic effect). This endpoint is very important to include in a battery of ecotoxicity tests for a better risk assessment of contaminated soils and of the impact of remediation techniques applied to them.Among techniques of in situ remediation, soil amendments with biochar, i.e. the solid product from biomass pyrolysis, have recently been investigated for decreasing the bioavailability of metals in industrial soils. Biochar has been shown to immobilize metals both by direct sorption at its surface and by an indirect effect through an increase of soil pH (Rees et al., 2014). The effect of biochar on the actual genotoxicity potential of metal-contaminated soils is however unknown.The aim of this study was to assess the genotoxic potential of a range of soils contaminated by heavy metals (Zn, Pb, Cd) and amended by a wood-derived biochar to create a gradient of metal availability. We recently obtained international standardization of the Vicia-micronucleus test (ISO 29200) and we performed it in this study by direct exposure of root tips to soils. The analysis of root morphology and root metal content completed the experiment. Results showed that these industrial soils induced genotoxicity, revealed by an increase of micronuclei frequency. Their genotoxic potential strongly decreased with the addition of biochar. Relationships between genotoxicity, soil metal extractability and root metal content will be discussed.