Remediation of polymetallic soils using biochar and earthworms: assessing heavy metal speciation, soil microbiological activities and earthworms’ responses

The use of biochar in soil remediation has received increasing attention in recent years. Furthermore, earthworms are known to influence pollutant mobility through their biological activity. However, the combined effects of biochar and earthworms on the speciation of heavy metals and the quality of the soil have not been sufficiently investigated. The present investigation evaluated the influence of Eisenia andrei in polymetallic contaminated soils amended with three biochar applications (5 %, 10 % and 15 %) on heavy metal speciation (Pb/Zn/Cd/Ni), soil physico-chemical properties (pH, cation exchange capacity), enzyme activities (β-glucosidase, alkaline phosphatase, urease, FDA) and bacterial functional diversity. In addition, the response of earthworms was assessed using biomarkers of cytotoxicity (stability of lysosomal membranes), genotoxicity (frequency of micronuclei), oxidative stress and neurotoxicity. Our results revealed that biochar and earthworms affect soil pH in different ways. When used together, they helped reduce the mobile, easily absorbed forms of metals in the soil while increasing their stable, less available forms. This suggests that the combination could make these metals less harmful to the environment. Soil enzyme activities varied with contamination level and biochar rate. Significant improvements were observed at the 5 % biochar treatment. In addition, as evidenced by reduced oxidative stress, cytotoxicity and genotoxicity, biochar mitigated the toxic effects of heavy metals on earthworms. Our findings demonstrate that the integration of biochar, particularly at a 5 % application rate, with earthworm activity can enhance soil remediation through stabilization of heavy metals and improvement of soil properties which offer a promising green strategy for the restoration of contaminated soils.