Indigenous earthworms and gut bacteria are superior to chemical amendments in the remediation of cadmium-contaminated seleniferous soils

The natural selenium (Se)-rich areas in China are generally characterized by high geological background of cadmium (Cd) which poses potential risks to human health. Therefore, immobilization of Cd is the prerequisite to ensure the safe utilization of natural seleniferous soil resources. A pot experiment was conducted to compare the effects of indigenous earthworm (Amynthas hupeiensis) and its gut bacteria (Citrobacter freundii DS strain) on the remediation of Cd-contaminated seleniferous soil with two traditional chemical amendments. The results indi-cated that earthworms and DS strain decreased DGT-extractable Cd by 25.52 -41.53% and reduced Cd accu-mulation in lettuce leaves by 20.83 -37.50% compared with control through converting the exchangeable Cd (EX-Cd) into residual Cd (RE-Cd) fractions. Overall, earthworms and DS strain were more effective in Cd immobilization, growth and quality promotion, oxidative stress alleviation, Cd accumulation and bioaccessibility reduction in the soil-lettuce-human continuum than biochar and lime. Moreover, all amendments induced the antagonism between Se and Cd through increasing bioavailable Se/Cd molar ratios in soil. However, all the Cd concentrations in lettuce exceeded the maximum permissible limit of Cd for leaf vegetables, indicating that soil amendment alone could not ensure food safety. This study confirmed that biological amendments were superior to chemical amendments in the remediation of Cd-contaminated seleniferous soil.