Biochar and Eisenia fetida (Savigny) promote sorghum growth and the immobilization of potentially toxic elements in contaminated soils

Biochar is a soil amendment capable of influencing plant growth and potentially toxic elements (PTEs) bioavailability in soils. At the same time Eisenia fetida (Savigny) is able to interact with biochar influencing its performance. As such they could constitute a resource for assisted phytostabilisation of PTE-polluted soils. To this end, a softwood-derived biochar was added at 2 and 5% (w/w) rate, with and without E. fetida, to a soil contaminated with Cd, Pb, Zn, As, and Sb, to evaluate the PTE phytostabilisation potentials when combined with Sorghum vulgare. The combination of sorghum, 5% biochar, and earthworms reduced the mobility of most PTEs in soil (e.g., up to 65% and 60% for Pb and Zn), while sorghum biomass was greatly increased (i.e., ~ 3- and 2-fold for roots and shoots, respectively). Biochar addition alone reduced the PTE uptake by plants, while the presence of earthworms slightly increased it. Overall, the joint action of biochar and earthworms increased the PTE removal efficiency by S. vulgare compared to control plants (e.g., the amount of root As, Pb and Sb was ~ 5-, 4- and 3-fold higher, respectively). Although 2% biochar didn’t affect E. fetida fitness, the highest biochar rate (5%) exhibited toxic effects (the survival rate reduced by ~2-fold; weight loss increased by ~3-fold). Taken together, these results indicated that S. vulgare, in combination with softwood biochar and E. fetida, could be used for the assisted phytostabilisation of PTEs contaminated soils