Positive Effects of the Combined Application of Compost and Biochar on Metal(loid) Immobilization and Plant Growth Depend on Their Application Quantity

A combination of compost and biochar has been proven to be a promising strategy to immobilize metal(loid)s and improve soil properties. To the best of our knowledge, no study has been conducted to investigate the effect of biochar and compost combination administered at different rates on mining technosol. Thus, we investigated the effect of different rates of compost (20%, 40%, 60% w/w) in combination with biochar (0%, 2%, 6% w/w) on soil physiochemical properties and the mobility of arsenic (As) and lead (Pb), the ability of Arabidopsis thaliana (ecotype Columbia-0) to grow and accumulate metal(loid)s. All modalities improved pH and electrical conductivity, stabilized Pb and mobilized As, but only the mixture of 20% compost and 6% biochar improved plant growth. Plants in all modalities showed a significant reduction in root and shoot Pb concentrations compared to the non-amended technosol. Whereas, As shoot concentration was significantly lower for plants in all modalities (except with 20% compost only) compared to non-amended technosol. For root As, plants in all modalities showed no significant reduction except for the mixture of 20% compost and biochar. Overall, the mixture of 20% compost with 6% biochar represents the optimum combination for improving plant growth and As uptake.