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Amending contaminated soils with organic wastes can influence trace element mobility and toxicity. Soluble concentrations of metals and arsenic were measured in pore water and aqueous soil extracts following the amendment of a heavily contaminated mine soil with compost and biochar (10% v:v) in a pot experiment. Speciation modelling and toxicity assays (Vibrio fischeri luminescence inhibition and Lolium perenne germination) were performed to discriminate mechanisms controlling metal mobility and assess toxicity risk thereafter. Biochar reduced free metal concentrations furthest but dissolved organic carbon primarily controlled metal mobility after compost amendment. Individually, both amendments induced considerable solubilisation of arsenic to pore water (>2500 μg l−1) related to pH and soluble phosphate but combining amendments most effectively reduced toxicity due to simultaneous reductions in extractable metals and increases in soluble nutrients (P). Thus the measure–monitor-model approach taken determined that combining the amendments was most effective at mitigating attendant toxicity risk.
•Compost and biochar addition to a mine soil decreased metal solubility.•Inorganic arsenic solubility was increased furthest by compost alone.•Combining amendments most effectively reduced toxicity, assessed by bio-assays.•Field study should verify the efficacy of the amendment in the longer term.
Individual compost and biochar amendments to a contaminated mine soil decreased metal and increased arsenic solubility, but combining amendments most effectively reduced toxicity.