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) 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 measureemonitor-
model approach taken determined that combining the amendments was most effective at
mitigating attendant toxicity risk.