Chemical fractionation of Cu, Ni, Pb and Zn in a mine soil amended with compost and biochar and vegetated with Brassica juncea L.

• Amending increased the metal retention by soil compounds.
• Organic matter and pH were the major component for immobilize metals.
• Amending decreased the mobility of Cu, Ni, and Pb in soils.

A greenhouse experiment was performed to evaluate the effects of organic amendments (compost and biochar) and vegetation (Brassica juncea L.) on the metal fractionation of Cu, Ni, Pb and Zn in a soil of a depleted copper mine. The influence of organic amendments alone (compost + biochar) and combined with mustard plants (B. juncea L.) was assessed. The addition of increasing doses (20, 40, 80 and 100%) of compost and biochar to the soil allowed plants both to survive and grow, as mustard plants were not capable of surviving in the untreated mine soil. These materials improved the soil conditions by increasing the soil pH values from 2.7 to 8.7, soil C from values below the detection limit to 149 g kg− 1, soil N from values below the detection limit to 11.1 g kg− 1, generally reduced the CaCl2-extractable concentration of metals in the soil, reduced the mobility factor of Cu (from 19.4 to 2.4), Ni (from 52.7 to 7.3) and Pb (from 29.9 to 4.6) and they also reduced metal concentrations in the mobile soil fractions. However, the used compost increased the Zn pseudototal concentration in the amended soils from 73.6 mg kg− 1 to 330 mg kg− 1. It was concluded that the combined use of compost and biochar with B. juncea provides scarce additional benefits for remediating a Cu-contaminated mine soil compared with only the addition of the organic amendments.