The effects of high concentrations of copper (Cu) and zinc (Zn) in a soil treated with biosolids previously spiked with these metals on poplar (Populus deltoides × yunnanensis) were investigated in a pot trial. The total soil metal concentrations in the treatments were 12, 46, 137, and 226 mg Cu/kg and 25, 141, 433, and 686 mg Zn/kg. Copper accumulation was lower in poplar leaves than Zn and the maximum bioconcentration factor was 0.8 for Cu and 10 for Zn. Copper was not found to be toxic to plants at any level of application or to mycorrhiza up to 137 mg/kg, but it was found to be toxic to soil microorganisms at all levels of Cu addition. Copper application increased mycorrhiza colonisation up to 137 mg Cu/kg and root dry matter at 226 mg Cu/kg, but had no effect on leaf dry matter. Increasing Zn rate decreased all plant and soil parameters. Lower percentages of Cu in the soil exchangeable fraction, and a lower Cu²⁺ concentrations in soil solution relative to Zn indicated lower bioavailability of Cu. Dehydrogenase activity was reduced by 50% at total solution-phase Cu and Zn concentrations of 0.1 and 27 mg/L, respectively, and solid-phase exchangeable Cu and Zn concentrations of 5 and 169 mg/kg, respectively.