When Cu(II) contaminated Ultisol was mixed with biochar derived from straw and incubated for 120 d, acid-soluble Cu(II) decreased by 0.08-0.33 mmol/kg due to the liming effect of biochar; 1.00-1.93 mmol/kg due to organic functional groups of biochar when it was added to the soil at 30 g/kg, and by 1.40-2.43 mmol/kg at 50 g/kg. The total functional groups and volatile matter (VM) were significantly related to Cu(II) immobilization (P < 0.01), suggesting that it is functional groups in VM that are essential to Cu(II) immobilization in soil. The percentage of acid soluble Cu(II) decreased from 43.07% for the control, to 18.83-27.45% and 11.03-20.97% for the treatments with 30 and 50 g/kg of crop straw biochars added, respectively. The immobilized Cu(II) was primarily transformed to reducible and oxidizable forms. Biochar could retain Cu(II) for at least 120 d, indicating the long-term stability of biochar in Cu(II) immobilization. (C) 2013 Elsevier Ltd. All rights reserved.