objective  The mobilization mechanisms of cadmium (Cd) and zinc (Zn) by solid and liquid phase of rhizosphere soil of Helianthus annuus L.(sunflower) and Ixeris polycephala Cass were investigated to compare the remediation potential of the two plants in light and moderate Cd-Zn contaminated farmland soil.

  Method   Sunflower and Ixeris polycephala Cass were planted in field trials. The pH value, organic acid concentration, total concentration and bioavailable concentration of heavy metals in soil from maturity period were determined. The concentration of dissolved organic matter (DOM), ions, water-soluble heavy metals and its morphological distribution were calculated. The concentrations and chemical forms of heavy metals in different parts of plants were determined. The bioconcentration factors (BCF) and transport factors (TF) of heavy metals in plants were calculated to compare the remediation potential between two plants for heavy metal pollution soil.

  Result  The pH value of rhizosphere soil was decreased significantly by low molecular organic acids secreted from the root of sunflower and Ixeris Polycephala Cass, which was significantly lower than those in non-rhizosphere (P < 0.05). The concentration of low molecular organic acids and DOM in the rhizosphere soil of Ixeris polycephala Cass were significantly higher than those in the rhizosphere soil of sunflower(P < 0.05). The Cd in the rhizosphere soil solution of the two plants was mainly in the ionic state and DOM-bound state, and the Zn was mainly in the ion state. No significant difference wwa observed for the concentration of bioavailable Cd in rhizosphere soil between the two plants, the bioavailable Zn in the rhizosphere of sunflower was significantly higher than that of Ixeris Polycephala Cass. The concentrations of bioavailable Zn and Cd in rhizosphere soil between two plants were correlated with Cd-DOM and Zn-DOM in soil solution. The root of Ixeris polycephala Cass showed an outstanding accumulation heavy metal ability, but sunflower could absorb and transport more Cd and Zn from root to shoot, and the heavy metals were accumulated with undissolved phosphate and oxalate form in leaves of sunflower.

  Conclusion  The organic acids secreted by rhizosphere of two plants could increase the concentration of Cd-DOM and Zn-DOM in rhizosphere soil and improve the availability of Cd and Zn in soil. The root of Ixeris polycephala Cass presented remarkable mobilize ability to Cd and Zn, but the shoot of sunflower had outstanding absorption and transport ability for Cd and Zn. Both sunflower and Ixeris polycephala Cass were have remediation potential for heavy metal in soil, but sunflower was more suitable for remediation of heavy metal contaminated farmland in China from the economic point of view.