Speciation transformation of Cd、Pb in rhizosphere and their effects on plant

A sequential extraction technique was adopted to study the speciation of Cd and Pb in rhizosphere and their effects on wheat by a rhizobag pot experiment. The results showed that, (1) most of Cd existed in the residual species at the slight pollution of Cd (≤1 mg/kg). The exchangeable Cd and carbonate-bound Cd became the predominant species, and the residual Cd significantly decreased at the higher level of Cd (≥5 mg/kg). These results demonstrated that the residual species of Cd transformed to ex-changeable and cabonate-bound species in rhizosphere with the increase in Cd pollution. Therefore, the phytotoxicity of Cd was stronger in rhizosphere. The predominant species of Pb were the organic-bound and Fe-Mn oxide-bound. There was a little amount of exchangeable Pb. The predominant species of Pb did not change with the increase in Pb pollution. (2) The absorption of Cd or Pb by wheat increased with the total amount and concentration of species of Cd or Pb in soils, whereas the fresh weight of wheat decreased. The toxic effect of Cd on the growth of wheat was more significant than that of Pb. As the added mass fraction of Cd reached 5 mg/kg the fresh weight of wheat markedly decreased. As the added mass fraction of Pb was 500 mg/kg, the fresh weight of wheat decreased.