Quantifying the effect of rhizosphere processes on the availability of soil cadmium and zinc

Our purpose was to quantify the effect of rhizosphere processes on the availability of soil cadmium and zinc to various plant species. E values for Cd and Zn were measured on two contaminated soils differing mainly in their pH (6.2 and 8.1). L values were measured for six plant species with contrasting metal uptakes. The difference between E and L values quantifies the mobilization of the element which was non-phyto-available prior to cultivation. For Zn, L values were 1.2 to 5.6 times greater than the E values, depending on the plant species. The increase in Zn availability was greater in the basic soil. For Cd, L values in the basic soil were 1.1 to 2 times greater than the E value. In the slightly acid soil, plants did not enhance the cadmium availability. The mobilization of non-labile metal could be due to exudates from roots or microflora, phytosiderophores being responsible for the highest mobilization. The lower availability of Fe or Zn could explain the greater mobilization in the basic soil.