Application of Zinc Oxide Reduces the Bioavailability and Specific Uptake of Cadmium by Alfalfa Grown in a Cadmium-contaminated Alkaline Soil

Zinc (Zn) and cadmium (Cd) have similar chemical properties and interact with each other in the soil-plant system. Application of Zn may be an effective way for phytoremediation of Cd-contaminated soils. A greenhouse pot experiment was carried out to grow alfalfa in an alkaline soil spiked with Cd (0, 20, and 80 mg kg –1 ) and Zn (0, 200, and 800 mg kg –1 ) as cadmium oxide (CdO) and zinc oxide (ZnO), respectively, to investigate the effects of ZnO application on soil Cd bioavailability, plant growth and Cd uptake. Adding ZnO resulted in a lower soil pH and greater DTPA-extractable Zn concentration, but a lower DTPA-extractable Cd concentration. The effect of ZnO addition on Cd bioavailability depended on both soil Cd level and Zn dose. Soil Cd contamination considerably inhibited plant growth and increased both root and shoot Cd concentrations. Adding ZnO significantly enhanced plant growth, reduced Cd uptake per unit root dry mass and plant Cd concentrations, but did not affect the total amount of Cd taken up by plants. The percentage of total plant Cd content allocated to roots was significantly lower, but the root-to-shoot translocation factor of Cd was higher when ZnO was added. Both CdO and ZnO addition had a small effect on root exudation of carboxylates. Application of ZnO to soil alleviated Zn deficiency and reduced Cd bioavailability, consequently enhanced plant growth and Cd concentrations, and it may be an effective way for phytoremediation of Cd-contaminated soils.