Behavior of Cd and Zn in the rhizosphere of Brassica plants grown in a sewage sludge-amended soil and in a soil contaminated with Cd and Zn

摘要

Behavior of Cd and Zn in the rhizosphere soil and the uptake of the metals by Brassica plants were investigated by using the rhizobox system filled with a paddy soil contaminated with Cd and Zn at the levels of 4.24 and 398 mg kg~(-1) dry soil, respectively (A soil), and a sewage sludge-amended soil contained Cd and Zn at the level of 0.57 and 302 mg kg~(-1) dry soil, respectively (B soil). The soil pH values in the root-growing zone compartment (R.C.) and 0-1 mm zone compartment (0-1 mm Z.C.) were decreased (A soil: from 5.50 to 4.70, B soil: from 6.43 to 6.24) after 34 days from seeding of the plant. One M NH_4Cl-extractable Zn concentrations in R.C. and 0-1 mm Z.C. of the rhizoboxes in A and B soils were found to increase, while the NH_4Cl-extractable Cd concentrations in A soil of these compartments decreased. Under the present experimental conditions, it is considered that Zn in both soils and Cd in B soil moved from the outer compartments toward R.C. by mass flow. On the other hand, it was suggested that Cd in R.C. and 0-1 mm Z.C. in A soil moved mainly by diffusion because the amount of Cd taken up by the plants was much larger than that supplied to the root surface. The difference in behavior of Cd in the two rhizosphere soils of Brassicaplants seems to be a result from the different concentration of the NH_4Cl-extractable Cd, different pH of the soils, and/or different percentage of NH_4Cl-extractable Cd. These findings concerning the difference in behavior of Cd in the rhizosphere maybe important for phytoremediation.