Behavior of Cd and Zn in rhizosphere ofBrassicaplants grown in an andosol contaminated with Cd and Zn

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Abstract
Behavior of Cd and Zn in the rhizosphere soil and their uptake by two species of Brassica plants were investigated by using a rhizobox system filled with an Andosol contaminated with Cd and Zn (4 .24 Cd and 398 Zn mg kg−1 dry soil). The Brassica plants used were hybrid 60 / 31 (BT, Brassica juncea × Thlaspi caerulescens) and komatsuna (Brassica campestris var. rapa). By 34 d of plant cultivation, the soil pH values in the root-growing zone compartment (R.C.) and 0–1 mm zone compartment (0–1 mm Z.C.) decreased from 5.5 to around 4.6 in BT and to around 4.9 in komatsuna. The 1 M NH4Cl-extractable Cd concentrations in the soils of R.C. and 0–1 mm Z.C. of the rhizobox decreased, while the NH4Cl-extractable Zn concentrations in the soils of these compartments increased. Under the present experimental conditions, it is considered that Zn moved from the outer compartments toward R.C. by mass flow. On the other hand, it was considered that Cd in R.C. and 0–1 mm Z.C. moved mainly by diffusion because the amount of Cd taken up by the plants was much larger than that supplied to the root surface by mass flow of Cd. The amounts of Cd absorbed by the plants were similar to those of NH4Cl-extractable Cd which decreased in R.C. and 0–1 mm Z.C., suggesting that plant uptake of Cd was mainly confined to R.C. and 0–1 mm Z.C. It is thus important to select plant species with a large root surface area for effective phytoremediation of soil Cd.