Root-induced changes of lead availability in the rhizosphere of Oryza sativa L.

Geogenic, as well as anthropogenic heavy metals from distant sources, gradually increase the level of toxic metals in natural environments and these will be increasingly taken up by plants and transferred further up the food chain. To control any detrimental environmental impacts of soil contaminants, it is desirable to investigate the solubility and mobility of soil chemical contaminants under land-based agricultural systems. In this paper, a greenhouse experiment was conducted to assess the effects of rhizospheric dynamics on the bioavailability of lead. Rice (Oryza sativa L.) and Ultisol were used as the test materials. The results showed NH4OAc extractable Pb in rice rhizosphere was much higher than in bulk soil. which meant that the activation processes in the rhizosphere was significant and the amount of bioavailable Pb increased. The distribution of extractable Pb at a distance of 0-10 mm from the root compartment. however. varied and had a little depletion compared to that in the rhizosphere. The significant difference of Pb forms between the rhizosphere and bulk soil further demonstrated the root-induced changes of lead bioavailability in the rhizosphere might also be related to the various levels of Pb treatment and the simultaneity of Cd. It was observed the accumulation of soluble plus exchangeable Pb (SE-Pb) in the 500 mg kg(-1) Pb treatment, and relatively stronger fixation of Pb to oxides in the 250 mg kg(-1) Pb treatment. The activation of SE-Pb in rice rhizosphere was weaker in the 5 mg kg(-1) Cd treatment and stronger in the 10 mg kg(-1) Cd treatment than that in the treatment without Cd. The activation of lead in the rhizosphere of Oryza sativa L., suggested a potential risk of heavy metal accumulation and phytotoxicity in plants and soil ecosystem. (C) 2004 Elsevier B.V. All rights reserved.