Chapter 10 - Fractionation and bioavailability of copper, cadmium and lead in rhizosphere soil

Abstract

Chemical forms of copper, cadmium, and lead in the rhizosphere of several plant species were investigated using rhizobox cultivation, sequential extraction, and a set of specially designed experiments to examine dynamic changes in metal fractionation and bioavailability in the rhizosphere, and to experimentally evaluate the factors governing the variation in metal fractionation. The results from the dynamic change experiments demonstrated that there were continuous changes in metal fractionation within the maize rhizosphere. Initially, the amount of exchangeable copper and cadmium increased before dropping toward or below the initial levels after 40–70 days. Carbonate-associated copper followed a similar trend, but at a slower pace than the exchangeable fraction, while carbonate-bound cadmium and lead leveled off after an initial increase. The accumulation of the metals in the maize plant showed biomass-dependent characteristics. The amount of metals accumulated inside the plant material exceeded the initial quantity of the exchangeable metals in the soil, indicating a transformation from less bioavailable to more bioavailable forms. During cultivation, a decrease in redox potential, increases in pH and microbial activity, and an increase followed by a decrease in dissolved organic carbon in the maize rhizosphere were observed. The increase pattern of exchangeable copper and cadmium was affected by the relative magnitude of mobilization and bioaccumulation, which occurred in opposite directions. The effects of acidification, alkalization, and generation of root exudates were studied by the addition of acid, alkali or root exudates from the solution cultures of several plants to soil prior to incubation, and metal fractionation measurements.