Metal uptake by plants from sludge-amended soils: caution is required in the plateau interpretation

Uptake of metals by plants growing in sewage sludge-amended soils frequently exhibits a plateau response at high sludge loading rates associated with high total concentrations of metals in the soil. This type of response has generally been attributed to attenuation of metal bioavailability by increased sorption sites provided by the sludge constituents at the high sludge loading rates. We grew Raphanus sativus L. in a soil historically amended with sewage sludge at different rates and examined concentrations of Cd and Zn in the plants and in corresponding rhizosphere soil solution. Metal concentrations in the plants displayed a plateau response. However, concentrations of total or free metals in the soil solution did not display a similar plateau response, therefore the pre-requisite for determining that metal uptake by plants was limited by sludge chemistry was not met. It was concluded that plant physiological factors were responsible for the plateau in plant metal concentrations observed in this study. Examination of data by other authors suggests that a plateau response due to plant physiological factors has routinely been misinterpreted as being the result of only attenuation by sludge chemistry. The serious implications of an incorrect interpretation of the factors underlying a plateau response are discussed.