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 frequentl y 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 increa sed sorption sites provided by the sludge constituents at the high sludge l oading rates. We grew Raphanus sativus L. in a soil historically amended wi th sewage sludge at different rates and examined concentrations of Cd and Z n in the plants and in corresponding rhizosphere soil solution. Metal conce ntrations in the plants displayed a plateau response. However, concentratio ns of total or free metals in the soil solution did not display a similar p lateau response, therefore the pre-requisite for determining that metal upt ake 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 interpretatio n of the factors underlying a plateau response are discussed.