Distribution, movement and plant availability of trace metals in soils amended with sewage sludge composts: application to low metal loadings

Abstract

Three soils, representative of various physico–chemical properties, an acid soil from a granitic arena with a sandy texture, a calcareous soil with a sandy silty texture and a clayey and silty one, were used in a lysimetric experiment to evaluate the ecotoxicological risks associated with the disposal of sewage sludge composts (SSC) containing trace metal contents below the recommended limit values established by the French legislation. A crop of colza ( Brassica napus) was grown on the amended soils. Based on the results from this experiment, SSC application enhanced the nutritional levels of the soils and stimulated the growth of plants. Chemical fractionation of composts and amended soils was performed using a sequential extraction technique. The pattern of metal distribution in the five fractions gave a good prediction of the trace metal behaviour in soils. Among the metals studied, Zn was the one that moved most readily in the soils (except in the calcareous soil) since compost application increased the proportion of the most labile forms of Zn and especially its pH 4.7 acid-soluble forms in the amended soils. The migration of Cu and Pb within the profiles of amended soils was significantly influenced by the release of soluble organics from the amended layers during the sludge–soil equilibration period. However, leachates were very little affected by SSC application as metal redistribution took place in deep layers of these amended soils. The movement of Pb within the profiles of the amended soils was not pH dependent whereas the mobility and plant availability of Cu and Zn increased as soil pH decreased. The sequence of trace metal concentrations in all plant parts was: Zn⪢Cu>Pb. Colza leaves were the organs of accumulation of trace metals and nutrients (except Na). Zn contents in leaves were governed by SSC loading rates. Principal component analysis performed on the ‘total’ metal and nutrient concentrations in plants showed on the one hand that no deficiency in nutrients caused by interactions with trace metals arose in the plant organs and on the other hand that the nutrient and trace metal contents in plants were more influenced by soil nature than compost application.