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

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 e xperiment to evaluate the ecotoxicological risks associated with the dispos al of sewage sludge composts (SSC) containing trace metal contents below th e recommended limit values established by the French legislation. A crop of colza (Brassica napus) was grown on the amended soils. Based on the result s from this experiment, SSC application enhanced the nutritional levels of the soils and stimulated the growth of plants. Chemical fractionation of co mposts and amended soils was performed using a sequential extraction techni que. The pattern of metal distribution in the five fractions gave a good pr ediction of the trace metal behaviour in soils. Among the metals studied, Z n was the one that moved most readily in the soils (except in the calcareou s soil) since compost application increased the proportion of the most labi le forms of Zn and especially its pH 3.7 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 amende d layers during the sludge-soil equilibration period. However, leachates we re very little affected by SSC application as metal redistribution took pla ce in deep layers of these amended soils. The movement of Pb within the pro files of the amended soils was not pH dependent whereas the mobility and pl ant 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 (excep t Na). Zn contents in leaves were governed by SSC loading rates. Principal component analysis performed on the 'total' metal and nutrient concentratio ns 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 othe r hand that the nutrient and trace metal contents in plants were more influ enced by soil nature than compost application. (C) 1999 Elsevier Science B. V. All rights reserved.