Influence of sewage sludge treatment and application to an acid and a neutral soil on the growth and metal uptake of Brassica Napus.

The use of sewage sludge in agriculture is an attractive alternative disposal technique as this waste is rich in organic C as well as macro- and micro-nutrients. However, its application can cause accumulation of toxic elements in soil and their transfer to plants and animals. The addition of FeCl3 as coagulant for P removal, can result in the precipitation of Fe (hydr)oxides that are known to be strong metals adsorbents, but could interfere with organic matter mineralization and affect nutrient release. The aim of our study was to quantify the effect of FeCl3 treatment of sewage sludge on growth and accumulation of potentially toxic elements of plants. Brassica napus was cultivated in pots for 50 days on an acid soil (pH 4.5) and on a calcareous soil (pH 7.8) added with 25 g kg-1 native sewage sludge and sewage sludge treated with FeCl3 as coagulant. The influence of sludge application on dry matter accumulation of Brassica napus was described with logistic functions. Both types of sludge improved dry matter accumulation significantly. Sludge coagulation with Fe did not affect plant growth in both soils, but in the acid soil its application enhanced the accumulation of Zn, Cd, Mn and Ni in plants. On the opposite, in the neutral soil this effect was not observed with the exception of Cd, which was higher in plants treated with Fe coagulated sludge on the 20-30 days of growth; this difference disappeared thereafter. Iron uptake was unexpectedly not influenced by sludge treatments.