One of the best options for final disposal of sewage sludge is its use for agricultural purposes. Excessive heavy metal concentrations in sludge, however, pose a risk to the environment. It is therefore important to determine not only the availability, but also the distribution of such metals in different soil fractions as a result of sludge application. Our study aimed at evaluating both the redistribution of Zn among soil fractions and the Zn-DTPA available concentration in sludge-incubated soils. Samples of two contrasting soils were used in the experiment in combination with five doses of sewage sludge (0; 40.5; 81; 162; and 243 Mg ha-1). Soil samples were collected to evaluate the available Zn content extracted by DTPA and the distribution in soil fractions after six periods of incubation (30, 60, 90, 120, 150, and 180 days). Results showed that sludge application increased the DTPA-extracted available Zn content, although the available Zn content showd no increase as a function of the incubation period. Zn mobility decreased in the sludge-incubated soils since the organically bound Zn was mainly redistributed to residual fractions. For the conditions of this study, the maximal annual loading rate of Zn through sewage sludge suggested by the USEPA (United States, 1993) did not lead to phytotoxic Zn concentrations in soils.
fractionation; Zn availability; biosolid
