Effects of medium-term amendment with sewage sludges on heavy metal distribution in soil

The effects of medium-term applications of liquid, dehydrated and composted sewage sludges on a silty loam soil were studied in a field trial in northern Italy. Once per year, randomized plots were added of liquid, dehydrated and composted sewage sludges. A sequential extraction of Zn, Cu, Ni and Pb from sludge and soil samples was performed after six years of sludge application to evaluate the heavy metal distribution in the different sewage sludges and their influence on the pool of potentially bioavailable metals in amended soils. Comparison of metal fractions in the three sludges (liquid, dehydrated and composted sludge) showed that the process of composting altered the chemical forms between the metals and the organic matrix, resulting in a shift of metals from the residual to the more labile and potentially bioavailable forms. In particular, an increase of the EDTA-extractable Zn, Ni, Pb and Cu was observed, as well as a significant increase of the NaOH-extractable Cu. With regard to soil, heavy metals were found mostly in the residual form (HNO 3 -extractable) and at the sixth year of sludge amendment, except for zinc, the soil fractionation of heavy metals was not affected by the amendment practices. A significant increase of the potentially bioavailable fractions (EDTA and NaOH-extractable) of Zn was observed.