How do properties and heavy metal levels change in soils fertilized with regulated doses of urban sewage sludge in the framework of a real agronomic treatment program?

Purpose This field study was performed to assess the variation in chemical and agronomic properties and total and extractable concentrations of heavy metals in soils fertilized with regulated doses of urban sewage sludge (USS) for 6 consecutive years in the framework of an agronomic treatment program. Materials and methods Chemical and agronomical properties, total contents and extractable concentrations of Cd, Cr, Cu, Hg, Ni, Pb and Zn were determined in agricultural soils treated with USS for 6 consecutive years, agricultural soils cultivated using mineral fertilizers and uncultivated soils representative of the local geochemical background. Results and discussion USS application caused a decrease in pH and an increase in extractable concentrations of Cr, Cu,Pb and Zn. No organic carbon, total nitrogen and total phosphorus enrichment trend was observed in the treated soils due to biodegradation of the organic compounds supplied by USS.The decomposition of USS organic matter was presumably the main process responsible for the pH decrease in the USS-fertilized soils. There was no heavy metal accumulation in treated soils, and total heavy metal contents were below the corresponding maximum threshold concentrations set by European and Italian legislation. Increased availability of Cr, Cu, Pb and Zn was found in treated soils due to an increase in their extractable concentrations in the treatment period. Conclusions The results of this study suggest that the environmental risks related to the accumulation and availability of heavy metals in agricultural soils fertilized with USS are limited when treatment observes recommended doses in agronomic treatment programs.