Sewage sludge effects to cropland have been evaluated in a long-term field experiment which started in 1988 and is still underway in the eastern part of the Po Valley (Italy), on a silty-loam soil. Each autumn municipal-industrial wastewater sludge has been applied at 5 and 10 Mg DM ha-1 yr-1 as anaerobically digested slurry (liquid), belt filtered material (dewatered) and composted with wheat straw, on a winter wheat - maize - sugar beet rotation.
The 15 years continuous application of sludge significantly increased organic matter, total N, Olsen P, Cu and Zn content in the topsoil, with greater effects at the highest rate. Composted sludge favoured the highest OM topsoil increase. Sludge spreading improved soil fertility and crop yields, but it was associated with possible negative environmental effects: on water, due to increased phosphorus availability, and on soil ecology, due to zinc build up in the topsoil.
The results confirmed that compulsory application rates based on real crop nitrogen requirements and matching Nitrate EU Directive, undoubtedly can guarantee a sound sustainability in sewage sludge recycling on cropland in the long run.