Reuse of liquid, dewatered and composted sewage sludge on agricultural land: effects of long-term applications on soil and crop.

To evaluate the effects of repeated sewage sludge applications in comparison to mineral fertilisers on a winter wheat - maize - sugar beet rotation, a field experiment on a silty-loam soil, in the eastern Po Valley (Italy), was carried out since 1988. Municipal-industrial wastewater sludge as anaerobically-digested, belt filtered (dewatered), and composted with wheat straw, has been applied at 5 and 10 Mg DM ha-1yr-1.. Biosolids gave crop yields similar to the highest mineral fertiliser dressing. However, with the higher rate of liquid and dewatered sludges, excessive N supply was harmful, leading to wheat lodging and poor quality of sugar beet and wheat crops. From this standpoint compost use was safer. Biosolids increased organic matter (OM), total N, and available P in the soil and reduced soil alkalinity, with more evident effects at the highest rate. Compost caused the most pronounced OM topsoil accumulation. Significant accumulations of total Zn and Cu were detected in amended topsoil, but no other heavy metals (Cd, Cr, Ni, Pb), whose total concentration remained well below the hazard limits. Biosolid applications significantly increased the content of N, P, Zn, and Cu in wheat grain, N and Cu in sugar beet roots, and only Cu in maize grain. Although, the application of biosolids brought about notable benefits to soil fertility, it is associated with possible negative effects on water quality due to increased P availability and on soil ecology due to Zn accumulation.