Repeated Land Application of Biosolids in Quebec: Impacts on Soils, Dairy Milk Quality and Greenhouse Gas Emissions

In the Saguenay region (Quebec, Canada), a field study was done to evaluate metal accumulations of the soil surface layer (0–20cm) with 26 farm fields that received a mean of 7 annual spreadings of municipal biosolids between 1991 and 2006. This corresponds to a cumulative mean loading of 20 t/ha (d.w.b.). Spreadings were in conformity with Quebec regulations and criteria. Metal accumulations were evaluated by comparing the surface layer to the 40–60 cm layer, as a control, and by using loading calculations.Cumulative spreadings of biosolids on soils had no impact on cadmium and aluminium contents, but they caused a significant increase in the upper layer with total mercury and with extractable copper, lead and zinc (Mehlich 3). However, final soil contents were well below agricultural soil reference criteria used in the province of Quebec, even soils that received up to 12 applications. Because lead and mercury contents of Saguenay biosolids are decreasing since 1990, further accumulations of these metals in receiving soils are likely to be lower in the future.Repeated land application of biosolids also increased the phosphorus saturation index of soils (P/Al), still most final values remained low (