Metal leaching following sludge application to a deciduous forest soil

Dried urban sewage sludge residues were applied to a sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) forest in Quebec at doses of 0, 200, 400 and 800 kg of available nitrogen per hectare (0, 23.1, 46.3 and 92.6 dry weight tonnes ha−1), applied in spring 1993, and doses of 200 and 400 kg available nitrogen per hectare (19.3 and 38.6 dry weight tonnes ha−1) applied in autumn 1993. The lowest dose added the following amounts of metal to the forest soil (kg ha−1): Al, 243; Fe, 682; Mn, 4.6; Cd, 0.16; Cu, 5.2; Pb, 3.0; and Zn, 15. Metal concentrations were measured in soil solution at a 30 cm depth, during the 1993 and 1994 growing seasons. Concentrations of Al, Fe, Mn and Zn reached 8, 0.1, 1, and 0.2 mg l−1, respectively, for plots receiving the lowest level of sludge treatment; these concentrations exceeded water quality criteria for the protection of aquatic life in the case of Al and Zn. Cd was only detectable for treatments of 400 kg available N ha−1 and greater, where it achieved concentrations of 18 μg l−1. Cu and Pb concentrations were less than the limits of detection. This lysimeter study indicates that there is a potential of metal leaching when sewage sludge is applied at doses equal to or greater than 200 kg available N ha−1 to soils with pH<4.5.