Effects of wastewater sludge and woodchip combinations on soil properties and growth of planted hardwood trees and willows on a restored site

Abstract

Sludge from wastewater treatment plants and woodchips produced from urban tree pruning residues were used to improve soil conditions of a degraded site restored by planting trees and shrubs. The release of soil nitrogen resources was set by the proportions of sludge and woodchips applied. Combining 187 kg N/ha of sludge in one application with 200 m 3/ha of woodchips instead of 100 m 3/ha reduced first year N mineralization by 50%. The same can be said for the application of 125 kg N/ha of sludge in two applications, over 2 years. The degradation of sludge–woodchips and nitrate leaching was reduced even with the smallest sludge application. Survival and growth of Acer saccharinum L., Fraxinus pennsylvanica Marsh. and Salix discolor Muhl. did not vary much with sludge and woodchips quantities. However, differential soil nitrogen availability and the degradation process of organic amendments induced by the treatments may result in better sustainable growth for planted woody species. Some undesirable elements were measured in the soil, particularly Fe, Cd, Pb and NO 3–N. Because of these, care should be taken when choosing sites to be restored using sludge and woodchips.