Rates of forest floor decomposition and soil forming processes as indicators of forest ecosystem functioning on a polluted dredged sediment landfill

Results of intensive monitoring of forest floor decomposition rates and soil forming processes after afforestation of a calcareous upland dredged sediment landfill with an oxidised surface soil are presented. Nutrient status of the sediment substrate favours tree growth and allows for afforestation and thus an integration in the landscape. Soil processes on the landfill resulted in small differences between the surface and the subsurface soil layer, although higher soil organic carbon and Cd concentrations in the surface soil were observed. So far, tree species had a minor impact on soil processes. Relative to the uncontaminated covered part of the site and to general references, forest floor decomposition was found to be relatively fast for sycamore maple and pedunculate oak. Despite the pollution status of the dredged sediment landfill, the sediment substrate was favourable for forest floor decomposition. This might indicate that the soil nutrition status and the high carbonate status override the negative impact of soil pollution with metals and other pollutants. Application of a covering layer composed of uncontaminated soil material resulted in lower Cd concentrations in earthworms, but concentrations were still higher than for references. We conclude from the observations that polluted but fertile soils allow for afforestation and for regular forest floor decomposition with normal or slightly elevated metal concentrations. Only long-term observations of such new forests will lead to a correct site-specific assessment of the actual ecological risks, but after 16 years of landfilling and 12 years of afforestation no adverse effects were observed.