After substantial loading with sewage sludge corresponding to 50e60 years of normal application, we observed no detrimental effects on the function of the system, using the given methodology. On the contrary the results indicate beneficial short to mediumterm effects on the microbial community by addition of urban waste based fertilizer, compared to no addition of organic substrates. Ill effects of heavy metal loading may be masked as long as there is a massive supply of organic matter (Giller et al., 1998, 1999), and should be considered in future experiments. It may be argued that contemporary organic urban waste must be recycled in the future to preserve nutrients, and in line with recent reviews (Clarke and Smith, 2011; Eriksen et al., 2009) these results indicate that it is possible to do so in an extended period on a given site, without compromising soil functioning as long as ecotoxicological guidelines for heavy metal content are observed.
Acknowledgements
The authors thank the Danish Research Centre for Organic Food and Farming (DARCOF) for supporting the project CRUCIAL ( Closing the Rural Urban Nutrient Cycle) during 2001e2006, The Danish Agency for Science, Technology and Innovation for partial support to Pernille Poulsens PhD project ‘Biological Management of Urban Waste and Organic Waste Imprint on Soil Quality’, and the Danish Agricultural Advisory Service for partial support to the field trial during 2008e9.