Ecotoxicological assessment of heavy metals in sewage sludge amended soils

Contamination of soils by potentially toxic elements (PTEs) (e.g. Zn, Cu, N i, Cr, Pb, Cd) from amendments of sewage sludge is subject to strict contro ls within the European Community in relation to total permissible metal con centrations, soil properties and intended use. This paper highlights the ne ed for ecotoxicological data for the assessment of PTE impacts in addition to geochemical data alone. The soil microflora plays an essential role in d etermining levels of soil fertility, being intimately associated with the b iogeochemical cycling of essential plant nutrients and the turnover of orga nic carbon. The measurement of soil microbiological parameters can provide insight into the impact of PTEs upon soil fertility, where geochemical anal ysis alone can often be inadequate to assess contaminant effects on essenti al components of the soil ecosystem. Microbial investigations were conducte d on soils sampled from a well-controlled field experiment previously amend ed with specific types and rates of sewage sludge. Key microbiological para meters measured included the activity of the dehydrogenase enzyme and the p resence and number of effective nitrogen fixing cells of Rhizobium legumino sarum biovar trifolii capable of nodulating the host plant, white clover (T rifolium repens). Results were evaluated with respect to maximum permissibl e concentrations of PTEs in sludge amended soils, as stipulated under UK li mit values and the European Directive 86/278/EEC. Important effects on the size of the Rhizobium population and dehydrogenase activity were apparent i n soils samples in relation to the soil pH, sludge type, addition rates and the concentrations of PTE present. (C) 2001 Elsevier Science Ltd.