The overall aim of this project will be to evaluate the effects of established soil heavy metal concentrations on soil microbial activity, and consequent implications for agricultural productivity and long-term soil fertility. Phase I of the study involved the application of metal rich sewage sludge cakes at 7 sites with contrasting clay content throughout England and Wales to establish dose-response curves for Zn, Cu, and Cd. During phase II, effects of the established soil metal concentrations on selected soil microbial, chemical and physical properties will be measured. Experimental plots on each site will receive different treatments (uncontaminated sludge cakes or sludge cakes high in Zn, Cu or Cd); all plots will be managed under a ley/arable cropping regime and planted with the same grass cultivar and wheat variety. During harvest years 1998 and 2000, first cut grass yields and metal concentrations in the harvested dry matter will be determined at all 7 sites. In harvest years 1999 and 2001, wheat yields and metal concentrations in the grain will be determined at 6 of the sites. Moreover, metal uptakes will be measured at the seventh site to quantify bioavailability of the sludge metal additions to growing crops in the field and any impacts on crops will be measured. In order to assess effects of Zn, Cu and Cd sludge cake additions on soil microbial processes, the following assays will be undertaken: biomass carbon, soil respiration, rhizobia (most probable numbers) and the Biolog™ microtitre plate system for identifying microorganisms. Soil chemical properties measured will be total and extractable metals, speciation of metals, organic carbon, cation exchange capacity and soil pH. The effect of the metal additions on metal bioavailability to crops, agricultural productivity and long term soil fertility will be assessed by analysing Zn, Cu, Cd and Ni in ryegrass and wheat grain grown at each site. The annual addition of metal-rich sludge cakes at maximum permissible annual loading rates on a long-term basis (implemented in phase I) will be continued. During phase I, metal amended sludges were also applied at 3 sites in England in order to establish Zn, Cu and Cd dose response curves and to provide contrasting metal availabilities. In this phase, metal bioavailability for microorganisms and crops, agricultural productivity and long-term soil fertility will be compared on these 3 sites, following the same protocols for soil microbial, chemical and physical property measurements used for the sludge cake experiments. Close collaboration between ADAS, Rothamsted and WRc will be essential in achieving the objectives of the project; this will be facilitated by annual project meetings and additional meetings as required to effectively coordinate the programme.