Declines in soil quality can be caused by trace metal contamination from a variety of sources. In addition to direct adverse effects on crop yield and/or quality, trace metals may cause more subtle effects on the structure and function of soil microbial communities. Various Abioassay® procedures have been advocated to measure directly the adverse biotic effects of trace metals, but few rigorous evaluations of their capabilities have been conducted. In this study we are quantitatively comparing five bioassay approaches to assess soil quality changes caused by Cu contamination. The assays are: (i) a short-term root elongation test; (ii) a direct solid-phase bioassay (MetPLATE) based on b-galactosidase activity of a specific strain of E. Coli; (iii) a measurement of microbial genetic diversity using DNA reassociation techniques; (iv) longer-term growth of plants (wheat) in columns designed to simulate field conditions; and (v) activities of selected enzymes in leaves of the same plants. In order to achieve a wide range of Cu stress levels, three California soils were spiked with varying rates of copper sulfate, a high-Cu sewage sludge, or a high-Cu swine manure. With the exception of Cu salt-amended Domino soil, results of the root-elongation assay consistently show a reduction in root length with increasing copper application. In general, the MetPLATE bacterial assay yielded similar results, but was more sensitive to Cu additions in sewage sludge. Initial results from the DNA reassociation assay indicate (i) an increase in diversity with increasing rates of organic amendment, and (ii) a smaller reduction of diversity in the CuSO₄ than expected based on its toxicity toward root growth and b-galactosidase activity. The results of these experiments will contribute to improved methods for the assessment of soil quality degradation in California.