Direct toxicity assessment of two soils amended with sewage sludge contaminated with heavy metals using a protozoan (Colpoda steinii) bioassay.

A rapid bioassay involving growth of the common soil ciliate Colpoda steinii was used to study the toxicity and bioavailability of heavy metals in soil solutions extracted from air-dried soils from two long-term experiments that were amended with sewage sludge. The sludges were predominantly contaminated with either Ni, Cu, or Zn. Growth of C.steinii was strongly inhibited in all the metal-amended soils compared with the equivalent unamended control soils. The bioassay responses were regressed with metal concentrations in soil and soil pore water to relate the biological response to heavy metal regulatory limits for soils receiving sewage sludges.