Comparison of different microbial bioassays to assess metal-contaminated soils

These experiments compared the sensitivity of four different types of bioassay over time after five metals were added to a wide range of soils at the maximum concentrations in the European Union Sewage Sludge Directive. Three were chronic assays (most probable number of Rhizobium leguminosarum, soil microbial C and Biolog substrate utilization). The fourth bioassay, an acute biosensor, employed a lux-marked luminescent bacterium (Escherichia coli) in the soil pore water. Five metals were added to 23 different soils as a mixture at Zn = 300, Cd = 3, Pb = 300, Cu = 135, and Ni = 75 mg/kg as nitrate salts and compared with unamended controls. Zinc and Cu were the metals most likely to be toxic at the concentrations used here. In the case of Rhizobium, the number of cells in soil was not affected after 11 d; however, by 818 d the numbers had decreased by four orders of magnitude with increasing concentrations of Zn and Cu in soil solution. Microbial biomass also was not affected after 11 d, but significantly decreased with increased Zn (p < 0.001) and Cu (p < 0.01) in soil solution after 818 d. Toxicity to the soil microbial biomass increased with time, whereas the toxicity to the biosensor remained the same. Biolog substrate utilization profiles were not responsive to the concentrations used here.