Assessments of the availability of toxic or nutrient metals in soils need to consider solution speciation, mobility, and the kinetics of exchange between solution and solid phase. The newly developed technique of diffusive gradients in thin films (DGT) accumulates metals in a well-defined geometry that allows quantification of their supply from the solid phase. DGT was used to measure potential maximum fluxes of available metals in old arable and ex-woodland soils variously treated with sewage sludge with and without metal amendments. The flux increased with increas ing sludge application. The results suggest that for Zn and Cd there are two separate pools of metal associated with the solid phase. In untreated soils or those with 100 m3 ha-1 yr-1 sludge treatments, Zn and Cd appear to have higher resupply rate constants from the solid phase than in soils with 300 m3 ha-1 yr-1 sludge treatments (assuming the ratio of labile solid-phase metal to soil solution metal does not change). However, the actual rate of resupply is higher in the treated soils due to the much higher concentration associated with the solid phase. For Ni and Cu there appears to be only one pool of metal irrespective of the extent of sludge treatment.

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Samples of a sandy loam soil taken from a long-term liming experiment in southeast England were amended with solutions of metal sulfate salts. Soils with a range of pHs were amended to contain Cu, Cd, or Zn at concentrations around the maximum permissible values for these metals in agricultural land receiving sewage sludge. After a 3-year equilibration period, the microbial biomass was determined by the fumigation-extraction technique. These results were compared with data from substrate utilization patterns of microbial populations extracted by using a weak salt solution. There was no reduction in microbial biomass due to pH or metal treatment in any of the soils except the Cu treatment. Principal-component analysis of the respiration patterns in Biolog plates demonstrated effects of both pH and metal treatment on the extracted microbial population which were independent of gross biomass size. pH and soil amendments with Cu and Zn were found to reduce the metabolic potential of the extracted soil microbial population.

The effects of increasing concentrations of Zn and Cu in soil pore water from soils of a long-term sewage sludge field experiment on microbial bioluminescence were investigated. Concentrations of total soluble Zn, free Zn 2+ , and soluble Cu increased sharply in soil pore water with increasing total soil metal concentrations above 140 mg of Zn kg -1 or 100 mg of Cu kg -1 . Two luminescence bioassays were tested, based on two bacteria (Escherichia coli and Pseudomonas fluorescens) with the lux genes encoding bacterial luminescence inserted into them. The bioluminescence response of the two microorganisms declined as total soil Zn, soil pore water soluble Zn, and soil pore water free Zn 2+ concentrations increased. The EC 25 values for E. coli and P. fluorescens were 1.3 ( 0.2 and 4.3 ( 0.5 mg L -1 on a free Zn 2+ basis, respectively. The EC 50 values were 2.5 ( 0.2 and 9.6 ( 0.9 mg of free Zn 2+ L -1 , respectively.Copper had no significant effect on bioluminescence in the two assays, even at the largest soil pore water concentration of about 620 µg L -1 , corresponding to a total Cu concentration in bulk soil of about 350 mg kg -1 . Thus, the decline in bioluminescence of the two assays was ascribed to increasing soil pore water free Zn 2+ and not soluble Cu.