Changes in tolerance of soil microbial communities in Zn and Cd contaminated soils

Trace metals are present in the soil matrix in different forms, and this obscures the relationship between the amounts of metals, their biological availability and effects. Chemical methods have been devised to directly measure the biological available pools of trace metals, but such methods need to be validated against measured exposure of organisms in the soil. We studied acquired Zn- and Cd tolerance of the soil microbial community as a reporter of its exposure, and compared it with chemical determination of Zn and Cd in 10 soils differing in pH, organic matter content, texture, vegetation-/cultivation history and metal contamination. The tolerance was measured as LC 50 (i.e. the metal concentration which inhibits 50% of the activity) in suspensions of extracted soil bacteria, by measuring the incorporation rate of [ 3H] thymidine at different metal concentrations. Chemical determination of Cd and Zn in soils included total concentrations by aqua regia extractions (AR), and total concentrations in extracted pore water (PW). In addition was the ‘effective concentration’ ( C E) determined using the Diffusion Gradients in Thins films method (DGT). The LC 50 values correlated better with PW ( r 2=0.90 for Cd and r 2=0.97 for Zn) and C E (0.90 for Cd and 0.98 for Zn) compared to the correlation with AR (0.72 for Cd and 0.82 for Zn). After excluding a single extremely contaminated soil from the analysis, the correlation of LC 50 with AR was much poorer ( r 2=0.03 (ns) for Cd and r 2=0.48 for Zn), whereas correlations remained significant for both PW (0.90 for Cd and 0.87 for Zn) and C E (0.54 for Cd and 0.84 for Zn). In conclusion, PW fraction of Cd and Zn appear to be the best predictor of trace metal exposure of the soil microorganisms.