Toxicity response of a new enzyme-based functional diversity methodology for Zn-contaminated field-collected soils

Assessing the effects of stressors, such as metals, on soils by microbial functional diversity (FD) indices based on soil enzymes is increasingly promoted. Various enzymatic FD indices were used in past studies but none generated toxicity thresholds such as EC50, the concentration of stressor affecting 50% of the maximum enzymatic activity. The aim of this study was to generate a realistic EC50 value from an enzyme-based FD index sensitive to Zn from long-term Zn-contaminated soils. Assays of 6 soil enzymes (arylsulfatase, phosphatase, protease, urease, β-glucosidase and invertase) and metal characterization (total, soluble and labile Zn) were measured in 10 pairs of Zn-contaminated field-collected soils. Two approaches were studied in parallel: enzymatic activity (EA) and enzymatic stability (ES) assessed by the RSSIb, a score based on the modified Relative Soil Stability Index (RSSI) method. The enzymatic results (activity and stability) were then aggregated by various FD indices (weighted and geometric means, Shannon, Simpson and Gini indices) and correlated to Zn concentrations in soils to obtain the EC50 of these concentration–response curves. The concentration–response curves constructed from the EA weighted means were highly correlated to the labile and soluble Zn concentrations, and the EC50 generated from these curves were in a realistic bioavailable Zn range as compared to other studies. The aggregated ES showed Zn-sensitivity similar to the aggregated EA. Therefore, although the use of the ES approach was judicious for long-term Zn contamination, the greater resources required to assess the ES by RSSIb scores does not seem justified.