Microbial properties for the derivation of critical risk limits in cadmium contaminated soil

• Microbial properties and derived indexes were measured along a Cd gradient.
• IC50s and ecological thresholds were calculated for the derivation of risk limits.
• The risk limits obtained were similar to those in European legislations.
• Issues to be solved before using them in soil quality evaluation are listed.

Soil microbial communities play essential roles in soil functioning. Therefore, soil microbial properties are increasingly being used as indicators of soil quality and ecological risk in contaminated soils. In this study, a variety of microbial parameters and indexes reflecting the activity, biomass and diversity of soil microbial communities were determined in two soils artificially contaminated with a gradient of cadmium (from 0 to 1000 mg Cd kg−1 dry weight soil). From the soil microbial properties whose values decreased at increasing Cd concentrations following an exponential pattern, IC50 (IC = inhibitory concentration) values for Cd contamination, as well as ecological thresholds, were calculated in order to determine their suitability for the derivation of critical risk limits. The most cautious IC50 value obtained for the two soils of this study was 254 mg Cd kg−1 dry weight soil. However, taking into account the hump-shaped exponential decay pattern observed as well as the calculated ecological thresholds, the safest risk limit was established at 44 mg Cd kg−1 dry weight soil (from data of the dehydrogenase/WSOC index), a value close to the risk limits established by legislation in many European countries. It was concluded that, although certainly relevant from an ecological point of view, before using soil microbial properties to derive critical risk limits, it is essential to reach an agreement regarding which specific microbial properties to use and to establish different risk limits for specific soil types.