Unexpected reduction in reproduction of Collembola exposed to an arsenic-contaminated soil

To assess the problems posed by ecological testing of the toxicity of polluted soils, we applied the Collembola reproduction test to the soil from a mining site polluted by metals. Our aim was to quantify the chronic toxicity of contaminated soil using the Folsomia candida reproduction test. Briefly, the polluted soil, which contains in particular large quantities of arsenic, is mixed in various proportions with a reference soil, and the number of juveniles is counted five weeks later. The results were quite unexpected in that a stronger effect was observed on reproduction with dilute contaminated soil, 0.78 to 3.1%, than with much higher concentrations. The pure contaminated soil is not highly toxic. Arsenic (As) dosage in pore water shows that a change in its bioavailability occurs that may explain the surprising toxicity results. As the main contaminant of the soil was arsenic, we showed that the lowest-observed-effect concentration and the median effective concentration of this metal for the reproduction of F. candida in International Organization for Standardization (Geneva, Switzerland) soil experimentally contaminated with arsenic were low (2.22 and 2.19 microg/g, respectively). Likewise, we showed that pH modifies the bioavailability and the toxicity of As, but we do not think that this parameter explains the test results. These results show that this bioassay should be used cautiously to test toxicity of polluted soils.