Toxicity of binary mixtures of cadmium with lead, copper, or zinc to Folsomia candida in relation to bioavailability in soil

Metal pollution in soils usually involves mixtures, hampering a proper assessment of potential risks as metals may interact at different levels, including sorption to the soil, uptake by organisms, and interaction with target sites resulting in adverse effects. This study aimed at clarifying the interaction of metals on sorption to soil and uptake and effects in springtails. Single and combined toxicity of Cd with lead, copper, or zinc to Folsomia candida was studied in a natural LUFA 2.2 soil and related to total, 0.01 M calcium chloride-exchangeable and water-extractable metal concentrations in the soil and internal concentrations in the springtails. Cadmium availability in soil extracts significantly increased in the presence of another metal, and Cd also increased the availability of Pb and Zn. This, however, did not lead to increased Cd, Pb, or Zn concentrations in F. candida. Combined effects of the metals on springtail survival, growth, and reproduction were overall antagonistic, with some dose ratio-dependent deviations from the concentration addition reference models. Most likely, the complexation of Cd with the excess chloride introduced in the soil solution by the counterion of the lead(II) chloride, copper(II) chloride, and zinc(II) chloride salts used reduced its bioavailability. This shows the importance of taking into account all steps in the intoxication process to improve understanding of the effects of metal mixtures in soil.