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Toxicity of cadmium to Folsomia candida was determined in soils at different pHs (3.5, 5.0, and 6.5). The Langmuir sorption constant (K(L)), based on pore-water or water-extractable concentrations, showed a pH-related increase of cadmium sorption that was most pronounced when using free Cd2+ ion activities ([Cd2+]s). Two-species Langmuir isotherms that used total cadmium concentration ([Cd]) or [Cd2+] and pH in the water-extractable fractions gave the best description of cadmium sorption on all soils together. Cadmium concentrations causing 50% reduction of growth and reproduction (median effective concentrations [EC50s]) differed by a factor of 4.5 to 20 when based on total soil concentrations and increased with increasing pH. However, when based on water-extractable or pore-water [Cd] or [Cd2+], EC50s decreased with increasing pH, but differences between soils were still a factor of 4.5 to 32. The EC50s differed by less than a factor of 2.2 when based on body [Cd] in the surviving animals. Two-species Langmuir isotherms were used to relate body [Cd] in survivors to [Cd2+], corrected for pH in water-extractable or pore-water fractions. An excellent description of effects on growth and reproduction was found when related to the body concentrations predicted in this way; the difference in EC50s between soils was reduced to a factor <2. This demonstrates that F. candida is mainly exposed to cadmium through the soil solution, and suggests that principles of a biotic ligand model approach may be applicable for this soil organism.

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