Water‐extractability, free ion activity, and pH explain cadmium sorption and toxicity to Folsomia candida (Collembola) in seven soil‐pH combinations

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 Cd 2+ ion activities ({Cd 2+ }s). Two‐species Langmuir isotherms that used total cadmium concentration ([Cd]) or {Cd 2+ } 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 {Cd 2+ }, 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 {Cd 2+ }, 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.