Predicting copper toxicity to different earthworm species using a multicomponent Freundlich model

This study aimed to develop bioavailability models for predicting Cu toxicity to earthworms (Lumbricus rubellus, Aporrectodea longa, and Eisenia fetida) in a range of soils of varying properties. A multicomponent Freundlich model, complying with the basic assumption of the biotic ligands model, was used to relate Cu toxicity to the free Cu²⁺ activity and possible protective cations in soil porewater. Median lethal concentrations (LC50s) of Cu based on the total Cu concentration varied in each species from soil to soil, reaching differences of approximately a factor 9 in L. rubellus, 49 in A. longa and 45 in E. fetida. The relative sensitivity of the earthworms to Cu in different soils followed the same order: L. rubellus > A. longa > E. fetida. Only pH not other cations (K⁺, Ca²⁺, Na ⁺, and Mg²⁺) were found to exert significant protective effects against Cu toxicity to earthworms. The Freundlich-type model in which the protective effects of pH were included, explained 84%, 94%, and 96% of variations in LC50s of Cu (expressed as free ion activity) for L. rubellus, A. longa, and E. fetida, respectively. Predicted LC50s never differed by a factor of more than 2 from the observed LC50s. External validation of the model showed a similar level of precision, even though toxicity data for other soil organisms and for different endpoints were used. The findings of the present study showed the possibility of extrapolating the developed toxicity models for one earthworm species to another species. Moreover, the Freundlich-type model in which the free Cu²⁺ activity and pH in soil porewater are considered can even be used to predict toxicity for other soil invertebrates and plants.