Abstract/Summary

Earthworms are ecosystem engineers, capable of modifying the soil environment they inhabit. Recent evidence indicates that they increase the mobility and availability of potentially toxic elements in soils, but a quantitative synthesis of the evidence required to understand mechanisms and identify soils most susceptible to earthworm-induced potentially toxic element mobilisation is lacking. We undertook a meta-analysis of 42 peer reviewed journal studies, comprising 1185 pairwise comparisons between earthworm-inhabited and earthworm-free soils to quantify the impact of earthworms on potentially toxic element mobility in bulk earthworm-inhabited soil and earthworm casts, and on plant uptake and concentration. We find that endogeic and epigeic earthworms increase the mobility of potentially toxic elements in the bulk soil, and earthworms from all ecological groups mobilise potentially toxic elements during passage of soil through the earthworm gut. We also observe an increase in the concentration and uptake of potentially toxic elements by plants growing on soils inhabited by epigeic (mostly Eisenia fetida) earthworms. Earthworms mobilise potentially toxic elements in geogenic soils to a greater extent than anthropogenically contaminated soils. Soils with very low (<2%) soil organic matter content are most susceptible to earthworm-induced potentially toxic element mobilisation. These findings have important implications for the ability of exotic earthworms to alter soil biogeochemical cycles when introduced to new environments. Mixing amendments with contaminated soils with the intention of reducing the mobility of potentially toxic elements may be aided by the activity of earthworms that accelerate the mixing processes. Furthermore, our findings also highlight a promising phenomenon that, if harnessed, may help to alleviate micronutrient deficiencies in degraded soils.