Lanthanum toxicity to five different species of soil invertebrates in relation to availability in soil

This study determined the toxicity of lanthanum, one of the most commonly used rare earth elements (REEs), to five representative soil invertebrates after 3-4 weeks exposure. Toxicity was related to total, 0.01 M CaCl2-extractable and porewater concentrations in the standard LUFA 2.2 soil, and for earthworms also to body concentrations. La sorption to LUFA 2.2 soil, estimated by relating total soil concentrations to CaCl2-extractable or porewater concentrations seemed to reach saturation at >1000 mg La/kg dry soil. Isopod (Porcellio scaber) growth was the most sensitive endpoint, followed by earthworm (Eisenia andrei), enchytraeid (Enchytraeus crypticus), springtail (Folsomia candida) and oribatid mite (Oppia nitens) reproduction, with EC50s of 312 (95% confidence interval: 5.6-619), 529 (295-762), 1010 ((>377 < 3133), 1220 (1180-1250) and 1500 (1250-1750) mg La/kg dry soil, respectively. EC50s related to CaCl2-extractable concentrations ranged between 1.3 (0.046-2.6) and 15.6 (5.6-25.7) mg La/kg dry soil, while porewater-based EC50s were 3.5 (−) and 10.2 (−) mg/L for the springtails and mites, respectively. La uptake in the earthworms linearly increased with increasing exposure concentration with bioaccumulation factors ranging between 0.04 and 0.53 (average ± SE: 0.24 ± 0.032). EC50 for effects on earthworm reproduction related to internal concentrations was 184 (61-301) mg La/kg dry body weight. A risk assessment based on the available toxicity for soil invertebrates, bacteria and plants resulted in an HC5 of approx. 50 mg La/kg dry soil, suggesting that La may affect soil ecosystems at concentrations slightly above natural background levels (6.6-50 mg La/kg dry soil) in non-polluted soils.