Bioavailability and toxicity of nanoscale/bulk rare earth oxides in soil: physiological and ultrastructural alterations in Eisenia fetida

Abstract

The accumulation of rare earth oxides (REOs) in soils is linked with application of nanoscale phosphatic fertilizers, but their effects on earthworm species are poorly understood. We investigated the impacts of nanoscale and bulk-REOs of lanthanum and ytterbium (La₂O₃, Yb₂O₃) at low (25, 50 mg kg⁻¹), intermediate (100, 200 mg kg⁻¹) and high (500, 1000 mg kg⁻¹) concentrations on soil quality and earthworm health. The bioaccumulation of nanoscale and bulk Yb₂O₃ was 12–28% greater than La₂O₃ and occurred in a dose-dependent manner. At 100 mg kg⁻¹, nanoscale and bulk La₂O₃ and Yb₂O₃ induced earthworm mortality by 33–35% and 13–15%, and reduced reproduction by 10–32% and 10–12%, respectively. Ultrastructural observations reveal that nanoscale and bulk REO at 500–1000 mg kg⁻¹ induced abnormalities in internal organelles, including mitochondria, Golgi apparatus and chloragosomes. Nanoscale REOs significantly reduced earthworm digestive and cast enzymes by 20–80% at medium and high concentrations as compared to bulk materials. Earthworms reduced REO toxicity in the soil by minimizing exposure to microbial biomass carbon and soil enzymes. The data show that REOs beyond 50–100 mg kg⁻¹ adversely impact soil microbiota; these findings provide important understanding of the fate and effects of REOs in agricultural systems.