Combined effects of Cd and Cr pollution on Eisenia fetida: Influence of soil texture and toxicological mechanisms

Soil heavy metal co-contamination poses a global threat to soil ecosystems, yet the texture-dependent interactions between cadmium (Cd) and chromium (Cr) in earthworm toxicity remain poorly quantified. This study integrates quadratic saturated D-optimal design with ecotoxicological assays to unravel how soil texture modulates Cd 2+ -Cr 6+ synergistic effects on Eisenia fetida . Median lethal concentrations (LC 50 ) of Cd 2+ in loam and loamy-clay soils increased by 69.4 and 87.5% compared to sandy soil, while Cr 6+ LC 50 values rose by 49.7 and 56.4%, respectively. Growth inhibition rates (GIR) under combined pollution reached 77.9, 70.8, and 25.3% in sandy, loam, and loamy-clay soils, respectively, inversely correlating with clay content ( p < 0.05). Synergistic Cd 2+ -Cr 6+ interactions dominated in low-clay soils, whereas clay-mediated adsorption attenuated toxicity via electrostatic binding, SEM-EDS analysis revealed significant differences in Cd and Cr distribution in earthworms among the three soils. GIR or biomarkers demonstrated significantly positive correlation with Cd 2+ or Cr 6+ concentration, and negative with clay content. This study establishes a quantitative framework linking soil texture to Cd/Cr bioavailability, offering critical guidance for region-specific ecological risk assessment. • Soil texture controls Cd/Cr bioavailability, altering earthworm LC 50 significantly • The maximum difference in GIR between sandy and loamy-clay is 52.63%. • Cd-Cr interaction exhibits antagonistic effect could decrease toxicity. • GIR of earthworms significantly decreases linearly with clay content. • SEM-EDS effectively characterized the distribution of Cd/Cr in earthworm.