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 +-Cr⁶⁺ synergistic effects on Eisenia fetida. Median lethal concentrations (LC₅₀) of Cd²⁺ in loam and loamy-clay soils increased by 69.4 and 87.5 % compared to sandy soil, while Cr⁶⁺ LC₅₀ 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 Cd²⁺-Cr⁶⁺ 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 Cd²⁺ or Cr⁶⁺ 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.