Biological monitoring of soil pollution caused by two different zinc species using earthworms

Zinc oxide nanoparticles (ZnO-NPs) are commonly used in both commercial and agricultural sectors. As a result, ZnO-NPs are extensively discharged into soil ecosystems, creating a significant environmental issue. Therefore, it is crucial to assess their influence on the soil ecology to ensure its secure and enduring utilization in the future. The exact degree of toxicity associated with ZnO-NPs and their ionic form is still uncertain. To address the challenges, the study used the soil bioindicator earthworm species Eudrilus eugeniae as an experimental model to evaluate the effects of two zinc species (ZnO-NPs and ZnCl₂) at 100, 250, 500, and 750 mg kg^-1 and control (0 mg kg^-1) in garden soil over 28 days. The investigation also examined the impact of exposure on survival, reproduction, neuro-biomarker, avoidance behavior, and accumulation. The highest avoidance rates were 27.5% for ZnO-NP and 37.5% for ZnCl₂ at 750 mg kg^-1. ZnCl₂ treatment reduced juvenile production by 3.73 ± 1.73, while ZnO-NPs showed 4.67 ± 1.15. At 750 mg kg^-1, soils with ZnCl₂ (63.3%) demonstrated lower survival rates than those with ZnO-NPs (53.3%), likely because of higher Zn ion levels. After 28 days of exposure, ZnCl₂ (536.32 ± 11 mol min^-1) activated AChE enzymes more than ZnO-NPs (497.7 ± 59 mol min^-1) at the same dose, compared to control (145.88 ± 28 to 149.41 ± 23 mol min^-1). Nanoparticles and zinc ions bioaccumulated and reacted negatively with the neurotoxic marker AChE, affecting earthworm reproduction and behavior. However, earthworms exposed to ZnCl₂ exhibited less intestinal Zn than those exposed to NPs. The present work contradicts the finding that ZnO-NPs have hazardous effects on soil organisms. The results indicate that earthworm E. eugeniae may significantly affect soil metal uptake from metallic nanoparticles (NPs). This may help design NP soil pollution mitigation strategies. The study offers valuable information for establishing a relationship between the environmental toxicity of ZnO-NPs and soil ecosystems.