Dual regulation of pakchoi–soil systems by zinc oxide nanoparticles under polyethylene microplastics stress: Dose-dependent effects, microbial cascades, and risk propagation

Agricultural and industrial activities have progressively increased the accumulation of zinc oxide nanoparticles (ZnO NPs) in soils contaminated with polyethylene microplastics (PE-MPs). However, under high PE-MPs residues, the mechanisms by which different ZnO NPs concentrations regulate the plant-soil-microbe system, and their interactive effects with PE-MPs, remain poorly understood. This study examined the dose-dependent regulation of the pakchoi-soil-microbe system by ZnO NPs under PE-MPs stress, focusing on plant responses, antioxidant enzyme activity, nutrient uptake, elemental distribution, and cascading impacts on soil bacterial communities. Relative to the control, sole exposure to PE-MPs increased pakchoi shoot fresh weight by 19.4 %, peroxidase activity by 17.37 %, and malondialdehyde content by 134 %. Additionally, it disrupted nutrient distribution and soil microbial diversity. Co-exposure to ZnO NPs and PE-MPs revealed a dual regulatory effect: low ZnO NPs levels (<400 mg kg) alleviated the adverse impacts of PE-MPs on the pakchoi-soil-microbe system, whereas higher concentrations intensified abiotic stress and dominated overall toxicity. Moreover, soil microbial taxa strongly correlated with plant-soil ecological variables, and structural equation modeling elucidated the pathways through which ZnO NPs influenced pakchoi growth and zinc accumulation. Overall, the findings highlight a threshold-dependent regulatory mechanism in which ZnO NPs shift from mitigating to aggravating PE-MPs-induced stress, providing a scientific basis for assessing ecological risks under combined nanoparticle and microplastic contamination in agroecosystems.