Uptake, transformation, and environmental impact of zinc oxide nanoparticles in a soil-wheat system

Zinc oxide nanoparticles (ZnO-NPs) are metal-based nanomaterials, but their long-term effects on plant growth and the soil environment in the field remain unclear with most previous studies using short-term laboratory and glasshouse studies. In this study, we used a field experiment to examine the long-term effects of ZnO-NPs in a soil-wheat ( Triticum aestivum ) system. It was found that although ZnO-NPs had no significant effect on either yield or the concentration of other nutrients within the grain, the application of ZnO-NPs significantly increased Zn concentrations. Indeed, for grain, the application of ZnO-NPs to both the soil and foliage (SFZnO) (average of 33.1 mg/kg) significantly increased grain Zn concentrations compared to the the control treatment (21.7 mg/kg). Using in situ analyses, nutrients were found to accumulate primarily in the crease tissue and the aleurone layer of the grain, regardless of treatment. Specifically, the concentration of Zn in the aleurone layer for the SFZnO treatment was 2–3 times higher than that in the control, being >300 mg/kg, whilst the Zn concentration in the crease tissue was ca. 600 mg/kg in the SFZnO treatment, being two times higher than for the control. Although the application of ZnO-NPs increased the total Zn within the grain, it did not accumulate within the grain as ZnO-NPs with this being important for food safety, but rather mainly as Zn-phytate, with the remainder of the Zn complexed with either cysteine or phosphate. Finally, we also observed that ZnO-NPs caused fewer changes to the soil bacterial community structure and that it had no nano-specific toxicity.