ZnO-loaded PVC microplastics increases soil Zn bioavailability and phytotoxicity

Agricultural plastic film mulches are major sources of macro- and microplastic pollution in agroecosystems. While metal additives such as zinc oxide nanoparticles (ZnO NPs) are commonly incorporated into plastics as ultraviolet (UV) protectants, their environmental fate and impacts following plastic disposal remain poorly understood. This study investigated how ZnO additives in polyvinyl chloride (PVC) films affects soil-plant-microbe interactions, with UV aging used to simulate environmental weathering. Macro- or microplastic PVC film fragments containing 0, 1, or 5 % (w/w) ZnO were subjected to UV ageing before incorporation into mesocosms growing maize, with their impacts on maize productivity and soil microbial communities evaluated over a 9-week period. Soil bioavailable Zn reflected the ZnO loading of the PVC films, with microplastics releasing more Zn than macroplastics, and UV aging further increasing Zn mobilization. Zinc bioaccumulation followed the order roots > stems ≈ leaves, and root biomass was reduced by 40-50 % in the 5 % ZnO-PVC treatment, relative to the control. Overall, we observed few effects of PVC or ZnO addition on key microbial diversity indices. In some treatments, higher ZnO concentrations reduced the abundance of key nutrient-cycling bacterial families (e.g., Nitrosomonadaceae and Xanthobacteraceae). Notably, 1 % ZnO PVC films showed minimal adverse effects on the microbial community, suggesting they could potentially serve as a slow-release Zn fertilizer. Our study demonstrates that UV-enhanced weathering of ZnO-containing agricultural plastics enhances bioavailable Zn in soil, particularly at high loading rates (5 % ZnO). Furthermore, the presence of high concentrations of ZnO in PVC films negatively affected plant development and soil microbial community composition. These findings highlight the need to re-evaluate the use of metal-based additives in agricultural plastics given their potential to impair soil and plant health at high concentrations.