Risks of microplastics from polyurethane and polyethylene-polycarbonate coated fertilizers to soil-crop system

The widespread use of polymer-coated fertilizers raises concerns about microplastics (MPs) pollution, yet their impacts on soil-crop systems remain unclear. This study evaluated the effects of MPs derived from polyurethane (PU) and polyethylene-polycarbonate (PE-PC) coated fertilizers on tomato growth and soil enzyme activity. Results showed that PU and PE-PC MPs significantly reduced 7-day germination rates by 12.5-17.5 % and 12.5-21.7 %, respectively, and inhibited early seedling growth. Interestingly, during the 45-day stage, neither PU nor PE-PC MPs exhibited apparent phytotoxicity, and in some cases even enhanced the total fresh weight of tomato plant. Chlorophyll analysis revealed that 1 % PU MPs slightly promoted chlorophyll content, whereas high PE-PC concentrations reduce it. Soil enzyme analysis revealed increases in β-glucosidase and acid phosphatase activity but no significant change in urease activity. Furthermore, distance-based redundancy analysis (db-RDA) indicated that MPs type and concentration jointly influenced plant physiological traits and soil enzyme activities. These findings suggest that coated fertilizer-produced MPs pose potential risks at germination stages, but their adverse effects appear negligible during the vegetative growth stage. These results can provide a support for scientific application and security risk assessment of coated fertilizers.