Spray paint-derived microplastics influence soil properties and microbial activity through their composite composition

Paint-derived microplastics (MPs) are increasingly recognized as an overlooked but potentially significant source of terrestrial pollution. To investigate their ecological effects on soil health, we conducted a controlled six-week soil incubation experiment using 10 commercially available spray paints with varying polymer compositions and colors. Paint-derived MPs were applied at environmentally relevant concentrations (0.01% and 0.4% w/w). Soil pH, water-stable aggregates, soil respiration, and enzyme activities related to carbon, nitrogen, and phosphorus cycling were measured. Results showed that paint-derived MPs increased soil pH at the high concentration and reinforced the stability of water-stable aggregates. Paints with distinct polymeric compositions generated MPs with different size distributions upon application; these paint-derived MPs modulated soil respiration dynamics. The influence of color on the toxicity of paint-derived MPs appears to be primarily associated with metal pigments. In particular, paint-derived MPs containing copper consistently showed significant negative effects on all measured parameters. Furthermore, paint-derived MPs exhibited concentration-related toxicity patterns, which may be attributed to the various additives embedded in their formulations. This study provides new insights into the environmental risks of paint-derived MPs and highlights the role of their composite composition in determining their ecological impact on soil ecosystems. These findings underscore the importance of greater transparency in paint formulations and stricter regulation of hazardous components.