Soil properties explain the variability in tire wear particle effects in soil based on a laboratory test with 59 soils.

Tire wear particles (TWPs) are among the most prevalent microplastics in the environment, with potential detrimental effects on ecosystem health and functionality. While little is known how the effects of TWPs on soil physicochemical and microbial properties vary across different soil types, and if so, which factors contribute to this variability. To address this knowledge gap, we conducted a laboratory experiment involving soils from 59 grassland plots across two sampling regions in Germany, each experienced varying land-use intensities. These soils were treated with (at a concentration of 10 mg g) and without TWPs. At harvest, we measured soil water-stable aggregates (WSA), pH, respiration, and decomposition rate. Our results revealed that TWPs negatively, neutrally, or positively impacted these parameters depending on soil types. Random forest analysis indicated that the variability in TWP effects was significantly explained by grazing frequency for WSA (14.5 %), by clay content for pH (9 %), by bulk density for respiration (7.9 %), and by silt content for decomposition rate (12 %). Partial dependence analysis and piecewise regression further suggested that low-intensity grazing (∼0.7-1.2) reduced TWP effects on WSA; clay content (420-550 g kg) increased TWP effects on pH; bulk density (0.75-0.88) decreased TWP effects, and silt content (460-620 g kg) enhanced TWP effects on decomposition rate, with the identified thresholds of 1.45, 353 g kg, 0.84, and 327 353 g kg, respectively. These results highlighted the context-dependent nature of TWP pollution, with significant variability observed across different sampling points. Additionally, our findings suggest that TWP pollution is particularly of concern in soils with high clay, silt, high bulk density, and areas with intensive land-use intensity. Our study contributes to a better understanding of the mechanisms by which TWPs impact soil, and how these effects are regulated by environmental factors.