Ageing conditions of tire wear particles alter the effects of their leachates on soil

Tire wear particles (TWP) are a complex mixture of rubber and chemical additives, and the release of these chemicals from TWP can be influenced by the physical properties and chemical composition of TWP as well as the degradation or ageing conditions of the particles. Given that soil is considered the largest reservoir of TWP emitted into the environment, we established a soil laboratory experiment using TWP leachates with three different ageing treatments (mechanical-, thermal-, and UV irradiation), and measured nine soil parameters (soil microbial activities and physicochemical properties) to investigate how ageing conditions influence the effects of TWP leachates on soil. We found that leachates from aged TWP induced considerable effects on soil parameters while non-aged treatment significantly influenced only one soil parameter. We found that the leachates of the mechanical-ageing treatments significantly decreased β-1,4-N-acetyl-glucosaminidase activity and C/N ratio, and the thermal-ageing treatment slightly increased litter decomposition but largely decreased water stable aggregates and pH. UV-ageing treatments decreased 1,4-N-acetyl-glucosaminidase activity, water stable aggregates and C/N ratio. TWP leachates of different ageing treatments induced distinct effects compared to leachates from pristine TWP, and each ageing treatment showed different response patterns in different soil parameters. This has further refined our knowledge of the ecological impacts of TWP, and highlighted the necessity of considering ageing condition as a crucial factor investigating the environmental effects of TWP.