This dissertation explores the context-dependent effects of tire wear particles and multiple concurrent anthropogenic stressors on soil ecosystems. In chapter 2, we address the impacts of TWPs with different delivery rates on soil physicochemical properties and microbial activities. By either adding TWPs to soils abruptly or gradually, this study showed that gradually-delivered TWPs negatively influenced the activity of carbon cycle-related enzymes, and abruptly-delivered TWPs had no significant effect. Abruptly-delivered TWPs increased the activity of nitrogen cycle-related enzymes, while gradually-delivered TWPs had no significant effect. It highlights that delivery rate of TWPs could be a key factor shaping the effects of TWPs on soil microbial activities. In chapter 3, we investigate if ageing influence the effects of TWP leachates on soil. TWPs were subjected to different ageing conditions, including mechanical-, thermal-, and UV-ageing. We found that comparing to pristine TWPs, the leachates of aged TWPs generated broader effects on soil physicochemical properties and microbial activities, and the response patterns were dependent on ageing conditions. This study suggested that ageing can alter the effects of TWP leachates on soil, and highlighted the necessity of considering ageing condition as a crucial factor when investigating the environmental effects of TWPs. In chapter 4, we explore how an increasing number of concurrent anthropogenic stressors affects plant-soil systems and the role of plant diversity in this process. Plant-soil systems with different plant diversity levels (3 vs. 9 species) were subjected to an increasing number of stressors (1, 2, 5 and 8) using random assemblages of plants and stressors from a predefined plant species pool and a stressor pool. We found not only directional changes in soil properties, functions and especially plant community composition, but also the diminishment of plant diversity effects as stressor number increased. This study broadens our insights into the ecological impacts of multifactorial environmental change and highlights the necessity of exploring the relationships between the number of anthropogenic stressors and the effect of plant diversity. Overall, this dissertation contributes to two critical areas of environmental research: the effects of TWPs on soil properties and functions, and key factors that alter these effects; how multiple anthropogenic stressors affect soil properties, functions, plant community composition, and the role of plant diversity in this process. Our work has advanced the knowledge about the ecological risks of global change, including microplastics and other anthropogenic stressors, in terrestrial ecosystems.
