Microplastics can bind with toxic metals via surface complexation and chelation, forming combined pollutants. However, research regarding the toxicological impacts of these combined pollutants on soil fauna remains limited. This study employed Folsomia candida in a 28-day incubation experiment to investigate the ecotoxicological effects of combined pollution by polylactic acid microplastics (PLA-MPs) and thallium (Tl) on the functional traits of Folsomia candida, including biology, morphology, and gut microbiota. The results showed that the combined effects of PLA-MPs and Tl on these functional traits were characterized by amplified toxicity and trait-specific responses. Morphological traits exhibited lower sensitivity to the pollution treatments compared to other indicators. Exposure to high-concentration PLA-MPs (10%) significantly affected mortality and fecundity, and reduced gut bacterial diversity. Conversely, low-concentration Tl (1 mg/kg) significantly inhibited body length and antenna length while increasing gut bacterial diversity. Structured equation modeling further revealed that the pollution treatments exerted significant negative effects on the functional traits of Folsomia candida, both directly and indirectly by altering soil properties and soil microbiota. These findings provide valuable insights into the ecotoxicological effects of combined PLA-MPs and Tl pollution on soil fauna, contributing to ecological health risk assessments of microplastics and toxic metals in terrestrial ecosystems.

Remnant forests are vital in urban ecosystems as they serve as a crucial link between organisms, inorganic environments, and human settlements. However, there is a lack of research on how urbanization affects the physical and chemical properties of soil in remnant forests, as well as the response of soil fauna to environmental changes within these forests. Our study utilized the urbanization gradient research method to investigate the characteristics of the soil fauna community in remnant forests across different urbanization gradients and to understand its intrinsic response to environmental changes. Our results indicate support for the “moderate disturbance hypothesis” based on the statistical values of diversity indices. Additionally, it was found that SOM and Pb are the primary factors influencing soil fauna diversity in the remnant forests, while SOM and Zn are the main influencing factors for the dominant soil fauna groups. To elucidate the impact of urbanization on soil fauna biodiversity in remnant forests, future studies should consider other urbanization factors.