Trends in the occurrence and accumulation of microplastics in urban soil of Nanjing and their policy implications

Urban soil is an important sink of terrestrial microplastics (MPs), and understanding their distribution over time is essential for effective pollution management. Here, based on soil MP data from Nanjing, a typical megacity in eastern China, this study analyzed MP accumulation trends using decision tree and time series network based on soil attributes, POI (point of interest), and human activity factors such as urban industrial structure, transportation, water use. We also evaluated the impact of plastic policy interventions. In the past 15 years, MPs in urban soil in Nanjing have gradually increased, and highly polluted areas have also grown. From 2010 to 2020, the concentration of MPs in urban soil increased from 326.7 items/kg to 480.9 items/kg, with high pollution areas expanding from only 2.0 km (0.7 %) to 48.7 km (14.9 %). The accumulation of MPs was also influenced by changing factors due to urbanization. In the early 21st century, residential areas had the largest effect, while in the later period, public passenger transport and domestic water consumption were the dominant factors. The scenarios simulation suggests recent plastic intervention policies have helped alleviate this rate of increase, but MP source management (e.g., laundry fibers, tire wear) still needs improvement. By the proposed method, the past trend of microplastics in urban soil and their relationship with soil properties and human activities can be accurately revealed, which will be helpful for the formulation of countermeasures to mitigate regional soil MP pollution.