Environmental factors influence airborne microplastic deposition in the soil of urban allotment gardens

The widespread contamination of urban soils by airborne microplastics (MPs) is an emerging environmental concern, particularly in urban allotment gardens (UAGs) where food production occurs. This study investigates the vertical distribution of MPs in three UAGs in Berlin, Germany, over 90 days. Soil samples were collected at three depths (surface, 1-15 cm, and 15-30 cm) and analysed for MPs composition and concentration, with a detection limit of 0.1 μm using Optical PhotoThermal InfraRed (O-PTIR) spectroscopy. Results revealed that fibres were the dominant MP type, with the highest concentrations found in surface layers. MPs concentrations decreased with soil depth, and fibres showed limited vertical penetration. We also identified 19 different polymer types, with polyethylene terephthalate (PET), polyester (PES), and polyethylene (PE) being the most common. Temporal variations in MPs deposition were observed, with a significant peak at 90 days. Environmental factors, including PM_2.5 concentration, precipitation, and wind velocity, played a role in MPs deposition. Precipitation and PM_2.5 were positively associated with increased deposition, while higher wind speeds reduced MPs accumulation. These findings highlight the pervasive presence of MPs in urban soils and suggest that environmental conditions significantly influence MPs distribution. Understanding these dynamics is essential for assessing the long-term ecological impacts of MPs on soil health in urban environments.