Urban soils are dynamic systems increasingly affected by atmospheric inputs and anthropogenic pressures. Traffic-derived particulate matter (PM) and associated trace metals, including platinum (Pt) from catalytic converters, represent emerging contaminants with potential impacts on soil ecological functioning and the sustainability of urban spaces. This study investigates soil chemical and biological responses under Quercus ilex L. canopies along an urban-to-rural gradient in the Caserta area (Southern Italy), including a high-traffic road with stop-and-go conditions, an urban canyon-like square, and a rural reference site. The aim is to evaluate how atmospheric deposition influences soil contamination patterns and microbial functional quality in a context relevant for sustainable soil management. Topsoil samples (0–5 cm) were analyzed for Fe, Zn, Cu, and Pt concentrations as indicators of traffic-related contamination. Soil biological functioning was assessed through dehydrogenase activity as a proxy of microbial metabolic potential.Leaf particulate deposition was used as an indirect indicator of air pollutant load and interception by the holm oak canopy. Results showed a clear gradient in pollutant concentrations, with increasing accumulation of Fe, Zn, Cu, and Pt in soils from rural to highly-traffic sites. Platinum exhibited the highest sensitivity to traffic intensity, acting as a strong tracer of vehicle emissions. Soil microbial activity showed an inverse relationship with metal enrichment, indicating a progressive decline in soil functional quality under higher contamination loads. Significant correlation patterns between contaminant accumulation and reduced enzyme activity highlight a functional link between atmospheric deposition and soil ecosystem health. These findings suggest that urban soils beneath tree canopies act as active interfaces for pollutant transfer rather than passive sinks. Integrating emerging contaminants such as platinum into soil ecological assessments can improve understanding of urban soil functioning and support evidence-based strategies for sustainable urban green and soil management.