Assessment of Heavy Metal Accumulation in Box Elder Acer negundo L. Leaves and Soil in Ecologically Transformed Urban Areas in Southern Poland

Urban soils along transportation routes are subjected to intense anthropogenic pressure, altering their physicochemical properties and promoting the accumulation of potentially toxic metals (PTM). This study aimed to assess soil contamination levels and evaluate the bioindicative potential of Acer negundo L. growing in urban green areas exposed to varying traffic intensities. Topsoil and leaf samples were collected from eight sites representing different levels of anthropogenic disturbance. Soil granulometric composition, pH, organic carbon content, selected nutrients, and concentrations of PTM (Cu, Pb, Zn, Ni, Co, Mn, Cd, Hg, Fe) were determined, and contamination was evaluated using pollution indices (Igeo, EF, CF, Er). The soils ranged from moderately acidic to slightly alkaline (pH_KCl 5.85–7.66). Elevated concentrations of Zn (1078 ± 3.07 mg kg−1), Pb (401.4 ± 2.51 mg kg−1), Mn (1816 ± 3.3 mg kg−1), and Cd (10.8 ± 2.06 mg kg−1) were recorded at most sites, frequently exceeding permissible limits for urban green areas (Zn: 500; Pb: 200; Mn: 240 and Cd: 2 mg kg−1). Correlation analyses revealed that zinc and cadmium are the two predominantly traffic-related origins. Pollution indices indicated moderate to very high enrichment, particularly for Cu and, locally, for Cd and Zn, while cadmium posed the highest potential ecological risk. The differences in the order of element abundance between the soil and plant tissues indicated a selective enrichment in plant leaves, with a preference for Fe, Zn, Mn, Cu and Pb. A strong positive correlation between soil and leaf cadmium concentrations indicates its high bioavailability and efficient transfer to plant tissues. These results demonstrate that Acer negundo is a valuable bioindicator of urban soil contamination, particularly for cadmium and zinc, and highlight the significant impact of road traffic on urban soil quality.