Study on the mobility and bioavailability of PTEs in soils from Urban Forest Parks in Sofia, Bulgaria

• Studied urban soils are not strongly contaminated with potentially toxic elements.
• Anthrosols contain more mobile Cu, Zn, Pb, Cd than the natural soils.
• CEC, clay, pH, SOM of urban soils have greater specific influence on PTEs mobility.
• Mobile Pb and Cd strongly correlate with their content in oak tree leaves.

Forest parks in many urbanized areas are affected by the anthropogenic pollution as demonstrated by the poor health status of many trees. The concentrations of harmful agents like Potentially Toxic Elements (PTEs) can be very high in urban soils and their availability is an important issue for biological components in these environments. This study focuses on the mobile forms of PTEs extracted by 1 М NH4OAc at pH = 7 in relation to determining their bioavailability for forest trees in oak ecosystems in urban areas. The area studied is along an urban–rural gradient from the Sofia region, Bulgaria. The NH4OAc extracts mainly “biologically active” fraction of elements in soils comprising primarily the soluble and exchangeable forms and in some cases showed good positive correlation with PTE accumulation in trees. Four experimental forest park sites (U1, U2, U3 and U4) created on Hortic/Technic Anthrosols were selected along the urban gradient in the Sofia region. The control sites located in rural zones with natural soils Cambisols and Luvisols (R1, R2, R3 and R4) were used as reference. Strong correlations were found between mobile forms of Cu, Zn, Pb, Cd in urban soils and a number of soil parameters (pH, CEC and texture). The mobility is related to soil pH and with more acidic soils a higher mobility is shown, which is statistically significant for Pb. The element Cu is less mobile in the soil and is localized in the top layer, where it is mainly associated with soil organic matter. The mobile (NH4OAc extractable) portions of Cu, Zn, Pb, and Cd (mg/kg soils) in Urban Soils ranged between: 0.74 and 2.61; 2.94 and 3.29; 1.34 and 3.28; and 0.02 and 0.16, respectively and those in Natural Soils ranged between: 0.73 and 1.65; 1.20 and 5.29; 1.10 and 6.80; and 0.02 and 0.07. The significance of the mobile proportion was greatest for Cd in urban soils where it reached 33.3% of the total content. Despite the determined correlation between mobile forms of Pb and Cd with their total content in leaves of oak trees there is no clear visible effect on the tree health characteristics using parameters as defoliation and decoloration of crowns.