Impact of road transport on soil physicochemical characteristics and heavy metal concentrations in the bark of purple willow (Salix purpurea L.)

The aim of this study was to determine the impact of road transport on soil physicochemical characteristics and concentration of heavy metals in the bark of purple willow. The study was carried out at two groups of sites situated along a national road at a distance of 5–10 m and ca. 100 m from the road. At each of the sites, annual willow shoots were cut and surface soil samples were taken. The concentrations of Zn, Pb, and Cd in bark samples were measured and of K, Ca, Mg, Zn, Cr, Cu, Ni, Mn, Pb, and Cd in the soil. The concentrations of Mn and Pb were predictably higher in soils located near the road, which may indicate an impact of road transport on the soil content of these metals. The concentrations of Zn, Pb, and Cd in S. purpurea bark from the sites located at distances of 5–10 m and ca. 100 m from the road were similar. However, the Cd concentration in the bark exceeded the maximum permissible concentration in the dried material, despite the low Cd concentrations in the soils at of all the sampling sites. It is important to pay due attention to the concentrations of this heavy metal in this plant material when it is intended for pharmaceutical use, even in that obtained from plants growing on soils qualifying as “uncontaminated”. Salix purpurea used for Salicis cortex should therefore be cultivated under controlled conditions.