Effects of Roadside Deposition on Growth and Pollutant Accumulation by Willow (Salix miyabeana)

Abstract

Roadside plants have the potential to accumulate pollutants and safeguard waterways. To assess growth and pollutant accumulation of roadside plants, the willow Salix miyabeana was grown (a) in a greenhouse on soil collected at different distances from an interstate highway to test the longer-term effects of pollutant deposition as manifested in soil, and (b) in the field on reference soil placed at different distances from that highway to test the shorter-term effects of proximity to pollutant sources during a single growing season. In the first experiment, relative growth rate (RGR) increased 150 % with distance of soil collection from the roadway, from a baseline near the highway to 100 m away. Relative nitrogen and phosphorus accumulation rates were positively correlated with RGR (P <0.0001), and total contents of zinc, strontium, copper, nickel, cadmium, and lead in new shoots were also positively correlated with RGR (P <0.05). Thus more rapidly growing plants accumulated more N, P, and metals. Reduced growth for plants grown on soils collected near the roadway was associated with very high tissue concentrations of sodium and soil concentrations of chloride, implicating the deposition of deicing agents in this northern temperate roadside ecosystem. In contrast, S. miyabeana showed the opposite pattern on reference soil in the field, with RGR decreasing 31 % as distance from the roadside increased. The latter trend appears to have resulted from greater soil moisture and reduced shading near the highway. We suggest that reducing road salt applications will promote growth and pollutant accumulation by roadside vegetation.