TRACE METAL UPTAKE BY THE GRASS MELINIS REPENS FROM ROADSIDE SOILS AND SEDIMENTS, TROPICAL AUSTRALIA

Abstract

This study reports on trace metal uptake by the grass species Melinis repens, growing in roadside soils and sediments in tropical northeastern Australia. Median total Cu, Pb, Ni and Zn concentrations were significantly (P<0.05) higher in road edge soils (Cu=61.1mg/kg, Pb=97.3mg/kg, Ni=28.6mg/kg, Zn=729mg/kg) than in background soils collected away from roads (Cu=5.8mg/kg, Pb=11.2mg/kg, Ni=3.7mg/kg, Zn=21mg/kg). Significantly (P<0.05) elevated Zn values were recorded in the stems of the M. repens specimens growing on roadside soils (231.6mg/kg dry weight of tissue) compared with those of grasses growing on background soils (40.8mg/kg dry weight of tissue). Moreover, median Cu, Ni and Zn values in the roots of roadside grasses (Cu=29.1mg/kg, Ni=2.73mg/kg, Zn=169mg/kg) were significantly (P<0.05) higher than their respective levels in the roots of background M. repens samples (Cu=5.98mg/kg, Ni=0.70mg/kg, Zn=22mg/kg). A greenhouse experiment showed that Cu and Zn in road sediments are labile and are available for uptake by M. repens. The studied roadside soils and sediments were leached with a diethylenetriaminepentaacetic acid–CaCl2–triethanolamine–HCl extraction solution, which proved to be a rudimentary indicator of Zn availability and uptake to the root tissue of M. repens. The results demonstrate that trace metals in roadside grasses have the potential to be directed up the food-chain as grasses are consumed by herbivores. In addition, bioavailable metal contaminants hosted by road sediments have the capacity to impact on ecosystems downstream of roads because these sediments are mobilised by road runoff waters from road surfaces into adjoining catchments.