Effects of proximity to highways on tissue properties of three tropical grazing grass species

Abstract

To obtain indicative, extreme scenario data on the possible impacts of expanding road networks in rangelands, this study examined aboveground tissue of three grazing grasses near four high traffic highways: Digitaria eriantha, Brachiaria nigropedata, and Eragrostis lehmanniana. Samples were collected monthly on three occasions at the same respective locations during the rainy season, in transects approximately perpendicular to the highways. For comparison, tissue samples were concurrently collected from a control site more than 3 km from a road. In the laboratory, a whole stem of each specimen was washed in a fixed volume of distilled water. The pH and electrical conductivity of the water containing the washed-off tissue surface material were then measured. Concentrations of heavy metals in dry tissue were determined using ICP-MS analysis. The results showed poorer grass quality near roads. Tissue surface pH was lower (acidic), and electrical conductivity higher, towards the roads. Rainfall tended to dilute the effects. Linear regression models of tissue surface pH and conductivity against distance showed that the effects were pronounced within at least 70 m and up to nearly 800 m from the road edge. The highly nutritious grasses seemed to present higher potential risk of road-sourced metal toxicity to grazers.