Abstract: To identify the influences of urbanization on the environmental safety of urban soil, a total of 77 topsoil samples were collected along an urban-suburban-rural gradient in Urumqi City, Xinjiang, northwest China. The concentrations of nine trace elements were measured, including As, Cd, Ni, Pb, Hg, Cu, Zn, V, and Co. We used the potential ecological risk index and the environmental risk early warning index to compare the ecological risks of heavy metals in soil along the urbanization gradient. The average concentrations of Hg in urban, suburban, and rural soils were higher than the background value of soil in Urumqi. The average concentrations of As in soils in urban and suburban zones were higher than the corresponding background. The average concentrations of Zn and Pb in the soils in urban zone exceeded the corresponding background values. The average concentrations of other elements were lower than the corresponding background value. The concentrations of all heavy metals in urban soils were higher than that in rural soils. The Hg element in all the sites presented a very strong ecological risk situation, whereas other eight elements showed a slight ecological risk situation. The ecological risk early warning index of Hg showed a heavy, medium and heavy warning situation in urban, suburban, and rural zones, respectively. As showed early ecological warning situation in urban and suburban zones. Zn showed early ecological warning situation in urban soils, while the other elements showed no ecological warning situation. On the whole, the concentrations and potential ecological risk of heavy metals in topsoil showed an obvious decreasing trend along the urban-suburban-rural gradient. Hg was the main ecological risk factor in the topsoil of each zone in the study area, and thus the potential ecological risk of Hg should be cautious.

Key words: urbanization gradient, soil, heavy metal, ecological risk, Urumqi.