Sources and distribution of heavy metal and C–N isotopes in topsoils across an urban-rural gradient in a typical hazy city, northern China

Abstract

Abstract Urban soils differ from rural soils in that they are more susceptible to both point-source or diffuse anthropogenic contamination associated with urbanization. However, few studies have attempted to identify local sources of C and N, and determine the degree of anthropogenic inputs to topsoils using C–N isotopic data collected across an urban-rural gradient in China. In this study, a combination of heavy metals and isotopic data were collected and analyzed from topsoils along three transects. One transect was located in an urban-suburban area of central Shijiazhuang (CS); the other two were located in northern Shijiazhuang (NS, county-rural) and southern Shijiazhuang (SS, rural). Statistical analysis of heavy metal concentrations suggests that Cu, Pb, Cd and Zn were strongly influenced by vehicular transport, whereas Hg was related to coal burning. In contrast, V, Cr and Ni mainly originated from natural sources. Isotopically, topsoils in SS mainly reflected natural sources. In contrast, generally low δ13CTOC in topsoils from CS and NS indicate the input of coals and vehicle exhausts with depleted values. δ13CTC in topsoils from CS and NS were about the same as those in street dust. Nitrogen isotopes also reflected the influence of coal burning; elevated values (as high as 8.8‰) occurred around urban and industrial sites, whereas low values (as low as −7.2‰) characterized suburban-rural topsoils were affected by vehicle exhausts. However, several abnormally high values were related to coal combustion by local residents in rural areas. This study demonstrates that the distribution of heavy metals and C–N isotopes is closely correlated to local anthropogenic activities, and could be effectively differentiated from natural sources in topsoil among urban, suburban and rural areas.