Nanjing region, with a population of 5.63 million and covering 6,597 km², lies in eastern China (31°14′–32°36′ N, 118°22′–119°14′ E). It is in the process of rapid economic growth, urbanization and industrialization. About 15.6% of this region is urbanized.

Fourteen vegetable plots were sampled from three concentric rings around the city of Nanjing. The centre-most, urban ring extends from the city wall (which surrounds the centre of the city) to a distance of 4.0 km and encompasses 5 of the plots. Next, 5 sites were sampled from the suburban ring which extends from 4.0 km to 7.5 km. Finally, 4 rural sites were sampled from the outer-most ring extending 7.5 to 15 km from the city wall (Figure 1). All the sample sites are far away from the point pollution sources (such as landfill regions, gas station and factories) and line pollution sources (about 200 meters away from rivers and roads). All sites were located with GPS in order for further investigation. Investigation results of the sample plots’ surrounding environment, such as geographical and geological conditions, hydrological distributions, and neighboring pollution sources, were recorded. The soil managing conditions including the cultivation time, fertilization frequencies and quantities were also investigated.

Each soil sample consists of 35 sub-samples random collected from the sampling plot of about 200 m × 200 m. At each sampling point, the top-layer sample (0–15 cm of the soil) and sub-layer sample (15–30 cm) was taken separately using a stainless steel sampling tube. The soil samples were then placed into polyethylene bags, and returned to the laboratory. All these samples were air dried at room temperature and sieved through a 0.850 mm nylon sieve to remove coarse debris. The soil samples were then ground with a agate pestle and mortar until all particles passed a 0.150 mm nylon sieve. For the total heavy metal content analysis, Total Zn, Pb and Cr contents in soils were determined by an Atomic Absorption Spectrophotometer after digesting in HNO₃–HF–HClO₄. The total Cu contents in soils were determined by the Atomic Absorption Spectrophotometer after digesting in HNO₃–HCl–HClO₄ mixture. Total Hg contents in soils were digested in H₂SO₄–KMnO₄ mixture and determined by a Cold Atomic Absorption Spectrometer (CAAS).

Pearson’s correlation coefficients and Principle Component analysis (PCA) of heavy metal elements in top-layer and sub-layer were calculated and one-way ANOVA method to test the significant differences of heavy metals content in urban, suburban and rural regions was employed by SPSS software for Windows, version 13.0 (SPSS Inc., USA).

The results showed that the top-layer (0–15 cm) heavy metal levels in the soil varied greatly in urban (0–4 km), suburban (4–7.5 km) and rural regions (7.5–15 km) (Table 1). Except for Zn, the mean values of all heavy metals were degressive from urban to rural environment. All heavy metal contents, including Zn, were higher in urban sites than those in suburban and rural sites. Analysis of variance (ANOVA, p < 0.05) showed that there were significant differences in heavy metals between the urban and suburban areas for Pb, Zn, Hg, and Cu, whereas no significant difference was found between suburban and rural region. There were no significant differences among the 3 types of regions for Cr.

Many researchers concluded that heavy metal pollution in soil is mainly concentrated in the upper layer (0–10 cm) of the top ploughed layer (0–15 cm) of the artificial cultivated soil profile [22–24]. Therefore, comparing the heavy metal contents in top-layer and sub-layer soils show the heavy metal distribution in the soil profiles. The mean levels were calculated by subtracting the sub-layer (15–30 cm) from the top-layer (0–15 cm). According the subtracted results, Significance tests (ANOVA in probability levels of <0.05) were done and the results are shown in Table 2. The mean contents of heavy metals, which were calculated by the top-layer (0–15 cm) subtracting the sub-layer (15–30 cm), are all above zero and large in absolute value in urban areas, but in suburban and rural areas the means are all near zero and small in absolute value. There are no significant differences between different regions.

This research suggests that almost all investigated heavy metals accumulated in the top-layer in vegetable soil in urban areas where, as we know, there is relatively more industrial activity and more automobile emission sources than in suburban and rural areas. Variations of heavy metals along the urban-rural gradient in soil profiles indicate a distribution pattern in urban areas unaffected by regional background sources, mainly controlled by local sources.

Industrialization and urbanization as major sources of heavy metal pollution are known by many authors [25,26]. Nanjing is one of the fastest economic growth areas and is in the process of rapid urbanization and industrialization [27]. Generally, the proximity of vegetable plot soil to urban areas could increase pollution from irrigation with polluted waters, fertilization with contaminated manure, atmospheric particle fall from industrial dust, combustion of fossil fuels and road traffic. But which were the main pollution sources?

As the above analyses show, almost all heavy metals’ mean are degressive along with the urban to rural gradient, and vegetable soils were contaminated with Zn, Pb, Hg and Cu, especially in urban region [5]. Urban farmland soil pollution with heavy metal has been reported in many cities in the world. Heavy metals can exert detrimental effects on human health and on the environment in big cities [26]. Pb, Zn, Cr, Cd and Hg, which are considered very toxic elements, are of primary concern in soil and food contamination in recent literature [11,37–39]. Data had demonstrated that both Pb, Cd and Hg in vegetables posed substantial risk to local residents in China [40,41]. Heavy metals, such as Pb and Zn from heavy traffic, Hg from thermoelectricity power stations and combustion of fossil fuels, accumulated in most big cities in China. However, in the current study, regions near to urban and the suburban primarily plant vegetable and are the main vegetable production areas of cities, because that planting vegetables in urban and the suburban is more convenient transportation and better economic benefit than rural regions. Metal concentrations in vegetables and vegetable consumption through the foodchain poses a substantial risk to local residents, although the actual risks to local populations remains to be examined. The procedures of soil remediation are expensive and sometimes difficult to perform. Therefore heavily polluted soils have to be excepted from food production [42] and land use planning should be based on heavy metal pollution in the soil. Generally, we should avoid production of leafy vegetable and root plants in polluted areas, whereas leguminous plants and fruit plants could be grown under certain conditions [43]. Further investigation should be done to determine whether and what kind of agricultural production could be established in areas in close proximity to urban areas. Heavily polluted areas, such as urban regions, should avoid food production according to relevant reference standards and should be altered to non-agricultural land-use (recreation areas, construction sites, trade and industry areas, etc.).