| Soil is the main part of the environment, and it not only provides diversified nutrients for the humanity survival, but also accepts pollutant from industry, domestic activities and the agricultural chemicals that contain heavy metals. In recent years, with the global economy development and industrialization, soil pollution from industry receive more and more attention, and the content of heavy metal in soil - crops system was significantly influenced by waste water, the waste gas and the waste residue from factories. Research on accumulation, distribution, transfer of heavy metal and their affecting factors in soil-crop systems around factories with different pattern of pollutant discharging, has important significanees in theory and practices to the reasonable design of agricultural production and the safety of food.In this study, distribution and transfer of heavy metal in soil-crop systems and their affecting factors were studied by collecting soil and crop samples around different factories in Zhangjiagang County of Yangtze River Delta in China and analyzed items including: soil pH, available and total heavy metals in soil, heavy metals in crops. Distribution maps of heavy metals in soils and crops around different factories were produced with the aids of GIS methods. Results are described as follows:1. Generally speaking, distribution of heavy metal in soils and crops was dependent on the types and discharging patterns of pollutants from different factories. Heavy metals were not markly accumulated in soils around the metallurgy factory (SG) through air discharging, but Pb concentrations in crops were obviously enhanced. According to the Maximum Allowable Safety Levels of Contaminants in Foods (GB 2762-2005), Pb concentrations of 25% wheat samples and 52.6% rice samples had exceeded the Pb limit in our collecting ediable crop samples. Due to the irrigation of wastewater of Pb oxide factory (TP) and the chemical factory (MF and GK), Pb, Cd and Hg was accumulated in soil around them, respectively. According to the second grade standard in the Chinese Environmental Quality Standards for Soils (GB 15618-1995), concentrations of Pb in soils around TP did not exceed the second grade standard even though high level of Pb were accumulated in soil around it; Cd concentrations of 9.4% soil samples around MF had exceeded the Cd standard; Hg of 83.4% soil samples around GK had exceeded the Hg standard. Because of the irrigation of wastewater, the concentrations of heavy metal in crops were also enhanced. Pb concentrations of 50% wheat samples and 78.9% rice samples had exceeded the Pb standard(GB 2762-2005) around TP; Cd concentrations of 17% wheat samples and 5.3% rice samples had exceeded the Cd limit around MF; and Cd concentrations of 7.7% wheat samples, Hg concentrations of 13% dee samples had exceeded the Cd and Hg limit around GK.2. Distribution of heavy metal in soil and crops around factories is described as follows: in soil profile, factitious heavy metal, which was entering soil with polluted air or water from factories, was mainly accumulated in the surface layer (0-20cm), and its distribution was highly correlated with soil clay content, organic matters, redox features and its plough layer in soil. The distribution of Hg was the uppermost soil horizon (0-5cm) had lower available and total Hg concentrations compared to that in 5-15 cm soil horizon, and Hg had migrated down to deeper layer until 50cm; this may pose a certain threat to ground water. In space, under the air pollution, there was no trend of heavy metal distributions in soil and crops, and it was difficult to decide the scope influenced by factory. Under the irrigation of wastewater, distribution of heavy metals in soils and crops was showed that the highest content of heavy metal in soils and crops was distributed nearby the waste water outlet, and decreased exponential with the distance away from the factory, and the scopes influenced by factory TP,MF,GK was about 400 m, 100 m and 400m, respectively.3. Availability of heavy metal entering with the irrigation of waste water was about twice as much as that entering with air deposition in our study for four factories. Under the air pollution, there was no significant relationship between the heavy metals in crops and soils. Under the irrigation of waste water, there was significant exponential or linear relationship between heavy metals in soils and crops. There was no significant relationship between Pb in wheat and soil, but significant exponential relationship between Pb in rice and soil around TP. For rice absorbed Pb from soil, the critical concentrations for available and total Pb were 13 mg kg-1 and 110 mg kg-1, respectively according to the relationship. There was significant exponential relationship between Cd in rice and soil, and significantly linear relationship between Cd in wheat and soil. For rice absorbed Cd from soil, the critical concentrations for available and total Cd were 0.25 mg kg-1 and 1.2 mg kg-1, respectively according to the relationship. When wheat Cd is at 0.20 mg kg-1 according the Maximum Allowable Safety Levels of Cd in Foods (GB 2762-2005), critical concentrations of available Cd and total Cd in soil are at 0.319 mg kg-1 and 1.42 mg kg-1, respectively according to the relationship. When rice Cd is at 0.20 mg kg-1 according the Maximum Allowable Safety Levels of Cd in Foods (GB 2762-2005), critical concentrations of available Cd and total Cd in soil are at 0.360 mg kg-1 and 1.50 mg kg-1, respectively according to the relationship. There was a weak relationship between Hg in soil and wheat; but Hg in rice had a significant linear relationship with soil Hg. When rice Hg is at 0.20 mg kg-1 according the Maximum Allowable Safety Levels of Hg in Foods (GB 2762-2005), critical concentrations of available Hg and total Hg in soil were 0.074 mg kg-1 and 1.00 mg kg-1 respectively according to the relationship.4. Content and distribution of heavy metal in soils and crops is influenced by soil properties, origin and form of heavy metal, types of crops and heavy metal and so on. Low pH enhanced the availability of heavy metal, and uptakes of crops. For example, in this study, concentrations of Cu and Cd around GK was low, about 1/3 to 1/5 as much as the Cu and Cd content around MF, but concentrations of Cu and Cd in crops reached the same level around the two factories. At the same time, Crop uptakes of heavy metal from soil were influenced by crop types and transfer ability of heavy metals. For Cu and Zn, crop uptakes were in the order of wheat＞rice; for Pb, Cd and Hg, crop uptakes were in the order of rice＞wheat. For different heavy metal, uptakes for wheat were in the order of Zn＞Cd＞Cu＞Hg＞Pb, uptake for rice was in the order of Zn＞Cu＞Cd＞Hg＞Pb.Therefore, we can choose crops with weak uptakes for the types of heavy metal from heavily polluted soil to curb the harm to human according to their different uptakes to different heavy metals.
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