| Through the investigation of typical soils of protected vegetable cultivation in Shouguang of Shandong Province, Shangqiu of Henan Province, Siping of Jilin Province, Wuwei of Gansu Province in the North China, the characteristics of heavy metal accumulation and their distributions in the soil profiles and the mobility of heavy metals were studied systematically via laboratory analysis and field experiments. The balance of heavy metals between inputs and outputs in the soils of the protected vegetable cultivation was calculated. The strategies for regulation of the accumulation and contamination of heavy metals in the soils of the protected vegetable cultivation were also highlighted. The obtained results were as follows:1. The contents of As, Cd, Cu, Zn, Cr, Ni, and Pb in the soils of the protected vegetable cultivation in Shouguang of Shandong Province was with 9.63, 0.52, 33.91, 124.20, 53.04, 29.04, and 17.96 mg·kg-1, respectively, while it was with 11.08, 0.30, 25.03, 73.53, 51.06, 26.68, and 14.53 mg·kg-1 respectively in Shangqiu of Henan Province. It was with 12.47, 0.47, 37.20, 87.68, 67.50, 25.17, and 17.98 mg·kg-1 respectively in Siping of Jilin Province, and with 13.33, 0.42, 33.85, 85.31, 53.12, 28.59, and 20.76 mg·kg-1 respectively in Wuwei of Gansu Province. Heavy metal accumulation occurred in soils of the protected vegetable cultivation for these areas in the North China.2. Compared to the second grade standard values of National Soil Quality, heavy metal contents in most samples did not exceed the recommended value with the exception of Cd, Zn and Ni in some soil samples. Generally speaking, the accumulation of heavy metals had relatively small environmental risk. Among all investigated heavy metals, the most serious accumulation was observed for cadmium. The percentage of soil samples exceeding the national standard from high to low was in order of Siping (39.8%), Shouguang (27.4%), Shangqiu (6.1%), and Wuwei (1.0%).3. The heavy metal accumulation in the soils of the protected vegetable cultivation generally increased with the cultivation time prolonging in all studied regions. It was estimated that the accumulating rate of As, Cd, Cu, Zn, Cr, Ni, and Pb in the soils of the protected vegetable cultivation in Shouguang was with 0.105, 0.033, 0.923, 2.010, 0.370, 0.281, and 0.324 mg·kg-1·a-1, respectively, while it was with 0.116, 0.001, 0.517, 1.628, 0.225, 0.122, and 0.203 mg·kg-1·a-1 respectively in Shangqiu, and with 0.164, 0.035, 1.223, 2.629, 1.076, 0.248, and 0.143 mg·kg-1·a-1 respectively in Siping. It was with 0.194, 0.025, 1.022, 2.691, 0.156, and 0.481 mg·kg-1·a-1 respectively in Wuwei. Generally speaking, the highest accumulation rate occurred for Cu and Zn, and the lowest rate for highly toxic element of Cd.4. The distribution of heavy metal contents in the soil profiles showed that the heavy metal accumulated in the different soil layers and the highest concentration of heavy metals appeared in the surface soil layer. With the increase of the soil depth, most heavy metal content continuously decreased in Shouguang and Wuwei. In contrast with that of wheat field and vegetable field (0 year), heavy metal contents increased in the same soil layer in protected cultivation, which indicated some environmental risk to local groundwater. 5. According to the field experiments for two years in Wuwei, Gansu Province, the main input sources were from chemical fertilizer and organic fertilizer, and the input amount from organic fertilizer was much more than that from chemical fertilizer. For different fertilization treatments, the highest input rate into the soils of the protected vegetable cultivation was with 0.043 kg·ha-1·a-1for Cd, 16.910 kg·ha-1·a-1 for Cu, 45.617 kg·ha-1·a-1 for Zn, 1.668 kg·ha-1·a-1 for Cr, 1.297 kg·ha-1·a-1 for Ni, and 1.431 kg·ha-1·a-1 for Pb, respectively, and all of them appeared in MNPK (manure plus chemical fertilizers) treatment. Meanwhile, the most important output channel for heavy metals from soils of the protected vegetable cultivation was through vegetable plant straws and fruits. The heavy metal amount removed by fruits was much more than that by straws.6. Balancing the inputs and outputs of heavy metals in soils of the protected vegetable cultivation, it was indicated that heavy metals under different treatments had different levels of surplus except few elements. The rather high surplus occurred in MNPK, 1/2MNPK, and M (manure) treatments, while much less surplus was for NPK (Chemical fertilizers) treatment. The highest accumulating rate occurred for MNPK treatment, which was with 0.042, 16.750, 44.849, 1.653, 1.271and 1.419 kg·ha-1·a-1 for Cd, Cu, Zn, Cr, Ni, and Pb respectively.7. By analyzing the sources of heavy metal accumulation in the soils of the protected vegetable cultivation in the four typical regions, it showed that the heavy metal accumulation was closely related to the amount and quality of fertilizer applied to the soil. In Shouguang, Shandong Province, logarithmic significant positive correlations were found between the content of Cd, Cr, Cu, Pb, and Zn in soils and the organic fertilizer application amount (P<0.05), as well as those between the chemical fertilizer application amount and the contents of Cd and Zn in soils. In Shangqiu of Henan Province, Logarithmic positive correlation was also found between the application amount of organic fertilizer and soil Cd content at 0.01 significant levels. The arsenic content was significantly correlated with the organic fertilizer application level (P<0.05), while logarithmic positive correlation was found between soil Zn and the chemical fertilizer application amount. Application of chemical and organic fertilizer was one of the main reasons for heavy metal accumulation in the surveyed soils of the protected vegetable cultivation.8. In order to reduce the amount of the heavy metals entering into soils of the protected vegetable cultivation, the availability of heavy metals in soils and uptake amount of heavy metals by vegetables, several methods were recommended including the base selection for vegetable production under protected cultivation, detection and control of heavy metals of input source, plantation of crops with low adsorption for heavy metals. For seriously contaminated soils with heavy metals, such techniques as physical, chemical and biological approaches can be adopted. Therefore, measures like source control, process block and soil remediation should be taken on one hand, investment addition for research on science and technology of safe utilization and regulation technique for farmland polluted by heavy metals, environmental legislation and strict law enforcement were also necessary on the other hand.
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