| Soil is not only the element of natural environment, but also the most important natural resource of agricultural production. Soil pollution and accumulation of heavy metals will take place as well due to modern agricultural process. Bio-lives and human health would be impacted by highly concentrated heavy metals in their surroundings. Thus soil heavy metal pollution has been received increased attention. Zhangjiagang City is situated at a flat alluvial plain on Yangtze River Delta (YRD) with rapid economy development. In recent years, with the adjustment of agricultural planting structure, vegetable industry has became an important source of farmers’income. However how is the impact on vegetable production environment which the development of urbanization, industrialization and agricultural intensification bring to, especially how is the accumulation of heavy metals in vegetable fields, and how much are environmental risk and economic risk during vegetable production. These issues must be answered in order to ensure healthy development of the local vegetable production. On the other hand, measuring heavy metals is an important part for investigation of heavy metals enrichment. However, the current methods require grinding, digestion, determination and other steps. Analysis cycle is long, time-consuming, and there may be some harmful impact on analysts and environment in the analysis process. Therefore, developing a simple, direct method is very important to improve the efficiency of soil surveys, save the survey cost and reduce risk of environmental pollution.In this study, heavy metal contents and spatial distribution of vegetable fields under different cultivating ages were analyzed based on soil samples from rice-wheat fields, vegetable fields and vegetables in Zhangjiagang City, Jiangsu Province. The concentrations and trends over time of heavy metals in the vegetable fields were predicted with the expansion of vegetable cultivation and the increase of planting years. The contents of heavy metals and their ecological effects and economic effects in soil-vegetable system were investigated. In addition, the vegetable soil samples were conducted using a portable X-ray fluorescence device to measure contents of heavy metal, and the results were compared to the extracted concentration by traditional lab analysis, the accuracy and precision of this method were evaluated. Results are described as follows:The statistical analysis of heavy metals under different cultivating ages showed that soil type and planting ages had significant impact on heavy metal contents. Compared with the heavy metal contents of rice-wheat rotation soils (0years of vegetable growing), As had not significantly changed in vegetable fields, while Cd, Cr and Cu were increased with vegetable cultivating ages, especially in the1-5years of facilities vegetable fields, the increase were more dramatic. In space, the distribution of heavy metals also clearly reflected soil type and vegetable cultivating ages. In general, the contents of As, Cd, Cr and Cu in Cambisols were higher than in Anthrosols. When vegetable samples were increased to the interpolation chart of rice-wheat rotation samples, the new maps were different. In the latter the area of high concentration significantly increased, and the low area reduced. The area which was higher than0.200mg·kg-1of Cd was from20401hm2to21163hm2, and the area which was between70.5mg-kg-1and77.8mg-kg-1of Cr was from788hm2to15640hm2, higher than the first standard limit soil (Cu:35.0mg-kg-1) increased in size from16845hm2to20484hm2for Cu with the addition of vegetable samples, locating in the northwest, northeast and central parts. The analysis about loadings of soil basic properties and heavy metals on principle components showed that accumulation of heavy metals in Zhangjiagang City vegetable fields mainly originated the fertilization process, especially livestock and poultry manure or poultry manure as a raw material of organic fertilizer, which accounted for vast majority of inputs. In the present circumstances, mean contents of As, Cd, Cr and Cu in the study were lower than the second grade standard in the Chinese environmental quality standards for soils. If the current trend continued, the Cu concentration of2.4%of soil area would be over the second grade standard in the Chinese environmental quality standards by next10years, and that of0.7%over the second grade standard of Cd by next20years.The research roughly estimated the heavy metal pollution loss rates, the soil damaging degree caused by heavy metal pollution (%), based on the soil geochemical baselines of this city and pollution loss rate model and the heavy metal pollution economic loss (million yuan-year-1) when soils were utilized to cultivate vegetables. The results showed that the individual heavy metal pollution loss rates were from0.77%to1.68%with7.00%of the integrated ones, indicating that the soil heavy metal pollution in the vegetable fields was weak and clear level (Ⅱ level) as a whole. However, the integrated heavy metal pollution economic loss was about27.8million yuan-year-1because of large area and high yield of vegetables in Zhangjiagang City.By monitoring and evaluating heavy metal contents in vegetables of Zhangjiagang City, the results showed that Cd, As, Cr, Cu and Zn in vegetables did not exceed the limits of heavy metals. The results by comparison of the enrichment factors indicated that the heavy metal enrichment ability in vegetables was different and the leaf vegetable was higher than others. Assessment of healthy risk indicated that there were a higher risk of heavy metal pollution existed in leaf vegetables than non-leaf vegetables. The relationship between heavy metals in vegetables and soils was complex, and only Cr could achieve significant positive correlation (r=0.436**, p<0.01). Heavy metals in vegetables and enrichment factors reached significant correlation, while the relationship between heavy metals in soils and enrichment factors was poor.The soil analysis using traditional lab analysis (TLA) and field portable X-ray fluorescence spectrometry (PXRF) device were conducted to measure contents of heavy metal in vegetable fields, and their results were compared to the extracted concentrations by TLA. As a result, the determination coefficients of linear regression analysis for As, Pb, Cu, Zn of soil standard samples obtained by the real contents of heavy metals and PXRF were above0.99, and the slopes were close to1.0. The analysis of vegetable samples showed that the determination coefficients of As, Cu and Zn were above0.7reaching significant correlation, expert for Cr and Pb. In general, the accuracy and precision using PXRF for most heavy metals were good and the results are reliable, which provides a new approach for rapidly measuring soil heavy metals. |
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