西南岩溶区果园土壤—作物系统重金属元素迁移特征与污染评价

Migration characteristics and pollution evaluation of heavy metal elements in orchard soil-crop system in karst area of southwestern China

  • 摘要: 【目的】分析西南岩溶地区地质高背景下土壤—作物系统的重金属迁移富集关系及其潜在生态风险,为岩溶区土壤合理开发利用和粮食安全生产提供科学依据。【方法】选取广西较典型的岩溶区果园和撂荒地作为研究区,同时采集不同母质的土壤进行对比,对果园和撂荒地内植物的根、茎、叶、果实及土壤剖面(0~100 cm)中的8种重金属元素[镉(Cd)、铬(Cr)、砷(As)、汞(Hg)、铅(Pb)、铜(Cu)、锌(Zn)和镍(Ni)]含量进行研究,并分别采用单因子污染指数、地累积指数、生物富集系数及相关分析等方法对土壤及作物的生态风险进行评价。【结果】研究区土壤中重金属元素Cd、Cr、Cu、Pb、Ni和Zn含量平均值均超广西土壤元素背景值,其中,Cd和Zn含量分别是广西土壤元素背景值的7.7和5.4倍。地累积指数结果表明,果园剖面土壤的Cd存在中度至重度污染,Zn的最大值存在中度污染;撂荒地剖面土壤的Cd存在中度污染,Zn存在无污染至中度污染。果园土壤剖面中Cd、Cu、Ni和Zn 4种元素最大迁移率均出现在50~60 cm处,撂荒地土壤中Ni和Cd元素最大迁移率出现在30~40 cm处,Cu和Zn元素最大迁移率则出现在60~80 cm处。果园和撂荒地土壤的Cd活化率最高,具有较强活性。果园中7种重金属元素的作物污染指数排序为Pb (22.09) >Ni(20.84) >Zn(3.24) >As(2.77) >Cr(1.43) >Cu(0.21) >Cd(0.01)。比较作物根、茎、叶和果实的生物富集系数,发现所有元素含量均在根系部位最大;果实中As元素的生物富集系数最高,为0.07。除As和Pb 2个元素外,其他元素(Cu、Cd、Zn、Ni和Cr)的生物富集系数最小值均出现在果实。研究区内土壤和果实中同一种重金属元素含量之间无显著相关性(P>0.05),即土壤重金属元素含量不会影响作物重金属元素含量。【结论】研究区土壤中Cd和Zn的超标率较高,存在Cd污染现象;与土壤相比,研究区作物果实中Cd和Zn的生物富集系数总体较低,表明研究区重金属元素总体表现出高含量、低活性和生态低风险的特点。

     

    Abstract: 【Objective】This paper analyzed the heavy metal migration and enrichment relationship of soil-crop system and its potential ecological risk under the geological high background of karst area in southwestern China,so as to provide scientific basis for the rational development and utilization of soil,and food security and production in karst area. 【Method】Orchards and wastelands in the relatively typical karst areas of Guangxi were selected as the study areas. Soils with different matrices were collected for comparative studies. The contents of eight heavy metal elements[cadmium (Cd),chromium(Cr),arsenic(As),mercury(Hg),lead(Pb),copper(Cu),zinc(Zn),and nickel(Ni)]in roots, stems,leaves,fruits,and soil profiles(0-100 cm)of the plants in the orchards and wastelands were investigated and evaluated for the ecological risks of the soil and the crops by using the single factor contamination index method,the geoaccumulation index,the bioconcentration coefficient,and the correlation analysis respectively.【Result】The average values of heavy metal elements Cd,Cr,Cu,Pb,Ni and Zn in the soils of the study area exceeded the Guangxi background values of the soil elements,of which the contents of Cd and Zn were 7.7 and 5.4 times as much as the Guangxi background values of the soil elements. The results of the geoaccumulation index showed that the Cd in the orchard soil profile was moderately to severely polluted,and the maximum value of Zn was also moderately polluted;the Cd in the wasteland profile was moderately polluted,and the Zn was non-polluted to moderately polluted. The maximum mobility of 4 elements,Cd,Cu,Ni and Zn,in the soil profile of the orchard appeared at 50-60 cm. The maximum mobility of Ni and Cd,Cu and Zn in the soil profile of the wasteland appeared at 30-40 cm and 60-80 cm respectively. Cd had the highest activation rate in orchard and wasteland soils with strong activity. The ranking of crop contamination indexes for the 7 heavy metal elements in the orchard was Pb(22.09)> Ni(20.84)> Zn(3.24)> As(2.77)> Cr(1.43)> Cu(0.21)> Cd (0.01). Comparison of the bioconcentration coefficients of roots,stems,leaves and fruits of the crop revealed that all the elements were found to be maximum in the root portion of the crop;the highest bioconcentration coefficient of the element As was found in the fruits,which was 0.07. The minimum bioconcentration coefficients of all elements(Cu,Cd, Zn ,Ni,Cr)except the 2 elements(As and Pb)were found in fruits. In addition,it was found that there was no significant correlation between the content of the same heavy metal element in soil and fruits in the study area (P>0.05), namely, soil heavy metal element content did not affect crop heavy metal element content.【Conclusion】The exceeding rates of Cd and Zn in the soil of the study area are relatively high,and there is Cd pollution;compared with the soil,the bioconcentration coefficients of Cd and Zn in the fruits of the study area are generally low,which indicates that the heavy metal elements in the study area generally show characteristics of high content, low activity and low ecological risk.

     

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