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

【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.