The evaluation of the potential risk of soil heavy metal pollution with conventional pesticide application in pear orchards

Abstract:【Objective】The pollution of heavy metals in soil is becoming a more and more concerned issue in pear orchards. It has huge impact on pear yield and fruit and environment safety. To investigate the contamination of heavy metals in soils of pear orchard with conventional chemical application (8-10 times a year) , this study was conducted in order to guide reasonable pesticide application and reduce the risk of heavy metal pollution in pear orchards and thus to ensure orchard sustainability.【Methods】Forty six pear orchards with conventional pesticide application were investigated for heavy metal contamination evaluation. These orchards were located in 5 suburb counties of Beijing, namely Pinggu, Daxing, Miyun, Shunyi and Fangshan. Fifteen trees in each pear orchard were randomly selected. Three soil samples at the depths of 0-20 cm, 20-40 cm and 40-60 cm were taken 0.5-0.6 m away from the trunk of each tree with a 4 cm diameter earth drill. The soil samples of each layer from 5 trees (15 sampling points) were pooled and about 1 kg of the pooled sample was put into a plastic bag as one replication.Three replications were designed in this study. All the samples were collected and carried to LEN laboratory at the same day. In the laboratory, the collected soil samples were put into a ventilated and cool place for air-drying. The totally dried soil samples were crushed in a mortar and sieved with 80 Nylon meshes. The atomic absorption spectrophotometer (single pass) in Vegetable Research Center of Beijing Agriculture and Forestry Academy was used to measure the contents of cadmium (Cd) , chromium (Cr) , lead (Pb) , srsenic (As) and mercury (Hg) in the soil samples. Single factor, Nemerow multi-factor pollution index, single pollutant and multiple pollutant potential ecological risk index were used for analysis and evaluation of the potential pollution degree and the risk of heavy metals in pear orchard soils.【Results】The contents of Cd in the 3 layers of soil in all pear orchards were higher than the soil background value of Beijing (0.12 mg· kg-1) , most of which were beyond the quality standard of national soil environmental (0.6 mg· kg-1) , and thus the soils had been polluted. Among them, the highest content of Cd was found in the soil layer of 40-60 cm in pear orchards in Shunyi district (6.88 mg · kg-1) . The contents of Cr and Hg were beyond the background values of Beijing soil, but no higher than the national standards of soil environmental quality. It showed that the accumulation of Cr and Hg occurred in Beijing.The other elements were in the normal levels. The single factor pollution indexes of Cd in the three soil layers of 46 pear orchards (except 40-60 cm soil layer in Daxing district) were higher than one, which could be regarded as being polluted. Others were not. The Nemerow multi-factor pollution indexes (multifactorial) in the five suburb counties were Shunyi> Miyun> Daxing> Pinggu> Fangshan. The 40-60 cm soil layer in the pear orchards in Shunyi district and 0-20 cm soil layer in pear orchards of Miyun district were both severely polluted. The potential ecological risk index of multiple pollutant RI (471.57) was at a serious level, and the potential ecological risk index of Cd reached the severe level (459.41) . The Nemerow multi-factor pollution index of the 0-20 cm soil layer in pear orchards in Miyun district showed that the soil was severely polluted too. The potential ecological risk index of multiple pollutant RI was in the medium level (202.5) . In addition, the potential ecological risk index of Cd reached the serious level (184.56) . Other heavy metal elements were all in the low risk level. The potential ecological risk indexes of Cd in 20-40 cm soil layer of Daxing and 20-40 cm and 0-20 cm soil layer in Shunyi were 157.50, 93.76 and 89.66, respectively. All of them were at the severe level. Others were at the low level. The potential ecological risks of single and multiple pollutants in the remaining suburb counties and in the other soil layers were all in the low level.【Conclusion】The results indicated that some pear orchards in Shunyi and Miyun regions might have Cd pollution, possibly due to the orchards located in a high Cd content area. The use of phosphorus fertilizers and pesticides should be reduced, and Cd hyperaccumulation plants such as Galinsoga parviflora, Cardamine hirsuta Linn. are recommended to be planted in the pear orchards.