Heavy metals in soils and assessment of environmental risk in Myrica rubra plantations in Zhejiang province

Abstract: 【Objective】Myrica rubra is a subtropical tree grown for its sweet, crimson to dark purple-red,edible fruit, which is very populous in China. It is native to eastern Asia, mainly in south-central China,where the tree has been cultivated for at least 2 000 years. Due to the high medicinal and edible values ofM. rubra, the planting area of the fruit has exceeded 5 million acres in China including 1.3 million acresin Zhejiang province. With people’s concerns about the healthiness of the diet, increasing attention is be⁃ing paid upon the production environment and fruit quality and safety of M. rubra. Heavy metal content inM. rubra soil is an important parameter for the safety of the product. In this study, the contents of sixheavy metals in soils sampled from orchards in Taizhou, Wenzhou, Shaoxing and other places of Zhejiangprovince were investigated. The aim was to provide a reference for the evaluation and management of soilsand environmental quality in the traditional M. rubra producing regions in China.【Methods】Two hundred and forty-one soil samples were collected from the M. rubra producing areas in Zhejiang province from 2007 to 2013. Soils were sampled from a depth of 0-40 cm, and each sample volume was 1 kg evenly com⁃posed of soils sampled from five sites randomly selected from M. rubra orchards. The Pb, Cd, Cu, Zn andCr contents were detected with an atomic absorption spectrophotometer and As and Hg contents were de⁃tected using an atomic fluorescence spectrometer. Correlation analysis was carried out to reveal the rela⁃tionships between heavy metals. The assessment of heavy metal pollution in the orchards was conductedusing single pollution index and comprehensive pollution index methods.【Results】The average contentsof heavy metals in the soil samples from the M. rubra plantations were in the order of Zn> Cr> Pb> Cu>As> Hg> Cd. The contents of Hg, As, Pb and Zn in some soil samples exceeded the limits. In case of Hg,there was only one sample with 0.21 mg kg^-1, which was slightly higher than the limit of 0.20 mg kg^-1. Thecontents of As in 6 soil samples were slightly higher than the limit. The Pb contents in 18 soil samples collected mostly in 2007 and 2008 were higher than the current limit of 45 mg kg^-1. In addition, 20 soil samples had a Zn content that exceeded the limit value of 100 mg kg^-1. The majority of heavy metals in the M.rubra soils showed a slow deceasing trend from 2007 to 2013. Correlation analysis of heavy metals and pHvalue in the soils showed that there was no significant correlation between Hg and other elements. Thecontent of Cu showed significant positive correlations with As, Zn, Cd and Cr. There was a positive correlation between Pb and Cd as well as between Cr and As (P<0.001). However, a significant negative correla⁃tion was found between pH and As and between pH and Cr. Except for Hg, As, Pb and Zn in a few of thesoil samples, the heavy metal contents in most of the soil samples were lower than the limits of the soil environment quality standard. Soil samples were also assessed by means of the single factor pollution indexmethod and the comprehensive pollution index method. The result showed that the highest single factorpollution index was found in Pb in soil samples collected in 2008, which was 0.90. For most of the years (except for 2009 and 2012), the pollution index of Zn was higher than 0.70, which was below the limit forfood safety. However, the single factor pollution indexes of the heavy metals were all lower than 1.0, andso was the comprehensive pollution index.【Conclusion】Although some soil samples contained heavy metals exceeding the limits, the pollution index showed low risk of heavy metal pollution in the soils of M. rubra plantations in Zhejiang province. However, the pollution index of zn was always on the alert level,which needs to be further studied with the method of source apportionment. Policy of source control andprevention should be adopted to control heavy metal pollution in M. rubra plantations.