Soil metal contamination and fractionation of tea plantations: case studies in a normal tea garden and in a restored mineland tea stand

Extensive soil samples were collected from two tea gardens (a noted tea garden in Guilin and a mineland tea stand in Bayi Mn mine) in Guangxi to assess the heavy metal contamination level. The concentrations of Cu, Zn, Mn, Pb, Cd, and Al in garden soil were determined by ICP-AES. The results showed that the total concentrations of 6 metals (Cu, Zn, Mn, Pb, Cd, Al) in mineland tea garden soils were all higher than their soil background concentrations in Guangxi. Except for Cu, the concentrations of 5 other heavy metals in mineland tea garden were all higher than those in Guilin tea garden. Although tea garden soils contained high concentrations of Cd, most of the Cd existed in the form of residual fractions that were unavailable to higher plants according to the results of the sequential extraction. Similarly, Mn was dominated by iron-manganese oxide and residual fractions in mineland tea garden soils, but dominated by residual fractions in Guilin tea garden soils. Pb was dominated by residual and iron-manganese oxide fractions in mineland tea garden soils but by residual, iron-manganese oxide and exchangeable fractions in Guilin tea garden soils. Cu, Zn, and Al were all dominated by residual fractions in soils. A simple pollution index and nemerow index assessment were used to assess heavy metal contamination in the two tea gardens. Results showed that Cd was the key pollutant in tea garden soils. According to simple pollution indexes, no Pb, Zn, or Cu pollution was found. However, because of the high contribution of Cd in composite pollution index, bulk soils (in-between and outside tea tree rows) in Guilin tea garden met the moderate pollution level with nemerow index assessment. All other tea garden soils reached the heavy pollution standard.