Effect of Exogenous Rare Earth Elements on Fraction of Heavy Metals in Soils and Bioaccumulation by Plants

Abstract

Effects of exogenous rare earth elements (REEs) in fertilizer on fraction of heavy metals chromium (Cr), nickel (Ni), zinc (Zn), copper (Cu), lead (Pb), and cadmium (Cd) in 15 Chinese soils and their bioaccumulation by wheat root and shoot were investigated in a greenhouse experiment. Using a three-stage fraction analysis method suggested by the Standard and Testing Program of the European Community (STM), metal fractions were defined as fraction B1 (water soluble, exchangeable, carbonate bound form), B2 (Fe–Mn oxide bound form) and B3 (organic and sulfide bound form). The results revealed that concentrations of elements Cu, Zn, and Pb in fraction B1 increased for all 15 soils in the presence of REEs fertilizer, Cr increased in B1 for 11 soils. Chromium, Cu, Ni, and Zn also universally increased in fraction B3, but no significant changes were found in fraction B2. One-way ANOVA analysis also suggested that the response of heavy metal fractions to REEs fertilizer was significant (P≤0.05) in the order of B3>B1>B2. Contrary to the increasing amount of heavy metals in the extractable soil fractions, bioaccumulation of heavy metals in wheat shows no universal increase despite increased Pb in the shoot for 15 soils (P≤0.05) after REEs fertilizer was used. Concentrations of Zn, Pb, and Cd in wheat roots decreased universally, and Cu decreased universally in the shoot after REEs fertilizer was applied. The results of this study indicated that agricultural practice of REEs fertilizers changed the fraction distribution of heavy metals in soils, which in turn affected their availability to plants. In other words, introduction of REEs into soil does interrupt the elemental balance in the environment.