Evaluation of plant availability of rare earth elements in soils by chemical fractionation and multiple regression analysis

Abstract

This case field study describes the distribution of rare earth elements (REEs) in different soil fractions obtained by a sequential extraction procedure and plant availability with single correlation and multiple regression analysis. Soil and plant samples were collected from a rural region of Beijing, China. Plant samples (corn, rice) were segmented into grain, stem, leaf and root. The results indicated that REE contents in different parts of plants followed the order: root>leaf>stem>grain. REEs in soils were extracted by a three-stage sequential extraction procedure into three fractions: water soluble, exchangeable and carbonate bound (B1), Fe–Mn oxide bound (B2), and organic matter and sulfide bound (B3). Chemical fractionation showed that the concentration of REEs in fraction B1 was close to or lower than 1% of the total REEs in soils, and in fractions B2 and B3 was in the range of 1.42–3.35 and 7.32–30.88% of total REEs in soil collected from corn fields, respectively. In soils from rice fields the concentration of REEs in fractions B1, B2 and B3 ranged from 0.58 to 1.05, 13.47 to 21.60 and 7.53 to 25.37% of total REEs, respectively. Correlation and multiple regression analysis indicated that total content of REEs in soils and the sum extracted with the sequential extraction procedure were poor indicators for REE uptake by plants. For La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb and Y, consistently significant positive correlations existed between contents in fraction B1 and concentrations in leaves of corn ( r=0.5662–0.7866) and rice ( r=0.5847–0.8262). As for other parts of rice and corn, significant correlations were observed sparsely. Multiple regression analysis was utilized to obtain the ‘best’ regression equations for predicting plant uptake of REEs. The study demonstrated that the sequential extraction procedure applied, though operationally defined, could provide valuable information on plant uptake of REEs in soils.