Effects of humic acid on speciation and bioavailability to wheat of rare earth elements in soil

Abstract

Humic acid (HA) was extracted from an organic-rich soil in Heilongjian Province of China by 0.5 mol L-1 NaOH. The effects of HA on the distribution of rare earth elements (REEs) in Red Soil, a major soil type in the south of China, were investigated. The speciation of REEs in soil was analyzed by a sequential extraction technique. The bioavailabity of REEs in the same soil was investigated by the use of wheat seedling. The results showed that organic fraction of REEs is highly related to the concentration of HA, whereas the exchangeable fraction and carbonate fraction decreased as more HA was added. This was possibly related to the cation exchange capacity (CEC), because HA and CEC had a significant linear relationship. The REEs in wheat pooled in the roots. Low concentration of HA(<0.2 g kg-1) promoted the bioaccumulation of REEs in wheat root, while high concentration (>0.2 g kg-1) decreased the REE concentrations. The REE concentration in roots was negatively correlated to the HA concentration.