Relationship between the distribution of rare earth elements in soil pools with plant uptake: a sequential extraction study

Many studies have considered whether all forms of rare earth elements (REEs) in the soil are potentially bioavailable. The general consensus is that the exchangeable and carbonate-bound mineral pool is bioavailable. However, within the rhizosphere, site-specific geochemical conditions and biological weathering (due to plants, animals and microbes) may cause insoluble pools of REEs in the soil to become plant bioavailable. This study categorized soil into four fractions using the Community Bureau of Reference (BCR) sequential extraction method. The geochemical associations of REEs with soil fractions were assessed to determine which mineral and organic pools were most bioavailable by comparing the similarity of REE fractionation patterns in local plant tissues with the BCR extraction results for compatible soils. The results showed that the residual or more recalcitrant fraction of the soil displays a convex REE pattern with distinct depletion of middle REEs (MREEs) compared with light REEs (LREEs) and heavy REEs (HREEs). Some evidence suggested that Eu may be excluded by plants, as anomalous Eu concentrations were observed when the belowground plant tissue concentrations were normalized to the BCR extraction data. This study demonstrates that the REEs in belowground plant tissue are closely related, not only to exchangeable and carbonate-bound phases, but also to reducible and oxidizable soil fractions. According to the REE patterns, MREEs are more mobile than LREEs and HREEs, indicating enhanced bioavailability of MREEs.