The variation of REE (rare earth elements) patterns in soil-grown plants: a new proxy for the source of rare earth elements and silicon in plants

Rare earth elements (REEs) in five species of soil-grown plants (Taxodium japonicum, Populus sieboldii, Sasa nipponica, Thea sinensis and Vicia villosa) and in the soil on which each plant grew were determined with an inductively coupled plasma mass spectrometer (ICP-MS) in order to observe the variation in the distribution of REEs and to elucidate their source in soil-grown plants. The plant samples were divided into root (secondary root and main root), trunk (stem) and leaf; the soils into water soluble (soil_{soluble fraction}), HCl and HNO₃ soluble (soil_{non-silicate fraction}) and HF soluble (soil_{silicate fraction}). The REE abundances of samples were compared using REE patterns where the abundances were normalized to those of a chondrite and plotted on a logarithmic scale against the atomic number. All the plants showed similar REE patterns independent of species and location, and a W-shape variation (W-type tetrad effect) and abundance depletion of cerium (negative Ce anomaly) were found in each REE patterns of plants, more conspicuous tetrad effect being observed in HREE (heavier rare earth elements) region than in LREE (lighter rare earth elements) region. The overall variation of REE patterns of each secondary root was not similar to that of soil_{soluble fraction}, but similar to that of soil_{silicate fraction} except for the tetrad effect and Ce anomaly. The REE patterns can be interpreted by the idea that plants of different species take in REEs and Si from different parts in the soil. The results of this study seem to imply that Sasa nipponica and Vicia villosa take in free REEs and Si rather directly from silicate in the soil, and that a majority of REEs and Si in Taxodium japonicum and Thea sinensis are originated from the soluble fraction in the soil.