Effects of exogenous rare earths (REs) on transformation and distribution of elements at the root-soil interface and their bioavailability: a review

Exogenous rare earths (REs) application could alter the physical, chemical and biological characteristics at the root-soil interface, especially root-induced changes in the rhizosphere of plants. For example, exogenous REs, induce the extent of changes in root growth, root enzyme activities and root cell membrane permeability. Root biomass, some relative root enzyme activities and root activity reflect the capacity for absorption of nutrient (N, P, K, Ca, etc) and REE (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, etc) and heavy metal (Fe, Mn, Cu, Zn, Pb, Cd, Cr, Ni, etc) ions from rhizosphere soil solution. Hence, the morphological and physiological changes of root system result in the changes in uptake of rhizosphere nutrient and metal ions into plant roots directly. On the other hand, the release status of root exudates (organic acid, hydrogen ions, etc) affected by the change in root cell membrane permeability reflects the influence of rhizosphere environment on the transformation and distribution behaviors of nutrients and metals in the rhizosphere of plants. Such release could also further induce changes in uptake of rhizosphere nutrient and metal ions into plant roots indirectly. In this paper, the effect of exogenous REs application on root-soil micro-ecosystem and its environmental and biological effects in soil-plant systems were elucidated from four aspects as follows: 1) exogenous REs application impacts on root growth and root enzyme activities; 2) exogenous REs application impacts on root cell membrane permeability and root exudation; 3) exogenous REs application impacts on rhizosphere pH and the transformation and distribution (accumulation or depletion) of nutrients and metals in the rhizosphere of plants; 4) exogenous REs application impacts on the uptake and distribution of nutrients and metals by the roots of plants. Furthermore, some possible explanations for the above mentioned aspects were provided respectively, among which more attention was paid to the fractionation mechanisms of REEs at the root-soil interface because of their significance to studies on the phytoavailability of REEs.