A Biotic Ligand Model for Phytotoxicity of Rare Earth Elements in Mixture

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Concentrations of rare earth elements (REEs) are increasing in the St. Lawrence River, resulting from agricultural runoff, from which increased soil concentrations in the St. Lawrence basin soils can be inferred. As soil pH is lower in eastern Canadian soils than in western Canada, the potential for toxicity of REEs in soil is growing, and the additional risk of REE mixtures to soil-dwelling biota, unknown. Cerium (Ce), Lanthanum (La), Neodymium (Nd), Samarium (Sm) are light REEs (LREEs) which are taken up by plants as divalent ions but may be translocated as trivalent ions. As trivalent cations, the lanthanides are thought to displace the divalent Ca from extra- and intra-cellular binding sites, with La being characterized as super-calcium. The nutrient effects of REEs on plants include increased photosynthesis and thus yield, enhanced uptake and utilization of nutrients, enhanced respiration and uptake of water and increased tolerance to stress. The first objective of the proposed work is to expand the knowledge of REE toxicity thresholds for terrestrial plants and endpoints. These thresholds will be determined in multiple soils that range in pH, effective cation exchange capacity (eCEC) and major soil cations, factors that are expected to vary the toxicity of REEs. These thresholds will be determined for both single metal and mixtures using two different types of concentration ratios: equitoxic (using the effect concentrations derived from single metal studies) and environmental (based on crustal and mining waste data). The REE mixtures responses will be characterized using a suite of statistical models that identify whether the responses follow concentration addition, response addition or a general linear model. These data will create the potency factors for the individual REEs. The data will also create testable hypotheses of how REE speciation and movement in the soil might be modifying the nominal exposure of plants to REE mixtures. The second objective is to explore these testable hypotheses of REE behavior in the soil by identifying the relationships among soil characteristics, bioaccessible REEs in soils, and tissue concentrations using batch extraction, DGT devices and geochemical speciation modelling software. The third objective is to integrate the data from the first two objectives into a BLM-type framework for plant toxicity that can be applied to REEs in soil, singly or in mixture, such as WHAM-FTOX. This will include the solution speciation of the REEs as modified by soil characteristics, competition among REEs and between REEs and other soil cations, for the biotic ligand, and how diffusive and bulk transport through the soil-plant continuum might modify exposure. The data from this project will be of practical use to Canadian regulators to inform soil quality guidelines for REEs. The data will also advance fundamental knowledge of soil metal toxicity to plants for elements that are trivalent in solution.