Uptake and Effects of Six Rare Earth Elements (REEs) on Selected Native and Crop Species Growing in Contaminated Soils

Rare earth elements (REEs) have become increasingly important metals used in modern technology. Processes including mining, oil refining, discarding of obsolete equipment containing REEs, and the use of REE-containing phosphate fertilizers may increase the likelihood of environmental contamination. However, there is a scarcity of information on the toxicity and accumulation of these metals to terrestrial primary producers in contaminated soils. The objective of this work was to assess the phytotoxicity and uptake from contaminated soil of six REEs (chloride forms of praseodymium, neodymium, samarium, terbium, dysprosium, and erbium) on three native plants (Asclepias syriaca L., Desmodium canadense (L.) DC., Panicum virgatum L.) and two crop species (Raphanus sativus L., Solanum lycopersicum L.) in separate dose-response experiments under growth chamber conditions. Limited effects of REEs were found on seed germination and speed of germination. Effects on aboveground and belowground biomass were more pronounced, especially for the three native species, which were always more sensitive than the crop species tested. Inhibition concentrations (IC25 and IC50) causing 25 or 50% reductions in plant biomass respectively, were measured. For the native species, the majority of aboveground biomass IC25s (11 out of 18) fell within 100 to 300 mg REE/kg dry soil. In comparison to the native species, IC25s for the crops were always greater than 400 mg REE/kg, with the majority of results (seven out of 12) falling above 700 mg REE/kg. IC50s were often not detected for the crops. Root biomass of native species was also affected at lower doses than in crops. REE uptake by plants was higher in the belowground parts than in the above-ground plant tissues. Results also revealed that chloride may have contributed to the sensitivity of the native species, Desmodium canadense, one of the most sensitive species studied. Nevertheless, these results demonstrated that phytotoxicity may be a concern in contaminated areas.

稀土元素（Rare earth elements, REEs）现已成为现代科技领域愈发关键的金属材料。采矿、石油炼制、丢弃含稀土元素的老旧设备以及施用含稀土元素的磷肥等过程，均可能提升环境污染发生的概率。然而，目前针对污染土壤中这类金属对陆生初级生产者的毒性与累积效应的研究信息仍较为匮乏。本研究旨在通过人工气候箱条件下的独立剂量-反应实验，评估六种稀土元素（镨、钕、钐、铽、镝及铒的氯化物形态）对三种本土植物（萝藦Asclepias syriaca L.、山蚂蝗Desmodium canadense (L.) DC.、柳枝稷Panicum virgatum L.）以及两种农作物（萝卜Raphanus sativus L.、番茄Solanum lycopersicum L.）的植物毒性及其从污染土壤中的吸收情况。研究发现稀土元素对种子萌发及萌发速率的影响较为有限，而对植物地上与地下生物量的影响则更为显著，尤其是三种本土植物，它们始终比本次试验中的农作物更为敏感。本研究测定了导致植物生物量分别降低25%和50%的抑制浓度（IC25与IC50）。对于本土植物而言，其地上生物量IC25值中有11组（共18组）处于100~300 mg REE/kg干土范围内；与本土植物相比，农作物的IC25值始终高于400 mg REE/kg干土，其中12组结果中有7组超过700 mg REE/kg干土。农作物的IC50值往往难以测得，本土植物的地下生物量在更低的暴露剂量下就会受到影响。植物对稀土元素的吸收量在地下部分高于地上组织。研究结果还显示，氯离子可能是导致本土植物山蚂蝗（Desmodium canadense）——本次试验中最为敏感的物种之一——敏感性提升的因素之一。尽管如此，本研究结果表明，污染区域内的植物毒性效应值得关注。