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Rare earth elements in an intercropping cover crop to evaluate the trace element transfer from soil to plant

November 2022
Biogeochemistry 161(3)

DOI:10.1007/s10533-022-00989-7

Authors:

Bryan Arbalestrie

Catholic University of Louvain

Nathan Bemelmans

Catholic University of Louvain

Show all 7 authorsHide

Transfer of trace elements, such as toxic metals, from soil to plant is a corner stone for risk assessment. Rare earth elements (REE) are frequently used as environmental tracers to understand biogeochemical processes in the soil–plant system. In this study, we combined trace element and REE measurements in the soil–plant continuum to evaluate the element transfer between different compartments. We specifically aimed at: (1) assessing the geochemical relevance and representativeness of intermediate compartments (soil solution and soil water-extract as a proxy of the bioavailable soil fraction) by comparing the REE normalized patterns; and (2) characterizing the environmental conditions that control the trace element transfer by quantifying the REE indices. For that purpose, we compared geochemical signatures in an intercropping cover crop (bean, Persian clover, and spelt) in Belgium, including soil, root, shoot, soil solution, soil water-extract, earthworm, and snow samples. Evaluation of the element mobility was performed using both soil extractability and transfer factors. The main result showed dissimilar REE patterns between soil/plant samples and soil solution/soil water-extract samples, indicating that the intermediate compartments (i.e., soil solutions or soil water-extracts) do not chemically represent the bioavailable fraction of elements without obvious propensity to biological accumulation (unlike Cd, Cu, or Zn). Compared to light REE, heavy REE were more extractable and thus transferred to plants unlike what is observed in the literature. According to their different extractabilities, Ce and Eu allowed to highlight distinct transfer from soil to plant due to possible adsorption or organic matter complexation that should be further confirmed by studying contrasted soils.

Transfer factors from soil to plant (TFsoil-plant; A, C, and E) and from root to shoot (TFroot-shoot; B, D, and F) of trace elements and sum of rare earth elements (ΣREE) in Persian clover (Trifolium resupinatum; A and B), bean (Vicia faba; C and D), and spelt (Triticum spelta; E and F) collected at three sampling dates at the centre de Marbaix. Letters indicate statistically significant differences between sampling dates

…

Principal component analysis of log-ratio transformed trace element (A) and rare earth element (C) concentrations and element ratios Cd/Mn vs. Cu/Mn (B) and Ce/Eu vs. La/Lu (D) in samples collected at the centre de Marbaix

…

Normalized profiles of rare earth elements (REE) from samples collected at the centre de Marbaix: UCC-normalized soil profiles (A), soil-normalized soil water-extract profiles (B), soil-normalized soil solution and snow profiles (C), and snow-normalized soil solution profiles (D)

…

Soil-normalized profiles of rare earth elements (REE) from plant (shoot and root) and earthworm samples collected at the centre de Marbaix: bean (Vicia faba; A), Persian clover (Trifolium resupinatum; B), spelt (Triticum spelta; C), and earthworms (D)

…

Relationship between soil extractability (soil water-extract/soil concentration ratio, 0–30 cm soil depth) and soil solution/soil concentration ratio (A) and between soil–plant transfer (TFsoil–plant) and soil extractability (B) of trace elements (red) and rare earth elements (REE, from green for LREE to blue for HREE), as well as the average REE (open circle), in an intercropping cover crop soil–plant system

…

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Biogeochemistry (2022) 161:373–387

https://doi.org/10.1007/s10533-022-00989-7

Rare earth elements inanintercropping cover crop

toevaluate thetrace element transfer fromsoil toplant

BryanArbalestrie · JulieFalys·

NathanBemelmans · AdilThami·

LaurenceMonin· ElodieDevos· YannickAgnan

Received: 9 August 2022 / Accepted: 23 October 2022 / Published online: 3 November 2022

performed using both soil extractability and transfer

factors. The main result showed dissimilar REE pat-

terns between soil/plant samples and soil solution/soil

water-extract samples, indicating that the intermedi-

ate compartments (i.e., soil solutions or soil water-

extracts) do not chemically represent the bioavailable

fraction of elements without obvious propensity to

biological accumulation (unlike Cd, Cu, or Zn). Com-

pared to light REE, heavy REE were more extractable

and thus transferred to plants unlike what is observed

in the literature. According to their diﬀerent extract-

abilities, Ce and Eu allowed to highlight distinct

transfer from soil to plant due to possible adsorption

or organic matter complexation that should be further

conﬁrmed by studying contrasted soils.

Keywords Rare earth elements· Trace elements·

Soil· Plant· Soil solution· Transfer

Introduction

Trace elements, whose concentration is below 0.1% in

the continental crust, include metals (Cd, Cu, Mn, Ti,

V, Zn…) and metalloids (As, Sb…) that may induce

potentially harmful eﬀects on human and ecosystem

health. The environmental cycling of trace elements

depends on both element sources and compartment

conditions (Rauch and Pacyna 2009; Kabata-Pendias

2010). In agricultural soils, they are originated from

(He etal. 2005): (1) natural sources, mainly resulting

Abstract Transfer of trace elements, such as toxic

metals, from soil to plant is a corner stone for risk

assessment. Rare earth elements (REE) are frequently

used as environmental tracers to understand biogeo-

chemical processes in the soil–plant system. In this

study, we combined trace element and REE meas-

urements in the soil–plant continuum to evaluate the

element transfer between diﬀerent compartments.

We speciﬁcally aimed at: (1) assessing the geochemi-

cal relevance and representativeness of intermediate

compartments (soil solution and soil water-extract as

a proxy of the bioavailable soil fraction) by compar-

ing the REE normalized patterns; and (2) characteriz-

ing the environmental conditions that control the trace

element transfer by quantifying the REE indices. For

that purpose, we compared geochemical signatures

in an intercropping cover crop (bean, Persian clover,

and spelt) in Belgium, including soil, root, shoot, soil

solution, soil water-extract, earthworm, and snow

samples. Evaluation of the element mobility was

Responsible Editor: Dale Johnson

Supplementary Information The online version

contains supplementary material available at https:// doi.

org/ 10. 1007/ s10533- 022- 00989-7.

B.Arbalestrie· J.Falys· N.Bemelmans· A.Thami·

L.Monin· E.Devos· Y.Agnan(*)

Earth andLife Institute, Université catholique de Louvain,

1348Louvain-la-Neuve, Belgium

e-mail: yannick.agnan@biogeoscience.eu

Content courtesy of Springer Nature, terms of use apply. Rights reserved.

Pollution and health risk assessment of rare earth elements in soil, water and vegetables from an ion-adsorption rare earth mining area in Ganzhou, China

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ENVIRON GEOCHEM HLTH

Rare earth elements (REEs) are increasingly recognized as significant environmental pollutants due to their environmental persistence, bioaccumulation, and chronic toxicity. This study assessed REEs pollution in soil, water, and vegetables in an ion-adsorption rare earth mining area in Ganzhou, and evaluated the associated health risks to the local population. Results indicated that the REEs content in soil ranged from 168.58 to 1915.68 mg/kg, with an average of 546.71 mg/kg, substantially surpassing the background level for Jiangxi Province (243.4 mg/kg) and the national average (197.3 mg/kg). Vegetables displayed an average REEs content of 23.17 mg/kg in fresh weight, far exceeding the hygiene standard of 0.7 mg/kg. Water samples contained REEs at a concentration of 4.09 μg/L. The estimated daily intake (EDI) of REEs from vegetables exceeded the threshold for subclinical damage, posing potential health risks, particularly for children and adolescents. Further analysis of the adjusted average daily intake (ADI) and non-carcinogenic risk suggested that while most vegetable consumption remains within safe threshold, the intake of REEs from high-risk vegetables such as pakchoi should be limited. Overall, carcinogenic risks associated with lifetime cancer risk (LCR) model for REEs exposure through vegetables and water were found to be low in this area.

Screening for Heavy Metal-Resistant Clones in the Xero-Halophyte Atriplex halimus L.: A Prerequisite for Phytoremediation of Polymetallic Mining Pollution in Arid Areas

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INT J ENVIRON RES

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Pollution and health risk assessment of soil, water and vegetables from an ion-adsorption rare earth mining area in Ganzhou, China

Preprint

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Aug 2024

Rare earth elements (REEs) are increasingly recognized as significant environmental pollutants due to their environmental persistence, bioaccumulation, and chronic toxicity. This study assessed REEs pollution in soil, water, and vegetables in an ion-adsorption rare earth mining area in Ganzhou, and evaluated the associated health risks to the local population. Results indicated that the REEs content in soil ranged from 168.58 to 1915.68 mg/kg, with an average of 546.71 mg/kg, substantially surpassing the background level for Jiangxi Province (243.4 mg/kg) and the national average (197.3 mg/kg). Vegetables displayed an average REE content of 23.17 mg/kg in fresh weight, far exceeding the hygiene standard of 0.7 mg/kg. Water samples contained REEs at a concentration of 4.09 µg/L. The estimated daily intake (EDI) of REEs from vegetables exceeded the threshold for subclinical damage, posing potential health risks, particularly for children and adolescents. Further analysis of the adjusted average daily intake (ADI) suggested that while most vegetable consumption remains within safe threshold, the intake of REEs from high-risk vegetables such as pakchoi and radish should be limited.

Influence of glyphosate and aminomethylphosphonic acid on the mobility of trace elements in uncontaminated and contaminated agricultural soils

Article

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Sep 2023
ENVIRON SCI POLLUT R

Glyphosate is one of the most widely used herbicides in the world. In addition to its herbicidal effect, glyphosate is a chelating agent that can form complexes with trace elements. Yet, agricultural soils can be contaminated with both organic and mineral substances, questioning the possible influence of glyphosate application on the trace element mobility. In this context, we specifically studied the extractability of trace elements in uncontaminated and metal-contaminated agricultural soils by adding glyphosate, formulated glyphosate, and aminomethylphosphonic acid (AMPA, a degradation product of glyphosate) in batch experiments from 0 to 100 mg L⁻¹. Results showed that, on average, glyphosate enhanced the extractability of the elements considered (e.g., As, Cd, Cu, Pb, and Zn) at 20 and 100 mg L⁻¹. Surprisingly, the uncontaminated soil highlighted the highest influence of glyphosate compared to the contaminated ones, likely resulting from a higher natural element extractability in the contaminated soils. Although formulated glyphosate presented an overall higher impact than unformulated glyphosate, it was evidenced that AMPA showed lower influence meaning that glyphosate degradation is beneficial to limit deleterious effects.

Glyphosate application may influence the transfer of trace elements from soils to both soil solutions and plants

Article

Oct 2024
CHEMOSPHERE

Vertical distributions and potential contamination assessment of seldom monitored trace elements in three different land use types of Yellow River Delta

Article

Jan 2024

Rare earth elements in tea garden soils and their bioavailability to tea buds in Taiwan

Article

Jun 2023
SCI TOTAL ENVIRON

Tea (Camellia sinensis) is a widespread beverage plant that prefers aluminum-enriched acidic soils. However, rare earth elements (REEs) might be highly phyto-available in these soils. With the increasing demands for REEs in high-technology industries, understanding the dynamics of REEs in the environment is essential. Thus, this study identified the total concentration of REEs in the root-zone soils and corresponding tea buds (n = 35) collected from tea gardens in Taiwan. Additionally, the labile REEs in the soils were extracted with 1 M KCl, 0.1 M HCl, and 0.05 M ethylenediaminetetraacetic acid (EDTA) to elucidate the fractionation tendency of REEs in the soil-plant system and the relationships between REEs and Al in the tea buds. The concentration of light REEs (LREEs) was higher than those of medium REEs (MREEs) and heavy REEs (HREEs) in all soil and tea bud samples. According to the upper continental crust (UCC) normalization, MREEs and HREEs were more abundant than LREEs in the tea buds. Furthermore, REEs remarkably increased with increasing Al in the tea buds, whereas the linear correlations between Al and MREEs and HREEs were stronger than between LREEs. Compared with LREEs, the extractabilities of MREEs and HREEs by all single extractants in the soils were higher, coinciding with their higher UCC-normalization-based enrichments in the tea buds. Moreover, the 0.1 M HCl- and 0.05 M EDTA-extractable REEs were affected by soil properties and significantly correlated with the total REEs in the tea buds. The concentration of REEs in the tea buds was successfully predicted by empirical equations of extractable REEs with 0.1 M HCl and 0.05 M EDTA, as well as general soil properties including pH, organic carbon, dithionite-citrate-bicarbonate-extractable iron, aluminum, and phosphorus. However, this prediction should be further verified using many soil and tea types in the future.

Pollution and health risk assessment of rare earth elements in Citrus sinensis growing soil in mining area of southern China

Article

Jun 2023

Background Analyzing the pollution and health risk of rare earth elements (REEs) in crop-growing soils around rare earth deposits can facilitate the improvement of REE mining-influenced area. In this study, pollution status, fraction and anomaly, plant accumulation characteristics, and potential risks of REEs (including heavy and light rare earth elements, HREEs and LREEs) in C. sinensis planting soil near ion-adsorption deposits in southern Ganzhou were analyzed. The influence of the soil environment on REEs in soil and fruit of C. sinensis was also explored. Methods The geo-accumulation index (I geo ) and ecological risk index(RI) were used to analyze the pollution potential and ecological risks of REEs in soils, respectively. Health risk index and translocation factor (TF) were applied to analyze the accumulation and health risks of REEs in fruit of C. sinensis . The influence of soil factors on REEs in soil and fruit of C. sinensis were determined via correlation and redundancy analysis. Results Comparison with background values and assessment of I geo and RI indicated that the soil was polluted by REEs, albeit at varying degrees. Fractionation between LREEs and HREEs occurred, along with significant positive Ce anomaly and negative Eu anomaly. With TF values < 1, our results suggest that C. sinensis has a weak ability to accumulate REEs in its fruit. The concentrations of REEs in fruit differed between LREEs and HREEs, with content of HREE in fruit ordered as Jiading > Anxi > Wuyang and of LREE in fruit higher in Wuyang. Correlation and redundancy analysis indicated that K 2 O, Fe 2 O 3 and TOC are important soil factors influencing REE accumulation by C. sinensis , with K 2 O positively related and Fe 2 O 3 and TOC negatively related to the accumulation process.

Effect of substrate properties and phosphorus supply on facilitating the uptake of rare earth elements (REE) in mixed culture cropping systems of Hordeum vulgare, Lupinus albus and Lupinus angustifolius

Article

Full-text available

Aug 2022
ENVIRON SCI POLLUT R

This study presents how phosphate (P) availability and intercropping may influence the migration of rare earth elements (REEs) in legume-grass associations. In a replacement model, Hordeum vulgare was intercropped with 11% Lupinus albus and 11% Lupinus angustifolius. They were cultivated on two substrates, A (pH = 7.8) and B (pH = 6.6), and treated with 1.5 g P m −2 or 3 g P m −2. Simultaneously, a greenhouse experiment was conducted to quantify carboxylate release. There, one group of L. albus and L. angustifolius was supplied with either 200 µmol L-1 P or 20 µmol L-1 P. L. albus released higher amounts of carboxylates at low P supply than L. angustifolius, while L. angustifolius showed the opposite response. Plants cultivated on substrate B accumulated substantially higher amounts of nutrients and REE, compared to substrate A. Higher P supply did not influence the leaf and stem P concentrations of H. vulgare. Addition of P decreased REE accumulation in barley monocultures on alkaline soil A. However, when H. vulgare was cultivated in mixed culture with L. angustifolius on alkaline substrate A with high P supply, the accumulation of REE in H. vulgare significantly increased. Conversely, on acidic substrate B, intercropping with L. albus decreased REE accumulation in H. vulgare. Our findings suggest a predominant effect of soil properties on the soil-plant transfer of REEs. However, in plant communities and within a certain soil environment, interspecific root interactions determined by species-specific strategies related to P acquisition in concert with the plant's nutrient supply impact REE fluxes between neighbouring plants.

The ‘europium anomaly’ in plants: facts and fiction

Article

Full-text available

Jul 2022
PLANT SOIL

Aims Rare earth elements (REEs) and normalized REE patterns determined in plant and soil samples represent powerful tools to trace biogeochemical processes during weathering, soil genesis and processes in the rhizosphere, and thus publications reporting REE concentrations and normalized REE patterns in soil systems and plants are rapidly increasing. Methods A normalized REE pattern allows for the recognition of anomalous concentrations of an individual REE. In the literature anomalies are predominantly reported/focused for/on the redox-sensitive elements cerium (Ce) and europium (Eu) that can shift their oxidation state during interactions with organic and inorganic soil phases and the biological processes affecting their mobility in soil and uptake by plants. Thus positive Eu anomalies in plants are often interpreted as a consequence of reduction of Eu3+ to Eu2+ in the rhizosphere followed by a preferential uptake of Eu2+. Results Due to an analytical artefact in ICP-MS analysis, a false Eu anomaly may be reported. This can be avoided by using a barium (Ba) interference correction. We draw attention to the possibility of this problem and to being aware of its potential occurrence when Eu anomalies are reported. Conclusions We recommend (i) including information on how this potential problem was dealt with in the Materials and Methods section of articles and (ii) how to implement Findable Accessible Interoperable and Reusability (FAIR) guiding principles in that section (including data availability in an open repository).

Composition of the Continental Crust

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Jan 2014

This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

Presence and possible origin of positive Eu anomaly in shoot samples of Juncus effusus L

Article

Nov 2019
J TRACE ELEM MED BIO

Background: The rare earth elements (REE) are non-essential elements for plants. They stimulate plant growth at low doses, but at high levels are phytotoxic. There are differences in concentrations of REE in various organs of the same plant species, but the normalized REE patterns can be very similar in samples of the same species collected in different locations. Here we compare normalized REE curves in above-ground samples of Juncus effusus L. (common rush, soft rush) collected from sites with different land-use types. Methods: The concentrations of rare earth elements were measured in 55 shoot samples of J. effusus L. The samples were collected from 15 sampling sites located in the Holy Cross Mts., south-central Poland and analyzed with the use of inductively coupled plasma mass spectrometry (ICP-MS). The results were normalized to the North American Shale Composite and anomalies of different elements were calculated. Results: Total REE concentrations varied from 0.028 mg/kg to 2.7 mg/kg. The samples were enriched in the light REE (from La to Eu) with the highest concentrations of La and Ce. The North American Shale Composite (NASC)-normalized REE curves were roughly similar in all samples except for two samples collected in the acid mine drainageaffected areas. Conclusion: All samples showed positive europium anomalies in NASC-normalized REE concentration patterns. The most probable explanation of this is that the uptake and translocation of Eu in J. effusus (and possibly in other wetland plants) is caused by a short-term decrease of the redox potential in a rhizosphere favoring reduction of Eu3+ to Eu2+ and thus enhancing Eu mobility in the soil-plant environment.

Trace elements in soils and plants: Fourth edition

Book

Oct 2010

Alina Kabata-Pendias

Still the Gold Standard Resource on Trace Elements and Metals in Soils This highly anticipated fourth edition of the bestselling Trace Elements in Soils and Plants reflects the explosion of research during the past decade regarding the presence and actions of trace elements in the soil-plant environment. The book provides information on the biogeochemistry of these elements and explores how they affect food quality. Incorporating data from over 1500 new resources, this edition includes the most up-to-date information on the relationship of trace elements to topics such as: •Soil natural/background contents •Sorption/desorption processes •Anthropogenic impact and soil phytoremediation •Phytoavailability and functions in plants •Contents of food plants The book discusses the assessment of the natural/background content of trace elements in soil, bioindication of the chemical status of environmental compartments, soil remediation, and hyperaccumulation and phytoextraction of trace metals from the soil. The table of contents reflects the IUPAC’s recommendation for numbering element groups, giving the new edition an updated organizational flow. Trace Elements in Soils and Plants, Fourth Edition illustrates why trace elements’ behavior in soil controls their transfer in the food chain, making this book an invaluable reference for agronomists, soil and plant scientists, nutritionists, and geochemists.

Effect of rare earth elements on rice plant growth

Article

May 2018
CHEM GEOL

The goals of this study were, (1) to quantify the effects of rare earth elements (REE) on rice plant growth and (2) to determine whether the presence of iron (III) oxides on the plant root surface (i.e. the iron oxide plaques) played a role in impeding any toxic effects caused by the presence of the REE. Hydroponic experiments were designed to grow rice plants in a greenhouse under controlled conditions, exposed to all rare earth elements simultaneously, and to iron (II) sulfate or iron (II) chloride. The results showed a significant decrease in root and plant height and biomass at rare earth element concentrations of 0.5 mg/L and 1 mg/L. Negative growth effects were observed for plant roots and shoots upon addition of 100 μmol/L Fe(II) chloride or Fe(II) sulfate. Even when the root biomass was enhanced upon addition of Fe (II) chloride at a 1 mg/L rare earth concentration, however, statistically significant decreases in root length and plant height were recorded. In the presence of Fe(II) sulfate, a negative growth effect was present for all REE concentrations, being more pronounced at the highest REE levels. For the Fe(II) chloride experiments, speciation modeling showed that the rare earth elements would remain “free” as hydrated ions (Ln³⁺) or would be complexed by Fe(III) oxyhydroxides. With Fe(II) chloride, the light rare earths (La, Ce, Pr, Nd; LREE) remained mostly soluble, whereas the middle (Sm, Eu, Gd; MREE) and heavy (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; HREE) elements were for the most part bound by Fe(III) solids. As negative growth effects were observed with Fe(II) chloride, the most soluble LREE could be concluded to play a role in rice plant growth inhibition. Furthermore, upon addition of Fe(II) sulfate, the MREE and HREE were significantly associated with SO4²⁻ and a regained toxic effect for rice plants was observed at the highest REE concentration, suggesting also an effect of the dissolved MREE-SO4⁺ and HREE-SO4⁺ species on rice growth. This observation, coupled to the knowledge that SO4²⁻ is an essential nutrient for plants, suggests absorption of the REE-SO4⁺ species by the rice plant. The recorded negative growth effects for both the Fe(II) chloride or Fe(II) sulfate conditions, strongly suggest that all REE are detrimental to the development of rice. For both the Fe(II) chloride and Fe(II) sulfate conditions the negative growth effects may have been attenuated as a consequence of REE sorption to Fe(III) oxyhydroxides (i.e. iron oxide plaques) identified on the root surface, as suggested by surface complexation modeling of the REE to iron (III) oxides, at the pH and ionic strength conditions in this study.

Rare earth elements in German soils - A review

Article

Apr 2018
CHEMOSPHERE

Extraction and Characterization of Pore Water in Contaminated Soils

Chapter

Jan 2018

Chemical elements that are either present naturally in the soil or introduced by pollution are more usefully estimated in terms of "availability" of the element, since it is this property that can be related to mobility and uptake by plants. A good estimate of the immediately available fraction can be achieved by measuring the concentration, or activity, of chemical species in soil pore water. Current analytical techniques enable the application of this approach to trace elements, such as plant and animal micronutrients and those defined as potentially toxic elements (PTEs) in environmental studies. A complete chemical analysis of soil pore water represents a powerful diagnostic tool for the interpretation of many soil chemical phenomena relating to soil fertility, mineralogy, and environmental fate. This chapter describes some of the current methodologies used to extract soil pore water. In particular, five laboratory-based methods are described and discussed in detail: (i) high speed centrifugation-filtration, (ii) low (negative-) pressure Rhizon samplers and passive diffusion samplers, (iii) high pressure soil squeezing, (iv) equilibration of dilute soil suspensions, and (v) diffusive gradients in thin-films (DGT). A number of operational factors are presented: pressure applicable (i.e., pore size accessed), moisture prerequisites of the soil, pore water yield, efficiency, duration of extraction, materials and possible sources of contamination for micronutrient and PTE studies. There is also consideration of the advantages and disadvantages of the methods, including costs and material availability.

R: A Language and Environment for Statistical Computing

Book

Jan 2015

Core R Team

Trace elements in the soil-plant interface: Phytoavailability, translocation, and phytoremediation–A review

Article

Jun 2017
EARTH-SCI REV

Trace elements (TEs) are deposited to soils mainly due to anthropogenic activities and pose a significant threat to human health. In this review we aimed at (a) discussing the phytoavailability of TEs as affected by various soil parameters, and by plant defense mechanisms related to uptake and translocation; (b) examining soil and plant indices related to TE phytoavailability; (c) clarifying the challenges and problems related to phytoremediation; and (d) exploring the often encountered discrepancies of lower-than-expected TE toxicity. We particularly discussed the soil-to-plant availability index (transfer coefficient, TC), because it encompasses all soil and plant factors related to TE phytoavailability. As for soil, we explored the effect of pH, redox potential, clay and organic matter contents, as well as aging of added elements. The latter is a key factor in interpreting the observed lower-than-expected toxicity to plants in real field conditions. This is because the discrepancy is very often generated by growth experiments that expose plants to TEs directly from TE-laden solutions or by studies that spike soils with TEs only days or weeks before planting. Also, the behavior of TEs depends on the nature and quantity of TEs. As for plant, TE absorption or exclusion is highly related to species-specific defense mechanisms developed by plants so that they are exposed to TE-induced stress. These mechanisms address TE exposure by operating both outside and inside the plant body; outside with the assistance of root exudates, and the rhizosphere microflora, and inside with selective translocation and storage processes. The absorption/exclusion behavior of plants also depends on root activities and related soil chemical processes which are highly localized within a spatial scale of a few mm from roots. Novel techniques for the imaging of TE biogeochemistry at the root-soil interface are therefore addressed and their explanatory power is demonstrated. Such plant behavior greatly affects phytoremediation, a process which also depends on the maximal TE uptake capacity of plants, especially of hyperaccumulators. However, phytoremediation also greatly depends on plant biomass yield, an important factor in determining the time required to complete the procedure. In conclusion, soil factors, as well as plant- and TE- related issues, may create discrepancies in TE phytoavailability and phytoremediation that need to be thoroughly understood and addressed.

Rare earth elements in an intercropping cover crop to evaluate the trace element transfer from soil to plant

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