Anastasia Bolotnikova

Pierre Gergondet

Arnaud Tanguy

Sébastien Courtois

Abderrahmane Kheddar

Task-Space Control Interface for SoftBank Humanoid Robots and its Human-Robot Interaction Applications

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION AND BACKGROUND

Quadratic Programming (QP) task-space control has become a golden standard in humanoid robot control [START_REF] Escande | Hierarchical quadratic programming: Fast online humanoid-robot motion generation[END_REF], [START_REF] Cisneros | Qpbased task-space hybrid/parallel control for multi-contact motion in a torque-controlled humanoid robot[END_REF], [START_REF] Bouyarmane | Quadratic programming for multirobot and task-space force control[END_REF]. One of the most powerful software control frameworks that implements QP task-space control, called mc rtc 1 , is now publicly available. This framework provides developers with useful tools for writing complex controllers for either individual or multiple interacting robots to perform wide variety of experiments, e.g. in aircraft automation [START_REF] Kheddar | Humanoid robots in aircraft manufacturing: The airbus use cases[END_REF] or in physically interacting with humans [START_REF] Otani | Generating assistive humanoid motions for co-manipulation tasks with a multi-robot quadratic program controller[END_REF], [START_REF] Bolotnikova | Autonomous initiation of human physical assistance by a humanoid[END_REF] that we technically explain and extend in this work.

In order to control any given complex robots with this framework, an interface must be developed to allow communication between the control framework and the robot's low-level controllers, sensors and devices. One example of such open-source interface is the mc openrtm 2 software that is used to control Humanoid Robotics Project robots from Kawada Robotics [START_REF] Hirukawa | Humanoid robotics platforms developed in hrp[END_REF]. However, for different types of robots a specific interface, as well as robot description and robot module, need to be developed in order to adapt to particularities of robot brand and low-level control strategies, devices and onboard operating system (OS). The idea of taskspace control is to lie exactly between low-level control and high-level planning with somewhat adjustable frontiers.

In this work, we present the open-source software interface, called mc naoqi 3 , that enables running the controllers implemented with mc rtc framework on widely used Soft-Bank Robotics Europe (SBRE) humanoid robots [START_REF] Gouaillier | Mechatronic design of nao humanoid[END_REF], [START_REF] Pandey | A mass-produced sociable humanoid robot: pepper: the first machine of its kind[END_REF], that are running NAOqi OS and a customly developed local module mc naoqi dcm (Sec. II). We also provide robot modules and description packages that serve as a representation of SBRE robots in the mc rtc framework (Sec. III). Additionally, we release a basic sample Finite State Machine (FSM) mc rtc controller for Pepper robot and its interaction with a human model in simulation (Sec. IV). Finally, we present our current instance use of the developed tools for running the mc rtc controllers developed for Human-Robot interaction (HRI) experiments with human subjects (Sec. V).

Fig. 1 shows an overview of the developed tools and their interconnection, described in detail in the rest of the paper.

II. CONTROL INTERFACE

Fig. 2 gives a schematic overview of the mc rtc control framework and its connection to the simulation and control interfaces, such as mc naoqi. Interested readers are invited to visit mc rtc project website for more information, installation instructions and tutorials. Fig. 2: mc rtc control framework architecture. Fig. 1 illustrates schematically the role of the mc naoqi interface as a communication layer between mc rtc control framework and NAOqi OS running onboard SBRE humanoid robots, such as NAO, Pepper or Romeo. In the following subsections, we describe the entire system in detail.

A. Forwarding device commands from controller to the robot

The main role of the mc naoqi interface is to forward the fixed frame rate control commands computed by the QP solver of mc rtc whole-body QP controller to the onboard low-level robot actuators control. The decision variable of the mc rtc QP controller is robot joint accelerations, which is integrated once to get desired velocity (e.g. for Pepper mobile base command), and then once more to get desired robot joint angles for joint actuator commands.

For social, user-friendly and interacting humanoid robots, such as Pepper or NAO, it is highly beneficial to endow mc rtc controller with the functionality to also forward other device commands, such as sentence to play from the speakers, desired tablet screen image or eye led color, from the mc rtc controller to the robot devices via mc naoqi interface. This functionality allows mc rtc framework users to develop controllers which can provide a richer interaction experience for HRI applications. For instance, when a certain mc rtc controller FSM state is terminated, the robot can indicate this event (and that it is going to transition to the next FSM state) with a comprehensive verbal message, illustrative tablet image and/or a specific LED color.

The mc naoqi interface is thus developed with rich HRI consideration in mind, and allows mc rtc controller to forward commands to all the robot devices, not only the joint actuators. We describe how it is implemented on the robot module side in detail in Sec III.

B. Forwarding sensor data from the robot to the controller

The mc naoqi interface is also responsible for getting the most up-to-date sensor readings from a robot low-level memory in real-time and forwarding sensor measurements in a suitable form to the mc rtc controller real robot state representation as a feedback. This way, the task-space QP controller keeps track of the real robot state and can use it to perform closed-loop QP control computations.

Besides common sensor readings, such as encoder values, force sensors or Inertial measurement unit (IMU) measurements, mc naoqi interface also allows to forward from the robot to the mc rtc controller such sensor readings as electric motor current and touch sensor readings (from tactile or bumper sensors). We describe how custom robot sensors are implemented on the robot module side in detail in Sec III.

For HRI applications, the touch sensor readings are especially beneficial to be forwarded to the controller, as they allow to detect when a human touches the robot. This signal can then be used inside the mc rtc FSM controller, for instance, to trigger an appropriate reaction of the robot to the touch or to trigger a transition to a specific FSM state of the controller. We demonstrate this use-case in our sample HRI experiment, presented in detail in Sec. V.

C. Local low-level robot module

A customized local robot low-level module, called mc naoqi dcm 4 , is cross-compiled for NAOqi OS and is set to run onboard the robot to read sensor values and set device commands synchronized with a robot control loop via Device Communication Manager (DCM) every 12 ms, fastest update rate currently available for SBRE robots.

When a user connects to the robot via mc naoqi interface, and commands to turn on robot motors, a preprocess function is connected to DCM loop, to start setting the actuator commands at a fixed time rate, synchronized with the other DCM operations. After the user commands to turn off robot motors via mc naoqi interface, the preprocess actuator command update function is disconnected from the DCM loop. This way, the user can safely use other control applications, such as Choregraphe, with the robot right after using mc naoqi interface, even though mc naoqi dcm module is still active on the robot.

In order to prevent the default high-level robot behaviours to interfere with the commands forwarded to the robot via mc naoqi from the mc rtc controller, the default robot safety reflexes are disabled when robot motors are turned on via mc naoqi. Once the motors are turned off via mc naoqi, the default robot safety reflexes are re-enabled. This has to be taken into account by an mc rtc controller designer, to ensure that the developed controller is safe to be run on the real robot, through extensive testing in simulation.

A fast access to the low-level robot memory is initialized when mc naoqi dcm starts to run on the robot. This allows to read a predefined set of sensor values from robot memory in the fastest way.

III. ROBOT REPRESENTATION IN mc rtc FRAMEWORK

A. Robot description packages

To control any robot with mc rtc framework, a basic description of this robot needs to be provided. Such robot description includes robot kinematic tree, dynamic properties of the links, description of robot contact surfaces (i.e. covers), and convex (eventually strictly convex [START_REF] Escande | A strictly convex hull for computing proximity distances with continuous gradients[END_REF]) anticollision shapes (these shapes can be automatically generated from existing robot links graphic representation). Examples of these robot description elements for SBRE humanoids are shown in Fig. 3. With this work, we release these robot description projects, implemented as Robot Operating System (ROS) packages, for NAO 5 and Pepper 6 robots.

B. Robot mc rtc modules

As illustrated in Fig. 1, the robot description packages are used by a robot module software layer to create a structure that provides a complete description of the robot: kinematic tree parsed from URDF files, visual and physical representations of robot links, surfaces attached to the robot bodies, sensors and other devices, strictly convex hulls and primitive shapes for collision avoidance, etc. The instance of this structure is used by the mc rtc framework, as control robot state representation, to formulate the QP control problem (objectives and constraints), that is then passed to the solver to compute the next desired robot state. 5 https://github.com/jrl-umi3218/nao_description 6 https://github.com/jrl-umi3218/pepper_description

We make the robot modules for NAO 7 and Pepper8 publicly available with this work. For fast prototyping and experiments, the robot description package and module can easily be augmented with any new robot hardware elements, e.g. new onboard camera, which we demonstrate in Sec. V.

The Pepper robot module exploits a recently introduced new mc rtc framework feature -generic robot devices. This feature enables developers to implement any kind of robot custom device representation as part of the robot module, which can then be used in the mc rtc controller. Currently implemented devices in Pepper robot module are:

• loud speaker: to forward speech commands to the robot directly from the mc rtc controller via mc naoqi; • visual display tablet: to set robot tablet image from the mc rtc controller via mc naoqi interface; • touch sensor: to forward tactile sensor readings from the robot to the mc rtc controller via mc naoqi Wheeled mobile base of Pepper is modeled as a floating base, constrained to move on a plane (i.e. the room ground), with limits imposed to the mobile base body maximum speed and acceleration according to physical hardware limitations.

In mc rtc framework, two main hierarchies for the robot control are the following: An example of how these custom Pepper robot specific tasks and constraints are loaded and used in a sample mc rtc controller can be seen in PepperFSMController open-source project, which we describe in detail in Sec. IV. In an analogous way, many other novel tasks and constraints can easily be implemented and tested.

IV. SAMPLE PEPPER mc rtc FSM CONTROLLER

A. Individual robot controller

To facilitate the process of writing a new mc rtc controller, especially for new potential users of the framework, we provide a basic sample PepperFSMController 9 . It can be used as a starting point for new controller development or as an example of how similar projects should be implemented.

The sample controller includes Pepper as the main controller robot, associated default posture task, kinematics and dynamics constraints, self-collision avoidance constraints, robot-ground contact constraint. The controller also includes implementation of few sample FSM states that allow to control robot posture, mobile base, camera orientation and right and left hand end-effector positions, either through predesined setpoints or interactively through RViz [START_REF] Kam | Rviz: a toolkit for real domain data visualization[END_REF] (Fig. 4).

B. Controller for HRI including a human model

A direct extension of the master branch of the sample controller project, is a branch called topic/withHumanModel. On this branch, a multirobot QP (MQP) control feature of the mc rtc framework is exploited by adding a human model and its state as part of the mc rtc controller (recall Fig. 1). A human model is integrated into mc rtc exactly the same way that any other robot model, by providing a description ROS package 10 and implementing a corresponding robot module 11 . Fig. 5 shows the human model description used in our projects. This MQP controller can be used to develop and simulate a wide variety of HRI scenarios including pHRI ones, e.g. using human model and robot contact surfaces description to define contact tasks and constraints.

In the open-source sample MQP controller project, we provide an example of an HRI controller state called Navi-gateToHuman. In this state, a Position Based Visual Servoing (PBVS) task of mc rtc framework is used to control Pepper robot mobile base to navigate in closed-loop to a set-point defined w.r.t human model torso frame (it can be any other human model frame), while using simulation data as a virtual visual feedback signal. At the same time, Image Based Visual Servoing (IBVS) task is used to control robot camera orientation to look at the human head. Fig. 6 illustrates the simulation of the NavigateToHuman FSM state at the start, middle and the end of this state. The sample controller project can also efficiently serve as a starting point for developing controllers for various real HRI applications, which we showcase in Sec. V

V. EXPERIMENTS WITH REAL HUMAN SUBJECT

Recently, the Covid-19 pandemic has put attention into robotics, which can help to deal with the problematics of human virus transmission, by transferring some risky labours and non-added value tasks of care-givers to robotic systems. Examples come from different companies, like disinfecting ultraviolet (UV) rovers and ground drones for hospitals, or even beaming videos to connect patients to their relatives. There are more examples in other fields, like flagging patients with suspected pneumonia during their hospital admission, or using robots and AI in logistics to transport daily groceries.

In our recent publication, we have already demonstrated how the developed tools, presented in the current work, can be used to enable Pepper robot to perform autonomous initiation of human physical assistance [START_REF] Bolotnikova | Autonomous initiation of human physical assistance by a humanoid[END_REF]. We also made the controller code 12 publicly available to serve as a software reference to our work and as an advanced FSM mc rtc HRI controller example. This controller shows how our developed software components allow to easily create complex controllers for rich, intuitive and efficient HRI. This includes closed-loop navigation toward human, verbal, visual and body language communication, and physical interaction with a human for an assistance process initiation.

In the present work, we develop another HRI application -inspired from the Covid-19 outbreak, where Pepper robot behaviour is regulated, with the help of the mc naoqi and the developed robot modules, to autonomously deliver medication to a human lying on the bed. In this scenario a bed height conforms that of Pepper for the given tasks.

The aim here is also to demonstrate how the controllers for new HRI scenarios can be easily prepared using our developed tools and re-using states and elements of the controllers written for other HRI scenarios. As such, for instance, the navigation toward a human state could be reused from our previous work in [START_REF] Bolotnikova | Autonomous initiation of human physical assistance by a humanoid[END_REF], for this new scenario with almost no modifications, despite the person lying in the bed instead of sitting on a chair. Other parts of this new controller FSM also needed only slight adjustments w.r.t our existing work (states present in the other HRI controller) to create a controller for this new HRI application. Fig. 7 shows excerpts screenshots from the experiment video. In the bottom row images, the scenes are visualized in RViz. Note, that additional hardware, namely Azure Kinect, which is used for human state feedback, and RealSense camera, which is included in the robot prototype, but not used in this experiment, are easily included in the robot description (Fig. 8) and processed by the robot module, and therefore are also included in the QP problem formulation (e.g. for more accurate robot center of mass computation). Full experiment can be seen in the accompanying video. Fig. 8: Extra hardware included in the prototype Pepper robot Once the robot reaches a position nearby the person, it communicates verbally its intention to pass the medicine (Fig. 7b). Then it proceeds to open its right gripper for the person to take the pills (Fig. 7c). The passing of the bottle with liquid is arranged with the help of the robot module feature, described in Sec. III, that allows to forward robot tactile sensor data to the mc rtc controller, which then triggers robot left hand gripper to open slightly to allow the person to get out the bottle more easily (Fig. 7d). 9 shows the progress of the closed-loop PBVS task, that uses Azure Kinect body tracking for feedback to make the robot navigate to the person (Fig. 7a). Task errors eventually converge near zero, although in a not very smooth way. This is due to the low quality of human detection (that prohibits constant usage in a continuous closed-loop way), and the low detection frame rate and latency issues (that limits the speed in reaching the person). Indeed, the existing human motion trackers are not robust enough for robotic usage. It is important to underline some shortcomings that prohibit current spreading of HRI in real practice:

• reliable human posture and body detection algorithms (even those advanced) often fail in robotics because of the usage perspective. In assistive robotics scenarios, it is the robot which moves toward a human that can be static (i.e. not moving much), e.g. lying in a home or hospital bed or sitting on a chair. Most of the human pose detection algorithms are trained with rather static camera and moving human and not with a moving camera and static (or moving) human. We have witnessed considerable effects on the robustness of the human pose acquisition by a moving robot with all the algorithms we tried. As a consequence, it is difficult to use them continuously in closed-loop control. • lack of ground-truth: most of the human pose detection matches well in an augmented-reality, their image or video counterpart, but no one provides measurement ground-truth concerning the pose results that are returned. That is to say, it is difficult to assess the precision of the 6D pose returned by most algorithms with ground-truth 6D measurements. In robotics, it is important to know precisely and in real-time the exact joint and floating base values of the human posture and configuration, namely when it comes to contact a person or to manipulate a given limb of a person [START_REF] Bolotnikova | Autonomous initiation of human physical assistance by a humanoid[END_REF]. • last but not least, as it is the most critical issue: in closecontact interaction with humans (i.e. when the robot reaches a person), tracking may be lost even when a wide fish-eye camera is used. This is even more critical when physical interaction causes an obstructed view of the person. This means that human pose estimation approaches in the framework of human-robot closecontact interactions have to be deeply reviewed. This being said, the approach we adopt is rather modular enough to live with such shortcoming without recalling into question the controller. Any improvement in human pose estimation w.r.t the performances mentioned previously will straightforwardly result in better robustness and performance of the controller, because of its conceptual simplicity.

VI. CONCLUSION

For robotics to gain more insight, trust and meet the challenge of future human societal stakes, such as home daily assistance for frail or elderly persons, or to be efficiently used in future outbreaks, research efforts shall be paired with important integration development ones. Any advances made in critical human-robot interaction technological bricks such as human perception, artificial intelligence chatbots, 5G, advanced SLAM. . . should be integrated in readily available and sustained task-space control frameworks. This is the very reason of this work: we do not only aim at sharing the opensource code for the robotic community in general and the HRI in particular, but also sharing experiences that led to existing developments which are made to be further used, improved and sustained. Our methodology in terms of control can be seen now under a different philosophy.

In computer science, algorithms, simple instructions such as if-then-else, while-for loops, variables... together with basic well-agreed routines and functions constitute the basic bricks of any modern algorithm that solve increasingly more complex problems. Our robot control framework should be seen under this spectrum: we shall provide elementary controllers and controller "routines", templates that form the common "instructions", and functions of more complex controllers that solve more and more complicated tasks. In this paper, we thoroughly exemplify what mr rtc may bring under a unified framework in terms of task specification, embedding straightforwardly the constraints... how a multi-(sensory,objectives,robots) task-space controller can be used to build sustainable controllers, and show that what we propose is consistent, as all these tasks are defined exactly the same way for any robot or multi-robot system. We hope that users of SBRE robots could assess, enrich and use our developments in even more challenging scenarios.

In future work, we are aiming to use this framework to conduct in-situ assistance scenarios in a retirement house with few real patients. This needs to be complemented with considerable software engineering efforts.

Fig. 1 :

 1 Fig. 1: mc naoqi interface enables communication between SBRE humanoid robots and mc rtc control framework. It can be used to steer the robot behaviour in HRI experiments.

Fig. 3 :

 3 Fig. 3: NAO and Pepper robot descriptions for the robot representation in the robot module of the mc rtc framework.

•

  Tasks: that are objectives 'describing' what the robot should do at the best it can; • Constraints: that are limits under which the tasks are to be performed, i.e. what the robot should always fulfill strictly. Many tasks and constraints are already implemented as robot-independent templates in mc rtc. For instance Pos-tureTask, CoMTask, EndEffectorTask, KinematicsConstraint, ContactConstraint, etc. However, in some cases it might be desirable to design and implement new custom tasks or constraints, not yet implemented in mc rtc. Such new tasks and constraints might be specific to a robot, use-case or research topic. In the Pepper robot module mc pepper, we provide an example of the implementation of a custom CoMRelativeBodyTask QP control objective. This task allows to specify the desired Pepper center of mass (CoM) target relative to the robot mobile base frame (as opposed to world frame in mc rtc CoMTask). The implementation of this Pepper specific objective was necessary to allow a controller to simultaneously compute a new mobile base position and keep the CoM objective as part of QP computations. An example of a Pepper specific QP constraint implementation is a custom BoundedAccelerationConstr constraint, included in the Pepper robot module. This constraint allows to impose acceleration bounds for Pepper mobile base link.

Fig. 4 :

 4 Fig. 4: RViz scenes of sample FSM Pepper controller states.

Fig. 5 :

 5 Fig. 5: Simulation human model used in mc rtc framework.

9

  https://github.com/jrl-umi3218/pepper-fsm-controller 10 https://github.com/jrl-umi3218/human_description 11 https://github.com/jrl-umi3218/mc_human

Fig. 6 :

 6 Fig. 6: Simulated NavigateToHuman state of a sample MQP mc rtc FSM controller with a human model included.

Fig. 7 :

 7 Fig. 7: Experiment screenshots (top), perceived robot state, Azure Kinect point cloud and human body markers (bottom).

Fig. 9 :

 9 Fig. 9: Evolution of Position Based Visual Servoing closedloop Pepper mobile base navigation to human task errors Fig.9shows the progress of the closed-loop PBVS task, that uses Azure Kinect body tracking for feedback to make the robot navigate to the person (Fig.7a). Task errors eventually converge near zero, although in a not very smooth way. This is due to the low quality of human detection (that prohibits constant usage in a continuous closed-loop way), and the low detection frame rate and latency issues (that limits the speed in reaching the person). Indeed, the existing human motion trackers are not robust enough for robotic usage. It is important to underline some shortcomings that prohibit current spreading of HRI in real practice:• reliable human posture and body detection algorithms (even those advanced) often fail in robotics because of

https://github.com/jrl-umi3218/mc_naoqi_dcm

https://github.com/jrl-umi3218/mc_nao

https://github.com/jrl-umi3218/mc_pepper

ACKNOWLEDGMENT

We thank Sébastien Barthelemy and Sébastien Dalibard from SBRE for providing useful information for this work.


Ruoyu Hu

Thierry Beguiristain

Alexis De Junet

Corinne Leyval
email: corinne.leyval@univ-lorraine.fr

Bioavailability and transfer of elevated Sm concentration to alfalfa in spiked soils

Keywords: Rare earth element, Transfer, Soil, Bioavailability, Plant, Ageing

Rare earth elements (REEs) have been widely used in recent decades and their exploitation has led to industrial REE emission and to contaminated soils especially in former mining areas. This raised people's concerns on the accumulation and toxicity of REEs in soils and plants, and consequences on plant health.

Although many studies dealt with REE in soils and plants, there is still a need to precise their toxicity, bioavailability and transfer to plants in contaminated sites in order to restore such ecosystems. We studied the bioavailability and transfer of a REE to Medicago sativa grown on two contaminated soils differing in their chemical characteristics. A pot experiment was set up in a growth chamber where two natural soils were spiked or not with samarium (Sm) as a model REE. Two chemical extractants were tested to estimate the bioavailability of Sm in the soil, its decrease with time and its transfer to the plants. Results showed that DTPA extractable Sm was well correlated with Sm uptake in alfalfa shoots. The experiment pointed out a significant ageing effect since DTPA extractable Sm significantly decreased within two weeks in the soils and was significantly lower in the less acidic soil than in the other. The uptake of Sm from soil to alfalfa shoots depended on the soil pH and on the spiking concentration. The soil to plant transfer factor was low (<0.08) but a 30% reduction of alfalfa biomass was observed when the soils were spiked with 100 to 200 mg kg -1 of Sm.

Introduction

Rare earth elements (REEs) are a group of elements in the periodic table consisting of 15 lanthanide group elements from lanthanum (La) to lutetium (Lu), along with the pseudo lanthanide [START_REF] Tyler | Rare earth elements in soil and plant systems -A review[END_REF]. According mainly to the geochemical behaviour of the REEs, they were divided into light (LREEs; La to Sm) and heavy REEs (HREEs; europium to Lu) [START_REF] Brioschi | Transfer of rare earth elements (REE) from natural soil to plant systems: Implications for the environmental availability of anthropogenic REE[END_REF]). Because of their magnetic, fluorescent and catalytic properties, REEs, especially LREEs are nowadays widely used in many industries especially high-technology industries like electronics and medicine (Loell et al. 2011a;[START_REF] Pagano | Rare Earth Elements in Human and Environmental Health[END_REF]. In natural soils, REE concentrations range from 40 to 700 mg•kg -1 all over the world and are not considered as toxic for the environment. However, the higher demand and mining of REEs in the past decades have led to a notable increase in REEs in the environment [START_REF] Macmillan | Environmental Drivers of Rare Earth Element Bioaccumulation in Freshwater Zooplankton[END_REF]) and high soil REE concentrations in former mining areas [START_REF] Chao | Structure, Variation, and Co-occurrence of Soil Microbial Communities in Abandoned Sites of a Rare Earth Elements Mine[END_REF][START_REF] Khan | Geochemical characteristics of rare earth elements in different types of soil: A chemometric approach[END_REF]. For example, REE concentration in soil of Bayan Obo rare earth mining area in China is above 2.7 ×10 4 mg•kg -1 due to the mining activity [START_REF] Liang | State of rare earth elements in different environmental components in mining areas of China[END_REF]. Landfills with improperly discarded materials containing REEs might also be a source of contamination. REE concentrations in plants are very low in natural soils [START_REF] Tyler | Rare earth elements in soil and plant systems -A review[END_REF]), but their phytotoxicity may be a concern in contaminated areas [START_REF] Carpenter | Uptake and Effects of Six Rare Earth Elements (REEs) on Selected Native and Crop Species Growing in Contaminated Soils[END_REF]. REEs are not considered as essential elements for plants, but positive effects of REEs at low concentrations were reported on plant growth (Diatloff et al. 1995a;Xu et al. 2002;[START_REF] Hong | Study of lanthanum on seed germination and growth of rice[END_REF]Xiangsheng et al. 2006;[START_REF] Cheng | Rare earth element transfer from soil to navel orange pulp (Citrus sinensis Osbeck cv. Newhall) and the effects on internal fruit quality[END_REF]. In China, REEs have even been used to promote plant growth and have been widely applied in agriculture since the 1990s. However, when a high dose of REE fertiliser has been applied, plant growth was limited and even a toxic effect was found [START_REF] Hong | Study of lanthanum on seed germination and growth of rice[END_REF]. Diatloff et al. (1995a, b) studied the toxicity of lanthanum (La) and cerium (Ce) on corn and mungbean seedlings in nutrient solutions and showed a strong inhibition of mungbean root elongation and plant growth with Ce (even at 0.2 μM). [START_REF] Thomas | Rare earth elements (REEs): Effects on germination and growth of selected crop and native plant species[END_REF] studied the phytotoxicity of La, Ce and Y (yttrium) on germination and growth of native plants and crops in two artificial soils with different pH. However, there is little information on the phytotoxicity of REEs in contaminated soils. The excessive amounts of REEs are not only harmful to plants but are also able to be taken up by plants and enter the food chain, resulting in their accumulation in organisms and finally in human beings (Loell et al. 2011a;[START_REF] Turra | Rare earth elements in citrus production systems[END_REF]. Therefore, studies regarding the effects of REEs in contaminated soils on plants, and their transfer from soil to plants should be carried out [START_REF] Gonzalez | Environmental fate and ecotoxicity of lanthanides: Are they a uniform group beyond chemistry?[END_REF].

The LREE uptake by plants involves calcium (Ca 2+ ion) channels and HREE uptake uses aluminium (Al 3+ ion) channels [START_REF] Gonzalez | Environmental fate and ecotoxicity of lanthanides: Are they a uniform group beyond chemistry?[END_REF]Yuan et al. 2017). The analysis of REEs in different plants on mining and control areas was performed for health risk assessment and showed an enrichment of LREEs in the former (Zhuang et al. 2017) or to identify potential hyperaccumulating plants [START_REF] Khan | Geochemical characteristics of rare earth elements in different types of soil: A chemometric approach[END_REF].

The bioavailability of REEs has been discussed in many studies but many of them considered natural soils where their concentrations are low, or agricultural soils with fertilizer inputs. The mobility of REEs in soils is affected by REE sources and forms, soil particle size, pH and organic matter content [START_REF] Tyler | Rare earth elements in soil and plant systems -A review[END_REF][START_REF] Babula | Uncommon heavy metals, metalloids and their plant toxicity: A review[END_REF][START_REF] Macmillan | Environmental Drivers of Rare Earth Element Bioaccumulation in Freshwater Zooplankton[END_REF]. The bioavailability of REEs and transfer to plants in soils contaminated due to industrial or mining activities has been less considered. Different methods or development of methods have been studied to estimate/predict the uptake of metals in different plants. Among these methods, simple extractants have not been widely investigated for REEs. Using sequential chemical extractions, Cao et al. (2000b) showed that bioavailability of REEs differed with the REEs. [START_REF] Li | Evaluation of single extractants for assessing plant availability of rare earth elements in soils[END_REF] compared different extractants and found that 0.1 M HCl extractable fraction of REEs was well correlated with their concentration in wheat roots and shoots in cultivated soils in China. [START_REF] Li | Lanthanum toxicity to five different species of soil invertebrates in relation to availability in soil[END_REF] used CaCl 2 to correlate lanthanide toxicity to soil invertebrates with bioavailable fraction in soil. None of these studies considered the ageing of REEs in soil and the consequence on their bioavailability. This work aimed to study the bioavailability and transfer of a REE to a crop plant Medicago sativa grown on two contaminated soils differing in their chemical characteristics. A pot experiment was set up in a growth chamber where two natural soils were spiked or not with elevated concentrations of Samarium (Sm) as a model REE. The bioavailability of Sm was estimated using two chemical extractants in the spiked and control soils just after spiking, after two weeks of ageing and at plant harvest. Sm phytoavailability was also estimated by plant accumulation at harvest. Plant growth, P and Ca plant uptake were also estimated.

Material and methods

Experiment design

The soils used for the experiments were collected from the surface horizon (5-25 cm) of two sampling sites in north-eastern France: Maron (48°38'45.75"N, 6°4'48.44"E) and Bouzule (48°44'7.95"N, 6°19'9.64"E).

They both belong to Luvisol with different pH values. The physicochemical parameters of the two sampled soils are shown in Table 1. Soil samples were air-dried, ground, sieved to 2 mm, homogenised and stored in a dry and dark room ready to be used in the experiments. The Bouzule soil was partially amended with pre-sieved (2 mm), washed and dried sand (30w/70w) to avoid compaction and allow seedling growth. A ten times concentrated solution of SmCl 3 •6H 2 O in deionized water was added to 1/10 of both soils. The concentrated spiked soils were dried at 50 °C, homogenised, and added to the rest of the unspiked soils and homogenised to obtain 100 and mg•kg -1 Sm. 1 kg of soils were placed in 1kg pots with eight replicates and three Sm conditions (0, 100, mg•kg -1 ) according to LREE concentrations in abandoned mining sites [START_REF] Chao | Structure, Variation, and Co-occurrence of Soil Microbial Communities in Abandoned Sites of a Rare Earth Elements Mine[END_REF]. Deionized water was added to reach 70% of water holding capacity (WHC) and pots were placed in a dark room at 4°C (T0) for weeks for the ageing of the Sm contamination.

After soil ageing, 4-5-day-old alfalfa (Medicago sativa) seedlings germinated on sterilised vermiculite were transplanted to the pots (T1) with 15 plants in each pot. Pots with plants were transferred to a growth chamber with 16 h photoperiod, 22°C/18°C day/night, 250 µmol photons m -2 s -1 , 80% relative humidity for weeks. During the 6-week growing period, the pots were watered twice a week and a nutrient solution [START_REF] Redon | Differential effects of AM fungal isolates on Medicago truncatula growth and metal uptake in a multimetallic (Cd, Zn, Pb) contaminated agricultural soil[END_REF]) was added once per week to maintain 70% WHC.

Sampling and analysis

Soil samples were collected at T0 and T1 in centrifuge tubes and dried at 80 °C for 72h. Alfalfa plants and soil samples were harvested (T2) 28 days (Maron) or 42 days (Bouzule) after transplantation. Pots with Maron soil were harvested earlier due to the poor growth of plants on this soil. Shoots and roots were harvested, gently rinsed with deionized water then dried at 60 °C for 72 h, and the dry weight of shoots and roots of each pot was recorded. Soil samples were collected at T2 in centrifuge tubes and dried at 80 °C for 72 h. All soil and shoot samples were ground in a mortar with a pestle before the analysis.

To analyse the total concentration of Sm in soil, 200 mg of ground soil sample was transferred into an acid-cleaned Teflon tube designed for CEM Mars 5 Digestion Microwave System® and 10 mL of Aqua regia (nitric acid: hydrochloric acid =1:3 v/v) was added. Digestion was processed under the adapted programme (15 min digestion at 25°C and 10 min digestion at 170°C). After the digestion and cooling down procedure, the digested samples were filtered (DigiFILTER, SCP Science® 010-500-070), and the filtered solutions were transferred into 15 mL centrifuge tubes, adjusted to 10 mL with ultra-pure deionized water 18 M W and preserved at 4 °C till the analysis.

The bioavailable concentrations of Sm in soils were extracted by two different extractants. The DTPA extracting solution consists of diethylenetriaminepentaacetic acid (DTPA) (0.005M), CaCl 2 (0.01M) and triethanolamine (0.1M) [START_REF] Carter | Soil Sampling and Methods of Analysis, Second Edition, 2nd Editio[END_REF][START_REF] Kim | Bioavailability of heavy metals in soils: definitions and practical implementation-a critical review[END_REF]. One gram of ground soil sample was transferred into a 15 mL centrifuge tube, with 10 mL of DTPA solution and homogenised on a rotary shaker with 99 rpm for 2 hours. Samples were centrifuged at 3227 g for 15 min (20 °C) and the supernatant was filtered in 15 mL centrifuge tubes, adjusted to 10 mL with ultra-pure deionized water 18 M W and preserved at 4 °C till the analysis. The second extractant is CaCl 2 . One gram of ground sample was transferred into a 15 mL centrifuge tube with 10 mL of 0.01 M CaCl 2 solution with the same homogenization and centrifugation method as above.

The supernatant was filtered in 15 mL centrifuge tubes, adjusted to 10 mL with ultra-pure deionized water 18 M W and preserved at 4 °C till the analysis.

Dried shoot samples were digested in nitric acid (HNO 3 ) and hydrogen peroxide (H 2 O 2 ) solution. Six mL of 69% HNO 3 (CAS: 7697-37-2) and 3 mL of 30 % H 2 O 2 (w/w H 2 O) (CAS: 7722-84-1) were added into ). One-way and two-way analysis of variance (ANOVA 1 and 2) and the Tukey honestly-significantdifference (HSD) post-hoc test were performed for the significance of the data with a 95% significant level (pvalue ≤ 0.05).

Results

Total and extractable Sm in both soils

Sm concentration in unspiked Maron and Bouzule soils was 14.83 ± 0.37 mg•kg -1 and 4.99 ± 0.46 mg•kg -1 , respectively. After spiking with 100 and 200 mg•kg -1 Sm, the concentrations were 82.76 ± 12.25 mg•kg - 1 and 169.48 ± 47.11 mg•kg -1 for Maron soil, and 86.44 ± 32.62 mg•kg -1 and 151.49 ± 63.09 mg•kg -1 for Bouzule soil, respectively.

Both CaCl 2 and DTPA extractants were used for Maron and Bouzule soils at 3 sampling dates: at spiking (T0), after 2 ageing weeks (T1) and at plant harvest (T2), and in unspiked soils. For Maron soil CaCl 2 extractable Sm was below the detection limit (0.1 mg•kg -1 ) in unspiked soil. In spiked soils, CaCl 2 extractable Sm was 0.2 ± 0.02 mg•kg -1 at T0, 0.1 ± 0.01 mg•kg -1 at T2 in 100 mg•kg -1 spiked soil, and 0.39 ± 0.03 mg•kg -1 at T0 and 0.18 ± 0.02 mg•kg -1 at T2 in 200 mg•kg -1 spiked soil. For Bouzule soil, CaCl 2 extractable Sm was below the detection limit in all samples.

The DTPA extractable Sm in unspiked Maron and Bouzule soils was also low: 4.16 ± 0.58 mg•kg -1 at T0, 4.13 ± 0.35 mg•kg -1 at T1 and 3.27 ± 0.45 mg•kg -1 at T2 in Maron soil, without significant difference between the 3 sampling dates. Much lower DTPA extractable Sm was found in Bouzule soil with 0.36 ± 0.05 mg•kg -1 at T0, 0.30 ± 0.03 mg•kg -1 at T1 and 0.20 ± 0.005 mg•kg -1 at T2, with a significant difference between T0 and T2.

The DTPA extractable concentration of Sm in the spiked soils decreased as a function of time (Figure 1). It decreased by 16% and 11% respectively in 100 and 200 mg•kg -1 spiked Maron soil during the 2-week ageing period from T0 to T1. For Bouzule soil, there was a significant 24% and 29% decrease of DTPA extractable Sm in 100 and 200 mg•kg -1 spiked soil respectively during the 2-week ageing (T0 to T1). A significant decrease was also observed between T1 and T2, during the 4 weeks' alfalfa growing time, with 24% and 26% decrease respectively in 100 and 200 mg•kg -1 spiked Maron soil. During the 6 weeks' alfalfa growing time (between T1 and T2), the available Sm concentration in Bouzule spiked soil also significantly decreased with a 48% and 40% decrease at 100 and 200 mg•kg -1 Sm spiked concentration.

Alfalfa growth in Sm spiked and unspiked soils

In Maron soil, the shoot biomass showed a significant decrease in the 200 mg kg -1 Sm spiked soil compared to the unspiked control but the root biomass showed no significant difference between control and spiked soils. Shoot and root biomass in Bouzule soil was significantly lower in Sm spiked than in the unspiked soil (Figure 2). It was significant already with 100 mg•kg -1 Sm spiked concentration (p-value ≤ 0.05) and no further decrease was observed at 200 mg kg -1 . For Maron soil, the shoot biomass showed a significant decrease in the 200 mg kg -1 Sm spiked soil compared to the unspiked control but the root biomass showed no significant difference between control and spiked soils.

Sm transfer to plants

The concentration of Sm in alfalfa shoots was significantly higher in the spiked soils than in the controls and was twice higher with 200 than with 100 mg kg -1 Sm spiked soils (p-value = 0.05) (Figure 3). The soil-plant transfer factor (SPTF) was around 0.05-0.08 in Maron soil, and it was not significantly different between spiked and unspiked soils. However, in Bouzule soil, the SPTF was around 0.02 to 0.035 and was significantly lower in spiked than in unspiked soil (Figure 3). The Sm uptake by plants as a function of DTPAextractable concentration of Sm in soil was also calculated (Figure 4) and showed a positive linear correlation (R 2 = 0.9334, linear regression model).

We analysed P and Ca concentrations in plants. There were no significant differences in P and Ca concentrations in alfalfa shoots between unspiked and spiked soils (data not shown). P and Ca content in shoots was also not significantly different between spiked and unspiked Maron soil. However, in Bouzule spiked soil, the content of P and Ca in shoots was significantly lower than in unspiked soil (Figure 5).

Discussion

Assessment of bioavailability of Sm using different extractants

Different extractants and extraction methods have been applied to study the bioavailability of REEs in the soil or their phytoavailability and potential uptake by plants. In our study, we performed a single extraction method using two neutral extractants: DTPA and CaCl 2 . Our results showed that CaCl 2 extractant had a low extracting ability, while the phytoavailable Sm in the soil was well predicted by DTPA extraction as the Sm plant uptake accounted for 22.9% of DTPA extractable Sm (with R 2 =0.93). According to [START_REF] Rao | Comparison of single and sequential extraction procedures for the study of rare earth elements remobilisation in different types of soils[END_REF], the bioavailability of REEs can be overestimated by strong acid reagents such as CH 3 COOH and HCl. Meanwhile, the complexing extractants such as DTPA and EDTA showed moderate extraction ability and have been applied in different studies. EDTA was used to indicate that 10% to 16% of REEs were phytoavailable in natural soils in central Germany (Loell et al. 2011a, b). EDTA showed a stronger extracting ability than DTPA as it can extract both carbonate and organically bound fractions of metals in soils with low carbonate content [START_REF] Sahuquillo | Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments[END_REF]. For [START_REF] Censi | Rare earths and trace elements contents in leaves: A new indicator of the composition of atmospheric dust[END_REF], DTPA gave a good indication of the plant-bioavailable fraction of REEs and other trace elements in rural, urban and industrial soils in Sicily. The neutral salt 0.01M CaCl 2 was recommended as a good extractant of phytoavailability [START_REF] Menzies | Evaluation of extractants for estimation of the phytoavailable trace metals in soils[END_REF][START_REF] Kim | Bioavailability of heavy metals in soils: definitions and practical implementation-a critical review[END_REF]. However, in our study, CaCl 2 could not be used to represent alfalfa available Sm in both soils due to its weak extraction ability, while DTPA showed a good indication of alfalfa available Sm in the soil.

Bioavailability of high concentration of Sm in soils and influence of ageing

In our study, the bioavailable Sm, estimated by DTPA extraction, accounted for 40% of the total Sm concentration (extracted by aqua regia) in freshly spiked Maron soil and 25% in freshly spiked Bouzule soil.

Despite the same spiking concentrations in both soils, it was significantly higher in the acidic Maron soil than the more neutral Bouzule soil. In a study of four types of Chinese soils, with both control and spiked (200 mg•kg -1 REEs) soils, the red earth with lowest pH had the highest exchangeable REEs [START_REF] Liang | Environmental biogeochemical behaviors of rare earth elements in soil-plant systems[END_REF]. A higher REE availability in lower pH soil was also reported in a study comparing total REE contents and the potentially bioavailable fraction using EDTA in three different soils in Germany (Diatloff et al. 1995b). [START_REF] Diatloff | Concentrations of rare earth elements in some Australian soils[END_REF] found that the Ca(NO 3 ) 2 extractable REEs decreased with the soil pH increase when adding CaCO 3 and increased when soil pH decreased by adding CaSO 4 •2H 2 O. Yufeng et al. (2003) applied artificial acid rainwater to Chinese red soil and showed an increase in the REE concentrations in the soil leaching solution after the acid rainwater application. Our result of higher bioavailable Sm in Maron soil further confirmed pH as an important parameter influencing the bioavailability of REEs.

Despite soil pH and soil composition, the organic matter in soil with a large amount of negatively charged groups is able to adsorb or chelate cations as REEs and thereby decrease their availability and transfer [START_REF] Tyler | Rare earth elements in soil and plant systems -A review[END_REF]Loell et al. 2011b;[START_REF] Fiket | Influence of soil characteristics on rare earth fingerprints in mosses and mushrooms: Example of a pristine temperate rainforest (Slavonia, Croatia)[END_REF]. In our study, the bioavailability of Sm is higher in Maron soil even though the organic matter was higher than in Bouzule soil, indicating that the pH had a major influence on REE bioavailability. Ca 2+ was reported to have an influence on the bioavailability of LREE to plant, as LREE are reported to use the same ion channels as Ca 2+ when entering plant root (Yuan et al. 2017). In our study, there was no indication that Ca concentration influenced the bioavailability of the LREE Sm, but calcium concentration in both soils was similar.

Other than the above-mentioned factors, the ageing significantly affected the bioavailability of Sm in the spiked soils. In our study, the bioavailable fraction of Sm significantly decreased within the two weeks of ageing. Cao et al. (2000b) studied the bioavailability of REEs in natural soils using the same model plant alfalfa (Medicago sativa) and sequential extractions. They showed that exchangeable REE fraction contributed to the bioavailable REEs, which explained the decreased bioavailability of REEs, including Sm, with time as REEs were adsorbed to soil particles. In the unspiked soils, the bioavailable fraction of Sm was much lower than in the spiked soils (26 and 6 % for Maron and Bouzule, respectively), which was also observed for natural and spiked soils by [START_REF] Liang | Environmental biogeochemical behaviors of rare earth elements in soil-plant systems[END_REF]. The bioavailable fraction of REEs accounted for only 10.1 % of total REEs and 13.8%

of Sm in natural soil (Loell et al. 2011b). We also observed a further decrease of the bioavailable Sm from T1 to T2 down to 25 % in Maron and 9 % in Bouzule soil which could be due to the ageing effect and the presence of alfalfa plants as well. Therefore, REE toxicity studies with spiked soils [START_REF] Thomas | Rare earth elements (REEs): Effects on germination and growth of selected crop and native plant species[END_REF] should include an ageing period of the soil before to introduce the plants.

Impact of high concentration of Sm on plant growth

Although some studies have mentioned a positive influence of REEs on plant growth at low concentration in soil [START_REF] Hong | Study of lanthanum on seed germination and growth of rice[END_REF][START_REF] Cheng | Rare earth element transfer from soil to navel orange pulp (Citrus sinensis Osbeck cv. Newhall) and the effects on internal fruit quality[END_REF], other studies reported the inhibition of plant growth and toxic effects of high concentrations REEs to plants [START_REF] Carpenter | Uptake and Effects of Six Rare Earth Elements (REEs) on Selected Native and Crop Species Growing in Contaminated Soils[END_REF]. However, most of the studies on the effects of REEs on plant growth were performed in liquid media [START_REF] Asher | Rare earth elements and plant growth: II. responses of corn and mungbean to low concentrations of lanthanum in dilute, continuously flowing nutrient solutions[END_REF]Diatloff et al. 1995a, b;d'Aquino et al. 2009) on two or three LREEs (yttrium, lanthanum and cerium) and not in soil. Few data are focusing on the effect of single elements [START_REF] Pagano | Rare Earth Elements in Human and Environmental Health[END_REF]. In our study, we focused on the effect of a single REE on alfalfa. REE toxicity and transfer to plants may differ with plant species and REEs. This was observed with elevated La, Y and Ce concentrations in artificial spiked soil where lower IC25s values for Ce and some plants were observed [START_REF] Thomas | Rare earth elements (REEs): Effects on germination and growth of selected crop and native plant species[END_REF]). However, we showed a strong decrease, around 30%, of alfalfa shoot biomass when the Sm concentration in soil reached 100 to 200 mg•kg -1 . [START_REF] Carpenter | Uptake and Effects of Six Rare Earth Elements (REEs) on Selected Native and Crop Species Growing in Contaminated Soils[END_REF] showed that the IC25s of native plants in artificial soils spiked with 6 REEs was between 100 and 300 mg•kg -1 REEs, but the ageing of the contamination was not considered neither in the experiment of [START_REF] Liang | Environmental biogeochemical behaviors of rare earth elements in soil-plant systems[END_REF] where ryegrass weight was inhibited by spiking REEs on natural soils. Since the spiking was performed with SmCl 3 , a potential effect of Cl addition might be considered. Salinity, for example addition of NaCl, can be harmful to alfalfa growth and lead to decreased concentration of shoot K, Ca and P [START_REF] Li | Effects of salt and alkali stresses on germination, growth, photosynthesis and ion accumulation in alfalfa ( Medicago sativa L.)[END_REF][START_REF] Ashrafi | Effect of salt stress on Growth and Ion accumulation of alfalfa (Medicago sativa L.) cultivars[END_REF]. However, compared to these experiments, the added Cl concentration in our study was much lower. Further, the concentration of Ca, and P, and Na and K in plants was not significantly different between SmCl 3 spiked and non-spiked soil (Online Resource 1), suggesting no side effect of the addition of Cl.

Transfer of Sm to plants

The SPTF of Sm was rather low: 0.05 and about 0.08 in the control and spiked Maron soil, respectively, and 0.035 and about 0.02 in the control and spiked Bouzule soil. The difference between control and spiked soils can be explained by the source and ageing time of Sm. In studies focussing on natural soils, the transfer of REEs to plants, especially to wheat seeds [START_REF] Liang | Environmental biogeochemical behaviors of rare earth elements in soil-plant systems[END_REF] or fruits [START_REF] Cheng | Rare earth element transfer from soil to navel orange pulp (Citrus sinensis Osbeck cv. Newhall) and the effects on internal fruit quality[END_REF] was also low (around 0.03).

According to [START_REF] Tyler | Rare earth elements in soil and plant systems -A review[END_REF], the concentrations of REEs in the plant are generally low due to the low SPTF, ranging between 0.04 to 0.09 and even lower in grass [START_REF] Tyler | Plant uptake of major and minor mineral elements as influenced by soil acidity and liming[END_REF] and vegetables [START_REF] Bibak | Concentrations of 63 elements in cabbage and sprouts in Denmark[END_REF]. In an experiment using REE-containing coal fly ash amendment [START_REF] He | Bioaccumulation and translocation of rare earth elements in two forage legumes grown in soils treated with coal fly ash[END_REF] showed a transfer factor from soil to alfalfa shoots of the light REEs La and Ce, and of the heavy REE Y ranging from 0.01 to 0.10. The transfer factor depended on the element and was higher for Ce than the two other REEs. Our results for Maron soil fit with this range of data and the SPTF is lower in Bouzule soil which was mainly influenced by the availability of Sm in the soil. Moreover, the preferable neutral soil for alfalfa lead to better growth in Bouzule soil and a dilution effect occurred. Other plants reported to be hyperaccumulators, such as Phytolacca Americana that can accumulate up to 623 mg•kg -1 REEs or the Chinese fern Dicranopteris dichotoma, showed higher SPTF values, up to 75, depending on the soil properties and REE concentrations [START_REF] Chen | Rare earth element migration in gullies with different Dicranopteris dichotoma covers in the Huangnikeng gully group, Changting County, Southeast China[END_REF]van der Ent et al. 2018). Our study focused on REE transfer from a contaminated soil to a widespread crop plant with rapid growth, where, although the SPTF value is lower, the plant can be strongly impacted by the presence of REEs.

Conclusions

Although the SPTF of Sm to alfalfa represented a low percentage, the presence of Sm at high concentration inhibited the growth of alfalfa. In areas with a high natural abundance of REEs, in post-mining sites, or with the increasing risk of uncontrolled landfills, non-adapted plants might then be strongly impacted.

pH was a key parameter of the bioavailability of Sm since higher Sm availability and higher SPTF were found in the acid soil. The bioavailable Sm decreased during the ageing process in the soils, suggesting interactions between Sm and soil constituents. A decrease with time of the bio-and phytoavailability of REE in contaminated soils can be expected and help native or crop plants to cope with the presence of REEs. However, in order to further reduce the bioavailability and the transfer to plants of REEs in contaminated soils, and restore such ecosystems, other soil parameters could be considered such as Ca content and organic matter. Rhizospheric processes, such as root exudation or the contribution of root-associated microorganisms, which are known to affect the mobility and availability of metallic trace elements, should also be investigated for REEs. Different letters within a soil indicate significant differences (two-way ANOVA, p<0.05).

  DigiTUBEs containing 200 mg of ground shoot sample at 20 °C for a night. Then the DigiTUBEs with the sample solution was placed on DigiPREP MS 48 & TS (SCP Science® 010-500-205 & 010-500-275) hot plate system covered with DigiPROBE Watch Glass (SCP Science® 010-500-081) processing the digestion programme adapted to plant tissue with 5 min solubilization at 45 °C, 10 min pre-digestion at 65 °C and 60 min digestion at 95 °C. After cooling down, the solution was filtered as previously described and kept at 4 °C until the analysis.The total and the bioavailable Sm concentrations in soil samples and the concentration of Sm in alfalfa shoots were analysed using coupled plasma-optical emission spectroscopy (ICP-OES) analytical technique using a Varian Inc.® (Part A) -720/730-ES ICP-OES. Calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg) concentrations in alfalfa shoots were also analysed using the same technique and are presented as supplementary data (Online Resource 1). All the analysis used NCS Certified Reference Material NCS DC 73030 soil (Sm recovery rate = 85.1%) and Labmix24 oriental basma tabacco leaves (track elements) -IC-INCT-OBTL-5 (Sm recovery rate = 70%) as references. The soil-plant transfer factor (SPTF)[START_REF] Stanford | Soil-to-plant transfer of 137Cs, 40K and 7Be[END_REF] Cao et al. 2000a;[START_REF] Chyi | Trace elements in soils and plants, 4th Editio[END_REF][START_REF] Šmuc | Geochemical characteristics of rare earth elements (REEs) in the paddy soil and rice (Oryza sativa L.) system of Kočani Field, Republic of Macedonia[END_REF] was calculated as the ratio between Sm concentration in alfalfa shoots and the total concentration of Sm in soils.Statistical analysis was performed with RStudio software(Version 1.1.463 -© 2009-2018 RStudio, Inc.

  van der Ent A, Echevarria G, Baker AJM, Morel JL (2018) Agromining: Farming for Metals. Springer International Publishing, Cham Xiangsheng L, Jiachen W, Jun Y, Yubin F, Yanping W, He Z (2006) Application of Rare Earth Phosphate Fertilizer in Western Area of China. J Rare Earths 24:423-426 Xu X, Zhu W, Wang Z, Witkamp GJ (2002) Distributions of rare earths and heavy metals in field-grown maize after application of rare earth-containing fertilizer. Sci Total Environ 293:97-105 Yuan M, Guo M-N, Liu W-S, Liu C, van der Ent A, Morel JL, Huot H, Zhao W-Y, Wei X-G, Qiu R-L, et al (2017) The accumulation and fractionation of Rare Earth Elements in hydroponically grown Phytolacca americana L. Plant Soil 421:67-82 Yufeng Z, Zhenghua W, Xiaorong W, Lemei D, Yijun C (2003) Mobility of the Rare Earth Elements with Acid Rainwater Leaching in the Soil Column. Bull Environ Contam Toxicol 67:399-407 Zhuang M, Zhao J, Li S, Liu D, Wang K, Xiao P, Yu L, Jiang Y, Song J, Zhou J, et al (2017) Concentrations and health risk assessment of rare earth elements in vegetables from mining area in Shandong, China. Chemosphere 168:578-582

Figure1

  Figure1 DTPA extractable Sm concentration (mg kg -1 ) in (a) Maron and (b) Bouzule spiked soils as a function of time (T0=0, T1=14 days, T2 = 42 days (Maron) and 56 days (Bouzule)) and spiked Sm concentration (100 and 200 mg kg -1 ). Different letters within a soil indicate significant differences (two-way ANOVA, p<0.05).

Figure 2

 2 Figure 2 Shoot and root dry weight (mg) of alfalfa plants in (a) Maron and (b) Bouzule soil as a function of spiked Sm concentration (0, 100 and 200 mg kg -1 ). Mean ± SD (n= 8). Different letters within a soil indicate significant differences (two-way ANOVA, p<0.05).

Figure 3

 3 Figure 3 Sm concentration (mg kg -1 ) in alfalfa shoots growing in (a) Maron and (b) Bouzule soil as a function of Sm spiked concentration (0, 100 mg kg -1 and 200 mg kg -1 ) and soil-plant transfer factor. Mean ± SD (n= 8). Different letters within a soil indicate significant differences (two-way ANOVA, p<0.05).

Figure 4

 4 Figure 4 Sm concentration (mg kg -1 ) in plants as a function of the DTPA extractable Sm in spiked and unspiked soils.

Figure 5

 5 Figure 5 Content of (a) Sm (µg), (b) Ca (mg) and (c) P (mg) in alfalfa shoots growing in two soils as a function of Sm concentration (0, 100 ppm and 200 ppm). Mean ± SD (n=8).

Table 1

 1 Physico-chemical characteristics of the soils used in the experiment

		Maron	Bouzule
	Soil pH	5.34	6.68
	Sand: silt: clay (%)	81:362:557	167:513:320
	CaCO 3 (g•kg -1 )	0	0
	Organic carbon (g•kg -1 )	40.1	15.8
	Total Nitrogen (g•kg -1 )	3.1	1.7
	Organic matter (g•kg -1 )	69.4	27.4
	C/N	13.0	9.6
	P (mg•kg -1 )	8	91
	CEC cobalthexamine (cmol + •kg -1 )	23.2	16.1
	Saturation rate (%)	85	100
	Ca (cmol + •kg -1 )	18.1	13.1
	Mg (cmol + •kg -1 )	1.07	1.79
	Na (cmol + •kg -1 )	0.07	0.11
	K (cmol + •kg -1 )	0.50	1.14
	Fe (cmol + •kg -1 )	0.03	0.01
	Mn (cmol + •kg -1 )	0.73	0.30
	Al (cmol + •kg -1 )	1.45	0.05

Acknowledgements

This work was supported by the French National Research Agency through the national program "Investissements d'avenir" with the reference ANR-10-LABX-21-RESSOURCES21. The authors would like to give thanks to the technical staff of LIEC especially David BILLET, Christine FRIRY and Hervé MARMIER, and to the Experimental phytotronic platform of Lorraine (PEPLor, Université de Lorraine).

Online supplementary material