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Root system scientists strive to understand how a single root, emerging from a plant’s seed, can form a complex, dynamic and plastic network of thousands of individual roots. They investigate how such a network is ideally suited to perform a number of functions required for the harmonious development of the whole plant. Everyone in the community also knows how complicated it can be to study root systems, with tasks ranging from digging plants out of the soil, creating experimental set-ups that allow the observation of the roots, to quantifying the root network itself or the processes underlying its formation. Within the community, there is one person, Dr Loïc Pagès, who has been working on all these tasks for many years, and who has moved the field forward numerous times. On the occasion of his soon-to-be retirement, we would like to express our appreciation to him via this editorial.

International audience; Introduction Rhizodeposition, i.e. the release of organic materials by roots, represents a significant portion of plant’s carbon (C) budget, ranging from 5% to 15% of net photosynthesized C (Pausch and Kuzyakov, 2018). Various rhizodeposits can be released by roots, e.g. soluble exudates, secreted mucilage, sloughed cells, or volatile organic compounds. Despite their short lifetime, some of these products have been shown to favor plant growth, e.g. by increasing water and nutrient uptake. Among rhizodeposition processes, exudation has been suggested to depend on the concentration of carbohydrates inside the roots (Personeni et al., 2007). However, rhizodeposition not only depends on the availability of C in the roots, but also on the architecture of the root system, and many have shown that rhizodeposits are more concentrated in specific areas, such as root tips. Consequently, a Functional-Structural Plant Model (FSPM) would theoretically represent the best framework for simulating the spatial and temporal dynamics of rhizodeposition, as it can describe the evolution of both the metabolism and the architecture of the plant. The objective of this work is to create such a framework by coupling a whole-plant FSPM, a 3D root architectural model, and a new model simulating rhizodeposition.Modelling approachOur strategy has been to combine the FSPM CN-Wheat (Barillot et al., 2016), which describes the main processes of C and nitrogen (N) acquisition and transformation by an individual wheat plant and the 3D growth and development of its aerial organs, with the model ArchiSimple (Pagès et al., 2014) that simulates the development of the 3D root architecture for a range of plant species, and the new model RhizoDep, which calculates a full C balance in each part of a root system in order to simulate local rhizodeposition fluxes. The complementarity of the three models is illustrated in Figure 1: i) CN-Wheat is used to calculate the amount of C allocated from the shoots to the roots, ii) ArchiSimple provides the 3D structure of the root system, and iii) RhizoDep distributes the C provided by the shoots within the 3D root system and simulates the actual growth, respiration and rhizodeposition of each root element based on C availability. The major link between the three models lies in the exchange of C between aboveground and belowground tissues, which is driven by gradients of sucrose concentration in the different compartments of the plant.Preliminary results & short-term perspectivesThe coupling of the three models has been started using the OpenAlea platform and its Multiscale Tree Graph formalism (Pradal et al., 2008). First simulations were done using the allocation of C to the roots simulated by CN-Wheat as an input to the root model based on the effective coupling of ArchiSimple and RhizoDep. These simulations show how rhizodeposition is intrinsically dependent on the architecture of the root system and on the total amount of available C. For completing the coupling, several issues still need to be tackled, e.g. how N uptake and metabolism should be spatialized in a 3D root system, how it may be regulated by local C and N availability, and how rhizodeposition can modify soil N availability. However, this modelling approach has already led to a first prototype able to simulate rhizodeposition processes on a dynamic, 3D root system that is fully integrated within the functioning of the whole plant. Its refinement will offer unique opportunities to study the possible link between rhizodeposition and the environmental factors affecting plant growth, e.g. atmospheric CO2 concentration or soil N availability.

International audience

International audience; Rhizodeposition, i.e. the release of any organic material by roots, has been suggested to represent a major input of carbon in soils and to shape biological activity in the rhizosphere. Various rhizodeposits can be emitted by roots, e.g. soluble exudates, secreted mucilage, sloughed cells, or volatile organic compounds. Despite their short lifetime, some of these products have been shown to favor plant growth by increasing water or nutrient uptake, by promoting plant resistance, or by acting as signals. Rhizodeposition also represents a significant share of plant’s carbon budget and may affect plant growth in this way. So far, no computational model has integrated the variety of rhizodeposition processes into plant functioning. Our goal is to develop such a model in order to investigate in a more mechanistic way the effects of contrasted plant traits, crop management or climate change on plant growth and soil carbon sequestration. As an important step in this direction, we focus here on the integration of the carbon fluxes related to rhizodeposition processes into a functional-structural plant model. After summarizing the current knowledge on the mechanisms and carbon-costs associated to rhizodeposition, we will present a 3D shoot-root model implemented in the OpenAlea platform, which integrates a new rhizodeposition model, a root architecture model (ArchiSimple) and a model of C and N metabolism in the whole plant (CN-Wheat). The possibilities opened up by this integrative model will be illustrated by simulations of net sugar exudation fluxes distributed across a dynamic, 3D wheat root system interacting with soil.

National audience; The potential of agricultural soils to store carbon (C) has been claimed to require a large amount of nitrogen (N) and therefore a high soil N availability. This assumption is based on the observation that stable soil organic compounds are characterized by a narrow range in C:N ratios. However, N-fertilizer additions have also been shown to increase respiratory C losses from soils. Furthermore, most of the organic C naturally stored in soils originate from plant C inputs, which occur both by aerial litter deposition and by root turnover and rhizodeposition. Root C inputs have been shown to contribute 2-3 times more to soil C sequestration than shoot-derived C inputs. However, root C inputs may be limited or even decrease when soil N availability increases beyond a certain level. A trade-off may therefore exist between the amount of soil available N and the potential of soil C sequestration associated to the inputs of root C. The existence of such trade-off and its possible consequences for the optimization of soil C sequestration will be discussed according to evidences from scientific literature and to a modelling approach describing how root C inputs to soil can evolve with soil N availability.

International audience

International audience

International audience

International audience

Plants are the main source of organic carbon in soils. Besides litter incorporation, most of plant carbon fluxes to soil occur belowground, through the release of organic compounds by roots and the decay of root tissues. Despite the importance of these processes for soil carbon sequestration and for soil biological functions, our understanding of such fluxes has been hampered by the difficulties associated to their measurement in actual soil environments and their integration to plant growth models. Our aim is to develop new modelling approaches in order to accurately describe trophic fluxes from roots to soil and their spatial and temporal evolution. Functional-structural plant models (FSPM), which take into account both plant physiology and plant architecture, may be well adapted to such a modelling strategy, but also bring new challenges in terms of processes coupling. This work will present our current strategy to simulate and integrate root exudation, mucilage emissions, root cells desquamation and root senescence into a common model, and will highlight some of the main knowledge gaps associated to the simulation of these processes.

International audience

International audience

International audience; Technosols are characterized by the presence of mineral and organic parent materials of technogenic origin (e.g. agricultural or urban wastes, industrial by-products, building materials, transported natural materials). In view of the continual increase of such man-made soils, there is a true need of understanding their functioning and evolution. Micropedology, i.e. morphological and analytical characterization of pedofeatures on soil sections, appears as a relevant approach to take into account the diversity and the specificity of Technosols in the knowledge of their pedogenetic processes. Micropedology was investigated at microscopic and submicroscopic scale on four Technosols. Therefore, it determined specific features of anthropogenic constituents allowing in situ monitoring until the early stages of Technosol pedogenesis. Organic matter dynamics, soil porosity evolution, impact of faunal activity or hydric conditions on Technosol structure were investigated. Moreover, as Technosol components and deposition modes are diverse, one can expect numerous interfaces. In that way, micropedology appeared particularly well adapted to study these local interfaces as sites of favoured pedogenesis. Supplemented with overall physico-chemical soil analyses, characterization of Technosol pedogenic features using micropedology improves the understanding of their functioning and evolution. In addition, according to the environmental context, such data also give useful information for the Technosol management.; RESUMEN Los Tecnosoles se caracterizan por la presencia de materiales orgánicos y minerales de origen tecnogenético (p.ej. residuos agrícolas o urbanos, subproductos industriales, materiales de construcción, materiales naturales transportados, etc.). Dado el aumento continuo de estos tipos de suelos antrópicos, es necesario comprender su funcionamiento y evolución. La micropedología, definida como la caracterización morfológica y analítica de microestructuras de láminas de suelo, es una herramienta estándar para el estudio de los mismos. La micropedología ofrece un enfoque relevante para el conocimiento de los procesos edafogenéticos de los Tecnosoles, ya que permite considerar la diversidad y la especificidad de los mismos. Cuatro tipos de Tecnosoles fueron investigados mediante técnicas de micropedología a escala fotónica y de ultraestructura. Así, fue posible determinar las características de los constituyentes antropogénicos y realizar un seguimiento in situ hasta las fases tempranas de la edafogénesis de estos Tecnosoles. Se investigaron procesos como la dinámica de la materia orgánica, la evolución de la porosidad y el impacto de la actividad de la fauna o de las condiciones hídricas en la estructura de los Tecnosoles. Además, ya que los Tecnosoles son muy diversos tanto en sus componentes como en el modo en que estos componentes se organizan, es esperable que existan numerosas interfaces entre horizontes de suelo. De este modo, la micropedología se adapta muy bien al estudio de estas interfaces locales donde se dan procesos edafogenéticos. La caracterización micropedológica de las microestructuras de los Tecnosoles, complementada con análisis físico-químicos generales, incrementa de modo sustancial la comprensión

International audience

International audience; The risk assessment of industrial contaminated soils is an important challenge in remediation process. Although chemical analyses reveal the typology of pollution in a given matrix, they do not give information about the real ecotoxic potential of the matrix, which takes into account the bioavailability of pollutants. This information requires the development of biological tests, and especially with plants. Plants present indeed a particular interest in ecotoxicology due to their (i) immobility, (ii) important roots network, and (iii) fundamental role in ecosystems as primary producers (Hock and Elstner, 2005). Ma (1999) described higher plants as the most sensitive organisms for the detection of mutagens and genotoxic effects of environmental pollutants. Although ecologically relevant for soil toxicity assessment, plants are surprisingly not the most commonly used organisms for genotoxicity tests (White and Claxton, 2004). This is the reason why genotoxicity tests with higher plants have been promoted (IPCS – United Nations Environment Programme, 1999).Genotoxicity - simply defined as the toxicity on the genome - is an indicator of dysfunctions appearing at sub-lethal concentrations. An easy endpoint to observe is the formation of micronuclei, that are small nuclei appearing whenever a chromosome fragment or a complete chromosome is not incorporated into the nuclei during mitosis. It therefore reveals a break of genetic material (clastogenic effect) or a dysfunction of mitotic spindles (aneugenic effect). This endpoint is very important to include in a battery of ecotoxicity tests for a better risk assessment of contaminated soils and of the impact of remediation techniques applied to them.Among techniques of in situ remediation, soil amendments with biochar, i.e. the solid product from biomass pyrolysis, have recently been investigated for decreasing the bioavailability of metals in industrial soils. Biochar has been shown to immobilize metals both by direct sorption at its surface and by an indirect effect through an increase of soil pH (Rees et al., 2014). The effect of biochar on the actual genotoxicity potential of metal-contaminated soils is however unknown.The aim of this study was to assess the genotoxic potential of a range of soils contaminated by heavy metals (Zn, Pb, Cd) and amended by a wood-derived biochar to create a gradient of metal availability. We recently obtained international standardization of the Vicia-micronucleus test (ISO 29200) and we performed it in this study by direct exposure of root tips to soils. The analysis of root morphology and root metal content completed the experiment. Results showed that these industrial soils induced genotoxicity, revealed by an increase of micronuclei frequency. Their genotoxic potential strongly decreased with the addition of biochar. Relationships between genotoxicity, soil metal extractability and root metal content will be discussed.

International audience

International audience

International audience; With the increase of global population and the threat of climate changes caused by anthropogenic gas emissions, high pressure has been put on arable lands in order to cover the needs for both food and bioenergy production. Conversion of degraded lands for such purposes represents a sustainable solution, implying the restoration of several ecosystem services which have been impaired. Construction of Technosols on degraded lands have been investigated in order to increase soil fertility and biological activity, using various organic and inorganic industrial by-products. Field trials have demonstrated the interest of this remediation technique for the production of non-food biomass and the improvement of soil biodiversity. Moreover, the construction of Technosols with materials rich in organic carbon may also represent a way to store carbon in soils for a long-time, thus alleviating the emissions of greenhouse gases associated to the traditional end-of-life of these products. However, the processes of organic matter transformation and mineralization in these newly formed soils are largely unknown. We collected data on soil organic C evolution over 12 years on two experimental sites in Lorraine, France, where different Technosol profiles were created from thermally-treated cockery soil, paper sludge and green waste compost. Data were compared to the one from natural soils in the same region, and to the available data from other constructed Technosols in the literature. RothC model (Coleman and Jenkinson, 1996), describing the evolution of soil organic carbon divided in five pools of organic matter, was adapted to the specific constraints of the constructed Technosols (e.g. with possible evolution of soil density and soil depth). The adapted model was used to predict the evolution of soil organic carbon on both experimental sites, both at short-term over the last 12 years and on the long-term over the whole 21st century. Our results suggest that the stock of organic C in the top 30 cm of constructed Technosols can reach the level usually observed in natural soils with similar climate and plant cover in less than 10 years. This evolution was well described by RothC model when taking into account the various types of organic matters introduced in the Technosols. Despite this fast degradation at the surface of the soil, the total amount of organic C stored in the whole profile of the Technosol (100 cm) remained significantly higher than the corresponding value in natural soils. Constructed Technosols could therefore be a way to optimize soil ecosystem services associated to carbon storage, e.g. global climate regulation. Further investigations and new modelling approaches are however needed to precise the dynamics of soil organic carbon in these newly formed soils, particularly regarding the influence of climate changes expected in the next decades.

International audience; Biochar has been proposed as an effective soil amendment for the management of metal(loid)s [1] via a number of mechanisms: directly sorbed on its surface, by the occlusion of soil particles and, in particular, by modifying soil chemistry such as pH or dissolved organic carbon (DOC). An increase of soil pH induced by biochar has been shown to reduce metal solubility, while a release of labile compounds from biochar has resulted in soil metal leaching [2-4]. Different results may therefore be expected depending on the use of "fresh" or "aged" biochar, subject to weathering which can modify the surface and chemical properties of the biochar. Column leaching experiments were conducted in August 2014 by Frédéric Rees and Dr. Luke Beesley with the help of Aline Peregrina Puga at the James Hutton Institute of Aberdeen (UK), with the support of COST Action TD1107. One agricultural soil contaminated by repeated sewage sludge amendments was tested together with one biochar, used as purchased or aged by exposure to natural rainfall. Instead of amending soils with biochar in a single column, a new protocol was tested (Fig. 1), using one column for each material, linked in a loop system so that the retention of metals from soils by the biochar and the modifications of the biochar on the eluent fed back directly to the contaminated soils could be elucidated. Samples of the leaching solution were regularly collected after the soil column and the biochar column, which enabled the chemical evolution of the soil-biochar systems towards equilibrium to be monitored. Chemical equilibration between soils and biochar was fast, as seen by the identical final pH at the different sampling points of the system. Fresh and aged biochar retained metals released from the agricultural soil and decreased further metal release by increasing soil pH. Fresh biochar became however saturated with Zn at the end of the experiment, while aged biochar did not, indicating that aged biochar had a greater sorption capacity for metals. In conclusion, this work indicated that the simple ageing of biochar before its use as a sorbent amendment can increase its metal retention capabilities.

International audience

International audience; Biochar, i.e. the solid product of biomass pyrolysis, has recently been investigated as a soil amendment in metal contaminated soils, mostly for its ability to decrease the phytoavailability of trace elements and to support plant growth. Several works have shown its efficiency for limiting the uptake of metals by plants, but the combination of biochar with metal-hyperaccumulating plants has never been tested. This work was conducted to examine the effects of biochar amendments on metal uptake by a Cd- and Zn-hyperaccumulator (Noccea caerulescens), compared to a non-hyperaccumulating plant (Lolium perenne). Plants were grown in controlled conditions on one acidic (A) and one alkaline (B) soil contaminated by Cd, Pb and Zn by smelter activities, both amended by a wood-derived biochar at variable rates up to 10 % (w/w). Biochar amendments in both soils decreased the availability of metals, but also of other major cations such as Ca2+, as shown by the analysis of pore water and soil extracts. This effect was linked to the observed increase of soil pH with increasing biochar dose. While shoot metal uptake of L. perenne constantly decreased with biochar addition in both soils, an increase of shoot Cd uptake of N. caerulescens with 5% (w/w) biochar was recorded on both soils, and of Zn uptake on soil B. We explain this increase of metal hyperaccumulation by a decrease of competition with Ca for metal uptake. Biochar therefore affects plant metal uptake by decreasing the availability of both cationic trace elements and major cations. This study reveals that biochar may be used not only as a sorbent or liming agent for decreasing the mobility of metals in contaminated soils, but also as an enhancer of phytoextraction to increase the removal of metals by hyperaccumulating plants.

Mobility of metallic trace elements in biochar-amended soils. 3.3Sun Yat-sen University 2015

International audience; Soil amendments of biochar, i.e. the solid product of biomass pyrolysis, have been increasingly investigated over the last few years as a way to store stable C in soils, to improve crop production and to remediate degraded and contaminated land. Many short-term effects of biochar on soil chemical and biological properties may be explained by the observed increase of soil pH, largely reported in the recent scientific literature for acid or neutral soils. However, both the mechanisms by which biochar is affecting soil pH and the duration of pH changes remain unclear. Using batch reactors and column leaching experiments, we examined the response of a wood-derived biochar pyrolyzed at 450 °C to the addition of various quantities of HNO3 or NaOH. Our results enable to identify both the dissolution of biochar’s mineral phases, e.g. calcite and Si/Al oxides, and cation exchange at the surface of biochar as the two major mechanisms explaining biochar’s buffering capacity. The importance of these mechanisms vary in dependence with time and pH evolution. Observation of old charcoal amendments at various sites suggest however that, depending on initial soil pH and soil buffering capacity, the effects of biochar caused by the modification of soil pH may not last more than a few years.

Remediation of industrial soils - a lysimeter research: Monitoring the evolution of biochar's influence in a metal-contaminated soil. 13.International Workshop at UGT South 2015: Lysimeters – separating processes in flux measurement

Accès restreint aux membres de l'Université de Lorraine jusqu'au 2015-12-10; Biochars play a central part in the mitigation of global changes. They also represent a challenge for the sustainable management of contaminated soils. This work was conducted in order to better understand the effects of biochar on the mobility of metals in soils and their uptake by plants. A range of experiments was set up following a gradual increase of complexity, with a wood-derived biochar obtained by pyrolysis at 450 °C and two soils, acidic or alkaline, contaminated by Cd, Pb and Zn from smelter activity. Various plant species with contrasting response to metals were tested. Batch sorption and column leaching experiments coupled to microscopic and spectroscopic characterization of biochars were conducted, together with plant growth experiment in pots, rhizoboxes and lysimeters. Results demonstrate that biochar's carbonated mineral phases play a dominant role in the immobilization of metals. They lead to metals direct sorption on the surface of biochar by co-precipitation. Their dissolution also contributes to the increase of soil pH, leading to an increased retention of metals on soil particles. Biochar alters the transfer of metals to the plants by decreasing the availability of metals, but also by decreasing the mobility of major cations and by modifying root surface. A decrease of metal transfer to the shoots was generally observed with Lolium perenne or Zea mays, while an increase of the uptake of Cd and Zn by the hyperaccumulator Noccaea caerulescens was evidenced. In conclusion, biochar controls the mobility of metals in soil-plant systems through a range of different mechanisms. From a practical point of view, biochar promotes both strategies of phytostabilization and phytoextraction of metals in contaminated soils; Les biochars sont au cœur de la lutte contre les changements globaux. Ils constituent aussi un enjeu pour la gestion durable des sols contaminés. Ces travaux ont été conduits afin de mieux comprendre les effets du biochar sur la mobilité des métaux dans les sols et leur transfert vers les plantes. Une série d’expériences de complexité croissante ont été conduites, impliquant un biochar obtenu par pyrolyse de bois à 450 °C, et deux sols, l’un acide et l’autre alcalin, contaminés par Cd, Pb et Zn suite à l'activité d'une fonderie. Une gamme d’espèces végétales aux réponses contrastées vis-à-vis des métaux a été testée. Des essais de sorption en batch et de lixiviation en colonnes couplés à une caractérisation microscopique et spectroscopique du biochar ont été réalisés, ainsi que des expériences de croissance végétale en vases de végétation, en rhizotrons et en lysimètres. Les résultats démontrent que les phases minérales carbonatées du biochar jouent un rôle prépondérant dans l'immobilisation des métaux. Elles interviennent dans la sorption directe des métaux à la surface du biochar par co-précipitation. Leur dissolution contribue également à l'augmentation du pH du sol, conduisant à une rétention accrue des métaux à la surface des particules du sol. Le biochar modifie le transfert des métaux vers la plante en diminuant la disponibilité des métaux, mais également en diminuant la mobilité de cations majeurs et en modifiant la surface racinaire développée par la plante. Une diminution du transfert du métal vers les parties aériennes de la plante a généralement été observée avec Lolium perenne ou Zea mays, alors qu'une augmentation du prélèvement de Cd et Zn par l'hyperaccumulateur Noccaea caerulescens a été mise en évidence. En conclusion, le biochar contrôle la mobilité des métaux dans les systèmes sol-plante grâce à une série de mécanismes différents. Au plan pratique, le biochar favorise les stratégies de phytostabilisation et de phytoextraction des métaux dans les sols contaminés

National audience

LORVER project: production chain of biomass for industrial purposes from former sites and materials. . Contaminated Site Management in Europe (CSME) & Sustainable Approaches to Remediation of Contaminated Land in Europe (SARCLE)