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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; 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; 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.

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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.

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.

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International audience; Biochar is a charred carbon-enriched material intended to be used as a soil amendment to sequester carbon and enhance soil quality. Addition of biochar to soil has many environmental and agricultural benefits, including waste reduction, energy production, carbon sequestration, water resource protection, and soil improvement. Biochar has been reported to boost soil fertility and improve soil quality by raising soil pH, increasing moisture holding capacity, attracting more beneficial fungi and microbes, improving cation exchange capacity (CEC), and retaining nutrients in soil. Biochar usually has a greater sorption ability than natural soil organic matter due to its greater surface area, negative surface charge, and charge density what can be an important set of properties for soil protection and reclamation procedures. Another major benefit associated with the use of biochar as a soil amendment is its ability to sequester carbon from the atmosphere-biosphere pool and transfer it to soil. Unfortunately many of described effects of biochar addition in soil are not well understood and bring many doubts about the potential cost and benefits of its use in agriculture. In many cases biochar studies bring very opposite results depended on many factors eg. feedstock, biochar production technology, soil properties, climate or plant used in the experiment. The main objectives of the session is to: • Highlight the potential cost and benefits of biochar as an effective soil amendment. • Discuss main factors influencing quality of biochar in context of its effective use in soil. • Present the state of knowledge about mechanism of BC-soil and BC-plant and microorganism interactions. • To identify the main problems of biochar analysis and its use in soil environment. The purpose of the session is to bring together researchers working on biochar use in agriculture, horticulture and soil reclamation interested in sharing their experiences in international, multidisciplinary group, developing collaboration for scientific projects and publications.”

International audience; Biochar has been regarded as a promising carbon sequestration and fertilizing agent in soils. Its ability to sorb pollutants also offers new possibilities to remediate contaminated sites. However, very few studies have investigated the actual effect of biochar on the transfer of heavy metal to plants. In order to evaluate the potential of biochar in this context, we examined the effect of a wood-derived biochar at different amendment rates on the growth of ryegrass in one acid and one basic soil, both similarly contaminated by Zn, Pb and Cd from smelter industry. After 12 weeks of growth in one-liter pots, both roots and shoots were harvested, weighted and analyzed for their total trace element content. In addition, we also sampled the pore solution during the first weeks and analyzed soil samples after harvest to monitor changes in pH and in metal or nutrient availability. The increasing addition of biochar lead to an increase of pH on both soils and a continuous decrease of Zn, Pb and Cd concentration in the soil solution, which resulted in a reduced transfer of metals to the plants. Positive effects of biochar on the plant growth were observed on both soils, but the increasing addition of biochar after 0.5 % on the basic soil resulted in a decrease of shoot biomass, which we attributed to a decrease of nutrient absorption. The ability of biochar to reduce the phytotoxicity of heavy metal contaminated soils may therefore be counterbalanced by the limitation of nutrient availability to plants.

National audience; L’utilisation comme amendement de sol de biochar, le charbon solide récupéré après pyrolyse de tout type de biomasse, fait actuellement l’objet de nombreuses recherches, en particulier en lien avec la séquestration de carbone dans les sols et l’augmentation des rendements agricoles [1]. De récentes études suggèrent aussi que le biochar est capable d’immobiliser certains métaux lourds, ce qui ferait de ce matériau faiblement biodégradable, renouvelable et peu coûteux un outil de choix dans la remédiation des sols pollués [2]. Les processus d’immobilisation des métaux lourds, ainsi que leur réversibilité, sont cependant très peu compris. Afin d’évaluer le potentiel d’immobilisation d’un biochar produit à partir de bois résineux et feuillus à 450°C, des expériences de sorption de différents métaux en solution ont été réalisées en batch. Suite à une étape d’adsorption et de désorption, les charbons ont été récupérés puis caractérisés par différentes techniques (e.g. diffraction des rayons X, spectroscopie infrarouge, microscopie électronique) afin d’identifier les modes par lesquels les métaux ont été immobilisés. Les premiers résultats suggèrent la concomitance de plusieurs mécanismes d’immobilisation, dont la précipitation des métaux avec des carbonates ou des phosphates, tandis que l’hystérèse observée entre la courbe d’adsorption et de désorption indique que l’immobilisation n’est que partiellement réversible. Des essais d’extraction sur plusieurs sols contaminés réels prélevés à proximité d’une fonderie montrent par ailleurs que la diminution de l’extractibilité des métaux en présence de biochar serait principalement due à l’augmentation du pH observée, entraînant une plus grande rétention sur les particules de sol. Des expériences en colonne ont donc été imaginées pour compléter ces résultats et distinguer les effets réels du biochar sur la mobilité des métaux dans les sols. L’association d’une colonne de sol contaminé et d’une colonne de biochar permettra ainsi de mesurer la dynamique de sorption des métaux propre au biochar et offrira la possibilité de distinguer la part d’immobilisation des métaux due à leur adsorption à la surface du biochar et celle liée à l’élévation de pH dans le milieu. L’ensemble de cette étude permettra de conclure quant aux possibilités d’utiliser le biochar comme un outil durable de remédiation des sols pollués.

International audience; Biochar has been intensively investigated over recent years, not only as a promising carbon sequestration or fertilizing agent in soils but also as a possible new sorbent to remediate contaminated soils. A few studies have revealed its high potential for heavy metals immobilization depending on the nature of biochar and trace elements. The mechanisms behind this immobilization remain however unclear: some authors have hypothesized a high sorption capacity due to biochar large surface area while others have suggested that this immobilization is mainly due to soil pH increase. In particular, the distinction between heavy metals specific sorption in biochar pores and heavy metals precipitation in or outside biochar particles is often impossible to make while it is of primary importance to evaluate biochar ability to retain these pollutants on a long-time scale. In order to evaluate the main heavy metal immobilization effects on a standard biochar and to identify the most successful biochar parts of the sample, we examined biochar particles after heavy metals immobilization in batch experiments designed to mimic real chemical processes in soils. A biochar derived from hard and soft wood and pyrolyzed at about 450°C was put in contact with relatively low concentrations of heavy metals (Pb, Cu, Cd, Zn, Ni) in an initially acidic Ca(NO3)2 solution. Following a one-week adsorption and a one-week desorption step, we recovered the biochar particles and observed them using scanning electron microscopy coupled to energy dispersive x-ray spectroscopy, focusing especially on the changes in mineral phases and the location of each of the retained heavy metals on biochar particles. We were able to distinguish different structures in the biochar samples which were linked to the degree of pyrolysis and the exact nature of the raw wood biomass. We detected the presence of concentrated metals zones (e.g. lead) in specific locations of the organic particles depending on the original plant tissues, and enlightened metal associations with newly-formed mineral phases such as calcite present on biochar surface. These observations provide new insights in the understanding of metal immobilization mechanisms on biochar such as precipitation and co-precipitation. Our findings also underline the need to consider the heterogeneity of biochar constitution for optimizing the remediation potential of biochar on contaminated sites.

National audience; Biochar, the residual solid of biomass pyrolysis, has recently drained considerable attention as a potential carbon sequestration agent in soils due to its high recalcitrance to biodegradation as long as a low-cost soil quality improver to increase plant growth 1. Recent studies suggest also biochar ability to immobilize heavy metals in soils and thus to reduce the toxic impact of these elements 2–4. However, the sorption mechanisms and reversibility degree of biochar effects for the remediation of contaminated soils are still unclear and must be addressed to evaluate potential risks of biochar use in these soils5. In a first attempt to evaluate biochar capacity to immobilize metals in solution, we set up batch sorption studies of cationic trace elements (Cu, Cd, Ni, Zn, Pb) on an alkaline biochar produced at about 450°C from conifers and deciduous wood. Kinetics results show that sorption speed is low and particles-size dependent, which may be attributed to limiting diffusion of metal inside biochar particles. Adsorption/desorption experiments with initial metal concentration of 10-500 μmol/L reveal different sorption trends depending on the elements: Freundlich-type isotherms of Cd and Zn are identical while Cu and Pb show a much higher affinity for biochar and Ni the lowest of all five elements. On the other hand, hysteresis with desorption isotherms reveal a marked sorption irreversibility especially for Cu and Pb. However, the sorption dynamics of heavy-metals on biochar remains complex and several environmental parameters have to be taken into account, e.g. the influence of pH variations of the system in order to mimic possible long-term acidification of contaminated soils amended with biochar. In a second time, we will discuss the results of continuous solution analysis in column experiments with biochar alone and biochar-amended contaminated soils leached with different extracting solutions to describe the actual sorption dynamics on biochar of heavy-metals originating from historical atmospheric pollution by smelters in order to assess biochar potential for long-term remediation of contaminated sites.

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