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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.

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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.

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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.

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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.

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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.

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.

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

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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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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.