Trace element biogeochemistry in the rhizosphere: why bother with plant-mediated physical-chemical processes?

The investigation of trace element biogeochemistry in the soil-plant system is a necessary step for many agricultural and environmental issues. For instance, an in-depth understanding of trace element biogeochemistry is helpful to support soil fertility by enhancing or alternatively alleviating the plant uptake of micronutrients and inorganic contaminants, respectively. The manipulation of trace element biogeochemistry is also a key component of phytoremediation strategies implemented on highly contaminated soils. The common driver of all these agronomic and environmental issues is intimately related to the interactions occurring at the soil-plant-microbial interfaces, i.e. in the so-called rhizosphere. The present lecture will focus on an updated overview of the many physical-chemical processes mediated by plant roots in the rhizosphere and how these processes drive trace element biogeochemistry in the soil-plant system. The first part of the lecture will be dedicated to a case study that demonstrates the importance of plant-mediated physical-chemical processes in determining copper toxicity to wheat in former vineyard soils in southern France. In this context, the occurrence of copper toxicity to wheat interestingly contradicted the prediction that has been made from bulk-soil chemistry, but was nicely supported by rhizosphere chemistry. The second part of the lecture will address an updated overview of the many physical-chemical processes (i.e. changes in pH, dissolved organic matters, and redox potential) that plant roots are able to implement in the rhizosphere with a brief illustration of analytical technics and experimental systems that enable to study them. The basic mechanisms involved will be first introduced at the soil-root scale, then scaled-up to the whole root system and finally to field-scale. The last part of the lecture will be dedicated to a second case study about arsenic phytostabilization that illustrates the importance of considering concomitantly a range of physical-chemical processes in determining arsenic dynamic in the rhizosphere. In this microcosm-scale study, we will see that arsenic dynamic in the rhizosphere is concomitantly related to the dynamics of iron, calcium, and protons. (Texte intégral

Revisiting the origins of root-induced pH changes in the rhizosphere

Several decades of research were dedicated to unravel the mechanisms involved in root-induced pH changes in the rhizosphere and finally to determine the major drivers. The most considered driver is unequivocally the plant nutritional status. The imbalance between cation and anion uptake, notably between ammonium and nitrate, leads to the net excretion of protons or hydroxyls by roots and consequently to the respective acidification or alkalization of the rhizosphere. Also, specific nutritional constraints related to the low availability in soil of mineral elements such as phosphorus (P) and iron (Fe) are usually considered to compel plants to acidify their rhizosphere. However, the importance of plant nutritional status in regulating pH changes in the rhizosphere was recently challenged by the comparison of results we obtained in rhizobox experiments performed with three plant species belonging to different families (i.e. a solanaceae, a brassicaceae, and a poaceae) and having a similar nutritional status (nitrate-fed plants without P or Fe starvation). pH patterns in the rhizosphere differed substantially between the three species and as a function of the initial bulk-soil pH. We therefore deepened these preliminary investigations by cropping under similar growing conditions three species belonging to the same three families already tested on 55 soil samples exhibiting an initial bulk-soil pH varying between 5 and 8. In agreement with the preliminary investigations, the lower the initial pH of a given bulk-soil, the higher the root-induced alkalization in the corresponding rhizosphere. In addition, the effect of the initial bulk-soil pH was the strongest for the poaceae, then the brassicaceae, and finally the solanaceae. These results consistently suggest that, in addition to the plant nutritional status, the initial bulk-soil pH and the specific metabolism of each plant species concomitantly drive pH changes in the rhizosphere

Temporal dynamic of trace elements in soils amended with contaminated organic wastes

Applications of so-called organic wastes are recommended in agriculture where the organic matter content of soil is low. However, before applying organic wastes to soil it is essential to assess that these materials do not pose any hazard to humans, animals, or to the environment. This study aims at evaluating the dynamic of trace elements in a soil amended with various organic wastes combining incubation and DGT techniques. Soil/waste samples were incubated to evaluate the impact of organic waste application on trace elements exchangeable fractions by exploring the DGT response. Organic wastes were incorporated at 80 t ha-1 of dry matter. During organic waste mineralization, we have detected increase in the exchangeable fractions of trace elements, which was more noticeable for Zn and Pb than for Cu, Cd, Ni and Cr. (Résumé d'auteur

Investigation of trace elements content in organic wastes used for market gardening

While agricultural recycling is recognized as an alternative to stockpiling or incineration, the benefits of the use of organic wastes as fertilizers and soil amendments should be assessed together with potential environmental and toxicological impacts due to the presence of trace elements (TE). While these considerations are common in Northern countries, issues and problems involved in waste management are increasing in developing countries. Within the framework of the ANR project ISARD, designed to set up methods to ensure suitable agricultural intensification based on the recycling of organic wastes, this study investigated the contents of major chemical elements and TE in various composts, from sewage sludge, household refuses, animals manure and garden rubbish, applied on market-garden crops on the outskirts of cities in various countries (Saint Denis, La Réunion, France; Majunga, Magacascar; and Dakar, Senegal). Organic waste contents are various and depend on the geographic origin and type of wastes (e.g. Pb = 0.82-2100 mg kg-1 dry matter). Half of the organic wastes that were examined exhibit very high TE concentrations, and are above the limits set by European legislation and found in the literature data for organic wastes designed for market gardening. Size fractionation of organic wastes exhibited a fairly large enrichment in TE in the smaller solid fraction (0.2-20 ?m) in comparison with raw wastes. This result suggests that TE were potentially associated with organic matter in the 0.2-20 ?m fraction, which is the most reactive to degradation of micro-organisms. The use of such organic wastes for market gardening could consequently be potentially harmful with respect to TE phytoavailability and phytotoxicity. However, total concentrations of TE in organic wastes and of TE dynamic in amended soils will be crucial to predict TE phytoavailability. (Texte intégral

Development of a new plant-based biotest to assess trace element phytoavailability in contamined soils : selection of target-plant species for standardisation

Concerning the threat of soil contamination, the assessment of trace element phytoavailability at an operational level still requires the identification, the development and the standardisation of a set of biological methods (i.e. biotest). The present abstract introduces the first step in the development of a new plant-based biotest, the RHIZOtest, focused on the selection of the target-plant species suggested for the standardisation of the RHIZOtest. The RHIZOtest is notably based on a complete physical separation between plant and soil compartments enabling an easy, fast and clean recovery of the roots. The RHIZOtest was deployed for ten plant species commonly used in agriculture and on three soils exhibiting a broad range of pH and a high concentration in several trace elements. The measurement of trace element phytoavailability was finally achieved as the mean flux of trace element to the plants during the exposure of the plants to the soil. Seven out of ten plant species exhibited homogenous growth of roots and shoots and consequently can be used adequately in the RHIZOtest experimental procedure. As expected, plant uptake flux of trace elements varied significantly and many-fold among the ten plant species tested. However, trace element phytoavailability also broadly varied among trace elements and soils. Finally, a procedure of ordination and scoring enabled us to select three plant species that maximised trace element phytoavailability according to a precautionary-like principle. This three plant species will be suggested for further standardisation of the RHIZOtest. This study supports the requirement of biological methods that enable to encompass the biological diversity in the assessment of trace element phytovailability whereas chemical methods are not able to take it into account. Such kind of validation procedure for a biotest is the unique opportunity for achieving operational methods based on a hard scientific background that could be standardised for the assessment of trace element phytoavailability. (Résumé d'auteur

Connaître les fertilisants et les outils disponibles pour un pilotage raisonné

La valorisation de la fertilité de son sol passe par une connaissance et une utilisation raisonnée des fertilisants (amendements et engrais) à sa disposition. Pour cela, nous disposons d'outils d'aide à la décision qui gagnent à être mieux connus et mieux utilisés

Entre contamination et pollution des sols agricoles par les éléments traces : plaidoyer pour une évaluation à long-terme du risque écotoxicologique

Les recherches menées ces quelques trente dernières années ont permis d'aboutir à un niveau de connaissances et à un arsenal réglementaire permettant de limiter sur le court-terme (< 10 ans) le niveau de contamination des sols par les éléments traces et les impacts écotoxicologiques qui en découlent dans la plupart des contextes agricoles. En revanche, nos approches expérimentales généralement mises au point sur des milieux très fortement et artificiellement contaminés ainsi que nos démarches de modélisation prédictive, rarement validées expérimentalement, ne permettent pas actuellement de fournir une évaluation quantitative du risque écotoxicologique sur le long terme. Après avoir éclairci quelques éléments de définition, l'intervention abordera successivement (i) les principales sources de contamination des sols agricoles par les éléments traces et leurs impacts écotoxicologiques potentiels, (ii) les nouveaux paradigmes expérimentaux nécessaires à une évaluation écotoxicologique en milieux modérément contaminés et (iii) les limites et perspectives des démarches de modélisation prédictive sur le long-terme. (Texte intégral

RHIZOtest: an innovative tool for phytoavailability assessment and risk management in polluted soils

Resulting from 25 years of research at Inra and Cirad in France, RHIZOtest is a plant-based test that was standardized at ISO to assess the phytoavailability of trace elements in polluted soils. Initially, mainly developed on moderately contaminated soils, the objective is now to apply the RHIZOtest as a tool to assess the phytoavailability in heavily Polluted Sites and Soils. The RHIZOtest concretely measures the transfer of soil contaminants (mainly trace elements) to the plant, i.e. the phytoavailability. It takes into account the interactions between several contaminants, i.e. the mixture effect. By mimicking soil-plant interactions specific to each type of soil and plant species, the RHIZOtest reproduces natural processes to assess the actual risk. Finally, the RHIZOtest allows to reproduce, in few weeks in a laboratory, widely generalizable studies hardly achievable in the field for reasonable implementation time and cost. Today operational for transfer analysis of trace elements in the agricultural context, the RHIZOtest deployed by the spin off MetRHIZLab, offers new perspectives for the risk assessment and management of polluted sites and soils, such as: (i) assessing risk of transfer according to use, (ii) proving the effectiveness of remediation techniques, (iii) optimizing a site, (iv) attending revegetation or (v) characterizing the state of contaminated soil to be recycled. In order to test these new perspectives for the risk assessment and management of polluted sites and soils, a preliminary study was conducted on three soils from polluted sites. Soil 1 and 2 were collected in an ancient mine in southern France. Soil 1 and 2 are exhibiting high concentrations in Zn, Pb and Cd. Soil 3 is highly contaminated with As, Cd, Cr, Cu, Ni, Zn and even more Pb by atmospheric fallouts from a battery recycling plant. The three reference plant species, cabbage, fescue and tomato, were cultivated on these soils according to the ISO 16198 standard. Firstly, no difference of plant biomass was observed between the three soils. This shows that the RHIZOtest was able to assess phytoavailability of trace elements without any phytotoxic artefact. Secondly, the three plant species highlighted the same phytoavailability for each soil and each trace element. Thus, the RHIZOtest showed its capacity to give a robust measure of the phytoavailability of trace elements in heavily contaminated soils. Based on this preliminary study, a R&D program is underway to confirm these results with soils exhibiting contrasted physical-chemical properties and contaminations. Soils will be selected among different types of polluted sites such as ancient mines and industrial wasteland managed with various techniques (washing, phytomanagement or bioremediation), to create a decision support method based on RHIZOtest. Dedicated to design offices, laboratories and experts, the RHIZOtest is an innovative decision support tool that should enable soon to assess risks and to optimize the management of polluted sites and soils

Mobilité et phytodisponibilité des éléments traces métalliques dans les sols de La Réunion : Rapport de synthèse en vue d'une demande de dérogation sur le critère " éléments traces métalliques " relativement à l'utilisation d'eaux usées épurées pour l'irrigation de cultures ou d'espaces verts à La Réunion

A partir d'une synthèse des acquis et des limites de l'étude de milieu de Collin et Doelsch (2008) qui a permis l'obtention d'une dérogation au critère ETM pour l'épandage des boues de station d'épuration sur les sols agricoles, le présent rapport a pour objectif de présenter et d'analyser un ensemble de données complémentaires . Cette analyse fournit d es éléments scientifiques permettant de soutenir de futures demandes de dérogation au critère ETM pour l'ensemble des sols, des cultures et des espaces verts sur lesquels l'utilisation des eaux issues du traitement des eaux résiduaires urbaines pour l'irrigation de cultures ou d'espaces verts (ci- après dénommée Reuse) pourrait être envisagée à La Réunion . L'analyse des gammes de concentrations totales en chrome (Cr), cuivre (Cu), et nickel (Ni) obtenues dans trois inventaires complémentaires a montré que les 17 unités de milieu ( UM ) de l'étude de milieu de Collin et Doelsch (2008) étaient au moins représentatives de 60 des 93 UM de La Réunion en matière de concentrations totales des sols en ETM. L'analyse de deux jeux de données supplémentaires a permis de montrer que les conclusions de l'étude de milieu de Collin et Doelsch (2008) concernant la faible mobilité et la faible phytodisponibilité des ETM dans les sols étaient également extensibles à 60 des 93 UM de La Réunion, à l 'exception des sols des UM 28, 29, et 48 issus du Piton de la Fournaise . Pour ce qui concerne les 33 UM pour lesquelles aucune donnée n'est disponible sur les ETM, l'analyse de leur localisation et leur nature géo - morpho - pédologique a permis d'estimer que ces 33 UM présentaient un risque nul à faible d'observer une concentration, une mobilité et une phytodisponibilité élevée des ETM dans les sols, à l'exception des UM 65 et 84 pour lesquelles un risque potentiellement élevé ne peut être exclu. L'analyse des données complémentaires proven ant de mesures de concentration en Cr, Cu, et Ni dans des végétaux issus de déchets verts (représentatifs des espaces verts), dans des plantes maraîchères et arboricoles, ainsi que dans des espèces prairiales conforte les conclusions faites par Collin et Doelsch (2008) sur la canne à sucre, en ne montrant aucune différence de concentrations pour des plantes ayant été cultivées sur des sols présentant des concentrations en ETM inférieures ou supérieures aux seuils définis par l'arrêté du 8 janvier 1998. Con sidérés dans leur ensemble, les résultats de ce rapport de synthèse suggèrent donc que les conclusions formulées par Collin et Doelsch (2008) seraient effectivement extensibles à l'ensemble des sols, des cultures, et des espaces verts sur lesquels la Reuse serait susceptible d'être mise en oeuvre à La Réunion, à l'exception des sols des UM 28, 29, 48, 65, et 84 issus du Piton de la Fournaise pour lesquels une incertitude subsiste quant à la mobilité et la phytodisponibilité des ETM (Ni en particulier) (Résumé d'auteur