Polluted sites are ubiquitous worldwide but how plant partition their biomass between different organs in this context is unclear. Here, we identified three possible drivers of biomass partitioning in our controlled study along pollution gradients: plant size reduction (pollution effect) combined with allometric scaling between organs; early deficit in root surfaces (pollution effect) inducing a decreased water uptake; increased biomass allocation to roots to compensate for lower soil resource acquisition consistent with the optimal partitioning theory (plant response). A complementary meta-analysis showed variation in biomass partitioning across published studies, with grass and woody species having distinct modifications of their root: shoot ratio. However, the modelling of biomass partitioning drivers showed that single harvest experiments performed in previous studies prevent identifying the main drivers at stake. The proposed distinction between pollution effects and plant response will help to improve our knowledge of plant allocation strategies in the context of pollution.

Because of their high content in toxic metals, steel slag dumps are potential threats for the environment and public health. Among management methods that could mitigate their hazard, aided-phytostabilization is a relevant, though challenging, option. Indeed, steel slags are very unfavorable for plant growth, due to metal toxicity and very alkaline pH (>10). In this work, we investigated how composted sewage sludge could alleviate slag's toxicity while improving its nutritional status. A pot experiment was performed to study biomass production and leaf ionome composition of five herbaceous species (Achillea millefolium, Bromus erectus, Festuca arundinacea, Melilotus officinalis and Medicago sativa), in relation to soil pore water's pH, concentration of trace and major elements and their chemical speciation. Results showed that pH had a clear-cut effect on plant development. Above pH 8.6, plant biomass was severely affected, due to accumulation of Cr above toxic threshold and deficiencies in Mn, Zn and P. Below pH 8.6, biomass increased significantly, together with a decrease in leaf Cr below toxic level, and an increase in Mn, Zn and P above deficiency levels. Thus, these results bring new insights into the causes of slag phytotoxicity and allow considering aided-pytostabilization as a realistic and efficient approach for the remediation of steel slag dumps, provided soil pH is carefully monitored before seeding.

Steel slags are major by-products of the steel and iron industry. Particularly rich in metallic trace elements, these materials are massively stored in slagheaps and can constitute potential nuisance for the environment and public health. Several studies [(Alvarenga et al., 2009), (Mench et al., 2010)] have shown the effectiveness of phytostabilization processes for rehabilitating sites contaminated by high levels of metals, both because of their sustainable and ecological aspects but also due to their low implementation costs. Recent studies have indeed demonstrated the benefits of using indigenous AMF (Arbuscular Mycorrhizal Fungi) isolates for the phytostabilization of metal polluted soils (Lacercat-Didier et al., 2016) and alkaline mine tailings [(Orlowska et al., 2010), (Giridhar Babu and Sudhakara Reddy, 2011)]. The slagheap studied in this work contains approximately 500 000 tons of slags spread over an area of about 4 ha. A previous study (Bouchardon et al., 2014) carried out on the same site has demonstrated that the use of “Composted Sewage Sludge” (CSS) as an organic amendment favors the installation of metallophytes herbaceous species naturally able to grow on this type of substrate. In this work, we evaluated the coupled effects of CSS and AMF inoculation on : 1) the soil pore water composition and metals bioavailability and 2) the vegetation in terms of biomass production as well as accumulation of metallic compounds in leaves. Results showed that AMF inoculation led to root colonization and improved K and P uptake. We also observed that the CSS amendment has a critical role on plant growth by decreasing the fluid pH (from 10.5 to 8.5) and thus, the speciation of Al in the system. Indeed, geochemical modelling run with the PhreeqC program (Parkhurst & Appelo, 2013) revealed that Al becomes soluble, and therefore toxic for plant development, at a pH higher than 9.

Steel slags are major by-products of the steel and iron industry. Particularly rich in metal trace elements, these materials are massively stored in slagheaps and can constitute potential nuisance for the environment and public health. Several studies have shown the effectiveness of phytostabilization processes for rehabilitating sites contaminated by high levels of metals, both because of their sustainable and ecological aspects but also due to their low implementation costs. However, implementing a phytostabilization project for metallurgical slags is a challenging issue. Besides the potential toxicity of metal elements (e.g. Cr, Zn, Cu, V, Ni, Mo, W, Al), slags are almost devoid of organic matter and major nutrients such as nitrogen and phosphorus. They also have a very low water holding capacity (sandy texture), as well as a high pH (>10) that greatly reduces the phytoavailability of essential micronutrients (e.g. Fe, Cu, Zn). The slagheap studied located near Rive-de-Gier (France) contains approximately 500 000 tons of slags spread over an area of about 4 ha. A previous study carried out on the same site has demonstrated that the use of Composted Sewage Sludge (CSS) as an organic amendment favors the installation of metallophytes herbaceous species naturally able to grow on this type of substrate. Recent studies have also demonstrated the benefits of using AMF (Arbuscular Mycorrhizal Fungi) isolates for the phytostabilization of metal polluted soils and alkaline mine spoils. In this work, we evaluated the combined effects of CSS and AMF inoculation on: i) the soil pore water composition and metals bioavailability and ii) the vegetation in terms of biomass production as well as accumulation of trace metals in leaves. Results showed that AMF inoculation led to root colonization and improved P uptake. We also observed that the CSS amendment has a critical role on plant growth by changing the fluid pH (from 10.5 to 8.5) and thus, the speciation of Al in the system. Indeed, geochemical modelling run with the PhreeqC program revealed that Al becomes soluble, and therefore toxic for plant development, at a pH higher than 9. We acknowledge the financial support of School of Mines and ANR “HYPASS” supervisors.

Steel slags are major by-products produced by the steel and iron industry. While they are considered as industrial waste, slags represent an important potential economic resource because they often contain significant amounts of valuable Strategic Metals (SMs). These metals are essentially used as alloying elements in the steel industry as well as in catalysts and pigments in the chemical industry and as raw material in green technologies (photovoltaic cells, wind turbines and electric motors). During the last decade the global steel production driven by China has doubled and the development of renewable energies (wind and solar) and electrics cars has dramatically increased. This has sharply affected global SMs demand, and there is no doubt today that production and consumption of SMs will must face a major economic pressure in upcoming years. In this context, the importance of hydrometallurgy in production and recycling of SMs cannot be stressed enough. However, improving or even optimizing these processes is clearly required. In addition, although steel slags are classified as non-hazardous thermal waste according to the decree 2002-540 and the United States Environmental Protection Agency, it is well known that most metals they contain are quite toxic to living organisms and may pose serious environmental issues. In France, the Centre Technique de Promotion des Laitiers siderurgiques (CTPL) estimated that the total available stock of slags (including all slag families) was about 17 480 600 tons at the end of 2015. Thus, reusing steel slags appears the option of choice to ensure their sustainable management as well as to decrease their environmental concerns.

Les laitiers siderurgiques sont des matieres minerales artificielles produites lors du processus de fabrication de l’acier. Au vu des quantites de laitiers siderurgiques annuellement produites et des stocks disponibles, la gestion de ces dechets necessite de trouver une solution qui soit satisfaisante tant d’un point de vue environnemental qu’economique. Ce travail propose de developper un procede hydrometallurgique de recuperation des metaux par voie basique et d'evaluer des methodologies de phytostabilisation aidee (utilisation d'amendements organiques et de champignons mycorhiziens) pour les residus secondaires issus de ce traitement. La recuperation des metaux, oxydes et/ou hydroxydes metalliques des laitiers siderurgiques a fait et fait toujours l’objet de nombreuses etudes et implique des operations de separation mecanique, magnetique, physico-chimique, de lixiviation, etc. dans des combinaisons diverses. Les methodes de mise en solution de la fraction metallique decrites dans la litterature sont generalement exclusivement acides ( Rao et Nayak , 1992). Ces traitements presentent l’inconvenient majeur de lixivier la chaux provoquant une surconsommation de reactifs et une production importante de sels. Une lixiviation par voie alcaline pourrait constituer une alternative satisfaisante. En effet, l’emploi de soude peut limiter la lixiviation du fer et du calcium presents quasi-systematiquement dans les laitiers d’acierie. De plus, la preservation de la matrice minerale du laitier siderurgique peut permettre sa valorisation dans divers domaines (agriculture, bâtiments et travaux publics, etc .) Une precedente etude realisee sur le meme site a demontre que l’emploi de Materiaux d’Interet Agronomiques Issus du Traitement des Eaux (MIATE) en tant qu’amendement favorise l’installation d’especes herbacees metallophytes capables de s’implanter naturellement sur ce type de substrat ( Bourchardon et al ., 2014). Une culture ex-situ realisee sur le materiau primaire a permis d’evaluer 1) les effets conjoints de l’apport de MIATE et de l’inoculation d’un champignon mycorhizien a arbuscules (AMF) sur la vegetation et sur sa capacite a limiter la diffusion des metaux dans la solution de sol et 2) la quantite optimale de MIATE a apporter en vue d’obtenir les meilleurs taux de mychorization (symbiose entre les plantes et l’AMF). La figure 1 montre que la modalite MIATE+MYC semble conduire a une diminution du pH et du chrome mobile des solutions de sol.