Search

Numerous studies in phytoremediation have tried to increase the uptake of metals by plants for soil cleanup. This work is about a new phytoremediation approach named Filtering Gardens using common wetland plants (Phragmites australis, Iris pseudacorus and Salix viminalis) in the vegetated cells irrigated so as to impose periodic flooding-drying conditions for enhancing the solubility of metals in the soil and their removal through leaching. In a pilot-scale sixteen-month experiment, this approach was applied for phytoremediation of Zn, Cu and Pb from an agricultural soil highly contaminated by sewage disposal. Metal mass balances for soil-plant systems showed that only a non-significant amount of metals was accumulated in plant tissues. An important amount of metals was removed from the soil via phytoleaching resulting from the interaction of plant roots with irrigation. An additional chemical treatment with citrate can enhance metal leaching. Mechanisms of Zn and Cu transformations involved in this phytoremediation were highlighted by combining synchrotron-based X-ray analytical tools at micro- (µXRF, µXRD, µEXAFS) and macro-scale (EXAFS) coupled with chemical analyses, allowing to determine the nature and amount of metal forms in the soil. In the initial soil, zinc was occurred mainly as secondary minerals (Zn-ferrihydrite, Zn-phosphate and Zn-kerolite-like-phyllosilicate) and copper was essentially bound to organic matter. The activity of plant roots under flooding-drying conditions clearly modified the original speciation of metals. In the phytoremediated soil, Zn-ferrihydrite, one of the initially major forms of Zn, was entirely dissolved. The reductive dissolution of this iron oxyhydroxide favored by flooding conditions was the major process inducing Zn leaching. One part of solubilized Zn coprecipitated with Fe into another less soluble iron oxyhydroxide, Zn-substituted goethite, under oxidizing conditions and with assistance by plant roots likely in defense against toxic dissolved metals, as evidenced by the formation of goethite plaques on and near roots. Moreover, the newly occurred particles of metallic Zn and ZnO were discovered in the rhizosphere, in small amount. The oxidation of organic matter likely enhanced by root oxygen release caused an excess of toxic cationic Cu. In response to oxidative stress, this Cu was biotransformed into metallic Cu nanoparticles, in important amount, by plant roots with evidence of assistance by endomycorrhizal fungi. This newly identified mode of metal biomineralization by plant roots may be typical of common wetland plants. This new way of making phytoremedation involves mainly phytoleaching inducing the solubilization of metals in the soil and their leaching and phytotransformation, due in part to phytodetoxication, driving the conversion of toxic metals into weakly soluble forms., De nombreuses études en phytoremédiation visent à accroître le prélèvement des métaux par les plantes pour dépolluer les sols. Ce travail porte sur une nouvelle approche de phytoremédiation appelée Jardins Filtrants qui consiste à traiter le sol dans des casiers végétalisés par des plantes de milieux humides (Phragmites australis, Iris pseudacorus et Salix viminalis) et irrigués de manière à imposer une alternance des conditions hydromorphie-assèchement afin d'accroître la solubilité de métaux dans le sol et de les extraire par lixiviation. Dans une expérience pilote de seize mois, cette approche a été appliquée pour la phytoremédiation de Zn, Cu et Pb d'un sol agricole fortement pollué par l'épandage d'eaux usées. Le bilan de masse des métaux dans les systèmes sol-plante a montré que seule une quantité non-significative des métaux a été accumulée dans la biomasse des plantes. Une quantité importante des métaux a été éliminée du sol via la phytolixiviation résultant de l'interaction de l'activité racinaire avec l'irrigation. Un traitement chimique complémentaire au citrate permet d'augmenter la lixiviation. Les mécanismes de transformations de Zn et Cu impliqués dans cette phytoremédiation ont été mis en évidence par combinaison des techniques analytiques sur la source synchrotron à micro- (µXRF, µXRD, µEXAFS) et macro-échelle (EXAFS) couplée aux analyses chimiques, permettant d'identifier et quantifier les formes des métaux dans le sol. Dans le sol initial, le zinc a été majoritairement sous formes de minéraux secondaires (Zn-ferrihydrite, Zn-phosphate et Zn-phyllosilicate modélisé par Zn-kérolite) et le cuivre a été associé essentiellement à la matière organique. L'activité racinaire dans les conditions hydromorphie-assèchement a profondément modifié la spéciation des métaux. Zn-ferrihydrite, une des formes majoritaires de Zn, a été complètement dissoute. La dissolution réductive de cet oxyhydroxyde de fer, favorisée par les conditions d'hydromorphie, a induit la lixiviation de Zn. Une partie de Zn solubilisé a coprécipité avec Fe en un autre oxyhydroxyde de fer zincifère moins soluble, Zn-goethite substituée, dans les conditions oxydantes et avec assistance des racines formant des plaques de goethite en défense contre l'excès de métaux dissous. De plus, les nouvelles particules de Zn métallique et ZnO ont été découvertes dans la rhizosphère, en faible quantité. L'oxydation de la matière organique a induit l'excès de Cu cationique toxique. En réponse au stress oxydant, ce cuivre a été biotransformé par les racines en association avec des mycorhizes en nanoparticules de Cu métallique, en quantité importante. Ce nouveau mode de biominéralisation peut être typique des plantes de milieux humides. Cette nouvelle voie de phytoremédiation implique principalement la phytolixiviation induisant la solubilisation des métaux et leur lixiviation et la phytotransformation, due pour une part à la phytodétoxication, conduisant la conversion des métaux toxiques en formes peu solubles.

International audience; Copper is an essential element in the cellular electron-transport chain, but as a free ion it can catalyze production of damaging radicals. Thus, all life forms attempt to prevent copper toxicity. Plants diminish excess copper in two structural regions: rare hyperaccumulators bind cationic copper to organic ligands in subaerial tissues, whereas widespread metal-tolerant plants segregate copper dominantly in roots by mechanisms thought to be analogous. Here we show using synchrotron microanalyses that common wetlands plants Phragmites australis and Iris pseudoacorus can transform copper into metallic nanoparticles in and near roots with evidence of assistance by endomycorrhizal fungi when grown in contaminated soil in the natural environment. Biomolecular responses to oxidative stress, similar to reactions used to abiotically synthesize Cu0 nanostructures of controlled size and shape, likely cause the transformation. This newly identified mode of copper biomineralization by plant roots under copper stress may be common in oxygenated environments.

International audience; Synchrotron-based X-ray radiation microfluorescence (μ-SXRF) and micro-focused and powder extended X-ray absorption fine structure (EXAFS) spectroscopy measurements, combined with desorption experiments and thermodynamic calculations, were used to evaluate the solubility of metal contaminants (Zn, Cu, Pb) and determine the nature and fractional amount of Zn species in a near-neutral pH (6.5–7.0) truck-farming soil contaminated by sewage irrigation for one hundred years. Zn is the most abundant metal contaminant in the soil (1103 mg/kg), followed by Pb (535 mg/kg) and Cu (290 mg/kg). The extractability of Zn, Pb, and Cu with citrate, S,S-ethylenediaminedisuccinic acid (EDDS), and ethylenediaminetetraacetic acid (EDTA) was measured as a function of time (24 h, 72 h, 144 h), and also as a function of the number of applications of the chelant (5 applications each with 24 h of contact time). Fifty-three percent of the Zn was extracted after 144 h with citrate, 51% with EDDS and 46% with EDTA, compared to 69, 87, and 61% for Cu, and 24, 40, and 34% for Pb. Renewing the extracting solution removed more of the metals. Seventy-nine, 65, and 57% of the Zn was removed after five cycles with citrate, EDDS and EDTA, respectively, compared to 88, 100, and 72% for Cu, and 91, 65, and 47% for Pb. Application to the untreated soil of μ-SXRF, laterally resolved μ-EXAFS combined with principal component analysis, and bulk averaging powder EXAFS with linear least-squares combination fit of the data, identified five Zn species: Zn-sorbed ferrihydrite, Zn phosphate, Zn-containing trioctahedral phyllosilicate (modeled by the Zn kerolite, Si4(Mg1.65Zn1.35)O10(OH)2 · nH2O), willemite (Zn2SiO4), and gahnite (ZnAl2O4), in proportions of not, vert, similar30, 28, 24, 11, and less than 10%, respectively (precision: 10% of total Zn). In contrast to Cu and Pb, the same fractional amount of Zn was extracted after 24 h contact time with the three chelants (40–43% of the initial content), suggesting that one of the three predominant Zn species was highly soluble under the extraction conditions. Comparison of EXAFS data before and after chemical treatment revealed that the Zn phosphate component was entirely and selectively dissolved in the first 24 h of contact time. Preferential dissolution of the Zn phosphate component is supported by thermodynamic calculations. Despite the long-term contamination of this soil, about 79% of Zn, 91% of Pb, and 100% of Cu can be solubilized in the laboratory on a time scale of a few days by chemical complexants. According to metal speciation results and thermodynamic calculations, the lower extraction level measured for Zn is due to the Zn phyllosilicate component, which is less soluble than Zn phosphate and Zn ferrihydrite.

International audience; Miscanthus is a C4 perennial grass originating from East Asia, the yields of which progressively increase in the first years of growth. Several species for bioenergy have been studied since the mid-1980s in Europe, in particular (Miscanthus × giganteus [M. × giganteus]), due to its high yields. M. × giganteus is mainly cultivated in France and established from rhizomes. Our study aimed to assess, in field conditions , alternative establishment methods combined with an alternative species, Miscanthus sinensis (M. sinensis). We set up a multi-environment experimental network. On each trial, we tested two treatments with M. × giganteus, established from rhizomes (G_r-sd) and from plantlets obtained from rhizomes (G_p-sd), and two treatments with M. sinensis seedlings transplanted in single (S_p-sd) and double density (S_p-dd). ANOVA was performed to compare establishment and regrowth rates across treatments, as well as yields across treatments and site-years. A logistic model was used to describe yield trends and to compare the maximum yield reached and the rate of yield increase of both species. Results showed that miscanthus establishment from plantlets resulted in higher establishment (between 87% and 92%) and regrowth (between 91% and 94%) rates compared to establishment from rhizomes. Treatments with M. × giganteus obtained higher average yields across site-years than those with M. sinensis, but more variable yields across site-years. We showed a strong species effect on yields, yield components (shoot weight, shoot density and shoot number per plant) and light interception (through leaf area index). Lastly, to use M. sinensis established from transplanted plantlets as an alternative to M. × giganteus, research would be required on the breeding of M. sinensis sterile seeds to avoid risks of invasiveness. K E Y W O R D S biomass, Miscanthus, multi-environment trial, rhizomes, transplanted plantlets, yield trend modelling

Miscanthus is a perennial C4 crop whose lignocellulose can be used as an alternative to the production of biosourced material. Miscanthus x giganteus (M. x giganteus) has demonstrated high maximum yields but also high yield variability across farmers’ fields. Miscanthus sinensis (M. sinensis) can be an alternative to M. x giganteus because it is considered to be more tolerant to water stress and to produce more stable yields. This study aimed to identify the main factors explaining the variability of yields across site-years for M. x giganteus and M. sinensis. A multi-local and multi-year trial network was set up in France (Ile de France and Center regions). Four treatments were established on seven sites, from spring 2013 to winter 2019: at each site, two treatments of M. x giganteus (a treatment from rhizome and a treatment from rhizome-derived plantlets) and two treatments of M. sinensis (a treatment from seed-derived plantlets established in single density and a treatment from seed-derived plantlets established in double density). We experienced 5 years of harvest because miscanthus was not harvested in 2014. First, we characterized yield variations across site-years for both genotypes. Second, we defined and calculated a set of indicators (e.g., water stress indicator, sum of degree-days of the previous year, number of frost days) that could affect miscanthus yields. Finally, we performed a mixed model with re-sampling to identify the main indicators that explained yield variability for each genotype specifically. Results showed that water stress and crop age mainly explained yield variability for both genotypes. M. sinensis yields were also affected by the sum of degree-days of the previous year of growth. Hence, genotype choice must take into account environmental characteristics. M. sinensis could indeed achieve higher and more stable yields than those of M. x giganteus in shallow sandy soils or in locations with a higher risk of low rainfall.

To achieve the EU’s targets for reducing energy production from fossil fuels, the use of energy crops, such as Miscanthus × giganteus, is increasing resulting in a corresponding increase in waste ash from incineration. The chemical properties of Miscanthus ash (e.g. phosphorus and potassium content) may allow this waste material (currently landfilled) to be used as a fertiliser, but no information exists on the effect of the ash on the biological properties of soil. The main aim of this study was to determine the potential impact of Miscanthus ash on earthworms by assessing the effect on survival, change in biomass, reproduction and avoidance behaviour of the geophagous, soil dwelling earthworm, Aporrectodea caliginosa. Tests utilised a range of Miscanthus ash doses from 0 to 50 t ha−1 (0, 1, 2.5, 5, 10, 25, 50). Results showed that Miscanthus ash had no significant impact on A. caliginosa survival, biomass and reproduction, but negative trends were observed for biomass from 2.5 t ha−1 and for reproduction from 10 t ha−1. In contrast, a significant avoidance response was observed in the 25 and 50 t ha−1 treatment and according to ISO guideline 17512 there is a negative impact of the Miscanthus ash on soil habitat function at 25 t ha−1 and above as more than 80% of earthworms were in the control soil. It is suggested that this negative effect on soil habitat function could be attributed to a range of factors including the presence of heavy metals in the ash and a change in substrate pH, texture and/or osmotic stress. Further laboratory-based studies conducted over extended time periods with a more refined range of ash doses and associated field-based studies are required to validate the results and determine a more precise assessment of the threshold ash value inducing a loss of soil habitat function.

International audience; Miscanthus is a C4 bioenergy perennial crop characterized by its high potential yield. Our study aimed to compare the carbon storage capacities of Miscanthus sinensis (M. sinensis) with that of Miscanthus × giganteus (M. × giganteus) in field conditions in different types of soils in France. We set up a multi‐environment experimental network. On each trial, we tested two treatments: M. × giganteus established from rhizomes (Gr) and M. sinensis transplanted seedlings (Sp). We quantified the soil organic carbon (SOC) stock at equivalent soil mass for both genotypes in 2014 and 2019 and for two sampling depths: L1 (ca. 0–5 cm) and L1‐2 (ca. 0–30 cm). We also calculated the total and annual variation of the SOC stock and investigated factors that could explain the variation and the initial state of the SOC stock. ANOVAs were performed to compare the SOC stock, as well as the SOC stock variation rates across treatments and soil layers. Results showed that the soil bulk density did not vary significantly between 2014 and 2019 for both treatments (Gr and Sp). The SOC concentration (i.e. SOC expressed in g/kg) increased significantly between 2014 and 2019 in L1, whereas no significant evolution was found in L2 (ca. 5–30 cm). The SOC stock (i.e. SOC expressed in t/ha) increased significantly in the superficial layer L1 for M. × giganteus and M. sinensis, by 0.48 ± 0.41 and 0.54 ± 0.25 t ha−1 year−1 on average, respectively, although no significant change was detected in the layer L1‐2 for both genotypes. Moreover, SOC stocks in 2019 did not differ significantly between M. × giganteus and M. sinensis in the soil layers L1 and L1‐2. Lastly, our results showed that the initial SOC stock was significantly higher when miscanthus was grown after set‐aside than after annual crops.

In the context of increasing biomass cultivation for energy production in Europe, the objective of this study was to carry out a multi-parameter assessment of soil quality under Miscanthus x giganteus crop at marginal sites. Chemical (pH, organic carbon, total nitrogen, C:N, metal trace elements), biological (microbial biomass, earthworm communities) and physical (aggregate stability, bulk density) soil properties were evaluated 5 years after planting Miscanthus on a polluted (CH) and unpolluted (MG) marginal sites and compared with adjacent undisturbed meadow areas and Miscanthus crop cultivated on an arable field (BF). The effect of Miscanthus on soil quality was site dependent and related to soil properties (texture), metal trace element (MTE) contamination and previous land use. At MG, where Miscanthus was cultivated on a previously undisturbed meadow, results suggested a negative effect of Miscanthus on soil biological properties with a lower earthworm abundance and biomass, and a worst functional and species structure in the Miscanthus than in the undisturbed meadow, despite a good yield. Results under Miscanthus at BF were similar in terms of soil quality to Miscanthus at MG and higher than under Miscanthus at the polluted site (CH). At CH, results suggested that Miscanthus may reduce the mobility of MTE, however further longer term studies, at a range of sites are needed to conclude about the impact of Miscanthus on soil quality at contaminated and/or uncontaminated sites.