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Marginal lands have been proposed to produce non-food crop biomass for energy or green materials. For this purpose, the selection, implementation, and growth optimization of plant species on such lands are key elements to investigate to achieve relevant plant yields. Stinging nettle (Urtica dioica) is a herbaceous perennial that grows spontaneously on contaminated lands and was described as suitable to produce fibers for material applications. Two mercury-contaminated soils from industrial wastelands with different properties (grassland soil and sediment landfill) were used in this study to assess the potential growth of stinging nettle in a greenhouse mesocosm experiment. Two organic amendments were studied for their impact on nettle growth. The solid digestate from organic food wastes significantly doubled plant biomass whereas the compost from green wastes had a lower impact. The highest doses of organic amendments significantly increased the number of fibers, which doubled following digestate application, while reducing leaf Hg concentration. Both amendments significantly improved soil respiration and enzymatic activities linked to the microbial biomass in the soil from the sediment landfill by the end of the experiment. In the context of a phytomanagement scenario, solid digestate would be a preferred amendment resource to improve nettle production on industrial wastelands.

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The stinging nettle Urticadioica L. is a perennial crop with low fertilizer and pesticide requirements, well adapted to a wide range of environmental conditions. It has been successfully grown in most European climatic zones while also promoting local flora and fauna diversity. The cultivation of nettle could help meet the strong increase in demand for raw materials based on plant fibers as a substitute for artificial fibers in sectors as diverse as the textile and automotive industries. In the present review, we present a historical perspective of selection, harvest, and fiber processing features where the state of the art of nettle varietal selection is detailed. A synthesis of the general knowledge about its biology, adaptability, and genetics constituents, highlighting gaps in our current knowledge on interactions with other organisms, is provided. We further addressed cultivation and processing features, putting a special emphasis on harvesting systems and fiber extraction processes to improve fiber yield and quality. Various uses in industrial processes and notably for the restoration of marginal lands and avenues of future research on this high-value multi-use plant for the global fiber market are described.

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Phytoextraction using hyperaccumulating plants is a method for the remediation of soils contaminated with trace elements (TEs). As a strategy for improvement, the concept of fungal-assisted phytoextraction has emerged in the last decade. However, the role played by fungal endophytes of hyperaccumulating plants in phytoextraction is poorly studied. Here, fungal endophytes isolated from calamine or non-metalliferous populations of the Cd/Zn hyperaccumulator Noccaea caerulescens were tested for their growth promotion abilities affecting the host plant. Plants were inoculated with seven different isolates and grown for 2 months in trace element (TE)-contaminated soil. The outcomes of the interactions between N. caerulescens and its native strains ranged from neutral to beneficial. Among the strains, Alternaria thlaspis and Metapochonia rubescens, respectively, isolated from the roots of a non-metallicolous and a calamine population of N. caerulescens, respectively, exhibited the most promising abilities to enhance the Zn phytoextraction potential of N. caerulescens related to a significant increase of the plant biomass. These strains significantly increased the root elemental composition, particularly in the case of K, P, and S, suggesting an improvement of the plant nutrition. Results obtained in this study provide new insights into the relevance of microbial-assisted phytoextraction approaches in the case of hyperaccumulating plants.

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Stinging nettle (Urtica dioica L.) raises growing interest in phytomanagement because it commonly grows under poplar Short Rotation Coppices (SRC) set up at trace-metal (TM) contaminated sites and provides high-quality herbaceous fibres. The mycobiome of this non-mycorhizal plant and its capacity to adapt to TM-contaminated environments remains unknown. This study aimed at characterizing the mycobiome associated with nettle and poplar roots co-occurring at a TM-contaminated site. Plant root barcoding using the fungi-specific ITS1F-ITS2 primers and Illumina MiSeq technology revealed that nettle and poplar had distinct root fungal communities. The nettle mycobiome was dominated by Pezizomycetes from known endophytic taxa and from the supposedly saprotrophic genus Kotlabaea (which was the most abundant). Several ectomycorrhizal fungi such as Inocybe (Agaricomycetes) and Tuber (Pezizomycetes) species were associated with the poplar roots. Most of the Pezizomycetes taxa were present in the highly TM-contaminated area whereas Agaricomycetes tended to be reduced. Despite being a known non-mycorrhizal plant, nettle was associated with a significant proportion of ectomycorrhizal OTU (9.7%), suggesting some connexions between the poplar and the nettle root mycobiomes. Finally, our study raised the interest in reconsidering the fungal networking beyond known mycorrhizal interactions.

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Dark septate endophytes (DSEs) can improve plant stress tolerance by promoting growth and affecting element accumulation. Due to its ability to accumulate high Cd, Zn, and Ni concentrations in its shoots, Noccaea caerulescens is considered a promising candidate for phytoextraction in the field. However, the ability of DSEs to improve trace element (TE) phytoextraction with N. caerulescens has not yet been studied. The aim of this study was therefore to determine the ability of five DSE strains, previously isolated from poplar roots collected at different TE-contaminated sites, to improve plant development, mineral nutrient status, and metal accumulation by N. caerulescens during a pot experiment using two soils differing in their level of TE contamination. Microscopic observations revealed that the tested DSE strains effectively colonised the roots of N. caerulescens. In the highly contaminated (HC) soil, a threefold increase in root biomass was found in plants inoculated with the Leptodontidium sp. Pr30 strain compared to that in the non-inoculated condition; however, the plant nutrient status was not affected. In contrast, the two strains Phialophora mustea Pr27 and Leptodontidium sp. Me07 had positive effects on the mineral nutrient status of plants without significantly modifying their biomass. Compared to non-inoculated plants cultivated on HC soil, Pr27- and Pr30-inoculated plants extracted more Zn (+ 30%) and Cd (+ 90%), respectively. In conclusion, we demonstrated that the responses of N. caerulescens to DSE inoculation ranged from neutral to beneficial and we identified two strains (i.e. Leptodontidium sp. (Pr30) and Phialophora mustea (Pr27)) isolated from poplar that appeared promising as they increased the amounts of Zn and Cd extracted by improving plant growth and/or TE accumulation by N. caerulescens. These results generate interest in further characterising the DSEs that naturally colonise N. caerulescens and testing their ability to improve phytoextraction.

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Growing lignocellulosic crops on marginal lands could compose a substantial proportion of future energy resources. The potential of poplar was explored, by devising a field trial of two hectares in 2007 in a metal-contaminated site to quantify the genotypic variation in the growth traits of 14 poplar genotypes grown in short-rotation coppice and to assess element transfer and export by individual genotypes. Our data led us to conclusions about the genotypic variations in poplar growth on a moderately contaminated site, with the Vesten genotype being the most productive. This genotype also accumulated the least amounts of trace elements, whereas the Trichobel genotype accumulated up to 170 mg Zn kg-1 DW in the branches, with large variation being exhibited among the genotypes for trace element (TE) accumulation. Soil element depletion occurred for a range of TEs, whereas the soil content of major nutrients and the pH remained unchanged or slightly increased after 10 years of poplar growth. The higher TE content of bark tissues compared with the wood and the higher proportion of bark in branches compared with the wood led us to recommend that only stem wood be harvested, instead of the whole tree, which will enable a reduction in the risks encountered with TE-enriched biomass in the valorization process.

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As plants and associated insects are at the bottom of some terrestrial food webs, they are the primary contributors to mercury (Hg) fluxes in ecosystems. In addition to the trophic position of these organisms, factors related to their life traits have been hypothesized to influence their exposure to Hg. This study investigates the transfer of Hg in a soil-nettle-insect system and the insect-related factors affecting their Hg concentrations in a revegetated chlor-alkali landfill. Twenty-three insect species were identified and classified according to their life traits, their relationship with nettle, and their morphological characteristics. We observed low total mercury (THg) concentrations in nettles, with only 1% methylmercury (MeHg) being detected, while concentrations ranged from 5 to 3700 µg/kg dry wt. in insects with a MeHg percentage of up to 75%. The nettle-related insects were primarily exposed to Hg through the food web with significant biomagnification, particularly at the level of secondary predators. Within the nettle-unrelated group, the insect habitat was the most explanatory factor, with the highest enrichment being for the insects that spent part of their cycle in direct contact with Hg sources. Therefore, these insects require special attention because they are an essential vector of Hg transfer for terrestrial top predators.

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Chlorinated ethenes (CE) are among the most common volatile organic compounds (VOC) that contaminate groundwater, currently representing a major source of pollution worldwide. Phytoscreening has been developed and employed through different applications at numerous sites, where it was generally useful for detection of subsurface chlorinated solvents. We aimed at delineating subsurface CE contamination at a chlor-alkali facility using tree core data that we compared with soil data. For this investigation a total of 170 trees from experimental zones was sampled and analyzed for perchloroethene (PCE) and trichloroethene (TCE) concentrations, measured by solid phase microextraction gas chromatography coupled to mass spectrometry. Within the panel of tree genera sampled, Quercus and Ulmus appeared to be efficient biomonitors of subjacent TCE and PCE contamination, in addition to the well known and widely used Populus and Salix genera. Among the 28 trees located above the dense non-aqueous phase liquid (DNAPL) phase zone, 19 tree cores contained detectable amounts of CE, with concentrations ranging from 3 to 3000 µg L-1. Our tree core dataset was found to be well related to soil gas sampling results, although the tree coring data were more informative. Our data further emphasized the need for choosing the relevant tree species and sampling periods, as well as taking into consideration the nature of the soil and its heterogeneity. Overall, this low-invasive screening method appeared useful to delineate contaminants at a small-scale site impacted by multiple sources of chlorinated solvents.

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Although current Hg emissions from industrial activities may be accurately monitored, evidence of past releases to the atmosphere must rely on one or more environmental proxies. We used Hg concentrations in tree cores collected from poplars and willows to investigate the historical changes of Hg emissions from a dredged sediment landfill and compared them to a nearby control location. Our results demonstrated the potential value of using dendrochemistry to record historical Hg emissions from past industrial activities.

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