The Uptake of Rare Trace Elements by Perennial Ryegrass (Lolium perenne L.)

Technological development has increased the use of chemical elements that have hitherto received scant scientific attention as environmental contaminants. Successful management of these rare trace elements (RTEs) requires elucidation of their mobility in the soil-plant system. We aimed to determine the capacity of Lolium perenne (a common pasture species) to tolerate and accumulate the RTEs Be, Ga, In, La, Ce, Nd, and Gd in a fluvial recent soil. Cadmium was used as a reference as a well-studied contaminant that is relatively mobile in the soil-plant system. Soil was spiked with 2.5-283 mg kg-1 of RTE or Cd salts, representing five, 10, 20, and 40 times their background concentrations in soil. For Be, Ce, In, and La, there was no growth reduction, even at the highest soil concentrations (76, 1132, 10.2, and 874 mg kg-1, respectively), which resulted in foliar concentrations of 7.1, 12, 0.11, and 50 mg kg-1, respectively. The maximum no-biomass reduction foliar concentrations for Cd, Gd, Nd, and Ga were 0.061, 0.1, 7.1, and 11 mg kg-1, respectively. Bioaccumulation coefficients ranged from 0.0030-0.95, and increased Ce < In < Nd ≅ Gd < La ≅ Be ≅ Ga < Cd. Beryllium and La were the RTEs most at risk of entering the food chain via L. perenne, as their toxicity thresholds were not reached in the ranges tested, and the bioaccumulation coefficient (plant/soil concentration quotient) trends indicated that uptake would continue to increase at higher soil concentrations. In contrast, In and Ce were the elements least likely to enter the food chain. Further research should repeat the experiments in different soil types or with different plant species to test the robustness of the findings.

Phytomanagement of trace element polluted fields with aromatic plants: supporting circular bio-economies

Trace elements pollution of soils became a global concern because of their persistence in the environment which can lead to accumulation in food chains up to toxic levels. At the same time, there is a shortage of arable land for growing food, fodder and industrial crops, which highlights the need for remediation/use of polluted land. Restoration of degraded lands has been included as a vital component of UN Sustainable Development Goals (SDGs). We summarize various sources of entry of important trace elements in the environment, available biological reclamation and management strategies and their limitations. Recent advances in phytomanagement approaches using aromatic crops to obtain economically valuable products such as essential oils and revalorize such polluted areas are reviewed. The worldwide application of this strategy in the last 10 years is illustrated through a choropleth map. Finally, the emerging concept of phytomanagement as a restorative and regenerative circular bio-economy is also discussed.

Drivers to improve metal(loid) phytoextraction with a focus on microbial degradation of dissolved organic matter in soils

Bioaugmentation of soils can increase the mobilization of metal(loid)s from the soil-bearing phases. However, once desorbed, these metal(loid)s are mostly complexed to the dissolved organic matter (DOM) in the soil solution, which can restrict their availability to plants (roots mainly taking up the free forms) and then the phytoextraction performances. Firstly the main drivers influencing phytoextraction are reminded, then the review focuses on the DOM role. After having reminding the origin, the chemical structure and the lability of DOM, the pool of stable DOM (the most abundant in the soil) most involved in the complexation of metal(loid)s is addressed in particular by focusing on carboxylic and/or phenolic groups and factors controlling metal(loid) complexation with DOM. Finally, this review addresses the ability of microorganisms to degrade metal(loid)-DOM complexes as an additional lever for increasing the pool of free metal(loid) ions, and then phytoextraction performances, and details the origin of microorganisms and how they are selected. The development of innovative processes including the use of these DOM-degrading microorganisms is proposed in perspectives.

Coriander (Coriandrum sativum) Cultivation Combined with Arbuscular Mycorrhizal Fungi Inoculation and Steel Slag Application Influences Trace Elements-Polluted Soil Bacterial Functioning

The cultivation of aromatic plants for the extraction of essential oils has been presented as an innovative and economically viable alternative for the remediation of areas polluted with trace elements (TE). Therefore, this study focuses on the contribution of the cultivation of coriander and the use of arbuscular mycorrhizal fungi (AMF) in combination with mineral amendments (steel slag) on the bacterial function of the rhizosphere, an aspect that is currently poorly understood and studied. The introduction of soil amendments, such as steel slag or mycorrhizal inoculum, had no significant effect on coriander growth. However, steel slag changed the structure of the bacterial community in the rhizosphere without affecting microbial function. In fact, Actinobacteria were significantly less abundant under slag-amended conditions, while the relative proportion of Gemmatimonadota increased. On the other hand, the planting of coriander affects the bacterial community structure and significantly increased the bacterial functional richness of the amended soil. Overall, these results show that planting coriander most affected the structure and functioning of bacterial communities in the TE-polluted soils and reversed the effects of mineral amendments on rhizosphere bacterial communities and their activities. This study highlights the potential of coriander, especially in combination with steel slag, for phytomanagement of TE-polluted soils, by improving soil quality and health.

Trace Element Uptake by Willows Used for the Phytoremediation of Biosolids

The land application of biosolids can result in the unacceptable accumulation of Trace Elements (TEs) in agricultural soil and potentially introduce xenobiotics and pathogens into the food chain. Phytoremediation of biosolids aims to minimize this risk, while producing valuable biomass. Willows, well known to accumulate zinc (Zn), are used extensively in farming systems for soil conservation, shelter and as feed supplements with demonstrable health benefits. Potentially, biosolids phytoremediation could occur on marginal lands adjacent to farmlands where willows are grown for supplementary fodder. We aimed to determine the uptake and distribution of Zn and other TEs in willows grown on soils amended with biosolids and biosolids blended with biochar, with a view to their use as stock fodder. In the Canterbury Region, New Zealand, we grew Salix 'tangaio' (S. matsudana X S. alba) in a greenhouse trial and field study. The biomass production of the willows was unaffected by biosolids and increased by the biosolids+biochar mixture. The addition of 4% biosolids (w/w) to the soil resulted in a foliar Zn concentration of 600-1000 mg kg^-1, some 25 times higher than the average New Zealand pasture. Zinc concentrations were highest in the bottom leaves and increased throughout the season. Biosolids addition doubled the copper (Cu) concentration to 10 mg kg^-1. Adding biochar to the system reduced the plant uptake of Cu and to a lesser extent Zn, while cadmium (Cd) uptake was unaffected. For Cd, Cu, and Zn, plant uptake was a function of the Ca(NO₃)₂-extractable concentration, both in greenhouse experiments and the field trial. Future work should determine the changes in plant TE uptake over several growing seasons.

Willow and Herbaceous Species' Phytoremediation Potential in Zn-Contaminated Farm Field Soil in Eastern Québec, Canada: A Greenhouse Feasibility Study

Phytoremediation shows great promise as a plant-based alternative to conventional clean-up methods that are prohibitively expensive. As part of an integrated strategy, the selection of well-adapted plant species as well as planting and management techniques could determine the success of a long-term program. Herein, we conducted an experiment under semi-controlled conditions to screen different plants species with respect to their ability to phytoremediate Zn-contaminated soil excavated from a contaminated site following a train derailment and spillage. The effect of nitrilotriacetic acid (NTA) application on the plants and soil was also comprehensively evaluated, albeit we did not find its use relevant for field application. In less than 100 days, substantial Zn removal occurred in the soil zone proximal to the roots of all the tested plant species. Three perennial herbaceous species were tested, namely, Festuca arundinacea, Medicago sativa, and a commercial mix purposely designed for revegetation; they all showed strong capacity for phytostabilization at the root level but not for phytoextraction. The Zn content in the aboveground biomass of willows was much higher. Furthermore, the degree of growth, physiological measurements, and the Zn extraction yield indicated Salix purpurea ‘Fish Creek’ could perform better than Salix miyabeana, ‘SX67’, in situ. Therefore, we suggest implementing an S. purpurea—perennial herbaceous co-cropping strategy at this decade-long-abandoned contaminated site or at similar disrupted landscapes.

Phytomanagement of a Lead-Polluted Shooting Range Using an Aromatic Plant Species and Its Effects on the Rhizosphere Bacterial Diversity and Essential Oil Production

This field study aimed to assess the baseline conditions of a long-term shooting range in Argentina polluted with 428 mg kg^-1 lead (Pb) to evaluate the establishment and development of Helianthus petiolaris plants and address the efficacy of the phytomanagement strategy through: (i) element accumulation in plant tissues; (ii) rhizosphere bacterial diversity changes by Illumina Miseq™, and (iii) floral water and essential oil yield, composition, and element concentration by GC-MS and ICP. After one life cycle growing in the polluted sites, in the roots of Helianthus petiolaris plants, Pb concentration was between 195 and 304 mg kg^-1 Pb. Only a limited fraction of the Pb was translocated to the aerial parts. The predominance of the genus Serratia in the rhizosphere of Helianthus petiolaris plants cultivated in the polluted sites and the decrease in the essential oil yield were some effects significantly associated with soil Pb concentration. No detectable Pb concentration was found in the floral water and essential oil obtained. Extractable Pb concentration in the soil reduced between 28% and 45% after the harvest.

Mycorrhizal inoculation effects on growth and the mycobiome of poplar on two phytomanaged sites after 7-year-short rotation coppicing

AIMS

Afforestation of trace-element contaminated soils, notably with fast growing trees, has been demonstrated to be an attractive option for bioremediation due to the lower costs and dispersion of contaminants than conventional cleanup methods. Mycorrhizal fungi form symbiotic associations with plants, contributing to their tolerance towards toxic elements and actively participating to the biorestoration processes. The aim of this study was to deepen our understanding on the effects of mycorrhizal inoculation on plant development and fungal community at two trace-element contaminated sites (Pierrelaye and Fresnes-sur-Escaut, France) planted with poplar (Populus trichocarpa x Populus maximowiczii).

METHODS

The 2 sites were divided into 4 replicated field blocks with a final plant density of 2200 tree h^-1. Half of the trees were inoculated with a commercial inoculum made of a mix of mycorrhizal species. The sites presented different physico-chemical characteristics (e.g., texture: sandy soil versus silty-loam soil and organic matter: 5.7% versus 3.4% for Pierrelaye and Fresnes-sur-Escaut, respectively) and various trace element contamination levels.

RESULTS

After 7 years of plantation, inoculation showed a significant positive effect on poplar biomass production at the two sites. Fungal composition study demonstrated a predominance of the phylum Ascomycota at both sites, with a dominance of Geopora Arenicola and Mortierella elongata, and a higher proportion of ectomycorrhizal and endophytic fungi (with the highest values observed in Fresnes-sur-Escaut: 45% and 28% for ECM and endophytic fungi, respectively), well known for their capacity to have positive effects on plant development in stressful conditions. Furthermore, Pierrelaye site showed higher frequency (%) of mycorrhizal tips for ectomycorrhizal fungi (ECM) and higher intensity (%) of mycorrhizal root cortex colonization for arbuscular mycorrhizal fungi (AMF) than Fresnes-sur-Escaut site, which translates in a higher level of diversity.

CONCLUSIONS

Finally, this study demonstrated that this biofertilization approach could be recommended as an appropriate phytomanagement strategy, due to its capacity to significantly improve poplar productivity without any perturbations in soil mycobiomes.

Nettle, a Long-Known Fiber Plant with New Perspectives

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.

Chemical Composition, Antioxidant and Anti-Inflammatory Activities of Clary Sage and Coriander Essential Oils Produced on Polluted and Amended Soils-Phytomanagement Approach

The potential of essential oils (EO), distilled from two aromatic plants—clary sage (Salvia sclarea L.) and coriander (Coriandrum sativum L.)—in view of applications as natural therapeutic agents was evaluated in vitro. These two were cultivated on a trace element (TE)-polluted soil, as part of a phytomanagement approach, with the addition of a mycorrhizal inoculant, evaluated for its contribution regarding plant establishment, growth, and biomass production. The evaluation of EO as an antioxidant and anti-inflammatory, with considerations regarding the potential influence of the TE-pollution and of the mycorrhizal inoculation on the EO chemical compositions, were the key focuses. Besides, to overcome EO bioavailability and target accession issues, the encapsulation of EO in β-cyclodextrin (β-CD) was also assessed. Firstly, clary sage EO was characterized by high proportions of linalyl acetate (51–63%) and linalool (10–17%), coriander seeds EO by a high proportion of linalool (75–83%) and lesser relative amounts of γ-terpinene (6–9%) and α-pinene (3–5%) and coriander aerial parts EO by 2-decenal (38–51%) and linalool (22–39%). EO chemical compositions were unaffected by both soil pollution and mycorrhizal inoculation. Of the three tested EO, the one from aerial parts of coriander displayed the most significant biological effects, especially regarding anti-inflammatory potential. Furthermore, all tested EO exerted promising antioxidant effects (IC₅₀ values ranging from 9 to 38 g L⁻¹). However, EO encapsulation in β-CD did not show a significant improvement of EO biological properties in these experimental conditions. These findings suggest that marginal lands polluted by TE could be used for the production of EO displaying faithful chemical compositions and valuable biological activities, with a non-food perspective.

The Aromatic Plant Clary Sage Shaped Bacterial Communities in the Roots and in the Trace Element-Contaminated Soil More Than Mycorrhizal Inoculation - A Two-Year Monitoring Field Trial

To cope with soil contamination by trace elements (TE), phytomanagement has attracted much attention as being an eco-friendly and cost-effective green approach. In this context, aromatic plants could represent a good option not only to immobilize TE, but also to use their biomass to extract essential oils, resulting in high added-value products suitable for non-food valorization. However, the influence of aromatic plants cultivation on the bacterial community structure and functioning in the rhizosphere microbiota remains unknown. Thus, the present study aims at determining in TE-aged contaminated soil (Pb - 394 ppm, Zn - 443 ppm, and Cd - 7ppm, respectively, 11, 6, and 17 times higher than the ordinary amounts in regional agricultural soils) the effects of perennial clary sage (Salvia sclarea L.) cultivation, during two successive years of growth and inoculated with arbuscular mycorrhizal fungi, on rhizosphere bacterial diversity and community structure. Illumina MiSeq amplicon sequencing targeting bacterial 16S rRNA gene was used to assess bacterial diversity and community structure changes. Bioinformatic analysis of sequencing datasets resulted in 4691 and 2728 bacterial Amplicon Sequence Variants (ASVs) in soil and root biotopes, respectively. Our findings have shown that the cultivation of clary sage displayed a significant year-to-year effect, on both bacterial richness and community structures. We found that the abundance of plant-growth promoting rhizobacteria significantly increased in roots during the second growing season. However, we didn't observe any significant effect of mycorrhizal inoculation neither on bacterial diversity nor on community structure. Our study brings new evidence in TE-contaminated areas of the effect of a vegetation cover with clary sage cultivation on the microbial soil functioning.

Phytomanagement Reduces Metal Availability and Microbial Metal Resistance in a Metal Contaminated Soil

Short rotation coppice (SRC) with metal tolerant plants may attenuate the pollution of excessive elements with potential toxicity in soils, while preserving soil resources and functionality. Here, we investigated effects of 6 years phytomanagement with willow SRC on properties including heavy metal levels, toxicity tested by BioTox, microbial biomass, enzyme activities, and functional gene abundances measured by GeoChip of soils contaminated by As, Cd, Pb and Zn, as compared to the same soils under non-managed mixed grassland representing no intervention treatment (Unt). Though metal total concentrations did not differ by SRC and Unt, SRC soils had lower metal availability and toxicity, higher organic carbon, microbial biomass, phosphatase, urease and protease activities, as compared to Unt soils. Significantly reduced abundances of genes encoding resistances to various metals and antibiotics were observed in SRC, likely attributed to reduced metal selective pressure based on less heavy metal availability and soil toxicity. SRC also significantly reduced abundances of genes involved in nitrogen, phosphorus, and sulfur cycles, possibly due to the willow induced selection. Overall, while the SRC phytomanagement did not reduce the total heavy metal concentrations in soils, it decreased the heavy metal availability and soil toxicity, which in turn led to less metal selective pressure on microbial communities. The SRC phytomanagement also reduced the abundances of nutrient cycling genes from microbial communities, possibly due to intense plant nutrient uptake that depleted soil nitrogen and phosphorus availability, and thus site-specific practices should be considered to improve the soil nutrient supply for phytomanagement plants.

Phytostabilization of Pb and Cd polluted soils using Helianthus petiolaris as pioneer aromatic plant species

The area of soils polluted with heavy metals is increasing due to industrialization and globalization. Aromatic plant species can be a suitable alternative way for agricultural valorization and phytomanagement of such soils by the commercialization of essential oils avoiding risks for the food chain. The potential of growing Helianthus petiolaris in heavy metal polluted soils was assessed in pot experiments using spiked soils and soils from a shooting range. In terms of phytostabilization, H. petiolaris could grow in soils containing 1000 mg/kg Pb²⁺, 50 mg/kg Cd²⁺, accumulating more than three times the soil Cd content in the aerial parts and translocating significant amounts of Pb to the aerial parts when growing in soils polluted with up to 500 mg/kg Pb. When phytostabilization is considered, phytotoxicity of heavy metals strongly depends on the rhizospheric microbial communities, either by mitigating trace element phytotoxicity or promoting plant growth via phytohormone production. So, the effects of heavy metals on the diversity of the rhizospheric bacterial community were assessed using DNA-fingerprinting.

Trace element transfer from two contaminated soil series to Medicago sativa and one of its herbivores, Spodoptera exigua

Alfalfa was cultivated in two potted soil series obtained from two sandy soils contaminated by Cu (SM) and metal(loids)/PAH (CD). Shoot production was monitored for 8 weeks. Then, larvae of Spodoptera exigua were reared on alfalfa of both soil series for eight days. A biotest (using Phaseolus vulgaris) was used to assess the soil phytotoxicity. Increasing soil contamination reduced P. vulgaris growth, but alfalfa growth was only reduced on the SM soil series. Exposure to the SM soil was mirrored by shoot Cu and Cr concentrations of alfalfa (respectively, in mg kg ^-1 DW, Cu and Cr ranged from 11.9 and 0.4 in the CTRL soil to 98.5 and 1.2 in the SM one). Exposure to the CD soil series was mirrored by shoot Zn concentrations (i.e., 48-91.6 mg kg^-1 DW). Internal metal(loid) concentrations of S. exigua remained generally steady across both soil series (respectively Cd 0.05-0.16, Cr 0.5-3.3, Cu 5.8-98.5, Ni 0.6-1.6, Pb 0.4-1.3, and Zn 57-337 mg kg^-1 DW), and most of the associated transfer factors were lower than 1. Here, due to the excluder phenotype of alfalfa across our TE contamination gradients, S. exigua could cope with high total metal(loid) concentration in both contaminated soils.

Differential growth promotion of poplar and birch inoculated with three dark septate endophytes in two trace element-contaminated soils

Dark septate endophytes (DSEs) are abundant in stressful environments, including trace element (TE)-enriched soils. However, knowledge about the effects of DSEs on plant growth in such soils is poor compared to the well-known mycorrhizal fungi. The aim of this work was to evaluate the effects of three DSE strains isolated from TE-contaminated soils on the growth and mineral nutrition of Betula pendula and Populus tremula x alba grown on two contrasting TE-polluted soils. The three DSEs evenly colonized the two plant species in both soils. Nevertheless, plant responses to DSE inoculation varied from neutral to beneficial depending on soil properties. Depending on fungal strain and plant species, different factors seemed to contribute to plant growth promotion. Phialophora mustea Pr27 and Leptodontidium Pr30 decreased lipid peroxidation in birch shoots. Chlorophyll, K, and P concentrations increased in the shoots of Leptodontidium Pr30-inoculated trees, whereas Cd concentration decreased in Cadophora Fe06-inoculated birch. The absence of a general DSE-mediated plant growth-promoting behavior could represent a limiting factor for a generic use of DSEs in the tree-based phytomanagement of TE-contaminated soils. Our results suggest that the selection of strains adapted to particular edaphic conditions should not be overlooked within the framework of phytomanagement.

Greenhouse gas emissions from a Cu-contaminated soil remediated by in situ stabilization and phytomanaged by a mixed stand of poplar, willows, and false indigo-bush

Phytomanagement of trace element-contaminated soils can reduce soil toxicity and restore soil ecological functions, including the soil gas exchange with the atmosphere. We studied the emission rate of the greenhouse gases (GHGs) CO₂, CH₄, and N₂O; the potential CH₄ oxidation; denitrification enzyme activity (DEA), and glucose mineralization of a Cu-contaminated soil amended with dolomitic limestone and compost, alone or in combination, after a 2-year phytomanagement with a mixed stand of Populus nigra, Salix viminalis, S. caprea, and Amorpha fruticosa. Soil microbial biomass and microbial community composition after analysis of the phospholipid fatty acids (PLFA) profile were determined. Phytomanagement significantly reduced Cu availability and soil toxicity, increased soil microbial biomass and glucose mineralization capacity, changed the composition of soil microbial communities, and increased the CO₂ and N₂O emission rates and DEA. Despite such increases, microbial communities were evolving toward less GHG emission per unit of microbial biomass than in untreated soils. Overall, the aided phytostabilization option would allow methanotrophic populations to establish in the remediated soils due to decreased soil toxicity and increased nutrient availability.

ECO-physiological response of S. vulgaris to CR(VI): Influence of concentration and genotype

The objective of this work is to study the response of Silene vulgaris to a range of environmentally relevant concentrations of Cr(VI) in order to evaluate its potential use in the phytomanagement of Cr polluted sites. Cuttings of six homogenous genotypes from Madrid (Spain) have been used as plant material. The eco-physiological response of S. vulgaris to Cr(VI) changed with the genotype. The yield dose-response curve was characterized by stimulation at low doses of Cr(VI). The effects of metal concentration were quantified on root dry weight, water content and chlorophyll content, determined by SPAD index. The response was not homogeneous for all studied genotypes. At high doses of Cr(VI), plants increased micronutrient concentration in dry tissues which suggested that nutrient balance could be implicated in the alleviation of Cr toxicity. This work highlights the importance of studying the eco-physiological response of metallophytes under a range of pollutant concentrations to determine the most favorable traits to be employed in the phytomanagement process.

Agronomic Practices for Improving Gentle Remediation of Trace Element-Contaminated Soils

The last few decades have seen the rise of Gentle soil Remediation Options (GRO), which notably include in situ contaminant stabilization ("inactivation") and plant-based (generally termed "phytoremediation") options. For trace element (TE)-contaminated sites, GRO aim to either decrease their labile pool and/or total content in the soil, thereby reducing related pollutant linkages. Much research has been dedicated to the screening and selection of TE-tolerant plant species and genotypes for application in GRO. However, the number of field trials demonstrating successful GRO remains well below the number of studies carried out at a greenhouse level. The move from greenhouse to field conditions requires incorporating agronomical knowledge into the remediation process and the ecological restoration of ecosystem services. This review summarizes agronomic practices against their demonstrated or potential positive effect on GRO performance, including plant selection, soil management practices, crop rotation, short rotation coppice, intercropping/row cropping, planting methods and plant densities, harvest and fertilization management, pest and weed control and irrigation management. Potentially negative effects of GRO, e.g., the introduction of potentially invasive species, are also discussed. Lessons learnt from long-term European field case sites are given for aiding the choice of appropriate management practices and plant species.