Dieldrin accumulation, distribution in plant parts and phytoextraction potential for several plant species and Cucurbita pepo varieties

Dieldrin, an organochlorine pesticide (OCP) widely used for crop protection in the second half of the 20th century till the 70's, is worldwide still present in arable soils. It can be transferred to crops, notably cucurbits, depending on plant species and cultivars. Finding strategies to decrease OCP bioavailability in soil is therefore a main concern. Phytomanagement strategies could provide (i) ready-to-use short term solution for maintaining the production of edible plant parts with dieldrin concentrations below the Maximum Residue Limits (MRL) and (ii) long-term solution for dieldrin phytoextraction reducing progressively its bioavailability in the soil. This field study aimed at determining dieldrin accumulation capacities and allocation pattern in 17 non-Cucurbitaceae species and 10 Cucurbita pepo varieties, and assessing the dieldrin phytoextraction potential of these plant species when grown to maturity in a historically dieldrin-contaminated soil. Out of the non-Cucurbitaceae species, vetiver was the only one able to accumulate significant amounts of dieldrin, which mainly remained in its roots. All C. pepo varieties were able to uptake and translocate high dieldrin amounts into the shoots, leading to the highest phytoextraction potential. Despite the intraspecific variability in dieldrin concentration in zucchini plant parts, mainly in the reproductive organs, the phytoextraction capacity for shoots and fruits was high for all tested varieties (147 to 275 μg dieldrin plant-1, corresponding to 5.6 % of the n-heptane extractable soil dieldrin), even for the one with low fruit dieldrin concentration. Both food safety and phytoextraction could be achieved by selecting productive zucchini varieties displaying low dieldrin concentration in fruits and high one in shoots.

Maintaining the cultivation of vegetables with low Pb accumulation while remediating the soil of an allotment garden (Nantes, France) by phytoextraction

Lead (Pb) is commonly found in urban soils and can transfer to vegetables. This entails a health risk for consumers of garden crops. The increasing demand of gardening on urban soil linked to the population increase and concentration in urban areas induces an increase in the risk, as people could be forced to cultivate contaminated soils. The aim of this study was to evaluate the performance of a cropping system that allows simultaneously (i) growing eatable vegetables that accumulate few Pb and (ii) cleaning up the soil with other plants by phytoextraction. The tests were carried out in an allotment garden (Nantes, France) where soils are moderately enriched by Pb from geogenic origin (178 mg.kg-1 of dry soil on average). Four vegetables known to accumulate slightly Pb (Solanum lycopersicum, Brassica oleracea cv. "Capitata," Solanum tuberosum, and Phaseolus vulgaris) were grown. The in situ ability of Brassica juncea L. to progressively absorb the phytoavailable Pb of the soil was assessed during four seasons. Analyses of the edible parts of the four vegetables confirmed that they can all be safely cultivated. The accumulation of Pb in B. juncea shoots was too low (ca. 1 mg.kg-1 of dry matter at best) for phytoextraction purposes. Our results confirm that it is possible to grow very low Pb-accumulating vegetables on soils moderately contaminated with Pb, although it was not possible to reduce phytoavailable Pb rapidly enough with B. juncea. This study identifies possible avenues of research to improve this cropping system by using appropriate vegetables that will allow food production to continue on moderately contaminated soil while cleaning it up.

Differential effects of tree species identity on rhizospheric bacterial and fungal community richness and composition across multiple trace element-contaminated sites

Soil microbial communities play a key role in plant nutrition and stress tolerance. This is particularly true in sites contaminated by trace metals, which often have low fertility and stressful conditions for woody plants in particular. However, we have limited knowledge of the abiotic and biotic factors affecting the richness and composition of microbial communities inhabiting the rhizosphere of plants in contaminated sites. Using high-throughput amplicon sequencing, we studied the rhizospheric bacterial and fungal community structures of 14 woody plant families planted in three contrasting sites contaminated by metals (Pb, Cd, Zn, Mn, Fe, S). The rhizospheric bacterial communities in the given sites showed no significant difference between the various woody species but did differ significantly between sites. The Proteobacteria phylum was dominant, accounting for over 25 % of the overall relative abundance, followed by Actinobacteria, Bacteroidetes and Gemmatimonadetes. Site was also the main driver of fungal community composition, yet unlike bacteria, tree species identity significantly affected fungal communities. The Betulaceae, Salicaceae and Fagaceae families had a high proportion of Basidiomycota, particularly ectomycorrhizal fungi, and the lowest diversity and richness. The other tree families and the unplanted soil harboured a greater abundance of Ascomycota and Mucoromycota. Consequently, for both bacteria and fungi, the site effect significantly impacted their community richness and composition, while the influence of plants on the richness and composition of rhizospheric microbial communities stayed consistent across sites and was dependent on the microbial kingdom. Finally, we highlighted the importance of considering this contrasting response of plant rhizospheric microbial communities in relation to their host identity, particularly to improve assisted revegetation efforts at contaminated sites.

Enhancing soil ecological security through phytomanagement of tailings in erosion-prone areas

Revegetation is effective in improving soil quality in ecologically fragile areas. However, little is known about the impact of diverse phytomanagement strategies of tailings on soil quality and ecological security in erosion-prone areas. We investigated the water stability, soil aggregate nutrients, and the risk of heavy metal contamination of abandoned tailings under phytomanagement and in adjacent bare land on the Loess Plateau. The results showed that phytomanagement significantly enhanced soil aggregate stability, as demonstrated by higher contents of soil organic carbon (SOC), glomalin-related soil protein (GRSP), aromatic-C, and alkene-C in macro-aggregates. The pollution load index (PLI) and ecological risk index (RI) of soil heavy metals were lower in shrub/herbaceous mixed forests than in natural grasslands and planted forests. The risk of heavy metal contamination was higher in macro-aggregates (>0.25 mm) than in micro-aggregates (<0.25 mm) and was significantly and positively correlated with the SOC and GRSP contents of the aggregates. Our study demonstrates that soil aggregate quality is closely related to the fate of heavy metals. Diversified tailing revegetation measures can improve soil quality and ensure ecological security.

Techno-economic analysis of phytoremediation: A strategic rethinking

Phytoremediation is a cost-effective and environmentally sound approach, which uses plants to immobilize/stabilize, extract, decay, or lessen toxicity and contaminants. Despite successful evidence of field application, such as natural attenuations, and self-purification, the main barriers remain from a "promising" to a "commercial" approach. Therefore, the ultimate goal of this paper is to examine factors that contribute to phytoremediation's underutilization and discuss the real costs of phytoremediation when the time and land values are considered. We revisit mechanisms and processes of phytoremediation. We synthesize existing information and understanding based on previous works done on phytoremediation and its applications to provide the technical assessment and perspective views in the commercial acceptance of phytoremediation. The results show that phytoremediation is the most suitable for remote regions with low land values. Since these regions allow a longer period to be restored, land vegetation covers can be established in more or less time like natural attenuation. Since the length of phytoremediation is an inherent limitation, this inherent disadvantage limits its adoption in developed business regions, such as growing urban areas. Because high land values could not be recovered in the short term, phytoremediation is not cost-effective in those regions. We examine the potential measures that can enhance the performance of phytoremediation, such as soil amendments, and agricultural practices. The results obtained through review can clarify where/what conditions phytoremediation can provide the most suitable solutions at a large scale. Finally, we identify the main barriers and knowledge gaps to establishing a vegetation cover in large-scale applications and highlight the research priorities for increased acceptance of phytoremediation.

Floristic survey, trace element transfers between soil and vegetation and human health risk at an urban industrial wasteland

This study aimed to determine the trace element accumulation in the soil and plants in an industrial wasteland and to estimate the extent of transfer to humans to measure the effects on and risks to vegetation and human health and find bioindicator plants representative of the levels of the main contaminants. In areas with the highest extractable trace element levels, we observed decreases in plant biodiversity explained by the disappearance of several families, favouring the coverage of tolerant species, such as Urtica dioica and Hedera helix. Trace elements were also found in the leaves of several plants, especially in a dominant species that is poorly studied, Alliaria petiolata. Indeed, this species had the highest contents of Zn (1750 mg.kg-1 DW), Ni (13.1 mg.kg-1 DW), and Cd (18 mg.kg-1 DW) found at the site and is a potential Zn bioindicator since its leaf contents were also representative of the Zn extractable contents in soil (R² = 0.94). The hazard quotient and carcinogen risk revealed that most of the site had an identified or possible risk, mainly due to Pb and As. Native species, especially A. petiolata, could be used in phytoextraction to manage and limit these human and environmental risks.

Cover crop response to increased concentrations of copper in vineyard soils: Implications for copper phytoextraction

The use of cover crops (CCs) in viticulture is threatened by the contamination of vineyard soils by copper (Cu). This study investigated the response of CCs to increased concentrations of Cu in soil as a way to assess their sensitivity to Cu and their Cu phytoextraction ability. Our first experiment used microplots to compare the effect of increasing soil Cu content from 90 to 204 mg kg^-1 on the growth, Cu accumulation level, and elemental profile of six CC species (Brassicaceae, Fabaceae and Poaceae) commonly sown in vineyard inter-row. The second experiment quantified the amount of Cu exported by a mixture of CCs in vineyards with contrasted soil characteristics. Experiment 1 showed that increasing the soil Cu content from 90 to 204 mg kg^-1 was detrimental to the growth of Brassicaceae and faba bean. The elemental composition of plant tissues was specific to each CC and almost no change in composition resulted from the increase in soil Cu content. Crimson clover was the most promising CC for Cu phytoextraction as it produced the most aboveground biomass, and, along with faba bean, accumulated the highest concentration of Cu in its shoots. Experiment 2 showed that the amount of Cu extracted by CCs depended on the availability of Cu in the topsoil and CC growth in the vineyard, and ranged from 25 to 166 g per hectare. Taken together, these results emphasize the fact that the use of CCs in vineyards may be jeopardised by the contamination of soils by Cu, and that the amount of Cu exported by CCs is not sufficiently high to offset the amount of Cu supplied by Cu-based fungicides. Recommendations are provided for maximizing the environmental benefits provided by CCs in Cu-contaminated vineyard soils.

Pleiotropic melatonin-mediated responses on growth and cadmium phytoextraction of Brassica napus: A bioecological trial for enhancing phytoremediation of soil cadmium

Melatonin (MT) has recently gained significant scientific interest, though its mechanism of action in enhancing plant vigor, cadmium (Cd) tolerance, and Cd phytoremediation processes are poorly understood. Therefore, here we investigated the beneficial role of MT in improving growth and Cd remediation potential of rapeseed (Brassica napus). Plants, with or without MT (200 µM L^-1), were subjected to Cd stress (30 mg kg¹). Without MT, higher Cd accumulation (up to 99%) negatively affected plant growth and developmental feature as well as altered expression of several key genes (DEGs) involved in different molecular pathways of B. napus. As compared to only Cd-stressed counterparts, MT-treated plants exhibited better physiological performance as indicated by improved leaf photosynthetic and gaseous exchange processes (3-48%) followed by plant growth (up to 50%), fresh plant biomass (up to 45%), dry plant biomass (up to 32%), and growth tolerance indices (up to 50%) under Cd exposure. MT application enhanced Cd tolerance and phytoremediation capacity of B. napus by augmenting (1) Cd accumulation in plant tissues and its translocation to above-ground parts (by up to 45.0%), (2) Cd distribution in the leaf cell wall (by up to 42%), and (3) Cd detoxification by elevating phytochelatins (by up to 8%) and metallothioneins (by upto 14%) biosynthesis, in comparison to Cd-treated plants. MT played a protective role in stabilizing hydrogen peroxide and malondialdehyde levels in the tissue of the Cd-treated plants by enhancing the content of osmolytes (proline and total soluble protein) and activities of antioxidant enzymes (SOD, CAT, APX and GR). Transcriptomic analysis revealed that MT regulated 1809 differentially expressed genes (828 up and 981 down) together with 297 commonly expressed DEGs (CK vs Cd and Cd vs CdMT groups) involved in plant-pathogen interaction pathway, protein processing in the endoplasmic reticulum pathway, mitogen-activated protein kinase signaling pathway, and plant hormone signal transduction pathway which ultimately promoted plant growth and Cd remediation potential in the Cd-stressed plants. These results provide insights into the unexplored pleiotropic beneficial action of MT in enhancing in the growth and Cd phytoextraction potential of B. napus, paving the way for developing Cd-tolerant oilseed crops with higher remediation capacity as a bioecological trial for enhancing phytoremediation of hazardous toxic metals in the environment.

Assessment for combined phytoremediation and biomass production on a moderately contaminated soil

Once previous industrial activity has ceased, brownfields are found in urban and suburban environments and managed in different ways ranging from being left untouched to total reconversion. These situations apply to large surface areas often impacted by residual diffuse pollution. Though significant and preventing any sensitive use, residual contamination does not necessarily require treatment. Moreover, conventional treatments show their technical and economic limits in these situations and gentle remediation options such as phytomanagement might appear more relevant to the management of those sites. Thus, these sites face up two major issues: managing moderate contamination levels and providing an alternative use of economic interest. This work proposes to assess a management strategy associating the phytoremediation of organic pollution along with the production of biomass for energy generation production. A 16-week controlled growth experiment was conducted on a soil substrate moderately impacted by multiple pollution (trace elements, mainly Zn and Pb, and hydrocarbons), by associating rhizodegradation with Medicago sativa or biomass production with Robinia pseudoacacia or Alnus incana in monocultures. The effect of a microbial inoculum amendment on the performances of these treatments was also evaluated. Results showed total hydrocarbons (TH), and to a lesser extent polycyclic aromatic hydrocarbons (PAH), concentrations decreased over time, whatever the plant cover. Good biomass production yields were achieved for both tree species in comparison with the control sample, even though R. pseudoacacia seemed to perform better. Furthermore, the quality of the biomass produced was in conformity with the thresholds set by the legislation concerning its use as a renewable energy source.

Developing field-scale, gentle remediation options for nuclear sites contaminated with 137Cs and 90Sr: The role of Nature-Based Solutions

The remediation of contaminated land using plants, bacteria and fungi has been widely examined, especially in laboratory or greenhouse systems where conditions are precisely controlled. However, in real systems at the field scale conditions are much more variable and often produce different outcomes, which must be fully examined if 'gentle remediation options', or GROs, are to be more widely implemented, and their associated benefits (beyond risk-management) realized. These secondary benefits can be significant if GROs are applied correctly, and can include significant biodiversity enhancements. Here, we assess recent developments in the field-scale application of GROs for the remediation of two model contaminants for nuclear site remediation (⁹⁰Sr and ¹³⁷Cs), their risk management efficiency, directions for future application and research, and barriers to their further implementation at scale. We also discuss how wider benefits, such as biodiversity enhancements, water filtration etc. can be maximized at the field-scale by intelligent application of these approaches.

Harnessing an arbuscular mycorrhizal fungus to improve the adaptability of a facultative metallophytic poplar (Populus yunnanensis) to cadmium stress: Physiological and molecular responses

Populus yunnanensis Dode, a facultative metallophytic poplar, exhibits afforestation potential in barren mine tailing areas. However, the interactions and functional roles of arbuscular mycorrhizal fungus (AMF) in P. yunnanensis adaptability to heavy metal stress remain unclear. Physiological and molecular responses of P. yunnanensis plantlets to AMF (Funneliformis mosseae) under cadmium (Cd) stress (50 mg kg^-1) were investigated. Results showed attenuation of Cd phytotoxicity effects on cell organelles upon AMF inoculation, which also reduced the Cd concentration in the poplar leaves, stems, and roots. Under Cd stress, AMF-blocking of metal transporter (e.g., Ca²⁺ channel) activity occurred, decreasing root cell Cd influx by reducing H⁺ efflux. Bioaugmentation of rhizosphere sediments by AMF to stabilize metals with a decreasing DTPA-extractable Cd also occurred. The AMF inoculation promoted Cd conversion into inactive, less phytotoxic forms, and helped to maintain ion homeostasis and relieve nutritional ion (e.g., Ca, Mg) disorders caused by excessive Cd. Leaf enzyme and non-enzyme antioxidant systems were triggered. Root and leaf physiological response patterns differed. The AMF regulated the poplar functional genes, and nine metal-responsive gene clusters were identified. We suggest that AMF is a functional component of P. yunnanensis phenotype extension, contributing to strong adaptability to unfavorable mine tailings conditions.

Water and soil contaminated by arsenic: the use of microorganisms and plants in bioremediation

Owing to their roles in the arsenic (As) biogeochemical cycle, microorganisms and plants offer significant potential for developing innovative biotechnological applications able to remediate As pollutions. This possible use in bioremediation processes and phytomanagement is based on their ability to catalyse various biotransformation reactions leading to, e.g. the precipitation, dissolution, and sequestration of As, stabilisation in the root zone and shoot As removal. On the one hand, genomic studies of microorganisms and their communities are useful in understanding their metabolic activities and their interaction with As. On the other hand, our knowledge of molecular mechanisms and fate of As in plants has been improved by laboratory and field experiments. Such studies pave new avenues for developing environmentally friendly bioprocessing options targeting As, which worldwide represents a major risk to many ecosystems and human health.

A risk management framework for Gentle Remediation Options (GRO)

Gentle Remediation Options (GRO) are remediation measures involving plants, fungi, bacteria, and soil amendments that can be applied to manage risks at contaminated sites. Several studies and decision-support tools promote the wider range of benefits provided by GRO, but there is still skepticism regarding GRO implementation. Key issues that need to be better communicated are the various risk mitigation mechanisms, the required risk reduction for an envisioned land use, and the time perspective associated with the risk mitigation mechanisms. To increase the viability and acceptance of GRO, the phytomanagement approach implies the combination of GRO with beneficial green land use, gradually reducing risks and restoring ecosystem services. To strengthen the decision basis for GRO implementation in practice, this paper proposes a framework for risk management and communication of GRO applications to support phytomanagement strategies at contaminated sites. The mapping of the risk mitigation mechanisms is done by an extensive literature review and the Swedish national soil guideline value model is used to derive the most relevant human health exposure pathways and ecological risks for generic green land use scenarios. Results indicate that most of the expected risk mitigation mechanisms are supported by literature, but that knowledge gaps still exist. The framework is demonstrated to support the identification of GRO options for the case study site given two envisioned land uses: biofuel park and allotment garden. A more easily understandable risk management framework, as proposed here, is expected to act as a communication tool to educate decision-makers, regulatory bodies and other stakeholders for better understanding of risk mitigation mechanisms and preliminary timeframes of various GRO, particularly in the early stages of a brownfield redevelopment project.

Variations in phytoremediation potential and phytoavailability of heavy metals in different Salix genotypes subjected to seasonal flooding

Climate-related flooding poses a potential challenge to phytoremediation of metal polluted areas. In the present study, uptake, translocation and accumulation of heavy metals (Cd, Zn and Cu), and their phytoavailability in six flood-tolerant Salix genotypes were investigated under simulated seasonal flooding conditions (non-flooding conditions were kept as the control). Plants were cultivated in a greenhouse with open windows using the soil polluted with Cd, Zn and Cu for 110 days. All the control (non-flooded) genotypes did not exhibit visible toxic symptoms, whereas the flooded genotypes showed leaf chlorosis and developed both lenticels and adventitious roots. Biomass production and metal accumulation in tissues varied with Salix genotypes. The flooded genotypes dramatically decreased aerial biomass production compared with corresponding non-flooded genotypes. All the control Salix genotypes showed relatively high accumulation for Cd, Zn and Cu in aerial parts due to high EDTA-extractable metals in the rhizosphere, exhibiting phytoextraction features. In contrast, the flooded genotypes drastically decreased uptake, translocation, accumulation, and extraction capacities for Cd, Zn and Cu in aerial parts, differing with genotypes, and tended to phyto-stabilize them in roots, especially Cu. This study indicated that flooding is a leading factor on phytoremediation efficiency for contaminated sites with willows.

Relationship between Leachate Pollution Index and growth response of two willow and poplar hybrids: Implications for phyto-treatment applications

Phytoremediation is a potentially suitable technique for the reclamation of toxic landfill leachate (LL) by decreasing its volume through water uptake and improving its composition by uptake, accumulation and amelioration of pollutants. We investigated the use of two parameters, the LL concentration and the Leachate Pollution Index (LPI), a method used to determine the phytotoxicity potential of a leachate source based on a weighted sum of its components, to set the best LL dilution to apply when poplar clone 'Orion' and willow clone 'Levante' are selected for phytoremediation. Cuttings were watered with five LL concentrations ranging from 0 to 100%. The poplar clone showed significantly higher values than the willow clone for lowest effective concentration index (LOEC) for leaf (i.e. 11.3% vs 10.5%; p = 0.0284) and total biomass (i.e. 10.9% vs 10.6%; p = 0.0402) and for lowest effective LPI for leaf (i.e. 12.3 vs 12.1; p = 0.0359) and total biomass (i.e. 12.8 versus 12.2; p = 0.0365), respectively, with effectiveness demonstrating the LOEC or LPI value at which the parameter is negatively affected. Photosynthetic rates were higher in poplar than willow in both control and the lowest LL dilution, but rapidly declined in both at higher LL dilutions. Although a direct translation of data from bench trials to field conditions should be investigated, we concluded that in the establishment phase, the poplar hybrid is more tolerant than the willow hybrid to LL. We also provide evidence for LPI as a potential predictor for setting LL irrigation levels in the initial phase of a phyto-treatment approach.

Co-planting Brassica napus and Salix nigra as a phytomanagement alternative for copper contaminated soil

Phytoremediation of copper contaminated soil poses particular difficulties because of the low Cu mobility in the soil. Although several plant species, such as willows or Brassicaceae, have been used in the phytomanagement of abandoned brownfields, certain trace elements, such as copper often remain difficult to treat or remove from contaminated soils. An experiment was conducted under semi-controlled conditions to test the phytoremediation potential of co-planting two crop species, Brassica napus L. and Salix nigra 'S05', in soil spiked with two concentrations of Cu (500 mg kg^-1 and 800 mg kg^-1). Particular attention was given to the potential of 1) the co-plantation design and 2) uprooting, to efficiently mimic the remediation of a Cu-contaminated soil. Results showed that most Cu was found in plant roots and that the polyculture treatment produced the most overall biomass and maximum stabilization and extraction of Cu of the three treatments tested, regardless of contamination level.

Biochar and urban solid refuse ameliorate the inhospitality of acidic mine tailings and foster effective spontaneous plant colonization under semiarid climate

Phytomanagement is considered a suitable option in line with nature-based solutions to reduce environmental risks associated to metal(loid) mine tailings. We aimed at assessing the effectiveness of biochar from pruning trees combined with compost from urban solid refuse (USR) to ameliorate the conditions of barren acidic (pH ~5.5) metal(loid) mine tailing soils (total concentrations in mg kg^-1: As ~220, Cd ~40, Mn ~1800, Pb ~5300 and Zn ~8600) from Mediterranean semiarid areas and promote spontaneous plant colonization. Two months after amendment addition were enough to observe improvements in chemical and physico-chemical tailing soil properties (reduced acidity, salinity and water-soluble metals and increased organic carbon and nutrients content), which resulted in lowered ecotoxicity for the soil invertebrate Enchytraeus crypticus. Recalcitrant organic carbon provided by biochar remained in soil whereas labile organic compounds provided by USR were consumed over time. These improvements were consistent for at least one year and led to lower bulk density, higher water retention capacity and higher scores for microbial/functional-related parameters in the amended tailing soil. Spontaneous growth of native vegetation was favored with amendment addition, but adult plants of remarkable size were only found after three years. This highlights the existence of a time-lag between the positive effects of the amendment on tailing soil properties being observed and these improvements being translated into effective spontaneous plant colonization.

Partial overlap of fungal communities associated with nettle and poplar roots when co-occurring at a trace metal contaminated site

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.

Beneficial traits of root endophytes and rhizobacteria associated with plants growing in phytomanaged soils with mixed trace metal-polycyclic aromatic hydrocarbon contamination

The diversity of cultivable bacteria associated with plants from phytomanaged soils with mixed trace metal (TM) and polycyclic aromatic hydrocarbon (PAH) contamination in Pierrelaye (France) was evaluated. The emphasis was on the cultivable bacterial community since the overall objective is to obtain inoculants to improve the remediation of this type of contaminated site. Root endophytic and rhizosphere soil bacterial counts were determined, and isolates were pooled by amplified rDNA restriction analysis and identified by 16S rDNA sequencing. Isolates were further characterized for the production of plant growth-promoting (PGP) substances, and resistance to TM. The selected strains were evaluated for their ability to degrade PAHs. The potential of cell-free microbial supernatant to increase the mobilisation of PAHs from the polluted soil of Pierrelaye was also evaluated. Proteobacteria and Actinobacteria dominated the collection of isolates, and differences in taxonomic diversity were observed between plant species (Populus or Zea mays) and depending on the remediation treatment (Populus inoculation with mycorrhizae or Populus intercropping with Alnus). The majority of isolates exhibited at least one of the tested PGP traits, as well as resistance to more than one TM. Several rhizosphere, endophyte and even one bulk soil isolate showed high rates of fluoranthene and pyrene reduction. The endophyte Rhizobium strain MR28 isolated from maize and degrading pyrene produced bioemulsifying molecules capable of improving the availability of PAHs from the soil of Pierrelaye. A selection of the most interesting strains was made for further re-inoculation experiments in order to assess their potential in rhizoremediation processes.

Cyclocarya paliurus for Phytomanagement of Lead-Contaminated Soils

Cyclocarya paliurus seedlings were cultivated in three types of lead (Pb)-contaminated soils with Pb concentration of 305 ± 17 mg/kg (T1), 1964 ± 59 mg/kg (T2) and 3502 ± 107 mg/kg (T3), respectively. The results showed that after 180 days of cultivation, the contents of exchangeable and carbonate-bound Pb fractions significantly decreased in T1 and T2, but increased in T3. The growth indices of C. paliurus seedlings decreased with increasing Pb concentration; however, no difference was found between that in T1 and in Pb-free soil. The Pb concentration in the roots was an order of magnitude higher than that in the stems and in the leaves. The bioconcentration factor (BCF) of the leaves was the lowest among the three tissues investigated, and decreased with the higher concentration of Pb in the soils. These results suggest that C. paliurus can be used as a sustainable and profitable plant for the phytomanagement of Pb-contaminated soil.

Insight into metal immobilization and microbial community structure in soil from a steel disposal dump phytostabilized with composted, pyrolyzed or gasified wastes

The soil system is a key component of the environment that can serve as a sink of pollutants. Using processed waste for aided phytostabilization of metals (HMs) in contaminated soils is an attractive phytoremediation technique that integrates waste utilization and recycling. In this study, we evaluated the effect of biologically and thermally processed wastes, i.e. sewage sludge compost (CSS), poultry feather ash (AGF) and willow chip biochar (BWC), on phytostabilization of contaminated soil from a steel disposal dump. Greenhouse experiments with Lupinus luteus L. and amendments (dosage: 3.0%, w/w) were conducted for 58 days. Soil toxicity was evaluated with Ostracodtoxkit and Phytotoxkit tests. At the end of the experiment, soil pH, plant biomass yield, and HM accumulation in plant tissues were determined. HM distribution, HM stability (reduced partition index) and potential environmental risk (mRI index) in the soil were assessed. During phytostabilization, changes in the diversity of the rhizospheric bacterial community were monitored. All amendments significantly increased soil pH and biomass yield and decreased soil phytotoxicity. AGF and BWC increased accumulation of individual HMs by L. luteus roots better than CSS (Cu and Cr, and Ni and Zn, respectively). The soil amendments did not improve Pb accumulation by the roots. Improvements in HM stability depended on amendment type: Ni and Pb stability were improved by all amendments; Zn stability, by AGF, and BWC; Cd stability, by AGF; and Cr stability, by BWC. AGF reduced the mRI most effectively. Microbial diversity in amended soils increased with time of phytostabilization and was up to 9% higher in CSS amended soil than in control soil. AGF application favored the abundance of the genera Arenimonas, Brevundimonas, Gemmatimonas and Variovorax, whose metabolic potential could have contributed to the better plant growth and lower mRI in that soil. In conclusion, AGF and BWC have great potential for restoring steel disposal dump areas, and the strategies researched here can contribute to achieving targets for sustainable development.

Use of native plants and their associated bacteria rhizobiomes to remediate-restore Draa Sfar and Kettara mining sites, Morocco

Soil and mine tailings are unreceptive to plant growth representing an imminent threat to the environment and resource sustainability. Using indigenous plants and their associated rhizobacteria to restore mining sites would be an eco-friendly solution to mitigate soil-metal toxicity. Soil prospection from Draa Sfar and Kettara mining sites in Morocco was carried out during different seasons for native plant sampling and rhizobacteria screening. The sites have been colonized by fifteen tolerant plant species having different capacities to accumulate Cu, Zn, and P in their shoots/root systems. In Draa Sfar mine, Suaeda vera J.F. Gmel., Sarcocornia fruticosa (L.) A.J. Scott., and Frankenia corymbosa Desf. accumulated mainly Cu (more than 90 mg kg^-1), Atriplex halimus L. accumulated Zn (mg kg^-1), and Frankenia corymbosa Desf. accumulated Pb (14 mg kg^-1). As for Kettara mine, Aizoon canariense L. mainly accumulated Zn (270 mg kg^-1), whereas Forsskalea tenacissima L. was the best shoot Cu accumulator with up to 50 mg kg^-1, whereas Cu accumulation in roots was 21 mg kg^-1. The bacterial screening revealed the strains' abilities to tolerate heavy metals up to 50 mg kg^-1 Cu, 250 mg kg^-1 Pb, and 150 mg kg^-1 Zn. Isolated strains belonged mainly to Bacillaceae (73.33%) and Pseudomonadaceae (10%) and expressed different plant growth-promoting traits, alongside their antifungal activity. Results from this study will provide an insight into the ability of native plants and their associated rhizobacteria to serve as a basis for remediation-restoration strategies.

Urea-enhanced phytoremediation of cadmium with willow in pyrene and cadmium contaminated soil

The phytoremediation of cadmium (Cd) and pyrene (PYR) in agricultural soil with willow was investigated by carrying out a pot-culture experiment in a greenhouse. The soil was incubated with urea 60 days before it was used for this experiment. The concentrations of Cd and PYR in soil and willow, the bioconcentration and transfer factors, the physiological and biochemical responses, and plant biomass production were determined at the end of the experiment. The phytoremediation with willow based on urea application was effective for enhancing the phytoremediation of Cd and PYR contaminated soil. Urea application did not affect the available Cd but increased the accumulation of soil Cd and the plant biomass of different parts of the willow. The removal rate (77.1-89.5%) of PYR in soil was not significantly affected although urea application decreased the accumulation of PYR in willow root and bark. Urea application significantly promoted the uptake of chlorophyll, carotenoid and malondialdehyde by willow leaves. The results of this study will provide scientific information for the effective phytoremediation of Cd in Cd and PYR contaminated soil.

Aided-phytostabilization of steel slag dumps: The key-role of pH adjustment in decreasing chromium toxicity and improving manganese, phosphorus and zinc phytoavailability

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.

Impact of O3 on the phytoremediation effect of Celosia argentea in decontaminating Cd

Elevated atmospheric O₃ can inhibit the growth rate of various plants and increase metal content in their tissues owing to the oxidative damage, thereby affecting their phytoremediation efficiency. In this study, a series of O₃ fumigation treatments were designed to evaluate the dry weight, Cd content, and transpiration rate responses of Celosia argentea to different levels of O₃ (40, 50, 55, 60, 65, and 80 ppb). The dry weight of C. argentea decreased as the atmospheric O₃ level increased, and the Cd concentration of the plant leaves increased until the level of O₃ reached 60 ppb before decreasing slightly. The variations in the transpiration rate followed a similar trend to the Cd content under different O₃ levels. The phytoremediation efficiency of C. argentea increased with O₃ fumigation at low (50 ppb) and moderate (55 and 60 ppb) levels, and significantly decreased at the highest level. The regression curves indicated that the plant species treated with 52 ppb of O₃ exhibited the highest Cd accumulation capacity. Overall, the phytoremediation effect of C. argentea cultivated in Cd-polluted soil might be improved under the high-O₃ conditions. This result might help to choose suitable plants for soil remediation in future atmospheric environment.

Willow and poplar for the phyto-treatment of landfill leachate in Mediterranean climate

Phytotechnological approaches using living plants are currently being proposed to address a wide range of environmental purposes including the treatment of landfill leachate (LL). Despite their popularity, few studies have investigated this possibility under actual Mediterranean conditions using fast-growing trees. This research reports the results of a two-year project where poplar and willow grown in mesocosm were tested for their ability to withstand and remove specific pollutants from different [Low: 7% (1st year) and 15% (2nd year); High: 15% (1st year) and 30% (2nd year)] amounts of LL. Results indicate that both species were able to treat 340 (Low) and 680 (High) m³ ha^-1 in the establishment year (70 days) and 2470 (Low) and 4950 (High) m³ ha^-1 in the second year (150 days). Both species yielded the same aboveground biomass, but under high LL treatment, poplar performed better than willow. Poplar showed on average significantly higher extraction rates for Cd, Cu, P, and N than willow. Moreover, under high LL treatment, poplar also seemed more efficient than willow in decreasing the concentration of specific pollutants (BOD₅, COD and As) in output effluent. However, with low LL loads both species were able to significantly reduce other compounds (i.e. NH₄-N, Cu and Ni). By contrast, Cl, surfactants, and NO₃-N, had a tendency to accumulate over time in the effluent and could still represent an actual constraint to large-scale application of the technique. The fate of such pollutants should be investigated with further research to better inform strategies used to manage low amounts of high-concentrated effluent.

Changes in plant growth, Cd partitioning and xylem sap composition in two sunflower cultivars exposed to low Cd concentrations in hydroponics

This study characterizes sunflower response to the levels of Cd encountered in moderately Cd-polluted soils. Two sunflower cultivars differing in their ability to sequestrate Cd in roots were exposed to low concentrations of Cd (0.5 nM or 100 nM) in hydroponics and sampled after 18 days (258 degree-days) when ten leaves were fully expanded. Plant growth, Cd uptake and partitioning among organs were monitored along with the ionomic (ICP-MS) and the metabolic (¹H-NMR) composition of the xylem sap. Sunflower tolerance to Cd differed between the two cultivars. The cultivar with the highest ability to sequestrate Cd in roots (Kapllan) was more tolerant to Cd than the one with the lowest ability (ES RICA). The 23% penalization of plant growth observed at 100 nM in cultivar ES RICA was associated with reduced xylem loading fluxes of soluble sugars, perhaps pointing to disruption of carbohydrate metabolism. Retention of Cd in the stem was higher at 100 nM than at 0.5 nM in the Cd-sensitive cultivar ES RICA, which can be seen as a sunflower strategy to restrict the amount of Cd delivered to the leaves under Cd stress. No direct connection was found between the speciation of Cd in the xylem sap and the Cd translocation efficiency, although significant changes in the free ionic fraction of Cd were observed between the two cultivars at 0.5 nM. The relevance of these results in promoting the use of sunflower in phytomanagement of Cd-polluted soils is discussed.

Phytosociology and antioxidant profile study for selecting potent herbs for phytoremediation of crude oil-contaminated soils

Crude oil exploration activities affect the surrounding vegetation. The present investigation deals with the study of phytosociology and biochemical profiles of the herbaceous community in the active and abandoned oil drilling sites of crude oil-explored area. For comparison, a similar investigation was also carried out in control sites where oil exploration activities were not evident. At first, a phytosociological investigation was carried out and based on the results obtained antioxidant enzyme profiles of dominant herbs were studied to understand their defense mechanism to crude oil-associated stress. A total of 69 plant species belonging to 20 families were recorded in the studied sites and the family Cyperaceae was the most dominant in the crude oil-contaminated sites. The results revealed that the plants growing near the oil-explored-contaminated sites exhibit a higher level of DPPH and H₂O₂ radical scavenging activities as compared to control plant samples. For DPPH assay, the lowest IC₅₀ value was exhibited by Cyperus rotundus which was recorded to be 31.49 and 55.31 respectively for the samples of contaminated and control sites. Again, in the case of H₂O₂ scavenging activity assay, Parthenium hysterophorus showed the lowest IC₅₀ values of 27.48 and 63.07 for the samples of contaminated and control sites respectively. As a whole, the findings confirm the superior defense mechanism of some dominant herbs of the contaminated sites that include Torenia flava, Croton bonplandianus, Eclipta alba, Cyperus rotundus, Cyperus brevifolius, and Parthenium hysterophorus and their suitability for use in phytomanagement practices.

Gentle remediation options for soil with mixed chromium (VI) and lindane pollution: biostimulation, bioaugmentation, phytoremediation and vermiremediation

Gentle Remediation Options (GROs), such as biostimulation, bioaugmentation, phytoremediation and vermiremediation, are cost-effective and environmentally-friendly solutions for soils simultaneously polluted with organic and inorganic compounds. This study assessed the individual and combined effectiveness of GROs in recovering the health of a soil artificially polluted with hexavalent chromium [Cr(VI)] and lindane. A greenhouse experiment was performed using organically-amended vs. non-amended mixed polluted soils. All soils received the following treatments: (i) no treatment; (ii) bioaugmentation with an actinobacteria consortium; (iii) vermiremediation with Eisenia fetida; (iv) phytoremediation with Brassica napus; (v) bioaugmentation + vermiremediation; (vi) bioaugmentation + phytoremediation; and (vii) bioaugmentation + vermiremediation + phytoremediation. Soil health recovery was determined based on Cr(VI) and lindane concentrations, microbial properties and toxicity bioassays with plants and worms. Cr(VI) pollution caused high toxicity, but some GROs were able to partly recover soil health: (i) the organic amendment decreased Cr(VI) concentrations, alleviating toxicity; (ii) the actinobacteria consortium was effective at removing both Cr(VI) and lindane; (iii) B. napus and E. fetida had a positive effect on the removal of pollutants and improved microbial properties. The combination of the organic amendment, B. napus, E. fetida and the actinobacteria consortium was the most effective strategy.

Field evaluation of one Cu-resistant somaclonal variant and two clones of tobacco for copper phytoextraction at a wood preservation site

A Cu-resistant somaclonal tobacco variant (NBCu 10-8-F1, C1), its BaG mother clone (C3), and the FoP tobacco clone (C2) were cultivated at a wood preservation site on Cu-contaminated soils (239-1290 mg Cu kg-1 soil range) and an uncontaminated control site (CTRL, 21 mg Cu kg-1) to assess their shoot DW yields and potential use for bioavailable Cu stripping. The Cu concentration in the soil pore water varied between 0.15 and 0.84 mg L-1. Influences of Cu exposure and soil treatments, i.e., untreated soil (Unt), soils amended with compost and either dolomitic limestone (OMDL) or zerovalent iron grit (OMZ), on plant growth and shoot ionome were determined. All transplants survived and grew even at high total soil Cu. Shoots were harvested after 3 months (cut 1). Subsequently, bottom suckers developed and were harvested after 2 months (cut 2). Total shoot DW yield (cuts 1 + 2) varied between 0.8 and 9.9 t DW ha-1 year-1 depending on tobacco cultivars, soil treatments, and soil Cu exposure. It peaked for all cultivars in the OMDL plots at moderate Cu exposure (239-518 mg kg-1 soil), notably for the C2 plants. Cut 2 contributed for 11-43% to total shoot DW yield. Increase in shoot DW yield diluted shoot Cu concentration. At low Cu exposure, total shoot Cu removal peaked for the variant. At moderate Cu exposure, shoot Cu concentrations were similar in all cultivars, but total shoot Cu removal was highest for the C2 plants. At high Cu exposure (753-1140 mg kg-1), shoot Cu concentrations peaked for the C2 plants in the Unt plots, the C1 and C2 plants in the OMZ plot, and the C3 ones in the OMDL plots. Shoot Cu removal (in g Cu ha-1 year-1) ranged from 15.4 (C2 on the CTRL soil) to 261.3 (C2 on moderately contaminated OMDL soils). The C2 plants phytoextracted more Cu than the C1 and C3 ones in the Unt plots and in the OMDL plots at moderate Cu exposure. In the OMDL plots with high Cu exposure, shoot Cu removal was highest for the C1 plants. Soil amendments improved shoot Cu removal through increase in either shoot DW yield (OMDL-3-fold) or shoot Cu concentration (OMZ-1.3-fold). Increased shoot Cu concentration induced an ionome imbalance with increased shoot Al, Fe, B, and Mg concentrations and decreased P and K ones. Copper concentrations in plant parts varied in decreasing order: roots > leaves > inflorescence (cymes including seeds) > stem, whereas Cu removal ranked as roots > stem = leaves > inflorescence.

Poplar rotation coppice at a trace element-contaminated phytomanagement site: A 10-year study revealing biomass production, element export and impact on extractable elements

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.

Effects of elevated CO2 on the phytoremediation efficiency of Noccaea caerulescens

Concentrations of atmospheric carbon dioxide have been continuously increasing, and more investigations are needed in regard to the responses of various plants to the corresponding climatic conditions. In particular, potential variations in phytoremediation efficiency induced by global warming have rarely been investigated. Objective of this research was to evaluate the changes in phytoremediation efficiency of Noccaea caerulescens exposed to different concentrations of CO₂. The concentrations of CO₂ in the elevated CO₂ treatments were adjusted to 550 ± 50 ppm to match the level of atmospheric CO₂ predicted in 2050-2070. Compared to ambient controls (400 ppm), biomass yields and metal concentrations of N. caerulescens increased under elevated CO₂ conditions, thus indicating that the phytoremediation efficiency of the species could increase in higher CO₂ environment. In addition, water soluble and exchangeable Pb and Cu concentrations in soils decreased under elevated CO₂ conditions, which reduced the leaching risks of the metals. The concentrations of malondialdehyde (MDA) of N. caerulescens decreased to different degrees with the increased CO₂ concentrations. The overall findings suggested that elevations in CO₂ can reduce the oxidative damage caused by metals in this species. The phytoremediation efficiency of N. caerulescens grown in multiple metal-enriched soils could be enhanced with global warming.

Potential use of biochar, compost and iron grit associated with Trifolium repens to stabilize Pb and As on a multi-contaminated technosol

Vegetation cover can be used in the phytomanagement of polluted areas by adding value to abandoned sites and reducing the dispersion of pollutants by erosion. Appropriate amendments, that allow both efficient plant growth and the immobilization of contaminants in the soil must be chosen in order to optimize the efficiency of this process. We used a mining technosol mainly contaminated by arsenic (1068 mg kg^-1) and lead (23387 mg kg^-1) to study the effect of three amendments (biochar, compost and iron grit) on (i) physico-chemical properties of the soil and soil pore water, (ii) metal(loid) mobility, bioavailability and bioaccessibility (CaCl₂ and Simple Bioaccessibility Extraction Test (SBET)), and (iii) the capability of Trifolium repens to germinate and grow. All the amendments used increased the pH and electrical conductivity of the SPW, resulting in a 90% decrease in the concentration of lead in the soil pore water (SPW). We also demonstrated a decrease in Pb phytoavailability. The amendments allowed the establishment of a plant cover, although the addition of iron grit alone did not allow any clover germination. For the Pontgibaud technosol, the combination of the three amendments resulted in a significant decrease in As and Pb concentrations in clover tissues, mainly in the aerial organs. The amendments also made it possible for some of them to halve the phytoavailable fraction of arsenic. However, for compost, both the As concentrations in the SPW, and the bioavailable fraction of As increased. All the amendments used had contrasting effects on the bioaccessible fractions of metal(loid)s. The most efficient amendment combination was the addition of 5% biochar and 5% compost.

Trace element partitioning in a poplar phytoextraction stand in relation to stem size

At an Italian field test site the efficiency of phytoextraction of toxic trace elements (TEs) from the soil is determined by uptake capacity, bioavailability of TEs in the soil and biomass yield of the plants involved. Altering the quantity and type of biomass produced, especially among fast-growing trees, may be one method of increasing phytoextraction efficiency. In poplar bark and wood show different TE concentration. Poplar also shows changing proportions of bark and wood with increasing diameter at breast height (DBH). Though it is often thought that the amount of TE accumulated in the biomass increases with the size of the plant, in the current study we show that this is only partially true. In fact while Zn is highly accumulated by the largest (60 mm DBH) poplar plants, Cd, Cu, and Ni were more concentrated in slightly smaller plants (50 mm DBH), and Pb in even smaller (40 mm DBH). These findings could open new strategies for managing a poplar phytoextraction stand in terms of coppicing techniques and planting cycles in order to address specific targeted TEs and enhance the overall performance of this green technology.

Capability of amendments (biochar, compost and garden soil) added to a mining technosol contaminated by Pb and As to allow poplar seed (Populus nigra L.) germination

The germination capacity of poplar seeds has never been studied in the context of metal(loid)-contaminated soils, even though poplars are present over a vast geographical area. In this study, black poplar seeds from the Loire Valley (France) were grown for 28 days in mesocosm on a heavily polluted soil that was subjected to different amendments. This phytomanagement process aimed to allow the revegetation of an As and Pb-contaminated mining soil by adding appropriate amendments, resulting in metal(loid) soil stabilisation and efficient plant growth. The objectives were to evaluate the effect of three amendments (garden soil, compost and biochar) when added alone or combined to a technosol on (i) the soil physicochemical properties, (ii) the mobility of As and Pb in the soil pore water (SPW), (iii) the capacity of poplar seeds to germinate and to grow and (iv) the metal(loid) distribution within the plant organs. The addition of amendments alone or combined allowed a 90% decrease in SPW Pb concentrations, while the arsenic concentrations were between 18 and 416 times higher. However, we were only able to obtain seed germination and plant growth on amended soils. These promising results will allow us to explore the use of such amendments in rehabilitating areas that are sources of significant metal(loid) dissemination, as well as allowing a natural plant recolonisation of these sites by seeds from the surrounding environment.

Biochar effect associated with compost and iron to promote Pb and As soil stabilization and Salix viminalis L. growth

Soil pollution by metal(loid)s is one of the most significant problems in Europe. To remediate and potentially rehabilitate these contaminated sites, phytoremediation procedures are being put into place, often using amendments to help offset the extreme conditions of such soils. The aim of this study was to define the best amendment to use on the field. This was done by studying how the addition of three different amendments (biochar, compost and iron grit), alone or in combination, could affect: (i) soil physico-chemical properties, (ii) Salix viminalis growth, and (iii) metal(loid) stabilization. A 69 day-mesocosm study was thus set up using a former mine technosol, the three amendments applied alone or combined, and S. viminalis cuttings. The results showed that biochar and/or compost improved the soil fertility and the soil pore water characteristics, with reductions of acidity, metal(loid) mobility and toxicity, while iron grit amendment presented negative effects on such parameters. Such ameliorations allowed better plant growth and higher biomass production. In addition, stress indicators (leaf pigment content and root guaiacol peroxidase activity) showed a reduction in plant stress following biochar and/or compost application. Finally, among the different treatments, the use of compost or a biochar-compost combination showed better results in terms of improvement of soil conditions, increase in plant growth and reduced translocation of metal(loid)s towards upper parts, making these two treatments a valuable option for a field trial.

Biowastes alone and combined with sulfur affect the phytoavailability of Cu and Zn to barnyard grass and sorghum in a fluvial alkaline soil under dry and wet conditions

Management of degraded soils (i.e., metal contaminated soils, salt affected soils, and soils with low organic matter content) by applying biowastes (e.g., biosolids and compost) and inorganic soil amendments such as sulfur is of great agro-environmental concern. Because Cu and Zn chemical behaviour may be altered with these additions, we aimed at studying the impact of mono- and co-application of different rates (1.25% and 2.5%) of biosolids, compost, and sulfur on the mobilization of Cu and Zn and their uptake in a fluvial soil contains low and high metal concentrations and under two distinct moisture regimes (wet, where we grew barnyard grass; dry, with sorghum). We measured metal fractions and potential availability, along with soil pH, as well as plant yield and metal content in both plants, in an attempt to identify differences in metal behaviour. We found that organic matter (OM) (increased with biosolids and compost application) and soil pH (dramatically reduced with added sulfur) highly affected Cu and Zn mobility. Plant yield increased with increasing soil OM content and decreased with decreasing soil pH, particularly in the 2.5% sulfur treatment. However, Cu absorption was different in the two studied moisture regimes, as it was higher in the wet soil (Cu-DOC complexes, encouraged under wet conditions, may explain this), while it was lower in the dry soil. The biosolid-added Cu was significantly more bioavailable to sorghum plants than the spiked Cu. Co-application of sulfur and biosolids showed significantly higher sorghum uptake of Cu than application of sulfur to the spiked soil with Cu. Zinc uptake decreased in the high compost application rate (2.5%). This behaviour can be explained with the altered geochemical metal fractionation: added metals were distributed mainly in the oxides and organic fraction, but in the wet soil the percentage was higher compared to the dry, possibly due to metal-DOC associations. Also the residual fraction was lower in the wet, denoting higher metal mobility. We conclude that the observed differences between wet and dry soil concerning the metal geochemical behaviour, as were induced by added OM (with biosolids and compost) and reduced pH (with sulfur), are mainly responsible for the markedly different metal uptake patterns. These results may be an aid for effective phyto-management of alkaline fluvial soils with low and high content of Cu and Zn under paddy- and upland cultivation systems.

Nutrient limitation determines the suitability of a municipal organic waste for phytomanaging metal(loid) enriched mine tailings with a pine-grass co-culture

The suitable phytomanaging of mine tailings not only requires an improvement of soil fertility but also the assessment of the biotic interactions between the selected plant species. This study aimed to evaluate the effect of an organic amendment on the response of two plant species of contrasting habit, a tree, Pinus halepensis and a grass, Piptatherum miliaceum growing on a metal(loid)-contaminated substrate collected from mine tailings. Pots containing single plant individuals or their combination, with and without organic amendment (at 10% rate), were established and grown in a greenhouse for 13 months. Plant biomass, foliar ionome, leaf δ¹⁵N and metal(loid) concentrations were measured at the end of the experiment. The amendment alleviated P deficiency in the substrate and strongly stimulated biomass production by both plant species (10-fold for pine; 90-fold for the grass), leading to more balanced N/P ratios in leaves (especially for the grass). Co-culture with the grass negatively affected pine growth, decreasing total biomass and leaf δ¹⁵N values and inducing severe N deficiency (leaf N/P ratio<10). In contrast, co-culture with pine improved the nutrient status and growth of the grass, but only under non-amended conditions. Needle metal(loid) concentrations in P. halepensis were affected by both amendment addition and co-culture with the grass. High biomass growth with low metal(loid) concentrations in P. miliaceum leaves for the amended treatment makes this grass species suitable for the phytomanagement of metal(loid) polluted tailings, since it achieves high biomass production together with low concentrations of metal(loid)s in edible/senescent parts.

Assisted phytostabilisation of As, Pb and Sb-contaminated Technosols with mineral and organic amendments using Douglas fir (Pseudotsuga menziesii (Mirb.) Franco)

Phytoremediation of metal(loid)s by conifers is not widely studied, although conifers may be interesting, particularly in temperate-cold areas and/or on acidic soils. In this study, seeds of Douglas fir were sown in greenhouse and cultivated for 3 months on two Technosols highly contaminated with different concentrations of Pb, As and Sb and collected in two French old former mines located in massif Central or close to it: a mine of gold at La Petite Faye and a mine of lead and silver at Pontgibaud. Two amendments, a nutrient solution (NS) and composted sewage sludge (CSS), were tested in order to stimulate Douglas fir growth and to reduce the metal(loid)s mobility and phytoavailability. The speciation determined by sequential extractions as well as mineralogy highlight different geochemical behaviours of Pb, As and Sb as a function of the Technosol. In all cases, CSS amendments significantly reduced Pb phytoavailability as well as the uptake and translocation of Pb and As. Moreover, CSS stimulated the growth of Douglas firs highlighting that this amendment could be a good strategy for a better phytostabilisation of these metal(loid)s.

Functional activity and functional gene diversity of a Cu-contaminated soil remediated by aided phytostabilization using compost, dolomitic limestone and a mixed tree stand

Trace elements (TEs) availability, biochemical activity and functional gene diversity was studied in a Cu-contaminated soil, revegetated for six years with a mixed stand of willow, black poplar, and false indigo-bush, and amended or not with compost plus dolomitic limestone (OMDL). The OMDL amendment significantly reduced Cu and As availability and soil toxicity, and increased the biochemical activity and microbial functional diversity assessed with the GEOCHIP technique, as compared to the unamended soil (Unt). The OMDL soil showed significantly higher abundance of 25 functional genes involved in decomposition organic compounds, and 11, 3 and 11 functional genes involved in the N, P and S biogeochemical cycles. Functional gene abundance was positively correlated with nutrient contents but negatively correlated with Cu availability and soil toxicity. The abundance of microbial functional genes encoding for resistance to various TEs also increased, possibly due to the microbial proliferation and lower Cu exposure in the presence of high total soil Cu concentration. Genes encoding for antibiotic resistance due to the co-occurrence of TEs and antibiotic resistant genes on genetic mobile elements. Overall, phytomanagement confirmed its potential to restore the biological fertility and diversity of a severely Cu-contaminated soil, but the increase of TEs and antibiotic resistant gene abundances deserve attention in future studies.

HCH phytoremediation potential of native plant species from a contaminated urban site in Turda, Romania

Current physical or chemical methods used for remediation of soils contaminated with hexachlocyclohexane (HCH), leave behind significant levels of pollutants. Given the compound's volatility and persistence in the environment, sites contaminated with HCH remain a concern for the population living in nearby areas. By making use of both the recovery capacity and the pollutant uptake ability of spontaneously growing vegetation, our study aimed to identify native plant species able to cover and moreover take up the HCH left at a former lindane production unit in Turda, Romania. The results showed that dominant species across the study site like Lotus tenuis, Artemisia vulgaris or Tanacetum vulgare, were capable of taking up HCH in their tissues, according to different patterns that combined at the scale of the plant community. Regardless of the proximity of the HCH contamination hotspots, the development of the plant cover was characteristic for vegetation succession on disturbed soils of the Central European region. Finally, we conclude that plant species which grow spontaneously at the HCH contaminated site in Turda and are capable of taking up the pollutant, represent a self-sustainable and low maintenance phytomanagement approach that would allow for the reintegration of the site in the urban or industrial circuit and nevertheless would reduce the toxicity risk to the neighboring human inhabitants.

Phytoremediation of mine tailings with Atriplex halimus and organic/inorganic amendments: A five-year field case study

Mine tailings have adverse chemical and physical conditions, including high concentrations of metals and salts, low organic matter content, and unbalanced rates of nutrients which limit the development of vegetation. A large scale field experiment was conducted to reclaim a tailing pond by triggering the growth of native species by spontaneous colonization by tilling (TL) the tailing pond surface and using marble waste (CaCO₃; MW), pig slurry (PS) and their combination (MW + PS) as soil amendments. Soil physicochemical properties and water and DTPA extractable metal concentrations of bulk and rhizosphere soils were analyzed after five year from the application of the treatments. In addition, plants of Atriplex halimus from each treatment were collected and metals in roots, leaves and stems analyzed. Before amendments application, the studied pond showed a neutral pH, high salinity and a moderate organic carbon content. After five years, the pH value was significantly increased only in MW plot. The results showed significant increases of DTPA-extractable Zn in MW and MW + PS plots, Pb in all treatments except MW plot, Cd only in PS plot, and Cu only in MW + PS plot. A. halimus was the most dominant species, growing spontaneously in all plots, with lower vegetation cover in CT and MW plots, 6% and 2% respectively. Application of MW increased leaf Pb accumulation by 2.5-fold and Cd by 55%, when compared to the CT. The high initial salinity and probable substitution of metals by Ca²⁺ on exchangeable surfaces of soil particles may be the reasons for higher uptake of metals in MW plot when compared to the other plots. Although this plant is widely utilized in contaminated sites for phytostabilization purposes, it may absorb and translocate high concentrations of metals to the aboveground tissues in saline contaminated sites.

Phytoremediation of sewage sludge contaminated by trace elements and organic compounds

Phytoremediation is a green technique being increasingly used worldwide for various purposes including the treatment of municipal sewage sludge (MSS). Most plants proposed for this technique have high nutrient demands, and fertilization is often required to maintain soil fertility and nutrient balance while remediating the substrate. In this context, MSS could be a valuable source of nutrients (especially N and P) and water for plant growth. The aim of this study was to determine the capacity willow (Salix matsudana, cv Levante), poplar (Populus deltoides × Populus nigra, cv Orion), eucalyptus (Eucalyptus camaldulensis) and sunflower (Helianthus annuus) to clean MSS, which is slightly contaminated by trace elements (TEs) and organic pollutants, and to assess their physiological response to this medium. In particular, we aimed to evaluate the TE accumulation by different species as well as the decrease of TEs and organic pollutants in the sludge after one cropping cycle and the effect of MSS on plant growth and physiology. Since MSS did not show any detrimental effect on the biomass yield of any of the species tested, it was found to be a suitable growing medium for these species. TE phytoextraction rates depended on the species, with eucalyptus showing the highest accumulation for Cr, whereas sunflower exhibited the best performance for As, Cu and Zn. At the end of the trial, some TEs (i.e. Cr, Pb and Zn), n-alkanes and PCBs showed a significant concentration decrease in the sludge for all tested species. The highest Cr decrease was observed in pots with eucalyptus (57.4%) and sunflower (53.4%), whereas sunflower showed the highest Cu decrease (44.2%), followed by eucalyptus (41.2%), poplar (16.2%) and willow (14%). A significant decrease (41.1%) of Pb in the eucalyptus was observed. Zn showed a high decrease rate with sunflower (59.5%) and poplar (52%) and to a lesser degree with willow (35.3%) and eucalyptus (25.4%). The highest decrease in n-alkanes concentration in the sludge was found in willow (98.3%) and sunflower (97.3%), whereas eucalyptus has the lowest PCBs concentration (91.8%) in the sludge compared to the beginning of the trial. These results suggest new strategies (e.g. crop rotation and intercropping) to be adopted for a better management of this phytotechnology.

Litter breakdown as a tool for assessment of the efficiency of afforestation and ash-aided phytostabilization on metal-contaminated soils functioning in Northern France

The main objective of the study was to assess the efficiency of phytoremediation methods implemented for 14 years on highly metal-contaminated soils. The different experimental strategies were plots planted with a tree mix or with a single tree species coupled or not with the use of fly-ashes as an amendment to limit metals mobility in soil. The breakdown of poplar litter on the four plots was monitored during 10 months. In parallel, colonization of litter bags by functional groups of mesofauna (Collembola and Acari) was followed. Two mesh-sized litter bags were used to allow distinguishing microbial and mesofaunal actions on the litter breakdown. We observed the breakdown of litter in four studied plots. Litter breakdown occurred faster in 3-mm litter bags than 250 μm ones during summer demonstrating the importance of mesofauna. Mixed plantation allowed faster litter breakdown than mono-specific plantation. A higher abundance of mesofauna and/or better abiotic conditions (moisture, shading…) could explain this result. Regarding litter breakdown and mesofauna, no significant difference was observed between the amended plots and those subjected to soil phytomanagement. However, communities of the studied area are disturbed since a low abundance of detritivores was observed. This could explain also the slower litter breakdown than expected in our study. To conclude, among the phytomanagement methods tested, mixed plantations could provide a benefit for the restoration of degraded soils. By contrast, the use of fly-ashes does not seem to have any effect on the functionality of ecosystem neither on the litter breakdown process nor on the abundance of mesofauna.

Bacterial diversity associated with poplar trees grown on a Hg-contaminated site: Community characterization and isolation of Hg-resistant plant growth-promoting bacteria

Industrial waste dumps are rarely colonized by vegetation after they have been abandoned, indicating biological infertility. Revegetation of industrial tailings dumps is thus necessary to prevent wind erosion, metal leaching and has been shown to restore soil functions and ecosystem services. However, little is known about the microbial colonization and community structure of vegetated tailings following the application of restoration technologies. In this study, we investigated the rhizosphere and phyllosphere bacterial communities of a poplar tree plantation within a phytomanagement-based restoration program of a Hg-contaminated site. We used Illumina-based sequencing combined with culture-dependent approaches to describe plant-associated bacterial communities and to isolate growth-promoting bacteria (PGPB) and Hg-resistant bacteria. The genus Streptomyces was highly specific to the root community, accounting for 24.4% of the relative abundance but only representing 0.8% of the soil community, whereas OTUs from the Chloroflexi phylum were essentially detected in the soil community. Aboveground habitats were dominated by bacteria from the Deinococcus-Thermus phylum, which were not detected in belowground habitats. Leaf and stem habitats were characterized by several dominant OTUs, such as those from the phylum Firmicutes in the stems or from the genera Methylobacterium, Kineococcus, Sphingomonas and Hymenobacter in the leaves. Belowground habitats hosted more cultivable Hg-resistant bacteria than aboveground habitats and more Hg-resistant bacteria were found on the episphere than in endospheric habitats. Hg-resistant isolates exhibiting plant growth-promoting (PGP) traits, when used as inoculants of Capsicum annuum, were shown to increase its root dry biomass but not Hg concentration. The N₂-fixing and Hg-resistant species Pseudomonas graminis, observed in the poplar phyllosphere, may be a key microorganism for the restoration of industrial tailings dumps.

Brassica napus has a key role in the recovery of the health of soils contaminated with metals and diesel by rhizoremediation

Contaminated soils are frequently characterized by the simultaneous presence of organic and inorganic contaminants, as well as a poor biological and nutritional status. Rhizoremediation, the combined use of phytoremediation and bioremediation, has been proposed as a Gentle Remediation Option to rehabilitate multi-contaminated soils. Recently, newer techniques, such as the application of metallic nanoparticles, are being deployed in an attempt to improve traditional remediation options. In order to implement a phytomanagement strategy on calcareous alkaline peri-urban soils simultaneously contaminated with several metals and diesel, we evaluated the effectiveness of Brassica napus L., a profitable crop species, assisted with organic amendment and zero-valent iron nanoparticles (nZVI). A two-month phytotron experiment was carried out using two soils, i.e. amended and unamended with organic matter. Soils were artificially contaminated with Zn, Cu and Cd (1500, 500 and 50mgkg^-1, respectively) and diesel (6000mgkg^-1). After one month of stabilization, soils were treated with nZVI and/or planted with B. napus. The experiment was conducted with 16 treatments resulting from the combination of the following factors: amended/unamended, contaminated/non-contaminated, planted/unplanted and nZVI/no-nZVI. Soil physicochemical characteristics and biological indicators (plant performance and soil microbial properties) were determined at several time points along the experiment. Carbonate content of soils was the crucial factor for metal immobilization and, concomitantly, reduction of metal toxicity. Organic amendment was essential to promote diesel degradation and to improve the health and biomass of B. napus. Soil microorganisms degraded preferably diesel hydrocarbons of biological origin (biodiesel). Plants had a remarkable positive impact on the activity and functional diversity of soil microbial communities. The nZVI were ineffective as soil remediation tools, but did not cause any toxicity. We concluded that rhizoremediation with B. napus combined with an organic amendment is promising for the phytomanagement of calcareous soils with mixed (metals and diesel) contamination.

Trace element phytoextraction from contaminated soil: a case study under Mediterranean climate

The current field study aims to assess the suitability of four different plant species (i.e. poplar, willow, hemp and alfalfa) to be used for trace element (TE) (i.e. Cd, Cu, Ni, Pb and Zn) phytoextraction under hot-arid Mediterranean climate conditions. Plants were grown for two consecutive years on a moderate TE contaminated soil, supplied with water and mineral nutrients. The growth and physiological parameters were assessed throughout the trial to compare the response of plants to the environmental pollution, and TE uptake rates were measured for aboveground plant tissues. The phytoextraction rate for each species was expressed as a function of aboveground biomass yield and the TE uptake and translocation within the plant. Alfalfa played a significant role in reducing extractable Ni (60.6%) and Zn (46%) in the soil, whereas hemp reduced 32% of extractable Cd and 46% of extractable Pb; poplar decreased extractable Cd (37%), Ni (49%), Pb (46%) and Zn (63%); and willow reduced the extractable Zn (73%) compared to the beginning of the trial. No change in total TE content was observed; however, poplar and willow were able to extract and accumulate the highest amount of Zn (3200 and 5200 g ha^-1 year^-1 respectively) and Cu (182 and 116 g ha^-1 year^-1), whereas hemp, with 36 g ha^-1 year^-1, showed the best phytoextraction potential for Pb. Overall, we found a positive correlation between the phytoextraction rate and biomass yield, extractable TE concentration and translocation factor (TF) and a negative relationship with Ca concentration in the soil.

Phytomanagement of radionuclides and heavy metals in mangrove sediments of Pattani Bay, Thailand using Avicennia marina and Pluchea indica

This study determines uptake and accumulation of radionuclides and heavy metals by Pluchea indica and Avicennia marina and evaluates phytoremediation potential via greenhouse and field experiments. P. indica and A. marina are considered excluders for ⁴⁰K and ²⁶²Ra, and Pb since roots accumulated them in higher quantities compared to other plant parts, and the bioconcentration factor (BCF) and transfer factor (TF) values for Pb, and ⁴⁰K and ²⁶²Ra were >1, respectively. Absorbed dose rate in air (D) showed significant values in sediments, which were generally over the maximum recommended value of 55nGyh^-1. Phytostabilization of radionuclides and heavy metals may serve as an appropriate strategy for mangrove-polluted areas. D values in sediments were considered sufficiently high to recommend long-term monitoring. Radionuclide activities may increase in the food chain via uptake and accumulation of edible plants, ultimately resulting in harm to human health.

Safe utilization of heavy-metal-contaminated farmland by mulberry tree cultivation and silk production

Heavy-metal-contaminated soil threatens human health and environmental safety. Complete remediation of contaminated soil is expensive; therefore, phytomanagement has emerged as a cost-effective alternative. The current study investigated mulberry tree (Morus alba) plantation, silkworm cultivation, and silk production as phytomanagement options. Results defined the safety of planting mulberry trees in soils with lead (Pb) and arsenic (As) concentrations lower than 369 and 180mgkg^-1, respectively. Silkworms fed with mulberry leaves collected from slightly contaminated soil exhibited productive growth and normal silk production. The silk, silkworm chrysalis, and silkworm fecal matter reached the national standards for textiles, feed, and agricultural sludge, respectively. Based on risk assessment, planting mulberry trees instead of rice significantly decreased the human health risk from contaminated soil. The total carcinogenic risk (Risk_d) and noncarcinogenic risk (HQ_d) derived from mulberry tree plantation and textile production were 2.4×10^-8 and 6.7×10^-5, respectively, whereas those derived from rice plantation and ingestion were 0.44 and 18.4, respectively. Cost-benefit analysis showed that a mulberry tree plantation can yield $25,675 for every 1ha soil, whereas a rice plantation can yield $8409 for the same area. Moreover, phytoextraction requires $50,000-$150,000 to remediate 1ha of soil. Therefore, constructing a mulberry tree plantation with silkworm culture and silk production is a safe means to utilize slightly contaminated soil.

Comparison of three types of oil crop rotation systems for effective use and remediation of heavy metal contaminated agricultural soil

Selecting suitable plants tolerant to heavy metals and producing products of economic value may be a key factor in promoting the practical application of phytoremediation polluted soils. The aim of this study is to further understand the utilization and remediation of seriously contaminated agricultural soil. In a one-year field experiment, we grew oilseed rape over the winter and then subsequently sunflowers, peanuts and sesame after the first harvest. This three rotation system produced high yields of dry biomass; the oilseed rape-sunflower, oilseed rape-peanut and oilseed rape-sesame rotation allowed us to extract 458.6, 285.7, and 134.5 g ha^-1 of cadmium, and 1264.7, 1006.1, and 831.1 g ha^-1 of lead from soil, respectively. The oilseed rape-sunflower rotation showed the highest phytoextraction efficiency (1.98%) for cadmium. Lead and cadmium in oils are consistent with standards after extraction with n-hexane. Following successive extractions with potassium tartrate, concentrations of lead and cadmium in oilseed rape and peanut seed meals were lower than levels currently permissible for feeds. Thus, this rotation system could be useful for local farmers as it would enable the generation of income during otherwise sparse phytoremediation periods.