Heavy metal accumulation by poplar in calcareous soil with various degrees of multi-metal contamination: implications for phytoextraction and phytostabilization

Recalcification stabilizes cadmium but magnifies phosphorus limitation in wastewater-irrigated calcareous soil

Long-term wastewater irrigation leads to the loss of calcium carbonate (CaCO3) in the tillage layer of calcareous land, which irreversibly damages the soil's ability to retain cadmium (Cd). In this study, we selected calcareous agricultural soil irrigated with wastewater for over 50 years to examine the recalcification effects of sugar beet factory lime (SBFL) at doses of 0%, 2.5%, 5%, and 10%. We found that SBFL promoted Cd transformation in the soil from active exchangeable species to more stable carbonate-bonded and residual species, which the X-ray diffraction patterns also confirmed results that CdSO4 reduced while CdS and CaCdCO3 increased. Correspondingly, the soil bioavailable Cd concentration was significantly reduced by 65.6-84.7%. The Cd concentrations in maize roots and shoots were significantly reduced by 11.7-50.6% and 13.0-70.0%, respectively, thereby promoting maize growth. Nevertheless, SBFL also increased the proportion of plant-unavailable phosphorus (P) in Ca8-P and Ca10-P by 4.3-13.0% and 10.7-25.9%, respectively, reducing the plant-available P (Olsen P) content by 5.2-22.1%. Consequently, soil P-acquiring associated enzyme (alkaline phosphatase) activity and microbial (Proteobacteria, Bacteroidota, and Actinobacteria) community abundance significantly increased. Our findings showed that adding SBFL to wastewater-irrigated calcareous soil stabilized Cd, but exacerbated P limitation. Therefore, it is necessary to alleviate P limitations in the practice of recalcifying degraded calcareous land.

Tradeoffs between pH, dissolved organic carbon, and mineral ions regulate cadmium uptake by Solanum hyperaccumulators in calcareous soil

Soil solution pH and dissolved organic carbon (DOC) influence cadmium (Cd) uptake by hyperaccumulators but their tradeoff in calcareous soils is unclear. This study investigated the mechanisms of Solanum nigrum L. and Solanum alatum Moench in calcareous soil using a combination of concentration gradient experiments (0.6-100 mg Cd kg-1) and soil solution composition analysis. The results showed that the soil solution pH of S. nigrum remained stable despite Cd stress. On average, the soil solution pH of S. alatum was 0.23 units higher than that of S. nigrum, although pH decreased significantly under high Cd stress. In addition, the concentrations of potassium (K) and calcium (Ca) in the soil solution of S. nigrum increased and decreased under low and high levels of Cd stress, respectively. In S. alatum, the K and Ca concentrations in the soil solution generally increased with increasing Cd stress levels. Moreover, the level of DOC in the soil solution of both plants was higher under Cd stress compared to the control, and a gradually increasing trend with Cd stress level was observed in S. alatum. Consequently, the bioconcentration factors of the roots (2.62-19.35) and shoots (1.20-9.59) of both plants were >1, while the translocation factors were <1, showing an obstacle of Solanum hyperaccumulators in transferring Cd into their aboveground parts. Redundancy analysis revealed that the Cd concentration in S. nigrum roots was significantly negatively correlated with the soil solutions of K and Ca. In contrast, Cd concentrations in S. alatum roots and shoots were significantly positively correlated with soil solution DOC, K, and Ca but negatively correlated with pH. Our results suggest that calcareous soil neutralizes the acidity of released protons but does not affect cation exchange, inhibiting DOC in assisting the translocation of Cd within plants.

Comparison of organic and synthetic amendments for poplar phytomanagement in copper and lead-contaminated calcareous soil

Soil remediation can be achieved through organic and synthetic amendments, but the differences in the phytomanagement of trace metal-contaminated land are unclear. We conducted an outdoor microcosm experiment to simulate the effects of organic amendment citric acid and synthetic amendments EDTA and EGTA on poplar phytomanagement of copper (Cu)- and lead (Pb)-contaminated calcareous land at doses of 0, 1, 3, and 9 mmol kg-1. We found that soil-bioavailable Cu and Pb contents increased by 2.11-27.27 and 1.48-269 times compared to the control, respectively. Additionally, synthetic amendments had a long-lasting (within 25 days) effect on metal bioavailability relative to organic amendments. Consequently, organic amendments increased the root Cu and Pb contents by 2.68-48.61% and 6.60-49.51%, respectively, whereas synthetic amendments increased them by 65.94-260% and 12.50-103%. The Cu and Pb contents in the leaves were lower than those in the roots, and increased significantly by 47.04-179% and 237-601%, respectively, only under synthetic amendments. Interestingly, none of the amendments increased the Cu and Pb content in poplar stems (<5 mg kg-1), which remained within the normal range for terrestrial plants. Regardless of the type and addition level, the amendments did not affect poplar growth. Nevertheless, synthetic amendments caused a significant redistribution of metals (Cu: 22-32%; Pb: 23-53%) from the topsoil into the subsoil within the root zone at medium and high levels relative to organic amendments. Therefore, organic and synthetic amendments can assist poplar phytomanagement with a phytostabilization strategy for Cu- and Pb-contaminated calcareous land and obtain marketable wood biomass. Moreover, collecting leaf litter is crucial when using synthetic amendments at optimum concentration levels.

Comparative performance of Populus spp. and Salix spp. for growth, nutrition, and heavy metal uptake in a wastewater hydroponic system

This research aimed to ascertain the growth, biomass, and phytoremediation capacity of poplars and willow cultivated using wastewater in a hydroponic system. The cuttings were exposed to two water treatments for eight weeks: (1) tap water supply with 1/4 strength Hoagland's solution (TW) as a control and (2) urban raw wastewater with 1/4 strength Hoagland's solution (WW) in a completely randomized experimental design and growth parameters and nutrient and heavy metal content distribution in various plant tissues were assessed. Our results suggest that compared with the TW, seven studied species/clones revealed an increase in growth and biomass parameters (stem height by 16.4%, and root, shoot, and total biomass by 37.3%, 26.9%, and 29.3%, respectively) caused by WW treatment, except the root length and stem diameter that remained the same under two water treatments. Poplars and willow showed a remarkable variability in growth and biomass parameters, with the highest stem diameter, stem height, and root, shoot, and total biomass in Populus nigra L. 62.154. Expression of growth responses to WW treatment with tolerance indices (Tis) indicated the tested poplars and willow as highly tolerant (Ti >100%) with no significant differences among them based on this index. Plant analysis showed that WW treatment increased the concentrations of nutrients and heavy metals in the shoots and roots. Overall, under both water treatments, the capability of the tested species/clones to uptake and accumulate micro-nutrients (except copper (Cu) in shoots) and heavy metals (except chromium (Cr) in shoots) in the plant tissues varied noticeably. However, for all tested plants, the roots had higher concentrations of micro-nutrients (iron (Fe), zinc (Zn), and Cu concentrations in the roots were about 36.8%, 107.6%, and 30.1% of that in the shoots, respectively) and heavy metals (nickel (Ni), Cr, and lead (Pb) concentrations in the roots were about 115.3%, 344.2%, and 198.9% of that in the shoots, respectively), suggesting their capability for micro-nutrients and heavy metals rhizofiltration. Concerning obtained results under hydroponic culture, it can be concluded that these poplars and willow might be promising candidates for wastewater applications. However, data obtained by a hydroponic system need to be confirmed in pot and field experiments.

Recent progress on emerging technologies for trace elements-contaminated soil remediation

Abiotic stresses from potentially toxic elements (PTEs) have devastating impacts on health and survival of all living organisms, including humans, animals, plants, and microorganisms. Moreover, because of the rapid growing industrial activities together with the natural processes, soil contamination with PTEs has pronounced, which required an emergent intervention. In fact, several chemical and physical techniques have been employed to overcome the negative impacts of PTEs. However, these techniques have numerous drawback and their acceptance are usually poor as they are high cost, usually ineffectiveness and take longer time. In this context, bioremediation has emerged as a promising approach for reclaiming PTEs-contaminated soils through biological process using bacteria, fungus and plants solely or in combination. Here, we comprehensively reviews and critically discusses the processes by which microorganisms and hyperaccumulator plants extract, volatilize, stabilize or detoxify PTEs in soils. We also established a multi-technology repair strategy through the combination of different strategies, such as the application of biochar, compost, animal minure and stabilized digestate for stimulation of PTE remediation by hyperaccumulators plants species. The possible use of remote sensing of soil in conjunction with geographic information system (GIS) integration for improving soil bio-remediation of PTEs was discussed. By synergistically combining these innovative strategies, the present review will open very novel way for cleaning up PTEs-contaminated soils.

Heavy metal uptake by plant parts of Populus species: a meta-analysis

Populus species are well documented for being potentially suitable for phytoremediation purposes regarding their accumulation characteristics. However, published results are contradictory. Based on the data gathered during an extensive literature search, we aimed to assess and revise the metal accumulation potential in the root, stem, and leaf of Populus species growing in contaminated soils, with meta-analysis. We evaluated the influences of pollution level, soil pH, and exposure time on the metal uptake patterns. We found accumulations of Cd, Cr, Cu, Pb, and Zn to be significant in each plant part, while that was only moderate for Ni, and limited for Mn. By calculating the soil pollution index (PI), we observed significantly intensive, PI-independent accumulation for Cd, Cr, Cu, Ni, Pb, and Zn. A decrease in soil pH significantly increased the uptake of Mn and significantly decreased the accumulation of Pb in the stem. Metal uptake was significantly influenced by exposure time as well; Cd concentration was significantly decreased in the stem, while concentrations of Cr in the stem and leaf, and Mn in the stem were significantly increased with time. These aforementioned findings support a well-founded metal-and-growth condition-specific application of poplars in phytoremediation processes, also triggering further in-depth assessments to enhance the efficiency of relevant poplar-based technologies.

Evaluation of Miscanthus × giganteus Tolerance to Trace Element Stress: Field Experiment with Soils Possessing Gradient Cd, Pb, and Zn Concentrations

Miscanthus × giganteus demonstrated good phytostabilization potentials by decreasing the trace elements (T.E.s) mobility and enhancing the degraded soil quality. Nevertheless, most of the published work was performed under controlled conditions in ex situ pot experiments and/or with soils being spiked. Hence, data about the plant's tolerance to increased T.E. concentrations in real conditions is still scarce and requires further investigation. For this sake, a field experiment was established by cultivating miscanthus plants in three different agricultural plots representing gradient trace element (Cd, Pb and Zn) concentrations. Another uncontaminated plot was also introduced. Results showed that T.E. concentrations in the leaves were tolerable to the plant. In addition, no variations were detected between the miscanthus cultivated in the contaminated and uncontaminated soils at the level of antioxidant enzymatic activities (ascorbate peroxidase and superoxide dismutase), photosynthetic pigments (chlorophyll a and b and carotenoids), and secondary metabolites (phenolic compounds, flavonoids, anthocyanins, and tannins). These outcomes validate the high capacity of miscanthus to resist and tolerate contaminated conditions. Such results may contribute to further understanding of the miscanthus tolerance mechanisms.

Effects of urban wastewater application on growth, biomass, nutrition, and heavy-metal accumulation of Populus nigra L. "62/154," P. alba L. "20/45," P. euramericana (Dode) Guinier "92/40," and Salix excelsa S.G. Gmel grown in heavy-metal contaminated soil

Iran is located in a dry climate zone, and climate change has substantially reduced its precipitation and water resources. Reusing wastewaters from urban communities can meet some requirements for irrigation and fertilization of tree plantations in arid environments, leading to sustainable wastewater recycling, enhanced biomass production, and reduced land degradation. The objective of this study was to test the growth, biomass, nutrition, and heavy-metal accumulation of poplars [Populus nigra L. "62/154," P. alba L. "20/45," P. euramericana (Dode) Guinier "92/40"], and willow (Salix excelsa S.G. Gmel) in a pot experiment at four and eight months after planting when grown in soils irrigated with tap water (SITW) and wastewater (SIWW). After four months, SIWW treatment had no significant effect on growth, biomass, nor absorption of macronutrients. After eight months, SIWW treatment of poplars and willow significantly (p = 0.000) increased: (1) height, (2) leaf area, (3) root, stem, leaf, and total biomass, and (5) phytoextraction and phytoaccumulation of macro-/micro-nutrients and heavy metals in tree tissues, over trees receiving the SITW treatment. There were significant differences in growth, biomass, and accumulation of micronutrients and heavy metals in poplar versus willow tissues, with the highest biomass production and tissue-specific content of heavy metals in P. nigra trees, and the greatest total concentrations of heavy metals in P. alba and S. excelsa trees. In contrast, uptake of Fe, Cu, Ni, Cr and Pb were similar between poplar and willow, and phytoaccumulation of these elements was primarily in the roots. Leaf concentrations were highest for Zn and Mn. While P. nigra outperformed all other species overall, tolerance index (TI; defined as the tolerance to the heavy metals as calculated by the ratio of the biomass of SIWW trees relative to SITW trees) values exceeding 100% for all one-year-old poplar and willow trees demonstrated that they can be considered for planting in soil affected by urban wastewaters with similar contaminant profiles as in the current study.

New approach strategy for heavy metals immobilization and microbiome structure long-term industrially contaminated soils

The progress of engineering technologies highly influences the development of methods that lead to the condition improvement of areas contaminated with heavy metals (HMs). The aided phytostabilization fits into this trend, and was used to evaluate HM-immobilization effectiveness in phytostabilized soils under variable temperatures by applying 16 freezing-thawing cycles (FTC). Diatomite amendment and Lolium perenne L., also were applied. Cd/Ni/Cu/Pb/Zn each total content in phytostabilized soils were determined, along with the verification for each metal of its distribution in four extracted fractions (F1 ÷ F4) from soils. Based on changes in HM distribution, each metal's stability was estimated. Moreover, HM accumulation in plant roots and stems and soil microbial composition were investigated. Independently of the experimental variant (no-FTC-exposure or FTC-exposure), the above-ground biomass yields in the diatomite-amended series were higher as compared to the corresponding control series. The evident changes in Pb/Zn-bioavailability were observed. The metal stability increase was mainly attributed to metal concentration decreasing in the F1 fraction and increasing in the F4 fraction, respectively. Diatomite increased Cd/Zn-stability in not-FTC-exposed-phytostabilized soils. FTC-exposure favorably influenced Pb/Zn stability. Diatomite increased soil pH values and Cd/Ni/Cu/Zn-bioaccumulation (except Pb) in roots than in stems (in both experimental variants). FTC-exposure influenced soil microbial composition, increasing bacteria abundance belonging to Actinobacteria, Gammaproteobacteria, and Sphingobacteria. At the genus level, FTC exposure significantly increased the abundances of Limnobacter sp., Tetrasphaera sp., Flavobacterium sp., and Dyella sp. Independently of the experimental variant, Sphingomonas sp. and Mycobacterium sp., which have a tolerance to HM contamination, were core bacterial groups, comprising about 6 ÷ 7% of all soil bacteria.

Response of Three Miscanthus × giganteus Cultivars to Toxic Elements Stress: Part 2, Comparison between Two Growing Seasons

The positive impact on restoring soil functionality, decreasing toxic elements (TE) bioaccessibility, and enhancing soil physicochemical and biological parameters established a consensus on considering a Miscanthus × giganteus convenient species for phytomanaging wide TE contaminated areas. Nevertheless, information about the plant's mode of reaction to elevated soil multi-TE concentrations is still scarce. For the sake of investigating the miscanthus response to stressful TE concentrations, an ex-situ pot experiment was initiated for 18 months, with three miscanthus cultivars referred to as B, U, and A planted in soils with gradient Cd, Pb, and Zn concentrations. A non-contaminated control soil was introduced as well, and plants were cultivated within. Results revealed that the long exposure to increasing soil TE concentrations caused the number of tillers per plant to decline and the TE concentrations in the leaves to boost progressively with the soil contamination. The photosynthetic pigments (chlorophyll a, b, and carotenoids) were negatively affected as well. However, the phenolic compounds, flavonoids, tannins, and anthocyanins, along with the antioxidant enzymatic activities of superoxide dismutase, ascorbate peroxidase, and glutathione reductase elevated progressively with the TE concentration and exposure duration. Conclusively, miscanthus plants demonstrated an intensified and synchronized antioxidative activity against the TE concentration.

Field Testing of Selected Salt-Tolerant Screened Balsam Poplar (Populus balsamifera L.) Clones for Use in Reclamation around End-Pit Lakes Associated with Bitumen Extraction in Northern Alberta

For the oil sands mine sites in northern Alberta, the presence of salty process affected water, a byproduct of the hot-water bitumen extraction process, is anticipated to pose a challenge on some reconstructed landforms. The fundamental challenge when re-vegetating these sites is to ensure not only survival, but vigorous growth where plants are subjected to conditions of high electrical conductivity owing to salts in process affected water that may be contained in the substrate. Finding plants suitable for high salt conditions has offered the opportunity for Alberta-Pacific Forest Industries Inc. (Al-Pac) to investigate the potential role of using native balsam poplar (Populus balsamifera L.) as a key reclamation species for the oil sands region. Two years of greenhouse screening (2012 and 2013) of 222 balsam poplar clones from Al-Pac’s balsam poplar tree improvement program, using process affected discharge water from an oil sands processing facility in Ft. McMurray, has suggested an opportunity to select genetically suitable native clones of balsam poplar for use in reclamation of challenging sites affected by process water. In consideration of the results from both greenhouse and field testing, there is an opportunity to select genetically suitable native clones of balsam poplar that are tolerant to challenging growing conditions, making them more suitable for planting on saline sites.

Redistribution of calcium and sodium in calcareous soil profile and their effects on copper and lead uptake: A poplar-based phytomanagement

Poplar serves as a phytostabilizator in phytomanagement of the trace metals (TMs) copper (Cu) and lead (Pb) contaminated land. In the process of long-term phytomanagement, it is not clear how the cycling of the mineral nutrients calcium (Ca) and sodium (Na) in calcareous soil will affect poplar remediation mechanisms. We selected a site contaminated by Cu and Pb and phytomanaged by Populus simonii Carr. stands of different ages (7, 14, and 28 years) to study the influencing mechanisms. The results showed that after afforestation, the Ca in the subsoil returned to the topsoil through fallen leaves, whereas the Na in the topsoil migrated downward to the subsoil by leaching, resulting in the redistribution of mineral nutrients in the soil profile. In addition, the Ca content in soil solution of the root-zone was significantly lower relative to that of the bulk soil, whereas the Na content in soil solution was significantly higher in all stands. As a result, because of the competitive adsorption of mineral nutrient and TM cations on the soil surface, the pool of bioavailable TM in root-zone soils did not significantly decrease with stand age. On the contrary, the TM content in poplar leaves (Cu: 31-37 mg kg^-1; Pb: 62-84 mg kg^-1) and litter (Cu: 230-790 mg kg^-1; Pb: 394-1366 mg kg^-1) increased significantly with stand age. Nevertheless, the TM content in poplar wood (Cu < 3 mg kg^-1; Pb < 12 mg kg^-1) remained at an extremely low level in all stands. Our results highlighted that strengthening leaf collection is necessary to eliminate ecological risks and ensure the safe production of poplar wood in the long-term phytomanagement of TM-contaminated land.

Extracting cadmium in the presence of salt: a study on three poplar clones under controlled conditions

This study aimed at determining the cadmium phytoextraction potential of three Populus alba L. clones cultivated in the presence of increasing sodium chloride concentrations. Plantlets of a commercial and two autochthonous poplar clones were grown in perlite with nutrient solution enriched in CdSO₄ (50 and 100 μM) and NaCl (25 and 50 mM), administered either alone or in combination. The three clones showed significant variation not only in cadmium and salt tolerance, accumulation and content, but also in the effect of the interaction between the two elements on these parameters. The toxic effect of Cd and salt excess on plants was mutually exacerbated by the presence of both. Even though the outcome of the joint treatment was always a decrease in shoot Cd or Na accumulation, the three clones showed variation in the extent of such reduction. Evaluating the total element content per plant shoot, the fast-growing commercial clone displayed the highest phytoextraction potential for Cd and Na, either alone or in mixture. Our results demonstrated for the first time that the Cd response in presence of salt can vary in the different clones.

Temporal changes of calcareous soil properties and their effects on cadmium uptake by wheat under wastewater irrigation for over 50 years

Calcareous soil has a strong buffering capacity for neutralizing acid and stabilizing cadmium (Cd) because of the high calcium carbonate (CaCO₃) content. However, it is not clear whether the buffering capacity of calcareous soil can be maintained after long-term wastewater irrigation. We selected a typical area in western China that has been irrigated with wastewater for over 50 years to study the temporal changes of soil properties and their effects on Cd uptake by wheat. The results showed that compared with the background level before the 1960s, the soil pH and CaCO₃ content in 2018 were lower by 0.80 units and 35%, respectively, while the soil organic matter (SOM) content, Olsen phosphorus (P) content, and soil total Cd content in 2018 increased by 1.54, 13.05, and 164 times, respectively. Due to the significant decrease in the soil pH and CaCO₃, the high load of soil total Cd and electrical conductivity, the low soil clay content, and the coupling of SOM with soil nitrogen and P, the input Cd was activated. Furthermore, the activated Cd was effectively taken up by wheat roots and transported to grains with the assistance of dissolved organic carbon. Our results highlight that long-term wastewater irrigation caused irreversible damage to soil buffering capacity, resulting in the Cd activation and the enhancement of Cd uptake by wheat.

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

Monitoring of a long term phytoremediation process of a soil contaminated by heavy metals and hydrocarbons in Tuscany

The purpose of this study was to monitor and model indicators of soil contamination, organic matter evolution and biochemical processes involved in a long-term phytoremediation process. Populus nigra L., Paulownia tomentosa Steud., Cytisus scoparius L. and natural vegetation were used in differently contaminated areas (high, medium and low levels of contamination). Parameters indicating contamination (total petroleum hydrocarbons (TPH) and heavy metals) and agronomic (C, N and P) and functional (enzyme activities) soil recovery were monitored for 3.5 years. Three subareas with different levels of contamination (high, medium and low) were identified according to the Nemerow Index. A considerable decrease in TPH (52% on average) over time in the whole site was measured, while the metal reduction was only of about 22% at surface level. A stimulation in metabolic soil processes and improvement in the chemical quality of the soil was also observed throughout the experimental site. Statistical analysis modelling showed that the contaminant content decreased following a one-phase decay model, while the dramatic increase in enzyme activities could be represented by an exponential growth equation. On the basis of our data, it is possible to conclude that the initial contamination level affected neither the decontamination process nor the improvement in soil quality, which occurred similarly in the three different contaminated areas.

Impact of poplar-based phytomanagement on metal bioavailability in low-phosphorus calcareous soil with multi-metal contamination

Bioavailability of trace metals (TMs) is the key component in the management of TM-contaminated soils; however, its impact mechanism is unclear in low-phosphorus (P) calcareous soils afforested by fast-growing tree species for a long duration (>10 years). We selected a site contaminated with multiple TMs and phytomanaged by poplar (Populus hopeiensis Hu & Chow) to study the impact mechanism of plant-soil interactions on TM bioavailability along a long-term chronosequence (i.e., 10, 15, 20, and 25 years). We found that phytomanagement significantly decreased soil organic carbon (SOC) content, soil total nitrogen (N) content, and soil C/P and N/P ratios with stand age, but did not significantly change soil total P content. In contrast, soil available P content significantly changed in rhizospheric soils compared with the bulk soil, suggesting the tight coupling between the amplification of P turnover and N availability. Soil pH in rhizospheric soils significantly decreased by 0.22 to 0.32 units, while calcium carbonate (CaCO₃) content decreased by 14% to 39%, as compared with the bulk soil. Bioavailable concentrations of cadmium, lead, and zinc were positively correlated with soil available P, whereas bioavailable nickel concentration was negatively correlated with soil pH. Furthermore, TM bioavailability in rhizospheric soils significantly increased with stand age, regardless of the metal type. Our results suggest that P mobilization associated with SOC depletion induced soil acidification followed by CaCO₃ dissolution, collectively leading to metal mobilization with stand age.

Anatomical and morphological changes of the juniper under the influence of heavy metals in condition of man-induced load

Reaction of the plant body in polluted conditions allows estimating the anthropogenic impact on the environment. The aim of this work is to study anatomical and morphological changes of juniper in terms of the polymetallic pollution of the environment and under the influence of heavy metals. The assessment of some heavy metals impact on juniper was conducted after the planting of seed samples in the soil and obtaining good germination. The juniper has been treated with heavy metal solutions in the lab for 4 months. To determine the anatomical structures of the leaves of the juniper, the samples were fixed by the Strasburger-Fleming method. Under the influence of urban air, negative anatomical and morphological changes of juniper were discovered, the significant increase in the thickness of the needles, the thickness of the conducting bundle and the thickness of the epidermis was noted, that, apparently, provides higher resistance to man-induced pollution. It is shown that the process of environmental restoration can be done due to self-purification by using adsorption properties of leaves of woody plants, for example juniper, that is able to absorb heavy metals (lead, cadmium, cobalt, chromium, nickel, zinc and copper) from urban air. It was first discovered that the effects of heavy metals (lead and nickel) as the nutrient medium leads to the significant changes of the leaves’ anatomical structure (the swelling of the phloem, some decondensation in the mesophyll). The research results allow recommending the juniper for phytoremediation, bioindication and landscape gardening of cities and settlements in different countries.

Temporal changes of metal bioavailability and extracellular enzyme activities in relation to afforestation of highly contaminated calcareous soil

Metal bioavailability and extracellular enzyme activity are two important indicators of soil quality in metal-contaminated soil. However, it is unclear how the chronosequence effect modifies these two factors in highly contaminated calcareous soils undergoing afforestation. We used Populus simonii Carr. and the calciphilous Ulmus macrocarpa Hance as contrasting tree species to study the chronosequence effect. We found that afforestation significantly increased soil total nitrogen (N) content as well as soil carbon (C)/phosphorus (P) and N/P ratios, but decreased soil total P content and soil C/N ratio, regardless of the tree species and stand age, suggesting strong P limitation. However, available P did not change significantly with stand age. In both tree species, P mobilization depleted soil organic matter through the priming effect of dissolved organic carbon, whereas the decrease in soil pH in the U. macrocarpa stands enhanced CaCO₃ dissolution, collectively reducing the capacity of the soil to immobilize metals, resulting in increased metal bioavailability with stand age. The activity of oxidase (dehydrogenase) was positively correlated with bioavailable zinc concentration, soil electrical conductivity, and soil total N content. Hydrolase activities (alkaline phosphatase, β-glucosidase, and urease) were significantly positively correlated with the ratios of soil C/N and C/P, soil pH, and CaCO₃, but negatively correlated with soil N/P ratio and bioavailable cadmium concentration. Increasing stand age was associated with the gradual recovery of oxidase activity and remarkable inhibition of hydrolase activity. Our results suggest that the combination of soil hydrolase activity and metal bioavailability can predict soil quality in the afforestation of highly contaminated soils.

Differential distribution of metals in tree tissues growing on reclaimed coal mine overburden dumps, Jharia coal field (India)

Opencast bituminous coal mining invariably generates huge amount of metal-polluted waste rocks (stored as overburden (OB) dumps) and reclaimed by planting fast growing hardy tree species which accumulate metals in their tissues. In the present study, reclaimed OB dumps located in Jharia coal field (Jharkhand, India) were selected to assess the accumulation of selected metals (Pb, Zn, Mn, Cu and Co) in tissues (leaf, stem bark, stem wood, root bark and root wood) of two commonly planted tree species (Acacia auriculiformis A.Cunn. ex Benth. and Melia azedarach L.). In reclaimed mine soil (RMS), the concentrations of pseudo-total and available metals (DTPA-extractable) were found 182-498 and 196-1877% higher, respectively, than control soil (CS). The positive Spearman's correlation coefficients between pseudo-total concentration of Pb and Cu (r = 0.717; p < 0.05), Pb and Co (r = 0.650; p < 0.05), Zn and Mn (0.359), Cu and Co (r = 0.896; p < 0.01) suggested similar sources for Pb-Cu-Co and Mn-Zn. Among the five tree tissues considered, Pb selectively accumulated in root bark, stem bark and leaves; Zn and Mn in leaves; and Cu in root wood and stem wood. These results suggested metal accumulation to be "tissue-specific". The biological indices (BCF, TF_leaf, TF_{stem bark} and TF_{stem wood}) indicated variation in metal uptake potential of different tree tissues. The study indicated that A. auriculiformis could be employed for Mn phytoextraction (BCF, TF_leaf, TF_{stem bark} and TF_{stem wood} > 1). The applicability of both the trees in Cu phytostabilization (BCF > 1; TF_leaf, TF_{stem bark} and TF_{stem wood} < 1) was suggested. The study enhanced knowledge about the selection of tree species for the phytoremediation of coal mine OB dumps and specific tree tissues for monitoring metal pollution.

Response to cadmium and phytostabilization potential of Platycladus orientalis in contaminated soil

The tolerance characteristics and phytostabilization potential of Platycladus orientalis grown in soil contaminated by cadmium (Cd) were studied using a greenhouse experiment. The results showed that the ornamental plant P. orientalis had high tolerance for Cd in contaminated soil at 24.6 mg·kg^-1 and its physiological activities were slightly affected after 203 days (d) of cultivation. Moreover, Cd in soil at 9.6 mg·kg^-1 was beneficial for P. orientalis growth, and the total biomass after 203 d cultivation was significantly (p < 0.05) increased by 35.03%, while the contents of chlorophyl a, chlorophyl b and carotenoid in leaves also increased by 20.84%, 44.06% and 28.25% compared to the control, respectively. Meanwhile, the Cd content in the tissues of P. orientalis was increased with both plant growth and the Cd content in the soil. The uptake of Cd in P. orientalis roots was greater than in shoots, with the Cd content in roots reaching 41.45 mg·kg^-1. P. orientalis, an ornamental plant, that accumulates Cd predominantly in its roots, can be suggested as a promising plant for phytostabilization in Cd-contaminated soil.

Non-enhanced phytoextraction of cadmium, zinc, and lead by high-yielding crops

Heavy metal soil contamination from mining and smelting has been reported in several regions around the world, and phytoextraction, using plants to accumulate risk elements in aboveground harvestable organs, is a useful method of substantially reducing this contamination. In our 3-year experiment, we tested the hypothesis that phytoextraction can be successful in local soil conditions without external fertilizer input. The phytoextraction efficiency of 15 high-yielding crop species was assessed in a field experiment performed at the Litavka River alluvium in the Příbram region of Czechia. This area is heavily polluted by Cd, Zn, and Pb from smelter installations which also polluted the river water and flood sediments. Heavy metal concentrations were analyzed in the herbaceous plants' aboveground and belowground biomass and in woody plants' leaves and branches. The highest Cd and Zn mean concentrations in the aboveground biomass were recorded in Salix x fragilis L. (10.14 and 343 mg kg^-1 in twigs and 16.74 and 1188 mg kg^-1 in leaves, respectively). The heavy metal content in woody plants was significantly higher in leaves than in twigs. In addition, Malva verticillata L. had the highest Cd, Pb, and Zn concentrations in herbaceous species (6.26, 12.44, and 207 mg kg^-1, respectively). The calculated heavy metal removal capacities in this study proved high phytoextraction efficiency in woody species; especially for Salix × fragilis L. In other tested plants, Sorghum bicolor L., Helianthus tuberosus L., Miscanthus sinensis Andersson, and Phalaris arundinacea L. species are also recommended for phytoextraction.

Opinion: Taking phytoremediation from proven technology to accepted practice

Phytoremediation is the use of plants to extract, immobilize, contain and/or degrade contaminants from soil, water or air. It can be an effective strategy for on site and/or in situ removal of various contaminants from soils, including petroleum hydrocarbons (PHC), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), solvents (e.g., trichloroethylene [TCE]), munitions waste (e.g., 2,4,6-trinitrotoluene [TNT]), metal(loid)s, salt (NaCl) and radioisotopes. Commercial phytoremediation technologies appear to be underutilized globally. The primary objective of this opinion piece is to discuss how to take phytoremediation from a proven technology to an accepted practice. An overview of phytoremediation of soil is provided, with the focus on field applications, to provide a frame of reference for the subsequent discussion on better utilization of phytoremediation. We consider reasons why phytoremediation is underutilized, despite clear evidence that, under many conditions, it can be applied quite successfully in the field. We offer suggestions on how to gain greater acceptance for phytoremediation by industry and government. A new paradigm of phytomanagement, with a specific focus on using phytoremediation as a "gentle remediation option" (GRO) within a broader, long-term management strategy, is also discussed.

Amelioration of iron mine soils with biosolids: Effects on plant tissue metal content and earthworms

The achievement of environmentally sound and economically feasible disposal strategies for biosolids is a major issue in the wastewater treatment industry around the world, including Swaziland. Currently, an iron ore mine site, which is located within a wildlife sanctuary, is being considered as a suitable place where controlled disposal of biosolids may be practiced. Therefore, this study was conducted to investigate the effects of urban biosolids on iron mine soils with regard to plant metal content and ecotoxicological effects on earthworms. This was done through chemical analysis of plants grown in biosolid-amended mine soil. Earthworm behaviour, reproduction and bioaccumulation tests were also conducted on biosolid-amended mine soil. According to the results obtained, the use of biosolids led to creation of soil conditions that were generally favourable to earthworms. However, plants were found to have accumulated Zn up to 346 mg kg^-1 (in shoots) and 462 mg kg^-1 (in roots). This was more than double the normal Zn content of plants. It was concluded that while biosolids can be beneficial to mine soils and earthworms, they can also lead to elevated metal content in plant tissues, which might be a concern to plant-dependant wildlife species. Nonetheless, it was not possible to satisfactorily estimate risks to forage quality since animal feeding tests with hyperaccumulator plants have not been reported. Quite possibly, there may be no cause for alarm since the uptake of metals from soil is greater in plants grown in pots in the greenhouse than from the same soil in the field since pot studies fail to mimic field conditions where the soil is heterogeneous and where the root system possesses a complex topology. It was thought that further field trials might assist in arriving at more satisfactory conclusions.

Maize (Zea mays L.) performance in organically amended mine site soils

Organic amendments play an important role in the eco-friendly remediation of degraded mine site soils. This study investigated the quality (essential nutrients and heavy metal content) of maize grown on organically amended soils from three active mines in Nigeria. Soil samples were collected randomly at 0-15 cm depth, air-dried and sieved. Five kg of soil were amended with poultry manure and sawdust (poultry manure only, sawdust only, poultry manure-sawdust mixtures in 3:1, 2:1 and 1:1 ratios) at 10 g kg(-1). Maize (Zea mays L.) seeds were planted and watered for two consecutive periods of 8 weeks, with the control and treatment experiments set up in the screenhouse in quadruples. Harvested tissues were weighed, dried, ground and digested. Chemical properties were determined using standard methods while atomic absorption spectrophotometry was used to determine total metal concentrations (Ca, Mg, Fe, Zn, Pb, Cd and Cu). ANOVA was used to test for significant differences among treatment groups in the various parameters. Application of poultry manure-sawdust mixtures significantly (p < 0.05) enhanced tissue dry matter yield, as well as N, P, K, and Na contents while Zn, Cd, Cu and Pb were immobilized to approximately 50-100%. Treatment with sawdust alone reduced tissue nutrient content resulting in depressed plant yield while poultry manure only though enhanced crop yield, contained higher heavy metal contents. Soil amendments comprised of poultry manure-sawdust mixtures can be effective remediation strategy for mine site soils, as these organic materials help replenish soil nutrients, immobilize heavy metals, and enhance food productivity.

Effect of heavy metals combined stress on growth and metals accumulation of three Salix species with different cutting position

This study aimed to compare growth performance and heavy metal (HM) accumulation at different cutting positions of Salix species grown in multi-metal culture. Three Salix species stems cut at different positions (apical to basal) were grown hydroponically for four weeks. The plants were then treated for three weeks with 0, 5, 10, and 20 μM Cd, Cu, Pb, and Zn, resulting in total metal concentrations of 0, 20, 40, and 80 μM. The growth parameters and HM content in shoots and initial cutting were measured. Results showed that, compared with S. fragilis, S. matsudana grew more poorly in uncontaminated condition but grew better and accumulated lower metal in shoots under mixed HM treatment. In addition, cuttings from apical parent stem position exhibited poorer growth performance before and after treatment, as well as greater metal content in shoots than base parts under the HM treatment. These results suggest that S. matsudana may undergo a special mechanism to hinder metals in the initial cutting, thus mitigating growth damage. The apical portion also showed poor resistance against the invasion of mixed HMs because of the immature structure. Therefore, in the selection of phytoremediation plants, metal accumulation ability is not proportional to growth performance.

A greenhouse trial to investigate the ameliorative properties of biosolids and plants on physicochemical conditions of iron ore tailings: Implications for an iron ore mine site remediation

An iron ore mine site in Swaziland is currently (2015) in a derelict state as a consequence of past (1964-1988) and present (2011 - current) iron ore mining operations. In order to control problems associated with mine wastes, the Swaziland Water Services Corporation (SWSC) recently (2013) proposed the application of biosolids in sites degraded by mining operations. It is thought that this practice could generally improve soil conditions and enhance plant reestablishment. More importantly, the SWSC foresees this as a potential solution to the biosolids disposal problems. In order to investigate the effects of biosolids and plants in soil physicochemical conditions of iron mine soils, we conducted two plant growth trials. Trial 1 consisted of tailings that received biosolids and topsoil (TUSB mix) while in trial 2, tailings received biosolids only (TB mix). In the two trials, the application rates of 0 (control), 10, 25, 50, 75 and 100 t ha(-1) were used. After 30 days of equilibration, 25 seeds of Cynodon dactylon were sown in each pot and thinned to 10 plants after 4 weeks. Plants were watered twice weekly and remained under greenhouse conditions for 12 weeks, subsequent to which soils were subjected to chemical analysis. According to the results obtained, there were significant improvements in soil parameters related to fertility such as organic matter (OM), water holding capacity (WHC), cation exchange capacity (CEC), ammonium [Formula: see text] , magnesium (Mg(2+)), calcium (Ca(2+)) and phosphorus ( [Formula: see text] ). With regard to heavy metals, biosolids led to significant increases in soil total concentrations of Cu, Zn, Cd, Hg and Pb. The higher concentrations of Zn and Cu in treated tailings compared to undisturbed adjacent soils are a cause for concern because in the field, this might work against the broader objectives of mine soil remediation, which include the recolonization of reclaimed sites by soil-dwelling organisms. Therefore, while biosolids contain important nutrients that may greatly improve physicochemical conditions and enhance vegetation reestablishment in mined soils, the threat of the build-up of higher levels of trace elements in treated tailings compared to surrounding adjacent soils must not be underestimated.

Assessment of arbuscular mycorrhizal fungi status and heavy metal accumulation characteristics of tree species in a lead-zinc mine area: potential applications for phytoremediation

To select suitable tree species associated with arbuscular mycorrhizal fungi (AMF) for phytoremediation of heavy metal (HM) contaminated area, we measured the AMF status and heavy metal accumulation in plant tissues in a lead-zinc mine area, Northwest China. All 15 tree species were colonized by AM fungi in our investigation. The mycorrhizal frequency (F%), mycorrhizal colonization intensity (M%) and spore density (SP) reduced concomitantly with increasing Pb and Zn levels; however, positive correlations were found between arbuscule density (A%) and soil total/DTPA-extractable Pb concentrations. The average concentrations of Pb, Zn, Cu and Cd in plant samples were 168.21, 96.61, 41.06, and 0.79 mg/kg, respectively. Populus purdomii Rehd. accumulated the highest concentrations of Zn (432.08 mg/kg) and Cu (140.85 mg/kg) in its leaves. Considerable amount of Pb (712.37 mg/kg) and Cd (3.86 mg/kg) were concentrated in the roots of Robinia pseudoacacia Linn. and Populus simonii Carr., respectively. Plants developed different strategies to survive in HM stress environment: translocating more essential metals (Zn and Cu) into the aerial parts, while retaining more toxic heavy metals (Pb and Cd) in the roots to protect the above-ground parts from damage. According to the translocation factor (TF), bioconcentration factor (BCF), growth rate and biomass production, five tree species (Ailanthus altissima (Mill.) Swingle, Cotinus coggygria Scop., P. simonii, P. purdomii, and R. pseudoacacia) were considered to be the most suitable candidates for phytoextraction and/or phytostabilization purposes. Redundancy analysis (RDA) showed that the efficiency of phytoremediation was enhanced by AM symbioses, and soil pH, Pb, Zn, and Cd levels were the main factors influencing the HM accumulation characteristics of plants.

The impact of poplar tree plantations for biomass production on the aquifer water budget and base flow in a Mediterranean basin

Poplar plantations are used for biomass production in many countries. These plantations are often located in areas where the tree roots can reach the water table of shallow aquifers to reduce irrigation costs and increase evapotranspiration, mainly during the summer. This study aims to assess the effects of these plantations on an aquifer water budget and on the stream flow of a Mediterranean basin (Santa Coloma River, 321.3 km(2) NE Spain). A numerical flow model was constructed to simulate shallow aquifers and to simulate the stream-aquifer interaction for a period of 9 years. Once the model was calibrated, different land use scenarios, such as deciduous forests, dry farming and irrigated farming, were simulated for comparison. The mass balance shows that poplar extracts an average of 2.40 hm(3) from the aquifer, i.e., approximately 18% of the average recharge of the modelled area. This effect reduces the groundwater flow to the main stream and increases the infiltration from the stream to the aquifer. As a result, there is an average reduction in the main stream flow by 46% during the summer, when the lowest flow occurs and when the river is most sensitive. The results indicate that these impacts should be considered in basin management plans and in evaluating the benefits of this type of biomass production.

An analytical deterministic model for simultaneous phytoremediation of Ni and Cd from contaminated soils

Soil contamination by heavy metals, due to human activities, is not often limited to a single contaminant. The objective of this study was to develop a simple model for phytoextracting separate and combined Ni and Cd from contaminated soils. The study was further aimed to study phytoextraction potential of ornamental kale and land cress grown in soils contaminated with separate and combined Ni and Cd metals. The results indicated that elevated Ni and Cd concentrations in soil inhibit growth of both ornamental kale and land cress plants. In Ni + Cd treatments, growth and development of both plants were more affected than in either Ni or Cd treatments. Further, in Ni + Cd treatments, Ni concentration in tissues of both plants was increased by increasing soil Ni concentration under various Cd concentrations. At constant Ni concentration, addition of Cd did not appreciably changed Ni content of plant tissues. Land cress demonstrated higher tolerance to soil contamination by Ni and Cd compared to ornamental kale. It also demonstrated higher phytoextraction potential for soil Cd than ornamental kale. Enhanced bioavailability of Ni and Cd ions, due to competitive adsorption and desorption reactions, had no reasonable effect on metal ion accumulation in plant tissues. This indicates that at relatively high soil contamination, metal ion adsorption is no longer a limiting factor for phytoremediation. The newly proposed model, which assumes that metal uptake rate inversely depends on total soil metal ion concentration, reasonably well predicted the cleanup time of Ni, Cd, and Ni at the presence of Cd from the contaminated soils. The model also predicts that phytoremediation process takes much longer time when soil is contaminated by multi-metal ions.

Phytoextraction of risk elements by willow and poplar trees

To characterize the phytoextraction efficiency of two clones of willow trees (Salix x smithiana Willd., Salix rubens) and two clones of poplar trees (Populus nigra x maximowiczii, Populus nigra Wolterson) were planted in contaminated soil (0.4-2.0 mg Cd.kg(-1), 78-313 mg Zn.kg(-1), 21.3-118 mg Cu.kg(-1)). Field experiment was carried out in Czech Republic. The study investigated their ability to accumulate heavy metals (Cd, Zn, and Cu) in harvestable plant parts. The poplars produced higher amount of biomass than willows. Both Salix clones accumulated higher amount of Cd, Zn and Cu in their biomass (maximum 6.8 mg Cd.kg(-1), 909 mg Zn.kg(-1), and 17.7 mg Cu.kg(-1)) compared to Populus clones (maximum 2.06 mg Cd.kg(-1), 463 mg Zn.kg(-1), and 11.8 mg Cu.kg(-1)). There were no significant differences between clones of individual species. BCs for Cd and Zn were greater than 1 (the highest in willow leaves). BCs values of Cu were very low. These results indicate that Salix is more suitable plant for phytoextraction of Cd and Zn than Populus. The Cu phytoextraction potential of Salix and Populus trees was not confirmed in this experiment due to low soil availability of this element.

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.

Different behaviours in phytoremediation capacity of two heavy metal tolerant poplar clones in relation to iron and other trace elements

Plant biodiversity and intra-population genetic variability have not yet been properly exploited in the framework of phytoremediation and soil reclamation. For this reason, iron and other metal accumulation capacity of two Cu and Zn tolerant poplar clones, namely AL22 (Populus alba L.) and N12 (Populus nigra L.), was investigated in a pot experiment. Cuttings of the two clones were planted in iron rich soil collected from an urban-industrial area. Concentrations of Cd, Cu, Fe, Pb and Zn were analysed in leaves (at different times), as well as in stems and in roots (at the end of the experiment), both in control plants and in plants grown on a soil whose Fe availability was artificially enhanced. Results showed that Cd and Zn were preferentially accumulated in leaves, whereas Cu, Fe and Pb were mainly accumulated in roots. The main differences in metal accumulation between clones were related to Cd (about tenfold higher concentrations in N12) and Cu (higher concentrations in AL22). Once soil Fe availability was enhanced, the uptake and accumulation of all metals declined, with the exception of Fe at the first sampling time in AL22 leaves. The different behaviour of the two poplar clones suggests that a thoughtful choice should be made for their use in relation to soil heavy metal remediation.