Root-induced changes of Zn and Pb dynamics in the rhizosphere of sunflower with different plant growth promoting treatments in a heavily contaminated soil

Combined effects of heatwaves and atmospheric CO₂ levels on Brassica juncea phytoremediation

Heatwaves, expected to become more frequent, pose a significant threat to plant biomass production. This experiment was designed to estimate heatwave influence on Brassica juncea phytoremediation when superimposed on different CO2 levels. A 7-day heatwave was generated during the species flowering stage. Heatwaves decreased all B. juncea dry weights. The lowest species dry weight was recorded when the heatwave was accompanied by 250 ppm CO2, in which the biomass significantly decreased by 40.0% relative to that of no heatwave under the same atmospheric CO2 conditions. Heatwave superposition with 250 ppm CO2 reduced the Cd content in B. juncea aerial parts by 28.1% relative to that of identical environmental conditions without heatwave, whereas the opposite result was observed under 550 ppm CO2 conditions. The heatwave caused oxidative damage to B. juncea under all CO2 conditions, as manifested by increased malondialdehyde levels in the plant shoots. With heatwave superposition, antioxidant enzyme activity was enhanced by exposure to 400 and 550 ppm CO2. Considering biomass yield generation and Cd uptake capacity, heatwave superposition decreased the B. juncea phytoremediation effects, and high atmospheric CO2 conditions could alleviate detrimental effects to a certain extent. This study uniquely examines the combined effects of heatwaves and varying CO2 levels on phytoremediation, providing microscopic insights into oxidative damage and enzyme activity, highlighting the potential for CO2 enrichment to mitigate heatwave impacts, and offering comprehensive analysis for future agricultural practices and environmental management.

Spraying or fertigation? looking for the better method in terms of the lower concentration of Pb and Ni in greenhouse cucumber

This study was conducted to investigate the status of Lead (Pb) and Nickel (Ni) in greenhouse cucumber affected by fertigation and spraying as a factorial split plot arrangement based on a randomized complete block design (RCBD) with three replications at Soil and Water Research Institute, Karaj, Iran in 2023. The main and sub-plots were respectively fertigation and spraying that was applied in two levels [the minimum permissible concentration of these metals in granular triple super phosphate fertilizer (five ppm Pb and five ppm Ni) as the control treatment, and the maximum permissible concentration of Pb and Ni in granular triple super phosphate fertilizer (50 ppm Pb and 100 ppm Ni)]. Spraying was done in three modes (spraying leaves, leaves + fruit, and fruit). Based on the results, the highest concentration of Pb in the fruit (fruit peel + fruit flesh) (0.295 mg/kg DW) was recorded in the fertigation + spraying with the maximum permissible concentrations of Pb and Ni treatments. A general comparison between the two fertilization methods showed that the spraying method accumulated a higher concentration of Pb in the fruit (i.e., peel + flesh) than the fertigation method. The reverse of this status happened for Ni, so the fertigation method accumulated a higher concentration of Ni in the fruit (skin + flesh) compared to the spraying method. On average, the order of Pb concentration in different organs was as follows: leaf (0.765 mg/kg) > fruit peel (0.232 mg/kg) > fruit peel + flesh (0.174 mg/kg) > fruit flesh (0.129 mg/kg). This order for Ni was as follows: fruit flesh (0.597 mg/kg) > fruit peel + flesh (0.345 mg/kg) > leaf (0.3 mg/kg) > fruit peel (0.175 mg/kg). These orders show that the status of the heavy metals in plant tissues is related to the nature of the metal, the type of the organ, and the method of fertilization.

Phytobial remediation advances and application of omics and artificial intelligence: a review

Industrialization and urbanization increased the use of chemicals in agriculture, vehicular emissions, etc., and spoiled all environmental sectors. It causes various problems among living beings at multiple levels and concentrations. Phytoremediation and microbial association are emerging as a potential method for removing heavy metals and other contaminants from soil. The treatment uses plant physiology and metabolism to remove or clean up various soil contaminants efficiently. In recent years, omics and artificial intelligence have been seen as powerful techniques for phytobial remediation. Recently, AI and modeling are used to analyze large data generated by omics technologies. Machine learning algorithms can be used to develop predictive models that can help guide the selection of the most appropriate plant and plant growth-promoting rhizobacteria combination that is most effective at remediation. In this review, emphasis is given to the phytoremediation techniques being explored worldwide in soil contamination.

Role of Plant-Growth-Promoting Rhizobacteria in Plant Machinery for Soil Heavy Metal Detoxification

Heavy metals migrate easily and are difficult to degrade in the soil environment, which causes serious harm to the ecological environment and human health. Thus, soil heavy metal pollution has become one of the main environmental issues of global concern. Plant-growth-promoting rhizobacteria (PGPR) is a kind of microorganism that grows around the rhizosphere and can promote plant growth and increase crop yield. PGPR can change the bioavailability of heavy metals in the rhizosphere microenvironment, increase heavy metal uptake by phytoremediation plants, and enhance the phytoremediation efficiency of heavy-metal-contaminated soils. In recent years, the number of studies on the phytoremediation efficiency of heavy-metal-contaminated soil enhanced by PGPR has increased rapidly. This paper systematically reviews the mechanisms of PGPR that promote plant growth (including nitrogen fixation, phosphorus solubilization, potassium solubilization, iron solubilization, and plant hormone secretion) and the mechanisms of PGPR that enhance plant-heavy metal interactions (including chelation, the induction of systemic resistance, and the improvement of bioavailability). Future research on PGPR should address the challenges in heavy metal removal by PGPR-assisted phytoremediation.

Integrated biochemical and transcriptomic analysis reveals the effects of Burkholderia sp. SRB-1 on cadmium accumulating in Chrysopogon zizanioides L. under Cd stress

Application of plant growth-promoting rhizobacteria plays a vital role in enhancing phytoremediation efficiency. In this study, multiple approaches were employed to investigate the underlying mechanisms of Burkholderia sp. SRB-1 (SRB-1) on elevating Cd uptake and accumulation. Inoculation experiment indicated that SRB-1 could facilitate plant growth and Cd tolerance, as evidenced by the enhanced plant biomass and antioxidative enzymes activities. Cd content in plant shoots and roots increased about 36.56%-39.66% and 25.97%-130.47% assisted with SRB-1 when compared with control. Transcriptomics analysis revealed that SRB-1 upregulated expression of amiE, AAO1-2 and GA2-ox related to auxin and gibberellin biosynthesis in roots. Auxin and gibberellin, as hormone signals, regulated plant Cd tolerance and growth through activating hormone signal transduction pathways, which might also contribute to 67.94% increase of dry weight. The higher expression levels of ATP-binding cassette transporter subfamilies (ABCB, ABCC, ABCD and ABCG) in Chrysopogon zizanioides roots contributed to higher Cd uptake in Cd15 B (323.83 mg kg^-1) than Cd15 (136.28 mg kg^-1). Further, SRB-1 facilitated Cd migration from roots to shoots via upregulating the expression of Nramp, ZIP and HMA families. Our integrative analysis provided a molecular-scale perspective on Burkholderia sp. SRB-1 contributing to C. zizanioides performance.

Selected rhizobacteria facilitated phytoremediation of barren and heavy metal contaminated gold mine tailings by Festuca arundinacea

Gold mine tailings pose a significant challenge for phytoremediation due to their poor nutrition and heavy metal pollution. Rhizobacteria-assisted phytoremediation is a promising method, yet limited research has been conducted on its application in gold mine tailings. In this study, rhizobacteria R1 (Bacillus paramycoides) and R2 (Klebsiella michiganensisW14T) were isolated from the rhizosphere of Festuca arundinacea (F. arundinacea) to enhance the phytoremediation of gold mine tailings. Our results showed that inoculation of R1 and R2 led to a significant increase in the average germination rates of F. arundinacea by 36.9% and 16.5%, respectively. Furthermore, the average plant height increased by 68.3% and 53.4%, respectively. Importantly, after inoculation with rhizobacteria, the contents of Mn, Pb, and As in F. arundinacea increased by 13.2-33.9%, 40.1-41.0%, and 98.1%-124.5%, respectively, indicating that the rhizobacteria enhanced the plant uptake of heavy metals. The improved nutrient content and enzyme activity in the tailings after inoculation with rhizobacteria were positively correlated with the heavy metal content in F. arundinacea. In addition, inoculation of rhizobacteria significantly altered the microbial community structure of the tailings, with Bacillus becoming the dominant genus in the rhizosphere tailings of F. arundinacea after R1 inoculation. Overall, our findings demonstrated that rhizobacteria R1 was better to enhance the phytoremediation of gold mine tailings. These results offer valuable insights into the mechanism of rhizobacteria-assisted phytoremediation and provide a practical method to enhance remediation of gold mine tailings.

Beneficial Microorganisms Improve Agricultural Sustainability under Climatic Extremes

The challenging alterations in climate in the last decades have had direct and indirect influences on biotic and abiotic stresses that have led to devastating implications on agricultural crop production and food security. Extreme environmental conditions, such as abiotic stresses, offer great opportunities to study the influence of different microorganisms in plant development and agricultural productivity. The focus of this review is to highlight the mechanisms of plant growth-promoting microorganisms (especially bacteria and fungi) adapted to environmental induced stresses such as drought, salinity, heavy metals, flooding, extreme temperatures, and intense light. The present state of knowledge focuses on the potential, prospective, and biotechnological approaches of plant growth-promoting bacteria and fungi to improve plant nutrition, physio-biochemical attributes, and the fitness of plants under environmental stresses. The current review focuses on the importance of the microbial community in improving sustainable crop production under changing climatic scenarios.

Composition and Structural Characteristics of Rhizosphere Microorganisms of Polygonum sibiricum (Laxm.) Tzvelev in the Yellow River Delta

The Polygonum sibiricum (Laxm.) Tzvelev, an important herbal species used to protect seawalls, has a solid resistance to salinity and alkali and can grow on alkali spots in saline–alkali soils. So far, the composition, population, and characteristics of its rhizosphere biological community related to the adaptation salt–alkali environment were still unknown. In the present study, rhizosphere and non-rhizosphere soil samples from the P. sibiricum on Chenier Island were collected. High-throughput sequencing was conducted to obtain the structural diversity of rhizosphere microbial communities. Our results showed that the dominant bacteria groups in the rhizosphere and non-rhizosphere were Proteobacteria, Actinobacteriota, Gemmatimonadota, and Actinobacteriota. The dominant fungi groups in the rhizosphere and non-rhizosphere soil samples were Ascomycota, Basidiomycota, Chytridiomycota, and Mortierellomycota. The results of the ASVs (amplicon sequence variants) showed that fungi have more ASVs in common. The PERMANOVA analysis showed that the bacteria among different groups were significantly different. The PCoA (principal coordinates analysis) study also showed that the structures of the bacterial and fungal communities between the rhizosphere and non-rhizosphere were distinct. Function results showed that the relative abundance in COG (Clusters of Orthologous Groups of proteins) functional annotation was significantly different between the two groups. In addition to the general function prediction and carbohydrate transport and metabolism, the COG of the non-rhizosphere was higher than that of the rhizosphere. Our findings benefited the knowledge for studying and conserving the molecule-level adaptive mechanisms of P. sibiricum.

Plant growth-promoting bacteria in metal-contaminated soil: Current perspectives on remediation mechanisms

Heavy metal contamination in soils endangers humans and the biosphere by reducing agricultural yield and negatively impacting ecosystem health. In recent decades, this issue has been addressed and partially remedied through the use of “green technology,” which employs metal-tolerant plants to clean up polluted soils. Furthermore, the global climate change enhances the negative effects of climatic stressors (particularly drought, salinity, and extreme temperatures), thus reducing the growth and metal accumulation capacity of remediating plants. Plant growth-promoting bacteria (PGPB) have been widely introduced into plants to improve agricultural productivity or the efficiency of phytoremediation of metal-contaminated soils via various mechanisms, including nitrogen fixation, phosphate solubilization, phytohormone production, and biological control. The use of metal-tolerant plants, as well as PGPB inoculants, should hasten the process of moving this technology from the laboratory to the field. Hence, it is critical to understand how PGPB ameliorate environmental stress and metal toxicity while also inducing plant tolerance, as well as the mechanisms involved in such actions. This review attempts to compile the scientific evidence on this topic, with a special emphasis on the mechanism of PGPB involved in the metal bioremediation process [plant growth promotion and metal detoxification/(im)mobilization/bioaccumulation/transformation/translocation] and deciphering combined stress (metal and climatic stresses) tolerance.

Vermicomposting of sewage sludge with organic bulking materials to improve its properties

Adverse properties of sewage sludge such as high concentration of toxic metals and the presence of pathogens have limited its use in agriculture. In this study, the effect of vermicomposting on some important chemical properties of sewage sludge was investigated as a factorial arrangement based on a completely randomized design with three replications. The studied factors were bulking agents in three concentrations (0, 5, and 10% v/v), earthworm (Eisenia fetida) in 2 different concentrations (0 and 50 adult earthworms/6 kg sewage sludge), and retention times in three levels (30, 60, and 90 days). Sewage sludge was obtained from the lagoons of Ghahdarijan refinery in Isfahan province in Iran. The results showed that vermicomposting significantly (p < 1%) reduced the number of coliforms and concentrations of Ni, Cr, and Pb and increased the organic carbon percent and N of the sewage sludge. When comparing different treatments, vermicomposting of sewage sludge with rice husk 10% v/v during retention times of 60 and 90 days were superior treatments in reducing the concentrations of Ni, Cr, and Pb and reducing the number of coliforms. Furthermore, the highest %OC was measured in the rice husk 10% + earthworm in the retention time of 90 days which was about 41% more than the control treatment. In general, it is concluded that vermicomposting significantly reduced the adverse properties of sewage sludge for using in agriculture as an organic fertilizer.

Helping plants to deal with heavy metal stress: the role of nanotechnology and plant growth promoting rhizobacteria in the process of phytoremediation

Heavy metals (HMs) are not destroyable or degradable and persist in the environment for a long duration. Thus, eliminating and counteracting the HMs pollution of the soil environment is an urgent task to develop a safe and sustainable environment. Plants are in close contact with the soil and can play an important role in soil clean-up, and the process is known as phytoremediation. However, under HM contaminated conditions, plants suffer from several complications, like nutrient and mineral deficiencies, alteration of various physiological and biological processes, which reduces the plant's growth rate. On the other hand, the bioavailability of HMs is another factor for reduced phytoremediation, as most of the HMs are not bioavailable to plants for efficient phytoremediation. The altered plant growth and reduced bioavailability of HMs could be overcome and enhance the phytoremediation efficiency by incorporating either nanotechnology, i.e., nanoparticles (NPs) or plant growth promoting rhizobacteria (PGPR) along with phytoremediation. Single incorporation of NPs and PGPR might improve the growth rate in plants by enhancing nutrient availability and uptake and also by regulating plant growth regulators under HM contaminated conditions. However, there are certain limitations, like a high dose of NPs that might have toxic effects on plants. Thus, the combination of two techniques such as PGPR and NPs-based remediation can conquer the limitations of individual techniques and consequently enhance phytoremediation efficiency. Considering the negative impacts of HMs on the environment and living organisms, this review is aimed at highlighting the concept of phytoremediation, the single or combined integration of NPs and PGPR to help plants deal with HMs and their basic mechanisms involved in the process of phytoremediation. Additionally, the complications of using NPs and PGPR in the phytoremediation process are discussed to determine future research questions and this will assist to stimulate further research in this field and increase its effectiveness in practical application.

Plants-Microorganisms-Based Bioremediation for Heavy Metal Cleanup: Recent Developments, Phytoremediation Techniques, Regulation Mechanisms, and Molecular Responses

Rapid industrialization, mine tailings runoff, and agricultural activities are often detrimental to soil health and can distribute hazardous metal(loid)s into the soil environment, with harmful effects on human and ecosystem health. Plants and their associated microbes can be deployed to clean up and prevent environmental pollution. This green technology has emerged as one of the most attractive and acceptable practices for using natural processes to break down organic contaminants or accumulate and stabilize metal pollutants by acting as filters or traps. This review explores the interactions between plants, their associated microbiomes, and the environment, and discusses how they shape the assembly of plant-associated microbial communities and modulate metal(loid)s remediation. Here, we also overview microbe-heavy-metal(loid)s interactions and discuss microbial bioremediation and plants with advanced phytoremediation properties approaches that have been successfully used, as well as their associated biological processes. We conclude by providing insights into the underlying remediation strategies' mechanisms, key challenges, and future directions for the remediation of metal(loid)s-polluted agricultural soils with environmentally friendly techniques.

Bioremediation of potentially toxic elements of sewage sludge using sunflower (Heliantus annus L.) in greenhouse and field conditions

The bioremediation of sewage sludge, containing potentially toxic elements (heavy metals), by the hyperaccumulator sunflower (Helianthus annus L.), was determined in greenhouse (G) and field (F) conditions in Isfahan, Iran. The soil pots, mixed with dried sewage sludge at 0, 15, 30, 45, and 60 mg/kg, were planted with sunflower seedlings and kept in the greenhouse (G) and in the field (F). Different soil physicochemical and plant biochemical properties including heavy metal uptake of nickel (Ni), chromium (Cr), lead (Pb), and cadmium (Cd) were determined. In contrast with the soil pH, soil salinity, organic matter, nitrogen, and not soil CaCO₃, were significantly enhanced by increasing sewage sludge. Sewage sludge was significant on plant uptake of Ni (2.27-4.25 mg/kg), Cr (3.27-4.75 mg/kg), Cd (13.85-15.27 mg/kg), and total chlorophyll (1.69-1.99 mg/g) in the greenhouse, and plant uptake of Ni (1.75-2.75 mg/kg) and Cd (1.37-2.25 mg/kg), and chlorophyll b (0.06-0.26 mg/g), total chlorophyll (0.57-1.16 mg/g), and carotenoids (1.10-1.61 mg/g) in the field. Although Pb was not significantly affected by sewage sludge, it showed the highest bioaccumulation factor of 0.96 at 15 mg/kg. Interestingly, the heavy metals were all positively and significantly correlated with each other and with plant carotenoids, similar to the positive and significant correlations between Pb with chlorophyll a and b. Accordingly, the increased levels of carotenoids, acting as antioxidant, may be an indicator of oxidative stress. Sunflower plants can be used as an efficient method for the bioremediation of the soils polluted with sewage sludge including Ni, Cr, and Cd.

Effects of decapitated and root-pruned Sedum alfredii on the characterization of dissolved organic matter and enzymatic activity in rhizosphere soil during Cd phytoremediation

Decapitation and root pruning, can impact plant morphological and physiological characteristics, which may determine the efficiency of phytoremediation. However, the effects of decapitated and root-pruned plants on the characterization of dissolved organic matter (DOM) and enzymatic activity, which determine the bioavailability of soil pollutants, have rarely been reported. This study aims to characterize DOM and enzymatic activity in the rhizosphere soil of Sedum alfredii when treated by decapitation and root pruning. Decapitation, slight pruning (10% root cutting), and their combination stimulated S. alfredii to secrete more DOM in the rhizosphere soil compared with the control. Furthermore, the proportions of hydrophilic increased from 42.7% in the control to 57.1% in the decapitation and slight pruning combination. Soil urease, invertase, and neutral phosphatase activities were higher in the rhizosphere soil of decapitated and root-pruned S. alfredii, and the highest values were observed with their combination. DOM from the soils of decapitated and root-pruned S. alfredii had significantly higher Cd extraction ability compared with that of the untreated species. Based on the findings of this study, we suggest that decapitation and root pruning can improve the phytoremediation efficiency of S. alfredii by increasing the bioavailability of Cd in its rhizosphere.

A Kinetic Approach for Assessing the Uptake of Ag from Pristine and Sulfidized Ag Nanomaterials to Plants

Nanomaterials (NMs) are thermodynamically unstable by nature, and exposure of soil organisms to NMs in the terrestrial environment cannot be assumed constant. Thus, steady-state conditions may not apply to NMs, and bioaccumulation modeling for uptake should follow a dynamic approach. The one-compartment model allows the uptake and elimination of a chemical to be determined, while also permitting changes in exposure and growth to be taken into account. The aim of the present study was to investigate the accumulation of Ag from different Ag NM types (20 nm Ag⁰ NMs, 50 nm Ag⁰ NMs, and 25 nm Ag₂ S NMs) in the crop plant wheat (Triticum aestivum). Seeds were emerged in contaminated soils (3 or 10 mg Ag/kg dry soil, nominal) and plants grown for up to 42 d postemergence. Plant roots and shoots were collected after 1, 7, 14, 21, and 42 d postemergence; and total Ag was measured. Soil porewater Ag concentrations were also measured at each sampling time. Using the plant growth rates in the different treatments and the changing porewater concentrations as parameters, the one-compartment model was used to estimate the uptake and elimination of Ag from the plant tissues. The best fit of the model to the data included growth rate and porewater concentration decline, while showing elimination of Ag to be close to zero. Uptake was highest for Ag⁰ NMs, and size did not influence their uptake rates. Accumulation of Ag from Ag₂ S NMs was lower, as reflected by the lower porewater concentrations. Environ Toxicol Chem 2021;40:1861-1872. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Isolation and characterization of salt-tolerant bacteria with plant growth-promoting activities from saline agricultural fields of Haryana, India

Background

Soil salinity has been one of the biggest hurdles in achieving better crop yield and quality. Plant growth-promoting rhizobacteria (PGPR) are the symbiotic heterogeneous bacteria that play an important role in the recycling of plant nutrients through phytostimulation and phytoremediation. In this study, bacterial isolates were isolated from salt-polluted soil of Jhajjar and Panipat districts of Haryana, India. The potential salt-tolerant bacteria were screened for their PGPR activities such as phosphate solubilization, hydrogen cyanide (HCN), indole acetic acid (IAA) and ammonia production. The molecular characterization of potent isolates with salt tolerance and PGPR activity was done by 16S rDNA sequencing.

Results

Eighteen soil samples from saline soils of Haryana state were screened for salt-tolerant bacteria. The bacterial isolates were analyzed for salt tolerance ranging from 2 to 10%. Thirteen isolates were found salt tolerant at varied salt concentrations. Isolates HB6P2 and HB6J2 showed maximum tolerance to salts at 10% followed by HB4A1, HB4N3 and HB8P1. All the salt-tolerant bacterial isolates showed HCN production with maximum production by HB6J2. Phosphate solubilization was demonstrated by three isolates viz., HB4N3, HB6P2 and HB6J2. IAA production was maximum in HB4A1 (15.89) and HB6P2 (14.01) and least in HB4N3 (8.91). Ammonia production was maximum in HB6P2 (12.3) and least in HB8P1 (6.2). Three isolates HB6J2, HB8P1 and HB4N3 with significant salt tolerance, and PGPR ability were identified through sequencing of amplified 16SrRNA gene and were found to be Bacillus paramycoides, Bacillus amyloliquefaciens and Bacillus pumilus, respectively.

Conclusions

The salt-tolerant plant growth-promoting rhizobacteria (PGPR) isolated from saline soil can be used to overcome the detrimental effects of salt stress on plants, with beneficial effects of physiological functions of plants such as growth and yield, and overcome disease resistance. Therefore, application of microbial inoculants to alleviate stresses and enhance yield in plants could be a low cost and environmental friendly option for the management of saline soil for better crop productivity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-021-00186-3.

Attapulgite and processed oyster shell powder effectively reduce cadmium accumulation in grains of rice growing in a contaminated acidic paddy field

Heavy-metal contamination is widespread in agricultural soils worldwide, especially paddy soils contaminated by Cd. Amendment-induced immobilization of heavy metals is an attractive and effective technique, provided that cost-effective materials are used. This field experiment compared three alkaline passivators (attapulgite, processed oyster shell powder, and mixed soil conditioner) at a rate of 2.25 t ha^-1 for their effectiveness in decreasing Cd bioavailability in soils and accumulation in rice plants in a paddy field contaminated by Cd (0.38 Cd mg kg^-1). The utilization of attapulgite and processed oyster shell powder decreased labile fractions but increased stable fractions of Cd in soils through ion exchange, precipitation and complexation. The addition of attapulgite decreased the concentration of bioavailable Cd in both bulk and rhizosphere soils, whereas the amendment of processed oyster shell powder decreased it only in bulk soil. The Cd accumulation in rice plants correlated significantly with acid-soluble and residual Cd fractions in the rhizosphere soil but not in the bulk soil. The addition of attapulgite and processed oyster shell powder decreased Cd accumulation in rice grains from 0.26 mg kg^-1 to 0.14 and 0.19 mg kg^-1, respectively, meeting the National Food Safety Standard (< 0.20 mg kg^-1). However, the mixed soil conditioner did not decrease the Cd accumulation in rice shoots or grains. This study demonstrated that attapulgite and processed oyster shell powder were economic agents in reducing Cd accumulation in rice grains.

In situ phytoremediation of heavy metal-contaminated soil and groundwater: a green inventive approach

The heavy metal contamination of soil and groundwater is a serious threat to environment worldwide. The survival of human being primarily relies upon soil and groundwater sources. Therefore, the remediation of heavy metal-contaminated soil and groundwater is a matter of utmost concern. Heavy metals are non-degradable and persist in the environment and subsequently contaminate the food chain. Heavy metal pollution puts a serious impact on human health and it adversely affects our physical body. Although, numerous in situ conventional technologies have been utilized for the treatment purpose, but most of the techniques have some limitations such as high cost, deterioration of soil properties, disturbances to soil native flora and fauna and intensive labour. Despite that, in situ phytoremediation is a cost-effective, eco-friendly, solar-driven and novel approach with significant public acceptance. The past research reflects rare discussion addressing both (heavy metal in situ phytoremediation of soil and groundwater) in one platform. The present review article covers both the concepts of in situ phytoremediation of soil and groundwater with major emphasis on health risks of heavy metals, enhanced integrated approaches of in situ phytoremediation, mechanisms of in situ phytoremediation along with effective hyperaccumulator plants for heavy metals remediation, challenges and future prospects.

Promotion of growth and phytoextraction of cadmium and lead in Solanum nigrum L. mediated by plant-growth-promoting rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) are a specific category of microbes that improve plant growth and promote greater tolerance to metal stress through their interactions with plant roots. We evaluated the effects of phytoremediation combining the cadmium accumulator Solanum nigrum L. and two Cd- and Pb-resistant bacteria isolates. To understand the interaction between PGPR and their host plant, we conducted greenhouse experiments with inoculation treatments at Nanjing Agricultural University (Jiangsu Province, China), in June 2018. Two Cd- and Pb-resistant PGPR with various growth-promoting properties were isolated from heavy metal-contaminated soil. 16S rRNA analyses indicated that the two isolates were Bacillus genus, and they were named QX8 and QX13. Pot experiments demonstrated that inoculation may improve the rhizosphere soil environment and promote absorption of Fe and P by plants. Inoculation with QX8 and QX13 also enhanced the dry weight of shoots (1.36- and 1.7-fold, respectively) and roots (1.42- and 1.96-fold) of plants growing in Cd- and Pb-contaminated soil, and significantly increased total Cd (1.28-1.81 fold) and Pb (1.08-1.55 fold) content in aerial organs, compared to non-inoculated controls. We also detected increases of 23% and 22% in the acid phosphatase activity of rhizosphere soils inoculated with QX8 and QX13, respectively. However, we did not detect significant differences between inoculated and non-inoculated treatments in Cd and Pb concentrations in plants and available Cd and Pb content in rhizosphere soils. We demonstrated that PGPR-assisted phytoremediation is a promising technique for remediating heavy metal-contaminated soils, with the potential to enhance phytoremediation efficiency and improve soil quality.

A high Mn(II)-tolerance strain, Bacillus thuringiensis HM7, isolated from manganese ore and its biosorption characteristics

Microorganisms play a significant part in detoxifying and immobilizing excessive metals. The present research isolated a strain (HM7) with high Mn(II) tolerance from Mn(II)-contaminated soil samples. The 16S rDNA sequence analysis showed that HM7 had a 99% similarity to Bacillus thuringiensis, which can survive under a high concentration 4,000 mg/L of Mn(II), and the highest removal rate was up to 95.04% at the concentration of 400 mg/L. The highest Mn(II) removal rate was detected at the contact time 72 h, temperature 30 °C, and pH 5.0, while the differences in strain growth and Mn(II) removal rate among different inoculation doses were insignificant. Scanning electron microscopy indicated B. thuringiensis HM7 cells appeared irregular and cracked under Mn(II) stress. Fourier transform infrared exhibited that functional groups like carboxyl, hydroxyl, amino, sulfhydryl groups, and amide bands might take part in the complexation of Mn(II). In addition, HM7 suggested the ability of indoleacetic acid production, siderophore production, and P’ solubilization potential. Therefore, HM7 might have a potential to promote metal absorption by changing the form of heavy metals, and the experiments supported the application of B. thuringiensis HM7 as a biological adsorbent in Mn(II) contaminated environment remediation.

Health risk assessment of trace elements exposure through the soil-plant (maize)-human contamination pathway near a petrochemical industry complex, Northeast China

The trace elements contamination of agricultural soils near petrochemical industry complexes is a concern due to the risk of accumulating in food systems and subsequently affecting human health. We measured representative trace elements (Cu, Ni, Cr, Pb, Zn, Pb, Hg and As) through the soil-plant (maize)-human contamination pathway near a petrochemical industry complexes in an agricultural region from September 20 to 28, 2016. We found that the soil was mildly to moderately polluted by multiple trace elements, which was also confirmed by the contamination factor and enrichment factor values. Cd (enrichment factor = 2.28), Cu (2.75), Zn (1.85) and Pb (1.70) should be given more attention and prioritized over the other trace elements due to their higher potential risks. Furthermore, the trace elements contamination in maize grains was lower than the corresponding limits. The sequence of the transfer coefficient values was Zn > Cd > Cu > Hg > Ni > As > Cr > Pb. Maize grain safety was threatened mainly by Zn, Cd and Cu. There was no risk to humans through soil ingestion, while a potential health risk from maize grain consumption existed. Children were more sensitive than adults to the non-carcinogenic risks of maize grain consumption. Trace element As was found to be the priority metal for risk control. For carcinogenic risk, adults were more sensitive than children; As, Cr and Cd were the priority metals for risk control, with CR_maize values exceeding the risk threshold (1 × 10^-4). Overall, strict, intensive monitoring, especially of Cr and Cd, and soil protection measures are needed to prevent any furthertrace elements contamination and to ensure food safety. This study also provides a reference for similar studies worldwide.

Root-induced changes in aggregation characteristics and potentially toxic elements (PTEs) speciation in a revegetated artificial zinc smelting waste slag site

Root-induced changes play a crucial role in influencing the fate and speciation of potentially toxic elements (PTEs) in contaminated soils, but their role in the phytostabilization of waste slag sites remain unclear. The aim of this study was to determine the effect of four phytostabilization plants, Broussonetia papyrifera, Arundo donax, Robinia pseudoacacia, and Cryptomeria fortunei, planted in a zinc smelting waste slag site for 5 years on PTEs speciation and the mineral and aggregation characteristics at the interface of the waste slag-plant system. The results showed that the presence of a higher content of oxalic acid in the rhizosphere slags of the four plant species than in the bare slag. Revegetation of the waste slag with the four plant species significantly changed the mineral composition and morphology of the waste slag. The mass percentage of large particles (1-5 mm) and small particles (0.5-1 mm, 0.25-0.5 mm, and <0.25 mm) in the rhizosphere slags decreased and increased, respectively. The PTEs (Cu, Pb, Zn, and Cd) in most of the rhizosphere slags were mainly distributed within the small particles, and the enrichment coefficients of PTEs in the large particles and small particles were less than and greater than 1, respectively. The bioavailability of the PTEs in the waste slag increased with decreasing particle size. Root-induced the transformation of acid-soluble PTEs into their reducible, oxidizable, and residual forms in the different waste slag particles weathered in the rhizosphere. These results suggested that there are root-induced changes in the aggregation characteristics and geochemical behaviours of PTEs in waste slag fractions during the phytoremediation of waste slag sites.

Effects of selenium combined with zinc amendment on zinc fractions and bioavailability in calcareous soil

Selenium (Se) and zinc (Zn) are two important trace elements for human being and animals. The interaction between Se and Zn on the bioavailability of Zn in soil is still unclear. Therefore, pot experiments exposed to different dosages of zinc sulfate (ZnSO₄) (0, 20, and 50 mg/kg soil) and sodium selenite (Na₂SeO₃) (0, 0.5, 1.0, and 2.5 mg/kg soil) were conducted to investigate the effects of selenite application on Zn bioavailability in calcareous soil and its related mechanisms. The total Zn content of different tissues (roots and shoots) of pak choi (Brassica chinensis L.) and the changes in Zn fraction distribution in soil before planting and after harvest were determined, and the mobility factor (M_F) and distribution index (D_I) of Zn in soils were calculated. In addition, the Pearson correlation and path analysis were conducted to clarify the relationships between Zn fractions in soil and the Zn uptake of pak choi. Results showed that Se amendment elevated soil Zn bioavailability at appropriate levels of Se and Zn. When 1.0 and 2.5 mg/kg of Se and 20 mg/kg of Zn were applied in soil, the proportion of exchangeable Zn (Ex-Zn) and Zn weakly bound to organic matter (Wbo-Zn) to the total content of Zn was significantly increased by 28.14%-82.52% compared with that of the corresponding single Zn treatment. Therefore, the Zn concentration in the shoots of pak choi was significantly increased by 27.2%-31.1%. High Zn (50 mg/kg) and Se co-amended treatments showed no significantly beneficial effect on the bioavailability of Zn. In addition, the potential available Zn content in soil (weakly bound to organic matter and carbonate bound Zn) and M_F and D_I values were all positively correlated with the Zn concentrations in pak choi, indicating that these indexes can be used to predict the bioavailability of Zn in soil. This study can provide a good reference for Se and Zn biofortification of plants in calcareous soil.

Tomato (Lycopersicon esculentum L.) nutrient and lead uptake affected by zeolite and DTPA in a lead-polluted soil

Development of alleviation strategies, which enhance plant growth under heavy metal stress, is important. Inorganic (zeolite) and organic (diethylene triamine penta-acetic acid, DTPA) amendments affecting the alleviation of lead (Pb) stress in a calcareous soil were tested by investigating leaf nutrient uptake of tomato (Lycopersicon esculentum L.) plants. Experimental quantities of lead (Pb) at 0, 50, 100 and 150 mg·kg^-1 soil, zeolite (clinoptilolite) at 0%, 0.5% and 1%, and DTPA at 0, 50 and 100 mg·kg^-1 soil were tested in a factorial experiment with three plant replicates. According to the anova, Pb, zeolite, DTPA and their interactions significantly affected plant concentrations of nitrogen (N), potassium (K), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn) and lead (Pb). With increasing DTPA concentration at different levels of zeolite and Pb, plant concentrations of macro- and micronutrients significantly increased. Increasing soil Pb increased leaf Pb concentration and decreased the uptake of N, K, Fe, Zn, Cu and Mn. Although with increasing Pb concentration the uptake of macro- and micronutrients decreased in tomato, the use of zeolite and DTPA alleviated this stress by increasing nutrient uptake compared to the control. Interestingly, however, increased levels of zeolite and DTPA led to a decreased uptake of nutrients by plants (compared with control), indicating the absorption of such nutrients by the two amendments and their partial release for further plant use. Zeolite and DTPA may alleviate the negative effects of soil Pb on tomato growth by decreasing nutrient leaching and increasing plant nutrient uptake.

Understanding the molecular mechanisms for the enhanced phytoremediation of heavy metals through plant growth promoting rhizobacteria: A review

Rapid industrialization, modern agricultural practices and other anthropogenic activities add a significant quantity of toxic heavy metals into the environment, which induces severe toxic effects on all form of living organisms, alter the soil properties and its biological activity. Remediation of heavy metal contaminated sites has become an urgent necessity. Among the existing strategies, phytoremediation is an eco-friendly and much convincing tool for the remediation of heavy metals. However, the applicability of phytoremediation in contaminated sites is restricted by two prime factors such as i) slow growth rate at higher metal contaminated sites and ii) metal bioavailability. This circumstance could be minimized and accelerate the phytoremediation efficiency by incorporating the potential plant growth promoting rhizobacterial (PGPR) as a combined approach. PGPR inoculation might improve the plant growth through the production of plant growth promoting substances and improve the heavy metal remediation efficiency by the secretion of chelating agents, acidification and redox changes. Moreover, rhizobacterial inoculation consolidates the metal tolerance and uptake by regulating the expression of various metal transporters, tolerant and metal chelator genes. However, the exact underlying molecular mechanism of PGPR mediated plant growth promotion and phytoremediation of heavy metals is poorly understood. Thus, the present review provides clear information about the molecular mechanisms excreted by PGPR strains in plant growth promotion and phytoremediation of heavy metals.

Biogeochemical distribution of Pb and Zn forms in two calcareous soils affected by mycorrhizal symbiosis and alfalfa rhizosphere

Using of arbuscular mycorrhizal fungi (AMF) has emerged as a new technique to alleviate the toxic metals stress through changing their chemical behavior. The present work was conducted as a factorial arrangement based on a completely randomized design to study the inoculation effects of Glomus intraradices, Glomus mosseae and Glomus etunicatum, on Pb and Zn fractions in the rhizosphere of alfalfa by using rhizobox technique in two agricultural soils with different Zn and Pb concentrations [with low (LH) and high (HH) concentration levels]. The results showed that AMF colonization promoted plant growth and lowered the shoot and root Pb and shoot Zn concentrations in the studied soils compared to uninoculated treatments. Mycorrhizal colonization significantly increased the Ca(NO₃)₂- extractable Zn and ORG-Zn (respectively 500 and 59.6% more than the uninoculated treatment) and decreased the OXI-Zn (20.32% less than the none inoculated treatment) in the HH soil. By contrast, mycorrhizae slightly increased the CARB, OXI and ORG-Zn forms in the LH soil compared to the uninoculation condition. In the AMF- treated HH soil, an increase was recorded in the Ca(NO₃)₂- extractable Pb, EXCH-Pb and CARB-Pb (respectively, 17.65, 3.09 and 14.22% compared to the none inoculated treatment) and a decrease in the OXI and ORG-Pb forms (respectively, 28.79 and 13.51% compared to the uninoculated treatment). A reverse status was observed for Pb changes in the LH soil. Depending on the contamination level, the mycorrhizal inoculation differentially affected the Pb and Zn fractions at different distances from the root surface. In the LH soil, at <5 mm distance (i.e. rhizospheric soil), the mycorrhizal inoculation decreased the CARB (about 17.99%) and OXI -Zn (about 29.63%) forms compared to bulk soil (i.e. > 5 mm distance) while ORG-Zn was increased up to 48.63%. However, Ca(NO₃)₂- extractable, CARB and ORG-Pb was increased in rhizosphere soil (respectively, 89.33, 3.84 and 6.14%) and OXI-Pb was decreased up to 10.36% compared to the bulk soil. In the HH soil, mycorrhizal inoculation increased the CARB and OXI-Zn (respectively, 1.76 and 5.71%) and OXI-Pb fractions (11.56%) compared to the <5 mm distances. Whereas, it reduced the Ca(NO₃)₂- extractable, EXCH, and ORG-Zn (Respectively, 52.70, 19.19 and 30.16%) and Ca(NO₃)₂- extractable, CARB and ORG-Pb (respectively, 47.18, 3.70 and 5.79%). These results revealed that depending on the soil contamination level and nature of the element, AMF colonization affects biogeochemical fractions of the metals and their accumulation in the plant tissues.

Nanomaterials and plants: Positive effects, toxicity and the remediation of metal and metalloid pollution in soil

Currently, the pollution of metals and metalloids in the soil has attracted considerable attention. Phytoremediation is considered an environmentally friendly means of remediating pollution, but often takes a long time to perform. Therefore, the combination of plants and nanomaterials in environmental management has attracted the attention of many researchers because some nanomaterials can promote the germination of plant seeds and the growth of whole plants. However, when the concentration of nanomaterials is not controlled properly, certain toxicity will be produced. This paper reviews research on the combination of plant and nanomaterials for the remediation of contaminated environments, as well as on the effects of nanomaterials on plants.

Mobility of heavy metals in sandy soil after application of composts produced from maize straw, sewage sludge and biochar: Discussion of Gondek et al. (2018)

Gondek et al. (2018) investigate the mobility of heavy metals in sandy soil after application of composts produced from maize straw, sewage sludge and biochar. The authors made a little attention to role of the soil properties and soil condition on the metals chemical behavior. In different section of the paper, the authors reported that the content of the metals extracted with water or 1 M NH₄NO₃ was changed compared to the control treatment affected by the mentioned biosolids. Nowadays, different researches have shown that the trace metals nature have an important role in determining their fate in presence of different treatments. In addition to, organic compounds entered to the soil due to biosolids applications have a duplicate role in trace metals mobility. Therefore, it is needed this facts are considered to interpret the measurements in the original paper. Also, whenever effects of different materials are studied on the metals mobility, the metals status in the plant tissues presents valuable information to the scientists and readers for carefully interpreting the measurements, which it is ignored by the authors.