Phytoextraction of toxic trace elements by Sorghum bicolor inoculated with Streptomyces pactum (Act12) in contaminated soils

Structure, Function, and Biosynthesis of Siderophores Produced by Streptomyces Species

Since the natural supply of iron is low, microorganisms acquire iron by secreting siderophores. Streptomyces is known for its abundant secondary metabolites containing various types of siderophores, including hydroxamate, catecholate, and carboxylate. These siderophores are mainly synthesized through the nonribosomal peptide synthase (NRPS) and non-NRPS pathways and are regulated by ferric uptake regulator and diphtheria toxin regulators. Although both NRPS and non-NRPS pathways adenylate substrates, they differ significantly in the catalytic logic. Siderophores produced by Streptomyces play important roles in fields of agriculture, medicine, and environment. However, their structure, function, and synthetic mechanisms have been inadequately summarized. Therefore, this Review aimed to provide an overview of the classification, structure, biosynthesis, regulation, and applications of siderophores produced by Streptomyces. Finally, the need for a comprehensive and well-defined mechanism for synthesizing siderophores from Streptomyces was highlighted to further promote their commercialization and application in agriculture, medicine, and other areas.

Removal of Heavy Metal Ions from Aqueous Solution Using Biotransformed Lignite

Heavy metal pollution caused by industrial wastewater such as mining and metallurgical wastewater is a major global concern. Therefore, this study used modified lignite as a low-cost adsorbent for heavy metal ions. Pingzhuang lignite was dissolved and modified using Fusarium lignite B3 to prepare a biotransformed-lignite adsorbent (BLA). The O, H, and N contents of the BLA increased after transformation, and the specific surface area increased from 1.81 to 5.66 m²·g^-1. Various adsorption properties were investigated using an aqueous solution of Cu(Ⅱ). The kinetic and isothermal data were well-fitted by pseudo-second-order and Langmuir models, respectively. The Langmuir model showed that the theoretical Cu(II) adsorption capacity was 71.47 mg·g^-1. Moreover, large particles and a neutral pH were favorable for the adsorption of heavy metal ions. The adsorption capacities of raw lignite and BLA were compared for various ions. Microbial transformation greatly improved the adsorption capacity, and the BLA had good adsorption and passivation effects with Cu(II), Mn(II), Cd(II), and Hg(II). Investigation of the structural properties showed that the porosity and specific surface area increased after biotransformation, and there were more active groups such as -COOH, Ar-OH, and R-OH, which were involved in the adsorption performance.

A preliminary study to explain how Streptomyces pactum (Act12) works on phytoextraction: soil heavy metal extraction, seed germination, and plant growth

Streptomyces pactum (Act12) can both promote plant growth and strengthen heavy metal mobilization. Nevertheless, the mechanisms of how Act12 works during the phytoextraction process are still unknown. The present work investigated whether the metabolites produced by Act12 could influence the seed germination and the growth of potherb mustard and explored its mobilizing effect on soil cadmium (Cd) and zinc (Zn). The results showed that the germination potential and rate of potherb mustard seed treated with Act12 fermentation broth were 1.0- and 0.32-folds higher than those of control, probably due to the interruption of seed dormant stage. We also found that Act12 inoculation not only promoted the dry biomass (6.82%) of potherb mustard, but also increased the leaf chlorophyll (11.8%) and soluble protein (0.35%) production. The boosted seed germination rate under Act12 treatment (up to 63.3%) indicated that Act12 enhanced the resistance of potherb mustard seeds to Cd and Zn and alleviated their physiological toxicity. The generated metabolites during the Act12 fermentation posed positive impact on the availability of soil Cd and Zn. These findings bring new insight into the Act12-assisted phytoextraction of Cd and Zn from contaminated soils.

Heavy metal stabilization remediation in polluted soils with stabilizing materials: a review

The remediation of soil contaminated by heavy metals has long been a concern of academics. This is due to the fact that heavy metals discharged into the environment as a result of natural and anthropogenic activities may have detrimental consequences for human health, the ecological environment, the economy, and society. Metal stabilization has received considerable attention and has shown to be a promising soil remediation option among the several techniques for the remediation of heavy metal-contaminated soils. This review discusses various stabilizing materials, including inorganic materials like clay minerals, phosphorus-containing materials, calcium silicon materials, metals, and metal oxides, as well as organic materials like manure, municipal solid waste, and biochar, for the remediation of heavy metal-contaminated soils. Through diverse remediation processes such as adsorption, complexation, precipitation, and redox reactions, these additives efficiently limit the biological effectiveness of heavy metals in soils. It should also be emphasized that the effectiveness of metal stabilization is influenced by soil pH, organic matter content, amendment type and dosage, heavy metal species and contamination level, and plant variety. Furthermore, a comprehensive overview of the methods for evaluating the effectiveness of heavy metal stabilization based on soil physicochemical properties, heavy metal morphology, and bioactivity has also been provided. At the same time, it is critical to assess the stability and timeliness of the heavy metals' long-term remedial effect. Finally, the priority should be on developing novel, efficient, environmentally friendly, and economically feasible stabilizing agents, as well as establishing a systematic assessment method and criteria for analyzing their long-term effects.

Cow bone-derived biochar enhances microbial biomass and alters bacterial community composition and diversity in a smelter contaminated soil

Bone waste could be utilized as a potential amendment for remediation of smelter-contaminated soils. Nevertheless, the influences of cow bone-derived biochar (CB) on soil microbial biomass and microbial community composition in multi-metal contaminated mining soils are still not clearly documented. Hence, the cow bone was used as feedstock material for biochar preparation and pyrolyzed at two temperatures such as 500 °C (CB500) and 800 °C (CB800), and added to a smelter soil at the dosage of 0 (unamended control), 2.5, 5, and 10% (w/w); then, the soil treatments were cultivated by maize. The CB effect on soil biochemical attributes and response of soil microbial biomass, bacterial communities, and diversity indices were examined after harvesting maize. Addition of CB enhanced total nutrient contents (i.e., total nitrogen up to 26% and total phosphorus P up to 27%) and the nutrients availability (i.e., NH₄ up to 50%; NO₃ up to 31%; Olsen P up to 48%; extractable K up to 18%; dissolved organic carbon up to 74%) in the treated soil, as compared to the control. The CB500 application revealed higher microbial biomass C (up to 66%), P (up to 41%), and bacterial gene abundance (up to 76%) than the control. However, comparatively a lower microbial biomass nitrogen and diversity indices were observed in the biochar (both with CB500 and CB800) treated soils than in the unamended soils. At the phylum level, the highest dose (10% of CB500 and CB800 resulted in contrasting effects on the Proteobacteria diversity. The CB500₁₀ favored the Pseudomonas abundance (up to 793%), Saccharibacteria (583%), Parcubacteria (138%), Actinobacteria (65%), and Firmicutes (48%) microbial communities, while CB800₁₀ favored the Saccharibacteria (386%), Proteobacteria (12%) and Acidobacteria (11%), as compared to the control. These results imply that CB500 and CB800 have a remarkable impact on microbial biomass and bacterial diversity in smelter contaminated soils. Particularly, CB500 was found to be suitable for enhancing microbial biomass, bacterial growth of specific phylum, and diversity, which can be useful for bioremediation of mining soils.

Deciphering soil amendments and actinomycetes for remediation of cadmium (Cd) contaminated farmland

Soil heavy metal pollution is one of the most serious environmental problems in China, especially cadmium (Cd), which has the most extensive contaminated soil coverage. Therefore, more economical and efficient remediation methods and measures are needed to control soil Cd contamination. In this study, different amendments (biochar (B), organic fertilizer (F), lime (L)) and actinomycetes (A) inoculants were applied to Cd contaminated farmland to explore their effects on wheat growth. Compared with Control, all treatments except A treatment were able to significantly increase the underground parts dry mass of wheat, with the highest increase of 57.19 %. The results showed that the B treatment significantly increased the plant height of wheat by 3.45 %. All treatments increased wheat SOD activity and chlorophyll content and reduced the MDA, which contributes to wheat stress resistance under Cd contamination. F, L and AF treatments can significantly reduce the Cd content in wheat above- and underground parts by up to 56.39 %. Soil amendments can modify the physical and chemical properties of the soil, which in turn affects the absorption of Cd by wheat. Moreover, the addition of soil amendments significantly affects the composition and structure of the rhizospheric soil bacterial community at the wheat jointing stage. The application of organic fertilizer increases the richness and diversity of the bacterial community, while lime makes it significantly decreases it. T-test and microbiome co-occurrence networks show that actinomycetes could not only effectively colonize in local soil, but also effectively enhance the complexity and stability of the rhizosphere microbial community. Considering the practical impact of different treatments on wheat, soil microorganisms, economic benefits and restoration of soil Cd contamination, the application of organic fertilizer and actinomycetes in Cd contaminated soil is a more ideal remediation strategy. This conclusion can be further verified by studying larger repair regions and longer consecutive repair cycles to gain insight into the repair mechanism.

Influence of magnetized water irrigation on characteristics of antioxidant enzyme, ferritin, and Cd excretion in Festuca arundinacea during phytoextraction

The magnetic field can alter the hydrogen-bond structure and polarity characteristics of water; therefore, we hypothesize that magnetized water can affect plant physiological functions, including metal detoxification and excretion. In this study, the amount of Cd excreted on the leaves of Festuca arundinacea was estimated using magnetized water and normal water irrigation patterns. Irrigation with magnetized water improved the shoot dry weight and Cd content in F. arundinacea by 13.6% and 52.8%, respectively, compared to the control. Magnetized water irrigation also increased antioxidant enzyme activities in plant leaves, thereby alleviating the oxidative damage. The concentration of ferritin was 0.91 folds higher than that of the control, increasing the Fe sequestration and detoxification capacity of F. arundinacea. The amount of Cd excreted was significantly higher under magnetized water irrigation, thereby increasing the annual Cd removal by 109.7% from soil by leaf washing compared with that of the control. In contrast, F. arundinacea irrigated with magnetized water excreted 38.1% less Fe owing to the increase in ferritin levels, compared with that of the control. This study suggests a novel pathway of Cd phytoremediation by rinsing excreted Cd from the leaf surface without harvesting and replanting F. arundinacea.

Streptomyces pactum and Bacillus consortium influenced the bioavailability of toxic metals, soil health, and growth attributes of Symphytum officinale in smelter/mining polluted soil

Soil microbes influence the uptake of toxic metals (TMs) by changing soil characteristics, bioavailability and translocation of TMs, and soil health indicators in polluted environment. The potential effect of Streptomyces pactum (Act12) and Bacillus consortium (B. subtilis and B. licheniformis; 1:1) on soil enzymes and bacterial abundance, bioavailability and translocation of Zn and Cd by Symphytum officinale, and physiological indicators in soil acquired from Fengxian (FX) mining site. Act12 and Bacillus consortium were applied at 0 (CK), 0.50 (T1), 1.50 (T2), and 2.50 (T3) g kg^-1 in a split plot design and three times harvested (H). Results showed that soil pH significantly dropped, whereas, electrical conductivity increased at higher Act12 and Bacillus doses. The extractable Zn lowered and Cd increased at each harvest compared to their controls. Soil β-glucosidase, alkaline phosphatase, urease and sucrase improved, whereas, dehydrogenase reduced in harvest 2 and 3 (H2 and H3) as compared to harvest 1 (H1) after Act12 and Bacillus treatments. The main soil phyla individually contributed ∼5-55.6%. Soil bacterial communities' distribution was also altered by Act12 and Bacillus amendments. Proteobacteria, Acidobacteria, and Bacteroidetes increased, whereas, the Actinobacteria, Chloroflexi, and Gemmatimonadetes decreased during the one-year trial. The Zn and Cd concentration significantly decreased in shoots at each harvest, whereas, the roots concentration was far higher than the shoots, implicating the rhizoremediation by S. officinale. Accumulation factor (AF) and bioconcentration ratio (BCR) of Zn and Cd in shoots were lower and remained higher in case of roots than the standard level (≥1). BCR values of roots indicated that S. officinale can be used for rhizoremediation of TMs in smelter/mines-polluted soils. Thus, field trials in smelter/mines contaminated soils and the potential role of saponin and tannin exudation in metal translocation by plant will broaden our understanding about the mechanism of rhizoremediation of TMs by S. officinale.

Bacillus subtilis and saponin shifted the availability of heavy metals, health indicators of smelter contaminated soil, and the physiological indicators of Symphytum officinale

Bacillus subtilis and saponin were tested for the uptake of heavy metals (HMs) by Symphytum officinale grown in a smelter-contaminated soil in completely randomized design. Soil pH and electrical conductivity increased by 0.11 unit (T3) and 754 mS cm^-1 (T2), respectively. The bioavailable Zn decreased by 5.80% (T2); Cd and Pb increased by 6.21% (T2) and 13.46% (T3), respectively. Soil urease increased by 24% (T3) and alkaline phosphatase, β-glucosidase, and dehydrogenase decreased by 20% (T2), 27.70% (T2), and 21% (T1), respectively. Soil amendments altered the microbial diversity. Fourier-transform infrared spectroscopy and X-ray diffraction reported no obvious changes, except saponin application, which led to possible release of HMs in soil. The fresh weight of Symphytum officinale increased by 21.3 and 5.50% in T2 and T3, respectively. Chlorophyll (a) and carotenoid decreased by the sole application of B. subtilis and saponin and vice-versa for chlorophyll (b). Mono-application of B. subtilis efficiently increased the peroxidase (POD: 27%) and polyphenol oxidase (PPO: 13.56%), whereas, co-application enhanced the phenylalanine ammonia-lyase (PAL: 6.50%) level in shoots. Zn concentration in the shoots and roots declined by 12.75 and 27.32% in T1, respectively. Cd increased (3.92%, T3) in shoots and decreased (39.25%, T1) in roots; Pb concentration remained below detection in shoots and increased by 40% (T3) in roots due to accumulation in dead cells and cell vacuoles. Overall, B. subtilis and saponin influenced the bioavailability of HMs, enzymatic activities, and bacterial abundance in the soil; plant growth indicators, antioxidants activities, and metal uptake in shoots and roots.

Interactive assessment of lignite and bamboo-biochar for geochemical speciation, modulation and uptake of Cu and other heavy metals in the copper mine tailing

This study was designed to examine the combined effect of bamboo-biochar (BC) and water-washed lignite (LGT) at copper mine tailings (CuMT) sites on the concentration of Cu and other metals in pore water (PW), their bioavailability, and change in geochemical speciation. Rapeseed (first cropping-season) and wheat (second cropping-season) were grown for 40-days each and the influence of applied-amendments on both cropping seasons was observed and compared. A significant increase in pH, water holding capacity (WHC), and soil organic carbon (SOC) was observed after the applied amendments in second cropping-seasons. The BC-LGT significantly reduced the concentration of Cu in PW after second cropping seasons; however, the concentration of Pb and Zn were increased with the individual application of biochar and LGT, respectively. BC-LGT and BC-2% significantly reduced the bioavailability of Cu and other HMs in both cropping seasons. The treated-CuMT was subjected to spectroscopic investigation through X-ray photoelectron spectroscopy (XPS), Fourier transform Infrared spectroscopy (FTIR), and X-ray powder diffraction (XRD). The results showed that Cu sorption mainly involved the coordination with hydroxyl and carboxyl functional groups, as well as the co-precipitation or complexation on mineral surfaces, which vary with the applied amendment and bulk amount of Mg, Mn, and Fe released during sorption-process. The co-application of BC-LGT exerted significant effectiveness in immobilizing Cu and other HMs in CuMT. The outcomes of the study indicated that co-application of BC-LGT is an efficacious combination of organic and inorganic materials for Cu adsorption which may provide some new information for the sustainable remediation of copper mine tailing.

Streptomyces pactum addition to contaminated mining soils improved soil quality and enhanced metals phytoextraction by wheat in a green remediation trial

Streptomyces pactum (Act12), an agent of a gentle in situ remediation approach, has been recently used in few works in phytoextraction trials; however, the impact of Act12 on soil quality and metal phytoavailability has not been assessed in multi-metal contaminated soils. Consequently, here we assessed the potential impact of Act12 on the wheat (Triticum aestivum L.) growth, antioxidants activity, and the metal bioavailability in three industrial and mining soils collected from China and contained up to 118, 141, 339, and 6625 mg Cd, Cu, Pb, and Zn kg^-1 soil, respectively. The Act12 was applied at 0 (control), 0.75 (Act-0.75), 1.50 (Act-1.5), and 2.25 (Act-2.25) g kg^-1 (dry weight base) to the three soils; thereafter, the soils were cultivated with wheat (bio-indicator plant) in a pot experiment. The addition of Act12 (at Act-1.5 and Act-2.25) promoted wheat growth in the three soils and significantly increased the content of Cd, Cu, and Zn in the roots and shoots and Pb only in the roots (up to 121%). The Act12-induced increase in metals uptake by wheat might be attributed to the associated decrease in soil pH and/or the increase of metal chelation and production of indole acetic acid and siderophores. The Act12 significantly decreased the antioxidant activities and lipid peroxidation in wheat, which indicates that Act12 may mitigate metals stress in contaminated soils. Enhancing metals phytoextraction using Act12 is a promising ecofriendly approach for phytoremediation of metal-contaminated mining soils that can be safely utilized with non-edible plants and/or bioenergy crops.

Impact of water deficit on the development and senescence of tomato roots grown under various soil textures of Shaanxi, China

PURPOSE

Water scarcity is expected to extend to more regions of the world and represents an alarming threat to food security worldwide. Under such circumstances, water holding capacity is an important agronomic trait, which is primarily controlled by soil texture.

METHODS

Our work examined three different soil textures from three cities of Shaanxi Province in China, i.e., silt-sandy loam from Yulin (north of Shaanxi), loam-clay loam from Yangling (middle and western part of Shaanxi), and clay loam-clay from Hanzhong soil (south of Shaanxi), at two moisture levels, i.e., field capacity of 70-75% (well-watered) and 50-55% (water deficit).

RESULTS

The differences in soil particle sizes altered the soil physiochemical properties and soil enzymatic activities. Soil urease and ß-glucosidase activities were significantly higher in the Yangling soil under the well-watered treatment, while the differences were nonsignificant under the water deficit conditions. The leaf photosynthesis rate and total chlorophyll content were significantly higher in Hanzhong soil after 15 days of treatment; however, the overall highest plant length, root cortex diameter, and xylem element abundance were significantly higher in Yangling soil under the water deficit conditions. Furthermore, comparable differences were observed in antioxidant defence enzymes and endogenous hormones after every 15 days of treatments. The auxin, gibberellic acid and cytokinin concentrations in leaves and roots were comparably high in Yangling soil, while the abscisic acid concentrations were higher in Hanzhong soil under the water deficit conditions.

CONCLUSIONS

Our findings concluded that soil compaction has a significant role not only in root morphology, growth, and development but also in the soil physicochemical properties and nutrient cycle, which are useful for the growth and development of tomato plants.

Green remediation of toxic metals contaminated mining soil using bacterial consortium and Brassica juncea

Microorganism-assisted phytoremediation is being developed as an efficient green approach for management of toxic metals contaminated soils and mitigating the potential human health risk. The capability of plant growth promoting Actinobacteria (Streptomyces pactum Act12 - ACT) and Firmicutes (Bacillus subtilis and Bacillus licheniformis - BC) in mono- and co-applications (consortium) to improve soil properties and enhance phytoextraction of Cd, Cu, Pb, and Zn by Brassica juncea (L.) Czern. was studied here for the first time in both incubation and pot experiments. The predominant microbial taxa were Proteobacteria, Actinobacteria and Bacteroidetes, which are important lineages for maintaining soil ecological activities. The consortium improved the levels of alkaline phosphatase, β-D glucosidase, dehydrogenase, sucrase and urease (up to 33%) as compared to the control. The bacterial inoculum also triggered increases in plant fresh weight, pigments and antioxidants. The consortium application enhanced significantly the metals bioavailability (DTPA extractable) and mobilization (acid soluble fraction), relative to those in the unamended soil; therefore, significantly improved the metals uptake by roots and shoots. The phytoextraction indices indicated that B. juncea is an efficient accumulator of Cd and Zn. Overall, co-application of ACT and BC can be an effective solution for enhancing phytoremediation potential and thus reducing the potential human health risk from smelter-contaminated soil. Field studies may further credit the understanding of consortium interactions with soil and different plant systems in remediating multi-metal contaminated environments.

Bone-derived biochar improved soil quality and reduced Cd and Zn phytoavailability in a multi-metal contaminated mining soil

Reusing by-products such as cow bones in agriculture can be achieved thorough pyrolysis. The potential of bone-derived biochar as a promising material for metals immobilization in contaminated mining soils has not yet been sufficiently explored. Therefore, cow bones were used as biochar feedstock were pyrolyzed at 500 °C (CBL) and 800 °C (CBH) and. The two biochars were applied to a mine contaminated soil at 0 (control), 2.5, 5 and 10%, w/w, dosages; then, the soils were incubated and cultivated by maize in the greenhouse. Cadmium (Cd) and zinc (Zn) bioavailability and their sequentially extracted fractions (acid soluble, reducible, oxidizable, and residual fraction), soil microbial function, and plant health attributes were analyzed after maize harvesting. Bone-derived biochar enhanced the content of dissolved organic carbon (up to 74%), total nitrogen (up to 26%), and total phosphorus (up to 27%) in the soil and improved the plant growth up to 55%, as compared to the control. The addition of CBL altered the acid soluble fraction of both metals to the residual fraction and, thus, reduced the content of Zn (55 and 40%) and Cd (57 and 67%) in the maize roots and shoots, respectively as compared to the control. The CBL enhanced the β-glucosidase (51%) and alkaline phosphatase activities (71%) at the lower doses (2.5-5%) as compared to control, while the activities of these enzymes decreased with the higher application doses. Also, CBL improved the antioxidants activity and maize growth at the 2.5-5% application rate. However, the activity of the dehydrogenase significantly decreased (77%), particularly with CBH. We conclude that CBL, applied at 2.5-5% dose, can be utilized as a potential low cost and environmental friendly amendment for stabilization of toxic metals in contaminated mining soils and producing food/feed/biofuel crops with lower metal content.

Application of amendments for the phytoremediation of a former mine technosol by endemic pioneer species: alder and birch seedlings

Metal(loid) pollution of soils has important negative effects on the environment and human health. For the rehabilitation of these soils, some eco-innovative strategies, such as phytoremediation, could be chosen. This practice could establish a plant cover to reduce the toxicity of the pollutants and stabilize the soil, preventing soil erosion and water leaching; this technique is called phytoremediation. For this, plants need to be tolerant to the pollutants present; thus, phytoremediation can have better outcomes if endemic species of the polluted area are used. Finally, to further improve phytoremediation success, amendments can be applied to ameliorate soil conditions. Different amendments can be used, such as biochar, a good metal(loid) immobilizer, compost, a nutrient-rich product and iron sulfate, an efficient arsenic immobilizer. These amendments can either be applied alone or combined for further positive effects. In this context, a mesocosm experiment was performed to study the effects of three amendments, biochar, compost and iron sulfate, applied alone or combined to a former mine technosol, on the soil properties and the phytoremediation potential of two endemic species, Alnus sp. and Betula sp. Results showed that the different amendments reduced soil acidity and decreased metal(loid) mobility, thus improving plant growth. Both species were able to grow on the amended technosols, but alder seedlings had a much higher growth compared to birch seedlings. Finally, the combination of compost with biochar and/or iron sulfate and the establishment of endemic alder plants could be a solution to rehabilitate a former mine technosol.

Performance of Streptomyces pactum-assisted phytoextraction of Cd and Pb: in view of soil properties, element bioavailability, and phytoextraction indices

Microbe-assisted phytoremediation provides an eco-friendly and cost-effective approach to reclaim Cd- and Pb-contaminated soils. In this work, incubation and pot experiments were established to investigate the effect of Streptomyces pactum (Act12) combined with compost on soil physicochemical properties, enzymatic activities, and thereby acted on phytoextraction of Cd and Pb by using potherb mustard (Brassica juncea Coss.). The addition of Act12 and compost increased EC (7.2%), available phosphorus (P) (14.9%), available potassium (K) (17.0 folds), DOC (37.7%), OM (2.8 folds), urease (49.8%), dehydrogenase (2.2 folds), and alkaline phosphatase (23.0 folds) of soil, while reduced pH (7.7%) compared with control. Significant decrease of available Cd and Pb uptake was observed after adding compost and Act12 by 29.1% and 32.2%. Presence of compost and Act12 enhanced the biomass by 3.98 folds and 1.83 folds in shoots and roots of plant. Results showed the assimilation of Cd and Pb in shoots was increased by 103.8% and 48.7% due to the increased of biomass. Meanwhile, the rhizosphere effect of soil microorganisms increased the uptake of Cd (60.4%) and Pb (19.2%) in roots. These findings suggested that Act12 joined with compost-strengthened potherb mustard phytoremediation of Cd- and Pb-polluted soils, which may provide new insights into the clean-up of mining-contaminated soils in field practice.

Performance of the emerging biochar on the stabilization of potentially toxic metals in smelter- and mining-contaminated soils

Soil potentially toxic metals (PTMs) pollution caused by anthropogenic activities has become serious concern with respect to the crop safety production. In this study, an emerging biochar derived from kiwi pruning branches waste was employed as amendment aiming to evaluate its remediation potential on smelter- and mining-contaminated soils. The effect of biochar on the soil physicochemical properties, leachability, and chemical fractions acted on stabilization practice of PTMs in soil was investigated. The results showed that the addition of biochar increased the soil pH, cation exchange capacity, organic matter, and enzymatic activities (dehydrogenase, urease, and sucrase) but reduced the extraction toxicity of PTMs in both smelter (Fengxian, FX) and mining (Tongguan, TG) soils. The fraction analysis showed that the maximum reduction of exchangeable fraction of Cd, Zn, and Pb in the 4% biochar amended soils decreased by 11.1, 13.3, and 24.7% in FX soil and 7.67, 22.8, and 7.89% in TG soil, respectively, in comparison with to control (no biochar added). Additionally, the residual fraction of Cd, Zn, and Pb increased by 55.9, 7.14, and 11.0% in FX soil and 23.7, 5.86, and 10.0% in TG soil, respectively. The further greenhouse experiment showed that the Indian mustard (Brassica juncea) production increased with the increasing application dosages of biochar, while the PTMs uptakes in plant notably decreased after amendments. Conversion of kiwi pruning branches waste into emerging biochar benefits the agricultural waste recycling utilization and enhances PTMs-contaminated soil remediation in practice. Graphical abstract.

Phytostabilization of Pb-Zn Mine Tailings with Amorpha fruticosa Aided by Organic Amendments and Triple Superphosphate

A greenhouse pot trial was conducted to investigate the effect of organic amendments combined with triple superphosphate on the bioavailability of heavy metals (HMs), Amorpha fruticosa growth and metal uptake from Pb-Zn mine tailings. Cattle manure compost (CMC), spent mushroom compost (SMC) and agricultural field soil (AFS) were applied to tailings at 5%, 10%, 20% and 30% w/w ratio, whereas sewage sludge (SS) and wood biochar (WB) were mixed at 2.5%, 5%, 10% and 20% w/w ratio. Triple superphosphate (TSP) was added to all the treatments at 4:1 (molar ratio). Amendments efficiently decreased DTPA-extracted Pb, Zn, Cd and Cu in treatments. Chlorophyll contents and shoot and root dry biomass significantly (p < 0.05) increased in the treatments of CMC (except T4 for chlorophyll b) and SMC, whereas treatments of SS (except T1 for chlorophyll a and b), WB and AFS (except T4 for chlorophyll a and b) did not show positive effects as compared to CK1. Bioconcentration factor (BCF) and translocation factor (TF) values in plant tissues were below 1 for most treatments. In amended treatments, soluble protein content increased, phenylalanine ammonialyase (PAL) and polyphenol oxidase (PPO) decreased, and catalase (CAT) activity showed varied results as compared to CK1 and CK2. Results suggested that A. fruticosa can be a potential metal phytostabilizer and use of CMC or SMC in combination with TSP are more effective than other combinations for the in situ stabilization of Pb-Zn mine tailings.

Phytoremediation of cadmium-contaminated soil by Sorghum bicolor and the variation of microbial community

In this paper, the growth of S. bicolor in Cd-polluted sandy clay loam soil in north China, Cd accumulation in plant and the corresponding soil microbial community were characterized when the plant matured (140 d of growth). Cadmium promoted the growth of mature S. bicolor with higher height and heavier dry mass, especially at the spiked level of 1 mg kg^-1 soil (P < 0.05). The higher microbial diversity was found under Cd stress at the spiked level of 15 mg kg^-1, which basically corresponded with its influence on the plant growth. High-throughput sequencing data demonstrated that the predominant bacterial phyla include Proteobacteria (35.99% for Cd-polluted soil and 35.22% for the control soil), Chloroflexi (21.33% and 20.58%), Actinobacteria (12.00% and 12.89%), Acidobacteria (7.47% and 11.14%), Bacteroidetes (7.37% and 6.96%), Gemmatimonadetes (5.60% and 6.65%), Firmicutes (2.82% and 1.86%), Planctomycetes (2.47% and 0.95%), Saccharibacteria (1.26% and 1.11%). The predominant fungal phyla was Ascomycota, with the relative abundance of 89.96% for the control soil and 86.2% for the Cd-polluted soil. S. bicolor could grow well in sandy clay loam soil in northern China at low Cd lvel, but it could not accumulate cadmium at higher cadmium level. S. bicolor could be used for phytoextraction of cadmium from the lightly Cd-polluted soil.

Streptomyces pactum combined with manure compost alters soil fertility and enzymatic activities, enhancing phytoextraction of potentially toxic metals (PTMs) in a smelter-contaminated soil

The effect of manure compost alone and combined with Streptomyces pactum (Act12) applied in the smelter-contaminated soil was investigated. The soil fertility, enzymatic activities, potentially toxic metals (PTMs) solubility, and phytoremediation efficiency of potherb mustard (Brassica juncea, Coss.) were assessed. Results showed that the application of compost reduced the soil pH, while significantly increased the soil electrical conductivity (EC) (7.0 folds), available phosphorus (AP) (10.8 folds), available potassium (AK) (2.81 folds), dissolved organic carbon (DOC) (5.22 folds), organic matter (OM) (4.93 folds), together with soil enzymatic activities viz. urease (UR) (4.39 folds), dehydrogenase (DEH) (45.0 folds) and alkaline phosphatase (ALP) (123.9 folds) in comparison with control. The inoculation of Act12 increased AP, AK, DOC, OM and UR values, but reduced EC, DEH and ALP values compared to corresponding lone compost amendment. Additionally, Act12 solubilized PTMs (Cd and Zn) in the soil, and accordingly enhanced the PTMs uptake in the plant. The phytoextraction indices viz. biological concentration factor (BCF), translocation factor (TF) and metal extraction amount (MEA) indicated that compost and Act12 had a synergistic role in enhancing the phytoremediation efficiency, among which MEA values of Cd and Zn maximally increased by 9.64 and 11.4 folds, respectively, compared to control. Redundancy analysis (RDA) indicated that phytoextraction indices correlated well with soil parameters. Our results suggested that manure compost associated with Act12 is a potential strengthening strategy in phytoremediation of PTMs contaminated soil.

Streptomyces pactum and sulfur mediated the antioxidant enzymes in plant and phytoextraction of potentially toxic elements from a smelter-contaminated soils

The toxic potentially toxic metals elements (PTEs) discharged from industrial activities and agricultural practices persistently pose multiple hazards to environment and living organisms. Microbe-assisted phytoremediation provide an effective approach to remediate PTEs-contaminated soils. A phytoextraction process involved the application of Streptomyces pactum (Act12, 1.0, 2.0 and 3.0 g kg^-1 dry soil, respectively) alone/jointly with sulfur was executed. The main texture of the tested soil was sandy loam and with a pH 8.27. The obtained results showed that the leaf pigments and plant biomass were improved after the application of the Act12, while the shoot fresh weight, chlorophyll a and chlorophyll b decreased by 57.8, 38.2 and 40.7%, respectively, after treatment with sulfur. Similarly, sulfur application facilitated the malondialdehyde (MDA) production by 18.4-33.6% compared to the control (no amendments). Both peroxidase (POD) and superoxide dismutase (SOD) activities were boosted, while the catalase (CAT) activity was suppressed with Act12 alone/jointly with sulfur treatment. The sulfur combined with elevated Act12 levels notably increased the cadmium (Cd) and zinc (Zn) concentrations both in shoots and roots, while the elemental extraction amount showed the removal efficiency following the order: Act12 alone ＞ control ＞ Act12 jointly with sulfur. Taken together, the results suggested that Streptomyces pactum and sulfur assisted the phytoremediation process, while further studies should be conducted in the field to test practical application.

Growth, accumulation, and antioxidative responses of two Salix genotypes exposed to cadmium and lead in hydroponic culture

Cd and Pb are a toxic environmental pollutant, and their elevated concentrations in the waters and soils could exert detriment effects on human health by food chain. In order to evaluate the capacity to heavy metal accumulation and the physiochemical responses of two Salix genotypes, a 35-day hydroponic seedling experiment was implemented with Salix matsudana Koidz. 'Shidi1' (A42) and Salix psammophila C. 'Huangpi1' (A94) under different concentrations of Cd (15 and 30 μM) or Pb (250 and 300 μM). The results showed that the biomass of A94 severely reduced more than that of A42. The accumulation ability of Cd in different plant organs followed the sequence of leaves > roots > stems. Pb primarily accumulated in the roots for both Salix genotypes (54.27 mg g^-1 for A42 and 54.52 mg g^-1 for A94). Translocation factors based on accumulation (TF') for Cd were more than 8.0, while TF's for Pb were less than 1.0 in both A42 and A94, implying they could be applied in the phytoremediation of Cd-contaminated sites due to their stronger ability to Cd phytoextraction. The stress of Cd or Pb significantly increased malondialdehyde (MDA) contents and increased photosynthetic rates in leaves of two Salix genotypes. Transpiration rates of willow were positively correlated with its Cd translocation. Both catalase (CAT) and peroxidase (POD) activities were suppressed, while the superoxide dismutase (SOD) was boosted with increasing Cd and Pb levels in the leaves and roots of the two willow genotypes, suggesting SOD plays an important role in the removal of ROS. The inconsistency of the changes in enzyme activity suggests that the integrated antioxidative mechanisms regulate the tolerance to Cd and Pb stress.

Foliar litters: Sources of contaminants in phytoremediation sites by returning potentially toxic metals (PTMs) back to soils

Phytoremediation is regarded as one of the most cost-effective and environmentally friendly strategies for potentially toxic metals (PTMs) contaminated soil remediation. However, uncertainties still existed about the contribution of foliar litter on metal accumulation and mobility in phytoremediation sites. Thus, in this study, fallen leaves, decomposed leaves, and soils at different depths (i.e., 0-5 and 5-10 cm) were collected from a phytoremediation site near a Zn smelter factory. Metals content and mobility were evaluated. Results indicated that upper-layer soils (0-5 cm) were higher in the electrical conductivity (EC) and soil organic matter (SOM) content than the deeper-layer soils (5-10 cm). However, the pH was relatively lower in the upper-layer soils. Fallen leaves were sources of metals in the phytoremediation site, and significantly high amounts of Cd (16.08 ± 0.21 mg kg^-1) and Zn (1130.30 ± 60.10 mg kg^-1) were found in the decomposed leaves. Metals in the upper-layer soils demonstrated higher accumulation and mobility than the deeper-layer soils. Moreover, the accumulated metals in leaves would gradually return to the soil as the contents of extractable metals increased with the rising decomposition degree of leaves (i.e., cold-water extraction < 80 °C hot water extraction < 1 M HCl extraction). Results from this research are helpful for the guidance of phytoremediation site management.

EDTA and organic acids assisted phytoextraction of Cd and Zn from a smelter contaminated soil by potherb mustard (Brassica juncea, Coss) and evaluation of its bioindicators

Phytoremediation of contaminated soil is an in-situ reclamation technique for removal of potentially toxic metals through hyperaccumulator plants. Potherb mustard (Brassica juncea, Coss.) is less explored for its assisted phytoextraction potential to restore and accelerate potentially toxic metals removal from smelter-contaminated soil. In this study, different levels of ethylene diamine tetraacetic acid (EDTA) alone and combined with citric acid (CA) and oxalic acid (OA) were applied in a greenhouse pot experiment. Chelates added on 25th d and 25/35th d after sowing, enhanced cadmium (Cd) and zinc (Zn) bioavailability in soil due to complexation. As a result, Cd and Zn in shoot and root were significantly amplified by 1.7, 2.15 and 1.93, 2.7 folds than control, respectively. Shoot and root dry weight significantly reduced and ranged between 4.13-9.91 and 0.21-0.77 g pot^-1, respectively. The toxicity induced by potentially toxic metals in plant imposed a series of biological responses. Plant antioxidants like Phenylalanine ammomialyase (PAL), polyphenol oxidase (PPO) Catalase (CAT) content increased, except the peroxidase (POD) with the addition of chelating agents. Besides, biological concentration factor (BCF) of Cd and Zn, translocation factor (TF) of Cd were notably elevated (>1.0), while TF of Zn was reduced. Pearson correlation analysis showed a positive relation between DTPA-extractable and shoot concentration of Cd and Zn, whereas it showed negative correlation with plant dry weight. In general, chelate-assisted phytoremediation of smelter contaminated soil proved effective in this study, and followed the order: EDTA > EDTA + CA ≈ EDTA + OA > CK.

Role of Streptomyces pactum in phytoremediation of trace elements by Brassica juncea in mine polluted soils

The industrial expansion, smelting, mining and agricultural practices have increased the release of toxic trace elements (TEs) in the environment and threaten living organisms. The microbe-assisted phytoremediation is environmentally safe and provide an effective approach to remediate TEs contaminated soils. A pot experiment was conducted to test the potential of an Actinomycete, subspecies Streptomyces pactum (Act12) along with medical stone compost (MSC) by growing Brassica juncea in smelter and mines polluted soils of Feng County (FC) and Tongguan (TG, China), respectively. Results showed that Zn (7, 28%), Pb (54, 21%), Cd (16, 17%) and Cu (8, 10%) uptake in shoot and root of Brassica juncea was pronounced in FC soil. Meanwhile, the Zn (40, 14%) and Pb (82, 15%) uptake in the shoot and root were also increased in TG soil. Shoot Cd uptake remained below detection, while Cu decreased by 52% in TG soil. The Cd and Cu root uptake were increased by 17% and 33%, respectively. Results showed that TEs uptake in shoot increased with increasing Act12 dose. Shoot/root dry biomass, chlorophyll and carotenoid content in Brassica juncea were significantly influenced by the application of Act12 in FC and TG soil. The antioxidant enzymatic activities (POD, PAL, PPO and CAT) in Brassica juncea implicated enhancement in the plant defense mechanism against the TEs induced stress in contaminated soils. The extraction potential of Brasssica was further evaluated by TF (translocation factor) and MEA (metal extraction amount). Based on our findings, further investigation of Act12 assisted phytoremediation of TEs in the smelter and mines polluted soil and hyperaccumulator species are suggested for future studies.