Combined biochar and metal-immobilizing bacteria reduces edible tissue metal uptake in vegetables by increasing amorphous Fe oxides and abundance of Fe- and Mn-oxidising Leptothrix species

Iron at the helm: Steering arsenic speciation through redox processes in soils

The toxicity and bioavailability of arsenic (As) in soils are largely determined by its speciation. Iron (Fe) is widely present in soils with a strong affinity for As, and therefore the environmental behaviors of As and Fe oxides (including oxides, hydrates and hydrated oxides) are closely correlated with each other. The redox fluctuations of Fe driven by changes in the environment can significantly affect As speciation and its fate in soils. The interaction between Fe and As has garnered widespread attention, and the adsorption mechanisms of As by Fe oxides have also been well-documented. However, there is still a lack of systematic understanding of how Fe redox dynamics affects As speciation depending on the soil environmental conditions. In this review, we summarize the mechanisms for As speciation transformation and redistribution, as well as the role of environmental factors in the main Fe redox processes in soils. These processes include the biotic Fe oxidation mediated by Fe-oxidizing bacteria, abiotic Fe oxidation by oxygen or manganese oxides, dissimilatory Fe reduction mediated by Fe-reducing bacteria, and Fe(II)-catalyzed transformation of Fe oxides. This review contributes to a deeper understanding of the environmental behaviors of Fe and As in soils, and provides theoretical guidance for the development of remediation strategies for As-contaminated soils.

A Review of Biochar from Biomass and Its Interaction with Microbes: Enhancing Soil Quality and Crop Yield in Brassica Cultivation

Biochar, produced from biomass, has become recognized as a sustainable soil amendment that has the potential to improve soil quality and agricultural production. This review focuses on production processes and properties of biochar derived from different types of biomass, including the synergistic interactions between biochar and soil microorganisms, emphasizing their influence on overall soil quality and crop production, particularly in cultivation of Brassica crops. It additionally addresses the potential benefits and limitations of biochar and microbial application. Biomass is a renewable and abundant resource and can be converted through pyrolysis into biochar, which has high porosity, abundant surface functionalities, and the capacity to retain nutrients. These characteristics provide optimal conditions for beneficial microbial communities that increase nutrient cycling, reduce pathogens, and improve soil structure. The information indicates that the use of biochar in Brassica crops can result in improved plant growth, yield, nutrient uptake, and stress mitigation. This review includes information about biochar properties such as pH, elemental composition, ash content, and yield, which can be affected by the different types of biomass used as well as pyrolysis conditions like temperature. Understanding these variables is essential for optimizing biochar for agricultural use. Moreover, the information on the limitations of biochar and microbes emphasizes the importance of their benefits with potential constraints. Therefore, sustainable agriculture methods can possibly be achieved by integrating biochar with microbial management measurements, resulting in higher productivity and adaptability in Brassica or other plant crop cultivation systems. This review aims to provide a comprehensive understanding of biochar's role in supporting sustainable Brassica farming and its potential to address contemporary agricultural challenges.

Combined biochar and wheat-derived endophytic bacteria reduces cadmium uptake in wheat grains in a metal-polluted soil

In this study, two wheat-derived cadmium (Cd)-immobilizing endophytic Pseudomonas paralactis M14 and Priestia megaterium R27 were evaluated for their effects on wheat tissue Cd uptake under hydroponic conditions. Then, the impacts of the biochar (BC), M14+R27 (MR), and BC+MR treatments on wheat Cd uptake and the mechanisms involved were investigated at the jointing, heading, and mature stages of wheat plants under field-plot conditions. A hydroponic experiment showed that the MR treatment significantly decreased the above-ground tissue Cd content compared with the M14 or R27 treatment. The BC+MR treatment reduced the grain Cd content by 51.5%-67.7% and Cd translocation factor at the mature stage of wheat plants and increased the organic matter-bound Cd content by 31%-75% in the rhizosphere soils compared with the BC or MR treatment. Compared with the BC or MR treatment, the relative abundances of the biomarkers associated with Gemmatimonas, Altererythrobacter, Gammaproteobacteria, Xanthomonadaceae, Phenylobacterium, and Nocardioides in the BC+MR-treated rhizosphere microbiome decreased and negatively correlated with the organic matter-bound Cd contents. In the BC+MR-treated root interior microbiome, the relative abundance of the biomarker belonging to Exiguobacterium increased and negatively correlated with the Cd translocation factor, while the relative abundance of the biomarker belonging to Pseudonocardiaceae decreased and positively correlated with the Cd translocation factor. Our findings suggested that the BC+MR treatment reduced Cd availability and Cd transfer through affecting the abundances of these specific biomarkers in the rhizosphere soil and root interior microbiomes, leading to decreased wheat grain Cd uptake in the contaminated soil.

Revitalizing Soybean Plants in Saline, Cd-Polluted Soil Using Si-NPs, Biochar, and PGPR

Excessive irrigation of saline-alkaline soils with Cd-contaminated wastewater has resulted in deterioration of both soil and plant quality. To an investigate this, a study was conducted to explore the effects of biochar (applied at 10 t ha-1), PGPRs (Bradyrhizobium japonicum (USDA 110) + Trichoderma harzianum at 1:1 ratio), and Si-NPs (25 mg L-1) on soybean plants grown in saline-alkali soil irrigated with wastewater. The results showed that the trio-combination of biochar with PGPRs, (as soil amendments) and Si-NPs (as foliar spraying), was more effective than individual or coupled applications in reducing Cd bioavailability in the soil, minimizing its absorption, translocation and bioconcentration in soybean tissues. The trio-combination reduced Cd bioavailability in the soil by 39.1% and Cd accumulation in plant roots, shoots, and seeds by 61.0%, 69.3%, and 61.1%, respectively. Physiological improvements in soybean plants were also observed, including 197.8% increase in root growth, 209.3% increase in chlorophyll content, and 297.4% increase in carotenoid levels. The trio-combination significantly improved soil physicochemical characteristics, enhanced soil microbial indicators and boosted soil enzymes activity, which in turn facilitated nutrient uptake and increased antioxidant enzymes activity. These positive outcomes enhanced photosynthesis, improved productivity and increased seed nutritional value. Overall, the trio-combination of biochar with PGPRs and Si-NPs are considered a reliable approach not only for revitalizing soybean growth but also for immobilizing Cd and improving soil health under wastewater irrigation.

Effect of natural aging on biochar physicochemical property and mobility of Cd (II)

This project utilized both field experiment and laboratory analyses to address the gap in understanding regarding the alterations in properties and functions of biochar, and the impact of heavy metal passivation in soil over long-term natural field aging. The study aimed to examine the changes in the physical and chemical characteristics of biochar over an extended period of natural aging. Additionally, it sought to analyze the impact and mechanisms of biochar in reducing of the harmful effects of the heavy metal cadmium (Cd) during the aging process. Both original and aged biochar conformed to the pseudo-second-order kinetics model and the Langmuir model. The aging process enhanced the adsorption of Cd by biochar and mitigated the leaching of Cd2+ into the soil. These findings provide a scientific basis for evaluating biochar's environmental behavior and its potential use in the remediation of soil contaminated with heavy metals.

Investigation of the effects of biochar amendment on soil under freeze‒thaw cycles and the underlying mechanism

Biochar (BC) is widely utilized as a soil amendment; however, for widely distributed seasonally frozen soils, the effect of BC on soil and the optimal utilization of BC during the freeze‒thaw process are still unclear. In this study, the effects of freeze‒thaw aged biochar (FT-BC) and BC on soil properties and wheat cultivation were systematically investigated, and the underlying interaction mechanism between BC and soil was explored. The results show that FT-BC dramatically reduces the adverse effects of freeze‒thaw cycles on soil, enhances wheat growth, and increases dry matter yield by 17.5 %, which is mainly attributed to the ability of FT-BC to maintain soil structure, reduce water loss rates to below 0.20 g/h, and decrease nitrogen leaching by more than 20 % during freeze‒thaw cycles. Additionally, fresh BC had a greater effect on the fixation of cadmium than FT-BC in the soil, reducing its accumulation in wheat by 22.5 %. Multiple characterizations revealed that the freeze‒thaw process increased the porosity and specific surface area of FT-BC, providing more sites for water and nitrogen adsorption, whereas the dissolved organic matter released from fresh BC had a better ability to trap cadmium. These findings provide insights into the interactions between BC and soil components during the freeze‒thaw process and suggest the optimized utilization of fresh BC and FT-BC for different soil repair purposes.

Layered double hydroxides for simultaneous and long-term immobilization of metal(loid)s in soil under simulated aging

Soil contamination by toxic metals and metalloids poses a grave threat to food security and human well-being. Immobilization serves as an effective method for the remediation of soils contaminated by metal(loid)s. Nevertheless, the ability of soil amendments for simultaneous immobilization of cations and oxyanions, and the long-term effectiveness of immobilization need substantial improvements. In this study, we used a series of layered double hydroxides (LDHs), including Mg-Al LDH and Ca-Al LDH fabricated from pure chemicals, and one waste-derived LDH synthesized using granulated ground blast furnace slag (GGBS), for the immobilization of Cu, Zn, As, and Sb in a historically contaminated soil obscured from a mining-affected region. The LDHs were first subjected to iron (Fe) modification to enhance their short-term immobilization performances toward metal(loid)s. Furthermore, the long-term effectiveness of Fe-modified LDHs was examined via two sets of experiments, including column experiments simulating 2-year water leaching, and accelerated aging experiments simulating 100-year proton attack. It was observed that Fe-modified LDHs, either made from pure chemicals or GGBS, demonstrated promising long-term immobilization performances toward metal(loid)s. Results from this study are encouraging for the future use of LDHs for simultaneous and long-term immobilization of metal(loid)s in soil.

Urease-producing bacteria combined with pig manure biochar immobilize Cd and inhibit the absorption of Cd in lettuce (Lactuca sativa L.)

The synergistic remediation of heavy metal-contaminated soil by functional strains and biochar has been widely studied. However, the mechanisms by which urease-producing bacteria combine with pig manure biochar (PMB) to immobilize Cd and inhibit Cd absorption in vegetables are still unclear. In our study, the effects and mechanisms of PMB combined with the urease-producing bacterium TJ6 (TJ6 + PMB) on Cd adsorption were explored. The effects of TJ6 + PMB on the Cd content and pH of the leachate were also studied through a 56-day soil leaching experiment. Moreover, the effects of the complexes on Cd absorption and microbial mechanisms in lettuce were explored through pot experiments. The results showed that PMB provided strain TJ6 with a greater ability to adsorb Cd, inducing the generation of CdS and CdCO3, and thereby reducing the Cd content (71.1%) and increasing the pH and urease activity in the culture medium. TJ6 + PMB improved lettuce dry weight and reduced Cd absorption. These positive effects were likely due to (1) TJ6 + PMB increased the organic matter and NH4+ contents, (2) TJ6 + PMB transformed available Cd into residual Cd and decreased the Cd content in the leachate, and (3) TJ6 + PMB altered the structure of the rhizosphere bacterial and fungal communities in lettuce, increasing the relative abundances of Stachybotrys, Agrocybe, Gaiellales, and Gemmatimonas. These genera can promote plant growth, decompose organic matter, and release phosphorus. Interestingly, the fungal communities were more sensitive to the addition of TJ6 and PMB, which play important roles in the decomposition of organic matter and immobilization of Cd. In conclusion, this study revealed the mechanism by which urease-producing bacteria combined with pig manure biochar immobilize Cd and provided a theoretical basis for safe pig manure return to Cd-polluted farmland. This study also provides technical approaches and bacterial resources for the remediation of heavy metal-contaminated soil.

Unlocking the potential of biochar in the remediation of soils contaminated with heavy metals for sustainable agriculture

Agricultural soils contaminated with heavy metals (HMs) impose a threat to the environmental and to human health. Amendment with biochar could be an eco-friendly and cost-effective option to decrease HMs in contaminated soil. This paper reviews the application of biochar as a soil amendment to immobilise HMs in contaminated soil. We discuss the technologies of its preparation, their specific properties, and effect on the bioavailability of HMs. Biochar stabilises HMs in contaminated soil, enhance the overall quality of the contaminated soil, and significantly reduce HM uptake by plants, making it an option in soil remediation for HM contamination. Biochar enhances the physical (e.g. bulk density, soil structure, water holding capacity), chemical (e.g. cation exchange capacity, pH, nutrient availability, ion exchange, complexes), and biological properties (e.g. microbial abundance, enzymatic activities) of contaminated soil. Biochar also enhances soil fertility, improves plant growth, and reduces the plant availability of HMs. Various field studies have shown that biochar application reduces the bioavailability of HMs from contaminated soil while increasing crop yield. The review highlights the positive effects of biochar by reducing HM bioavailability in contaminated soils. Future work is recommended to ensure that biochars offer a safe and sustainable solution to remediate soils contaminated with HMs.

MnFe2O4-biochar decreases bioavailable fractions of thallium in highly acidic soils from pyrite mining area

The prevalence of toxic element thallium (Tl) in soils is of increasing concern as a hidden hazard in agricultural systems and food chains. In the present work, pure biochar (as a comparison) and jacobsite (MnFe2O4)-biochar composite (MFBC) were evaluated for their immobilization effects in Tl-polluted agricultural soils (Tl: ∼10 mg/kg). Overall, MFBC exhibited an efficient effect on Tl immobilization, and the effect was strengthened with the increase of amendment ratio. After being amended by MFBC for 15 and 30 days, the labile fraction of Tl in soil decreased from 1.55 to 0.97 mg/kg, and from 1.51 to 0.88 mg/kg, respectively. In addition, pH (3.05) of the highly acidic soil increased to a maximum of 3.97 after the immobilization process. Since the weak acid extractable and oxidizable Tl were the preponderantly mitigated fractions and displayed a negative correlation with pH, it can be inferred that pH may serve as one of the most critical factors in regulating the Tl immobilization process in MFBC-amended acidic soils. This study indicated a great potential of jacobsite-biochar amendment in stabilization and immobilization of Tl in highly acidic and Tl-polluted agricultural soils; and it would bring considerable environmental benefit to these Tl-contaminated sites whose occurrence has significantly increased in recent decades near the pyrite or other sulfide ore mining and smelting area elsewhere.

Techniques and mechanisms of bacteria immobilization on biochar for further environmental and agricultural applications

Bacteria immobilization on biochar is a promising approach to achieve high concentration and stability of microbial cells for several applications. The present review addressed the techniques utilized for bacteria immobilization on biochar, discussing the mechanisms involved in this process, as well as the further utilization in bioremediation and agriculture. This article presents three immobilization techniques, which vary according to their procedures and conditions, including cell growth, adsorption, and adaptation. The mechanisms for cell immobilization are primarily adsorption and biofilm formation on biochar. The favorable characteristics of biochar immobilization depend on the pyrolysis methods, raw materials, and properties of biochar, such as surface area, pore size, pH, zeta potential, hydrophobicity, functional groups, and nutrients. Scanning electron microscope (SEM) and colony forming unit (CFU) are the analyses commonly carried out to verify the efficiency of bacteria immobilization. The benefits of applying biochar-immobilized bacteria include soil decontamination and quality improvement, which can improve plant growth and crop yield. Therefore, this emerging technology represents a promising solution for environmental and agricultural purposes. However, it is important to evaluate the potential adverse impacts on native microbiota by introducing exogenous microorganisms.

Metal-immobilizing Pseudomonas taiwanensis WRS8 reduces heavy metal accumulation in Coriandrum sativum by changing the metal immobilization-related bacterial population abundances

Metal-immobilizing bacteria play a critical role in metal accumulation in vegetables. However, little is known concerning the mechanisms involved in bacteria-induced reduced metal availability and uptake in vegetables. In this study, the impacts of metal-immobilizing Pseudomonas taiwanensis WRS8 on the plant biomass, Cd and Pb availability and uptake in two coriander (Coriandrum sativum L.) cultivars, and bacterial community structure were investigated in the polluted soil. Strain WRS8 increased the biomass of two coriander cultivars by 25-48% and reduced Cd and Pb contents in the edible tissues by 40-59% and available Cd and Pb contents in the rhizosphere soils by 11.1-15.2%, compared with the controls. Strain WRS8 significantly increased the pH values and relative abundances of the dominant populations of Sphingomonas, Pseudomonas, Gaiellales, Streptomyces, Frankiales, Bradyrhizobium, and Luteimonas, while strain WRS8 significantly decreased the relative abundances of the dominant populations of Gemmatimonadaceae, Nitrospira, Haliangium, Paenibacillus, Massilia, Bryobacter, and Rokubacteriales and the rare bacterial populations of Enterorhabdus, Roseburia, Luteibacter, and Planifilum in the rhizosphere soils, compared with the controls. Significantly negative correlations were observed between the available metal concentrations and the abundances of Pseudomonas, Luteimonas, Frankiales, and Planifilum. These results implied that strain WRS8 could affect the abundances of the dominant and rare bacterial populations involved in metal immobilization, resulting in increased pH values and decreased metal availability and uptake in the vegetables in the contaminated soil.

Biochar-immobilized Bacillus spp. for heavy metals bioremediation: A review on immobilization techniques, bioremediation mechanisms and effects on soil

Heavy metals contamination present risks to ecosystems and human health. Bioremediation is a technology that has been applied to minimize the levels of heavy metals contamination. However, the efficiency of this process varies according to several biotic and abiotic aspects, especially in environments with high concentrations of heavy metals. Therefore, microorganisms immobilization in different materials, such as biochar, emerges as an alternative to alleviate the stress that heavy metals have on microorganisms and thus improve the bioremediation efficiency. In this context, this review aimed to compile recent advances in the use of biochar as a carrier of bacteria, specifically Bacillus spp., with subsequent application for the bioremediation of soil contaminated with heavy metals. We present three different techniques to immobilize Bacillus spp. on biochar. Bacillus strains are capable of reducing the toxicity and bioavailability of metals, while biochar is a material that serves as a shelter for microorganisms and also contributes to bioremediation through the adsorption of contaminants. Thus, there is a synergistic effect between Bacillus spp. and biochar for the heavy metals bioremediation. Biomineralization, biosorption, bioreduction, bioaccumulation and adsorption are the mechanisms involved in this process. The application of biochar-immobilized Bacillus strains results in beneficial effects on the contaminated soil, such as the reduction of toxicity and accumulation of metals in plants, favoring their growth, in addition to increasing microbial and enzymatic activity in soil. However, competition and reduction of microbial diversity and the toxic characteristics of biochar are reported as negative impacts of this strategy. More studies using this emerging technology are essential to improve its efficiency, to elucidate the mechanisms and to balance positive and negative impacts, especially at the field scale.

Marked changes in biochar's ability to directly immobilize Cd in soil with aging: implication for biochar remediation of Cd-contaminated soil

To investigate the change in biochar's ability to directly immobilize Cd in soil, a successive wheat cultivation experiment was conducted. Three biochars with different Cd adsorption mechanisms were added to the soils, and a mesh bag was used to separate the soil particles (> 1 μm) from the biochar. The results showed that the ash contents and anionic contents (CO₃^2- and PO₄^3-) of the biochar decreased with the cultivation time, while the oxygen-containing functional group content and CEC of the biochar increased. As a result, the Cd concentration on biochar decreased, by 68.9% for WBC300, while unstable Cd species (acid soluble and reducible fraction of Cd) on biochar increased with successive cultivation, increasing from 3 to 17% for WBC300 in FS. Correspondingly, the ability of biochar to inhibit Cd accumulation in wheat decreased. The results of this study illustrated that the ability of biochar to directly immobilize Cd in soil is not permanent; it gradually decreases with aging in soil. The adsorption mechanism of Cd on biochar changed from precipitation to complexation, and ion exchange processes could be the main reason.

Immobilization of microbes on biochar for water and soil remediation: A review

Biochar has caught great attention over the last decade with the loose and porous structure, and carbon stability provides suitable living conditions for the growth and activity of microorganisms. This review provided a comprehensive summary of biochar immobilization microbe (BIM) in water and soil decontamination. Firstly, the bacterial immobilization techniques including adsorption, entrapping, and covalence methods were exhibited. Secondly, the applications of BIM in water and soil environmental remediation were introduced, mainly including the treatment of organic pollutants, heavy metals, and N/P, among which the most frequently immobilized microorganism was Bacillus. Then, the mechanisms of adsorption, redox, and degradation were analyzed. Finally, pertinent questions for future research of BIM technology were proposed. The purpose of this paper is to provide useful background information for the selection of better biochar fixation microorganisms for water and soil remediation.

Unlocking the potential of co-applied biochar and plant growth-promoting rhizobacteria (PGPR) for sustainable agriculture under stress conditions

Sustainable food security is a major challenge in today's world, particularly in developing countries. Among many factors, environmental stressors, i.e., drought, salinity and heavy metals are major impediments in achieving sustainable food security. This calls for finding environment-friendly and cheap solutions to address these stressors. Plant growth-promoting rhizobacteria (PGPR) have long been established as an environment-friendly means to enhance agricultural productivity in normal and stressed soils and are being applied at field scale. Similarly, pyrolyzing agro-wastes into biochar with the aim to amend soils is being proposed as a cheap additive for enhancement of soil quality and crop productivity. Many pot and some field-scale experiments have confirmed the potential of biochar for sustainable increase in agricultural productivity. Recently, many studies have combined the PGPR and biochar for improving soil quality and agricultural productivity, under normal and stressed conditions, with the assumption that both of these additives complement each other. Most of these studies have reported a significant increase in agricultural productivity in co-applied treatments than sole application of PGPR or biochar. This review presents synthesis of these studies in addition to providing insights into the mechanistic basis of the interaction of the PGPR and biochar. Moreover, this review highlights the future perspectives of the research in order to realize the potential of co-application of the PGPR and biochar at field scale.

Graphical Abstract