Cadmium-resistant phosphate-solubilizing bacteria immobilized on phosphoric acid-ball milling modified biochar enhances soil cadmium passivation and phosphorus bioavailability

Phosphate-solubilizing bacteria reduce Cd accumulation in spinach by forming P-Ca adhesive films in the roots and altering the structure of soil macroaggregates

Phosphate-solubilizing bacteria are crucial for plant growth promotion and heavy metal remediation in contaminated soils. However, the mechanisms underlying phosphate-solubilizing bacteria-mediated regulation of phosphorus (P) and calcium (Ca) availability and cadmium (Cd) bioavailability reduction remain unclear. This study investigated the effects and mechanisms of phosphate-solubilizing bacteria Kluyvera sp. M8 on P and Ca regulation, soil aggregate structure modification, and Cd uptake inhibition in spinach. Untargeted metabolomics showed that strain M8 secreted substances such as 3-Indolepropionic acid, N1-Acetylspermidine, and uric acid for the release of P and Ca and the immobilization of Cd. Strain M8 facilitated P and Ca enrichment on spinach root surfaces, forming a protective P-Ca film that reduced Cd into the root. Furthermore, it enhanced Cd immobilization in root cell walls and increased macroaggregate proportions in rhizosphere soil. Notably, strain M8 enriched active inorganic P and Ca components within macroaggregates while disrupting CaCO3 structure, promoting insoluble CdCO3 formation, enhancing Cd immobilization and decreasing the concentration of active Cd in the pore water. These findings provide valuable insights for Cd pollution management in agricultural fields and Cd uptake reduction in vegetables.

The effect of phosphate solubilizing bacteria on the fate of cadmium immobilized by microbial induced phosphate precipitation

Microbial induced phosphate precipitation (MIPP) is an environmentally friendly method for Cd immobilization. MIPP precipitates were mainly insoluble phosphates which were inevitably affected by phosphate solubilizing bacteria (PSB). However, effect of PSB on the fate of Cd immobilized by MIPP still remained unclear. Here, we investigated the transformation of Cd and MIPP precipitates with PSB strain Enterobacter sp. QY1. The results showed that Enterobacter sp. QY1 could secrete D-gluconic acid and acetic acid to release Cd, Ca and phosphorus from MIPP precipitates. The concentration of released Cd reached a peak on 5-7 d, then decreased, indicating that some of released Cd was re-immobilized. Sorption of Enterobacter sp. QY1 and substitution of CaxCd10-x(PO4)6(OH)2 played a major role in re-immobilization of released Cd. Finally, about 7.0-8.7 % Cd immobilized by MIPP was released after 28 d. The fate of Cd immobilized by MIPP in soil was also explored, Sphingomonas might dissolve MIPP precipitates and release Cd, then the concentration of exchangeable Cd increased. Overall, PSB mobilized Cd immobilized by MIPP, posing a threat to the long-term effectiveness of MIPP technology. Our findings are beneficial to understand the effect of phosphorus biogeochemical cycle on durability of MIPP in Cd remediation.

Two strains of cadmium (Cd)-resistant bacteria isolated from soils and their ability to promote oilseed rape (Brassica juncea L.) to grow and absorb Cd in soils

As a highly toxic, mobile, and persistent heavy metal, cadmium (Cd) in soils is becoming a crucial environmental problem. Most of classical physical and chemical remediation measures for Cd-contaminated soils possibly cause some dangers to soil structure and characteristics and potential secondary pollution, however, Cd-resistant microbial which can sequestrate Cd by releasing extracellular polymeric substances (EPS) capable of ion exchange, coordination, and adsorption and improve plant growth should be favorable for remediation of Cd-contaminated soils due to being environmentally friendly and cost-effective. Therefore, the plant-microbe combination is becoming a priority option in the remediation of Cd-contaminated soils. Here, we isolated two strains of Cd-resistant bacteria from soils and investigated the ability of the two strains to promote growth of oilseed rape (Brassica juncea L.) and Cd uptake by the plants. Citrobacter farmeri and Cupriavidus gilardii were isolated from soils via culture media containing 30 and 50 mg/L Cd, respectively, which could release EPS including proteins, polysaccharide, and DNA. The EPS from C. gilardii was significantly higher than that from C. farmeri, and the proportion of protein in EPS was the highest for two strains. Additionally, two strains secreted indole-3-acetic acid (IAA) and could solubilize phosphorus, and the ability of C. gilardii to secret IAA was significantly higher than that of C. farmeri. The pot experiment indicated that C. farmeri and C. gilardii significantly enhanced oilseed rape biomass (by 81.99% and 76.57%, respectively), C and N contents, Cd accumulation in plants by 229.03% and 264.63%, respectively, and remediation efficiency at 40 days after emergence (flowering stage). However, the difference in promoting plant growth and Cd uptake and phytoremediation efficiency of Cd-contaminated soils between the two strains was not significant. Overall, C. farmeri and C. gilardii isolated from soils might be promising strains in enhancing phytoremediation of Cd-contaminated soils.

Acid-modified corn straw biochar immobilized Pseudomonas hibiscus CN-1 facilitated the bioremediation of carbendazim-contaminated soil

Carbendazim application in agroecosystems has posed potential threats to ecosystems and human health. The utilization of biochar-based materials for immobilizing microorganisms offers a sustainable strategy for effective bioremediation. In this study, a novel highly efficient carbendazim-degrading bacterium Pseudomonas hibiscus CN-1 was isolated and immobilized using corn straw-based biochar as a carrier. The effects of degrading strain CN-1, biochar materials, and biochar-CN-1 composite on carbendazim degradation, soil enzyme activities, and community structure diversity were investigated. Under the optimal conditions (pH 6.6, 31 °C, and 6.5% inoculum volume), strain CN-1 metabolized carbendazim into benzimidazole-2-carbamic acid and 2-hydroxybenzimidazole, indicating that demethylation was a major metabolic pathway. Among the biochar materials, acid-modified biochar pyrolyzed at 700 °C proved to be the most effective carrier for strain CN-1 immobilization and efficient removal of carbendazim, achieving a removal rate of 78.7% in water. Compared to the control, the degradation half-lives of carbendazim in the soil with biochar, strain CN-1, and the biochar-strain CN-1 composite were reduced from 30.97 to 23.23, 19.46, and 10.99 days, respectively. Soil enzyme activities and bacterial community diversity results demonstrated that the biochar-strain CN-1 composite not only mitigated the adverse effects of carbendazim on soil enzyme activities but also had the most positive impact on soil microbial richness and diversity. This study highlights the importance of selecting appropriate biochar materials and offers insights into an environmentally friendly method for the efficient bioremediation of soils contaminated with carbendazim.

Root acid phosphatases and microbial biomass phosphorus induced Cd tolerance and P acquisition in wheat inoculated with P solubilizing bacteria

Microbial bioremediation has emerged promisingly to improve crop tolerance to cadmium (Cd). Moreover, Cd tolerance and phosphate acquisition in plants positively correlated under P solubilizing bacteria inoculation, yet there is no evidence on specific mechanisms influencing Cd tolerance and plant P acquisition. The present study evaluates Cd tolerance in rock P-amended durum wheat in response to inoculation with P solubilizing bacteria (PSB) [three individual isolates Bacillus siamensis, Rahnella aceris, Bacillus cereus and their consortium (PSBCs)] and consequently reveals key rhizosphere mechanisms involved in both Cd tolerance and P use efficiency. Results show that inoculation overall improved plant growth, rhizosphere parameters and nutrient uptake (P, N, K) under increasing Cd concentrations [8 (Cd8) and 16 (Cd16) ppm Cd2+]. Under Cd16, Rahnella aceris induced the most significant plant responses in terms of biomass [shoots (31 %), roots (40 %), and spikes (92 %)], rhizosphere available P (234 %) and root inorganic P (109 %) compared to uninoculated plant. Microbial biomass P (MBP) and root acid phosphatases (APase) were 33-and 13-times higher, respectively, than in uninoculated plants. In addition, inoculation (particularly using PSBCs) significantly decreased Cd translocation factor (TF) (Cd8: -17 % and Cd16: -22 %) and Cd bioaccumulation factor (BAF) (Cd8: -6 % and Cd16: -40 %) concomitantly to enhanced root morphological traits and P contents in shoots and spikes. Furthermore, PSB inoculation under Cd constraint increased (rhizosphere available P / MBP) and (Root APase / Rhizosphere Apase) ratios that significantly (p < 0.05) correlate with plant P uptake in shoots and spikes. Increase in both ratios was concomitant to a significant decrease in TF and BAF of Cd exemplified by negatively significant correlations (r2=0.70 and r2=0.57, p < 0.05). This finding elucidates the key role of bacterial inoculation that presumably triggered Cd tolerance and aboveground P owing to increased (rhizosphere available P / MBP) and (Root / Rhizosphere APase) ratios in PSB-inoculated wheat.

Synergistic Effect of Biochar, Phosphate Fertilizer, and Phosphorous Solubilizing Bacteria for Mitigating Cadmium (Cd) Stress and Improving Maize Growth in Cd-Contaminated Soil

Cadmium (Cd) contamination threatens human health and plant growth due to its accumulation in edible parts. The sole application of phosphorus-solubilizing bacteria (PSB), biochar (BC), and phosphorus (P) effectively mitigates Cd's adverse effects in contaminated agricultural systems. However, further investigation into their combined impacts on Cd toxicity and maize (Zea mays) production is essential. This study evaluates the synergistic effects of PSB (10 g kg-1 of Bacillus megaterium), BC (5% w/w), and P (0.8 g kg-1) on soil properties and the morphological and physiological traits of maize cultivated in agricultural soil contaminated with Cd (20 mg kg-1). The study revealed that Cd toxicity negatively impacts soil properties, reducing shoot and root biomass, lowering chlorophyll content, and heightening oxidative stress levels. Conversely, the combined use of P, PSB, and BC markedly improved soil properties, increasing the organic matter by 175.94%, available K by 87.24%, and available P by 306.93% compared to the control. This combination also improved maize growth metrics, with increases in aboveground dry biomass (92.98%), root dry biomass (110.33%), chlorophyll a (28.20%), chlorophyll b (108.34%), and total chlorophyll (37.17%). Notably, the treatment reduced Cd concentrations in maize leaves by 61.08% while increasing soil Cd levels by 31.12% compared to the control group. Overall, the synergistic effect of P-BC-PSB is an eco-friendly strategy for mitigating Cd toxicity in contaminated soil. However, further studies are required to explore its effects and molecular mechanisms on other crops.

Reclamation of co-pyrolyzed dredging sediment as soil cadmium and arsenic immobilization material: Immobilization efficiency, application safety, and underlying mechanisms

The safe management of toxic metal-polluted dredging sediment (DS) is imperative owing to its potential secondary hazards. Herein, the co-pyrolysis product (DS@BC) of polluted DS was creatively applied to immobilize soil Cd and As to achieve DS resource utilization, and the efficiency, safety, and mechanism were investigated. The results revealed that the DS@BC was more effective at reducing soil Cd bioavailability than the DS was (58.9-73.2% vs. 21.8-27.4%), except for the dilution effect, whereas the opposite phenomenon occurred for soil As (25.5-35.7% vs. 35.7-42.8%). The DS@BC immobilization efficiency was dose-dependent for both Cd and As. Soil labile Cd and As were transformed to more stable fractions after DS@BC immobilization. DS@BC immobilization inhibited the transfer of soil Cd and As to Brassica chinensis L. and did not cause excessive accumulation of other toxic metals in the plants. The appropriate addition of the DS@BC (8%) sufficiently alleviated the oxidative stress response of the plants and enhanced their growth. These findings indicate that the DS@BC was safe and effective for soil Cd and As immobilization. DS@BC immobilization decreased the diversity and richness of the rhizosphere soil bacterial community because of the dilution effect. The DS@BC immobilized soil Cd and As via direct adsorption, and indirect increasing soil pH, and regulating the abundance of specific beneficial bacteria (e.g., Bacillus). Therefore, the use of co-pyrolyzed DS as a soil Cd and As immobilization material is a promising resource utilization method for DS. Notably, to verify the long-term effects and safety of DS@BC immobilization, field trials should be conducted to explore the effectiveness and risk of harmful metal release from DS@BC immobilization under real-world conditions.

Building microbial consortia to enhance straw degradation, phosphorus solubilization, and soil fertility for rice growth

Straw pollution and the increasing scarcity of phosphorus resources in many regions of China have had severe impacts on the growing conditions for crop plants. Using microbial methods to enhance straw decomposition rate and phosphorus utilization offers effective solutions to address these problems. In this study, a microbial consortium 6 + 1 (consisting of a straw-degrading bacterium and a phosphate-solubilizing bacterium) was formulated based on their performance in straw degradation and phosphorus solubilization. The degradation rate of straw by 6 + 1 microbial consortium reached 48.3% within 7 days (The degradation ability was 7% higher than that of single bacteria), and the phosphorus dissolution rate of insoluble phosphorus reached 117.54 mg·L- 1 (The phosphorus solubilization ability was 29.81% higher than that of single bacteria). In addition, the activity of lignocellulosic degrading enzyme system was significantly increased, the activities of endoglucanase, β-glucosidase and xylanase in the microbial consortium were significantly higher than those in the single strain (23.16%, 28.02% and 28.86%, respectively). Then the microbial consortium was processed into microbial agents and tested in rice pots. The results showed that the microbial agent significantly increased the content of organic matter, available phosphorus and available nitrogen in the soil. Ongoing research focuses on the determination of the effects and mechanisms of a functional hybrid system of straw degradation and phosphorus removal. The characteristics of the two strains are as follows: Straw-degrading bacteria can efficiently degrade straw to produce glucose-based carbon sources when only straw is used as a carbon source. Phosphate-solubilizing bacteria can efficiently use glucose as a carbon source, produce organic acids to dissolve insoluble phosphorus and consume glucose at an extremely fast rate. The analysis suggests that the microbial consortium 6 + 1 outperformed individual strains in terms of both performance and application effects. The two strains within the microbial consortium promote each other during their growth processes, resulting in a significantly higher rate of carbon source consumption compared to the individual strains in isolation. This increased demand for carbon sources within the growth system facilitates the degradation of straw by the strains. At the same time, the substantial carbon consumption during the metabolic process generated a large number of organic acids, leading to the solubilization of insoluble phosphorus. It also provides a basis for the construction of this type of microbial consortium.

Ball milling nano-sized biochar: bibliometrics, preparation, and environmental application

Nano-sized biochar, which is a small structure prepared from biochar by grinding, has surpassed traditional biochar in performance, showing enhanced effects and potential for a wide range of environmental applications. Firstly, this paper visualizes and analyzes the literature in this field by CiteSpace to clarify the development trend of nano-sized biochar. The review intuitively shows the most influential countries, the most productive institutions, and the most concerned hot spots in the field of nano-sized biochar. Secondly, these hotspots in environment management are summarized by keywords and clustering: (1) The application of ball milling is a modification scheme that researchers have paid attention to, and it is also a key method for preparing biochar nanomaterials. It has a more dispersed structure and can support more modified materials. (2) Nano-sized biochar in the comprehensive utilization of water, soil, and plants was discussed and is a small range of application modification methods. (3) The bidirectional effects of nano-sized biochar on plants were analyzed, and the challenges in its application were listed. Finally, the economic management of nano-sized biochar and the relationship between microorganisms are the focus of the next research.

Enhanced degradation of phthalate esters (PAEs) by biochar-sodium alginate immobilised Rhodococcus sp. KLW-1

In this study, a new strain of bacteria, named Rhodococcus sp. KLW-1, was isolated from farmland soil contaminated by plastic mulch for more than 30 years. To improve the application performance of free bacteria and find more ways to use waste biochar, KLW-1 was immobilised on waste biochar by sodium alginate embedding method to prepare immobilised pellet. Response Surface Method (RSM) predicted that under optimal conditions (3% sodium alginate, 2% biochar and 4% CaCl2), di (2-ethylhexyl) phthalate (DEHP) degradation efficiency of 90.48% can be achieved. Under the adverse environmental conditions of pH 5 and 9, immobilisation increased the degradation efficiency of 100 mg/L DEHP by 16.42% and 11.48% respectively, and under the high-stress condition of 500 mg/L DEHP concentration, immobilisation increased the degradation efficiency from 71.52% to 91.56%, making the immobilised pellets have strong stability and impact load resistance to environmental stress. In addition, immobilisation also enhanced the degradation efficiency of several phthalate esters (PAEs) widely existing in the environment. After four cycles of utilisation, the immobilised particles maintained stable degradation efficiency for different PAEs. Therefore, immobilised pellets have great application potential for the remediation of the actual environment.

The application of P-modified biochar in wastewater remediation: A state-of-the-art review

Phosphorus modified biochar (P-BC) is an effective adsorbent for wastewater remediation, which has attracted widespread attention due to its low cost, vast source, unique surface structure, and abundant functional groups. However, there is currently no comprehensive analysis and review of P-BC in wastewater remediation. In this study, a detailed introduction is given to the synthesis method of P-BC, as well as the effects of pyrolysis temperature and residence time on physical and chemical properties and adsorption performance of the material. Meanwhile, a comprehensive investigation and evaluation were conducted on the different biomass types and phosphorus sources used to synthesize P-BC. This article also systematically compared the adsorption efficiency differences between P-BC and raw biochar, and summarized the adsorption mechanism of P-BC in removing pollutants from wastewater. In addition, the effects of P-BC composite with other materials (element co-doping, polysaccharide stabilizers, microbial loading, etc.) on physical and chemical properties and pollutant adsorption capacity of the materials were investigated. Some emerging applications of P-BC were also introduced, including supercapacitors, CO2 adsorbents, carbon sequestration, soil heavy metal remediation, and soil fertility improvement. Finally, some valuable suggestions and prospects were proposed for the future research direction of P-BC to achieve the goal of multiple utilization.

Study on the solidification performance and mechanism of heavy metals by sludge/biomass ash ceramsites, biochar and biomass ash

Industrial by-products are stored in large quantities in the open, leading to wasted resources and environmental pollution, and the natural environment is similarly faced with phosphate depletion and serious water and soil pollution. This study uses these by-products to produce a new sludge/biomass ash ceramsite that will be used to adsorb nitrogen and phosphorus from wastewater, and solidify heavy metals in the soil while releasing Olsen P. The sludge/biomass ash ceramsites are made using sewage sludge and biomass ash in a certain ratio calcined at high temperatures and modified for the adsorption of nitrogen and phosphorus from wastewater. Sludge/biomass ash ceramsites before and after phosphorus adsorption, biochar and biomass ash were compared to analyze their heavy metal adsorption capacity and potential as phosphate fertilizer. After phosphorus adsorption, the sludge/biomass ash ceramsites released effective phosphorus steadily and rapidly in the soil, with a greater initial release than biochar and biomass ash, and the ceramsites were in a granular form that could be easily recycled. Biochar and biomass residue, due to their surface functional groups, are better at solidifying heavy metals than sludge/biomass ash ceramsites. Biochar, biomass ash and sludge/biomass ash ceramsites significantly reduced the concentrations of Cd, Cu, Pb and Zn in the soil. Correlation analysis demonstrated that there was a synergistic relationship between the increase in soil Olsen P content and the change in pH, with the increase in soil Olsen P content and the increase in pH contributing to heavy metal solidification.

Effects of Priestia aryabhattai on Phosphorus Fraction and Implications for Ecoremediating Cd-Contaminated Farmland with Plant-Microbe Technology

The application of phosphate-solubilizing bacteria has been widely studied in remediating Cd-contaminated soil, but only a few studies have reported on the interaction of P and Cd as well as the microbiological mechanisms with phosphate-solubilizing bacteria in the soil because the activity of phosphate-solubilizing bacteria is easily inhibited by the toxicity of Cd. This paper investigates the phosphorus solubilization ability of Priestia aryabhattai domesticated under the stress of Cd, which was conducted in a soil experiment with the addition of Cd at different concentrations. The results show that the content of Ca2-P increased by 5.12-19.84%, and the content of labile organic phosphorus (LOP) increased by 3.03-8.42% after the addition of Priestia aryabhattai to the unsterilized soil. The content of available Cd decreased by 3.82% in the soil with heavy Cd contamination. Priestia aryabhattai has a certain resistance to Cd, and its relative abundance increased with the increased Cd concentration. The contents of Ca2-P and LOP in the soil had a strong positive correlation with the content of Olsen-P (p < 0.01), while the content of available Cd was negatively correlated with the contents of Olsen-P, Ca2-P, and LOP (p < 0.05). Priestia aryabhattai inhibits the transport of Cd, facilitates the conversion of low-activity P and insoluble P to Ca2-P and LOP in the soil, and increases the bioavailability and seasonal utilization of P in the soil, showing great potential in ecoremediating Cd-contaminated farmland soil with plant-microbe-combined technology.

Enhanced cadmium phytoextraction efficiency of ryegrass (Lolium perenne L.) by porous media immobilized Enterobacter sp. TY-1

Although studies on immobilized microorganisms have been conducted, their performance remains unclear for enhancing plants to remediate cadmium (Cd)-contaminated soil. In this study, a Cd-resistant strain TY-1 with good plant growth promotion traits was immobilized by biochar (BC) or oyster shell (OS) power to strengthen ryegrass to remediate Cd-contaminated soil. SEM-EDS combined with FTIR showed that TY-1 could tolerate Cd toxicity by surface precipitation, and functional groups such as hydroxyl and carbonyl groups might be involved. In the biocomposite treatments, soil pH increased, and the activity of fertility-related enzymes such as dehydrogenase increased by 109.01%-128.01%. The relative abundance of genus Saccharimonadales decreased from 7.97% to 3.35% in BS-TY and 2.61% in OS-TY, respectively. Thus, a suitable environment for ryegrass growth was created. The fresh weight, dry weight, plant height and Cd accumulation of ryegrass in TY treatment increased by 122.92%, 114.81%, 42.08% and 8.05%, respectively, compared to the control. Cd concentration in ryegrass was further increased in BC-TY and OS-TY by 24.14% and 40.23%, respectively. The improvement in soil microcosm and plant biomass forms an ongoing virtuous cycle, demonstrating that using carrier materials to improve the efficiency of microbial-assisted phytoremediation is realistic and feasible.