Role of Root Exudates on the Soil Microbial Diversity and Biogeochemistry of Heavy Metals

Harnessing the rhizosphere sponge to smooth pH fluctuations and stabilize contaminant retention in biofiltration system

Fluctuating pH conditions can affect heavy metal mobility, thereby limiting the efficiency of biofiltration systems (BS). To address this, we developed an innovative rhizosphere sponge, biochar-based bioreactor (RBB), designed to stabilize Cd2+ removal across a pH range of 5 to 9. RBB consistently outperformed the control, achieving a notable 91.3 % Cd2+ removal at pH 5. By creating optimized oxygen and redox zoning, the rhizosphere sponge enhanced both biochar surface reactions and microbial activity. Under acidic conditions, biochar facilitated Fe2+/Mn2+ precipitation into stable (oxy)hydroxides, a process further driven by microbial oxidation. Consequently, RBB accumulated 1.54 times more Fe-Mn oxide-bound Cd than the control, effectively reducing Cd2+ mobility. Additionally, loosely bound extracellular polymeric substances claimed preferential Cd2+ sequestration after acidification. The stabilized microecology and increased ecological niches, allowing RBB to better buffer against pH fluctuations, presenting it as a robust solution for sustainable heavy metal remediation in variable environments.

Synergistic effects of allantoin and Achyranthes japonica-biochar profoundly alleviate lead toxicity during barley growth

Lead (Pb), a toxic metal, causes severe health hazards to both humans and plants due to environmental pollution. Biochar addition has been efficiently utilized to enhance growth of plants as well as yield in the presence of Pb-induced stress. The present research introduces a novel use of biochar obtained from the weed Achyranthes japonica to enhance the growth of plants in Pb-contaminated soil. An experiment was performed with 7 treatments: Control, Pb2+ (10 mg kg-1) only, biochar (4 %) only, allantoin (4 g kg-1) only, biochar combined with Pb2+, allantoin combined with biochar, as well as a combination of allantoin and biochar with Pb2+. Lead toxicity alone markedly diminished plant growth metrics, including root and shoot length, biomass (wet and dry), chlorophyll concentration, and grain production. The application of biochar, allantoin, or their joint administration markedly enhanced the length of shoots (by 50.3 %, 29 %, and 70 %), length of roots (by 69 %, 50 %, and 69 %), and fresh biomass of shoots (by 5 %, 29 %, and 5 %), respectively. This enhancement is ascribed to improved soil characteristics and more efficient absorption of nutrients. The application of biochar, allantoin and their combination improved the tolerance against Pb2+ by increasing the total chlorophyll level by 12 %, 16 %, and 17 %, respectively, vs. the control. Likewise, these amendments significantly (p < 0.05) improved the activity of antioxidant enzymes, including SOD, POD, and CAT by 49 %, 29 %, and 49 %, respectively. The resistance towards Pb2+ was enhanced by biochar, allantoin, and their combined application, with lower Pb2+ concentrations in shoots (59.9 %, 40.1 %, and 49.8 %), roots (48.2 %, 24.1 %, and 58.3 %), and grains (60.2 %, 29.7 %, and 40.1 %) compared to solely Pb-stress, respectively. In summary, converting the weed Achyranthes japonica into biochar and integrating it with allantoin provides an eco-friendly approach to control its proliferation while efficiently alleviating Pb-induced toxicity in plants.

Microplastic-induced changes in Cd and Cr behavior in the agricultural soil-wheat system: Insights into metal bioavailability and phytotoxicity

Microplastics (MPs) and heavy metals widely coexist in agricultural soils, posing significant risks to soil-plant ecosystems. This study explores the effects of five common MPs-polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polylactic acid (PLA)-and environmental-simulating microplastics (EMPs), composed based on the composition of local MPs in agricultural soils, on the bioavailability and phytotoxicity of Cd and Cr in soils. Pot experiments demonstrated that MPs, particularly PE and EMPs at a 5 % dosage, markedly decreased soil pH, water-holding capacity, and soil organic carbon content. This decrease in pH led to enhanced Cd and Cr mobility and bioavailability, especially with PE and EMPs increasing Cr bioavailability in 15 cm depth soil by up to 43.9 % and 37.8 %, respectively. In soils with 2.1 mg/kg of Cd and 390 mg/kg of Cr, both 1 % and 5 % doses of MPs inhibited wheat growth while enhancing the uptake and translocation of Cd and Cr in wheat. Notably, PE, PS, PLA, and EMPs exposure significantly elevated levels of oxidative stress markers (SOD, POD, CAT, and MDA) in wheat. These findings highlight the importance of further research on the combined impacts of MPs and heavy metals on soil health and plant safety.

Enhanced Phytoextraction Technologies for the Sustainable Remediation of Cadmium-Contaminated Soil Based on Hyperaccumulators-A Review

Heavy metal pollution in soil is a significant challenge around the world, particularly cadmium (Cd) contamination. In situ phytoextraction and remediation technology, particularly focusing on Cd hyperaccumulator plants, has proven to be an effective method for cleaning Cd-contaminated agricultural lands. However, this strategy is often hindered by a long remediation cycle and low efficiency. To address these limitations, assisted phytoextraction has been proposed as a remediation strategy based on the modification of certain traits of plants or the use of different materials to enhance plant growth and increase metal absorption or bioavailability, ultimately aiming to improve the remediation efficiency of Cd hyperaccumulators. To thoroughly understand the progress of Cd hyperaccumulators in remediating Cd-polluted soils, this review article discusses the germplasm resources and assisted phytoextraction strategies for these plants, including microbial, agronomic measure, chelate, nanotechnology, and CO2-assisted phytoextraction, as well as integrated approaches. This review paper critically evaluates and analyzes the numerous approaches and the remediation potential of Cd hyperaccumulators and highlights current challenges and future research directions in this field. The goal is to provide a theoretical framework for the further development and application of Cd pollution remediation technologies in agricultural soils.

Applying cross-scale regulations to Sedum plumbizincicola for strengthening the bioremediation of the agricultural soil that contaminated by electronic waste dismantling and revealing the underlying mechanisms by multi-omics

Electronic waste dismantling has induced the surrounding agricultural soils suffered from combined contamination of heavy metals and organic pollutants. Lower efficiency and complex mechanisms of bioremediation remain to be resolved. Here, we adopted regulations to Sedum plumbizincicola cross aboveground and belowground scales to strengthen the bioremediation efficiency. Results showed that the S. plumbizincicola intercropping with the Astragalus sinicusL. that inoculated with Rhizobium had the highest performance in reductions of Cd, PBDEs and PCBs from soils by 0.11 mg/kg, 67.93 μg/kg and 38.91 μg/kg, respectively. Rhizosphere soil metabolomics analysis demonstrated that reductions in Cd and PBDEs significantly associated with 2-Methylhippuric acid and L-Saccharopine, which were involved in phenylalanine metabolism, biosynthesis of amino acids and lysine. Metagenomics analysis revealed that these functional pathways were mediated by Frankia, Mycobacterium, Blastococcus, etc. microbial taxa, which were also significantly altered by regulations. Moreover, regulation regimes significantly affected transcription genes of S. plumbizincicola. Functional annotation revealed that cross-scale regulations significantly improved bioremediation efficiency through microorganisms and metabolites in the rhizosphere and transcription genes of S. plumbizincicola, which were illustrated to promote plant growth and tolerance to environmental stress. Our integration of multi-omics provides comprehensive and deep insights into molecular mechanisms in the cross-scale regulations of S. plumbizincicola, which would favor remediation techniques advances for the soil contaminated by electronic waste dismantling.

Innovative solutions for coastal wetlands pollution: Application of Spartina alterniflora Loisel. - biochar for cadmium removal and carbon sequestration

Coastal wetland ecosystems are extremely fragile, with heavy metal contamination and uncontrolled growth of invasive plants (e.g. Spartina alterniflora Loisel.) could result in threaten the ecological security and resource wastage. In this study, Spartina alterniflora-biochars at two different temperatures (400 and 600 °C), as well as phosphorus-modified biochars (400BC, 600BC, 400PBC, and 600PBC) were produced, and evaluated for their removal capacity in cadmium (Cd)-contaminated sediments. The maximum sorption capacity, as determined by the Langmuir equation, was in the order of 600BC (838.81 mg/g) > 400BC (707.25 mg/g) > 600PBC (581.69 mg/g) > 400PBC (455.82 mg/g) for Cd, with adsorption occurred through single-layer chemical processes. In the Cd removal tests, increasing incubation times led to a significant reduction in bioavailable Cd (from 84.74 mg/kg at 1 h to 34.79 mg/kg at 7 day). There were significantly negative correlation (p < 0.01) and highly linear regression correlation (R2 > 0.81) between bioavailable Cd and three enzymes' activities (urease, sucrase, and acid phosphatase). Moreover, 63.52%-98.75% of the desorbed Cd can be reabsorbed and fixed in the presence of biochar after 56 d, which effectively reducing the potential for secondary release of heavy metals into the environment. Notably, a comprehensive evaluation through cost-benefit analysis (CBA) and life cycle assessment (LCA) revealed that the biochar system can generate an overall profit of $302.8/tonne and result in a net reduction of CO2 emissions by 1455.479 kg/tonne. The application of biochar could reduce carbon footprints and promote carbon neutrality, and be an effective in-situ remediation strategy for Cd-contaminated coastal wetland.

Synergistic effects of tree-herb intercropping on the phytoremediation efficiency of cadmium and lead contaminated soil

Tree-herb intercropping has emerged as an effective strategy for the remediation of soil contamination. In this study, the effects of intercropping willow with herbaceous plants Lolium perenne L., Iris lactea Pall. and Bidens pilosa L. were investigated on the phytoremediation of Cd- and Pb-contaminated soil. After a 90-day of cultivation, the results showed that intercropping stimulated the phytoremediation efficiency through increased metal accumulation in plants. Intercropping caused a significant (p < 0.05) increase in willow biomass ranging from 48.07 % to 95.58 % by promoting photosynthesis activities and antioxidant responses. Metal contents in willow leaves and roots were also observably (p < 0.05) enhanced, indicating a beneficial effect in tree-herb intercropping systems. The biomass and metal accumulation of I. lactea Pall. and B. pilosa L. decreased due to competitive interactions with willow in the intercropping treatments. However, intercropping with willow (p < 0.05) significantly increased the Pb contents of L. perenne L. Intercropping improved the absorption of bioavailable fractions of Cd and Pb by willow and herbs in comparison to the monoculture. The decrease in soil Cd contents was partly due to the chemical changes induced by root exudates, which enhanced the transfer of Cd from the soil to the plants. Willow showed a tendency for Cd accumulation, whereas herbs exhibited Pb accumulation, reflecting the complementarity of metal accumulation in tree-herb intercropping patterns. Intercropping willow with B. pilosa L. was found to be an effective method for the remediation of Cd-contaminated soil, whereas the combination of willow with L. perenne L. proved suitable for the Pb-contaminated soil. These findings might support the potential of tree-herb intercropping as an effective strategy for enhancing the phytoremediation of contaminated soils.

Small peptides: novel targets for modulating plant-rhizosphere microbe interactions

The crucial role of rhizosphere microbes in plant growth and their resilience to environmental stresses underscores the intricate communication between microbes and plants. Plants are equipped with a diverse set of signaling molecules that facilitate communication across different biological kingdoms, although our comprehension of these mechanisms is still evolving. Small peptides produced by plants (SPPs) and microbes (SPMs) play a pivotal role in intracellular signaling and are essential in orchestrating various plant development stages. In this review, we posit that SPPs and SPMs serve as crucial signaling agents for the bidirectional cross-kingdom communication between plants and rhizosphere microbes. We explore several potential mechanistic pathways through which this communication occurs. Additionally, we propose that leveraging small peptides, inspired by plant-rhizosphere microbe interactions, represents an innovative approach in the field of holobiont engineering.

Modulating phytoremediation: How drip irrigation system affect performance of active green wall and microbial community changes

A recent innovation in air phytoremediation is active green walls, which utilises biofiltration technology with airflow from mechanical ventilation. While this novel technology is gaining traction, the influence of irrigation on soil moisture, and subsequently the microbial communities that play a role in air filtration is untested. In this study, the application of drip irrigation techniques in active green walls were investigated for their influence on system performance. A modular green wall system was tested, with tests across 7 different plant species, as well as a substrate only control. Water distribution across the modules, the water-carrying capacity and airflow through the substrate were measured. The microbial community present, which is critical to the phytoremediation process, was quantified by identifying individual microbial phospholipid fatty acids (PLFA) within the substrate. Results demonstrated that the lower-speed drip irrigation reduced water consumption compared to the rapid system, and had generally more uniform moisture distribution. High flow drip irrigation resulted in a water pathway phenomenon, leading to uneven moisture distribution within the green wall, and this effect was accentuated with fibrous root plant species. Drip irrigation did not change microbial community composition across planted modules, apart from increasing fungi by 6%, but did wash out bacteria at the high flow rate used (-56.67%), thus low flow rate irrigation rate is more beneficial for both plant growth and microbial community composition. The current work provides evidence that drip irrigation has considerable effects on both substrate airflow rate and substrate microbial density: both key to system air cleaning performance.

Root exudate-assisted phytoremediation of copper and lead contamination using Rumex acetosa L. and Rumex K-1

Heavy metal pollutants can be effectively removed from soil through phytoremediation using root exudates. Herein, experiments were conducted to assess the phytoremediation capabilities of Rumex acetosa L. and Rumex K-1 root exudates for copper (Cu) and lead (Pb) contamination. Results indicated that these root exudates effectively adsorbed Cu and Pb. Furthermore, the optimal adsorption conditions of Cu by the root exudates of both plants were as follows: light duration of 36 h, light intensity of 8000 Lx, temperature of 25 °C and CO(NH2)2 concentration of 0 %. Moreover, the optimal adsorption conditions of Pb by Rumex acetosa L. and Rumex K-1 root exudates were light duration of 48 h and 24 h, respectively, light intensity of 8000 Lx, temperature of 25 °C and CO(NH2)2 concentration of 0 %. In addition, the root exudates from both plants enhanced the enrichment and transport of Cu and Pb. Moreover, the root was found to be the main accumulation site of Pb, while the stems and leaves were the main accumulation sites of Cu. With the application of root exudates, plant growth increased, with growth indices in Rumex acetosa L. and Rumex K-1 groups treated with exudates being 1.08-1.81-fold and 1.06-1.9-fold higher, respectively, compared with the untreated ones; physiological indexes showed 1.14-2.62-fold and 1.14-2.71-fold improvements, respectively. Remediation efficiency indexes showed 1.05-1.62-fold and 1.10-1.89-fold improvements, respectively. Rumex acetosa L. and Rumex K-1 exhibited promising potential for the phytoremediation of Cu and Pb, with root exudates playing a critical role in metal adsorption and stabilisation, suggesting their potential for enhancing remediation capabilities. This study sheds light on the mechanisms of root exudate-assisted phytoremediation and provides insights into alleviating heavy metal pollution.

Compost-diatomite-based phytostabilization course under extreme environmental conditions in terms of high pollutant contents and low temperatures

The effects of changes in environmental temperatures on the immobilization or removal of cationic potentially toxic elements (PTE) in heavily polluted soils are often poorly understood, although both are widely studied in the context of phytostabilization. To address this issue, a novel compost-diatomite hybrid (CDH) amendment was developed and applied for assisted phytostabilization at two external temperature regimes. (Cd/Ni/Cu/Zn)-extremely polluted soils (unenriched and CDH-enriched) were cultivated with perennial ryegrass and native soil microbiome under greenhouse conditions and then transferred to freeze-thaw conditions (FTC). The decrease in metal potential toxicity in soils subjected to phytostabilization following both temperature treatments was characterized by a combination of sequential extraction and atomic absorption measurements. The soil microbiome was characterized by high-throughput sequencing. In a relative comparison, the greatest decrease in the content of all PTEs in CDH-enriched soil (compared to unenriched soil) appeared in FTC. Furthermore, under the influence of FTC, in the relative comparison between two CDH-enriched soils (exposed-, and not-exposed- to FTC) and two unenriched soils (exposed-, and not-exposed- to FTC), the content of all PTEs decreased more sharply in the CDH-enriched series than in the unenriched series. The largest redistribution into four sequentially extracted fractions in CDH-enriched soil was found for Zn. Based on the distribution pattern, Zn immobilization was greater in CDH-enriched soil in FTC. CDH increased species richness in the soil, while FTC stimulated the growth of Bacteroidia, Alphaproteobacteria, Theromomicrobia, and Gammaproteobacteria. The analysis of the functionalities of the microbiome indicated enhanced metal transportation and defense systems in samples exposed to FTC. The current research is crucial for understanding how extreme environmental conditions in both cases high pollutant levels and low temperatures affect the movement and transformation of PTEs in polluted soils during phytostabilization.

Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice of early rice was below 0.2 mg·kg-1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, β-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including roots removal, is an economical and effective technique to reduce Cd accumulation in rice plants.

Investigation of arsenic contamination in soil and plants along the river of Xinzhou abandoned gold mine in Qingyuan, China

The exploitation of mineral resources is very important for economic development, but disorderly exploitation poses a serious threat to the ecological environment. However, investigations on the advantages of plant species and environmental pollution in polluted mining areas are limited. Thus, a survey was conducted to evaluate the impacts of abandoned mines on the surrounding ecological environment along rivers in polluted areas and to determine the Arsenic (As) pollution status in soil and plants. The results showed that the soil and vegetation along the river in the survey area were seriously polluted by As. The total As content of the 15 samples was significantly greater than the national soil background value (GB 15618-2018), and degree of pollution was nonlinearly related to the distance from the mine source, R2 = 0.9844. B. bipinnata, P. vittata and B. nivea were predominant with degrees of dominance of 0.01-0.33, 0.05-0.11, and 0.06-0.14 respectively. The As enrichment capacities of Juncus and P. vittata were significantly greater than those of the other plants, while the bioaccumulation factors (BCFs) were 21.81 and 7.04, respectively.

Exploring Phytoremediation Potential: A Comprehensive Study of Flora Inventory and Soil Heavy Metal Contents in the Northeastern Mining Districts of Morocco

Mining activities produce waste materials and effluents with very high metal concentrations that can negatively impact ecosystems and human health. Consequently, data on soil and plant metal levels are crucial for evaluating pollution severity and formulating soil reclamation strategies, such as phytoremediation. Our research focused on soils and vegetation of a highly contaminated site with potentially toxic metals (Pb, Zn, and Cu) in the Touissit mining districts of eastern Morocco. Vegetation inventory was carried out in three mine tailings of the Touissit mine fields using the "field tower" technique. Here, 91 species belonging to 23 families were inventoried: the most represented families were Poaceae and Asteraceae, and the biological spectrum indicated a predominance of Therophytes (55.12%). From the studied areas, 15 species were selected and collected in triplicate on the tailings and sampled with their corresponding rhizospheric soils, and analyzed for Pb, Zn, and Cu concentrations. Reseda lutea, lotus marocanus, and lotus corniculatus can be considered as hyperaccumulators of Pb, as these plants accumulated more than 1000 mg·kg-1 in their aerial parts. According to TF, these plant species could serve as effective plants for Pb phytoextraction.

Rhizospheric bacteria from the Atacama Desert hyper-arid core: cultured community dynamics and plant growth promotion

The Atacama Desert is the oldest and driest desert on Earth, encompassing great temperature variations, high ultraviolet radiation, drought, and high salinity, making it ideal for studying the limits of life and resistance strategies. It is also known for harboring a great biodiversity of adapted life forms. While desertification is increasing as a result of climate change and human activities, it is necessary to optimize soil and water usage, where stress-resistant crops are possible solutions. As many studies have revealed the great impact of the rhizobiome on plant growth efficiency and resistance to abiotic stress, we set up to explore the rhizospheric soils of Suaeda foliosa and Distichlis spicata desert plants. By culturing these soils and using 16S rRNA amplicon sequencing, we address community taxonomy composition dynamics, stability through time, and the ability to promote lettuce plant growth. The rhizospheric soil communities were dominated by the families Pseudomonadaceae, Bacillaceae, and Planococcaceae for S. foliosa and Porphyromonadaceae and Haloferacaceae for D. spicata. Nonetheless, the cultures were completely dominated by the Enterobacteriaceae family (up to 98%). Effectively, lettuce plants supplemented with the cultures showed greater size and biomass accumulation. We identified 12 candidates that could be responsible for these outcomes, of which 5 (Enterococcus, Pseudomonas, Klebsiella, Paenisporosarcina, and Ammoniphilus) were part of the built co-occurrence network. We aim to contribute to the efforts to characterize the microbial communities as key for the plant's survival in extreme environments and as a possible source of consortia with plant growth promotion traits aimed at agricultural applications.IMPORTANCEThe current scenario of climate change and desertification represents a series of incoming challenges for all living organisms. As the human population grows rapidly, so does the rising demand for food and natural resources; thus, it is necessary to make agriculture more efficient by optimizing soil and water usage, thus ensuring future food supplies. Particularly, the Atacama Desert (northern Chile) is considered the most arid place on Earth as a consequence of geological and climatic characteristics, such as the naturally low precipitation patterns and high temperatures, which makes it an ideal place to carry out research that seeks to aid agriculture in future conditions that are predicted to resemble these scenarios. Our main interest lies in utilizing microorganism consortia from plants thriving under extreme conditions, aiming to promote plant growth, improve crops, and render "unsuitable" soils farmable.

Silicon and iron nanoparticles protect rice against lead (Pb) stress by improving oxidative tolerance and minimizing Pb uptake

Lead (Pb) is toxic to the development and growth of rice plants. Nanoparticles (NPs) have been considered one of the efficient remediation techniques to mitigate Pb stress in plants. Therefore, a study was carried out to examine the underlying mechanism of iron (Fe) and silicon (Si) nanoparticle-induced Pb toxicity alleviation in rice seedlings. Si-NPs (2.5 mM) and Fe-NPs (25 mg L-1) were applied alone and in combination to rice plants grown without (control; no Pb stress) and with (100 µM) Pb concentration. Our results revealed that Pb toxicity severely affected all rice growth-related traits, such as inhibited root fresh weight (42%), shoot length (24%), and chlorophyll b contents (26%). Moreover, a substantial amount of Pb was translocated to the above-ground parts of plants, which caused a disturbance in the antioxidative enzyme activities. However, the synergetic use of Fe- and Si-NPs reduced the Pb contents in the upper part of plants by 27%. It reduced the lethal impact of Pb on roots and shoots growth parameters by increasing shoot length (40%), shoot fresh weight (48%), and roots fresh weight (31%). Both Si and Fe-NPs synergistic application significantly elevated superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione (GSH) concentrations by 114%, 186%, 135%, and 151%, respectively, compared to plants subjected to Pb stress alone. The toxicity of Pb resulted in several cellular abnormalities and altered the expression levels of metal transporters and antioxidant genes. We conclude that the synergistic application of Si and Fe-NPs can be deemed favorable, environmentally promising, and cost-effective for reducing Pb deadliness in rice crops and reclaiming Pb-polluted soils.

Progress in Microbial Fertilizer Regulation of Crop Growth and Soil Remediation Research

More food is needed to meet the demand of the global population, which is growing continuously. Chemical fertilizers have been used for a long time to increase crop yields, and may have negative effect on human health and the agricultural environment. In order to make ongoing agricultural development more sustainable, the use of chemical fertilizers will likely have to be reduced. Microbial fertilizer is a kind of nutrient-rich and environmentally friendly biological fertilizer made from plant growth-promoting bacteria (PGPR). Microbial fertilizers can regulate soil nutrient dynamics and promote soil nutrient cycling by improving soil microbial community changes. This process helps restore the soil ecosystem, which in turn promotes nutrient uptake, regulates crop growth, and enhances crop resistance to biotic and abiotic stresses. This paper reviews the classification of microbial fertilizers and their function in regulating crop growth, nitrogen fixation, phosphorus, potassium solubilization, and the production of phytohormones. We also summarize the role of PGPR in helping crops against biotic and abiotic stresses. Finally, we discuss the function and the mechanism of applying microbial fertilizers in soil remediation. This review helps us understand the research progress of microbial fertilizer and provides new perspectives regarding the future development of microbial agent in sustainable agriculture.

Genomic mechanisms of plant growth-promoting bacteria in the production of leguminous crops

Legumes are highly nutritious in proteins and are good food for humans and animals because of their nutritional values. Plant growth-promoting bacteria (PGPR) are microbes dwelling in the rhizosphere soil of a plant contributing to the healthy status, growth promotion of crops, and preventing the invasion of diseases. Root exudates produced from the leguminous plants' roots can lure microbes to migrate to the rhizosphere region in other to carry out their potential activities which reveals the symbiotic association of the leguminous plant and the PGPR (rhizobia). To have a better cognition of the PGPR in the rhizosphere of leguminous plants, genomic analyses would be conducted employing various genomic sequences to observe the microbial community and their functions in the soil. Comparative genomic mechanism of plant growth-promoting rhizobacteria (PGPR) was discussed in this review which reveals the activities including plant growth promotion, phosphate solubilization, production of hormones, and plant growth-promoting genes required for plant development. Progress in genomics to improve the collection of genotyping data was revealed in this review. Furthermore, the review also revealed the significance of plant breeding and other analyses involving transcriptomics in bioeconomy promotion. This technological innovation improves abundant yield and nutritional requirements of the crops in unfavorable environmental conditions.

Combination of intercropping maize and soybean with root exudate additions reduces metal mobility in soil-plant system under wastewater irrigation

The effects of root exudates and irrigation with treated wastewater on heavy metal mobility and soil bacterial composition under intercropping remain poorly understood. We conducted a pot experiment with maize and soybean grown in monocultures or intercultures, irrigated with either groundwater or treated wastewater. In addition, the pre-collected root exudates from hydroponic culture with mono- or inter-cropped maize and soybean were applied to the soil at four levels (0 %, 16 %, 32 % and 64 %). The results showed that application of root exudates increased plant growth and soil nutrient content. The analysis of "Technique for Order of Preference by Similarity to Ideal Solution" for higher plant biomass and lower soil Cd and Pb concentrations indicated that the best performance of soybean under treated wastewater irrigation was recorded under intercropping applied with 64 % of exudates, with a performance score of 0.926 and 0.953 for Cd and Pb, respectively. The second-best performance of maize under treated wastewater irrigation was also observed under intercropping applied with 64 % of exudates. Root exudate application reduced heavy metals migration in the soil-plant system, with a greater impact in intercropping than in monocropping. In addition, certain soil microorganisms were also increased with root exudate application, regardless of irrigation water. This study suggests that appropriate application of root exudates could potentially improve plant growth and soil health, and reduce toxic heavy metal concentrations in soils and plants irrigated with treated wastewater.