Employment of pqqE gene as molecular marker for the traceability of Gram negative phosphate solubilizing bacteria associated to plants.

Insoluble phosphorous compounds solubilization by soil bacteria is of great relevance since it puts available the phosphorus to be used by plants. The production of organic acids is the main microbiological mechanism by which insoluble inorganic phosphorus compounds are solubilized. In Gram negative bacteria, gluconic acid is synthesized by the activity of the holoenzyme glucose dehydrogenase-pyrroloquinoline quinine named GDH-PQQ. The use of marker genes is a very useful tool to evaluate the persistence of the introduced bacteria and allow to follow-up the effect of biotic and abiotic factors on these beneficial microorganisms in the soil. In previous studies we detected the presence of the pqqE gene in a great percentage of both non-culturable and culturable native soil bacteria. The objective of this study was to analyze the phylogeny of the sequence of pqqE gene and its potential for the study of phosphate solubilizing bacteria from pure and mixed bacterial cultures and rhizospheric soil samples. For this, the presence of the pqqE gene in the genome of phosphate solubilizing bacteria that belong to several bacteria was determined by PCR. Also, this gene was analyzed from mixed bacterial cultures and rhizospheric soil associated to peanut plants inoculated or not with phosphate solubilizing bacteria. For this, degenerate primers designed from several bacterial genera and specific primers for the genus Pseudomonas spp., designed in this study, were used. DNA template used from simple or mixed bacterial cultures and from rhizospheric soil samples was obtained using two different DNA extraction techniques. Results indicated that pqqE gene amplification product was found in the genome of all Gram negative phosphate solubilizing bacteria analyzed. It was possible to detect this gene in the DNA obtained from mixed cultures where these bacteria grew in interaction with other microorganisms and in that obtained from rhizospheric soil samples inoculated or not with these bacteria. The phylogenetic analysis indicated that pqqE gene is a conserved gene within related genera. In conclusion, pqqE gene could be a potential marker for the study of phosphate solubilizing bacterial populations.

Gongronella sp. w5 hydrolyzes plant sucrose and releases fructose to recruit phosphate-solubilizing bacteria to provide plants with phosphorus.

The mechanisms of how plant-beneficial rhizospheric fungi interact with the soil microbial community to promote plant growth by facilitating their phosphorus acquisition are poorly understood. This work supported that a Mucoromycotina fungus, Gongronella sp. w5 (w5), could promote phosphorus uptake of Medicago truncatula by increasing the available phosphorus (P) in the soil. The abundance of phosphate-solubilizing bacteria (PSB) and the activity of alkaline phosphatase (ALP) in alfalfa rhizosphere soil increased after w5 inoculation. Further analysis showed that w5 donated a portion of ALP activity and also stimulated the PSB to secrete ALP during plant-w5-PSB interaction to help release more available P in the rhizosphere of M. truncatula. Unlike most plant-beneficial rhizospheric fungi that mainly acquire hexoses from plants, w5 gained sucrose directly from the host plant and then recruited PSB to aid P acquisition by hydrolyzing sucrose and releasing mainly fructose to induce PSB to secrete ALP. IMPORTANCE: This work supported that after absorbing plant sucrose, Gongronella sp. w5 mainly releases sucrose hydrolysis product fructose into the environment. Fructose was used as a carbon source and signaling molecules to induce PSB to co-produce higher alkaline phosphatase activity, releasing soil-available phosphorus and promoting M. truncatula growth. This is the first report that plant-beneficial fungi could directly metabolize sucrose from plants and then recruit PSB to aid P acquisition by providing fructose. Our findings revealed the diversity in pathways of plant-fungi-PSB interactions on soil P acquisition and deepened our understanding of the cooperation of growth-promoting microorganisms in plant rhizosphere.

Sex-specific phosphorus acquisition strategies and cycling in dioecious Populus euphratica forests along a natural water availability gradient.

Soil phosphorus (P) availability affects plant growth and distribution. However, it is still unknown how sex-specific variation in functional traits affects plants' P acquisition and soil P transformation. On wet sites, female poplars had a greater specific root length (SRL), and a higher diversity of arbuscular mycorrhizal fungi (AMF) and phosphate-solubilizing bacteria (PSB). Male poplars living on wet sites increased the abundance of AMF and PSB communities and enhanced moderately labile and highly resistant organic P mineralisation via increased phosphatase activity. In contrast, on the dry site, the abundance and diversity of AMF and PSB communities increased in females, enhancing moderately labile and highly resistant organic P mineralisation via elevating phosphatase activities. Males maintained greater SRL and promoted Ca-P mobilisation via the release of root carboxylic acids and rhizosphere acidification on the dry site. The AMF community diversity followed a similar pattern as that of the PSB community when altering the P availability of different-sex plants. Our results indicated that organic P and Ca-P are the major sources of plant-available P in natural P. euphratica forests. Seasonal shifts and geographic locations affected the share of organic and inorganic P pools, and AMF and PSB diversities, ultimately altering sex-specific P acquisition strategies of plants.

Revealing the phosphate-solubilizing characteristics and mechanisms of the plant growth-promoting bacterium Agrobacterium deltaense C1.

AIMS: This study explores the phosphate (Pi)-solubilizing characteristics and mechanisms of a novel phosphate-solubilizing bacterium, Agrobacterium deltaense C1 (C1 hereafter). METHODS AND RESULTS: The growth-promoting effects of C1 were investigated by gnotobiotic experiments, and the Pi-solubilizing mechanism was revealed by extracellular metabolomics, liquid chromatography analysis, and reverse transcription quantitative polymerase chain reaction. Results showed that C1 significantly increased Arabidopsis biomass and total phosphorus (P) content under P deficiency. Under Ca3(PO4)2 condition, the presence of C1 resulted in a significant and negative correlation between available P content and medium pH changes, implying that Pi dissolution occurs through acid release. Metabolomics revealed C1's ability to release 99 organic acids, with gluconic acid (GA), citric acid, and α-ketoglutaric acid contributing 64.86%, 9.58%, and 0.94%, respectively, to Pi solubilization. These acids were significantly induced by P deficiency. Moreover, C1's Pi solubilization may remain significant even in the presence of available P, as evidenced by substantial pH reduction and high gcd gene expression. Additionally, C1 produced over 10 plant growth-promoting substances. CONCLUSIONS: C1 dissolves Pi primarily by releasing GA, which enhances plant growth under P deficiency. Notably, its Pi solubilization effect is not significantly limited by available Pi.

Phosphate solubilization and plant growth properties are promoted by a lactic acid bacterium in calcareous soil.

On the basis of good phosphate solubilization ability of a lactic acid bacteria (LAB) strain Limosilactobacillus sp. LF-17, bacterial agent was prepared and applied to calcareous soil to solubilize phosphate and promote the growth of maize seedlings in this study. A pot experiment showed that the plant growth indicators, phosphorus content, and related enzyme activity of the maize rhizospheric soils in the LF treatment (treated with LAB) were the highest compared with those of the JP treatment (treated with phosphate solubilizing bacteria, PSB) and the blank control (CK). The types of organic acids in maize rhizospheric soil were determined through LC-MS, and 12 acids were detected in all the treatments. The abundant microbes belonged to the genera of Lysobacter, Massilia, Methylbacillus, Brevundimonas, and Limosilactobacillus, and they were beneficial to dissolving phosphate or secreting growth-promoting phytohormones, which were obviously higher in the LF and JP treatments than in CK as analyzed by high-throughput metagenomic sequencing methods. In addition, the abundance values of several enzymes, Kyoto Encyclopedia of Genes and Genomes (KEGG) orthology, and Carbohydrate-Active Enzymes (CAZys), which were related to substrate assimilation and metabolism, were the highest in the LF treatment. Therefore, aside from phosphate-solubilizing microorganisms, LAB can be used as environmentally friendly crop growth promoters in agriculture and provide another viable option for microbial fertilizers. KEY POINTS: • The inoculation of LAB strain effectively promoted the growth and chlorophyll synthesis of maize seedlings. • The inoculation of LAB strain significantly increased the TP content of maize seedlings and the AP concentration of the rhizosphere soil. • The inoculation of LAB strain increased the abundances of the dominant beneficial functional microbes in the rhizosphere soil.

Spartina alterniflora invasion altered phosphorus retention and microbial phosphate solubilization of the Minjiang estuary wetland in southeastern China.

Spartina alterniflora invasion is considered a critical event affecting sediment phosphorus (P) availability and stock. However, P retention and microbial phosphate solubilization in the sediments invaded with or without S. alterniflora have not been fully investigated. In this study, a sequential fractionation method and high-throughput sequencing were used to analyze P transformation and the underlying microbial mechanisms in the sediments of no plant (NP) zone, transition (T) zone, and plant (P) zone. Results showed that except for organic phosphate (OP), total phosphate (TP), inorganic phosphate (IP), and available phosphate (AP) all followed a significant decrease trend from the NP site to the T site, and to the P site. The vertical decrease of TP, IP, and AP was also observed with an increase in soil depth. Among the six IP fractions, Fe-P, Oc-P, and Ca10-P were the predominant forms, while the presence of S. alterniflora resulted in an obvious P depletion except for Ca8-P and Al-P. Although S. alterniflora invasion did not significantly alter the alpha diversity of phosphate-solubilizing bacteria (PSB) harboring phoD gene, several PSB belonging to p_Proteobacteria, p_Planctomycetes, and p_Cyanobacteriota showed close correlations with P speciation and IP fractions. Further correlation analysis revealed that the reduced soil pH, soil TN and soil EC, and the increased soil TOC mediated by the invasion of S. alterniflora also significantly correlated to these PSB. Overall, this study elucidates the linkage between PSB and P speciation and provides new insights into understanding P retention and microbial P transformation in the coastal sediment invaded by S. alterniflora.

Comparison of bio-beads combined with Pseudomonas edaphica and three phosphate materials for lead immobilization: Performance, mechanism and plant growth.

Phosphate materials (PMs) combine with phosphate solubilizing bacteria play an essential roles in lead (Pb) immobilization, but their resulting ability to reduce Pb bioavailability may vary depending on PMs used. In this study, Pseudomonas edaphica GAU-665 and three PMs: tricalcium phosphate, calcium phytate and nano-hydroxyapatite were respectively encapsulated into bio-beads by sodium alginate, which immobilization efficiency of Pb2+ were 99.11%, 97.76% and 99.02% at initial Pb2+ concentration of 200 mg L-1, respectively. The Pb2+ immobilization performance of bio-beads under different conditions and their organic acids secreted were examined. Most Pb2+ was immobilized by bio-beads through combined functions of adsorption, precipitation, ion exchange and biomineralization, accompanied by the formation of more stable compounds such as Pb3(PO4)2, Pb5(PO4)3OH and Pb5(PO4)3Cl. Meanwhile, pot experimental results indicated that the inoculation of CPhy (calcium phytate) bio-beads with PSB have highest biomass and root growth of oat (Avena sativa L.) in Pb-stressed compared with CK, which increased the content of chlorophyll b (167.51%) in shoot. In addition, the CPhy bio-beads enhance the peroxidase, catalase activities and reduce the malondialdehyde content to alleviating lead physiological toxicity in oat, which reductions the Pb accumulation in shoot (52.06%) and root (81.04%), and increased the residual fraction of Pb by 165.80% in soil. These findings suggest the bio-beads combined with P. edaphica GAU-665 and calcium phytate is an efficient Pb immobilization material and provided feasible way to improve safety agricultural production and Pb-contaminated soil remediation.

Polycyclic aromatic hydrocarbon: underpinning the contribution of specialist microbial species to contaminant mitigation in the soil.

The persistence of PAHs poses a significant challenge for conventional remediation approaches, necessitating the exploration of alternative, sustainable strategies for their mitigation. This review underscores the vital role of specialized microbial species (nitrogen-fixing, phosphate-solubilizing, and biosurfactant-producing bacteria) in tackling the environmental impact of polycyclic aromatic hydrocarbons (PAHs). These resistant compounds demand innovative remediation strategies. The study explores microbial metabolic capabilities for converting complex PAHs into less harmful byproducts, ensuring sustainable mitigation. Synthesizing literature from 2016 to 2023, it covers PAH characteristics, sources, and associated risks. Degradation mechanisms by bacteria and fungi, key species, and enzymatic processes are examined. Nitrogen-fixing and phosphate-solubilizing bacteria contributions in symbiotic relationships with plants are highlighted. Biosurfactant-producing bacteria enhance PAH solubility, expanding microbial accessibility for degradation. Cutting-edge trends in omics technologies, synthetic biology, genetic engineering, and nano-remediation offer promising avenues. Recommendations emphasize genetic regulation, field-scale studies, sustainability assessments, interdisciplinary collaboration, and knowledge dissemination. These insights pave the way for innovative, sustainable PAH-contaminated environment restoration.

An insight into the role of the organic acids produced by Enterobacter sp. strain 15S in solubilizing tricalcium phosphate: in situ study on cucumber.

BACKGROUND: The release of organic acids (OAs) is considered the main mechanism used by phosphate-solubilizing bacteria (PSB) to dissolve inorganic phosphate in soil. Nevertheless, little is known about the effect of individual OAs produced by a particular PSB in a soil-plant system. For these reasons, the present work aimed at investigating the effect of Enterobacter sp. strain 15S and the exogenous application of its OAs on (i) the solubilization of tricalcium phosphate (TCP), (ii) plant growth and (iii) P nutrition of cucumber. To this purpose two independent experiments have been performed. RESULTS: In the first experiment, carried out in vitro, the phosphate solubilizing activity of Enterobacter 15S was associated with the release of citric, fumaric, ketoglutaric, malic, and oxalic acids. In the second experiment, cucumber plants were grown in a Leonard jar system consisting of a nutrient solution supplemented with the OAs previously identified in Enterobacter 15S (jar's base) and a substrate supplemented with the insoluble TCP where cucumber plants were grown (jar's top). The use of Enterobacter 15S and its secreted OAs proved to be efficient in the in situ TCP solubilization. In particular, the enhancement of the morpho-physiological traits of P-starved cucumber plants was evident when treated with Enterobacter 15S, oxalate, or citrate. The highest accumulation of P in roots and shoots induced by such treatments further corroborated this hypothesis. CONCLUSION: In our study, the results presented suggest that organic acids released by Enterobacter 15S as well as the bacterium itself can enhance the P-acquisition by cucumber plants.

Nematodes and their bacterial prey improve phosphorus acquisition by wheat.

Plant growth is greatly influenced by the rhizosphere microbiome, which has been traditionally investigated from a bottom-up perspective assessing how resources such as root exudates stimulate microbial growth and drive microbiome assembly. However, the importance of predation as top-down force on the soil microbiome remains largely underestimated. Here, we planted wheat both in natural and in sterilized soils inoculated with the key microbiome predators - bacterivorous nematodes - to assess how plant performance responds to top-down predation of the soil microbiome and specific plant growth-promoting bacteria, namely phosphate-solubilizing bacteria. We found that nematodes enriched certain groups (e.g. Actinobacteria, Chloroflexi, and Firmicutes) and strengthened microbial connectance (e.g. Actinobacteria and Proteobacteria). These changes in microbiome structure were associated with phosphate-solubilizing bacteria that facilitated phosphorus (P) cycling, leading to greater P uptake and biomass of wheat in both soils. However, the enhancement varied between nematode species, which may be attributed to the divergence of feeding behavior, as nematodes with weaker grazing intensity supported greater abundance of phosphate-solubilizing bacteria and better plant performance compared with nematodes with greater grazing intensity. These results confirmed the ecological importance of soil nematodes for ecosystem functions via microbial co-occurrence networks and suggested that the predation strength of nematodes determines the soil bacteria contribution to P biogeochemical cycling and plant growth.

Screening of Phosphate Solubilization Identifies Six Pseudomonas Species with Contrasting Phytostimulation Properties in Arabidopsis Seedlings.

The interaction of plants with bacteria and the long-term success of their adaptation to challenging environments depend upon critical traits that include nutrient solubilization, remodeling of root architecture, and modulation of host hormonal status. To examine whether bacterial promotion of phosphate solubilization, root branching and the host auxin response may account for plant growth, we isolated and characterized ten bacterial strains based on their high capability to solubilize calcium phosphate. All strains could be grouped into six Pseudomonas species, namely P. brassicae, P. baetica, P. laurylsulfatiphila, P. chlororaphis, P. lurida, and P. extremorientalis via 16S rRNA molecular analyses. A Solibacillus isronensis strain was also identified, which remained neutral when interacting with Arabidopsis roots, and thus could be used as inoculation control. The interaction of Arabidopsis seedlings with bacterial streaks from pure cultures in vitro indicated that their phytostimulation properties largely differ, since P. brassicae and P. laurylsulfatiphila strongly increased shoot and root biomass, whereas the other species did not. Most bacterial isolates, except P. chlororaphis promoted lateral root formation, and P. lurida and P. chlororaphis strongly enhanced expression of the auxin-inducible gene construct DR5:GUS in roots, but the most bioactive probiotic bacterium P. brassicae could not enhance the auxin response. Inoculation with P. brassicae and P. lurida improved shoot and root growth in medium supplemented with calcium phosphate as the sole Pi source. Collectively, our data indicate the differential responses of Arabidopsis seedlings to inoculation with several Pseudomonas species and highlight the potential of P. brassicae to manage phosphate nutrition and plant growth in a more eco-friendly manner.

The effects of herbicide application on two soil phosphate solubilizing bacteria: Pantoea agglomerans and Serratia rubidaea.

The application of synthetic pesticides is one of the fastest acting tools at farmers' disposal to prevent and mitigate the threats posed by plant pests in agriculture. However, the effects of these above-ground applications of pesticides are known to be detrimental to some belowground, non-target soil biota. At present, the effects many pesticides have on key functional microbial groups associated with phosphate (P) solubilization in the soil are still largely unknown. The purpose of this study was to compare the effects of two herbicides, glyphosate, and paraquat, on phosphate solubilizing bacteria (PSB) with and without pH adjustment (after herbicide addition) since pH is a major indicator of P solubilization. In our assay, two PSB strains (Pantoea agglomerans and Serratia rubidaea) were chosen to assess their ability to solubilize tricalcium phosphate (TCP) by using the vanadate-molybdate method (to measure the amount of P solubilized) in the presence of glyphosate (5.4 g/L and 10.8 g/L) or paraquat (2 g/L and 4 g/L) separately. To assess the effect of PSB treated by the herbicides, a growth experiment using PSB inoculated wheat seedlings was performed under greenhouse conditions (25 °C, light 16 h/8 h dark). After four weeks, wheat above-ground growth parameters were measured. Our results showed that even under recommended doses of glyphosate (5.4 g/L) and paraquat (2 g/L), a decrease in P solubilization activity was observed in P. agglomerans and S. rubidaea. Whilst paraquat affected TCP solubilization more than glyphosate with and without pH adjustment, there was a significant decrease (p < 0.05) in TCP solubilization, up to 39% and 93% in the presence of glyphosate and paraquat, respectively, for S. rubidaea, and up to 45% and 95% in the presence of glyphosate and paraquat, respectively, for P. agglomerans. The effect of the herbicides on the PSB had the same results as in the greenhouse test on wheat seedling growth, confirming that these herbicides have both above and belowground negative effects, despite being used at recommended doses.

Study on the mechanism of biochar affecting the effectiveness of phosphate solubilizing bacteria.

Low phosphorus utilization and phosphorus fertilizer pollution are serious issues primarily affecting soil health. To investigate the effects of biochar on the growth, phosphorus solubilization, and metabolites of phosphorus-solubilizing bacteria (PSB), rice husk biochar (RH) and rice straw biochar (RS) were incubated with Bacillus megatherium (BM1) and Bacillus mucilaginosus (BM2), respectively. The highest phosphorus solubilization was observed in BM2 following the addition of RS. The dissolved amount of phosphorus was 244.99 mg/L, which was 43.86% higher than that of the control group. Hence, biochar can improve the phosphorus solubilization capacity of PSB by affecting the organic acid and polysaccharide contents, and phosphatase activity secreted by the PSB, as the porous structure and surface characteristics of biochar ensured the adsorption of PSB. This study can help improve the functional activity of PSB and provide basis for improving the utilization of soil phosphorus, which in turn, aid in the development of biochar-based microbial fertilizers.

Role of Mutant Phosphate Solubilizing Microorganism on Growth of Brassica juncea (L.) cv RH 30.

Phosphorus is the second major plant nutrient and the availability of this element to plants is a major challenge due to conversion to insoluble form by chemical reactions with metal cations depending upon the soil pH. Total of sixty eight isolates of phosphate solubilizing bacteria from rhizosphere of mustard grown in different region of Haryana was studied. The isolates were mutagenised by giving treatment of Nitrosoguanidine (50 ug/ml). Three PSB strains (15M, 22M and 25M) and six mutants (15M2, 15M6, 22M28, 22M29, 25M11 and 25M30) were evaluated for their establishment in the rhizosphere, their effect on biomass production in mustard (Brassica juncea). Total bacterial count in rhizosphere increased after 30 and 40 days of sowing while decrease in growth was observed at 60 days of sowing. The phosphate solubilizing bacterial count in the rhizosphere varied from 1 to 24, 2 to 20 and 1 to 11 at 30, 45 and 60 days after sowing. Phosphate uptake also increased upto 11-21% which shows thatmutants had significant effect on increase in plant dry biomass and P-uptake under pot house conditions.

Screening of phosphate-solubilizing bacteria and their abilities of phosphorus solubilization and wheat growth promotion.

BACKGROUND: Phosphate-solubilizing bacteria (PSB) can enhance plant growth and phosphorus (P) solubilization, it also has been reported to reduce the negative effects of overused agricultural fertilizer in farmland and protect the soil environment. However, the mechanism behind this interaction has not been fully elucidated. RESULTS: In this study, we screened out Pseudomonas moraviensis, Bacillus safensis, and Falsibacillus pallidus which can both solubilize P efficiently and produce indole-3-acetic acid (IAA) from sandy fluvo-aquic soils. The yield of wheat (Triticum aestivum) under PSB inoculation significantly increased up to 14.42% (P < 0.05) compared with the control treatment in phosphate fertilizer-used farmland. Besides promoting wheat growth, we found the labile P fraction in soil was significantly increased by over 122.04% (P < 0.05) under PSB inoculation compared with it in soils without, in parallel, the stable P fraction was significantly reduced by over 46.89% (P < 0.05). Furthermore, PSB inoculation increased the soil microbial biomass and activity, indicating that PSB screened out in this work performed a remarkable ability to colonize the soils in the wheat field. CONCLUSION: PSB from sandy fluvo-aquic soil improve wheat growth and crop productivity by increasing the labile P fraction and IAA content in the greenhouse and wheat field. Our work provides an environment and economy-friendly bacterial resource that potentially promotes sustainable agricultural development in the long term.

Phosphate solubilizing epilithic and endolithic bacteria isolated from clastic sedimentary rocks, Murree lower Himalaya, Pakistan.

Rock microbes are capable to solubilize phosphate present in the rocks.. In this study, we focused on the isolation of phosphate solubilizing bacteria from rocks of Murree, Pakistan. Both endolithic and epilithic bacteria were screened for phosphate solubilization. Three bacterial strains were selected based on halozone formation inNational Botanical Research Institute for phosphate) medium supplemented with TCP (tribasic calcium phosphate). The solubilization index for these bacteria was recorded as 4.29, 4.03 and 3.99. The pH of the medium dropped from 7.0 to 4.0 after 5 days with continuous shaking at 150 rpm, which facilitate the phosphate solubilization. The strains P26, P4 and N27 were identified as Pseudomonas putida strain (KT004381), Pseudomonas grimontii (KT223621) and Alcaligenes faecalis (KT004385). Strain P26 showed maximum phosphate solubilization (367.54 µg/ml), while P4 and N27 showed 321.88 and 291.36 µg/ml after 3 days of incubation. Such inorganic phosphate solubilization could be attributed to the organic acids production by bacteria. The presence of organic acids is determined by high-performance liquid chromatography. Three different types of acids, gluconic, oxalic and malic acid were the dominant acids found in the culture medium. It may be assumed that these bacteria can play a role in weathering of rocks as well. PSB is likely to serve as an efficient biofertilizer, especially in areas deficient in P to increase the overall performance of crops.

Effectiveness of compost inoculated with phosphate solubilizing bacteria.

AIMS: The present study was carried out to design a phosphate solubilizing bacterial (PSB)-based biofertilizer using locally produced fruit waste. METHOD AND RESULTS: Two PSB strains Pseudomonas aeruginosa CMG4 and AAC1 were inoculated into compost. Six compost piles were prepared with carbon:nitrogen (C:N) ratio 30:1. Four piles were inoculated with PSB and two piles served as a control. After 125 days, composts were considered mature at 29-31°C in the pH range of 7.1-7.3 and 32%-35% moisture content (MC). Accessible calcium (Ca) content increased up to 50 g kg-1 . Microbial analysis showed the survival of P. aeruginosa species in the maturing compost even at higher temperature (~53°C). Native bacterial load was retrieved in the range of 109 -1011 CFUg-1 . Heavy metal concentrations including copper (Cu), lead (Pb), chromium (Cr) and cadmium (Cd) were found to occur below critical thresholds. Seed germination index for compost toxicity was found to be >80%, significantly higher than animal manure and chemical fertilizer, that is, 78% and 31%, respectively, suggesting non-toxicity. CONCLUSIONS: The evaluation of prepared compost by physicochemical parameters revealed that inoculation of P. aeruginosa does not affect the temperature, MC, carbon to nitrogen ratio, organic matter and Mg content but significantly increased the accessible Ca content, suggesting the solubilization of inorganic Ca bound phosphate. Compost was safe in terms of heavy metal concentration and seed germination. SIGNIFICANCE AND IMPACT OF STUDY: This study encourages that the PSB-rich tailored compost can be utilized as a phosphatic biofertilizer to fulfil the demand for phosphorus which would improve and sustain soil fertility.

Screening and evaluation of phosphate-solubilizing bacteria isolated from aquaculture ponds in a step-by-step strategy as potential biofertilizer.

AIMS: The application of phosphate-solubilizing bacteria (PSB) has received little attention in aquaculture. In addition, the low efficiency of PSB as a biofertilizer in farm conditions is a major concern. Therefore, this study aims to isolate the PSB from sediment of earthen fishponds and evaluate with a more appropriate approach to ensure their effectiveness in increasing the bioavailability of phosphorus (P) in farm conditions. METHODS AND RESULTS: PSB was first isolated and selected using the National Botanical Research Institute's Phosphate (NBRIP) medium-containing tri-calcium phosphate (TCP) in solid and liquid media. Among 96 strains that were isolated, 11 strains identified by 16 s rRNA, belonging to the genera Pseudomonas and Acinetobacter, showed a higher ability to release P from TCP (48-170 mg L-1 ). Then, the efficiency of 11 strains was evaluated by combining different criteria. Among 11 selected strains, based on the ability to dissolve TCP and Ca-Phytate in culture medium, release P in sediment microcosm, and growth in a wide range of environmental conditions in fishponds, especially optimum growth at 4°C and pH above 8, Ps. deceptionensis strain Persian10 was selected as the most efficient strain for testing in aquarium conditions. In the last step, incubation of Persian10 in aquarium sediment (sterilized under gamma-ray) increased soluble P and had an impact on calcium phosphate, organic P and alkaline phosphatase activity; however, Persian10 had no impact on the concentration of iron phosphate and aluminium phosphate. CONCLUSIONS: Persian 10 strain can be considered a bio-fertilizer candidate in earthen fishponds as it expressed the ability to solubilize P in different conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: It is very important to evaluate PSB in an appropriate process using a combination of different criteria to ensure optimal performance of PSB in farm conditions.

Bioremediation of lead-contaminated soil by inorganic phosphate-solubilizing bacteria immobilized on biochar.

In this study, a bio-composite (IBWS700) was prepared using inorganic phosphate-solubilizing bacteria (iPSB), which were immobilized on biochar produced from wheat straw (WS700). Further, the bio-remediation effects of the composite for lead (Pb) in soil were also investigated. The presence of different Pb species, physicochemical properties, enzyme activities, and immobilization mechanisms of Pb in soil were also evaluated. Compared to free iPSB and biochar, IBWS700 significantly decreased the lead bio-availability whereas increased the residual fraction, also affected available phosphorus (AP), cation exchange capacity (CEC), organic matter (OM) and activity of urease, alkaline phosphatase, sucrase and catalase. Interestingly, the changes in the enzyme activity, AP and OM performed twice increases with increasing Pb concentration, which was rarely reported. The reason might be attributed to the reconstruction of bacteria communities with high Pb load. Further, the immobilization mechanisms mainly included bio-adsorption and bio-precipitation. SEM revealed that the surface of IBWS700 covered with a large number of heterogeneous colonization of iPSB and white stack after Pb2+ adsorption. FTIR spectra showed that O-H, C-O-P, CO, and C =C could play important roles in bio-adsorption. Moreover, XRD analysis indicated that bio-precipitates were mainly Pb5(PO4)3Cl. In general, the use of IBWS700 could effectively immobilize Pb2+ and improve soil quality.

Mobilization of soil phosphate after 8 years of warming is linked to plant phosphorus-acquisition strategies in an alpine meadow on the Qinghai-Tibetan Plateau.

Phosphorus (P) is essential for productivity of alpine grassland ecosystems, which are sensitive to global warming. We tested the hypotheses that (1) mobilized 'calcium-bound inorganic P' (Ca-Pi ) is a major source of plant-available P in alpine meadows with alkaline soils after long-term warming, (2) mobilization of Ca-Pi is linked to effective plant carboxylate-releasing P-acquisition strategies under warming, and (3) the mobilization is also related to plant nitrogen (N)-acquisition. We conducted an 8-year warming experiment in an alpine meadow (4635 m above sea level) on the Qinghai-Tibetan Plateau. A significant increase in P concentration in both aboveground and belowground biomass indicates an increased mobilization and assimilation of P by plants under warming. We observed a significant decrease in Ca-Pi , no change in moderately-labile organic P, and an increase in highly resistant organic P after warming. There was no increase in phosphatase activities. Our results indicate that Ca-Pi , rather than organic P was the major source of plant-available P for alpine meadows under warming. Higher leaf manganese concentrations of sedges and forbs after warming indicate that carboxylates released by these plants are a key mechanism of Ca-Pi mobilization. The insignificant increase in Rhizobiales after warming and the very small cover of legumes show a minor role of N-acquisition strategies in solubilizing phosphate. The insignificant change in relative abundance of mycorrhizal fungi and bacteria related to P cycling after warming shows a small contribution of microorganisms to Ca-Pi mobilization. The significant increase in leaf N and P concentrations and N:P ratio of grasses and no change in sedge leaf N:P ratio reflect distinct responses of plant nutrient status to warming due to differences in P-acquisition strategies. We highlight the important effects of belowground P-acquisition strategies, especially plant carboxylate-releasing P-acquisition strategies on responses of plants to global changes in alpine meadows.