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Background and aims Dark septate endophytes (DSE) are a group of multifunctional fungi that ubiquitously colonize the roots of numerous plants worldwide, benefiting the nutrition and adaptation of hosts to various stressed environments. How DSE benefit phosphorus absorption by their host plants remains unclear. Methods We established pot cultures and compartmentalized microcosms in the greenhouse and used transcriptomic analysis to examine the response of maize to inoculation with a DSE strain, Exophiala pisciphila H93, which has been found to be capable of promoting plant growth and enhancing the tolerance of maize to heavy metals. Results H93 promoted the biomass, phosphorus absorption and photosynthesis of inoculated maize by enhancing phosphorus enzyme activities in the rhizosphere and by decreasing the pH of the rhizosphere compared with non-inoculated controls, either in sufficient or deficient phosphorus conditions in pot cultures. H93 inoculation changed the transcriptional pattern of maize related to 8 metabolic pathways. While H93 did not induce particular phosphate transporter gene expression under deficient phosphorus conditions, it dissolved the sparingly soluble phosphates (tricalcium phosphate) in the rhizosphere. Conclusions Enhanced phosphorus absorption by the host plant by DSE (H93) seemed to occur independently of DSE-induced genes involved in phosphorus absorption. The greatest contribution of DSE to phosphorus utilization of hosts might be their efficiency in transferring sparingly soluble phosphorus into soluble phosphate (Pi).

Arbuscular mycorrhizae (AM) are thought to improve crop growth by enhancing phosphorus (P) uptake via scavenging and enhancing dissolution. However, AM-mediated crop growth responses to P forms of varying solubility are often crop-species and soil-context dependent. The relative importance of AM associations and P source solubility on crop growth is not conclusively understood, and requires controlled factorial experiments to test their relative and interactive effects. We conducted a meta-analysis to evaluate how AM impact crop growth responses to rock phosphate relative to soluble phosphates across diverse crop species and soil characteristics. A total of 83 observations utilizing a 2 × 2 factorial design of relative presence or absence of AM and fertilization with rock phosphate vs. soluble phosphates were identified. We found that AM similarly improved crop growth with rock phosphate and soluble phosphates. A distinguishable crop growth benefit from AM coupled with rock phosphate was observed for soils with a low degree of weathering, at soil pH < 6.5 and > 7.5, and when soils were heat-sterilized prior to inoculation with AM. Shoot biomass of legumes was uniquely greater than non-legumes with rock phosphates and AM as compared to soluble phosphates and AM. However, crop growth under rock phosphate fertilization relative to soluble phosphates was still lower irrespective of AM. This meta-analysis reveals that crop growth is more dependent on P fertilizer solubility than AM. Moreover, AM do not appear to close the solubility gap of rock phosphate vs. soluble phosphate fertilizers to support similar crop growth under rock phosphate relative to soluble phosphates. Studies assessing crop growth responses to AM-crop associations effect on contrasting solubility P fertilizers should expand to the field, and greenhouse experiments should be conducted under realistic field growing conditions, such as agronomically appropriate P application rates.

Organisms intended to solubilise soil phosphate are chosen on their ability to produce a clear halo on a plate containing a sparingly soluble phosphate. This involves production of acidity. However, if these organisms establish on plant roots any benefit from increased acidity is likely to be as a result of increased rate of uptake of phosphate by plant roots rather than from increased desorption of phosphate from soil.

Phosphorus (P) is an essential element for all life forms, and P-availability thus an important driver of a functioning agriculture. However, phosphate rock resources for P-fertilizer production are only available in a few countries. Therefore, P-recovery from waste materials has become of increasing interest during the last decade and has been investigated worldwide. In order to characterize potential novel P-fertilizers made from recycled materials, a large array of P-compound characterizations, chemical extractions and growth experiments were performed. This review bundles the work carried out in that field over the last years. Overall, P-fertilizers from recycled materials show a broad range of P-compounds with very different chemical structure and solubility. Growth experiments performed to assess their fertilizing effects display high variations for most of the products. While these experiments have demonstrated that some fertilizers made of recycled materials may reach P effects in the same order of magnitude as water-soluble phosphate rock-based fertilizers, an important limitation in their interpretation is the fact that they often vary considerably in their experimental design. The existing data show clearly that standardization of growth experiments is urgently needed to achieve comparable results. Standard chemical extractants used to assess the chemical solubility of P-fertilizers were found to be of limited reliability for predicting plant P uptake. Therefore, alternative methods such as sequential fractionation, or the extraction of incubated soil/fertilizer mixtures with standard soil extractants or with P sink methods should be tested more intensively in the future to provide alternative options to predict the P-availability of fertilizers from recycled materials.

Background and scope Enriching phosphorus (P) fertilizers with phosphate-solubilizing microorganisms (PSM) can be a promising strategy to enhance P-use efficiency in agriculture. However, the effects of these microorganisms when applied in mixtures or coating P fertilizers needs further investigation. In this work, we coated a Rock Phosphate (RP) or soluble P fertilizer (Triple Superphosphate -TS) with Trichoderma harzianum , to enhance plant P uptake and growth of corn ( Zea mays ). Results The addition of T. harzianum to RP and TS reduced soil pH and increased the activity of soil acid and alkaline phosphatases which are important mechanisms involved in P mobilization by PSM. Trichoderma harzianum increased plant root length, root surface area and total root dry mass (17%). Trichoderma harzianum also increased plant P content (34%) and total dry matter content (22%) of plants fertilized with soluble phosphate (TS). Conclusion Our main findings provide evidence that coating fertilizers with T. harzianum can be an effective way to enhance plant growth and P-use efficiency in agricultural systems. Importantly, we show that joint application with fertilizer did not impair important mechanisms of P mobilization by T. harzianum . Further research, in the field and under different environmental conditions, is necessary to validate the technology.

Phosphorus (P) is one of the essential elements that are necessary for plant development and growth. However, the availability of soluble forms of P for plants in the soils is limited, because a large proportion of it is bound to soil constituents. Thus, the concentration of P available to plants at any time is very low and, moreover, its availability depends on the soil pH. As a solution, phosphate-solubilizing microorganisms (PSMs) are employed that render inorganic P available to plants in soluble form. Thus far, research into PSMs has been insufficient, and only few such organisms have been considered for exploitation as microbial fertilizer strains. The characteristics of plant growth promotion with the plant-PSMs coculture system remain to be elucidated. In the current study, we report on the isolate Rhodosporidium paludigenum JYC100 that exhibits good performance for solubilizing calcium phosphate. We found that it can be regulated by the amount of soluble phosphate. Furthermore, R. paludigenum JYC100 promotes plant growth under specific conditions (P deficiency, but with insoluble phosphate) in different media and soil pots. In contrast, the yeast Aureobasidium pullulans JYC104 exhibited weak phosphate-solubilizing capacities and no plant growth-promoting ability. Compared to control plants, the biomass, shoot height, and cellular inorganic P content of plants increased in plants cocultivated with R. paludigenum JYC100. In addition, histochemical GUS and qRT-PCR assays of phosphate starvation-induced (PSI) genes showed that the transcript levels of these PSI genes are decreased in the plants cocultured with R. paludigenum JYC100. These findings reflect the unique ability of R. paludigenum JYC100 to convert insoluble P compounds to plant-available P, thereby leading to growth promotion. Our study results highlight the use of yeasts as potential substitutes for inorganic phosphate fertilizers to meet the P demands of plants, which may eventually improve yields in sustainable agricultures.

Isolation and identification of temperature tolerant phosphate solubilizing bacteria (TTPSB) and their use as microbial fertilizers was the main goal of the study. In this study, TTPSB were isolated from soil samples treated for 16 h at 55 °C. Their phosphate solubilizing activity was either evaluated in solid media by forming a clear zone (halo) or in liquid media by quantification of the soluble phosphate in the growth medium. Five colonies (RPS4, RPS6, RPS7, RPS8 and RPS9) were identified to be able to form a halo and two of the isolates (RPS9 and RPS7) tolerated a temperature of 55 °C. With tricalcium phosphate (TCP) as the sole P-source, the phosphate solubilizing capacity of RPS9 and RPS7 was determined to be 563.8 and 324.1 mg P L−1 in liquid Sperber medium, respectively. Both bacterial isolates were identified as Pantoea agglomerans by molecular and biochemical characterization. To be used as a microbial fertilizer a carrier system for the temperature tolerant bacteria consisting of rock phosphate, sulfur and bagasse was used. It could be established that the bacterial cell counts of the microbial fertilizers were acceptable for application after storage for 4 months at 28 °C. In a greenhouse experiment using pot cultures, inoculation of maize (S.C.704) with the microbial fertilizers in an autoclaved soil resulted in a significant effect on total fresh and dry weight of the plant root and shoot as well as on the P content of the root and shoot. The effects observed with RPS9 as a component of the microbial fertilizer on plant growth and P nutrition was comparable with the addition of 50% of recommended triple superphosphate (TSP) dose. Using temperature tolerant bacteria in microbial fertilizers will overcome limitations in production and storage of the microbial fertilizers and contribute to a environmentally-friendly agriculture. The temperature tolerant P. agglomerans strain RPS9 was shown to be effective as part of a microbial fertilizer in supporting the growth and P uptake in maize.

Purpose The co-pyrolysis of biomass and soluble phosphates generates biochar-based phosphate fertilizers (BBF), which may enhance phosphorus (P) input in soil and P uptake by plants. Conversely, pyrolysis of biomass impregnated with rock phosphate results in low P solubility and may not supplement plant requirement in short term. However, bacterial strains promoting rock phosphate solubilization increases P use efficiency and can be applied to BBFs. Methods An in vitro assay was conducted to investigate the solubilization profile of five bacterial strains ( Pseudomonas sp.—UFPI-B5-8A, Burkholderia fungorum —UFLA 04-155, Acinetobacter sp.—UFLA 03-09, Paenebacillus kribbensis —UFLA 03-10, and Paenibacillus sp.—UFLA 03-116) isolated from common bean and cowpea nodules in a rock phosphate BBF. Additionally, a pot trial was carried out aiming to investigate the influence on maize growth by inoculation of three selected strains under a rock phosphate BBF fertilization. Results Inoculations with UFPI B5-8A, UFLA 04-155, and UFLA 03-09 were efficient in solubilizing P in vitro, being closely associated with pH decrease, likely due to the release of organic acids. As for the pot trial, the dose of 400 mg kg −1 of P in the BBF using UFPI B5-8A significantly increased maize shoot dry matter. All strains significantly enhanced P availability in the soil. Conclusions Bacterial inoculation in biochar-based rock phosphate aiming to improve its fertilizer value is an inexpensive and sustainable strategy to improve maize growth and enhance available P in soil and should be further explored.

Core Ideas Phosphorus recycling by cover crops in a typical Brazilian cropping system. Maize response to phosphate sources under no‐till management. Residual effects of phosphate sources in tropical weathered soils. Rock phosphate was more effective than soluble phosphate in supplying P for maize over time. This research evaluated the potential benefits of winter cover crops on the utilization and cycling of P in Brazilian tropical cropping systems. The effect of P fertilizer [none, rock phosphate (RP), and soluble phosphate (single superphosphate, SSP)] in combination with cover crop residues (common vetch [Vicia sativa L.], white lupin [Lupinus albus L.], forage radish [Raphanus sativus L.], ryegrass [Lolium multiflorum Lam], black oat [Avena strigosa Schreb.], red clover [Trifolium pratense L.], and fallow) were evaluated on maize (Zea mays L.) yield and P use efficiency over three maize cropping seasons under no‐tillage, from 2009 to 2012. Cover crop yields and P uptake were higher under phosphate fertilizers than nil‐P across all seasons evaluated. The highest amounts of P recycled in cover crops over the period were under white lupin, followed by radish and ryegrass, but without any significant cover crop effect on maize yield. The largest response and greatest P use efficiency (30 kg grain per kg P applied) was obtained in the third year of evaluation, when maize yield was restricted by low rainfall. In this year, RP promoted greater maize yield than SSP and the nil‐P. Soil available P at the end of the experiment was higher under RP than SSP. It is concluded that RP solubility is higher than currently predicted (9% P2O5 in citric acid). Cover crops were not able to affect maize yield after 3 yr of leaving the residues on the surface, however they can reduce the soil loss by erosion and runoff.

Excess sludge (ES) and plant waste (PW) are two typical organic solid wastes in urban areas, and their co-fermentation is one of the key strategies in the circular economy. This study innovatively investigated the impact of the oxidant potassium ferrate (PF) on the anaerobic co-fermentation of ES and PW for the production of volatile fatty acids (VFAs) and elucidated the underlying mechanisms. The results indicated that PF could enhance the co-fermentation of ES and PW to produce VFAs, with an optimal dosage of 0.06 g/g (based on total suspended solids), yielding a maximum VFAs production of 261 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS), which is 2.1 times that of the control group. High doses of PF inhibited microbial metabolism, reducing VFAs production, but still higher than the control group. PF effectively promoted the solubilization of organic matter in the ES and PW co-digestion system, increasing the concentrations of soluble COD, soluble protein, and soluble polysaccharides, with higher PF concentrations leading to more significant solubilization of available organic matter. PF increased the content of loosely bound extracellular polymeric substances (EPS) but decreased the content of tightly bound EPS. The oxidizing nature of PF suppressed the production of biogas, with only 99.5 mL/g VSS produced in the 0.08 g/g PF group. PF stimulated the concentration of ammonium nitrogen in the fermentation liquid of the co-digestion system but decreased the concentration of soluble phosphate.