The ability of plants to take up phosphorus (P) from soil depends on root morphology and root exudates release and can be modulated by beneficial soil microbes. These microbes can solubilize P, affect root elongation and branching, and lead to a higher uptake of P and other nutrients. However, coordination of these mechanisms is unclear, especially the mechanism for changing the available form of P. We aimed to dissect the effects of two different beneficial microbial taxa (plant growth-promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF)) on root morphological traits, plant nutrient content, and growth in tomato and corn fertilized with either Gafsa rock phosphate (RP) or triple superphosphate (TSP), which have contrasting solubility levels. Tomato and corn were grown in pots and inoculated with one of three PGPB species or a mix of two AMF species or were not inoculated. Root traits, botanical fractions, and the contents of various mineral nutrients were measured. TSP stimulated tomato biomass accumulation compared to RP but did not stimulate corn biomass accumulation. PGPB improved the growth of both plant species under RP, with limited differences among the strains, whereas AMF only improved tomato growth under TSP. These differences between microbial systems were explained by a bacterial effect on the total root length but not on the mean root diameter and by the ability of AMF to improve the mineral nutrient content. The effects of PGPB were less dependent on the plant species and on P form than the effects of AMF.These results have implications for the improvement of the early plant growth through the management of beneficial microbes.

Growth-promoting bacteria and arbuscular mycorrhizal fungi differentially benefit tomato and corn depending upon the supplied form of phosphorus.

This work was funded by the Basque Government (IT-932-16) and the Spanish Ministry of Economy and Competitiveness (Grants BIO2017-84035-R and AGL2017-86293-P, both co-funded by FEDER).

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Can Ammonium Stress Be Positive for Plant Performance

Rock powders are low-cost potential sources of most of the nutrients required by higher plants for growth and development. However, slow dissolution rates of minerals represent an obstacle to the widespread use of rock powders in agriculture. Rhizosphere processes and biological weathering may further enhance mineral dissolution since the interaction between minerals, plants, and bacteria results in the release of macro- and micronutrients into the soil solution. Plants are important agents in this process acting directly in the mineral dissolution or sustaining a wide diversity of weathering microorganisms in the root environment. Meanwhile, root microorganisms promote mineral dissolution by producing complexing ligands (siderophores and organic acids), affecting the pH (via organic or inorganic acid production), or performing redox reactions. Besides that, a wide variety of rhizosphere bacteria and fungi could also promote plant development directly, synergistically contributing to the weathering activity performed by plants. The inoculation of weathering bacteria in soil or plants, especially combined with the use of crushed rocks, can increase soil fertility and improve crop production. This approach is more sustainable than conventional fertilization practices, which may contribute to reducing climate change linked to agricultural activity. Besides, it could decrease the dependency of developing countries on imported fertilizers, thus improving local development.

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Use of Mineral Weathering Bacteria to Enhance Nutrient Availability in Crops: A Review.

Microbial indicators are better predictors of wheat yield and quality than N fertilization.

The excessive use of fertilizers in agriculture is mainly due to the recognized plant requirements for soluble phosphorus. This problem has limited the implementation of sustainable agriculture. A viable alternative is to use phosphate solubilizing soil microorganisms. This work aimed to isolate inorganic phosphorus-solubilizing bacteria from the soils of agroecosystems, to select and identify, based on sequencing and phylogenetic analysis of the 16S rRNA gene, the bacterium with the highest capacity for in vitro solubilization of inorganic phosphate. Additionally, we aimed to determine its primary phosphate solubilizing mechanisms and to evaluate its effect on Habanero pepper seedlings growth. A total of 21 bacterial strains were isolated by their activity on Pikovskaya agar. Of these, strain ITCB-09 exhibited the highest ability to solubilize inorganic phosphate (865.98 µg/mL) through the production of organic acids. This strain produced extracellular polymeric substances and siderophores that have ecological implications for phosphate solubilization. 16S rRNA gene sequence analysis revealed that strain ITCB-09 belongs to the genus Enterobacter. Enterobacter sp. ITCB-09, especially when immobilized in beads, had a positive effect on Capsicum chinense Jacq. seedling growth, indicating its potential as a biofertilizer.

https://www.mdpi.com/2077-0472/10/9/383/pdf

Inorganic phosphate solubilization by a novel isolated bacterial strain enterobacter sp. Itcb-09 and its application potential as biofertilizer

Phosphate-(P)-solubilizing microorganisms (PSM) are important drivers of P cycling in natural and agro-ecosystems. Their use as plant inoculants to improve P acquisition of crops has been investigated for decades. However, limited reproducibility of the expected effects, particularly under field conditions, remains a major challenge. This study demonstrates that the form of nitrogen fertilization has a significant impact on the performance of various fungal and bacterial PSM inoculants in maize grown on neutral to alkaline soils with limited P availability. Under these conditions, a high soil pH-buffering capacity frequently limits the efficiency of nutrient mobilization, mediated by plant roots and microorganisms via rhizosphere acidification. In a soil pH range between 7.0 and 8.0, nitrate fertilization promoting rhizosphere alkalinisation further aggravates this problem. Accordingly, in greenhouse experiments, six strains of Pseudomonas, Bacillus, Paenibacillus, Streptomyces, and Penicillium with proven P-solubilizing potential, completely failed to promote P acquisition in maize grown on a calcareous Loess sub-soil pH 7.6 with nitrate fertilization and rock phosphate (Rock-P) as a sparingly soluble P source. However, after replacement of nitrate fertilization by ammonium, stabilized with the nitrification inhibitor 3,4-dimethylpyrazole-phosphate (DMPP), five out of seven investigated PSM inoculants (comprising 12 fungal and bacterial PSM strains) exerted beneficial effects on plant growth and reached up to 88% of the shoot biomass production of a control supplied with soluble triple-superphosphate (TSP). Stabilized ammonium combined with PSM-inoculants improved P acquisition (Trichoderma harzianum T22, Pseudomonas sp. DMSZ 13134), while other strains particularly stimulated root growth (T. harzianum OMG16, Bacillus amyloliquefaciens FZB42), which promoted the acquisition also of other mineral nutrients, such as N, K, and Mn. A similar effect was recorded under field conditions on an alkaline clay-loam soil pH 8.6. The combination of stabilized ammonium with a range of consortium products based on T. harzianum OMG16, B. amyloliquefaciens, micronutrients, and humic acids completely compensated the effect of a TSP fertilization on field establishment, nutrient acquisition, and yield formation in maize, while non-stabilized urea-di-ammonium phosphate fertilization was largely ineffective. These findings suggest that the efficiency of PSM-plant interactions can be influenced by the form of N fertilization, offering promising perspectives for synergistic effects with stabilized ammonium fertilizers.

https://www.mdpi.com/2076-2607/7/2/38/pdf

The Form of N Supply Determines Plant Growth Promotion by P-Solubilizing Microorganisms in Maize

Fertilization management affects not only plant performance but also soil microbiota, with specific rhizosphere interactions influenced by root activity, still poorly understood. We assumed that long-term fertilization exerts selective effects on rhizodeposition with consequences for recruitment of rhizosphere microbiota and plant performance. To address this hypothesis, we conducted a minirhizotron-experiment with lettuce as model plant, using field soils with contrasting properties from two long-term field experiments (HUB-LTE: loamy sand, DOK-LTE: silty loam) with organic and mineral fertilization history. Although no consistent fertilization effects were detectable on alpha-diversity of fungal and prokaryotic/archaeal rhizosphere communities, increased abundance of plant-beneficial arbuscular-mycorrhizal fungi was characteristic for the rhizospheres of the organically managed soils (HU-org; BIODYN2). However, their rhizosphere microbiota also comprised more fungal pathotrophs, and the plants systemically expressed defense-related genes in shoot tissues. However, there was no consistent effect of fertilization history on shoot biomass and root development, which remained unaffected in HUB-LTE. In the BIODYN2 soil, shoot and root growth was severely suppressed and affected by Olpidium infection, coinciding with the highest rhizosphere abundance of the pathogen (89 %). This was associated with a reduction of potentially plant growth-promoting microorganisms, such as Pseudomonaceae and Mortierella elongata by 87 and 97 %. These profound alterations of rhizosphere microbial communities were linked with characteristic changes in the composition of the rhizosphere solution. Olpidium-induced growth suppression and low rhizosphere abundance of Pseudomonadaceae were associated with reduced rhizosphere concentrations of the antifungal root exudate benzoic acid. This metabolite was described in the literature to be increased by Pseudomonas inoculation. By contrast, high abundance of Pseudomonadaceae (Gammaproteobacteria) in the rhizosphere of plants with long-term mineral fertilization (61-74 %) coincided with high rhizosphere concentrations of chemotactic dicarboxylates (succinate, malate) and a high C (sugar)/N (amino acid) ratio, known to support the growth of Gammaproteobacteria. Under these conditions no plant growth depressions were recorded even in presence of a high Olpidium abundance (76 %) in the rhizosphere. Our results suggest a complex network of belowground interactions between crop roots, site-specific factors and rhizosphere microbiota, modulating the impact of fertilization management with consequences for plant health and crop performance.

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Jörg Geistlinger

Papers

The Form of N Supply Determines Plant Growth Promotion by P-Solubilizing Microorganisms in Maize

Impact of long-term organic and mineral fertilization on rhizosphere metabolites, root-microbial interactions and plant health of lettuce