Background: Increasing the presence of beneficial soil microorganisms is a promising sustainable alternative to support conventional and organic fertilization and may help to improve crop health and productivity. If the application of single bioeffectors has shown satisfactory results, further improvements may arise by combining multiple beneficial soil microorganisms with natural bioactive molecules.Methods: In the present work, we investigated in a pot experiment under greenhouse conditions whether inoculation of two phosphate-solubilizing bacteria,Pseudomonasspp.(B2) andBacillus amyloliquefaciens(B3), alone or in combination with a humic acids (HA) extracted from green compost and/or a commercial inoculum (M) of arbuscular mycorrhizal fungi (AMF), may affect maize growth and soil microbial community. Phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) fingerprinting analysis were performed to detect changes in the microbial community composition.Results: Plant growth, N and P uptake, and mycorrhizal root colonization were found to be larger in all inoculated treatments than in the uninoculated control. The greatest P uptake was found whenB. amyloliquefacienswas applied in combination with both HA and arbuscular mycorrhizal fungi (B3HAM), and whenPseudomonaswas combined with HA (B2HA). The PLFA-based community profile revealed that inoculation changed the microbial community composition. Gram+/Gram− bacteria, AMF/saprotrophic fungi and bacteria/fungi ratios increased in all inoculated treatments. The greatest values for the AMF PLFA marker (C16:1ω5) and AMF/saprotrophic fungi ratio were found for the B3HAM treatment. Permutation test based on DGGE data confirmed a similar trend, with most significant variations in both bacterial and fungal community structures induced by inoculation of B2 or B3 in combination with HA and M, especially in B3HAM.Conclusions: The two community-based datasets indicated changes in the soil microbiome of maize induced by inoculation of B2 or B3 alone or when combined with humic acids and mycorrhizal inoculum, leading to positive effects on plant growth and improved nutrient uptake. Our study implies that appropriate and innovative agricultural management, enhancing the potential contribution of beneficial soil microorganisms as AMF, may result in an improved nutrient use efficiency in plants.

Background and aims: Biostimulants of natural origin represent a growing ecological strategy to increase crops productivity, especially when applied in combination with microbial bioeffectors. We studied the effect of biostimulants such as Potassium Humates (KH) from Leonardite and Compost Tea (CT) from green compost on both productivity and nutritional status of lettuce plants, as well as on the primary and secondary metabolism of treated plants, when amended either alone or in combination with a commercial microbial inoculum (M+), mainly based on arbuscular mycorrhizal fungi (Micosat TabPlus). Results: The biomass production as well as the uptake of both macro- and micronutrients by lettuce plants significantly increased when amended by the mixture of both humic materials (MIX) combined with the microbial inoculum. Similarly, the synergic MIX_M+ treatment significantly affected both the primary and secondary metabolism of lettuce more than their individual applications, by increasing, respectively, the biosynthesis of essential amino acids and carbohydrates, and that of antioxidant polyphenolic compounds, such as hydroxycinnamic acids, flavonols and coumarins. Conclusions: Our findings suggest that a calibrated mixture of humic bioactive molecules in combination with microbial consortia represents a potential tool to improve crop productivity and its nutritional and metabolic status.

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.