The application of plant beneficial microorganisms has been widely accepted as an efficient alternative to chemical fertilizers and pesticides. Isolation and selection of efficient microorganisms, their characterization and testing in soil-plant systems are well studied. However, the production stage and formulation of the final products are not in the focus of the research, which affects the achievement of stable and consistent results in the field. Recent analysis of the field of plant beneficial microorganisms suggests a more integrated view on soil inoculants with a special emphasis on the inoculant production process, including fermentation, formulation, processes, and additives. This mini-review describes the different groups of fermentation processes and their characteristics, bearing in mind different factors, both nutritional and operational, which affect the biomass/spores yield and microbial metabolite activity. The characteristics of the final products of fermentation process optimization strategies determine further steps of development of the microbial inoculants. Submerged liquid and solid-state fermentation processes, fed-batch operations, immobilized cell systems, and production of arbuscular mycorrhiza are presented and their advantages and disadvantages are discussed. Recommendations for further development of the fermentation strategies for biofertilizer production are also considered.

The application of plant beneficial microorganisms is widely accepted as an efficient alternative to chemical fertilizers and pesticides. It was shown that annually, mycorrhizal fungi and nitrogen-fixing bacteria are responsible for 5 to 80% of all nitrogen, and up to 75% of P plant acquisition. However, while bacteria are the most studied soil microorganisms and most frequently reported in the scientific literature, the role of fungi is relatively understudied, although they are the primary organic matter decomposers and govern soil carbon and other elements, including P-cycling. Many fungi can solubilize insoluble phosphates or facilitate P-acquisition by plants and, therefore, form an important part of the commercial microbial products, with Aspergillus, Penicillium and Trichoderma being the most efficient. In this paper, the role of fungi in P-solubilization and plant nutrition will be presented with a special emphasis on their production and application. Although this topic has been repeatedly reviewed, some recent views questioned the efficacy of the microbial P-solubilizers in soil. Here, we will try to summarize the proven facts but also discuss further lines of research that may clarify our doubts in this field or open new perspectives on using the microbial and particularly fungal P-solubilizing potential in accordance with the principles of the sustainability and circular economy.

Piriformospora indica, a mycorrhizal-like fungus able to establish associations with roots of a wide range of plants, supporting plant nutrition and increasing plant resistance and tolerance to stress, was shown to solubilise phosphate applied in the form of animal bone char (HABO) in fermentation systems. The process of P solubilisation was caused most likely by proton extrusion and medium pH lowering. The fungal mycelium was successfully immobilized/retained in a polyurethane foam carrier. Further employment of the immobilized mycelium in repeated-batch fermentation process resulted in at least 5 cycles of P solubilization. The concentration of soluble P increased during the experiment with 1.0 and 3.0 g HABO l−1 and at the end of the 5th batch cycle reached 40.8 and 120 mg l−1, respectively. The resulting final liquid product, without or with solubilized phosphate, was found to significantly increase plant growth and P plant uptake. It can be used as a biostimulant containing microbial plant growth-promoting substances and soluble P derived from renewable sources (HABO) thus supporting the development of sustainable agro-ecosystems.

Sustainable crop production includes methods of growing food in a responsible manner avoiding application and dependence on chemically produced fertilizers and pesticides. The latter means development of approaches that lead to environmentally mild inputs based on the production-consumption-recycling principle. Biofertilizers are an important tool to achieve sustainable crop production. In this work, we report the results of experiments on growth and spore/mycelium production of plant growth promoting A. niger applying standard nutritional medium (potato-dextrose broth, PDB) enriched with 3% insoluble phosphate (20 to 200 mesh hydroxyapatite of animal-bone origin, HABO) and 0 to 80 g/L glycerol (a by-product of biodiesel production. Results showed the ability of A. niger to acidify the medium with the highest titratable acidity of 28.9 mmol/1 (at 5% of glycerol) and solubilize animal bone char under these conditions. As a second stage of the experimental work, the resulting final products were used to formulate gel-based inoculant. Both, the spores and mycelium produced during the fermentation process were further used as a base for formulation to make the biofertilizer production the key in the Sustainable Agriculture. Storage of the resulting products reported here was facilitated by the presence of glycerol in the formulation system.

Sustainable crop production includes methods of growing food in a responsible manner avoiding application and dependence on chemically produced fertilizers and pesticides. The latter means development of approaches that lead to environmentally mild inputs based on the production-consumption-recycling principle. Biofertilizers are an important tool to achieve sustainable crop production. In this work, we report the results of experiments on growth and spore/mycelium production of plant growth promoting A. niger applying standard nutritional medium (potato-dextrose broth, PDB) enriched with 3% insoluble phosphate (20 to 200 mesh hydroxyapatite of animal-bone origin, HABO) and 0 to 80 g/L glycerol (a by-product of biodiesel production. Results showed the ability of A. niger to acidify the medium with the highest titratable acidity of 28.9 mmol/1 (at 5% of glycerol) and solubilize animal bone char under these conditions. As a second stage of the experimental work, the resulting final products were used to formulate gel-based inoculant. Both, the spores and mycelium produced during the fermentation process were further used as a base for formulation to make the biofertilizer production the key in the Sustainable Agriculture. Storage of the resulting products reported here was facilitated by the presence of glycerol in the formulation system.