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.

During fungal rock phosphate (RP) solubilization, a significant quantity of fluoride (F ⁻ ) is released together with phosphorus (P), strongly inhibiting the process. In the present study, the effect of two F ⁻ adsorbents [activated alumina (Al ₂ O ₃ ) and biochar] on RP solubilization by Aspergillus niger was examined. Al ₂ O ₃ adsorbed part of the F ⁻ released but also adsorbed soluble P, which makes it inappropriate for microbial RP solubilization systems. In contrast, biochar adsorbed only F ⁻ while enhancing phosphate solubilization 3-fold, leading to the accumulation of up to 160 mg of P per liter. By comparing the values of F ⁻ measured in solution at the end of incubation and those from a predictive model, it was estimated that up to 19 mg of F ⁻ per liter can be removed from solution by biochar when added at 3 g liter ⁻¹ to the culture medium. Thus, biochar acted as an F ⁻ sink during RP solubilization and led to an F ⁻ concentration in solution that was less inhibitory to the process. In the presence of biochar, A. niger produced larger amounts of citric, gluconic, and oxalic acids, whether RP was present or not. Our results show that biochar enhances RP solubilization through two interrelated processes: partial removal of the released F ⁻ and increased organic acid production. Given the importance of organic acids for P solubilization and that most of the RPs contain high concentrations of F ⁻ , the proposed solubilization system offers an important technological improvement for the microbial production of soluble P fertilizers from RP.