Comparative Efficiency of Fungal Organic Acids and Pure Acids in Tricalcium Phosphate Solubilisation

Phosphorus (P) is a vital macronutrient involved in key biochemical processes that support plant growth; however, its low bioavailability in agricultural soils remains a major constraint on crop productivity. This limitation is commonly addressed through the application of chemical P fertilisers produced by acidulation of phosphate rock (PR), a process that is costly, energy-intensive, and environmentally hazardous. This study evaluated the P-solubilising potential of culture filtrates from three fungal strains (Aspergillus flavus JKJ7, Talaromyces purpureogenus JKJ12, and Trichoderma koningiopsis JKJ18) grown in National Botanical Research Institute's Phosphate (NBRIP) liquid medium supplemented with tricalcium phosphate (TCP), and compared their TCP solubilisation efficiency with that of pure acids (citric and sulfuric acid). All three fungal strains solubilised TCP in NBRIP medium, with A. flavus JKJ7 producing the highest concentration of soluble P (259.81 mg L^-1), followed by T. koningiopsis JKJ18 (166.41 mg L^-1) and T. purpureogenus JKJ12 (47.07 mg L^-1). Soluble P concentrations were inversely correlated with pH and positively correlated with titratable organic acidity (TOA). High-performance liquid chromatography (HPLC) identified citric, succinic, tartaric, and gluconic acids as the dominant organic acids associated with P solubilisation. In pure acid treatments, sulfuric acid exhibited concentration-dependent increases in soluble P, whereas citric acid showed reduced solubilisation efficiency at higher concentrations. Although fungal culture filtrates achieved lower maximum TCP solubilisation than strong mineral acids, their higher TOA contributed to improved stabilisation of soluble P by limiting calcium-mediated reprecipitation. These findings demonstrate that crude fungal organic acid mixtures can complement or partially substitute inorganic acids for mobilising P from low-reactivity PR, offering a potentially cost-effective and environmentally sustainable alternative for P fertiliser production. This study supports the development of biologically derived P inputs aligned with circular bioeconomy and sustainable agriculture goals.