Fungal soil bioinoculants and their skills to improve phosphorus slow solubilisation and release

Phosphorus is one of the most important nutrients for plant development and deficiencies and in soil it limits agricultural crop yields worldwide since its soluble forms rapidly becomes insoluble or unavailable to organisms. This forces growers to repeated applications of fertilizers to crops, resulting in eutrophication and other ecological challenges. The global P reserves are projected to be depleted in 50 years, raising concern over food production for the human population which is thought to increase in next decades. The use of microorganisms, fungi and bacteria, as biological fertilizers is a growing practice and the subject of intense research. Fungi are ubiquitous in soil and can be dominant components of the microbiota in many soil types. Among the potentialities of fungi to be employed in innovative forms of soil amendment, the ability of some fungal species in the leaching of minerals and elements' chelation and translocation has been yet very little evaluated and even less exploited. The aim of this study was to identify the best P biofertilizers among the tested fungal species for their application in agricultural soil, in order to increase the P availability for plant development from insoluble tricalcium phosphate (TCP). We tested selected fungal isolates (29 species: 4 Zygomycota and 25 Ascomycota) with different life strategies on Pikovskaya’s medium containing 5% TCP. Fungal growth was evaluated by diametric extension of colonies and biomass yield in order to also consider the density of fungal mycelium. The surface pH of the agar was measured at specific intervals across the diameter of the Petri dish with a pH meter. The halo zones around the colonies were assessed and the solubilising efficiency was calculated according to different indexes. The solubilised phosphorus in broth cultures and the P content of fungal biomass were estimated by colorimetric methods. Different patterns of solubilisation by fungi were described and related to fungal growth. The results of the study provided evidence on some saprotrophic fungi ability to mobilize P from insoluble compounds, and thus release it with different speeds. P forms slowly released by fungi in soil can become available to plants at a sustainable rate. This study integrates the number of P solubilising fungal species and shed the light on mechanisms and parameters that are important to be considered in the choice of efficient soil P-biofertilizers.