The Role of Microbial Mechanisms on the Availability of Soil Phosphorus from Fixed and Structural Mineral Fractions

This review deals with the role of soil microorganisms on the availability of phosphorus (P) derived from its two predominant low-solubility soil sources: 1) fixed P via inner-sphere adsorption onto Fe oxides (e.g., goethite), and 2) structural P sequestered within Ca-phosphate minerals (e.g., hydroxyapatite). The processes addressed the exudation of organic acids, siderophores, and reduction reactions. Organic acids, notably citric acid, facilitate the liberation of inner-sphere bound P by mechanisms such as acidification. Hydroxyapatite solubility at pH 4.0 exceeds that at pH 6.0 by a factor of nearly 40,000. Siderophores form inner-sphere complexes with Fe oxides (siderophore-Fe³⁺), reducing the concentration of Fe in the soil solution. The organic acid/siderophore-Fe³⁺ complex favors the dissolution of Fe oxides by mitigating the soil’s electrical potential. Reduction reactions emerge as pivotal players in liberating inner sphere-bound P complexes. The presence of organic molecules capable of shuttling electrons from microorganisms to solid phase Fe minerals augments reduction conditions and influences Fe reduction kinetics. The environment that favors the synergistic interaction of these factors, leading to the enhanced release and increased P availability for plants, can be highlighted: i) the application of low-solubility phosphates (hydroxyapatite), along with the association and incorporation of animal and plant residues (organic matter) to soils, favors microbial growth and the release of organic acid. The organic matter and hydroxyapatite should be distributed across the land surface rather than being confined to the planting furrow and the incorporation of these products carried out by agricultural implements.; ii) the Brazilian agriculture occur mainly in oxidic soils (Oxisols with high contents of Fe and Al oxides). The applying of organic residues is also recommended in this production system to increase the microbial population, promote the release organic acid and siderophore, and reduce the soil’s redox potential. The major challenge is to solubilize the residual soil P from previous plant cultives and convert it into forms that are more readily available to plants (outer-sphere and soluble). In this context, the microbial mechanisms discussed in this review are of great significance.