Role of crystalline iron oxides on stabilization of inositol phosphates in soil

Inositol phosphates selective retention in soil is related to their great affinity to iron (Fe) oxides, with which they interact by the formation of ligand exchange complexes. Soils contain from amorphous to crystalline Fe oxides forms, as goethite (Gt), whose effect on inositol phosphates retention is well-known. The contribution of other widespread crystalline Fe oxides as haematite (Hm), magnetite (Mt) and maghemite (Mh) is instead less known. These oxides display different phosphorus (P) retention and release capacities due to their specific surface properties and stability towards protons, organic acids and complexing agents. We therefore investigated: i) the extent and mechanisms of myo-inositol hexaphosphate (InsP6) and inorganic phosphate (Pi) adsorption on Gt, Hm, Mt and Mh, also studying the computational geometries of InsP6 molecule; ii) the reversibility of the reaction as affected by pH and in the presence of citrate; iii) the efficiency of NaOH-EDTA on P extraction. Compared to Gt, Hm retained less InsP6, due to the distribution of superficial –OH groups allowing for the interaction of two phosphate groups instead of four, as for Gt. Mt and Mh retained more InsP6 than Hm, also through precipitation processes due to a partial dissolution of the oxides caused by InsP6. Desorption of InsP6 in the pH range 3.5–8.5 was negligible, except for Hm, while citrate led to larger desorption, with no release from Gt. NaOH-EDTA extraction of InsP6 was instead particularly effective from Gt and less from the other oxides. The four crystalline Fe oxides thus showed different capacity to retain organic P in the form of InsP6 as a function of surface properties and stability of the mineral-P complexes, therefore influencing the extent of release, accumulation and turnover in soils.