Kinetics of phytate adsorption and response of phosphorus forms initially present in alkaline soils

Aims: Although phytate is the most abundant organic P compound in soil, its sorption makes it poorly bioavailable through enzymatic hydrolysis. The sustainable use of the P resource in agriculture will require improving the supply of P to plants from organic P forms. To this end, further knowledge on dynamics of phytates in the soil is necessary, particularly in alkaline soils. This work aimed to study the factors that control phytate sorption and hydrolysis by phytases in alkaline soils and how this sorption affects other P forms present in soil. Methods A phytate sorption experiment was carried out using six soils with basic pH. Subsequently, the hydrolysis by two phytases (from Aspergillus niger and Bacillus subtilis) of organic P remaining in solution and extracted from the soil was assessed. Results Phytate was rapidly sorbed by soils with an estimated maximum sorption of 603 mg kg 1 at 360 min. Phytate sorption was positively correlated with carbonates, poorly crystalline Fe oxides, organic C, pH, and electrical conductivity in soils. Phytate adsorption promoted the release of other organic and inorganic P forms with less affinity for sorbent surfaces. For inorganic P, this release was more evident in soils with a low poorly crystalline Fe oxide content, while for organic P, the release increased with carbonates and clay content. Most of the phytate adsorbed was not extracted by NaOH, particularly in soils with high carbonate and poorly crystalline oxide content. This reveals that a significant portion of adsorbed phytate can be bound to carbonates, precipitated as metal phytates, or was not desorbed from Fe oxides. The insights gained from this research contribute to increasing our understanding of organic P cycling in soils and how soil properties influence the dynamics of these P forms.