Crystallization of single and binary iron‐ and aluminum hydroxides affect phosphorus desorption

In acidic soils, phosphorus availability is affected by its strong affinity for mineral surfaces, especially Fe‐ and Al‐hydroxides. Plant roots have developed adaptive strategies to enhance the availability of phosphorus, including producing and exuding low molecular weight organic acids with a high affinity for phosphorus that competes with high molecular weight organic ligands formed during humification and mineralization. The aim of this study was to characterize the kinetics and mechanism of phosphorus desorption from Fe‐ and Al‐hydroxides of variable crystallinity, as well as binary Fe:Al‐hydroxide mixtures. Long‐term desorption experiments (56 days) were conducted with CaCl₂, CaSO₄, citric acid, and humic acid as competitive sorptives. CaCl₂ and CaSO₄ were selected as general inorganic sorptives and citric and humic acids were selected as organic ligands produced by organisms in the rhizosphere or following humification. The cumulative phosphorus desorption increased following the order CaCl₂ < CaSO₄ < humic acid < citric acid. Amorphous ferrihydrite and Fe‐rich Fe:Al‐hydroxides exhibited much less desorption when exposed to inorganic solutions than the crystalline and Al‐rich Fe:Al‐hydroxide mixtures. Models of the desorption data suggest phosphorus desorption with citric acid is diffusion‐controlled for ferrihydrite and Fe‐rich amorphous Fe:Al‐hydroxides. When humic acid was the sorptive, metal‐organic complexes accumulated in the solution. The results suggest organic compounds, especially citric acid, are more important for liberating phosphorus from Fe‐ and Al‐minerals than inorganic ions present in the soil solution.