Phosphate mineral solubility controls on cassava root exudates, rhizosphere nutrient availability, and plant nutrient accumulation

Phosphorus (P) is the most limiting essential nutrient in soil-plant systems; its availability is mainly dependent on host cationic metals/nutrients. However, P mineral solubility impacts plant root exudates and P solubility in the rhizosphere zone, with both P and host plant nutrient accumulation in the plant being poorly understood. This work developed the acid-washed-sand-hydroponic-hybrid technique to examine root exudates (protons and organic acid), P fractions, and nutrient accumulation in cassava exposed to P mineral with different solubilities (FePO4, Ca3(PO4)2, KH2PO4, and to no P addition). The root-to-shoot biomass data demonstrated that the cassava responded equally to different P minerals. Nevertheless, the P minerals substantially influenced the root proton and organic acid effluxes, with protons being the primary constituent released from the roots, corresponding to 99.75% of the total release of protons and organic acid. The sparingly soluble FePO4 and Ca3(PO4)2 treatments induced cassava to release fewer protons (449 and 480 μmol L−1 h−1 g−1 fresh weight root, respectively) from the roots than the highly soluble KH2PO4 treatment (630 μmol L−1 h−1 g−1 fresh weight root). The KH2PO4 treatment promoted higher oxalic acid release from the roots (3.31 μmol L−1 h−1 g−1 fresh weight root) than other treatments. The P fractions based on the sequential extraction technique revealed that the easily dissolved H2PO4− ion from the KH2PO4 compound could readily react with dissolved Fe in the sand material, possibly forming P bound to the Fe hydroxide fraction. Nevertheless, the positive relationship between the P bound to the Fe hydroxide and the P concentration in different plant parts (R2 = 0.39–0.89) showed that the newly formed Fe-bound P fraction could be available for plant assimilation. Notably, the Fe, Ca, and K concentrations in the plant parts were abundant in the respective FePO4, Ca3(PO4)2, and KH2PO4 treatments. Overall, the different P minerals considerably regulated the release of protons and ligands from cassava roots, with the released protons potentially promoting the release of the host metal of P minerals through the proton-promoted dissolution of the host metal oxides. In contrast, the released organic anions could enhance plant P availability by a ligand-exchange mechanism. This study highlighted the importance of phosphate minerals and their host metals in triggering root proton and organic ligand effluxes and in improving P availability and the P and cationic uptake by plants.