Comparative analysis of crystalline hydroxyapatite and amorphous calcium phosphate for dissolution and plant nutrition

The performance of fertilizers for supplying nutrients to plants depends on their dissolution characteristics in soils. Here, we compared the dissolution kinetics and compositional changes at surfaces between citrate-stabilized amorphous calcium phosphate (ACPc) and crystalline hydroxyapatite nanoparticles (HANPs) when exposed to citric and acetic acid buffers, two organic acids commonly present in root exudates. A series of complementary orthogonal characterization techniques, including FTIR, Raman, and PXRD, were combined to elucidate the evolution of the Ca/P ratio, pH change, and recrystallization of calcium phosphate. We found that changes in pH and Ca/P ratio during dissolution in HANP and ACPc were largely due to differences in the formation of surface complexes between the acids and the intra-particle migration of protons (confirmed from H₂O/D₂O isotope exchange). A greenhouse pot trial experiment was performed using commercial lettuce to ground-truth how these characteristics influence the plant nutrition. Results showed a higher crop yield in HANP and ACPc treatments compared to the commercial fertilizer (monocalcium phosphate (MCP)), by 20 and 33%, respectively. The major difference was in resource use efficiency (RUE), a ratio of crop yield to P lost after irrigation, which was about six times higher in HANP than commercial MCP. These outcomes correlate well with dissolution characteristics that the leaching loss of dissolved P could be a major reason for the low yield and highly diminished RUE of ACPc and MCPs compared to those of HANPs. These outcomes provided multiple reasons for the need for the development of next-generation phosphorus fertilizers that are dually capable of enhanced nutrient as well as high resource use efficiency.