The Fertilizing Potential of Nanohydroxyapatite Coated with Polyacrylic Acid Based on Their Mobility and Bioavailability

Phosphorus (P) nanoparticles (NPs) have been proposed as a promising alternative to conventional water-soluble inorganic P (WSP) fertilizers, based on the hypothesized advantages of higher mobility and bioavailability in soils. However, systematic quantification of the mobility and bioavailability of P NPs in soils remains lacking. The transport (saturated and unsaturated), diffusion, and plant response of spherical polyacrylic acid-coated nano-hydroxyapatite (PAA-nHAP) and WSP were evaluated in two low-P soils: an alkaline sand (Alk) and an acidic sandy clay loam (Ac), targeting scenarios where P is surface-applied without incorporation into the root zone. After systematically comparing 36 one-site or two-site kinetic models in HYDRUS-1D, a model selection process was established for the first time. The results showed that two-site kinetic models can capture the transport pattern, with distinct retention mechanisms across soils: reversible and time-dependent at both sites for the Alk soil; reversible and depth-dependent at one site and irreversible and time-dependent at the other for the Ac soil. Because of the air-water-solid or air-water interface, greater retention of the PAA-nHAP was observed in the unsaturated transport than in the saturated transport. The diffusion capacity of the PAA-nHAP was lower than that of the WSP in both soils, primarily due to NPs aggregation. Tomato bioassays further demonstrated that the bioavailability of PAA-nHAP was not superior to, and in most cases, lower than that of conventional P fertilizers. The limited fertilization efficacy of the PAA-nHAP was attributed to the low solubility of hydroxyapatite and the restricted diffusion of aggregated NPs, which prevented the macroscale transport advantage from translating into sustained plant-available P supply. The framework of this study can be used to assess the agronomic benefits of nanofertilizers and the environmental risks posed by NPs in surface soils.