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

Abstract

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.