Effects of phosphorus fertilizer application rate on transformation processes of phosphorus fractions in the purple alluvial soil of a riparian zone

Effects of phosphorus (P) fertilizer application rate on soil transformation processes of P fraction are still unclear in the riparian zone. Purple alluvial soils in the riparian zone of the Three Gorges Reservoir were collected to conduct a 21-day incubation executed by two hydrological environments (drying and flooding) and ten application rates of P fertilizer. Transformation percentages of P fertilizer (TPPF) were calculated as content differences of soil P fractions between fertilizer addition and none fertilizer addition divided by soil total P increases caused by fertilizer addition. TPPF to inorganic P extracted by sodium hydroxide (NaOH-Pi) and hydrochloric acid (HCl-Pi) increase by 20.91% (9.71%) and 24.26% (40.72%) under the drying (flooding) environment. Instead, TPPF to the other fractions decrease. Phosphorus fertilizer input mainly has indirect positive and negative effects on organic P via precipitated P under the drying and flooding environments and finally has indirect positive effects on labile P (p < 0.001). Percentage changes of water-soluble inorganic P (H₂O-Pi) and HCl-Pi under the flooding environment are higher than that under the drying environment, and percentage changes of organic P extracted by sodium hydrogen carbonate (NaHCO₃-Po) and NaOH-Pi show an opposite trend (p < 0.01). ∆ (differences in soil P fraction content between flooding and drying incubations) H₂O-Pi is negatively correlated with ∆NaHCO₃-Po, and ∆NaHCO₃-Po is positively correlated with ∆NaOH-Pi (p < 0.001). In conclusion, P fertilizer is transformed more into precipitated P than into other P fractions with an application rate increase. Phosphorus fertilizer input mainly increases organic P via precipitated P under the drying environment and decreases organic P via precipitated P under the flooding environment, and organic P is further transformed into labile P. With P fertilizer input, P release caused by flooding is derived from NaHCO₃-Po release triggered by NaOH-Pi release. The results can be helpful for the understanding of P fertilizer migration processes from the riparian zone soil to the Three Gorges Reservoir under rain leaching and flooding.