Objective  To explore the rhizosphere process of soil phosphorus (P), the profile vertical variation characteristics and differences of bioavailable P components in red soil under different utilization modes were analyzed.

  Method  Based on the biologically-based P (BBP) method, the quantitative changes of soluble P (CaCl₂-P), organic acid activated P (Citrate-P), Organic P mineralized by acid phosphatase and phytase (Enzyme-P), potential inorganic P (HCl-P), total P (TP), and available P (Bray-P) of pedogenic horizons were determined in sloping farmland, which located at the observation station of the typical red soil small watershed in Sunjia, Yingtan, Jiangxi Province. The correlations among soil pH, total nitrogen (TN), soil organic matter (SOM), iron-aluminum oxides and P components were analyzed by structural equation model.

  Result  The results showed that TP and Bray-P contents were higher in tillage layer than those in the bottom layers of sloping red farmland soil, while the TP and Bray-P in the red paddy field decreased significantly with the slope, while the Bray-P in the peanut upland showed the opposite change, and the TP in the middle slope was significantly higher than those in the top and bottom slopes. And the contents of each biological effective P component were in the following order: HCl-P > Citrate-P > CaCl₂-P > Enzyme-P, and all of them decreased with increasing depth of the profile, but there was no obvious regularity by slope position. The TP content in the red soil profile of paddy field was generally higher than that of peanut upland, while the Bray-P was opposite. However, only the Enzyme-P in the Ap1 layer of the paddy field was significantly higher than that of the peanut upland. Correlation analysis showed that HCl-P, Citrate-P and CaCl₂-P were significantly positively correlated with Bray-P, highly significantly positively correlated with TN, TP, and SOM, and negatively correlated with Fe_d (P < 0.01), while HCl-P and Citrate-P in red soil of paddy field were significantly negatively correlated with Al_d (P < 0.05).

  Conclusion  The P of each fraction in the red soil pedogenic horizons of sloping farmland in Sunjia watershed decreased with increasing depth of the profile, but there was no obvious regularity by slope position. The Bray-P in peanut upland and paddy field was mainly from HCl-P, Citrate-P, CaCl₂-P, and Citrate-P. TN, TP, SOM, and iron-aluminum oxides were the main factors affecting the P bioavailability of red soil sloping land. In practical production, soil P bioavailability could be improved by increasing application of organic fertilizer and adjusting the morphology of iron-aluminum oxides.