Abstract: 【Objective】This study is to probe the key declining process of soil available phosphorus (SAP) and its relationship with  P fertilizer application rate in red soil, and to verify the mathematical relationships among soil available P indexes in agronomy and environment. 【Method】 An laboratory incubation experiment was conducted with artificial fertilization and stable temperature of 25℃ using the material of red soil collected from subtropical area. The dynamic process of SAP was studied through testing SAP and phosphorus sorption index (PSI) in fresh soil samples in 1 h, 3 h, 12 h, 24 h, 3 d, 6 d, 9 d, 15 d, 30 d, and 60 d after fertilizer application into soil. Two types of test methods including Olsen- (OP), Bray 1-(BP), and Mehlich 3- (MP) extraction methods (agronomic type) and distilled water (HP) and 0.01M CaCl2 (CP) extraction methods (environmental type) were used. 【Result】 There was an obvious critical time (tc) during the declining process of SAP in red soil, ranging from 0.1 h to 18.7 h. The declining rate of SAP was rapid before tc but turned to be markedly slow since then. The impact of fertilizer application rate on tc was significant. The values of tc for OP, BP, and MP linealy delayed with the increment of fertilizer amount. However, the values of tc for HP and CP tended to delay in a power function pattern. Environmental index was nonlineally related to agronomic ones and there was a “change point” between them. Depending on this relationship, the threshold of soil P for environmental risk could be estimated. The thresholds of OP, BP, and MP based on CP were 49.97 mg•kg-1, 91.07 mg•kg-1, and 30.54 mg•kg-1, respectively, but these values based on HP were 60.78 mg•kg-1, 82.74 mg•kg-1, and 39.65 mg•kg-1, respectively. PSI ranged from 17.92 to 26.29 with the increasing fertilizer application rate and the value of PSI versus the threshold of OP for environmental risk was 23.46. 【Conclusion】There was an obvious critical time during the declining process of SAP in red soil. The value of critical time delays with increasing fertilizer application amount and subsequently the risk of soil P moving to the environment is more serious. There is a non-linear relationship between agronomic and environmental index. The critical value of SAP for environmental risk could be calculated according to this. The Olsen-P threshold of the tested red soil is 50 - 60 mg•kg-1and the relevant PSI is 23.46.

Key words: soil available phosphorus, fertilizer application amount, P threshold for environmental risk, phosphorus sorption index (PSI), red soil