Abstract: An anaerobic incubation test was conducted to study the effects of different P application rate on the P concentrations in paddy soil and its surface water. The results showed that soil available P (Olsen-P) decreased rapidly at the beginning, but approached to stable after 60 days of P application. Both Olsen-P and residual P increased with increasing P application rate, and Olsen-P had a positive correlation with P application rate, suggesting that the test soil had a strong P adsorption capacity. After P application, the total P (TP) in soil surface water increased rapidly, and then decreased slowly, showing that there was a P exchange between soil and its surface water. After 120 days of P application, there was an exponential relationship between soil surface water TP and P application. The TP in soil surface water increased rapidly when the P application rate was 400～800 kg·hm^-2, and easy to be lost when the P application rate was higher than 800 kg·hm^-2. The simulation with split line model on the relationship between soil Olsen-P and soil surface water TP showed that the change point of soil OlsenP, which induced a sharp increase of soil surface water TP concentration, was 82.7 mg·kg^-1, corresponding to a P application rate being about 712 kg·hm^-2. Soil Olsen-P could be a good indicator in forecasting the P loss from soil surface water.

Key words: Brachionus angularis, Food concentration, Population growth rate, Body size, Egg size