Abstract: Transportation and utilization of Azolla potassium in rice-Azolla-fish symbiotic system were researched by using ⁸⁶Rb as tracer. The experiment showed that the utilization rate of Azolla potassium by Ctenopharynodon idellus and Tilapia wore 39% and 31% respectively. With the prolongation of feeding time to Tilapia, the utilization rate of Azolla potassium is reduced. It's because of the overplus of potassium in the body fluid of fish be excreted by urine. Excrement sunken on the soil surface would released its potassium to the water in1—2 days, and then, the potassium would gradually penetrate to the soil layer. In30 days, about 83% of the potassium from excrements released to the soil and its penetrating depth was only about 10 cm. The utilization rate of Azolla potassium by early and late rices was 14.1% and 17.6% respectively. In our view, there exists an available problem of excrement potassium to rice plant in space and time. The cycling transportation and distribution model for Azolla potassium in rice-Azolla-fish system was laid down. On the basis of the analysis on the structure and function of this system, advantages and problems of the rice-Azolla-fish symbiotic system were commented.

Key words: Rice-Azolla-fish symbiotic system, Azolla potassium, Cycling utilization, Labelled technique