Objective Being an element promoting the biological nitrogen fixation (BNF) of the potentially applicable aquatic green fertilizer Azolla for rice farming, phosphorus (P) application was evaluated to optimize the practice.

Method In a greenhouse hydroponic experimentation, low P (LP as control at 1.1 mg·L⁻¹), medium P (MP at 11 mg·L⁻¹ ), or high P (HP at 22 mg·L⁻¹) was applied to grow azolla plants. Biomass, growth rate, BNF rate, BNF amount, morphology, soluble sugar, nitrogen, population thickness, growth area, root length, and leaf spacing of the azolla plants were measured.

Result P applications affected significantly on the azolla growth and BNF. Weekly, the applications yielded the fresh plant biomass at 747 g·m⁻² on LP, 967 g·m⁻² on MP, and 1184 g·m⁻² on HP, and the BNF amount at 1.78 g·m⁻² on LP, 2.41 g·m⁻² on MP, and 3.05 g·m⁻² on HP. MP and HP boosted the growth rate by 29% and 58%, respectively, over LP. The path analysis attributed the enhancements to the up to 36% rises on BNF rate and 71% on BNF amount. And the BNF rate was increased due to an accelerated soluble sugar conversion and expanded growth area of the plants on the pond.

Conclusion The increased supply of P significantly enhanced the BNF and growth of azolla. Thereby, rapid propagation and extended coverage on the water surface of the plants were achieved at the initial stage of cultivation. By virtue of co-cultivation with azolla in a paddy field, rice production could be significantly improved through the highly efficient BNF of the aquatic fern.