Objective Yield and nitrogen uptake of rice plants on a rice-azolla co-cultivation paddy field were investigated to understand how the highly efficient nitrogen-fixing Azolla might help to combat CO₂ rising from climate change and improve rice production.

Method Along with the current atmospheric CO₂ level as control (CK), CO₂ concentrations of Azolla grown under CK (A), addition of 200 ppm of CO₂ over CK (E), and Azolla grown under E (E+A) were automatically regulated using the open-top chamber for the experimentation. Biomass, biological nitrogen fixation (BNF) rate, and total BNF amount of azolla as well as tiller number, grain yield, and nitrogen uptake of rice plants grown with or without azolla co-cultivation under the atmospheric CO₂ conditions were determined.

Result The elevated CO₂ concentration in the atmosphere as in E significantly promoted the growth and BNF of azolla plants at the jointing, flowering, and grain filling stages of rice under rice-azolla co-cultivation. The BNF rate of azolla under E+A was 5.2%-46.7% higher, and the total BNF amount 11.5% greater than under A, resulting in an increased rice yield. Over CK, E increased the rice yield by 11%, A by 7%, and E+A by 18%. Furthermore, E+A significantly raised the effective rice tiller numbers by 16%, grain nitrogen uptake by 19%, straw nitrogen uptake by 17%, and total nitrogen uptake by 19%.

Conclusion As the concentration of atmospheric CO₂ rose due to climate change, azolla-rice co-cultivation in a paddy field could boost the BNF of azolla plants by the enhancing effect of CO₂ to increase the nitrogen fertilization for rice cultivation. Thus, through the symbiosis, rice crop yield could be significantly improved with the green practice.