Abstract:[Background] The long-term flooding conditions in paddy fields and root exudates of rice plants create a favorable environment for nitrogen-fixing microorganisms, among which nitrogen-fixing cyanobacteria are the main contributors to biological nitrogen fixation in paddy ecosystems. [Objective] To obtain efficient nitrogen-fixing cyanobacterial strains and provide strain resources for enhancing biological nitrogen fixation in paddy fields. [Methods] The BG-110 nitrogen-free medium was used to isolate nitrogen-fixing cyanobacteria from different paddy soil samples. The isolates were screened based on nitrogenase activity, indole-3-acetic acid (IAA) secretion ability, and siderophore production ability. The selected efficient nitrogen-fixing strain was identified by morphological observation and methods of molecular biology. The plant growth-promoting effect of the selected strain was evaluated by hydroponic and pot experiments. [Results] An efficient nitrogen-fixing [239.27 nmol/(min·mg)] strain with high IAA production (7.70 μg/mL) and robust siderophore secretion ability was screened out. It was identified as a strain belonging to Aliinostoc of Nostocaceae and named Aliinostoc sp. d389-10. This strain significantly promoted the growth of rice seedlings in hydroponic experiments, increasing the plant height, root length, and dry weight by 22.42%, 54.76%, and 57.80%, respectively. In pot experiments, inoculation with d389-10 significantly improved soil nitrogenase activity and rice yield, while significantly increasing soil organic carbon, total nitrogen, and available nutrients. [Conclusion] The strain d389-10 screened out is an efficient nitrogen-fixing cyanobacterial strain with rice growth-promoting effect, with research value and application potential.
