【objective】Nitrogen-fixing bacteria are crucial for biological nitrogen fixation in paddy fields. Returning straw to these fields increases the soil carbon-to-nitrogen ratio (C/N), potentially impacting the diversity and functionality of nitrogen-fixing microorganisms. The rice rhizosphere, a highly active microbial zone, experiences rapid oxygen depletion from straw decomposition, which promotes anaerobic nitrogen fixation. Conversely, the growth of rice may release oxygen, thereby potentially inhibiting these microorganisms. Research on these intricate interactions within rice cultivation systems remains limited.【Methods】 In this study, the rhizosphere bag method was employed to explore the impact of varying straw return levels (0%, 1%, 2%) on the abundance and community structure of nitrogen-fixing bacteria, as well as on nitrogenase activity in the rhizosphere and bulk soil of two distinct paddy soil types (high sandy soil and yellow mud soil).【Result】The results indicated that, compared to no straw return, a 2% straw return significantly increased the abundance of nifH genes in the bulk of high sandy soil by approximately 86.2%, and in the rhizosphere of both soil types by 154% to 179%. In both types of paddy fields, the abundance of nifH genes in the rhizosphere soil was higher than in the bulk soil, particularly under the 2% straw return condition, with increases ranging from 68.3% to 101%. Principal Coordinates Analysis (PCoA) revealed significant changes in the composition of nitrogen-fixing microorganisms in the bulk soil of both soil types compared to their respective rhizospheres. Additionally, the community compositions of nitrogen-fixing bacteria in the rhizosphere were distinctly separated from those in the bulk soil. The response of soil nitrogenase activities to straw return was inconsistent with that of nitrogen-fixing bacteria. Under a 1% straw return, nitrogenase activity in the bulk of high sandy soil showed no significant change, but increased by nearly 33.8% with a 2% return. Conversely, nitrogenase activity in the rhizosphere of high sandy soil decreased significantly by 18.3% to 37.2% under both straw return conditions. In contrast, in yellow mud soil, nitrogenase activity in both the rhizosphere and bulk soil significantly increased by 8.69% to 20.4% under a 1% straw return, but decreased under a 2% return.【Conclusion】In summary, the rhizosphere enhances the abundance of soil nitrogen-fixing bacteria and their enzyme activities by optimizing the soil environment. Meanwhile, straw return primarily increases soil organic carbon and the carbon-to-nitrogen (C/N) ratio, thereby boosting the abundance of nitrogen-fixing bacteria and altering microbial community diversity. The response of soil nitrogenase activity to straw return varies depending on the soil type and the amount of straw returned.