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Background and aims: Nitrogen fertilization is one of the most important practices for improving of crop yield and soil quality, which also affects soil CO2 emission. CO2 emission is strongly influenced by soil bacterial communities. It is still unknown precisely how nitrogen fertilization affects soil bacterial communities in the semiarid Loess Plateau and how its effects are related to CO2 emissions. Methods: A field experiment was conducted with four nitrogen fertilization rates (0 (N0), 100 (N1), 200 (N2), and 300 (N3) kg N ha–1). Nitrogen fertilization effects soil bacterial communities, CO2 emission, carbon emission efficiency (CEE), and their relationships were investigated. Results: As a result of nitrogen fertilization, soil bacterial abundance increased, and the bacterial community composition and network changed. Soil CO2 emission under N1, N2, and N3 were increased by 17.3%, 24.8%, and 28.9% compared to the N0 treatment, respectively. N2 and N3 exhibited the highest maize grain yield and CEE. CEE under N2 and N3 increased by 96.9%-140.9% compared to the N0 treatment. Soil bacterial abundance had a positive correlation with CO2 emission. CO2 emission were also affected by the bacterial composition and network, which were related to the Actinobacteria. Multiple regression analysis further suggested that CO2 emission was jointly regulated by the abundance, composition, and network modules of the bacterial community. Conclusion: Overall, our results indicated that nitrogen fertilization promoted CO2 emission via increasing bacterial abundance and altering the composition of their community, and provided a new insight into the connection between soil microorganisms and agricultural CO2 emission in the semi-arid region. [ABSTRACT FROM AUTHOR]