Abstract: Ammonia (NH₃) volatilization is one of the significant causes of nitrogen (N) loss in farmland. When NH₃ is released into the atmosphere, it reacts with acid gases to form secondary aerosols, which has a critical impact on air quality. This study aimed to simultaneously evaluate the effects and identify key mechanisms of combined applications of microbial organic fertilizer and chemical fertilizer on reducing ammonia volatilization in paddy fields. A two-year field experiment was conducted in a typical double-cropping rice field in Changsha County, Hunan Province. There were four fertilization treatments: no nitrogen fertilizer (CK), surface application of chemical nitrogen fertilizer (CON), a substitution of 40% chemical fertilizers with microbial organic fertilizers and surface application of chemical fertilizer (CB), and 30% reduction of chemical fertilizer with a substitution of 40% chemical fertilizers with microbial organic fertilizers and deep application of chemical fertilizer (RBD). NH₃ volatilization was measured using the intermittent closed chamber ventilation method in a two-year rice growing period (2019−2020), and the ammonium-N (NH₄⁺-N) and nitrate-N (NO₃⁻-N) concentrations in the surface water were also measured. The results showed that under the same nitrogen application rate, NH₃ volatilization was significantly (P<0.05) reduced in CB treatment compared to CON treatment, and the rice grain yield for CB treatment was not significantly different from that for CON treatment in all the four rice seasons. NH₃ volatilization was lowest in RBD treatment compared to CON and CB treatments. The differences in rice grain yield between CON and RBD treatments was significant (P<0.05) for the late-rice season in 2019, while the differences were not significant for the remaining three seasons. In the early-rice season, the average cumulative NH₃ volatilization losses of CON, CB, and RBD were 33.1 kg(N)∙hm⁻², 24.8 kg(N)∙hm⁻² and 12.2 kg(N)∙hm⁻², respectively. The NH₃ volatilization losses of CB and RBD decreased by 25.2% and 63.2%, respectively, compared to CON. In the late-rice season, the average cumulative NH₃ volatilization losses of CON, CB, and RBD treatments were 50.4 kg(N)∙hm⁻², 32.4 kg(N)∙hm⁻² and 14.7 kg(N)∙hm⁻², respectively. The NH₃ volatilization losses of CB and RBD decreased by 35.6% and 70.9%, respectively, compared to CON. The magnitude of NH₄⁺-N concentrations in the surface water showed the same trend with the NH₃ volatilization across the treatments in the rice seasons. Furthermore, there were significantly (P<0.01) positive correlations between these two parameters, which indicated that application of microbial organic fertilizer as well as deep application of chemical nitrogen fertilizer played a role in reducing NH₄⁺-N concentrations in the surface water, and thus, reduced NH₃ volatilization. Based on the two-year field experiment conducted here, this study revealed that microbial organic fertilizer combined with deep application of nitrogen-reduced fertilizer can reduce ammonia volatilization by 60%, while maintaining rice yields. Thus, in conclusion, microbial organic fertilizers combined with deep applications of reduced nitrogen fertilizer can effectively reduce the application rate of nitrogen fertilizer and mitigate ammonia volatilization in double-cropping paddy fields.