This research investigated the N 2 O fluxes and community compositions of nitrifiers and denitrifiers in a winter wheat-summer maize rotation system on the North China Plain. The experiment included three treatments: 1) a control treatment (CK); 2) biochar at 10.0 t ha–1 yr−1 (C); and 3) organic fertilizer at 7.5 t ha−1 yr−1 (M). The application of biochar reduced the cumulative N 2 O emissions by 47.7% and 62.2% in the maize and wheat seasons, respectively, compared with those of the CK treatment. Organic fertilizer increased the cumulative N 2 O emissions by 311.1% in the maize season and had no significant effects on them in the wheat season. Organic fertilizer reduced the cumulative N 2 O emissions by 39.8% in the nonfertilizer period of the maize season. The cumulative N 2 O emissions in the maize season accounted for 75.2–90.0% of the annual emissions among all the treatments. Biochar and organic fertilizer affected soil N 2 O emissions mainly by changing soil denitrifiers. In the maize season, the lower abundances of Candidatus Nitrosoarchaeum (AOA- amoA), Nitrosomonas (AOB- amoA), Mesorhizobium (nirK), Magnetospirillum (nirS) and Halomonas (nirS) may result in lower N 2 O emissions in the biochar treatment, and organic fertilizer had a similar influencing mechanism on N 2 O emissions during the nonbasic fertilizer and nontopdressing periods of the maize season. In the wheat season, the dominant genus Alicycliphilus (nosZ) was the major contributor to decreasing N 2 O emissions in the biochar treatment, while there was no biomarker related to N 2 O emissions in the M treatment. The soil pH, NO 3 −-N content and water-filled pore space (WFPS) were the key factors shifting the community compositions of nitrifiers and denitrifiers in this study. The application of biochar could be a better practice to improve saline-alkali soil with lower N 2 O emissions. These findings will improve our understanding of the nitrifiers and denitrifiers response to biochar and organic fertilizer. [Display omitted] • Both biochar and organic fertilizer had potential to reduced N 2 O emissions. • The reduction of N 2 O was related to AOA, AOB, nirK / nirS and nosZ -type denitrifiers. • The two additives had similar mechanism on N 2 O in non-fertilization stage of maize. • Soil pH was the key factor affecting nitrifiers and denitrifiers communities. [ABSTRACT FROM AUTHOR]