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Nitrous oxide (N 2 O) is a potent greenhouse gas that is released from agricultural ecosystems where nitrogen fertilizers are poorly used and contributes to global climate warming. Large nitrogen fertilizer inputs in summer maize fields on the North China Plain (NCP) lead to large N 2 O emissions. N 2 O emissions can be mitigated using 3,4-dimethylpyrazole phosphate (DMPP), but the efficiency and microbial mechanisms of this mitigation at different nitrogen rates remain poorly understood. To evaluate the efficiency of N 2 O emission mitigation by DMPP at different nitrogen rates and to explore its mechanisms at the microbial level, we monitored the dynamic changes in mineral and N 2 O fluxes and analysed the abundance and community structure of ammonia oxidizers. We found that DMPP significantly inhibited the oxidation of ammonium (NH 4 + ) to nitrate (NO 3 − ) and thus maintained NH 4 + concentrations but decreased the peak value of NO 3 − concentrations. Total N 2 O emissions were increased by 6.5-fold, 13.2-fold and 26.7-fold in fields with 112.5 kg N ha −1 , 225 kg N ha −1 and 337.5 kg N ha −1 , respectively, compared with fields with no nitrogen applied. Nitrogen increased the abundance of ammonia-oxidizing bacteria (AOB) and decreased that of ammonia-oxidizing archaea (AOA), whereas DMPP exhibited the opposite effects. Therefore, AOB could be the dominant N 2 O emission contributors in nitrogen-treated soils. Sequencing results suggested that all AOB belong to the Nitrosospira and Nitrosomonas nitrosa groups, and all AOA fell within the Nitrososphaera group. In both AOB and AOA communities, changes in the second-largest cluster after the application of nitrogen fertilizer and DMPP were more pronounced than those in the

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