Abstract: The first step of nitrification (i.e., the oxidation of ammonia to nitrate) is catalyzed by nitrifiers, such as ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). However, the impact of soil warming on the activity and abundance of nitrifiers under different nitrogen (N) fertilization conditions remains poorly understood. A long-term field warming experiment has been conducted since October 2008 at the Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences in the North China Plain, during which soil temperature was increased by 1.5℃ using infrared heaters (power, 1 000 W) placed 2 m above the soil surface. In 2018, we investigated soils from the control (no warming) and warming treatment plots for potential nitrification rate (PNR), abundance of AOB and AOA at 10 cm and 20 cm soil depth under two N fertilization conditions:without N fertilization (N0) and with 240 kg(N)·hm^-2·a^-1 fertilization (N1). Soil PNR, nitrate (NO₃^--N), and ammonium (NH₄⁺-N) contents were spectrophotometrically assessed, and the abundance of functional genes was investigated via real-time quantitative PCR. Warming increased PNR and NO₃^--N content under N1 treatment and decreased them under N0 treatment (P < 0.05). Moreover, warming significantly increased AOB abundance under N1 treatment (P < 0.05), whereas it decreased the abundance of both AOA and AOB under N0 treatment, at both soil depths. Compared with N0, N1 exhibited substantial decrease in AOA/AOB ratio, suggesting that compared with warming without N fertilization, warming with N fertilization exhibited higher stimulation of AOB growth than of AOA growth. Conclusively, this study suggests that AOB significantly and positively responded to warming with N fertilization, whereas both AOA and AOB significantly and negatively responded to warming without N fertilization. This study provides an understanding of nitrifier activity and the response of ammonia-oxidizing microorganisms to warming conditions and N availability.