Ammonia oxidation is the rate-limiting step of nitrification and an important indicator for assessing how nitrogen (N) deposition affects soil N cycling processes. After a five-year N addition experiment, we collected soil samples from a subtropical Moso bamboo forest during the wet season (June) and dry season (December). In conjunction with a phosphorus (P) addition experiment in vitro, we investigated the effects of N addition on soil nitrification potential and ammonia-oxidizing microorganisms in different seasons, along with their underlying mechanisms. The results showed that N addition significantly increased soil ammonium N content and decreased soil P availability in both seasons. Additionally, N addition significantly reduced soil pH only in the dry season. N addition had more pronounced effects on nitrification and ammonia-oxidizing microorganisms in the dry season compared to the wet season. High N addition significantly increased soil nitrification potential, abundance, and diversity of ammonia-oxidizing archaea (AOA), and altered the community structure of AOA in the dry season. In contrast, N addition only significantly altered the community structure of ammonia-oxidizing bacteria (AOB) in the wet season. In both seasons, the gene abundance of AOA and AOB, as well as nitrification potential, were significantly higher in soil with combined N and P additions compared to soil with P additions alone. The above results indicated that P addition altered the effects of N addition on ammonia-oxidizing microbial activity and nitrification potential. Moreover, the regression analyses revealed that the nitrification potential was significantly positively correlated with the abundance of AOA gene in the absence of P addition. However, the nitrification potential was significantly positively associated with the gene abundance of both AOA and AOB when P was added. It suggests that the decrease in soil P availability caused by N addition suppressed AOB activity and its role in nitrification potential to some extent. The study collectively provides a theoretical basis for a better understanding of nitrification and nitrogen cycling processes in P-poor subtropical soils under climate change.