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• Responses of alpine soil N transformations to warming and N input were examined. • Warming alone did not significantly affect soil N transformations. • N input significantly increased AOB and N 2 O emissions in the late growing season. • Warming amplified the effects of N input on nitrification and N 2 O emissions. • Ammonia-oxidizing bacteria dominate alpine soil N transformations and N 2 O emission. Alpine ecosystems worldwide are characterized with high soil organic carbon (C) and low mineral nitrogen (N). Climate warming has been predicted to stimulate microbial decomposition and N mineralization in these systems. However, experimental results are highly variable, and the underlying mechanisms remain unclear. We examined the effects of warming, N input, and their combination on soil N pools and N-cycling microbes in a field manipulation experiment. Special attention was directed to the ammonia-oxidizing bacteria and archaea, and their mediated N-cycling processes (transformation rates and N 2 O emissions) in the third plant growing season after the treatments were initiated. Nitrogen input (12 g m−2 y−1) alone significantly increased soil mineral N pools and plant N uptake, and stimulated the growth of AOB and N 2 O emissions in the late growing season. While warming (by 1.4 °C air temperature) alone did not have significant effects on most parameters, it amplified the effects of N input on soil N concentrations and AOB abundance, eliciting a chain reaction that increased nitrification potential (+83%), soil NO 3 −-N (+200%), and N 2 O emissions (+412%) across the whole season. Also, N input reduced AOB diversity but increased the dominance of genus Nitrosospira within the AOB community, corresponding to the increased N 2 O emissions. These results showed that a small temperature increase in soil may significantly enhance N losses through NO 3 − leaching and N 2 O emissions when mineral N becomes available. These findings suggest that interactions among global change factors may predominantly affect ammonia-oxidizing microbes and their mediated N-cycling processes in alpine ecosystems under future climate change scenarios. [ABSTRACT FROM AUTHOR]