Changes in nitrification rates due to climate change have the potential to influence soil nitrogen availability, water quality, and greenhouse gas emissions. However, the mechanisms through which temperature and precipitation affect nitrification and the nitrifying microbial community in the field are largely unknown. We examined the effects of warming (up to ~4°C) and altered precipitation (−50%, ambient, +50%) on potential nitrification kinetics, or potential nitrification rates over a range of ammonium (NH4+) concentrations. We also examined responses of the abundance and composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), which play a critical role in nitrification. This work took place over two years in an old-field ecosystem in Massachusetts, USA, as part of the Boston-Area Climate Experiment (BACE). Across all dates and during June and August 2010, drought decreased the half-saturation constant, Km, or the concentration of NH4+ at the half-maximal potential nitrification rate. AOB composition responded to the main and interactive effects of warming and precipitation, and warming decreased AOA abundance by 82% during January 2009. Although Km, AOB composition, and AOA abundance responded to the treatments to some degree, potential nitrification kinetics were generally uncorrelated with AO composition or abundance. Sampling date also had a greater effect on potential nitrification kinetics and AO than the treatments themselves, and these larger temporal fluctuations may have masked any correlations between nitrification kinetics and AO. Our results demonstrate that the effect of warming and altered precipitation on AO and nitrification kinetics must be considered in the context of broader temporal variations in AO composition, AO abundance, and nitrification kinetics.