Emissions of the trace gas nitrous oxide (N 2 O) play an important role for the greenhouse effect and stratospheric ozone depletion, but the impacts of climate change on community structure of denitrifiers and the underlying microbial drivers of N 2 O fluxes remain unclear. The aim of this study was to determine the effects of sustained climate change on field community structure of denitrifiers and associated N 2 O fluxes, microbial enzymatic activities, and microbial population abundance in an extensively managed, upland grassland. We simulated global warming effect by exposing a grassland for 4 years to elevated atmospheric CO 2 (+200 ppm), elevated temperature (+3.5 °C) and reduction of summer precipitations (-20%) as part of a long-term, multifactor climate change experiment. While recording N 2 O fluxes, potential nitrification and denitrification, microbial population size involved in these processes, we assessed the community structure of nitrite reducers ( nir K) that perform the first step of denitrification. Our results showed that specific lineages of nir K denitrifier communities responded significantly to temperature. In addition, nir K community composition showed significant changes in response to drought. Both warming and simultaneous application of warming, summer drought and elevated CO 2 had a positive effect on N 2 O fluxes, nitrification, N 2 O release by denitrification and the population size of N 2 O reducers and NH 4 oxidizers. In situ N 2 O fluxes showed a stronger correlation with microbial population size under warmed conditions compared with the control site. Path analysis explained more than 85% of in situ N 2 O fluxes variance by specific denitrifying lineages, soil temperature and denitrification activity. Overall, our study underlines that climate-induced changes in grassland N 2 O emissions reflect climate-induced changes in microbial community structure, with a potential selection of more adapted types. These in turn may modify microbial processes.