Four years of experimental climate change modifies the community structure of denitrifiers and the related microbial drivers of N2O fluxes in an upland grassland ecosystem.

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.