Controversy is brewing about the potential greenhouse gas (GHG) savings resulting from the displacement of fossil energy sources by bioenergy, which mostly hinges on the uncertainty on the magnitude of nitrous oxide (N2O) emissions from arable soils occuring during feedstock production. The life-cycle GHG budget of bioenergy pathways are indeed strongly conditioned by these emissions, which are related to fertilizer nitrogen input rates but also largely controlled by soil and climate factors. The IMAGINE project, funded by by the ENERBIO/Tuck Foundation from January 2010 to December 2011 aimed at improving the estimation of N2O emissions from local to regional scales using ecosystem models and measurements and modeling of atmospheric N2O in the greater Paris basin, by using ecosystem models and measurements and modeling of atmospheric N2O. Ground fluxes were measured in two locations to assess the effect of soil type and management, crop type (including lignocellulosics such as triticale, switchgrass and miscanthus), and climate on N2O emission rates and dynamics. High-resolution maps of N2O emissions were generated over the Ile-de-France region (around Paris) with a generic ecosystem model, O-CN, and an agro-ecosystem model, CERES-EGC, using geographical databases on soils, weather data, land-use and crop management. The models were tested against ground flux measurements, and the emission maps were fed into the atmospheric chemistry-transport model CHIMERE. The maps were tested by comparing the CHIMERE simulations with time series of N2O concentrations measured at various heights in the planetary boundary layer in two locations in 2007. The emissions of N2O, as integrated at the regional scale, were used in a life-cycle assessment of representative biofuel pathways : bioethanol from wheat and sugar-beet (1st generation), and miscanthus (2nd generation process) ; biodiesel from oilseed rape. Compared to the standard methodology currently used in LCA, based on fixed emissions for N2O, the use of model-derived estimates leads to a 10 to 40 % reduction in the overall GHG emissions of biofuels. This emphasizes the importance of regional factors in the relationship between agricultural inputs and emissions (altogether with biomass yields) in the outcome of LCAs. When excluding indirect land-use change effects (iLUC), 1st generation pathways enabled GHG savings ranging from 50 to 73 % compared to fossile-derived equivalents, while this figure reached 88 % for 2nd generation bioethanol from miscanthus.