IMproved Assessment of the Greenhouse gas balance of bioeNErgy pathways (IMAGINE)
Résumé
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 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 (France) basin. Ground fluxes of N2O were measured in two locations to assess the effect of soil type and management (in particular drainage), crop type (including lignocellulosics such as triticale, switchgrass and miscanthus), and climate on emission rates and dynamics. Atmospheric concentrations of N2O were monitored with high precision in 3 sites relevant to the Paris basin, using Radon tracing to produce source estimates. High-resolution maps of N2O emissions over France were simulated 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 the 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 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 wehat, sugar-beet and miscanthus; biodiesel from oilseed rape). Effects related to direct and indirect land-use changes (and their impact on soil carbon stocks) were also included in the assessment. The spatial distribution of the N2O emission generated with the O-CN and CERES-EGC ecosystem models differed markedly: O-CN simulated higher emissions in the west of France due to livestock farming whereas CERES-EGC emphasized the greater Paris basin with intensive cereal farming. This was partly due to variations in forcing such as N application rates, cropland area, and soil properties, but also to modelling concepts. In particular, the simulation of soil water balance and the response of N2O emissions to surface moisture follow different approaches in both models. On an annual basis, N2O emissions from agricultural soils over France totalled 17, 56 and 69 Gg N2O-N with the CERES-EGC, O-CN, and EDGAR32 maps, respectively. In both atmospheric measurement sites, simulations with the EDGAR32 map were closest to the observed concentrations, especially in spring when fertilizers are applied. This points to an underestimation by the ecosystem models by 20 to 80%, although both models compared well with measured ground fluxes in a few cropland test sites. Various causes for this pattern may be explored. First, indirect emissions via nitrate leaching were about half of the direct emissions according to the EDGAR database but were ignored by the ecosystem models. These may be easily introduced based on their simulations of nitrate leaching fluxes. Secondly, the regional inputs of mineral N-fertilizer were generally lower than fertilizer sales, by up to 50%, and this should be corrected. Lastly, models were mostly parameterized in sites with low N2O emissions rates, and should be tested in sites with higher emission potentials. The main results of the project may be summarized as follows: - an ambient air monitoring network was established for high accuracy N2O measurements, compatible with existing networks elsewhere in the world, in 3 sites in France; - high time resolved N2O surface concentrations were made available for two stations, along with estimates of N2O sources based on Rn-tracing over the 2 years of the project; ground fluxes were measured on arable crops in two locations (Orléans and Grignon), emphasizing the effect of rainfall patterns, drainage, fertilizer input rates and timing on N2O emissions. Measurements carried out over perennial lignocellulosic crops evidenced much smaller (up to an order of magnitude) N2O emission rates from these plants. two ecosystem models were improved and tested for the prediction of daily N2O emissions from agricultural and forest soils, resulting in prediction errors similar to the uncertainties in the observation erros. - simulated regional and global N2O emission maps are derived using a validated, process based N-cycle model implemented in the dynamic vegetation model; - The impact of higher model resolution is documented on simulated N2O mixing ratios with partitioning between the different emission sources; - A feasibility study for N2O inversions has been achieved; - A database contains the results open to the scientific community for further analysis; This report synthesizes the findings of the project, for each work-package, and closes with a tentative budget for N2O emissions in France in 2007 combining bottom-up and top-down estimates for the various sources (biogenic and non-biogenic).
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