• Energy Research

The greenhouse gas (GHG) balance of biofuels largely hinges on the magnitude of nitrous oxide (N2O) emissions from arable soils during feedstock production, which are highly variable. Here, used an agro-ecosystem model to generate these emissions at a high resolution over the Ile-de-France region in Northern France, for a range of feedstocks. The emissions were input to a life-cycle assessment of candidate biofuel pathways: bioethanol from wheat and sugar-beet, biodiesel from oilseed rape, and ethanol from miscanthus. Compared to the widely-used methodology based on fixed emission factors, ecosystem modelling lead to 55% to 70% lower estimates for N2O emissions, emphasizing the importance of regional factors. The life-cycle GHG emissions of 1st generation biofuels were 50% to 70% lower than fossile-based equivalents, and 85% lower for cellulosic ethanol. Indirect land-use change effects negated these savings for bio-diesel and wheat ethanol, but were offset by direct effects for cellulosic ethanol.

We address the question of potential climate change effects on interactions betweenwheat and brown rust. The paper is based on biophysical modelling approaches anda climate scenario for four contrasting French sites provided within a project of theFrench National Research Agency. Predicted distributions of surface wetness durationand infection risk were calculated with the help of a big-leaf model of mass and energytransfer, while plant-pathogen interactions were simulated with the biophysical model,Ceres-wheat, enriched with recent knowledge on wheat growth in the face of lateepidemics. No particular trend was discernible concerning the infection rates due toopposite evolutions of two major variables: temperature and surface wetness duration.Yield of diseased crops remained also nearly constant due to a slight decrease of thehealthy crop yield combined with a lesser evolution of the disease.

Cereal straw, a by-product in the production of agricultural crops, is considered as a potentially large source of energy supply with an estimated value of 47 10^18 J worldwide. However, there is some debate regarding the actual amounts of straw which could be removed from arable soils without jeopardizing their quality, as well as the potential trade-offs in the overall straw-to-energy chain compared to the use of fossil energy sources. Here, we used a deterministic model of C and N dynamics in soil-crop systems to simulate the effect of straw removal under various sets of soil, climate and crop management conditions in northeastern France. Model results in terms of nitrate leaching, soil C variations, nitrous oxide and ammonia emissions were subsequently inputted into the life cycle assessment (LCA) of a particular bio-energy chain in which straw was used to generate heat and power in a plant producing bio-ethanol from wheat grains. Straw removal had little influence on simulated environmental emissions in the field, and straw incorporation in soil resulted in a sequestration of only 5 to 10% of its C in the long-term (30 years). The LCA concluded to significant benefits of straw use for energy in terms of global warming and use of non-renewable energy. Only the eutrophication and atmospheric acidification impact categories were slightly unfavourable to straw use in some cases, with a difference of 8% at most relative to straw incorporation. These results based on a novel methodology thereby confirm the environmental benefits of substituting fossil energy with straw.

The objective of the work reported here was to reduce the uncertainty on the greenhouse gas balances of biofuels using agro-ecosystem modeling at a high resolution over the Ile-de-France region in Northern France. The emissions simulated during the feedstock production stage were input to a life-cycle assessment of candidate biofuel pathways: bioethanol from wheat, sugar-beet and miscanthus, and biodiesel from oilseed rape. Compared to the widely-used methodology based on fixed emission factors, ecosystem modeling lead to 55-70% lower estimates for N2O emissions, emphasizing the importance of regional factors. The life-cycle GHG emissions of first-generation biofuels were 50-70% lower than fossil-based equivalents, and 85% lower for cellulosic ethanol. When including indirect land-use change effects, GHG savings became marginal for biodiesel and wheat ethanol, but were positive due to direct effects for cellulosic ethanol.

Soil erosion poses a significant threat to agricultural production worldwide, with a still-debated impact on the current increase in atmospheric CO2. Whether erosion acts as a net carbon (C) source or sink also depends on how it influences greenhouse gas (GHG) emissions via its impact on crop yield and nutrient loss. These effects on the environmental impacts of crops remain to be considered. To fill this gap, we combined watershed-scale erosion modeling with life cycle assessment to evaluate the influence of soil erosion on environmental impacts of wheat production in the Ebro River basin in Spain. This study is the very first to address the full GHG balance of erosion including its impact on soil fertility and its feedback on crop yields. Two scenarios were simulated from 1860 to 2005: an eroded basin involving conventional agricultural practices, and a non-eroded basin involving conservation practices such as no-till. Life cycle assessment followed a cradle-to-farm-gate approach with a focus on recent decades (1985–2005). The mean simulated soil erosion of the eroded basin was 2.6 t ha−1 year−1 compared to the non-eroded basin. Simulated soils in both eroded and non-eroded basins lost organic C over time, with the former emitting an additional 55 kg CO2 ha−1 year−1. This net C source represented only 3% of the overall life cycle GHG emissions of wheat grain, while the emissions related to the increase of fertilizer inputs to compensate for N and P losses contributed a similar percentage. Wheat yield was the most influential parameter, being up to 61% higher when implementing conservation practices. Even at the basin scale, erosion did not emerge as a net C sink and increased GHG emissions of wheat by 7–70%. Nonetheless, controlling erosion through soil conservation practices is strongly recommended to preserve soils, increase crop yields, and mitigate GHG emissions.

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.