Bioenergy strategies and the global carbon cycle. / Stratégies bioénergétiques et cycle global du carbone

Abstract

Bioenergy strategies are seen as effective options to reduce net CO₂ emissions to the atmosphere. Carbon emissions from fossil fuel burning can be avoided if biomass is used as an energy carrier instead, because in a bioenergy system CO₂ is taken up by the plants and released to the atmosphere again when biomass is burned for energy. However, a carbon balance of bioenergy also needs to take into account the associated changes of С stored in the biosphere, i.e. in the vegetation, in plant litter and soils. For example, a net reduction of the С content in the biosphere leads to an increase of the atmosphere's С content by the same amount, and vice versa. Bioenergy is often produced along with other goods such as wood products. In such cases the carbon balance has to include the effect of С storage in biomass products and the substitution of other, energy consuming materials like steel or concrete with biomass. GORCAM (Graz -Oak Ridge -Carbon Accounting Model) is a spreadsheet model that has been developed to calculate the carbon balance of land management and biomass utilization strategies in forestry and agriculture. Input parameters describe the management régime (rotation length, harvesting intensity, growth rate...), previous land use, soil and litter carbon dynamics, fate of the harvest (biomass for energy, biomass products with varying lifetimes), fossil fuel substitution and its efficiency (bioenergy instead of fossil energy ; biomass products instead of products from other, more energy intensive materials), fossil fuel requirements for land management and biomass conversion. Model results reveal that the carbon balance of land management and biomass utilization strongly depends on the initial С storage on the site, the growth rate, the efficiency with which the harvest is used, and the time period of consideration. For high growth rates and efficient use of the harvest, the dominant component of the С balance is seen to be fossil fuel substitution. Some strategies result in a significant net reduction of carbon emissions from their very beginning {e.g. short-rotation forestry on previously agricultural land). Harvest of existing forests for biofuels and wood products can lead to a net source or sink at the beginning, depending on the efficiency of harvest use. In the case of a net initial source, the С balance can return to positive values as the forest regrows, the «payback period» being determined by the forest growth rate and other factors mentioned above.