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Associated Title(s): Global Change Biology
N2O emissions from biofuel production in an organic farming system
Replacing fossil fuels with biofuel produced from organic matter reduces the emissions of climate change-causing greenhouse gases (GHG). However, there are concerns that the land use change (LUC) associated with the expansion of cropland for biofuel production could be a significant source of GHG emissions. Land use change can be either direct, the conversion of land from some previous use (or disuse) to cultivated land for biofuel crop production, or indirect, the impacts associated with LUC elsewhere as an unintended consequence of biofuel crop production.
The effects of indirect LUC are especially difficult to quantify so modeled emission values vary greatly. Some studies report that factoring LUC into biofuel production results in negligible GHG savings and others report increased emissions.
Njakou Djomo and coauthors summarized and compared the models and approaches used to estimate the carbon dioxide emissions from direct and indirect LUC associated with biofuel production from 15 studies, and assessed the mechanisms that led to variation in emission values.
The authors found that most of the variation in LUC carbon emissions depended on the feedstock planted and type of land used or displaced. In general, high yielding biofuel crops such as oil palm, sugarcane, and cellulosic crops had better overall GHG performance because less land is required than low for yielding crops. With regard to direct LUC, cellulosic crops such as trees and grasses sequestered carbon while sugarcane released carbon0. When land is converted from degraded land or pasture to cropland, carbon is likely to be sequestered. However, when land is converted from a natural ecosystem (e.g., forest or grassland) to cropland, carbon is emitted. Carbon emitted from indirect LUC was negligible when biofuel crops were grown on marginal or degraded lands since there is no new land brought into production. However, substantial carbon dioxide was released when land displaced by biofuel crop production was relocated to forests. The total amount of carbon released due to LUC for bioethanol production was -29% to 384% of that of gasoline. This means that in some cases, LUC could cancel out the GHG benefits of biofuels.
The authors conclude that there is currently no way to determine which of the many models produce the most reliable LUC carbon emission values. They suggest that several key elements for an accurate assessment of carbon gain or loss due to LUC must be standardized to ensure more consistency and meaningful comparisons across studies in the future. These include the types and area of land to be converted and their carbon stocks before and after conversion, biomass crop yield, agricultural management practices, and the influences of climate, temperature, soil quality and rainfall. This might then enable policy makers to make better justified judgments on the sustainability of biofuels.
Njakou Djomo S, Ceulemans R (2012) A comparative analysis of the carbon intensity of biofuels caused by land use changes, GCB Bioenergy