Consequences of agro-biofuel production for greenhouse gas emissions

Currently CO2 from fossil fuel combustion accounts for 57% of the global greenhouse gas emissions, whereas the strong greenhouse gases nitrous oxide (N2O) and methane (CH4) contribute with 8% and 14%, respectively (IPCC, 2007). Agricultural activity is the dominant source of N2O, which is mainly associated with the use of nitrogen based fertilizers in agricultural production. Replacing fossil fuel-derived energy by biomass-derived energy is commonly and with increasing emphasis proposed as a mean to mitigate the CO2 emissions. However, a recent analysis of global emission data proposes that accelerated emissions of N2O associated with the production of biomass for bio-fuel purposes will outweigh the avoided emissions of fossil fuel-derived CO2 (Crutzen et al., 2008). In the present study we examined the effects on N2O and CH4 emissions when residues from bio-energy production were recycled as fertilizer for a maize energy crop within an organic cropping system. Furthermore, we assessed sustainability in terms of greenhouse gasses for co-production of bio-ethanol and bio-gas from maize. This was compared to estimated greenhouse gas balances for rye and grass-clover as alternative raw materials