Consequences of field N2O emissions for the environmental sustainability of plant-based biofuels produced within an organic farming system

Summary

One way of reducing the emissions of fossil fuel-derived carbon dioxide (CO2) is to replace fossil fuels with biofuels produced from agricultural biomasses or residuals. However, cultivation of soils results in emission of other greenhouse gasses (GHGs), especially nitrous oxide (N2O). Previous studies showed that field emissions of N2O may contribute significantly to total GHG emissions during biofuel production. Furthermore, N2O may counterbalance a considerable part of the global warming reduction, which is achieved by fossil fuel displacement. In the present study, we related measured field emissions of N2O to the reduction in fossil fuel-derived CO2, which was obtained when agricultural biomasses were used for biofuel production. The analysis included five organically managed feedstocks (viz. dried straw of sole cropped rye, sole cropped vetch and intercropped rye-vetch, as well as fresh grass-clover and whole crop maize) and three scenarios for conversion of biomass into biofuel. The scenarios were i) bioethanol, ii) biogas and iii) co-production of bioethanol and biogas. In the last scenario, the biomass was first used for bioethanol fermentation and subsequently the effluent from this process was utilized for biogas production. The net GHG reduction was calculated as the avoided fossil fuel-derived CO2, where the N2O emission was subtracted. This value did not account for fossil fuel-derived CO2 emissions from farm machinery and during conversion processes that turn biomass into biofuel. The greatest net GHG reduction corresponding to 700-800 g CO2 m-2 yr-1 was obtained by biogas production or co-production of bioethanol and biogas on either fresh grass-clover or whole crop maize. Maize has become increasingly popular as a feedstock for biofuel production in several regions of Europe. However, we suggest that more emphasis should be placed on perennial N2-fixing crops, like grass-clover swards, as suitable alternatives to maize when producing feedstock for biofuels within organic farming.