The Greenhouse Gas Balance of a Dairy Farm as Influenced by the Uptake of Biogas Production

Abstract

The production of energy crops for farm-scale anaerobic digestion (AD) can affect emissions of greenhouse gases (GHG) in a number of ways. Some examples are: fugitive CH4 emissions from the digester and the storage of the digestate, emissions of N2O from soil and emissions of CO2 from farm machinery. Moreover, uptake of AD may be accompanied by changes in the way the farm is operated, which may affect GHG emissions. The scale of these emissions was assessed from published data for the biogas feedstocks cattle slurry and grass silage. Emissions were compared to references representing current farm operation and energy generation by fossil fuels. Feeding the digester with cattle slurry for the entire year did not result in reduced emissions due to relatively high emissions from stored raw slurry in summer. If grass was used for digester feedstock, the level of N2O emissions from the crop was the most important factor for the GHG balance of farm-scale AD. If N2O emissions were low, biogas realised substantial savings of GHG in the order of 1 t CO2 equivalents per hectare per year. At a high level of N2O emissions, energy cropping might even result in increased GHG emissions compared to fossil fuels.