Background: The area used for bioenergy crops (annual row crops (e.g., wheat, maize), herbaceous perennial grasses, and short-rotation woody crops (e.g., poplar)) is increasing because the substitution of fossil fuels by bioenergy is promoted as an option to reduce greenhouse gas (GHG) emissions. However, biomass used for bioenergy production is not per se environmentally benign, since bioenergy crop production is associated with negative side effects such as GHG emissions from soil (dominated by N 2 O). N 2 O emissions vary greatly in space and time; thus, direct comparison of soil N 2 O fluxes from various agro-ecosystems is certainly crucial for the assessment of the GHG reduction potential from energy crops.