Producing fuels from crops such as rapeseed is one strategy being explored to replace fossil fuels, with a view to reducing greenhouse gas (GHG) emissions from the transport sector. However, annual crops such as rapeseed and maize require significant inputs of nitrogen-based fertilisers and this in turn will lead to higher N2O emissions. New research suggests that these N2O emissions are 3-5 per cent of the nitrogen fertiliser input, which is substantially higher than generally considered.
N2O is a major greenhouse gas. Although it has a lower warming effect than CO2, it persists in the atmosphere for longer. Over a 100 year time frame, each molecule of N2O has 296 times more impact on global warming than a molecule of CO2. Furthermore, N2O reacts in the atmosphere to create nitrogen oxides (NOx), which can damage the ozone layer.
Not all biofuels are the same in terms of N2O emissions. The research indicates that for bioethanol produced from maize, for example, the warming effect associated with N2O emissions would balance or slightly exceed the benefits achieved through CO2 reductions, from the replacement of fossil fuels. Rapeseed, currently the source of over 80 per cent of the world's transportation biofuels and widely used in Europe, has a value of 1.0-1.7 times, which means it is has an overall warming effect, compared with fossil fuels.
Sugar cane, used to make bioethanol, particularly in Brazil, has a value of 0.5-0.9, which means that it performs better than fossil fuels when both CO2 reductions and N2O emissions are considered. However, this does not take into account any fossil fuels used in the cultivation of the biofuel crop, for example in fertiliser manufacture.
Biofuels could be produced with a smaller impact on the climate by increasing the efficiency of nitrogen fertiliser use by plants and employing agricultural practices that minimise the amount of excess nitrogen that escapes into the environment. Other plants such as switchgrass, elephant grass, and lignocellulosic plants such as eucalyptus, poplar and willow would result in much smaller N2O emissions, which make them attractive alternatives for biofuel production leading to genuine GHG reductions.