The other approximately 50% of GHG emissions associated with N fertilizers are more directly under farm management control as these emissions are from the use of the fertilizer. See illustration courtesy ofYara:
These N2O emissions are characterised as 'direct' and 'indirect'. The direct emissions are when N2O is formed directly from the fertilizer by denitrification by microbial action and chemical processes of nitrate (NO3) to nitrite (NO2) and then to N2O.
The indirect emissions occur when urea (NH3) is hydrolysed by soil enzymes to ammonium (NH4), which is then susceptible to nitrification by soil bacteria to N2O – it is a less direct pathway and depending on soil temperature, moisture content, pH etc will occur to different extents.
It is estimated that between 10 – 30% of all nitrogen applied as fertilizer is not utilised by crops either through being leached out into water courses as NO3 or volatilised to the atmosphere as NH4. Approximately 3- 5% of this 'lost' N is lost as N2O emissions.
Because the pathways for N2O emissions are both multiple and complex, depending on both biological and chemical processes, changing different variables will result in a range of potentially different emissions. See the graphic below for different potential routes