Agricultural nitrous oxide emissions: How animal excreta and fertiliser management fit together

Agriculture produces 90% of N2O in Ireland, mainly through synthetic fertilisers, animal excreta and slurry. All these materials contain nitrogen (N), and part of this N is lost to the atmosphere as a result of agricultural activities. Author: Dominika Krol, Teagasc Johnstown Castle

Sources of N₂O in agriculture: Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) not only responsible for climate change but also contributing to the destruction of the ozone layer. Agriculture produces 90% of N₂O in Ireland, mainly through application of synthetic fertilisers to land, animal excreta deposited on pasture during grazing and manure management (slurry storage and land spreading). All these materials contain nitrogen (N), and part of this N is lost to the atmosphere as a result of agricultural activities. These losses are driven by the microbial activity in soils and manures that converts various forms of N into N₂O, environmental conditions (such as temperature and rainfall) and of course, the type and intensity of the agricultural activity (for example N application rate of fertilisation).

Importance of excreta and fertiliser N₂O in agriculture:  The Intergovernmental Panel on Climate Change (IPCC) provides default values for emissions that can be used in the absence of local data, but also encourages countries to derive their own country-specific data, especially for important emission categories. Based on this, we carried out our research which has shown that N₂O emissions from animal excreta are lower than the default emission factors provided by (IPCC) - View it here.

At the same time we have also been researching N₂O emissions from synthetic fertilisers, thus covering the two most important sources of N₂O in Irish agriculture. Nitrous oxide from synthetic fertiliser sources also turned out to vary from the IPCC default emission factor, however here emissions depended hugely on the type of fertiliser used (emissions from Calcium Ammonium Nitrate (CAN) were higher than the default, while emissions from protected urea were lower). On average, fertiliser emissions were higher than originally calculated using the IPCC default. As a result of this research, Irish N₂O emissions profile evolved from the one in Figure 1a to the one in Figure 1b.

Figure 1. Irish agricultural N₂O emission profile calculated using a) IPCC default emission factors and b) country-specific approach.

Mitigation of N₂O in agriculture: Applying results of our recent studies to improve calculations of Irish N₂O emissions from agricultural activities revealed changes in the profile and magnitude of emissions. Regarding magnitude of emissions, N₂O reduced by 10.8%, which in turn meant approximately 3.6% reduction in overall agricultural GHGs. Regarding profile of emissions, the share of N fertilisation rose to 38% making it the single most important source of agricultural N₂O emissions, while N₂O emitted from animal excreta deposited on pasture fell to 23%. This should be considered good news because while it would be difficult to mitigate N₂O from excreta of grazing animals, there is a great potential to mitigate fertiliser-driven N₂O emissions through the use of novel fertiliser formulations such as protected urea. Therefore, these two studies form the basis of the Teagasc advice in recent years to switch fertiliser source from CAN to protected urea and were followed by a number of further work on protected urea products. Other Teagasc experiments also provided evidence that switching straight urea to protected urea reduces another gaseous N loss-ammonia (NH₃) under Irish conditions. Ireland has committed to reducing national GHG emissions under the EU Energy and Climate Package and the Climate Action Plan to Tackle Climate Breakdown (2019) and there is a clear and easy solution as to where these savings can come from in terms of N₂O emissions.