How does nitrogen fertiliser contribute to emissions?
When un-protected urea fertiliser is applied to the soil surface, the naturally occurring soil enzyme – urease – converts the urea into ammonium in a process called hydrolysis. During this process, carbon dioxide gas is released and the pH around the urea granule starts to increase.
Nitrogen in the ammonium form the nitrogen is available for plant uptake, but it is also at risk of being lost as ammonia gas in a process called volatilisation. The risk of volatilisation is higher in warm, dry, windy conditions; the higher the soil pH and the longer the urea is on, or near, the soil surface.
Ammonium is also converted into nitrate by the soil’s nitrifying bacteria in the nitrification process. Nitrate nitrogen is available to be taken up by the plant, but in anaerobic soils – for example when soils are compacted or water-logged – it can be de-nitrified into nitrous oxide gas which is lost to the atmosphere.
Ammonia NH3 – the problem
UK government NT26 research in the 2000s showed that different types of nitrogen fertilisers had a range of ammonia emission factors with urea having the largest average at 24%.
DEFRA’s Clean Air strategy was published in January 2019 and identifies Agriculture as one of the main sources of ammonia pollution, accounting for 88% of UK emissions. Ammonia combines in air with secondary particulate matter to form a potent pollutant linked to respiratory health issues.
Of the 88% of ammonia emissions attributed to agriculture, 23% are thought to be associated with inorganic fertiliser application.
The Clean Air Strategy identifies a range of measures that will help farmers control ammonia emissions. A key one for fertiliser is:
“Switching from urea based fertilisers to ammonium nitrate, which has lower emissions, injecting or incorporating urea into soil or applying it alongside a urease inhibitor”
Nitrous Oxide N2O – the problem
Ireland – with a similar climate to much of the UK – have identified Nitrous Oxide, a potent greenhouse gas with a 100-year global warming potential 298 times greater than carbon dioxide, as being a big issue.
Ireland’s agriculture contributes almost 90% of the total nitrous oxide emissions, mainly due to nitrogen fertiliser use and emissions from animal waste.
Tegasc and Agrifood and Biosciences Institute conducted a nitrogen fertiliser comparison study that measured efficiencies and emissions. The field trials were conducted over a three-year period in the Republic of Ireland and in Northern Ireland.
The research showed that Nitrates had an N2O emission factor of 1.5% compared to urea at less than 0.5%. It concluded that a simple switch by Ireland’s farmers from calcium ammonium nitrate to a urease inhibitor could reduce nitrous oxide emissions associated with fertiliser application by an average of 73 percent across grassland sites.