by Arable Farming July 2020
In the second part of Arable Farming’s net zero special feature series, Alice Dyer explores what fertiliser decisions might mean for a crop’s environmental footprint.
Emissions relating to nitrogen are still the major contributor of greenhouse gas (GHG) emissions in the UK’s crop production systems, prompting the conversation there will need to be major changes to meet net zero targets by 2040.
In recent years, public perception has changed, and the role of food and agriculture in climate change is now being recognised.
This is according to Dr Daniel Kindred, head of agronomics at ADAS, who says: “That consciousness wasn’t there a few years ago. And while improving nitrogen efficiency is important, I’m not sure the four Rs [see box] are enough to get us to where we need to be.”
There are two main sources of GHG emissions associated with fertiliser: the manufacturing process and nitrous oxide (N2O) emissions from soil.
Dr Kindred says: “The task of taking nitrogen from the air and turning it into ammonia is an energy-intensive process. In the past it was responsible for a
lot of N2O emissions in that process itself. However, we’ve seen the industry invest heavily in the last 10 years.
“In my first calculations 12 years ago, there were emissions of 7.1kg CO2 equivalent per kilo N. This is now down to 3.5kg CO2e/kg N.”
But for soil emissions, the situation is more challenging.
Dr Kindred says: “The reality is the more N we apply, the more N2O we lose from soil. Though the loss of N2O is so small to be agronomically unimportant, the impact from a climate perspective is great, because every 1kg of N lost as N2O has a global warming potential of 170kg CO2.
“We led a project involving the industry to get a handle of what the N2O emissions are in UK agriculture.
“Our measurements showed that generally they were about half the IPCC default of 1% of the nitrogen applied lost as N2O. Emissions are greater where soils are wetter and less aerobic.
“The data from UK studies has now been combined to give country-specific emission factors which are lower than the IPCC defaults, and are related to rainfall.
A decade ago, N related emissions were calculated to be responsible for about 70% of GHG from a crop of wheat, says Dr Kindred. Now it is around 50%.
“However, N fertiliser is still responsible for most emissions in our crop production system so we must still think about what we can do about that,” he adds.
“Potentially, nitrification inhibitors have a role to slow the conversion of nitrate back into N2O and ammonium into nitrate. They can reduce N2O emissions very substantially, but their effectiveness is variable and depends on soil type and weather conditions after application.
“They don’t have a strong agronomic benefit either so, if we’re going to use them, there needs to be some incentive to do so from a GHG policy
point of view.”