Cutting fertiliser emissions in crop production
Cutting fertiliser emissions in crop production
by Arable Farming July 2020
In the second part of Arable Farmings net zero special feature series, Alice Dyer explores what fertiliser decisions might mean for a crops environmental footprint.
Emissions relating to nitrogen are still the major contributor of greenhouse gas (GHG) emissions in the UKs 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 wasnt there a few years ago. And while improving nitrogen efficiency is important, Im not sure the four Rs 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, weve 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 dont have a strong agronomic benefit either so, if were going to use them, there needs to be some incentive to do so from a GHG policy
point of view.
Fall in productivity would be harmful to climate
Despite the key role fertilisers play in the battle against climate change, a drop in farm productivity in the transition to net zero would have further detrimental effects on the environment, says Dr Kindred.
We have done work on optimising nitrogen rates to minimise GHG emissions, but as soon as you start applying less, yields are lower. If you are then going to have to produce grain somewhere else and incur land use change by ploughing up grassland somewhere else in the world, that has a massive effect. Simply applying radically less nitrogen is not the answer.
Net zero and carbon farming will only be possible if we can maintain or increase productivity to be able to spare land for carbon storage and biodiversity.
The perception nitrogen is a dominating factor in productivity is what drove the Yield Enhancement Network (YEN) to explore yield variability further.
Dr Kindred says: From YEN data, weve seen there is a relationship between N fertiliser and yield, but there is a huge amount of variation at any given nitrogen rate. The people achieving high yields are not achieving them by plastering on N fertiliser.
The farms with the lowest GHG intensities are where we would also see the highest nitrogen use efficiencies, and this is where were getting the high yields. So high yields are not a bad thing.
Theres this concept that productivity is something to be scared of but we see that high yields are not necessarily related to high inputs.
ADAS trials at Flawborough Farms, Nottinghamshire, looked at how nitrogen rates can vary across a field by applying different rates across five hectares to assess N response curves.
Dr Kindred says: Large variations of 200kg per hectare in nitrogen requirements were seen from one part of the field to the next.
However, when we looked at what the yields would have been with optimum fertiliser rates, there was still huge variation in that field, so getting the nitrogen fertiliser rate right doesnt solve the problem of variation in yield. There are still variations of three to four tonnes per hectare in a field like that.
Combating ammonia emissions
Ammonia emissions, which can impact human health and contribute to urban smog, are a key focus in the Governments Clean Air Strategy 2019, set
out to deal with air pollution.
Agriculture accounts for 88% of UK ammonia emissions emitted from livestock housing, the storage and spreading of manures and slurries and from the application of some inorganic fertilisers. However, it is required to cut that figure by 16% by 2030, says Dr Kindred.
Government is worried about how we can meet that target, as the savings must come from agriculture.
Although emissions largely come from livestock manures, 23% is related to fertiliser application, mostly dominated by the use of urea-based products, he adds.
Government has said it will regulate to minimise pollution from nitrogen fertiliser use, so urea is obviously a target for that, and from an arable side, thats where we must focus.
Defra-funded trials 15 years ago found ammonia emissions from urea were 20-25% compared to ammonia nitrate, where they were less than 5%. This is because the conversion process for urea into ammonium and then nitrate can lose large amounts of nitrogen to the atmosphere through volatilisation, says Dr Kindred.
When you apply ammonium nitrate, only about 1.5% of the nitrogen is lost by volatilisation. For urea, the average is 11% and for UAN its 5.5%.
We know there is a difference in efficiency as well. The amount of extra N you should apply if youre using urea compared to AN is around 10%.
However, urease inhibitors have a large effect on reducing ammonia loss and restoring agronomic efficiency, he says. The use of these urease inhibitors is something likely to be mandated in the future.