Reducing emissions in agriculture: nitrous oxide

CLA's Land Use Policy Adviser for Climate and Water, Alice Green blogs on emissions in agriculture, taking a focus on nitrous oxide.
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There is growing pressure on all industries to reduce their carbon emissions. The UK has a net zero deadline of 2050, with an interim carbon reduction target of 78 per cent by 2035. As such, a core part of any businesses sustainability plan will be to understand and reduce their carbon emissions.

But for agriculture, carbon dioxide is only a tiny part of the picture. A much larger proportion of agricultural emissions take the form of methane or nitrous oxide. This blog is the first in a two-part series looking at agricultural greenhouse gas emissions and how to reduce them. In this edition we are looking at nitrous oxide.

What is nitrous oxide?

Nitrogen is an essential component for life – found widely in the environment driving important cellular functions. Unfortunately, in the right – or perhaps it would be more apt to say wrong – conditions, processes occur which convert nitrogen to nitrous oxide (N2O).

Nitrous oxide is a potent greenhouse gas. Carbon dioxide is often used a benchmark for understanding the relative impact of other greenhouse gases. So, how does nitrous oxide compare?

Nitrous oxide has a warming potential around 300 times greater than carbon dioxide, remaining in the atmosphere for over 100 years. Just for good measure, it also depletes the ozone layer.

Where does nitrous oxide come from?

Taking the latest available agri-climate data, agriculture was responsible for 68 per cent of annual nitrous oxide emissions in the UK. Breaking this down further, 55 per cent of total UK emissions come from agricultural soils, and 12 per cent from other agricultural sources, such as manure management[i].

Other non-agricultural sources of nitrous oxide emissions in the UK include road transport, other fuel combustion sources and waste processes.


Why are agricultural soils such a big source of emissions?

The production of nitrous oxide from soil is a natural, biological process. In healthy soils with a high oxygen content, bacteria produce nitrate from ammonium in a process called nitrification, and in doing so they also create nitrous oxide. In the absence of oxygen, a different process called denitrification occurs. This time bacteria in the soil reduce nitrates to gaseous nitrogen. Denitrification is more likely in wet or compacted soils. Both processes have the same result: the production of nitrous oxide, although larger amounts result from denitrification.

When nitrogen is added to the soil, these bacteria have everything they need to pump out nitrous oxide. And of course, that’s exactly what has to happen for a productive agricultural system: the application of nitrogen fertilisers to land. This is widely accepted to be the key driver for agricultural nitrous oxide emissions.

How can agriculture reduce nitrous oxide emissions?

Defra estimate that nutrient management techniques have the potential to reduce greenhouse gas emissions by 957 Kt of carbon dioxide equivalent (CO2e). By February 2021, they calculate that 353 Kt CO2e of this 957 Kt potential has been realised. Mechanisms to achieve this include careful nutrient planning and fertiliser application methods. For example, using a location-based fertiliser recommendation system, and integrating fertiliser and manure nutrient supply.

Another 702 Kt CO2e reduction could be achieved from increasing plants with improved nitrogen use efficiency. This includes using clover in place of nitrogen fertilisers. Such plants are able to remove more nitrogen from the soil, preventing bacteria using it to create nitrous oxide. Of this 702 Kt potential reduction, a 91 Kt CO2e reduction has been achieved by February 2021.

Looking ahead

New technologies and better data are enabling the industry to cut nitrogen use, and with it, nitrous oxide emissions.

Nitrous oxide emissions from agriculture have been on a downward trend over the last 20 years. This is largely attributed to reductions in the overall application of nitrogen fertilisers. It shows that with careful management, and new application technologies, farmers are making progress to reduce fertiliser use and with it, emissions.

Nitrous oxide is often forgotten in the zero carbon debate, but reducing it will be a key step for the agricultural sector to decarbonise.

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