Nitrous oxide’s contribution to climate change is no laughing matter, says Keith Goulding.

Keith Goulding

Carbon dioxide is the most commonly recognised enemy in terms of its contribution to greenhouse-gas (GHG) emissions, and certainly the biggest culprit in terms of volume, but there are other gases, closely tied with food production, that are also major targets for reduction.

Farming is responsible for about 8% of the UK’s GHG emissions (up to about 19% when the road to consumption is included) but about 40% of its methane emissions, which mainly come from livestock, and 76% of its nitrous oxide emissions, which are mainly due to fertiliser use.

There is a lot of pressure on agriculture to reduce emissions of these gases because methane is about 20 times and nitrous oxide around 300 times a more powerful GHG than carbon dioxide.

The devil in the details

To address the nitrous oxide (N2O) problem, a recent event at the Royal Society, ‘Nitrous oxide: the forgotten greenhouse gas’, reviewed our current understanding of the processes by which N2O can be produced or destroyed and discussed approaches for combating N2O release.

I work on GHGs too. At Rothamsted Research my group’s work focuses on N2O, which is mostly produced by two processes in soils carried out by microbes.

First, N2O is a small but important by–product of a process called nitrification, which is the conversion of ammonium to nitrate. It’s part of a natural cycle and an essential soil process as dead plant and animal material decays and is converted back to the building blocks of new organisms. The second process is denitrification, the conversion of nitrate to nitrite, N2O and N2 — the nitrogen gas that forms 78% of the air we breathe. It is also a natural process and happens when oxygen is in short supply, especially when soils are very wet, and produces large but short–lived peaks of N2O.

The ‘Need for nitrogen’ to make crops grow was also detailed on this blog by Ian Crute. It can come from fertiliser, legumes (biological fixation) or recycled manures, but fossil fuel-based fertiliser nitrogen dominates and is used to produce about half the world’s food. We will need more food and more nitrogen as the population increases toward nine billion, or more.

Unfortunately, some of this nitrogen will end up as N2O whether it comes from synthetic fertilisers or natural manure. The question is can we help our farmers to be more efficient and get more nitrogen into their cops and less into N2O?

Working the problem

Obviously we should not (and could not) try and stop natural processes such as nitrification or get rid of the microbes. But we wonder if we could find ways, when the soil is wet and denitrification happens, to encourage the microbes to convert nitrate to dinitrogen (N2) instead of N2O, all the time?

We have used our 168–year-old Broadbalk experiment on wheat production at Rothamsted, and a very special laboratory system that enables us to collect and analyse all the gases that come from the soil, to find out what controls the microbes and their production of N2O.

Our research shows that, perhaps not surprisingly, the amounts of nitrate and carbon (the microbes’ energy source) are very important: they must be in balance. So farmers need to get the right amount of nitrogen to their crops.

The structure of the soil is also important – a soil that holds enough water to supply crops but does not easily become saturated (waterlogged) and so deprive the plant roots of oxygen is important. And we were surprised to see that a soil made wet before it becomes really saturated with water produces less N2O than one dried before suddenly becoming saturated. It seems that the microbes behave better when accustomed to being wet!

However, if climate change happens as predicted then in the UK we will get more very dry weather followed by sudden wetter periods which our research suggests will increase N2O production and exacerbate climate change even further.

No laughing matter

Elsewhere at the meeting, Robert Portman brought us the good news that the GHGs methane and carbon dioxide reduce the depletion of ozone. Unfortunately N2O increases ozone depletion as well as being a very potent GHG, so it’s bad news all round.

Paul Crutzen, the Nobel Prize winner in 1995, and Keith Smith have made some detailed life cycle calculations that reinforce the view that most first generation bioenergy crops, such as wheat and oilseed rape, don’t deliver any fossil fuel saving. But sugar cane does because of the biological nitrogen–fixing microbes associated with sugar cane roots. (Not everyone agrees with this story so there is some good research to be done understanding what is happening.) Second generation bioenergy crops such as willow and Miscanthus) have a good carbon balance, but only if no or very little nitrogen fertiliser is used.

Some long–term opportunities for reducing nitrous oxide emissions were suggested. For example, Liz Baggs and colleagues at the new James Hutton Institute (also a subject of this blog post) have a range of barley varieties that seem to emit different amounts of N2O.

But, at the moment, as I and Lars Bakken said at the meeting, the best practical options to mitigate N2O release are managing soil pH and excess nitrogen inputs, and maintaining good soil structure to avoid soils becoming saturated with water and depleted of oxygen.

About Professor Keith Goulding

Professor Keith Goulding joined Rothamsted Research in 1974 after completing a Master’s degree in Soil Chemistry at Reading University and then a PhD in soil chemistry at Imperial College in 1980. He studies how plant foods (nutrients) in soils become available to growing plants and the best ways of augmenting these with fertilisers and manures without polluting air and water. He is a visiting Professor at the University of Nottingham, a Fellow of the Institute of Professional Soil Scientists and a Chartered Scientist. He was awarded the Royal Agricultural Society of England’s (RASE) Research Medal in 2003 for his research into diffuse pollution from agriculture and elected an Honorary Fellow of the RASE in 2010. He received a Nobel Peace Prize certificate for his contribution to the work of the Intergovernmental Panel on Climate Change, for which the Panel and Al Gore were jointly awarded the Prize in 2007. He is currently Vice–President of the British Society of Soil Science.

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