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.

Plants must secure high levels of nitrogen, and in conventional agriculture nitrogen is added at high concentrations in the form of inorganic fertilisers. Artificial nitrogenous fertilisers can increase yield by as much as 50% and the global farming system, and hence our own food supply, is now dependent on them. We would face very severe food shortages if nitrogen fertilisers were to become unavailable.

However, their use comes with high economic and environmental costs.  Farmers, especially in developing countries, spend a high proportion of their income on fertilisers that account for a significant proportion, sometimes the majority, of the costs of crop production.  Fertiliser synthesis and application leads to high amounts of nitrous pollution in aquatic systems causing algal blooms and dead zones in shallow seas as well as nitrous pollution of the atmosphere causing poor air quality and significant greenhouse gas emissions.

But we cannot stop using fertilisers and meet a food security agenda; nor can we afford to keep using them and meet an environmental sustainability agenda.

Producing nitrogenous fertilisers requires lots of energy that currently comes from the burning of fossil fuels. It is anticipated that by 2050 2% of global energy will be used in fertiliser production [ref 1]; this represents the single largest energy input into intensive agriculture. This is unsustainable, and if the price of oil increases, so does the price of fertilisers, and so our food. Add to this the environmental costs of these fertilisers and it is clear that we need to find another way. I believe the answer lies in plants themselves – finding a biological and sustainable means of fertilising plants.

My research looks at leguminous plants, such as peas and beans. On the roots of these plants are small growths called nodules which are factories that supply all of the nitrogen the plant needs. Within the nodules are specialised bacteria that form a mutually beneficial relationship with the plant. The bacteria take nitrogen from the air and covert it into a form that the plant can use. In exchange the bacteria are supplied with sugars produced by the plant. It’s a beautiful and elegant system, and I’m interested in understanding the fundamental science behind this association. 

This interaction involves signals between the bacteria and the plant. The signals trigger the plant to produce nodules to house the bacteria and also control the exchange of nutrients. Getting a complete understanding of the process will take a long time, but the driving force behind it is that if we can get a better understanding of the process we can look to transfer it into non-leguminous crops like wheat, rice or maize, the world’s three most cultivated crops. This would slash the amount of oil needed to grow them, and the amount of pollution caused by the fertilisers they currently need. However, transferring this process can only occur with the use of genetic modification (GM).

I see GM as a natural and biological solution to this huge problem. However, I know many people have a negative perception of GM. In this case I think the benefits are clear.

We are working very carefully and thoroughly to understand the process [ref 2,3], and then to predictably and safely transfer nitrogen fixation to crops. We know the effects of nitrogen fertiliser pollution on the environment, and we know the effect that burning huge amounts of fossil fuels has on our climate. But we do this anyway out of necessity to support current food supplies.

Balancing these very detrimental impacts against the perceived dangers of GM will, in my opinion, be the key to delivering the second, greener revolution in farming that we need to secure our food supply now and into the future.

References

1. Is it possible to increase the sustainability of arable and ruminant agriculture by reducing inputs?
2. Nodulation Signaling in Legumes Requires NSP2, a Member of the GRAS Family of Transcriptional Regulators
3. Nodulation independent of rhizobia induced by a calcium-activated kinase lacking autoinhibition

About Dr Giles Oldroyd

Dr Giles Oldroyd leads the Plant Perception and Response to the Environment Programme at the John Innes Centre. He received a David Phillips Fellowship from the BBSRC and has received a number of awards for his research, including European Molecular Biology Organisation young investigator, European Research Council young investigator, Society of Experimental Biology President’s medal and a Royal Society Wolfson Research Merit award.

Contact details

Dr Giles Oldroyd
John Innes Centre
Norwich Research Park
Colney
Norwich
NR4 7UH

Tel: 01603 450000
Email: giles.oldroyd@bbsrc.ac.uk

Crookes was aware of the pioneering work of Sir John Lawes and Sir Henry Gilbert who showed that wheat yields of up to four tonnes per hectare could be produced year after year by application of nitrogen fertilisers. Crookes proposed that the power of Niagara Falls should be harnessed for “oxidating free nitrogen of the air” and thereby enabling “twelve million tons of nitrate of soda to be applied to the global wheat crop”. 

Crooke’s idea was fanciful but just 10 years later Fritz Haber in Germany mastered the chemical synthesis of ammonia from gaseous nitrogen and hydrogen which led to the industrial-scale Haber-Bosch production of nitrogen fertilisers. 

Today, 55% of reactive nitrogen in the global nitrogen cycle has been fixed chemically and, had it not been for Haber’s discovery, it is estimated that global population would be at least 25% fewer than now (it was 1.7 billion in 1908 and is now 6.5 billion). A world without the Haber-Bosch process would have been one ravaged by even more deprivation, human conflict and misery than has been suffered by so many over the last century and indeed is still experienced by many millions. 

Nitrogen availability remains one of the primary drivers of secure global food production, along with sufficient water, essential mineral nutrients, and an ability to restrict losses due to weed competition and attacks by pests and disease. The so-called ‘green revolution’, which resulted in a further doubling of crop yields in many parts of the world, was also founded on the ability of new disease-resistant plant varieties to exploit higher levels of nitrogen fertiliser use. 

However, this short history poses some extremely testing questions. 

Sufficient reactive nitrogen, in the right place at the right time, is an essential component of being able to produce enough food for a future global population of more than 9 billion. But the chemical and biological fixation of nitrogen is energetically demanding. Furthermore, the natural process of microbe-mediated denitrification yields a greenhouse gas, nitrous oxide, which is 300 times more potent in this context than carbon dioxide. It seems probable that the quantity of nitrous oxide released to the atmosphere will be proportional to the amount of reactive nitrogen in the system – regardless of its source. 

Nevertheless, we need to generate more plant biomass for our food and animal feed as well as to increase the size of our terrestrial carbon sinks in soil and vegetation. Increased atmospheric concentrations of carbon dioxide assist this process but only if nitrogen available for plants is not limiting.

The more food we need and the more carbon dioxide we want to fix, the more reactive nitrogen is required and the more nitrous oxide will be released. This strikes at the heart of sustainability and requires resolution. 

In due course, the production of ammonia using renewable sources of electricity and hydrogen will undoubtedly become economic and feasible; biological nitrogen fixation in association with non-legume crops may also become a reality. Therefore, it is fair to assume that as we look to the future the availability of appropriate forms of reactive nitrogen should not be a primary constraint on our ability to produce enough food. However, the management of the nitrogen cycle and the ability to restrict emissions of nitrous oxide is a challenge that must be confronted.

Alternatively, can we constrain carbon dioxide and methane emissions from agriculture sufficiently to compensate for our continued need for nitrogen inputs? 

This will surely require the establishment of strong synergistic interactions between scientists who understand the complexity of biogeochemical cycles and scientists who are more at home studying the fundamental biology of denitrification with a view to seeking plant-based or chemical inhibition of the process. 

Without a major scientific breakthrough somewhere in this complex landscape I am left, like Crookes, to ponder if our ability to produce sufficient food in a truly sustainable way for the global population of 30 years hence will be possible.

About Professor Ian Crute, Chief Scientist, Agriculture and Horticulture Development Board

In 2009, Professor Ian Crute became the Agriculture and Horticulture Development Board’s first Chief Scientist. Prior to this appointment, he was Director of Rothamsted Research and took responsibility for all scientific, operational, commercial and external liaison activities of the institute, a post he held since 1999. He has a First Class Honours degree in botany and a PhD in plant pathology from the University of Newcastle and his committee and board memberships include Chairman of the Sainsbury Laboratory Council and membership of the ‘Future of Food and Farming’ Foresight project.

Contact details

Professor Ian Crute, Chief Scientist
Agriculture and Horticulture Development Board
Stoneleigh Park
Kenilworth
Warwickshire
CV8 2TL

Tel: 0247 669 2051
Email: Ian.Crute@ahdb.org.uk