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.

In achieving global food security, agriculture is part of the problem and part of the solution to climate change.

While we need to better understand greenhouse-gas emissions from agriculture we do know they are significant. The Intergovernmental Panel on Climate Change (IPCC) estimates that direct emissions are about 14% of global emissions (similar to those from transport) and emissions from deforestation are 17% of global emissions – but because farming is a major driver of deforestation the majority of these are due to agriculture.

The total of emissions from agriculture is even higher when the indirect emissions from energy for fertilisers and other agrochemicals, irrigation, agro-processing and packaging, and transportation are taken into account.

While we don’t have accurate or definitive figures we can realistically say that over a third of global emissions are due to agriculture.

However, using our existing knowledge on better land practices and husbandry we sequester carbon into soils and plant biomass. Again, it is estimated that potentially agriculture could sequester up to 6000Mt CO2 per year, or 88%, of its total annual CO2 emissions.

We know that agriculture, especially in developing countries, will be seriously affected by climate change but on the degree and locations of these impacts we are less certain. But unless we put in place adaptation to climate change many millions of the poorest in the world will suffer the most.

Game plan

We therefore need to look for triple wins –mitigation, adaptation and food security benefits.

To achieve this we are going to need radical change. As Richard Jacobs mentioned in his post ‘Don’t write off organics’ on this blog, organic and ecological approaches can improve yields especially in places where yields are low.

However, while these approaches are essential we will need to marry these techniques with the best of the most modern scientific approaches to produce the food we need. We need to become more efficient in how we use energy to produce our food and consider emissions from the whole food chain, which is much more than simple food or air miles.

This is a common agenda for developed and developing countries even though smallholder farmers and their communities in the developing world have specific challenges and lower carbon footprints than their developing world counterparts.

We need to put aside polarised arguments on whether GM crops and ‘industrial agriculture’ or ecological and organic approaches are the solution. It’s time for a radical rethink on how we can feed the world and to do it sustainably this has to combine the best of all approaches.

Out of Africa

We are looking to help do this at the Africa College – a research partnership between the University of Leeds and research institutions in Africa that are working on food security and human health.

Africa College is tackling these problems in a number of ways. For example, at a ‘landscape scale’ what matters is production of both food and the level of biodiversity (especially for the services that help produce food such as pollination).

The optimal way to design a landscape that produces food and biodiversity depends on a number of factors. Sometimes, using the whole land farmed extensively does best (e.g. organic agriculture). Sometimes, you get both more food and more wildlife if you separate areas out for specialised conservation areas and conventionally farm the rest; this allows some areas to be farmed for high productivity.

This potentially gives a route forwards: we need both high productivity and sustainability. Moving to a greener agriculture is necessary for sustainability (e.g. precision farming, low-input, no-till) but this need not entail a wholesale conversion to organic methodologies.

At the scale of the plot, some of our research indicates that GM crops and those developing non-GM biotechnologies can help make farming more sustainable. Crops can be modified to enable them to grow in conditions they otherwise wouldn’t, places prone to drought for example, and be resistant to pests that would otherwise require spraying with insecticides. Such crops may provide high yields and also require less chemical and energy input.

GM crops will carry a risk that needs proper evaluation; however, not using them also carries a cost: more land will be required to grow the food needed, and conversion of the extra land to farming may impact heavily on the local environment.

We are also working with our African partners on the use of biotechnology, including GM, to benefit smallholder farmers. For example bananas are a staple food for over 60 million Africans whose food security is at threat when yields are reduced by plant diseases and pests. This includes nematodes, and the growth of bananas at high density for several years over large areas increases nematode populations and consequently the severity of the crop damage; it is estimated that losses exceed 35% in sub-Saharan Africa.

Most edible bananas are sterile and produce no seeds slowing their natural evolution and improvement by conventional plant breeding. Africa College partners are working together in public research using plant biotechnology to provide nematode resistant cooking bananas and plantains to benefit smallholder farmers in Africa.

About Mr David Howlett

David Howlett is Executive Director of Africa College and a visiting senior research fellow in climate change and agriculture at the University of Leeds. He is currently working with research scientists across different faculties at Leeds and with African research partners to increase the impact of their research. He is working to turn research results into evidence to inform agriculture and climate change policies.

Before joining Leeds, David worked for the UK Government’s Department for International Development (DFID) where he worked on food and climate change policies. Prior to this he led DFID’s agriculture research team. David has lived and worked in Asia and Africa and most recently was a United Nations Development Programme adviser based in the Vice President’s Office in Tanzania. He has undertaken research on sustainable land management while working for international and national research organisations in Africa, Asia as well as the Pacific.

Livestock enterprises contribute substantially to the world’s greenhouse gases, largely through deforestation to make room for livestock grazing and feed crops, the methane ruminant animals give off, and the nitrous oxide emitted by manure.  Estimates of this contribution vary widely (10-18% (PDF), or more, of global greenhouse-gas emissions) and are still being researched – it’s a complex question and hotly debated.  

Whatever the exact figure, many worry these greenhouse-gas emissions will only grow due to increasing livestock production to meet the surging demand for meat and milk in developing countries.

But significant livestock-related greenhouse gas reductions could be quickly achieved in tropical countries by modifying production practices, which were recently detailed in a paper by myself and a colleague published in the Proceedings of the National Academy of Sciences. For example, switching to more nutritious pasture grasses, supplementing diets with even small amounts of crop residues or grains, restoring degraded grazing lands, planting trees that both trap carbon and produce leaves that cows can eat, and adopting more productive breeds can all be employed relatively quickly to reduce emissions.

Such changes could increase the amount of milk and meat produced by individual animals, thus reducing emissions because farmers would require fewer animals.

For example, in Latin America switching cows from natural grasslands to more nutritious sown pastures can increase daily milk production and weight gain by a factor of three. Fewer animals would then be needed to satisfy demand, while farmers’ incomes could be raised substantially.

There are several other well-documented options that could increase incomes for smallholders while at the same time reducing overall emissions.

Supplementing grazing with feed consisting of crop residues, such as the leaves and stalks of sorghum or maize plants, is one example. There is also potential to boost production per animal by crossbreeding local with genetically improved breeds, so providing more milk and meat than traditional breeds while emitting less methane per kilo of meat or milk produced. Supplementing cattle diets with the leaves of certain trees, such as Leucaena leucocephala, has similar effects on meat and milk production and incomes.

These options could not only reduce methane emissions, but some of them, such as planting improved pastures and agroforestry tree species, can also sequester carbon directly.  For example, if about 30 percent of livestock owners in the tropical regions of Latin America switch from natural grass to improved grasses such as some of the Brachiaria species, this alone could reduce carbon dioxide emissions by about 30 million tons per year.

Payback time

It would be a useful incentive if these farmers were allowed to sell the reductions they achieve as credits on global carbon markets. Overall, at US$20 per ton, which is roughly what carbon is currently trading for on the European Climate Exchange, we calculate that poor livestock keepers in tropical countries could generate about US$1.3 billion each year in carbon revenues.  Such carbon payments could make a meaningful contribution to many livestock-keeping households.

My colleague and I have calculated that, for a range of readily-available options for poor livestock keepers to increase production in the tropics, these could save about 7 percent of all livestock-related global greenhouse-gas emissions (a conservative estimate, as we did not consider the full range of options available to livestock keepers, nor did we consider nitrous oxide emissions). Consumption of milk and meat may double in the developing world by 2050, so it’s critical to adopt sustainable approaches now that reduce the negative effects of increasing livestock production while allowing countries to realize the benefits, such as better nutrition and higher incomes for livestock-producing households.

At the same time, reductions in the consumption of livestock products in developed countries could result in additional and substantial reductions in emissions. 

But back in the tropics, many livestock keepers are highly dependent on their animals for food, for income, as ‘engines’ to prepare their land, and as tradable assets. They need technological options and economic incentives that help them intensify their production in sustainable ways. Hence, carbon payments would be a welcome additional incentive that could help to bring about useful and much-needed changes in smallholder livestock production as well as bringing about a more conducive enabling environment.

About Philip Thornton

Philip Thornton is a Theme Leader and Senior Scientist with the Challenge Programme on Climate Change, Agriculture and Food Security (CCAFS) at the International Livestock Research Institute (ILRI) in Nairobi, Kenya, and an Honorary Research Fellow in the Institute of Atmospheric and Environmental Sciences at the University of Edinburgh.  He has worked mostly in Latin America, Europe, North America and Africa, on systems modelling and impact assessment. His current research interests revolve around integrated assessment at different scales and evaluating the possible impacts of global change on agricultural systems in developing countries.

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