In the world's effort to cut greenhouse gas emissions, the source of our food is coming into the spotlight. There's good reason for that: Agriculture accounts for 16 to 27% of human-caused climate-warming emissions. But much of these emissions are not from carbon dioxide, that familiar climate change villain. They're from another gas altogether: nitrous oxide (N2O).
Also known as laughing gas, N2O does not get nearly the attention it deserves, says David Kanter, a nutrient pollution researcher at New York University and vice-chair of the International Nitrogen Initiative, an organisation focused on nitrogen pollution research and policy making.
Yet molecule for molecule, N2O is about 300 times as potent as carbon dioxide at heating the atmosphere. And like CO2, it is long-lived, spending an average of 114 years in the sky before disintegrating. It also depletes the ozone layer. In all, the climate impact of laughing gas is no joke. Scientists at the Intergovernmental Panel on Climate Change (IPCC) have estimated that nitrous oxide comprises roughly 6% of greenhouse gas emissions, and about three-quarters of those N2O emissions come from agriculture.
But despite its important contribution to climate change, N2O emissions have largely been ignored in climate policies. And the gas continues to accumulate. A 2020 review of nitrous oxide sources and sinks found that emissions rose 30% in the last four decades and are exceeding all but the highest potential emissions scenarios described by the IPCC. Agricultural soil – especially because of the globe's heavy use of synthetic nitrogen fertiliser – is the principal culprit.
Today, scientists are looking at several ways to treat the soil or adjust farming practices to cut back on N2O production.
"Anything that can be done to improve fertiliser use efficiency would be big," says Michael Castellano, an agroecologist and soil scientist at Iowa State University.
Humanity has tipped the Earth's nitrogen cycle out of balance. Before the rise of modern agriculture, most plant-available nitrogen on farms came from compost, manure and nitrogen-fixing microbes which take nitrogen gas (N2) and convert it to ammonium, a soluble nutrient that plants can take up through their roots. That all changed in the early 1900s with the debut of the Haber-Bosch process that provided an industrial method to produce massive amounts of ammonia fertiliser.
This abundance of synthetic fertiliser has boosted crop yields and helped to feed people around the globe, but this surplus nitrate and ammonium comes with environmental costs. Producing ammonia fertiliser accounts for about 1% of all global energy use and 1.4% of CO2 emissions (the process requires heating nitrogen gas and subjecting it to pressures of up to 400 atmospheres, so it's very energy-intensive). More importantly, the fertiliser drives increased emissions of nitrous oxide because farmers tend to apply the nitrogen to their fields in a few large batches during the year, and crops can't use it all.
A Guest Editorial