A Forgotten Greenhouse Gas
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. “It’s a forgotten greenhouse gas,” he says.
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.
When plant roots don’t mop up that fertiliser, some of it runs off the field and pollutes waterways. What remains is consumed by a succession of soil microbes that convert the ammonia to nitrite, then nitrate and, finally, back to N2 gas. N2O is made as a by-product at a couple of points during this process.
Carefully dispensing fertiliser right when plants need it or finding ways to maintain yields with reduced nitrogen fertiliser would reduce these N2O emissions. Scientists are looking at various ways to do that. One strategy under investigation is to use precision agriculture techniques that use remote sensing technology to determine where and when to add nitrogen to fields, and how much. Another is to use nitrification inhibitors, chemicals that suppress the ability of microbes to turn ammonia into nitrate, impeding the creation of N2O and keeping the nitrogen in the soil for plants to use over a longer span of time.
Widely adopting these two practices would reduce nitrous oxide emissions about 26% from their current trajectory by 2030, according to a 2018 estimate by researchers at the International Institute for Applied Systems Analysis in Austria. But the authors say it will take more than that to help meet greenhouse gas targets such as those set forth in the Paris Agreement. So, scientists are exploring additional strategies.
One option involves harnessing the potential of certain microbes to directly supply nitrogen to plants, much as nitrogen-fixing bacteria already do in partnership with beans, peanuts and other legumes. “There’s really a gold mine living in the soil,” says Isai Salas-González, an author of an article on the plant microbiome in the 2020 Annual Review of Microbiology and a computational biologist who recently completed a PhD at the University of North Carolina at Chapel Hill.
In that vein, since 2019 the company Pivot Bio has marketed a microbial product called Pivot Bio Proven that, they say, forms a symbiosis with crops’ roots after an inoculant is poured in the furrows where corn seeds are planted. (The company plans to release similar products for sorghum, wheat, barley and rice.) The microbes spoon-feed nitrogen a little at a time in exchange for sugars leaked by the plant, reducing the need for synthetic fertiliser, says Karsten Temme, chief executive of Pivot Bio.
Temme says that company scientists created the inoculant by isolating a strain of the bacterium Kosakonia sacchari that already had nitrogen-fixing capabilities in its genome, although the genes in question were not naturally active under field conditions. Using gene editing technology, the scientists were able to reactivate a set of 18 genes so the bacterium makes the enzyme nitrogenase even in the presence of synthetic fertiliser. “We coax them to start making this enzyme,” Temme says.
Steven Hall, a biogeochemist at Iowa State University, is now testing the product in large, dumpster-sized containers with corn growing in them. Researchers apply the inoculant, along with different amounts of synthetic fertiliser, to the soil and measure corn yields, nitrous oxide production and how much nitrate leaches from the base of the containers. Though results of the trial are not yet out, Hall says there’s “good initial support” for the hypothesis that the microbes reduce the need for fertiliser, thereby reducing nitrous oxide emissions.
But some soil scientists and microbiologists are sceptical of a quick microbial fix. “Biofertilisers” like these have had mixed success, depending on the soil and environment in which they are applied, says Tolu Mafa-Attoye, an environmental microbiology graduate student at the University of Guelph in Canada. In one field study of wheat, for example, inoculating the crops with beneficial microbes enhanced growth of the plants but only resulted in slightly greater yields. Unknowns abound, Mafa-Attoye’s Guelph colleagues wrote in February in Frontiers in Sustainable Food Systems – such as whether the microbes will negatively affect the soil ecology or be outcompeted by native microbes.
Instead of adding in a microbe, it may make more sense to encourage the growth of desirable microbes that already exist in the soil, says Caroline Orr, a microbiologist at Teesside University in the UK. She has found that cutting back on pesticide use led to a more diverse microbial community and a greater amount of natural nitrogen fixation. In addition, production of nitrous oxide is influenced by the availability of carbon, oxygen and nitrogen – and all are affected by adjusting fertiliser use, irrigation and ploughing.
Take tillage, for example. An analysis of more than 200 studies found that nitrous oxide emissions increased in the first 10 years after farmers stopped or cut back on ploughing their land. But after that, emissions fell. Johan Six, a co-author of the analysis and an agroecologist at ETH Zürich in Switzerland, thinks that’s because the soils start out in a heavily compacted state after years of equipment driving over them. Over time, though, the undisturbed soil forms a cookie-crumb-like structure that allows more air to flow in. And in high oxygen environments, microbes produce less nitrous oxide. Such no-till systems also result in more carbon storage because less ploughing means reduced conversion of organic carbon to CO2– thereby providing an additional climate benefit.
It may even be possible for farmers to save money on fertiliser and water and reduce emissions, all while maintaining yields. In research on tomato farms in California’s Central Valley, Six found that study plots with reduced tillage and a drip irrigation system that slowly oozed nitrogen to plants – reducing how much of the nutrient pooled in the soil – lowered N2O emissions by 70% compared with conventionally managed plots. The farmer who implemented those changes was also compensated for his greenhouse gas reduction through the state’s cap-and-trade program. With the right incentives, persuading farmers to cut their emissions might not be that hard, says Six.
In Missouri, farmer Andrew McCrea grows 2,000 acres of corn and soy in a no-till system. This year, he plans to trim back his fertiliser use and see if the Pivot Bio inoculant can keep his yields more or less the same. “I think all farmers certainly care about the soil,” he says. “If we can cut costs, that’s great too.”
And if policymakers turn to tackling nitrous oxide, there should be rippling benefits for all of us, says Kanter of New York University. Some of them could be more rapid and tangible than addressing climate change. The same measures that lower N2O levels also reduce local air and water pollution as well as biodiversity losses. “Those are things that people will see and feel immediately,” Kanter says, “within years as opposed to within decades or centuries.”
Written By Ula Chrobak