The world's forgotten greenhouse gas
(Image credit: Getty Images)
Chrobak3rd June 2021
From Knowable Magazine
Emissions of the greenhouse gas commonly
known as laughing gas are soaring. Can we cut emissions from its
greatest anthropogenic source?
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.
Synthetic fertilisers are a large source of N2O emissions in
agriculture (Credit: Getty Images)
Today, scientists are looking at several
ways to treat the soil or adjust farming practices to cut back on N2O
"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 take up all the nutrients from fertiliser, the
greenhouse gas N2O is released
(Credit: E. Verhoeven et al/California Agriculture 2017/Knowable Magazine)
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.
There's really a gold mine living in the soil – Isai Salas-González
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
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.
Microbes in the soil break ammonia down through a series of reactions,
releasing N2O in the process, which can be measured in the field
(Credit: Getty Images)
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
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.
Switching to minimal ploughing could help reduce N2O emissions from
soils (Credit: Getty Images)
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."
This article originally appeared in Knowable
Magazine, and is republished here with permission. This is also why
this story does not have an estimate for its carbon emissions,
as Future Planet stories usually do.
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