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The world's forgotten
Credit: Getty Images)
By Ula 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 production.
"Anything that can be done to improve fertiliser use efficiency would be
big," says Michael Castellano, an agroecologist and soil scientist at Iowa
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 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.
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 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.
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
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