CLIMATEWIRE | Scientists are sounding the alarm that certain agricultural practices aimed at reducing greenhouse gas emissions are likely causing more harm than good.
Research published last month in Science warns that certain efforts to reduce methane emissions could backfire by increasing emissions of nitrous oxide — a greenhouse gas that is less potent than methane in the short term but does more damage overall.
“Even though there’s better recognition that methane is a great target to control to bring the rate of temperature increase into some sort of order, a lot of the mechanisms ignored the nitrogen input,” said Lisa Stein, a professor at the University of Alberta and one of the paper’s authors.
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“So when you’re trying to mitigate methane emissions, these technologies might inadvertently stimulate nitrous oxide production — and we don’t want that trade-off,” added Stein, who specializes in climate change microbiology.
Separate researchers have found that methane-reducing irrigation techniques backed by several nongovernmental organizations are dramatically increasing nitrous oxide from rice paddies and could worsen climate change on balance.
Curbing the methane output of rice paddies has been a longstanding goal of farmers and scientists. The Environmental Defense Fund estimates that annual global rice cultivation has a long-term greenhouse gas impact equal to as many as 600 coal plants.
More than nine-tenths of atmospheric methane is produced by microbes that, when separated from oxygen, turn organic matter into methane. But introducing oxygen to the environment will slow — or even halt — methane emissions.
Several sustainability efforts, such as those supported by the USA Rice Federation industry group, have advocated agricultural techniques that do just that: add oxygen to methane-producing soil microbes. Those methods include intermittent flooding and furrow irrigation.
But there’s a catch, says Joe Rudek, lead senior scientist at the Environmental Defense Fund.
When you add oxygen to the environment, “then the bacteria that are cycling nitrogen convert the nitrogen to ammonia,” Rudek said in an interview, “In that process, nitrous oxide is one of the byproducts.”
Nitrous oxide accounts for 6 percent of global emissions compared to 76 percent for carbon dioxide. Yet each metric ton of nitrous oxide contributes 265 times the amount of warming as one metric ton of CO2 over 100 years, according to the Intergovernmental Panel on Climate Change.
For more than a decade, some microbiologists have wondered whether reducing methane through farming techniques that employed oxygen could have the side effect of increasing the production of nitrous oxide emissions.
Proof of the effect was long limited to laboratory observations, but a 2018 study co-authored by Rudek and led by the Environmental Defense Fund found compelling real-world evidence.
“There were quite a few nitrous oxide and methane measurements made in various rice fields across India” assessing the impact of intermittent flooding, Rudek said.
Intermittent flooding — also known as alternate wetting and drying (AWD) — is where rice fields are periodically irrigated and then dried rather than being continuously flooded. Drying the soil exposes the microbes to oxygen, slowing methane emissions and increasing nitrous release.
The practice is encouraged by the USA Rice Federation, the Rockefeller Foundation and the Sustainable Rice Platform.
The Sustainable Rice Platform is an initiative co-convened by the United Nations Environment Programme and the International Rice Research Institute, with more than 100 members dedicated to promoting sustainable and ethical rice cultivation methods.
The platform calls intermittent flooding a “better way,” citing studies that find it decreases overall greenhouse gases.
“[Crop] yields were almost identical, and the AWD method emitted less greenhouse gases and required fewer seeds and fertilizer,” their website says.
The U.N. Food and Agriculture Organization, a member of the Sustainable Rice Platform, doesn’t share the program’s endorsement of intermittent flooding.
“We stay neutral at this stage as we are piloting the technique accompanied with other agricultural water management measures,” the FAO told POLITICO’s E&E News in an email.
The USA Rice Federation and the Sustainable Rice Platform did not respond to a request for comment before publication.
Based on their measurements, Rudek and his co-authors concluded: “The Indian subcontinent’s N2O emissions from intermittently flooded rice fields could be 30–45 times higher than reported under continuous flooding.”
Depending on the frequency and intensity of intermittent flooding, the study found that the practice actually could worsen climate change within 30 years because the new nitrous oxide would outweigh lowered methane.
There is, however, at least one potential alternative. Stein says a substance called biochar can serve as a methane mitigator without inadvertently upping nitrous oxide.
Biochar is a type of charcoal made from burning organic material, usually wood. It’s often used as a fertilizer for crops — including rice — and has the added bonus of storing carbon.
“Biochar is like the favorite idea of what you can add to soils because it’s a combusted organic matter — meaning that the methane-producing microbes can’t access it because it’s already burned up,” Stein said. “Biochar keeps all of the nutrients in the soil at a nice level so that all of the carbon and nitrogen cycling can stay internalized.”
A 2019 Chinese study published in Nature found that biochar reduced both methane and nitrous oxide emissions in tobacco and rice paddies, regardless of season or flooding level.
One New York startup is going a step further by sequestering methane in its biochar. Northeastern Biochar Solutions’ process involves incinerating treated sewage — rather than wood — to create biochar.
“Our process actually uniquely deals with the methane issue because when you take (uncharred sewage) and land apply it, it rots and emits a tremendous amount of methane. If you put it in a landfill, it emits a tremendous amount of methane,” said Raymond Apy, CEO of Northeastern Biochar.
But by scorching the sewage in an oxygen-free environment, Apy says they sequester the sewage’s methane in the biochar and stop future emissions.
“We hired chemical engineers to do a full, 360 life-cycle analysis on our first proposed facility, and they did that,” Apy told E&E News. “The methane reductions are tremendous. Overall greenhouse gas reduction is net -137 percent. That’s verified, third party.”
While he thinks nitrous oxide needs attention, Rudek with the Environmental Defense Fund stresses that methane’s heavyweight greenhouse gas effect — about 80 times as potent as carbon dioxide over 20 years — makes it the critical short-term polluter to address.
“One of the most important tools we have to reduce the rate of global warming over the next couple of decades (is) by controlling methane,” Rudek said. “You need to also do carbon dioxide and nitrous oxide, but methane is unique in that way.”
Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2024. E&E News provides essential news for energy and environment professionals.
