After the rain, desert microbes exhale powerful greenhouse gases

New research from UC Riverside shows how after rain, desert soil microbes convert one form of pollution into another – laughing gas.

No laughing matter, nitrous oxide or N2O is the third most powerful greenhouse gas. The scientists conducting the research were surprised to measure the production of N2O in the desert heat.

Rain in a desert landscape can trigger nitrous oxide emissions from the ground.

“This only happens in waterlogged soils. Since the desert is dry most of the year, we didn’t think this process could happen in arid soils, said Alex Krichels, UCR environmental scientist and first author of the new study.

This study, published in the journal Biogeochemistry, examines how and why desert bacteria produce N2O emissions. It builds on work published in 2020, when a team led by UCR landscape ecologist Darrel Jenerette found that desert soils produced substantial amounts of N2O after rainfall.

The traditional view, Krichels explained, is that N2O comes from heavily fertilized agricultural fields like those in the Midwest. Growers add more nitrogen, ammonium, and nitrates than the plants need, and after a rain, bacteria convert the excess to N2O, a process called denitrification.

“It’s a strategy for the bacteria to survive after adding a ton of water and there’s no oxygen for them in the soil,” Krichels said. “When this happens, instead of oxygen they use nitrate and exhale nitrous oxide, a process called denitrification.”

Unlike fertilization in agricultural fields, denitrification in deserts may have a different source of nitrate. “Nitrate pollution in deserts comes from burning fossil fuels, not from fertilization,” Krichels said. “Combustion releases pollution that hangs around, settles in soils over time and reappears after a rain as N2O.”

Automobiles or industrial processes release a few different forms of nitrogen into the atmosphere. “Combined, they’re called NOx, and they can produce ground-level ozone, which is bad for your lungs and also a greenhouse gas. It should not be confused with the good ozone higher up in the stratosphere which protects us from UV rays,” explained Peter Homyak, UCR environmental scientist and co-author of the paper.

To determine if fossil fuel byproducts could cause desert denitrification, the researchers chose two Southern California sites in the University of California Nature Preserve System. They used a “coffin-like” box, Krichels said, with instruments to measure the chemical composition of the air coming out of the ground after adding nitrate.

tools to analyze the exhalations of desert microbes
Instruments used by scientists to measure the composition of the air leaving the ground. (Alex Krichels/UCR)

The box also contained an air conditioning unit, as temperatures often reached 120 degrees. “Temperatures well above 100 degrees Fahrenheit are thought to inhibit microbial processes. Given the heat at our sites, it was surprising to see so much N2O,” Krichels said.

Krichels, who has previously studied similar processes in Illinois cornfields, said what emerges from deserts after rains is 10 times higher than anything he’s seen in the Midwest. “Emission rates are really high, but short-lived,” he said. “It only happens when water is added to dry soils.”

There is plenty of evidence to suggest that droughts are becoming more common around the world and that these droughts will be punctuated by heavy rainfall. Since droughts dry out soils, these climate changes will make cycles of drying and wetting more frequent and increase the likelihood that these processes will become more significant sources of greenhouse gases.

In the future, the researchers will replicate the study with sites in Riverside and Joshua Tree, to measure whether proximity to cities increases post-rainfall nitrous oxide emissions from soils.

In general, Krichels said he hopes awareness of these findings will inspire people to limit fossil fuel emissions that lead to denitrification of desert soils.

“On a larger scale, many people don’t know that these processes occur in soils in general, or that the nitrogen that humans add to the atmosphere can end up affecting climate change and human health in this way. “, Krichels said. “There is a lot of life in these soils, and it can affect the whole world.”

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