Soil microbes accelerate global warming

Artist's rendering
Artist's rendering
The increasing amount of carbon dioxide absorbed by plants is triggering the soil to release nitrous oxide and methane, two potent greenhouse gases. Artist’s rendering by Victor Leshyk

Ecosystems are not as efficient in counteracting the effects of global warming as once believed, according to a study released in this week’s edition of Nature.

The study, “Increased Soil Emissions Of Potent Greenhouse Gases Under Increased Atmospheric Carbon Dioxide,” details how the research team found the opposite is occurring in several regions in the world—higher levels of carbon dioxide in the atmosphere triggers soil to release methane and nitrous oxide, two potent greenhouse gases.

“This feedback to our changing atmosphere means that nature is not as efficient in slowing global warming as we previously thought,” said Kees Jan van Groenigen, lead author of the study and visiting research fellow at Northern Arizona University.

Van Groenigen, along with co-authors Bruce Hungate, professor of biological sciences at Northern Arizona University, and Craig Osenberg, professor of ecology at the University of Florida, analyzed all published research to date from 49 different experiments conducted in forests, grasslands, wetlands, and agricultural fields, including rice paddies, in North America, Europe and Asia.

The researchers measured how extra carbon dioxide in the atmosphere affects how soils take up or release methane and nitrous oxide.

The research team used a statistical technique called meta-analysis, or quantitative data synthesis, a powerful tool for finding general patterns in a sea of conflicting results.

“Until now, there was no consensus on this topic because results varied from one study to the next,” Osenberg said. “Two strong patterns emerged when we analyzed all the data—more carbon dioxide boosted soil emissions of nitrous oxide in all the ecosystems, and in rice paddies and wetlands, extra carbon dioxide caused soils to release more methane.”

The analysis pointed to wetlands and rice fields as two major sources of methane emissions in the atmosphere.

The culprits responsible for the greenhouse gas emissions are specialized microscopic organisms in soil that respire the chemicals nitrate and carbon dioxide like humans respire oxygen. The microbes also produce methane, a greenhouse gas 25 times more powerful than carbon dioxide, and nitrous oxide, 300 times more potent than carbon dioxide. Their oxygen-free habit is one of the reasons these microorganisms flourish when atmospheric carbon dioxide concentrations increase.

“The higher concentrations of carbon dioxide reduce plant water use, making soils wetter and reducing the availability of oxygen in soil, which provides favorable conditions for these microorganisms,” van Groenigen said.

The other reason these microorganisms become more active is that increasing carbon dioxide makes plants grow faster, and the extra plant growth supplies soil microorganisms with extra energy, pumping up their metabolism. This extra plant growth is one of the main ways ecosystems could slow climate change: with more carbon dioxide, plants grow more, soaking up carbon dioxide through photosynthesis, and, the hope is, locking away carbon in wood and soil.

But this new work shows at least some extra carbon also provides fuel to microorganisms whose byproducts end up in the atmosphere and counteract the cooling effects of more plant growth.

“It’s an ecological point and counterpoint—the more the plants soak up carbon dioxide, the more microbes release these more potent greenhouse gases,” Hungate said. “The microbial counterpoint is only partial, reducing the cooling effect of plants by about 20 percent.”

It’s an ecological surprise too, and one that climate models will need to reckon with as they further refine pictures of the climate of the future.

“By overlooking the key role of these two greenhouse gases, previous studies may have overestimated the potential of ecosystems to mitigate the greenhouse effect,” van Groenigen said.