Microbes in soil pose global-warming challenge

LAWRENCE — A grim hazard of climate change is that “positive feedbacks” will augment the warming induced by human activities.

One such a positive feedback loop that could spiral out of control lies directly underfoot. When microorganisms in the soil break down organic matter like dead plants, they also emit heat-trapping gases into the atmosphere. The warmer the world gets, the busier the microorganisms in many places could become, producing relatively more greenhouse gases.

“There is great concern that with human-induced climate change in the coming century, warmer temperatures will increase the rates at which microbial activity in the soil occurs, and that in turn will drive increased fluxes of greenhouse gases into the atmosphere,” said Sharon Billings, associate professor of ecology and evolutionary biology at the University of Kansas.

Microbial activity in the soil generates carbon dioxide, nitrous oxide and methane, all of which add to global warming. Indeed, emissions from soil-dwelling microorganisms worldwide constitute a “huge governing factor in the Earth’s climate,” according to the KU researcher.

Thus, with a hotter climate, increased emissions by soil biota could contribute severely to yet more global warming in the coming decades — at levels akin to other major systems that threaten positive feedback loops in response to a warming planet, such as melting reflective sea ice or methane-rich Siberian permafrost.

But according to Billings, who also works at the Kansas Biological Survey, scientists who forecast the Earth’s changing climate urgently require a better grasp on these microorganisms’ role in global warming and the reasons behind any change in their behavior with warming.

“Our work highlights what we really need to study – and model – to understand what is going to happen in the future,” Billings said. “We already know that if you warm soils, you’re going to get more CO2 in most cases. The question is how much, and why.”

Billings stressed that scientists must understand the specific means by which warming boosts microbial respiration in soil – a key process that produces CO2.

“When you warm soils in the field or warm soils in the lab and you measure how much CO2 comes off, that CO2 is the end result of many different processes prior to cells’ respiration,” she said. “There’s a big effort in my lab to try to parse those different processes out and find out what the temperature influence is on each of them.”

Billings and her team are researching several possible mechanisms behind increased greenhouse-gas production from soil microbes. “You have to ask, ‘Is the microbial population increasing in size? Is that a feature driving warmer soils to emit more CO2?’” she said. “Or, are the microbes changing what they’re going after? Are they going after cellulose instead of sugar, or going after lignin instead of cellulose? We don’t know the answer to those questions, and that’s really where we need to go.”

With such an understanding, a more finely tuned estimation of the microbial influence on Earth’s warming climate is possible, taking into account precipitation and regional climate differences as well.

There might be little that humankind can do to change the rates at which microorganisms break down organic matter in soil around the planet. “We have very little control of that relative to human-driven processes,” Billings said. “These ecosystems are going to function in response to environmental features like temperature and precipitation. And human activities are changing those patterns.”