Hungry microbes stop warming soils from releasing extra carbon

Soil is a living system. It’s home to tiny organisms – bacteria, fungi, and viruses – that go about their business beneath our feet, breaking down organic matter and releasing carbon dioxide (CO₂) into the air as they “breathe.”

These microscopic organisms play a huge role in Earth’s carbon cycle. But what happens when the planet heats up? Do they breathe harder and pump out more CO₂ just because it’s warmer? You might think so, but the story is more complicated.

A recent study found that simply heating up soil doesn’t automatically cause microbes to release more carbon dioxide (CO₂). Warming by itself had little to no effect.

Heat fails to drive soil carbon

The research was done in Athens, Georgia, at one of the world’s oldest long-term soil-warming sites. Scientists collected soil from former cotton fields that had been turned into forest land. These aren’t rich, healthy forest soils. They’re worn-out soils, low in nutrients and organic matter – hardly a buffet for microbes.

The researchers brought the soil into a lab and cranked up the heat by about 4.5°F. They also added carbon and nutrients in various combinations. Then they measured how much CO₂ was released.

“When things warm up, there is more plant photosynthesis, more ‘food’ for microbes to metabolize on, more activity for microbes,” said Debjani Sihi from North Carolina State University, the lead author of the study.

“The question here is whether warming was enough to cause more carbon dioxide release from soil. The findings show that if you don’t have the carbon and nutrients in easily available forms that soil microbes need to grow and thrive, then heating alone will not increase the loss of carbon.”

Soil carbon follows strict rules

We usually think about human emissions – cars, factories, power plants – when we talk about CO₂ and climate change. But about one-third of CO₂ emissions from land come from soil. That’s natural, and it’s part of a cycle. But scientists need to understand how that cycle responds to a warming world.

“Microbes are breathing and they are getting their energy from carbon,” Sihi said. “Then they’re also fulfilling their demand of nutrients from the same food that they’re getting.”

“Like humans who need a balanced diet – an energy source, proteins, fiber – you can think about a similar parallel with microbes. They use some of the carbon to build biomass.”

They invest energy to build enzymes that break down complex organic matter into usable carbon and nutrients, while expelling the rest as part of their metabolism.

So if soils don’t have enough “microbe food” to begin with, heating things up doesn’t cause a big release of CO₂. But if the carbon and nutrients are there? Then warming gives the microbes what they need to get to work.

Focus moves to warmer soils

Most soil-warming studies until now have focused on cold places like the Arctic or northern forests. Those areas are thought to be especially sensitive to climate change. But this study looked at subtropical soil – hot, humid, and already struggling with fertility.

“This study occurs in former cotton fields converted to forest land, not in native forest land,” Sihi said. “Cotton is an exhaustive crop, so the soil doesn’t contain many nutrients or carbon; the soil is not fertile or healthy.”

That matters because not all soil is created equal. What works – or doesn’t work – in the Arctic might play out very differently in a tropical forest or a worn-out field in Georgia.

“It appears in this case that warming alone may not stimulate microbial activities because these microbes actually don’t have a lot of resources to thrive in,” Sihi said. “In other words, depleted microbial resources constrain warming effects.”

Soil science has more to learn

The team behind this study isn’t stopping here. They’re running similar experiments in Puerto Rico and Panama to see how tropical ecosystems respond to warming. Every bit of information helps build a better picture of what to expect as global temperatures keep rising.

“Nature emits carbon, but it also absorbs carbon. If you know how much CO₂ comes from the natural system, then you can identify targets for different other industries or economic sectors to reduce emissions,” Sihi said.

Understanding how soil works – when it stores carbon and when it lets it go – is key to managing Earth’s carbon budget. This research adds one more important piece to the puzzle. And it shows us one thing for sure: when it comes to soil, heat isn’t the whole story.

The full study was published in the journal Biogeochemistry.

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