Underground fungi absorb up to a third of our fossil fuel emissions

The relationship between plants and the fungi that colonize their roots is responsible for trapping large amounts of carbon underground, perhaps representing more than one-third of Earth’s emissions from fossil fuels.

Nearly all land plants on Earth have a symbiotic relationship with fungi that live in the soil around their roots, exchanging carbon uptake from the air for nutrients such as nitrogen and phosphorus.

These mycorrhizal fungi store carbon from partner plants in their tissues and surrounding soil, keeping it out of the atmosphere. But despite interest in nature-based solutions to climate change, mycorrhizal fungi have been largely ignored, says Heidi Jayne Hawkins of the University of Cape Town in South Africa. So she and her colleagues decided to calculate how much carbon the plants were transferring to these fungi.

By reviewing data from dozens of scientific studies of plant-fungi relationships, the researchers estimated that between 3 and 13 percent of carbon dioxide is present. What the plant pulls out of the atmosphere ends up in the fungal tissue.

Using global data on which plants live where, how productive they are, and which fungi they associate with, the researchers then estimated that about 13.1 gigatonnes of CO would be produced.₂ Transfer to fungi every year – this equates to about 36%. Annual emissions from fossil fuels worldwide.

What’s not clear is how long the carbon is trapped underground, Hawkins said. After the fungus dies, some remains there, bound to soil particles or reused by other plants, while some is released into the atmosphere. And since most of the data is based on snapshots of fungal activity at specific locations and times, there was little data from Africa, for example. There are large gaps, adding a lot of uncertainty to the estimates.

Melanie Jones of the University of British Columbia, Canada, said the study highlights the key role of fungi in the carbon cycle. “This is the first time anyone has put a number on how much carbon we are talking about on a global scale,” she says.

But by focusing solely on mycorrhizal fungi, research misses part of the picture, says Ellie Mollien of the University of Amsterdam in the Netherlands. Saprophytic fungi (the kinds that eat dead organic matter) make up the majority of fungal populations and play a major role in the carbon cycle by releasing carbon dioxide.₂ through decomposition. “It determines how much carbon is actually put back into the atmosphere,” she says.

Hawkins hopes that a better understanding of the relationship between plants and fungi will help us better plan nature-based climate solutions such as reforestation. “There are a lot of failed projects that cost a lot of money to plant and then die,” she says. “Knowing more about which trees grow best with which fungal partners will help the success of these projects.”