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

The relationships between plants and the fungi that colonise their roots are responsible for locking away a huge amount of carbon underground – maybe equivalent to more than one-third of global emissions from fossil fuels.

Almost all land plants on Earth have a symbiotic relationship with fungi that live in the soil around their roots, trading the carbon they draw from the air for nutrients like nitrogen and phosphorus.

These mycorrhizal fungi store the carbon they get from their plant partners in their tissues and the surrounding soil, thus keeping it out of the atmosphere. But despite the interest in nature-based solutions to climate change, mycorrhizal fungi have been largely overlooked, says Heidi-Jayne Hawkins at the University of Cape Town, South Africa. So, she and her colleagues set out to calculate just how much carbon plants might be transferring to these fungi.

By scouring data from dozens of scientific studies on the relationships between plants and fungi, the researchers estimated that between 3 and 13 per cent of the carbon dioxide that plants pull out of the atmosphere ends up in the fungal tissue.

The team then used global data on which plants live where, how productive they are and which fungi they are associated with to estimate that about 13.1 gigatonnes of CO₂ is transferred to fungi each year – equivalent to around 36 per cent of annual emissions from fossil fuels around the world.

What isn’t clear is how long that carbon stays locked up underground, says Hawkins. While some will remain there even after the fungi die, bound to soil particles or reused by other plants, some will be released back into the atmosphere. And since most of the data was based on snapshots of fungal activity at a certain place and time – there was almost no data from Africa, for example – there are big gaps that add a great deal of uncertainty to the estimates.

Melanie Jones at the University of British Columbia in Canada says the work highlights fungi’s important role in the carbon cycle. “This is the first time someone has come up with numbers for how much carbon we’re talking about globally,” she says.

But Elly Morriën at the University of Amsterdam in the Netherlands points out that by focusing only on mycorrhizal fungi, the research is missing part of the picture. Saprotrophic fungi – the kind that feast upon dead organic material – make up a much larger portion of the fungal population and play a huge role in the carbon cycle by releasing CO₂ through decomposition. “They really determine how much carbon is returned to the atmosphere,” she says.

Hawkins hopes that an improved understanding of the relationship between plants and fungi will help us to better plan nature-based climate solutions like forest restoration. “There are a lot of failed projects, where trees were planted at vast expense but then died,” she says. “Knowing more about which trees grow best with which fungal partners can help these projects succeed.”