Fungi take up more mass than people—see how they stretch across the Earth

We might only think of fungi as the mold in our walls, the fleshy protrusions on rotting logs, or the button mushrooms for sale in the grocery store—but most fungal life takes place far out of human sight.

The “roots” of the fungus under the soil form the organism’s mycelium. There, tiny filaments called hyphae form networks among plants and supply them with the essential nutrients they need to grow.

A new study, published today in Science, has attempted to quantify the extent of these hyphae networks. For the first time, scientists have created a global map showing where these types of fungi are most likely to be found.

Using machine-learning models and evidence from thousands of soil cores, scientists also found that a single type of fungus—called arbuscular mycorrhizal fungi—collectively weigh 300 megatons, four to six times the mass of every human on the planet. The tiny hyphal fingers of these networks reach far enough to stretch from Earth to the sun and back nearly a billion times.

“It really challenges the assumption of plants as stand alone organisms and forces us to think about how organisms are involved in a myriad of interactions that we don’t see,” says Toby Kiers, a study author and evolutionary biologist at Vrije University in Amsterdam.

Kiers is also the executive director of SPUN, the Society for the Protection of Underground Networks, and says knowing where these fungi hotspots are, and protecting them, could even limit climate change.

Why fungi networks are essential

Plants take in water and nutrients such as phosphorus and nitrogen through their roots. But most plants also have help absorbing these nutrients. They are in a symbiotic relationship—a type of partnership—with arbuscular mycorrhizal fungi.   

“Mycorrhizal symbiosis is a cooperation between plants and fungi that's quite ancient, about 450 million years old,” says Justin Stewart, an evolutionary biologist with SPUN, who is also a study author. “You have plants that take carbon dioxide in through photosynthesis, and then they feed this to these bodies of mycelium in soil,” Stewart, who’s also a study author, says.

After a fungus receives carbon from a plant, it sends out “thin tubular threads. They’re super, super small, a tenth or a twentieth of a human hair,” they explain. “They go deep into soils and extract nutrients, such as nitrogen and phosphorus, and they trade it back to the plant.”

In a way, Stewart says, it’s an economy: fungi give plants nutrients in exchange for carbon. Plants even grow larger when they have these partnerships, and it can help plants like wheat resist drought, supplied from below with extra water from fungi.

It’s such a good arrangement that more than 70 percent of plants partner with arbuscular mycorrhizal fungus—including crops such as wheat, corn, and rice. Fungi can provide up to 80 percent of the phosphorus these plants need, and up to 20 percent of the nitrogen.

The partnership is deeply, physically intimate. It begins when plant roots send out chemicals to attract fungi and welcome them into the delicate tips of their roots.

They “form this beautiful structure called an arbuscule, which looks like a mini tree inside the actual root cell,” says Kiers..

The tiny underground trees are the site of this economic exchange. But to obtain the nitrogen and phosphorus the plants crave, the fungi, like all good merchants, must search for new sources of nutrients as they exhaust ones nearest the plant. To do so they send out their hyphae, which can increase the nutritional “reach” of plant roots by up to a hundred times.

Measuring the world’s mushrooms

Even though arbuscular mycorrhizal fungi are so critically important, scientists didn’t know how much mass they take up in the soil, or how much carbon they might take in.

“It’s hard to see fungi growing in soil,” says Anne Pringle, a mycologist at the University of Wisconsin, Madison and National Geographic Explorer who was not involved in the study. “Until we invent magical glasses…they basically appear invisible to us.”

But SPUN has been developing techniques to take a look. For previous studies, Kiers, Stewart, and their colleagues developed an imaging robot that could photograph and analyze more than 300,000 hyphae in petri dishes and plant pots in the lab. They used this robot to analyze samples of fungal networks found in more than 16,000 soil cores across most of the ecosystems found on Earth.

Using machine learning, the researchers then used the results of their lab work to estimate what the density of arbuscular mycorrhizal fungi might look like for every ecosystem on Earth

The result is a truly massive estimate of fungi.

In only the top 15 centimeters of soil, scientists estimate that every square centimeter of soil packs around 4.4 meters of hyphae, 50 times longer than the fine roots of the plants they parent with. Stretched end to end, that’s 110 quadrillion kilometers, enough to stretch between Earth and the sun nearly a billion times.

“This is like when you have a ball of yarn,” says Giuliana Furci, a field mycologist, executive director of the Fungi Foundation and National Geographic Explorer who was not involved in the study. If you unravel the ball, “it could reach from your house to your grandmother’s house,” she says. “Before that, it fit in your backpack.”

Yet, if you were to scoop up a handful of dirt, you wouldn’t see a network of fungal threads.

“They are so small that to the naked eye you cannot see these filaments,” says Furci. “They are intertwined in everything that you think of as soil.”

Fungi thrive where the green grass grows

The study’s estimate of fungal length and mass are a global average, but some regions have more fungi than others. The densest networks were in dry grasslands, such as the Tibetan steppe, and flooded grasslands like the Everglades in Florida and the Sudd wetland in South Sudan.

Grassland areas alone made up 40 percent of the global hyphae biomass. “People tend to think of grasslands as just weeds,” Furci says. But “grasslands can be extremely ancient and extremely diverse.”

That figure partially results from the type of fungal network the scientists were counting, Pringle says.

“Trees can associate with arbuscular mycorrhizal fungi,” she says. But “many trees associate with ectomycorrhizal fungi,” in which the fungi stay on the outside of the plant root, instead of being invited in. That means fungal mass could be even higher in where plants form partnerships with different fungal types.

Studies like this one, “set the stage potentially for other kinds of work on other groups of fungi,” Pringle says. “It's sort of a roadmap, potentially, of what to do.”

Not all grasslands were hotspots of fungal biomass, the study found.

Where those grasses had been converted to croplands, the arbuscular mycorrhizal network was 47 percent less dense than naturally occurring grasslands.

“At the global scale, we see agriculture reduces the density of these networks,” Stewart says. That could be due to several factors. Tilling might disturb the hyphae underground, and plants treated with fertilizer might rely less on arbuscular mycorrhizal fungi for nutrients.

Stewart thinks Indigenous farming practices, no-till soil tending, and growing food organically might preserve some of the soil’s fungal biomass.

The study’s map contains are other areas of uncertainty, such as the Sahara Desert and Greenland, where there simply isn’t enough soil data to know if there are fungi present.

“That is always the limitation in this sort of studies,” says Matthias Siewert, who uses remote sensing to study ecosystems at Umeå University in Sweden and was not involved in the study. “We cannot sample unlimited everywhere. So, we have to make some base assumptions.”

Why fungi are important for fighting climate change

The mass of fungal bodies in soil is more than just living matter. As hyphae die, the carbon they absorbed from plants stays underground.

Kiers, Stewart, and their colleagues estimate that fungi help pack around four billion tons of carbon dioxide into soil. That’s about 11 percent of the carbon dioxide humans pump into the atmosphere in a year.

The hyphae also give structure to the soil, Kiers says. “They're habitat builders for other soil species,” she explains. Around 59 percent of Earth’s biodiversity is underground. Earthworms, insects, nematodes and many more species thrive without ever seeing the sun.

This could mean that as the hyper-fungal grasslands get converted into croplands, far more than grass could be lost and more planet-warming carbon could stay in the atmosphere.

The researchers working for SPUN ultimately want to create legally protected areas in ecosystems where fungal diversity and abundance is high.

The work “was really interesting and adds a lot of information,” Pringle says. “A lot of it fills a rather large knowledge gap.”

Understanding more about underground fungal networks could completely change how we approach and understand ecosystem conservation, Furci says. Like veins circulating blood through a human body, Furci says these fungal networks are Earth’s lifeblood.  

"They are the circulatory system of the planet underground,” she says. "We're starting to realize that we cannot diagnose, treat, or heal the ailments of Earth without looking at the mycorrhizal circulatory system.”