New maps reveal how vast fungal networks cool Earth

Researchers estimate that arbuscular mycorrhizal fungi stretch across the world’s soils for 110 quadrillion kilometers and move about 4 billion metric tons of carbon every year. The new machine-learning maps also show where these underground networks are dense

In soils beneath your feet. there’s a living infrastructure most people will never see—but it’s working at planetary scale. Arbuscular mycorrhizal fungi form vast subterranean networks of tubes called hyphae. hooking up with the roots of plants to exchange nutrients. Earth is so verdant in large part thanks to these partnerships. and the fungi are associated with nearly three-quarters of all plant species.

The trouble is that the networks sprawl underground. Scientists have struggled to measure how much arbuscular mycorrhizal fungi exists across the planet. because digging everywhere and collecting samples is impractical. Now researchers have used a workaround. estimating the scale of the hidden system with machine-learning models trained on thousands upon thousands of samples taken around the world.

The resulting numbers are staggering. Worldwide, the arbuscular mycorrhizal network stretches for 110 quadrillion kilometers—almost a billion times the distance from Earth to the sun. The study estimates that every year. these fungi shuttle around 4 billion metric tons of carbon. equal to 11 percent of humanity’s CO2 emissions. As a rough sampling lesson. the researchers note that if you scoop up just a teaspoon of soil. you might find 10 meters of fungal strands.

The maps are built from those training data. predicting where arbuscular mycorrhizal fungi are more or less concentrated. even in the most remote environments. “We have started to have a clear picture of the full extent of these hidden living infrastructures that circulate carbon and nutrients in the soils beneath our feet. ” said Toby Kiers. executive director of the Society for the Protection of Underground Networks and coauthor of the new paper. which published today in the journal Science.

In the study’s visualization, brighter yellow spots indicate higher densities of arbuscular mycorrhizal fungi.

Two big categories of mycorrhizal species exist. Ectomycorrhizal fungi grow as sheaths around a plant’s roots, especially conifer trees. Arbuscular mycorrhizal fungi—center stage in the new work—penetrate plant roots. Either way, these fungi operate like an extension of the root system, helping plants absorb more water and nutrients. Kiers compares the relationship to a circulatory system: “Just as a circulatory system moves resources through a body. these sort of microscopic fungal pipes are connected to plants.”.

The exchange is reciprocal. Mycorrhizal species receive energy in the form of carbon that plants have drawn from the atmosphere. In return. they help plants grow in ways that can further sequester carbon—benefiting humans as well because the partnership helps keep the planet from warming even further.

What’s changed with these new maps is not just scale, but clarity about distribution. The density of arbuscular mycorrhizal fungi isn’t uniform across the planet’s biomes. Researchers found that tropical rainforests are not the dominant share. Instead, grasslands account for 40 percent of the predicted global arbuscular biomass. The study suggests a reason: herbaceous plants like grasses tend to allocate more carbon to their symbiotic fungi than trees do. and grasslands also have vast root systems that translate into “loads of hidden biomass.”.

Kiers points to why that matters for carbon storage after disturbance: “Even if grasslands get burned above ground, that carbon tends to remain underground, and they can come back again, which is different than forests.”

Still, the new study also flags a vulnerability that can be easy to miss when a system is invisible. Kiers added that just 5 percent of arbuscular mycorrhizal fungal biodiversity hot spots lie in environmentally protected areas. The aim of these maps. he says. is to help scientists and policymakers identify where fungi are thriving and where they’re threatened—so protections can support plant life and biodiversity overall. including birds. insects. and herbivores that depend on vegetation.

The maps also point to a different kind of pressure—how modern land use may be changing the underground ecology. In areas with large-scale agriculture, the study found that fungal network densities are about 50 percent lower on average. One explanation offered in the paper is that synthetic fertilizers provide crops all the nutrients they need. reducing reliance on arbuscular mycorrhizal fungi. Tillage may add another hit, tearing fungal networks apart at the end of a growing season.

Other research has found that tilling also disrupts soil’s ability to retain water, adding another reason farmers and land managers may want to think carefully about how disturbance reverberates through ecosystems.

“Maybe we can do better to have more fungal biomass in our agricultural systems, and in our terrestrial ecosystem as a whole, and capture more carbon dioxide,” said ecologist Smriti Pehim Limbu of Dartmouth College, who studies mycorrhizal fungi but wasn’t involved in the new paper.

Humanity has to feed itself, of course. But the new maps put a number on what’s been happening largely out of sight: a living network that circulates carbon and nutrients in the soils beneath everyday landscapes. The question now is how to protect the parts we can’t see without losing the benefits they provide.

Kiers frames it as a shift in what researchers can do with basic existence knowledge. “This map is for mycorrhizal fungi what the first detailed maps were for. I don’t know. ocean currents or river systems. ” he said. “Where you go from knowing a system exists to knowing where it is. how dense it is. and where it’s threatened.”.

The team’s work includes field sampling far from labs—Toby Kiers and Merlin Sheldrake taking soil samples in the mountains of Bhutan. photographed courtesy of Tomás Munita. And if the networks are truly as widespread as these estimates suggest, the stakes are not confined to remote places. They reach into how soils store carbon. how plants regrow after fire. and how agriculture shapes the ecosystems that keep the ground alive.

arbuscular mycorrhizal fungi hyphae networks soil carbon machine learning biodiversity hotspots grasslands agriculture carbon sequestration mycorrhizal fungi Science journal