Published In The Journal Science And Built From 16,669 Soil Cores Spanning Every Continent And Nine Biomes, The Map Shows That Agricultural Croplands Have Approximately 50% Lower Fungal Density Than Wild Grasslands And That The Very Practices Destroying This Network Are Eliminating One Of Earth’s Most Powerful And Least Understood Carbon Sinks
According to Live Science and a paper published Thursday June 11 in Science by lead author Justin Stewart, an evolutionary biologist at the Society for the Protection of Underground Networks, researchers have produced the first global map of Earth’s arbuscular mycorrhizal fungal networks; the vast system of microscopic branching threads that runs beneath virtually every patch of land on Earth and underpins the nutrient cycle, carbon storage, and plant health of the entire terrestrial biosphere.
The map was built from 16,669 soil cores collected across 322 previous studies spanning every continent and nine biomes, and used artificial intelligence to predict fungal distribution for every 0.4 square miles (1 square kilometre) of topsoil worldwide. The result is a picture of a living system whose scale is almost impossible to comprehend.
The number that stops you. The average hyphal density in land topsoil is 237 feet per cubic inch (4.4 metres per cubic centimetre). If all hyphae on Earth were laid end to end in a straight line, the researchers estimate they would span approximately 68 quadrillion miles; 110 quadrillion kilometres.
That is nearly a billion times the distance from Earth to the sun. It is approximately 10% of the width of the Milky Way galaxy. The Milky Way is approximately 100,000 light years wide. Ten percent of that, expressed as a fungal thread, is hiding in the top layer of soil beneath your feet.
What arbuscular mycorrhizal fungi actually are and what they do. Arbuscular mycorrhizal fungi are made up of tiny branching threads called hyphae. These threads extend outward from the roots of host plants into the surrounding soil, forming dense networks that function as two-way pipes.
In one direction, they channel nitrogen and phosphorus from the soil into the plant; nutrients the plant cannot easily access on its own. In the other direction, they channel carbon from the plant into the soil. This exchange is not incidental. It is the operating mechanism of most of the world’s land plant life.
An estimated 80% of all land plant species form symbiotic relationships with arbuscular mycorrhizal fungi of this kind. The fungi are not passengers in the soil ecosystem. They are foundational infrastructure.
The carbon implications of this infrastructure are staggering. One previous estimate found that arbuscular mycorrhizal fungi absorb around 4.3 billion tons of carbon dioxide equivalent each year; representing roughly 11% of global fossil fuel emissions as of 2021. This is not carbon absorbed by trees, or grasslands, or peatbogs. This is carbon absorbed specifically by the fungal threads themselves, channelled out of plant roots and into the soil, where it contributes to long-term carbon storage in ways that remain incompletely understood.
Understanding where these threads are most densely packed; which is what the new map reveals for the first time, is a prerequisite for understanding how much carbon they are currently sequestering, how much they could sequester under different land management conditions, and how much of that capacity is being destroyed by agricultural practices.
Where the fungi are most concentrated and why it matters. Wild grasslands dominate the map’s highest-density regions. In high-altitude or flooded grasslands such as those in the Everglades in Florida; the top 6 inches (15 centimetres) of soil are especially dense, containing around 40% of the global fungal biomass in that thin surface layer.
The overall density in wild grasslands averages 355 feet per cubic inch (6.6 metres per cubic centimetre); the highest of any biome studied. Cultivated trees returned the lowest density of any category, at 204 feet per cubic inch (3.8 metres per cubic centimetre).
The contrast between wild grasslands and croplands is the most practically urgent finding in the study. Agricultural croplands contain approximately 50% lower hyphal densities than wild grasslands. The authors suggest fungicides, and the phosphorus and nitrogen fertilisers used in industrial agriculture, as the most likely explanation; these inputs either kill hyphal networks directly or reduce the plant’s dependency on them by providing nutrients externally, causing the fungi to be selected against over time.
The map cannot yet identify which specific agricultural practices cause the greatest damage. But the pattern is clear: intensive agriculture is associated with a dramatic reduction in the fungal network that normally occupies the same topsoil.
“These are areas that people are really ripping up because it’s much easier to rip up a grass than it is to rip up a tree,” Stewart told Live Science. The implication is not merely ecological. If wild grassland topsoil contains 40% of global fungal biomass, and that biomass is associated with absorbing 11% of annual fossil fuel emissions through carbon channelling, then the conversion of wild grassland to cropland is not just a biodiversity event; it is a carbon event, removing a major sequestration mechanism from the climate system.
The first map of a system we knew existed but could not locate. Before this study, the scale of arbuscular mycorrhizal fungal networks was known only in the aggregate. The total estimated length of the hyphae had been calculated; the carbon absorption had been estimated; the symbiotic relationship with land plants had been well established for decades.
What was not known was where the networks were densest, where they were absent, and how their distribution related to land use, climate, and vegetation type across the whole planet. Stewart used an analogy to capture the absurdity of that gap: “That’s like saying we know every day 100 million cars move across Earth but we have no idea what road network facilitates that.”
The new map is the road network. It was made possible by combining the largest curated dataset of soil core hyphal density measurements ever assembled; 16,669 cores from six continents with AI prediction models trained on climate, soil chemistry, and vegetation data to fill in the spaces between measurement points. Every 0.4 square miles of topsoil on Earth now has an estimated fungal density on record for the first time.
What scientists outside the study said. The external scientific response was unusually emphatic. Andrea Genre, an expert in arbuscular mycorrhizal fungi at the University of Turin in Italy, told Live Science that the map was “urgently needed” and that it “can inform more efficient strategies for biodiversity conservation and restoration, agricultural management, and climate change mitigation.”
Edouard Evangelisti, a plant scientist at Côte d’Azur University in France, called it “seminal” research that “makes part of the invisible visible” and described the map as a “major milestone.” Evangelisti also identified the next question the map opens up: “The abundance of living hyphae is important, but for the carbon cycle, we also need to know how quickly these hyphae grow, die, and contribute to stable soil carbon.”
What the map cannot yet show — and where it goes next. The current version of the map has meaningful gaps. Some regions of the world, including tropical rainforests and deserts have insufficient soil core data to reduce uncertainty to acceptable levels. Stewart acknowledged this directly, telling Live Science: “Within the next five years, this map will be updated and we’re going to have a better picture of the distribution of these fungi.” The dynamic properties of the network; how quickly hyphae grow, die, and regenerate are also not captured by the static density measurements the map is built from.
But these limitations do not diminish what has been accomplished. A system of living threads so vast it would span a tenth of the Milky Way has been, for the first time, mapped to a global spatial resolution. It has been shown to concentrate in wild grasslands. It has been shown to collapse under industrial agriculture. And it has been shown to absorb carbon at a rate equivalent to 11% of annual fossil fuel emissions. The invisible has been made visible. What happens next depends on whether it is treated as the infrastructure it actually is.
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