Beneath the soil, a vast fungal network is moving nutrients, water and carbon on a scale scientists are only now starting to map.

Researchers have produced what they say is the first global estimate of arbuscular mycorrhizal fungi, or AM fungi, threads in soil, calculating a network that stretches about 110 quadrillion kilometres through the top 15 centimetres of Earth’s soils.

“It is hard to overstate the importance and enormity of these fungi,” said evolutionary ecologist Justin Stewart of the Society for the Protection of Underground Networks, SPUN, and the Vrije Universiteit Amsterdam. “There could be up to 10 meters (32 feet) of mycorrhizal network in just a teaspoon of soil.”

Mycorrhizal networks are made up of underground threads called hyphae. They run below the surface in forests and other plant communities, carrying nutrients such as phosphorus and water that plant roots cannot reach as easily, in exchange for carbon from plants.

Around 70 percent of all plant species rely on mycorrhizal symbiosis, according to the research.

To build the map, Stewart and his colleagues assembled data from 322 studies covering more than 16,000 soil cores across nine global biomes. The studies included more than 4,000 measurements of AM hyphal densities.

The team then used machine learning to predict the density of unseen AM networks around the planet. They also used robotic imaging to measure the thickness of more than 300,000 living fungal threads so they could convert network length into biomass.

“We have provided the first, to our knowledge, global estimate of AM hyphal densities, predicting 110 quadrillion kilometers of AM hyphae in the top 15 centimeters of Earth’s soils,” the researchers wrote in their paper.

The researchers estimated the network weighs about 300 million tons and acts as a pathway for roughly 4 billion tons of carbon dioxide equivalent moving from plants into underground ecosystems each year.

The map also pointed to an unexpected pattern. The highest densities were found in grasslands, prairies, steppes and wetlands, rather than tropical rainforests. The researchers estimated that 40 percent of the world’s total AM fungal biomass is concentrated in those areas.

They said herbaceous plants such as grass may channel more carbon to mycorrhizal fungi than woody plants do.

The study also found fungal network density was, on average, 47 percent lower in cultivated croplands. The researchers linked that to the use of fertilisers such as phosphorus and nitrogen, along with fungicides and farming practices that limit fungal presence.

The research also pointed to major gaps in what is known. Deserts, tropical forests and tundra remain poorly sampled, and most measurements come from the uppermost soil layers.

“Mycorrhizal fungi have shaped life on earth for hundreds of millions of years, but we still understand too little about how the infrastructure of these living transport systems is distributed across the planet,” said mycologist Merlin Sheldrake of SPUN and the Vrije Universiteit Amsterdam.

“This study is an exciting step towards understanding how this planetary circulatory system operates and suggests ways that we can better work with fungi to help address many of the unfolding challenges of our times, from food security to climate change.”

The research was published in Science.

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