Global Map Reveals 110 Quadrillion Kilometres of Mycorrhizal Fungi

The soil underfoot is far from still.

It crackles and throbs with intense activity, much of which is powered by an alliance between plants and fungi that began at least 450 million years ago.

Known as the mycorrhizal network, this immense web of fungal filaments has a mutually beneficial relationship with the vegetation covering Earth: it carries nutrients out of the soil while receiving carbon made through plant photosynthesis in return.

This system is both enormous and essential. About 70 percent of all plant species depend on mycorrhizal symbiosis.

Global map of the mycorrhizal network

For the first time, researchers have created a worldwide map of this concealed infrastructure. It shows that threads of arbuscular mycorrhizal fungi (AM) extend for an estimated 110 quadrillion kilometres through the planet's soils.

That distance could cover the 150 million-kilometre (93 million-mile) journey between Earth and the Sun almost a billion times.

"It is hard to overstate the importance and enormity of these fungi," says 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."

Fungal networks are formed from subterranean strands known as hyphae, which spread beneath the soil in forests and other plant communities. They are frequently vital to the success of those communities.

In exchange for carbon supplied by plants, they move nutrients including phosphorus, along with water that plant roots would normally be unable to reach.

As a result, the 'wood wide web' is a crucial component of the global carbon cycle. Yet its contribution has been difficult to measure without knowing the network's true scale.

Stewart and his team set out to answer that question in an ambitious study.

They brought together evidence from 322 studies, covering more than 16,000 soil cores from nine global biomes. Across those studies were over 4,000 measurements of AM hyphal density, enabling the scientists to identify the circumstances in which these networks are most likely to occur.

The team then applied machine learning to estimate the density of unobserved AM networks around the world. Robotic imaging was also used to assess the thickness of more than 300,000 living fungal threads, making it possible to translate network length into biomass.

The figures were vast.

"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 write in their paper.

AM fungal biomass and carbon transport

The network is estimated to weigh 300 million tonnes - four to six times the total biomass of living humans - and provides a route for roughly 4 billion tonnes of carbon dioxide equivalent to pass from plants into underground ecosystems annually.

The most striking result, however, concerned where the networks were densest.

Instead of being concentrated most heavily in tropical rainforests, the greatest densities appeared in grasslands, prairies, steppes and wetlands. These environments contain an estimated 40 percent of global AM fungal biomass.

The researchers think this reflects the fact that herbaceous plants, including grasses, direct more carbon towards mycorrhizal fungi than woody plants.

More concerningly, fungal network density was on average 47 percent lower in cultivated croplands.

This may be linked to phosphorus and nitrogen fertilisers, fungicides, and agricultural methods that restrict fungal populations. Over time, this could diminish soils' ability to store carbon and transfer nutrients.

Gaps in knowledge of underground fungal networks

Even so, major uncertainties remain.

Substantial regions are still inadequately sampled, among them deserts, tropical forests and tundra. Furthermore, most observations have been taken from the very top layer of soil, leaving deeper levels poorly understood.

Consequently, it remains hard to establish precisely how much of the underground network has not yet been counted. There is also still much to discover about the 'wood wide web'.

Nevertheless, the project is the most comprehensive map yet produced. It illuminates the unseen processes occurring in the darkness below us, while underscoring how much there is left to understand.

"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," says 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.