Hidden underground around the world lie 110 quadrillion kilometers of arbuscular mycorrhizal fungal networks—webs of ultra-thin threads that, if connected in a single line, would stretch almost a billion times thge distance between the Earth and the sun, according to new research published in Science on Thursday.
These fungal communities form intimate relationships with the roots of plants, which they provide with nutrients like phosphorus and nitrogen in exchange for carbon, 1 billion tons of which the networks sequester underground annually, previous research has found. If the fungal network wasn’t storing it, that carbon would be warming the atmosphere.
But those networks have never been mapped globally until now. The
new study led by Society for the Protection of Underground Networks, or SPUN, an organization founded to map mycorrhizal fungi networks, used a combination of literature review, soil samples from around the globe, machine learning and laboratory testing to estimate the distribution and mass of these systems and map where they are densest.
“This is the moment where we went from knowing that this system exists to really knowing where it is, how dense it is and where it’s been,” said Toby Kiers, executive director and co-founder of SPUN and a co-author of the study.
For decades, researchers have known arbuscular mycorrhizal fungi form intimate symbiotic relationships with roughly 80 percent of the globe’s plant species and are found nearly everywhere plants are. But the extent of those networks and where they are densest, such as grasslands, and where they are being lost, like in agricultural areas, hasn’t been well understood until now.
“[The study] helps us come to grips with how important these below ground organisms can be to everything that we see above ground,” said James Bever, a professor of ecology and evolutionary biology at the University of Kansas who studies the interactions between plants and microbes like fungi in soils but was not involved in the new study.
Justin Stewart, an evolutionary ecologist at SPUN and lead author of the study, said previous studies the team had done on biodiversity of fungi were similar to asking someone to describe the forest outside their home.
Justin Stewart, an evolutionary ecologist at SPUN, samples soil in Tucson, Ariz. Credit: John Burcham/SPUN
“They could say ‘well there are three tree species in it.’ That’s great. That tells me about the biodiversity,” he said. “But you don’t actually know how big the forest is, how far apart the trees are. You don’t have information on its structure.”
Mycorrhizal fungal networks are made up of hyphae, each smaller than a strand of human hair. These living pipes transport the nutrients and carbon between the plants and fungi.
Because they are so long and thin, Stewart said, they can reach deeper into soils than roots, getting nutrients deep underground that plants can’t reach, while simultaneously storing carbon where it can stay put for a long time under the right conditions.
“You’re getting a win-win,” Stewart said. “The plants are growing better, and carbon’s being drawn down. That all depends on having dense fungal networks and soils that are active and alive.”
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Quantifying these fungal networks started with a review of existing studies on mycorrhizal fungi. Those studies contained 16,000 core samples taken from ecosystems around the world to understand the length of the fungal threads in a volume of soil. Each sample was geolocated, and from there the team was able to use machine learning to create predictive maps of fungal networks globally, and identify where the model is performing well and where uncertainties show more data is needed.
Working with AMOLF, a research institute in Amsterdam, they developed a technique using a robot with a camera that recorded fungal networks growing over time in a lab, to get better estimates of their widths. From there, the team was able to calculate the network’s mass, which amounted to about five times the weight of all humans on Earth.
The study only covers living arbuscular mycorrhizal fungal networks, Stewart said, and doesn’t include dead fungal networks, which also help to store carbon and add to the total biomass and influence of the networks on ecosystems. Research into dead fungal networks is still being explored.
The study also found where these networks are most threatened. Fungal network densities across croplands are about half of what they are in wild ecosystems. Meanwhile, wild grassland ecosystems hold about 40 percent of the world’s arbuscular mycorrhizal biomass. Yet those grasslands are among Earth’s least protected ecosystems, and they are converted into farmland at four times the rate of forests, posing a potential threat to these networks and the benefits they bring to plant life and carbon storage.
What exactly is driving mycorrhizal fungi losses, and the consequences of that decline, need to be explored next, the researchers said, which is why the SPUN team will be at this year’s United Nations Climate Change Conference—COP31—to present to policymakers about the importance of the networks and the role they could play in protecting ecosystems and sequestering carbon.
Understanding mycorrhizal fungi more deeply at the ground level is key, said Corentin Bisot, an AMOLF biophysicist and co-author of the study.
“We’re still far from completely understanding how, if you have a grassland next door, and you want to [increase] microbes and fungi there,” Bisot said. “We don’t have the toolbox for you to do it.”
This study, Stewart said, is just the first map. And like the first maps the Spaniards drew of California—which presented the state as an island, he said, there will be new discoveries about the density of fungi networks around the globe to grow the public’s understanding of them.
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