The ground beneath us is far from static. It teems with intense activity, largely driven by a partnership between plants and fungi that dates back at least 450 million years.
This is the mycorrhizal network – an extensive web of fungal threads in constant exchange with the plant life covering our planet. It transports nutrients from the soil, receiving carbon from plants in return, a byproduct of their photosynthesis.
This is a vast and vital process. Approximately 70 percent of all plant species rely on mycorrhizal symbiosis.
Now, for the first time, scientists have created a global map of this hidden infrastructure. They’ve revealed an underground network of arbuscular mycorrhizal (AM) fungal hyphae stretching across approximately 110 quadrillion kilometers of Earth’s soil. The findings of this study were published in the journal Science.
That distance is more than enough to traverse the 150 million kilometers from Earth to the Sun nearly a billion times.
“It’s hard to overstate the importance and the scale of these fungi,” remarked evolutionary ecologist Justin Stewart of the Society for the Protection of Underground Networks (SPUN) and the Vrije Universiteit Amsterdam. “A single teaspoon of soil can contain up to 10 meters of mycorrhizal network.”
Fungal networks consist of thread-like structures called hyphae that run beneath the surface in forests and other plant communities, often playing a crucial role in their flourishing.
They transfer nutrients like phosphorus, along with water that plants’ roots might otherwise struggle to access, in exchange for carbon acquired from the plants.
This makes the “wood wide web” an integral part of our planet’s carbon cycle, but its overall influence is difficult to gauge without understanding the network’s sheer size.
Stewart and his colleagues embarked on an ambitious effort to discover just that.
They synthesized data from 322 studies, encompassing over 16,000 soil samples from nine distinct global biomes. These studies included more than 4,000 measurements of AM plant hyphal density, enabling the researchers to identify patterns in the conditions where these networks are most likely to be found.
Subsequently, they employed machine learning to predict the density of unmapped additive microorganism networks globally. They also utilized robotic imaging to measure the thickness of over 300,000 living fungal threads, allowing them to translate network length into biomass.
The results were striking.
“To our knowledge, we have provided the first global estimate of AM-mycorrhizal hyphal density, predicting the presence of 110 quadrillion kilometers of AM-mycorrhizal hyphae in the top 15 centimeters of the Earth’s crust,” the researchers wrote.
This network is estimated to weigh 300 million tons – an amount four to six times the total living biomass of humans. Annually, it’s projected to facilitate the transfer of approximately 4 billion tons of carbon dioxide equivalent from plants into subterranean ecosystems.
However, the most surprising revelation came from observing where these networks were most prevalent.
Instead of being concentrated primarily in tropical rainforests, the highest density of mycorrhizal fungi was discovered in areas like grasslands, prairies, steppes, and wetlands. It’s estimated that about 40 percent of the world’s total mycorrhizal fungal biomass resides in these locations.
The researchers hypothesize this is because herbaceous plants, such as grasses, channel more carbon to mycorrhizal fungi compared to woody plants.
More concerning is the finding that fungal network density was, on average, 47 percent lower in cultivated agricultural land.
This is likely attributable to fertilizer use (phosphorus and nitrogen), fungicides, and farming practices that impede fungal presence. The long-term repercussions could include a diminished capacity for soil to store carbon and transport nutrients.
Nevertheless, much remains unknown.
Significant portions of the globe, including deserts, rainforests, and tundra, are still poorly studied. Furthermore, most measurements are confined to the uppermost soil layers, implying that deeper strata hold substantial mysteries.
All these factors complicate precise quantification of the unexplored underground network. It’s also important to note that our understanding of the “wood wide web” is far from complete.
Despite these limitations, this work offers the most comprehensive map to date, shedding light on the hidden activity beneath our feet and highlighting the vast amount we still have to learn.
“Mycorrhizal fungi have shaped life on Earth for hundreds of millions of years, yet we still have a very limited understanding of how this living transport system’s infrastructure is distributed across the planet,” stated mycologist Merlin Sheldrake of SPUN and the Vrije Universiteit Amsterdam. “This study marks a critical step towards comprehending how this planetary circulatory system functions and presents avenues for more effective engagement with fungi to address many of our era’s pressing challenges, from food security to climate change.”
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