What thread is longer than the galaxy is wide but is hidden six inches underground, holds soil together, and has been in a successful relationship for close to 500 million years?
The Mycorrhizal Fungi!
My adventure with planting rose bushes has activated a learning path I didn’t know I needed! As I learn more about the ways my foliage does or doesn’t survive, I’m learning more and more about soil. I never really gave much thought to the significant activity happening below the surface and the massive impact it has on the world above.
Threads, Not Roots
Mycorrhizal fungi aren’t one species, or even a single fungus. They’re a whole group of fungi that form symbiotic partnerships with most land-based plant and tree roots. The threads wrap around roots and spread out into the dirt like an underground spider web. Some of the fungi wrap around the outside of the roots and some grow into root cells and branch out from there. In exchange for the sugar the plant provides to the fungi, the fungi help the plants get water and nutrients.
The Fungal Version of a Meet-Cute
Like any good relationship, it’s all about the meet up and the underground version of “how YOU doin’?” The hyphae (the tips of the fungal threads) sense chemicals coming from a nearby plant root. The hyphae then curl toward the root and wrap around it and may even grow into the root cells. Inside the cell, the fungi form bushy, branching structures called arbuscules, which is where the magic happens. In the arbuscules, the plant sends sugars made in its leaves down to the roots and into the fungi and the fungi use their long network of threads to forage for water and nutrients like phosphorus, nitrogen, and potassium in pockets of soil the root can’t reach. They then share those resources back into the plant through the same connection. Like any healthy relationship, the give-and-take isn’t fixed; both partners adjust how much they give based on what they’re getting. If the plant sends more sugar, the fungi typically send more nutrients back. If nutrients are scarce, the fungi may hold onto more of them. Over time, the fungi develops into extended arms for the plant’s root system, letting the plant access resources it could never reach alone.
The Hidden Carbon Bonanza
Worldwide, plants and trees send an enormous amount of carbon underground to mycorrhizal fungi every year. Researchers estimate that, globally, roughly 13 billion tons of carbon dioxide are sent underground to mycorrhizal fungi every year. Sometimes the carbon returns immediately through respiration. Other times, it gets folded into fungal biomass and soil processes instead.
Regardless, the scale is significant. Soil stores far more carbon than the atmosphere, and the fungi help move, shape, and stabilize that carbon in ways climate models are still working to capture properly. The fungi also help bind soil into stable clumps. The clumps, called aggregates, create pockets for air and water to move freely, supporting plant roots and soil organisms. They hold onto carbon and nutrients, making them a vital indicator of soil fertility. They are also far less prone to washing or blowing away compared to loose, unstructured soil. Scientists are still exploring this function, but put simply, mycorrhiza fungi do not just live in the soil. They actively build soil fertility.
How Ecosystems Hold Together
Ecosystems are not just a collection of separate plants or trees. They are networks whose stability depends on strong root systems and good soil structure that helps keep water in the ground. Mycorrhiza fungi are critical to keeping the networks connected. They improve nutrient uptake, especially for phosphorus and other hard-to-access resources, which help plants and trees grow more strongly and establish more reliably in poor or disturbed soils. They also help plants and trees handle drought, disease, and other stress, which is why they are so important in places where conditions are changing fast.
When plants do better, the insects, birds, mammals (including humans), and microbes that depend on them do better too. So, while the fungus may be hidden underground, the ripple effect of its benefits extend far and wide.
Diversity Matters
Different species create different root environments, leak different kinds of carbon into the soil, and support different fungal partners. Mixed ecosystems usually build a more varied underground community than a simple one. The resulting fungal diversity improves nutrient cycling efficiency by spreading the work across multiple species and strategies.
Some fungi are better at breaking down organic matter, some are better at moving phosphorus or nitrogen, and some are better at functioning under different moisture or soil conditions. When all of these species coexist, the soil can more richly capture, transform, and recycle nutrients through more pathways. For example, a forest with richer tree diversity will usually have richer fungal diversity, with the relationships described above reinforcing one another.
Underground diversity is especially important in drought. Mycorrhizal networks increase the effective reach of roots, so plants and trees can keep accessing water and mineral nutrients from soil pockets that roots alone would miss. In some cases, fungi may also help move water through shared underground connections, giving stressed plants and seedlings a better chance of hanging on when the surface soil dries out.
Once we understand these relationships, we can see that forest resilience isn’t as abstract as it may seem. A forest with a mix of species and mycorrhizal types will likely handle drought better simply because the trees are not all relying on the same pathway for survival. Fungi don’t “solve” drought issues, but they can help soften the impact and make recovery faster once hydration returns.
Underground ecosystems are hard to map and the risk of fungal depletion in degraded ecosystems is significant. Depleted soils are less stable, slow plant recovery, increase erosion, and are harder to restore. The microbial balance is also disturbed, raising the possibility of disease. (View a map showing the predicted density of underground networks created by arbuscular mycorrhizal fungi).
Mycorrhizal fungi have been shaping plant life for hundreds of millions of years and likely helped early species make the jump from the ocean onto land. That kind of staying power matters. An organism this old, this widespread, and this essential is worth our attention.
Every time I water or trim my rose bushes, I’m reminded that so much more is happening than I will ever see, and it makes having these living ecosystems right out my front window so much more special.
Sources
- Bioneers / SPUN: Mapping Earth’s Hidden Allies: SPUN’s Mission to Protect Mycorrhizal Fungi and Combat Climate Change
- Eos.org: Soil Fungi May Be a Carbon Pool
- Frontiers in Forests and Global Change: Mycorrhizal Symbiosis for Better Adaptation of Trees to Abiotic Stress Caused by Climate Change in Temperate and Boreal Forests
- Hawkins et al. (2023): Mycorrhizal Mycelium as a Global Carbon Pool — Current Biology
- Inside Climate News: Threads of Earth’s Underground Fungal Networks Are Long Enough to Reach Beyond the Solar System
- Mongabay: Mycorrhizal Fungi Hold CO2 Equivalent to a Third of Global Fossil Fuel Emissions
- Mongabay: Mapping Underground Fungal Networks — Interview with SPUN’s Toby Kiers
- Society for the Protection of Underground Networks (SPUN)
- Wikipedia: Society for the Protection of Underground Networks
