According to the fossil record, plants took their first steps onto land 500 million years ago. They hadn’t yet evolved roots or vascular tissues, and they possessed anatomies more suited for buoyancy than gravity. While they eventually adapted into the diverse array of flora we know today, early botanical pioneers were largely ill-equipped for life out of—or far from—water.
Thankfully for them, their transition onto land wasn’t done alone. Fungi appear in the fossil record long before land plants emerge, and they served as important partners needed for plant survival on land. They played the role of a rudimentary root system, helping many early plants absorb water and nutrients from the barren soils of our ancient earth. In exchange, plants rewarded these fungi with the energy-rich carbon they produce through photosynthesis. As time went on, this relationship between plant and fungus grew intimate and co-dependent.
In fact, most modern plants still aren’t that great at absorbing nutrients on their own. More than 90 percent of plant species still depend on mutualistic fungi today. Likewise, these fungi are also dependent on their plant hosts. They’re so specialized in foraging for nutrients like nitrogen and phosphorus to “trade” with their host for carbon that they have largely lost their ability to acquire carbon from traditional sources like wood, leaves, and other organic materials. It’s almost as if they’ve lost their ability to chew and digest solid food, so they derive energy rich carbon via an IV connected to the plant. This saves them from having to compete for carbon with other fungi and bacteria, of which there are plenty.
These mutualistic fungi are more properly known as mycorrhizal fungi. Derived from Greek, mycorrhiza translates to “fungus root.” They’ve garnered public attention due to their ability to form extensive networks in the soil that allow plants to communicate and exchange nutrients with each other. Without mycorrhizal fungi, we wouldn’t have forests, woodlands, jungles, prairies, or most forms of vegetation as we know them today. So many of our agricultural crops depend on them too.
As of late, researchers have begun recognizing another exciting role of mycorrhizal fungi that extends beyond a single plant or ecosystem; they’re notable players in our planet’s carbon cycle.
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Some recent estimates suggest that mycorrhizal fungi are given the equivalent of 13.12 gigatons of carbon dioxide by their plant partners every year—an amount comparable to 36 percent of yearly fossil fuel emissions. This finding emphasizes a need to further understand the intricacies of these fungi and protect them in the face of global climate change. Yet, understanding how they fit into the broader carbon cycle remains a complex and evolving challenge.
Understanding Carbon
Carbon is the fundamental building block to all life as we know it, the central element on which life’s chemistry depends. With its ability to form strong and complex bonds, it offers the properties needed for the development of even the simplest life forms. Carbon is a vital source of fuel for life on Earth and that energy can be stored in simple sugars as well as fossil fuels.
Beyond biology, carbon also plays a crucial role in shaping our planet’s climate. In the form of carbon dioxide, it functions as a potent greenhouse gas, trapping heat and impacting global temperatures. While carbon dioxide comprises just 0.042 percent (424 ppm) of our atmosphere—up from 0.028 percent in the 1700s—its influence is profound. With too little carbon dioxide, Earth can become a frozen wasteland, and with too much, a sweltering inferno. Indeed, life thrives best within a thin range of carbon dioxide in the atmosphere.
This is why scientists are urgently studying the dynamics of carbon on our planet. Where carbon is located and how it moves between different pools is fundamental to understanding our climate. By measuring and quantifying the flow of carbon, scientists build models which can help us protect carbon resources and predict future climate scenarios.
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At the heart of these equations lies Earth’s soils. They are one of our planet’s largest carbon reservoirs, storing more of it than all living plants and the atmosphere combined. Despite their vast importance, soils remain one of the least understood and difficult to predict factors in global carbon dynamics.
The Missing Carbon
When you walk through the forest, it’s easy to sense that it holds a significant quantity of carbon. You see evidence of its presence in the trees and the shrubbery beneath the canopy. It’s in the leaf litter and the woody debris rotting on the soil surface. About half the dry weight of all this plant matter is composed of carbon. Even if only faintly, it’s possible to grasp the magnitude of its volume and mass.
Yet, the same can’t be said for the carbon that exists below ground. Unless you begin to dig, it’s out of sight, and even then, you only open a small window into the vast subterranean organic structure of the soil.
Fungi, and their network of mycelium, are even more difficult to put in perspective. Mycelium, the white branching network of fungi, is the actual body of the organism. You’ve likely seen it under logs when gathering fuel for a campfire, or beneath mulch in your garden. It’s where most fungal mass is held and where all day-to-day activity occurs. The mushrooms which we see above-ground, are only ephemeral reproductive structures akin to a fruit. In some cases, individual threads of mycelium can be many times thinner than a human hair and delicately interwoven into sediments and organic debris. Plant roots can be unearthed and examined—mycelium is delicate and fragile. Once in the soil, this carbon can reside there for years, decades, and potentially longer.
Quantifying Mycorrhizal Carbon
The challenge then, is quantifying the hidden flow of carbon. In a 2019 study researchers from the University of New Hampshire and Stanford attempted this directly. They wanted to see whether mycorrhizal fungi might be responsible for carbon that could not be accounted for by previous research cited in their publication. Researchers had already suspected that this “missing carbon” could be due to mycorrhizal fungi.
Among others methods, the team used what’s called the “mass balance approach,” which relies on measuring carbon emissions from soil to get an approximate estimate of the total carbon it receives from plants and fungi. It relies on the literature-backed assumption that the quantity of carbon entering the soil is roughly equivalent to that naturally emitted by the soil as carbon dioxide—a concept known as the “total belowground carbon flux.”
Simultaneously, the researchers measured all possible carbon inputs contributed by plants. They set up tarps and baskets in their marked forest plots to measure leaf litter and branch fall. They also accounted for the contribution of roots by measuring their growth in soil plugs filled with a rootless substrate. Finally, they calculated mycorrhizal carbon by subtracting the measured plant inputs from the total carbon input calculated by soil emissions. Put simply, total carbon minus plant carbon equals mycorrhizal carbon.
To supplement their findings, the team analyzed plant tissues for molecular fingerprints that reflect the contribution of nutrients by mycorrhizal fungi to the plants. The team also utilized data from previous surveys on truffle production in the forest which added an additional metric to their analysis.
The study found that some of the sampled forests allocate up to 30 percent of all their carbon to mycorrhizal fungi. This is aligned with numbers previously seen in the scientific literature. Yet not all forests in their plots were this generous. More specifically, coniferous forests, which tend to occur in low nutrient soils, allocated much more carbon to mycorrhizal fungi than broadleaf forests. In essence, conifer-dominant forests, which had less above-ground biomass in this study, have much more carbon invested in mycorrhizal fungi. The reason is that they, like early land plants, heavily depend on fungi to acquire nutrients from infertile soils.
Mycorrhizal Carbon on The Global Scale
Single studies rarely provide enough information to understand systems like these on a global scale. It’s essential to have as many data points as possible to capture the immense variability and complexity of fungal networks around the world. To this end, researchers are pioneering efforts to protect, map, and understand mycorrhizal fungi globally. As Michael Van Nuland, Lead Data Science Lead at the Society for the Protection of Underground Networks (SPUN) explains, “Our mission, broadly speaking, is to help protect, harness, and map these mycorrhizal fungi around the world.” By building predictive maps using machine learning and collaborating with a global network of scientists, SPUN is looking for hotspots of mycorrhizal activity and identifying gaps in current conservation efforts.
Van Nuland emphasizes the potential applications of these findings, stating, “If we had better information about fungi and how they intersect with soil carbon cycles, we might be able to build better, more sensitive, more specific parameters into global climate models that could reduce some of that uncertainty and lead to better predictions.”
Protecting Mycorrhizal Networks
Given their critical role in carbon storage, protecting and understanding mycorrhizal fungi is vital for addressing the threat of climate change. Unfortunately, fungi have often been overlooked in conservation frameworks due to their hidden nature and the historical focus on plants and animals.
In October 2024, at the UN Convention on Biological Diversity, the governments of Chile and the UK made a collaborative call to action for the conservation of fungi. In their “Fungal Conservation Pledge,” which was supported by an additional nine countries, they emphasize the importance of fungi for climate change, biodiversity loss, and pollution, and the urgent need to implement policies to protect them. In particular, they seek the recognition of fungi as an independent kingdom that should be included in conservation and regulatory frameworks.
The Fungal Conservation Pledge was spearheaded by the Chilean-born organization Fungi Foundation, a partner of SPUN. For years, they’ve advocated for fungi to be included in biodiversity targets and conservation frameworks, noting that the concept of “flora and fauna” excludes fungi from the discussion. So far, they’ve seen the addition of “funga” in numerous national frameworks and the adoption of the term by the International Union for Conservation of Nature (IUCN).
While fungi are pivotal for biodiversity and ecosystem health, their role in the carbon cycle inserts additional urgency to their conservation needs. Understanding their important contribution as a carbon sink and their role in climate-carbon dynamics further highlights why we must do what we can to protect these vital organisms.
Researchers in collaboration with conservation groups can help our global community incorporate fungi into climate models, conservation strategies, and the development of sustainable land-use strategies. Perhaps it’s time we learn from the early botanical pioneers and begin forming a strong alliance with fungi.
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