Underground fungal networks have been overlooked as one of Earth's largest terrestrial carbon sinks

In June 2023, SPUN and members of our Science Associates program published a paper called “Mycorrhizal mycelium as a global carbon pool.” The work was led by Dr. Heidi-Jayne Hawkins, and it’s open access, so anyone can read it at this link. This paper was covered by major media outlets such as Bloomberg, the Los Angeles Times, and The Atlantic among others. These efforts are really important for our field because it’s the first time we have been able to quantify and evaluate the total amount of carbon traded from plants to mycorrhizal fungi on Earth.

Mycorrhizal mycelium as a global carbon pool

We've known for quite some time that carbon flows from plants into mycorrhizal fungi. It’s one of the central pieces to this type of plant-fungal symbiosis. But until now, we haven't had a good global estimate of how much that flow of carbon is. There have been some back-of-the-envelope calculations and small-scale studies, but the numbers varied a lot. With this review, our goal was to synthesize all the data currently out there to try and better understand this overlooked component of the carbon cycle.

Before I get into the numbers, I want to lay out how we know that mycorrhizal fungi are holding carbon. Plants photosynthesize using sunlight and carbon dioxide from the atmosphere and convert them into energy. During that process, the plants fix carbon – turning it from its gaseous form into organic carbon compounds. The plants then use this carbon to build their structures. Flowers, leaves, stems – those are all made from organic carbon compounds.

Plants allocate carbon to fungal partners

But plants don’t use all the carbon they fix. They are left with a surplus, and they send that surplus belowground through their roots. Mycorrhizal fungi are there to receive it. Between the plant roots and the mycorrhizal fungi they are associated with, there is a complex pattern of trading, selling, and sharing. The plants give their carbon to the fungi, and the fungi funnel them nutrients that are crucial for plant growth. Since fungi can’t photosynthesize, this relationship is how they get basically all the carbon they need to grow and build up their biomass.

Harmonizing data: global carbon flows

What we did with this paper was estimate how much carbon plants are allocating to their fungal partners every year, pulling together as much published data that we could find. We looked primarily at three different types of mycorrhizal fungi – arbuscular, ectomycorrhizal, and ericoid, and were able to find that collectively, these three groups of fungi have 13.12 billion tons of carbon dioxide allocated to them every year.

To put this number in perspective: 13.12 billion tons of CO2 is about 36% of global fossil fuel emissions last year. China is by far the biggest emitter of greenhouse gasses – its annual emissions in 2021 were 12.47 billion tons. The U.S. emitted 4.75 billion tons of carbon dioxide in 2021 – mycorrhizal fungi take up nearly three times that each year. It’s a tremendous amount of carbon (source).

Underground carbon drawdown solutions

This has really changed the way I think about carbon solutions. Aboveground climate solutions are only going to get us so far. For example, electrifying transportation, decarbonizing energy grids, cutting emissions, improving agricultural practices, increasing forest stocks, protecting existing natural systems, etc - these are all important and necessary goals. But even if they are achieved, there will still be a gap in the carbon budget that needs closing. In other words, we also need to be innovating for new carbon drawdown solutions.

The big insight from this paper is that there is a living system underground, evolved to pump carbon from plants into soil, already operating at a scale that is making a real impact on our collective climate story. It’s really exciting to imagine how we can somehow work with, or partner with, or innovate with these organisms to help reach net zero goals.

We've known for a long time that soils store immense amounts of carbon, more than plants and the atmosphere combined. But what we are now realizing is that the position of fungi, straddling above and belowground worlds, is potentially a way for us to harness connections between above and belowground carbon flows.

The microbial life underground has largely been ignored in carbon calculations in the past. When people count up how much carbon is in an ecosystem, they almost exclusively focus on aboveground carbon accounting and plant biomass stocks. But we know that most plants on the planet partner with mycorrhizal fungi and send carbon to their fungal partners in the soil. So, we think that focusing on these mycorrhizal fungi, because of their position as a main entry point of carbon into soil, provides ways we can work with natural systems. 

Belowground mycorrhizal fungi in carbon management plans

This work, by itself, does not provide a solution yet. But it does show us the raw scale and potential for mycorrhizal fungi to draw carbon down into the soil. It’s important to understand that we are talking about dynamic systems, with carbon flowing in and out. Not all of this carbon is locked underground forever. For example, some of the carbon allocated to mycorrhizal fungi flows back out of soil through respiration, rejoining the atmospheric carbon dioxide pool. We don’t know how much yet, but this (among other things) will be key for considering mycorrhizal fungi in any carbon management plan.

What we can learn from underground ecosystems

I am amazed by how little we still know about carbon trading between plants and fungi. It’s incredible that even with Earth’s carbon cycle – something we've spent decades studying and quantifying – there is still room for major discoveries. This study has helped make something invisible, visible, and it’s only the beginning – there’s still so much we don’t know about underground ecosystems.