Trees tend to get the spotlight when we talk about natural organisms absorbing carbon dioxide and protecting our world from harmful emissions, but underground fungi deserve some credit too! According to a new study, fungi have the ability to store a large amount of carbon underground, which sums up to 36 percent of yearly global fossil fuel emissions. This extraordinary ability highlights fungi’s important role in alleviating carbon emissions from our atmosphere.
Mycorrhizal fungi have a significant symbiotic relationship with the roots of almost all land plant species. They form a vast underground network with thread-like filaments called mycelium, connecting with plant roots and transferring water and nutrients. In exchange, the fungi receive sugars from the plants produced by photosynthesis. This partnership between plants and fungi has existed for over 400 million years and is critical to the development and functioning of ecosystems worldwide.
Though science has determined how these intricate connections work between plants and fungi, only recently have they explored how fungi help more carbon into the soil. Over 75% of the Earth’s carbon pool is stored underground (1). Fungi are a key gateway for carbon to enter soil systems. When plants photosynthesize, they take in carbon dioxide from the atmosphere and convert it into several organic compounds, such as sugars. The mycorrhizal fungi take some of these sugars and carbon, though a large portion is transferred into the soil. The carbon is then stored underground for up to several centuries. Mycorrhizal fungi work strategically and provide more nutrients to the plants offering them more carbon. As a result, plants will give more carbon to these fungi, effectively removing more CO2 from the air and locking it into the ground rather than allowing it to fill our atmosphere.
Mycorrizhal fungi are classified into two types: endotrophic and ectotrophic. Endotrophic fungi, also known as arbuscular mycorrhiza, penetrate the cell walls of their plant host to form internal networks with their host. They associate with most plants and help with their growth, creating more plant biomass and leading to higher absorption of CO2 emissions. Ectomycorrhizal fungi do not penetrate the cell walls of their hosts but instead form a sheath around the roots of the plants, extending out into the soil. These types of mycorrhizal fungi only interact with around 2% of plant species; however, soils that predominantly consist of these fungi contain 70% more carbon per unit of nitrogen. Ectomycorrhizal fungi draw out nitrogen faster and more efficiently, allowing them to outcompete other microbes in the soil. As a result, they slow down the decomposition of plant matter, which keeps carbon in the ground for longer periods of time (2).
For the first time, scientists have determined an estimate for the amount of carbon that fungi have been helping store beneath the ground. The study, published in Current Biology, conducted a systematic review of almost 200 datasets to make the first worldwide quantitative estimate of how much carbon mycorrhizal fungi receive from plants. Researchers analyzed studies focusing on the global soil carbon pools from different types of mycorrhizal functions. They found that fungi use three main mechanisms to increase carbon in the soil.
First, by building and supporting an active mycelial network, mycorrhizal fungi draw plant-fixed carbon into the soil. Not only does this network transport carbon, but it also moves it away from zones with high respiratory activity into areas that can be stored more safely. Next, when this network dies, it leaves behind a complex structure or organic material known as necromass. Although the mycelium no longer transports carbon, it helps form and stabilize soil aggregates. When soil attaches to mycelial necromass, organic matter is protected from further decomposition, stabilizing carbon within the ground. Finally, mycorrhizal fungi exude compounds as they grow through soil environments that help retain carbon. These exudates are made from carbon and nitrogen, which are used by other microbes within the soil. When other microbes consume the exudates, they immobilize them into a more stable form of soil carbon called mineral-associated organic matter. Once carbon is in this form, it gets attached to tiny bits of mineral in the soil, protecting it from being further broken down.
Researchers estimate that over 13 gigatons of carbon dioxide move through plants into carbon which is then allocated in mycorrhizal mycelium every year. Though the study has methodological limitations that may impact the exact amount of this figure, the researchers have concluded that mycorrhizal fungi significantly contribute to worldwide carbon fluxes and should be studied further within both global climate and carbon cycling models and used in conservation practices (3).
The findings from this study are just the beginning of a new focus on investigating fungi’s role in carbon sequestration. One current project, led by the University of Sheffield’s School of Biosciences, is looking more into detail about the role of mycorrhizal fungi in soil carbon and other nutrient cycles. Specially designed outdoor field experiments that imitate future climates will help the researchers understand how long fungi and other microbes can store carbon in the ground and how climate change will affect these results.
Dr. Heidi Hawkins, the lead author of the study from the University of Cape Town, shared with EurekAlert:
“We always suspected that we may have been overlooking a major carbon pool. Understandably, much focus has been placed on protecting and restoring forests as a natural way to mitigate climate change, but little attention has been paid to the fate of the vast amounts of carbon dioxide that are moved from the atmosphere during photosynthesis by those plants and sent belowground to mycorrhizal fungi.”
This oversight of fungi’s role in ecosystem health has slowed our efforts in safeguarding these fungi and the soils they enrich. There is still much to uncover about the permanence of carbon residing in mycorrhizal structures. However, the new knowledge of this massive carbon pool catalyzes a greater understanding of how fungi significantly support all life on earth and reduce fossil fuel emissions.
Co-author of the study, Dr. Katie Field from the University of Sheffield, explains, “Soil ecosystems are being destroyed at an alarming rate through agriculture, development, and other industry, but the wider impacts of disruption of soil communities are poorly understood. When we disrupt the ancient life support systems in the soil, we sabotage our efforts to limit global heating and undermine the ecosystems on which we depend.”
Scientists are now urging that fungi be included in policies regarding biodiversity and conservation due to their significance in reducing carbon emissions. According to the UN, 90% of the earth’s topsoil could be degraded by 2050, which would not only be a disaster for reducing climate change but also seriously harmful to food crops and plant life.
Now more than ever, it’s crucial to apply these recent findings into actionable policies that will safeguard our soils and preserve the fungi that inhabit them. This approach will not only reduce carbon emissions from our atmosphere into the ground but will also ensure the protection of global biodiversity.