Mycorrhizal fungi hold CO2 equivalent to a third of global fossil fuel emissions

Plants and fungi struck a deal way back when. More than 400 million years ago, plants began trading sugar made from sunlight (a.k.a. carbon) for some of the soil nutrients gathered by mycorrhizal fungi. Nearly 90% of all land plants are now part of this arrangement, so scientists estimated that the amounts of carbon flowing through underground fungi must be significant. However, they didn’t realize how much carbon was in the system until now.

According to a recently published study in Current Biology, more than 13 billion metric tons of CO2 is passed from plants to mycorrhizal fungi each year — equivalent to around 36% of all annual global fossil fuel emissions.

“We always suspected that we may have been overlooking a major carbon pool,” said study co-author Heidi Hawkins, research lead at Conservation South Africa and research associate on plant-soil-microbe interactions at the University of Cape Town.

“Understandably, much focus has been placed on protecting and restoring forests as a natural way to mitigate climate change,” Hawkins said. “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.”

Researchers examined 194 data sets from 61 peer-reviewed papers and four unpublished studies to determine how much carbon plants allocate to fungi. They found that plants pass between 1% and 13% of their carbon to mycorrhizal fungi, depending on the type of fungi. Most of these estimations came from isotope-tracing techniques, which involve labeling carbon and measuring the proportion that becomes part of the fungi.

The conclusion that more than 13 billion metric tons of CO2 is passed from plants to mycorrhizal fungi each year may be an underestimate, meaning the actual figure could be even larger.

“The author’s conservative method of quantifying carbon allocation only to extraradical mycelium [on the outside of the roots] implies that the values reported underestimate the role of mycelium inside or around plant roots to the carbon pool, magnifying the implications of their results,” Mareli Sanchez-Julia, a Ph.D. candidate at Tulane University who studies soil and microfungi, told Mongabay.

Mycorrhizal fungi use carbon to build far-reaching networks of fine filaments known as hyphae. Fungal hyphae form a vast web that allows plants to source water and nutrients from a much larger area. This living underground network contributes to the evolution of plants, the engineering of soil ecosystems, and the maintenance of the global carbon balance.

“We know that mycorrhizal fungi are vitally important ecosystem engineers, but they are invisible,” said senior author Toby Kiers, a professor of evolutionary biology at Vrije University Amsterdam and co-founder of the Society for the Protection of Underground Networks (SPUN). “Mycorrhizal fungi lie at the base of the food webs that support much of life on Earth, but we are just starting to understand how they actually work. There’s still so much to learn.”

The authors say that while this study uses the best available data, they acknowledge limitations due to the differences in experimental designs among studies and the lack of studies from many different ecosystems, especially in the Global South. They emphasize the need for further research to better understand carbon and nutrient exchanges between plants and mycorrhizal fungi.

“One of the things that struck me most about this study was the care the authors took to report and discuss uncertainty in their results,” Sanchez-Julia said “The study is a tremendous contribution to our understanding of mycorrhizal fungi in wild and agricultural systems, and directly informs conservation efforts prioritizing soil carbon sequestration.”

Approximately 75% of all land carbon is stored in the soil, and mycorrhizal fungi are vital in pumping carbon into the soil food web, where it can be locked away. However, the U.N. Food and Agriculture Organization has warned that by 2050, 90% of soils could be degraded.

This study confirms the significant role of mycorrhizal fungi in global carbon flow and, the authors say, highlights the need to include mycorrhizal fungi in climate and carbon models, as well as in conservation policies and practices.

Organizations such as SPUN, the Fungi Foundation and GlobalFungi are at the forefront of global initiatives to sample soil and to create open-source maps of the planet’s fungal networks and areas of high carbon capture to better inform conservation and research.

“Mycorrhizal fungi represent a blind spot in carbon modeling, conservation, and restoration,”  said co-author Katie Field, a professor of plant-soil processes at the University of Sheffield in the U.K.

“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,” Field said. “When we disrupt the ancient life support systems in the soil, we sabotage our efforts to limit global heating and undermine the health and resilience of the ecosystems on which we depend.”

Citation:

Hawkins, H.-J., Cargill, R. I. M., Van Nuland, M. E., Hagen, S. C., Field, K. J., Sheldrake, M., … & Kiers, E. T. (2023). Mycorrhizal mycelium as a global carbon pool. Current Biology, 33(11), R560-R573. doi:10.1016/j.cub.2023.02.027

Banner image of mycelium of arbuscular mycorrhizal fungi with false color. Image courtesy of Oyarte-Galvez (AMOLF) (CC BY-SA).

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