- An international research team, including scientists from the University of Sheffield, has revealed that Earth's top soils hide a vast network of arbuscular mycorrhizal (AM) fungi - spanning a distance nearly a billion times from the Earth to the Sun
- AM fungal networks transport an estimated 4 billion tons of carbon dioxide equivalent into soils each year, equivalent to 11 per cent of all human-related carbon-dioxide emissions)
- Grassland ecosystems are home to an estimated 40 per cent of Earth's AM fungal infrastructure. The flooded grasslands of South Sudan, the Everglades in Florida, and the Tibetan plateau have exceptionally high predicted network density
For the first time, an international team of researchers has mapped the global distribution and mass of arbuscular mycorrhizal (AM) fungal networks. These vast underground systems sustain plant life and play a critical role in regulating Earth's climate by locking carbon into the soil.
Published alongside an interactive visualisation that helps reveal the scale of this underground fungal infrastructure, the research will help scientists and decision makers understand where these vital fungal systems are thriving and where they are threatened.
AM fungi form symbiotic relationships with approximately 70 per cent of plant species on Earth. The fungi provide nutrients and water in exchange for carbon fixed by plants. As ecosystem engineers, these networks form a critical living infrastructure that draws carbon into soils and supports much of life on Earth. Last year, in Nature, researchers published global analyses of the diversity patterns of underground mycorrhizal fungal communities accompanied by a digital tool, the Underground Atlas, to help decision-makers locate predicted underground biodiversity hotspots. But until now, no-one has attempted to predict and visualize the physical density and global distribution of AM fungal networks.
Study contributor Katie Field, a Professor of Plant-Soil Processes at the University of Sheffield and a Royal Society Faraday Discovery Fellow, said: "These findings are exciting and significant because they provide the first global view of AM fungal networks, revealing an extensive but largely invisible living infrastructure that underpins plant productivity, nutrient cycling, ecosystem resilience, and the health of terrestrial ecosystems worldwide.
"This study not only represents a major advance in quantifying these hidden fungal networks and identifying key hotspots such as grasslands, but it also highlights important knowledge gaps surrounding the ecological functioning of these networks."
The researchers assembled data on the density of AM networks from over 16,000 soil-cores collected across Earth. They developed machine-learning models that incorporated data layers from deserts and tundra to forests to predict network density in unsampled ecosystems. In collaboration with the Physics of Behaviour group at research institute AMOLF, the team calibrated their model with robotic imaging of over 300,000 living AM fungal hyphae grown in the lab.
Using these datasets, they estimate that AM fungal networks have a total length of approximately 110 quadrillion kilometres and a mass of around 300 megatons of carbon (4-6x the mass of all living humans).
"It is hard to overstate the importance and enormity of these fungi," said lead author Dr Justin Stewart, with the Society for the Protection of Underground Networks (SPUN).
"There could be up to 10 meters of mycorrhizal network in just a teaspoon of soil."
Often called one of the Earth's circulatory systems, mycorrhizal networks move carbon, water, and nutrients across underground ecosystems. In healthy soils, mycorrhizal networks can increase the foraging area of plant roots by up to 100 times, while providing more than 80 per cent of a plant's phosphorous.
"With the emergence of new technologies in high-resolution imaging, machine learning and robotics, we are starting to reveal what has long been hidden under our feet," said co-lead author, Dr. Corentin Bisot, an AMOLF biophysicist.
"We are learning how the complex bodies of network-forming fungi transport nutrients and help regulate the climate."
The team worked with award-winning data visualisation designer Moritz Stefaner to build the Mycorrhizal Infrastructure Map. It is the first time the Earth's fungal infrastructure has been seen at this scale and resolution. The underlying data are available to download for governments and decision-makers to begin monitoring the health of critical underground fungal communities.
Last year, several of the same authors published a cover story in Nature in which they described how mycorrhizal fungal networks and their plant partners build hyper-efficient supply chains for exchange of carbon and nutrients, measuring carbon flows inside these living transport systems that can reach speeds of up to 120 micrometres per second. The current study is a critical step towards understanding how carbon and nutrient flows unfold on a global scale.
The study also documented potential threats. Mycorrhizal densities across croplands are predicted to be roughly half those in wild ecosystems. Wild grassland ecosystems were found to contain approximately 40 per cent of the world's AMl biomass. Yet grasslands are among Earth's least protected ecosystems and are being transformed into farmlands four times faster than forests. This reinforces a finding published by SPUN researchers last year showing that 95 per cent of the biodiversity hotspots for AM fungi are located outside protected areas.
For evolutionary biologist Dr. Toby Kiers, Executive Director of SPUN, this growing body of research is critical in developing more precise climate policies: "Fungi have been ignored in climate and conservation for too long. Now is the time to change that trajectory." Kiers was recently named a prestigious MacArthur Fellow and winner of the Tyler Prize, known as the 'Nobel Prize for the Environment' for her work on plant-fungal systems.
"Mycorrhizal fungi have shaped life on earth for hundreds of millions of years, but we still understand too little about how the infrastructure of these living transport systems is distributed across the planet," added co-author and biologist Dr Merlin Sheldrake.
"This study is an exciting step towards understanding how this planetary circulatory system operates and suggests ways that we can better work with fungi to help address many of the unfolding challenges of our times, from food security to climate change."
This study helps quantify the extraordinary extent of AM fungal networks, but it also reveals how much remains unknown by pinpointing many regions of the planet which remain unsampled.
Professor Katie Field added: "Importantly, the research shows that large-scale agricultural croplands exhibit an approximately 50% reduction in AM fungal network density compared with less intensively managed systems. On the face of it, this seems alarming.
"However, as stated by the authors, our understanding of how specific environmental conditions and agricultural practices influence mycorrhizal health, carbon storage, and nutrient cycling remains very limited. In other words, although we are now starting to understand where these networks occur, we still know relatively little about what they do, how effectively they function, and how their roles vary across environments and host plants."
Addressing this gap is one of the central aims of Professor Field's new Royal Society Faraday Discovery Fellowship based at the University of Sheffield in collaboration with Lancaster University and the Natural History Museum. Building on recent advances, Professor Field's project will investigate the ecological performance and functional capacity of mycorrhizal networks across diverse environmental and management contexts.
Read the study in full: https://www.science.org/doi/10.1126/science.adu4373
