Mycorrhizal fungi form underground networks that sustain plant life and help regulate Earth's climate by drawing carbon into soils. In a study published today in Science, an international team of researchers produced the first global maps estimating the distribution and mass of the Earth's arbuscular mycorrhizal fungal networks. Published alongside an interactive visualization 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.
Researchers found:
Global topsoils contain ~110 quadrillion kilometers of arbuscular mycorrhizal fungal network – made up of tubular cells known as hyphae. This distance is almost a billion times the distance from the Earth to the sun.
Grassland ecosystems are home to an estimated ~40% of Earth's arbuscular mycorrhizal fungal infrastructure. The flooded grasslands of South Sudan, the Everglades in Florida, and the Tibetan plateau have exceptionally high predicted network density.
AM fungal networks transport an estimated ~4 billion tons of CO2e into soils each year (equivalent to 11% of all human-related carbon-dioxide emissions).
On average, large-scale agricultural crop lands are predicted to be associated with ~50% lower network densities. While more work is needed to link specific farming practices to mycorrhizal health, scientists worry that less dense networks diminish a soils' ability to store carbon, cycle nutrients, and resist stress.
Arbuscular mycorrhizal fungi (known as AM fungi) form symbiotic trade relationships with ~70% of plant species on Earth. The fungi provide nutrients and water in exchange for carbon produced 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.
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 AMOLF Biophysics Institute, 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 ~110 quadrillion kilometers and a mass of ~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 (32 feet) 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 > 80 percent 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 visualization 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 (estimates are calculated for every 1km2 of terrestrial land, excluding ice caps and areas lacking enough data to predict). 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 to trade carbon and nutrients, measuring carbon flows inside these living transport systems that can reach speeds of up to 120 um/sec (if one was inside the network, these speeds would feel like ~400km/hr). 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 ~40% of the world's arbuscular mycorrhizal 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% of the biodiversity hotspots for arbuscular mycorrhizal 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.
