"A single teaspoon of soil may contain up to 10 meters of mycorrhizal network."

The Hidden Web Beneath Our Feet: Scientists Map 110 Quadrillion Kilometers of Fungal Networks

A global study published in Science on June 11, 2023, has produced the first comprehensive map estimating the distribution and length of arbuscular mycorrhizal (AM) fungal networks in topsoils worldwide. The research—led by the Society for the Protection of Underground Networks (SPUN) in collaboration with international scientists—utilized machine learning models and data from over 16,000 soil samples to calculate the extent of these microscopic fungal filaments, known as hyphae.

Key Findings and Scale

The study estimates that the total length of AM fungal hyphae in the topsoil (approximately the top 15 centimeters) is roughly 110 quadrillion kilometers. For context, this distance is equivalent to approximately 10% of the width of the Milky Way galaxy, or about 750 million times the distance from Earth to the Sun.

The estimated mass of carbon stored within these networks is approximately 300 megatons—4 to 6 times the total mass of all living humans. The fungi are estimated to absorb and channel approximately 3.12 to 4.3 billion tons of carbon dioxide equivalent into the soil annually, a figure representing about 11% of global fossil fuel emissions in 2021.

Methodology

Researchers assembled and analyzed data from 16,669 soil cores collected from 322 previous studies worldwide. Machine learning algorithms, incorporating environmental data layers, were used to predict hyphal density in unsampled ecosystems. The model was calibrated using robotic imaging of over 300,000 living AM fungal hyphae.

Distribution and Densities

• Highest Densities: The highest hyphal densities were found in wild grasslands, including high-altitude and flooded ecosystems. Specific hotspots identified include the Florida Everglades, the Sudd wetlands in South Sudan, the Tibetan Plateau, the Anatolian steppe, and the North American Prairie.
• Grassland Concentration: Grassland ecosystems are estimated to contain approximately 40% of the global AM fungal biomass. The top six inches of soil in these grasslands contain an average hyphal density of 355 feet per cubic inch (6.6 meters per cubic centimeter).
• Cropland Reduction: Agricultural topsoil exhibits a significantly lower density of AM fungal networks. On average, hyphal density in croplands is approximately 47.3% to 50% lower than in wild ecosystems. The average density in croplands is 204 feet per cubic inch (3.8 meters per cubic centimeter), compared to 237 feet per cubic inch (4.4 meters per cubic centimeter) globally. Researchers point to agricultural practices such as tilling, the use of phosphorus and nitrogen fertilizers, and fungicides as contributing factors.

Ecological and Climate Implications

AM fungi form symbiotic relationships with approximately 70% of land plants, exchanging water and nutrients (such as phosphorus and nitrogen) for carbon derived from plant photosynthesis. The networks connect multiple plants, facilitating resource exchange. The research provides a baseline for understanding the role of these networks in carbon storage, nutrient cycling, and plant productivity.

Conservation Context

"Fungi have historically been overlooked in climate and conservation discussions." — Dr. Toby Kiers

• Protected Area Gaps: An estimated 95% of biodiversity hotspots for AM fungi are located outside of protected areas. Less than 10% of the densest fungal network clusters fall within current conservation zones.
• Grassland Conversion: Grasslands, which house the highest fungal densities, are being converted to farmland at a rate reported as four times faster than forests.
• IUCN Status: Of the more than 8,000 known species that participate in these networks, almost none have been evaluated for endangered status by the International Union for Conservation of Nature (IUCN).

Statements from Researchers

• Dr. Justin Stewart, lead author (SPUN): Noted that a single teaspoon of soil may contain up to 10 meters of mycorrhizal network. He also expressed that the findings aim to support the protection of wild grasslands.
• Dr. Toby Kiers, co-author (SPUN and Vrije University Amsterdam): Stated that the data aim to highlight hidden underground patterns. He added that fungi have historically been overlooked in climate and conservation discussions, and that this research is critical for developing precise climate policies.
• Dr. Katie Field, co-author (University of Sheffield): Described the findings as the first global view of these networks, revealing an extensive living infrastructure that underpins plant productivity and ecosystem resilience. She highlighted significant knowledge gaps regarding the ecological functioning of these networks.
• Dr. Merlin Sheldrake, co-author: Characterized the study as an exciting step toward understanding how this planetary circulatory system operates, potentially helping address challenges such as food security and climate change.
• Andrea Genre, (University of Turin): Called the map "urgently needed" and stated it can inform more efficient strategies for biodiversity conservation, agricultural management, and climate change mitigation.

Data and Availability

An interactive visualization, the Mycorrhizal Infrastructure Map, was created in collaboration with data visualization designer Moritz Stefaner. The underlying data are available for download for researchers and decision-makers. Researchers have noted that more sampling is needed in tropical rainforests and deserts, and that updates to the map are expected within the next five years.