Global Map of Mycorrhizal Fungi Reveals Underground Networks and Their Role in Carbon Storage

Episode snapshot

In this New Scientist episode of World, the Universe and Us, experts discuss the first global map of arbuscular mycorrhizal fungi, how these underground networks trade nutrients for carbon, and what the findings mean for carbon storage and desertification prevention. The team highlights advances in automated imaging, machine learning, and soil-core sampling that make global predictions possible, and they explore how grasslands harbor a large portion of fungal biomass.

• Global maps of fungal density reveal extensive underground networks across Earth's vegetated biomes.
• Fungal biomass and carbon storage capacity are substantial, with grasslands hosting around 40% of network mass.
• Automated robotic imaging and machine learning enabled scaling from local cores to global predictions.
• Conservation and agricultural practices impact these networks and climate-related processes, prompting policy engagement.

Introduction and background

The programme revisits the idea of the Wood Wide Web, the underground network by which plants exchange carbon for nutrients with soil fungi. The focus is on arbuscular mycorrhizal fungi, a widespread group forming intimate interfaces with plant roots. The guests explain how these networks extend beyond single sites, potentially altering our understanding of carbon storage, water movement, and nutrient cycling across ecosystems.

Global mapping approach

To create global predictive maps of arbuscular mycorrhizal fungal density, the study compiled data from about 16,000 soil cores collected around the world, covering roughly nine vegetated biomes. Each core is geolocated, enabling integration with spatial data layers such as elevation, soil nutrients, and precipitation. Researchers used machine learning models trained on the core data to predict fungal density in places where no cores were taken, effectively producing global density maps.

The project also needed to translate density into biomass. That required knowing the physical dimensions of fungal hyphae, especially the hyphal tube radius, which impacts volume and thus biomass. Automated robotic imaging allowed complete sampling of hypha dimensions, revealing substantial variation in width both within and between species. This refinement dramatically reduces uncertainty in biomass estimates, which previously relied on rough width assumptions drawn from a subset of images.

Key findings and implications

The density maps illuminate the global distribution of arbuscular mycorrhizal networks and their connections to soil and plant systems. The networks are a crucial infrastructure for terrestrial life, akin to a circulatory system for carbon and nutrients. A striking quantitative result is that arbuscular mycorrhizal fungi are responsible for drawing roughly 4 billion tonnes of carbon into soils each year, which is about 11% of emissions from fossil fuels. This carbon can become stabilized in soil horizons, especially when associated with minerals, contributing to long-term carbon storage.

A central finding is that roughly 40% of the biomass of these networks is found in grassland ecosystems, highlighting grasslands as particularly important for underground carbon dynamics and ecosystem resilience. Unexpectedly dense networks were also found in flooded wetlands and high-altitude regions, such as the Tibetan Plateau and Everglades, underscoring the global reach of these relationships beyond what many people typically consider.

Agriculture, landscape change, and uncertainty

The analysis also examined human land use. Large agricultural crop lands were associated with about a 50% decrease in network density on average. While this does not identify specific farming practices, prior work links reductions to fungicides, high-nutrient fertilizer regimes, and crop breeding that reduces root carbon transfer to fungal partners. The researchers stress that agriculture is not inherently incompatible with healthy fungal networks; practices like crop rotations that maintain soil biology can sustain or even enhance networks.

Another important aspect is uncertainty. The study presents maps of uncertainty alongside density maps, highlighting regions with limited sampling. Even with harmonized global data, many places require more cores and local scientists to refine the models. There is a call for better temporal data to understand how networks shift with climate change and land-use dynamics.

Future directions and policy impact

The team is collaborating on expanding the temporal and spatial coverage, including deeper soil layers, where carbon storage mechanisms might differ considerably from the uppermost soil. There is also interest in how warming temperatures affect flows of nutrients within these networks, an area that automated high-throughput imaging helps explore by enabling many environmental scenarios to be tested efficiently.

To translate these findings into policy and practice, the researchers launched the Society for the Protection of Underground Networks (SPUN) to map biodiversity patterns, threats, and carbon drawdown patterns, and to deliver datasets to decision-makers. This aims to reframe conservation from an above-ground focus to include underground ecosystems, with potential applications in restoration and desertification prevention.

Conclusion

The first global map of mycorrhizal networks marks a milestone in understanding how subterranean ecosystems shape climate and soil health. By combining thousands of soil cores, automated imaging, and machine learning, scientists can quantify and locate these invisible networks, informing management actions and public policy that support soil health, carbon storage, and ecosystem resilience.

Takeaway for readers

This discussion emphasizes that protecting underground ecosystems is essential for global environmental health, climate mitigation, and sustainable agriculture. The Wood Wide Web is not just a curiosity but a fundamental component of Earth’s biogeochemical cycles that warrants attention from farmers, land managers, policymakers, and researchers alike.