Hidden networks: The mycorrhizal fungi connected to plant roots form a gigantic, global network. In total, these fungal threads are around 110 quadrillion kilometers long and contain four to six times more carbon than all of humanity, as a global analysis in “Science” now reveals. For the first time, it shows the true extent of this network of symbiotic root fungi, but also in which ecosystems this network is particularly dense – it is not the forests.
Around 70 percent of all land plants have secret helpers: their roots are connected to symbiotic fungi that provide the plants with important nutrients. To do this, the mycorrhizal fungi receive sugar and other organic substances from the plant, which they can use to further expand their network of fungal threads. On average, there are 32 meters of this mycorrhizal network in every teaspoon of plant-covered soil.
Hidden helpers
In forests, this “Wood Wide Web” makes a crucial contribution to supplying trees, but plants in other ecosystems also benefit: “These networks of fungal hyphae increase the catchment area of plant roots a hundredfold,” explain Justin Stewart from the Free University of Amsterdam and his colleagues. “The arbuscular mycorrhizal fungi provide up to 80 percent of the phosphorus required by the plant and around 20 percent of the nitrogen.”
At the same time, the root fungi are also important players in the climate system; their underground networks are estimated to bind around a billion tons of carbon worldwide. “It’s hard to overstate the importance of these mushrooms,” emphasizes Stewart. But until now it was unclear how dense and extensive these mycorrhizal networks are worldwide and which plant communities support particularly dense fungal networks.
To clarify this, the researchers evaluated more than 16,000 core samples from 100 different ecoregions and nine different biomes worldwide. Using this data, they developed a model that can determine mycorrhizal density depending on plant species, climate, soil chemistry and other factors. They used this to infer the global distribution of root fungi and their networks from the sample data.
Four meters of fungal threads in every cubic centimeter of soil
The result is a first global map of mycorrhizal density. It reveals that on average four meters of fungal threads are hidden in every cubic centimeter of soil. “If you were to place all the fungal threads of the global arbuscular mycorrhiza together, you would get a length of around 110 quadrillion kilometers in the top 15 centimeters of the earth’s soil alone,” report Stewart and his colleagues. That is 50 times more than the length of all plant roots combined.
But the biomass of these underground fungal networks is also impressive. The team determined this by measuring more than 300,000 hyphae from three globally occurring mycorrhizal fungi species. Together with the density of the fungal networks, they determined that the carbon contained in the living root fungi alone amounts to around 300 million tons globally. “The total living biomass of the mycorrhizal hyphae exceeds the carbon stored in humanity as a whole four to six times,” writes the team.
Grassland ecosystems are richest in fungi, while fields are more impoverished
What’s surprising, however, is that, contrary to popular belief, the hotspots of root fungus density are not in the forests of our planet, but in grassland ecosystems: “The hyphal densities, for example, are 39 percent higher in mountain meadows than in tropical rainforests,” report Stewart and his team. “Ecoregions with particularly high densities of root fungi also include the Everglades in Florida, the Sudd wetlands in South Sudan and many prairie and steppe ecosystems worldwide.”
The reason for the dominance of root fungi in grassland systems is, on the one hand, the dense, extensive root system of meadows and grasslands. On the other hand, almost all grasses enter into a symbiosis with mycorrhizal fungi, as the researchers explain. Taken together, this makes grassland ecosystems important reservoirs of mycorrhizal biomass – and increases their role as carbon sinks. “But these ecosystems have so far been barely protected and are particularly at risk of degradation,” emphasizes the team.
On the other hand, the mycorrhiza in the fields and fields looks rather poor: “In agriculturally cultivated soils, the density of fungal hyphae is on average 47.3 percent below the global average,” report the researchers. Agricultural practices such as fertilization and tillage help destroy the delicate fungal networks. “This could help explain why agricultural soils are among the most impoverished soils on earth in terms of mineral carbon.”
Living infrastructure with enormous importance
According to Stewart and his team, their results support the enormous global importance of mycorrhizal fungi. “These fungi have shaped life on our Earth for hundreds of millions of years, but we still know little about how this living infrastructure is distributed across our planet,” says co-author Merlin Sheldrake from the Society for the Protection of Underground Networks (SPUN) in the US. “Our study is a first step toward understanding how this planetary transport system works.”
But many questions still remain unanswered. The researchers have not yet been able to determine how dense the mycorrhiza fungal networks are in deeper soil layers below a depth of 15 centimeters. But you expect that the root fungus networks are also present there. The specific influence of mycorrhiza on various ecosystems has only been partially researched so far. “Thanks to new technologies, we are just beginning to reveal what has been hidden beneath our feet for so long,” says co-author Corentin Bisot from the AMOLF Institute in Amsterdam.
Source: Justin Stewart (Vrije Universiteit Amsterdam) et al., Science, 2026; doi: 10.1126/science.adu4373