Just under Earth鈥檚 surface, 110 quadrillion kilometres of carbon-rich fungi intersect with plant roots. This vast network has been laid bare in the of our planet鈥檚 mycelial networks. Not only do these fungi exchange nutrients with plants, they also help regulate our climate.
Arbuscular mycorrhizal fungi, an ancient group of soil fungi found throughout most terrestrial ecosystems, form symbiotic relationships with about 70 per cent of the world鈥檚 plant species, providing nutrients and water in exchange for carbon. 鈥淪ome people call plants the saviours of these fungi, but these fungi are also the saviours of plants,鈥� says at the . 鈥淚f you鈥檙e not in symbiosis with arbuscular mycorrhizal fungi, you鈥檙e kind of the weirdo of the plant world.鈥�
Given the fungi鈥檚 importance, Stewart and his colleagues set out to quantify this hidden infrastructure. 鈥淲e asked the question: can we map the Earth鈥檚 underground circulatory system?鈥� says team member , also at the Society for the Protection of Underground Networks.
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First, the researchers analysed data from 16,000 soil samples from across Earth, derived from 322 previous studies. They also used robotic imaging to measure more than 300,000 fungal threads grown in the lab, allowing them to estimate the total biomass and carbon stored in the networks. Next, the team combined that data and used it to extend estimates across deserts, tundra, forests and other regions where direct measurements were sparse or unavailable.
The results suggest that worldwide arbuscular mycorrhizal fungi networks harbour a mass of carbon equivalent to about five times that of all living humans combined. 鈥淭hey鈥檙e very important for a lot of the different functions of our planet,鈥� says Stewart. 鈥淔or example, they pull carbon underground 鈥� that鈥檚 important for climate change.鈥�
The researchers also estimate that around 40 per cent of the world鈥檚 arbuscular mycorrhizal fungi live in grassland ecosystems, especially those of South Sudan, the Florida Everglades and the Tibetan plateau. That鈥檚 concerning, they say, since .
Croplands, meanwhile, show significantly reduced fungal presence, with large-scale crop-growing soils associated with about 50 per cent lower network densities than those in uncultivated ecosystems, despite their high presence of plants. That could be because fungicides can kill fungi directly, while tilling can break apart their networks and heavy fertiliser use may undermine the nutrient-for-carbon trade that normally sustains the symbiosis, says Stewart.
Last year, at the University of Leeds, UK, and her colleagues revealed that azole antifungals 鈥� a widely employed class of chemicals used to control fungal diseases like mildew and rot in crops 鈥� (the long, straw-like tube cells of fungi) by around 70 per cent and reduced the extent to which beneficial fungi colonise plant roots by up to 80 per cent. That, combined with the new findings, 鈥渟uggests current practices may be undermining a key natural ally of crops鈥�, says Carter. 鈥淪upporting arbuscular mycorrhizal fungi isn鈥檛 just an ecological issue; it is a practical route to improving soil health, resilience and long-term crop productivity.鈥�
at the University of California, Irvine, says he is particularly concerned about the thinner network under croplands. 鈥淲ith that biomass being knocked down so much, our agricultural crops may be missing out on some key benefits like nutrient access, drought resilience and carbon storage.鈥�
But there is a positive side as well, he adds. Now the researchers have quantified the scale of the losses, it should be easier to design interventions to rebuild fungal biomass. 鈥淔armers could add fungal spores to their soils,鈥� says Allison. 鈥淭he study might also prompt farmers to adjust their practices, by reducing tillage or adding less fertiliser.鈥�
While the work reveals expansive networks, it doesn鈥檛 mean all fungi are connected in a global 鈥�wood wide web鈥� 鈥� a hypothesised underground network through which plants exchange resources and information, says Stewart. 鈥淥ur research here was just asking how many threads are there on Earth, not if this is one large, continuous network.鈥�
Alongside the study, the researchers have launched an interactive map that reveals the global distribution of fungal networks in unprecedented detail. Kiers says they plan to present the findings to policy-makers at the United Nations desertification summit in Mongolia this August.
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