This unfathomably huge fungal network keeps Earth cool and green

Even if you don’t like eating mushrooms, you’re in debt to fungi. One group of them, known as arbuscular mycorrhizal fungi, form vast subterranean networks of tubes called hyphae, hooking up with the roots of plants to exchange nutrients. Earth is so verdant in large part thanks to these partnerships, as this expansive infrastructure is associated with nearly three-quarters of all plant species. But because the network sprawls underground, it’s been difficult for scientists to determine just how much arbuscular mycorrhizal fungi is out there. (Good luck digging everywhere on the planet and taking samples.) Scientists have developed a workaround, which has produced some astonishing numbers. In this map, brighter yellow spots indicate higher densities of arbuscular mycorrhizal fungi. Courtesy of the Society for the Protection of Underground Networks There are two major classes of mycorrhizal species. The ectomycorrhizal fungi grow as sheaths around a plant’s roots, especially conifer trees, whereas the arbuscular ones in this new paper penetrate them. Either way, these fungi act as an extension of the roots, helping them absorb more water and nutrients. “Just as a circulatory system moves resources through a body, these sort of microscopic fungal pipes are connected to plants,” Kiers said.  In exchange, mycorrhizal species get energy in the form of carbon that the plants have drawn from the atmosphere. They help the plants grow to sequester still more carbon, a mutually beneficial partnership that benefits humans, too, as it keeps the planet from warming even further. However, the density of arbuscular mycorrhizal fungi isn’t uniform across the planet’s biomes. You might assume that it would be highest in tropical rainforests, but in fact grasslands account for 40 percent of the predicted global arbuscular biomass, the study found. That might be because herbaceous plants like grasses tend to allocate more carbon to their symbiotic fungi than trees do. You can’t see it, but grasslands have vast root systems, meaning there’s loads of hidden biomass. “Even if grasslands get burned above ground, that carbon tends to remain underground, and they can come back again, which is different than forests,” Kiers said. Yet, Kiers added, just 5 percent of arbuscular mycorrhizal fungal biodiversity hot spots lie in environmentally protected areas. The idea with these new maps is for scientists and policymakers to identify where fungi might be thriving, and protect them. That will simultaneously support plant life and biodiversity overall — all kinds of birds, insects, and herbivores depend on this vegetation, too — and capture still more carbon in the soil. (Some savannas, like Brazil’s cerrado, also store enormous amounts of carbon underground in peat, or dead plant material that resists decay and accumulates over centuries.) Toby Kiers and Merlin Sheldrake take soil samples in the mountains of Bhutan.