Just beneath Earth’s surface, a carbon-rich network of fungi spans approximately 110 quadrillion kilometers. This extensive infrastructure is part of our planet’s mycelial network. These fungi not only facilitate nutrient exchange with plants but also play a crucial role in climate regulation.

Arbuscular mycorrhizal fungi, an ancient group of soil fungi found in nearly all terrestrial ecosystems, forge symbiotic relationships with around 70% of the world’s plant species. They provide essential nutrients and water in exchange for carbon. “Plants are often seen as saviors of these fungi, but in reality, it’s a mutual relationship—these fungi also support plant life,” states Justin Stewart from the Association for Underground Network Protection. “Those plants not partnered with arbuscular mycorrhizal fungi are anomalies in nature.”

Recognizing the significance of fungi, Stewart and his team aimed to quantify this hidden infrastructure. “We set out to answer: Can we map Earth’s subsurface circulation system?” remarks team member Toby Kiers from the same association.

The researchers analyzed data from 16,000 soil samples worldwide, pulling insights from 322 past studies. They also utilized robotic imaging to assess over 300,000 fungal threads cultivated in the lab, enabling them to estimate the total biomass and carbon stored within this vast network. By merging this data, they broadened their estimates across various ecosystems, including deserts, tundra, and forests where direct measurements were scarce.

The findings indicate that the global arbuscular mycorrhizal fungal network sequesters roughly five times more carbon than all current human biomass combined. “They are pivotal for numerous Earth functions,” Stewart explains. “For instance, they sequester carbon underground, which is vital in combating climate change.”

Researchers also estimate that approximately 40% of the world’s arbuscular mycorrhizal fungi thrive within grassland ecosystems, particularly in regions like South Sudan, the Florida Everglades, and the Tibetan Plateau. This is concerning, as grasslands are rapidly converting into farmland.

Conversely, the prevalence of fungi significantly diminishes in agricultural settings, resulting in about 50% lower network density in heavily cultivated soils compared to untouched ecosystems. This trend arises because fungicides can directly eliminate fungi, while tillage disrupts fungal networks, and excessive use of fertilizers can hinder the nutrient and carbon exchanges critical to sustaining these symbiotic relationships, according to Stewart.

Last year, Laura Carter from the University of Leeds uncovered that azole antifungals, commonly used to combat fungal diseases such as mold and rot in crops, reduced mycelial density by approximately 70%. Moreover, the beneficial fungi’s colonization of plant roots decreased by up to 80%. These findings, alongside the current research, suggest that existing agricultural practices may be damaging crucial natural allies in crop growth. “Supporting arbuscular mycorrhizal fungi isn’t just an ecological concern, but a viable strategy for enhancing soil health, resilience, and long-term agricultural productivity,” Carter asserts.

Stephen Allison, a professor at the University of California, Irvine, expressed alarm over the thinning fungal network beneath farmland. “With significant biomass loss, our crops could be deprived of vital benefits, including nutrient access, drought resilience, and effective carbon storage.”

Despite the challenges, there are hopeful prospects. With the quantification of the loss, designing interventions to restore fungal biomass becomes more feasible. “Farmers can introduce fungal spores back into the soil,” Allison suggests. “This research may also encourage farmers to modify practices, such as reducing cultivation intensity or minimizing fertilizer use.”

While the study highlights a vast fungal network, Stewart clarifies that it does not imply a universal “wood wide web” exists—an underground network for plants to share resources and information. “Our research measured the density of threads on Earth, not their linkage into a singular network.”

Alongside the study, the researchers released an interactive map, detailing the global distribution of fungal networks with unprecedented clarity. Kiers intends to present these findings to policymakers at the upcoming United Nations Desertification Summit in Mongolia this August.