Sequestration may be questionable fiscal policy, but it means good news in the context of carbon cycles. Vast underground networks of fungi may sequester heaps of carbon in boreal forest soil, a study suggests. By holding onto the element, the fungi do the environment a favor by preventing carbon dioxide from escaping into the atmosphere and warming the planet.

Mycorrhizal fungi, which grow underground in and on tree roots, hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden, researchers report March 28 in Science. The new findings poke holes in a long-held idea that carbon in boreal forests accumulates mainly above ground in a litter of pine needles, mosses and leaves. The researchers suggest instead that trees direct carbon deeper into the soil via their root systems.

“It’s hard to quantify how important mycorrhizal fungi are in ecosystems,” says forest ecologist Erik Hobbie of the University of New Hampshire in Durham, who was not involved with the study. “This paper presents hard evidence about their importance.”

Trees suck carbon dioxide from the air and turn the carbon into sugars to fuel growth of branches, leaves and roots. Because trees are so good at capturing carbon dioxide, ecosystems stash loads of carbon in forests. Cold boreal forests are thought to be carbon-storing superstars mostly because litter stacks up on their floors and takes a long time to decompose. These forests stow away nearly a quarter of all the carbon stocked in the Earth’s land surfaces.

But scientists have not understood where exactly trees put their carbon. The issue becomes important when researchers build computer simulations that track carbon cycling. “People talk about how plants shuttle half their carbon to the belowground root system, but it has kind of been neglected in carbon storage models,” says study coauthor Karina Clemmensen, a fungal ecologist at the Swedish University of Agricultural Sciences in Uppsala.

Clemmensen and colleagues measured the age and accumulation of carbon in soil samples from 30 islands sprinkled across two lakes in northern Sweden. At each island, Clemmensen’s team plunged long, hollow tubes with sharpened ends into the ground to extract soil cores.

Since new, carbon-rich litter settles layer by layer on forest floors, prevailing wisdom suggested the scientists would find older carbon as they went down. But instead, researchers found stores of young carbon deep beneath the surface. Only sugars shipped down trees to their roots could explain the young carbon’s presence in soil below ground, says Clemmensen.

Next, her team analyzed DNA and fungal molecules in the soil samples. Beneath the forest floor, soil was packed with traces of mycorrhizal fungi, which stretch long filamentous fingers through the earth and sop up water and nutrients. The fungi pull in carbon in the form of sugars from tree roots. Above ground, fungi that break down leaf litter dominated.

The study’s findings could help global carbon modelers tweak their simulations, says climatologist Victor Brovkin of the Max Planck Institute for Meteorology in Hamburg. “How to model soil carbon is poorly understood,” he says. “That’s why this new evidence is important to us.”