Fungi as carbon keepers

A new study suggests that tiny organisms living underground may hoard most of the carbon in Swedish forest floors. That’s a good thing for the environment: If the carbon remains stashed in the soil, then it’s not escaping into the atmosphere. There, it could contribute to global warming.

And what’s happening in Sweden may be happening in forests elsewhere around the world.

These organisms, called mycorrhizal fungi, grow on tree roots. They provide trees with nutrients in exchange for their sugars, which contain carbon. The new study finds that these fungi hold between half and almost three-quarters of the carbon found in the forest floor.

Scientists had long believed that most of the carbon sits above ground, in pine needles, leaves and mosses. But the new study suggests most carbon is trapped in the soil.

“It’s hard to quantify how important mycorrhizal fungi are in ecosystems,” Erik Hobbie told Science News. “This paper presents hard evidence about their importance.” Hobbie is a forest ecologist, or a scientist who studies how a forest’s inhabitants relate to each other and their environment. He works at the University of New Hampshire in Durham and was not involved with the new study.

Carbon is a building block of every living organism, from people and fish to bacteria and trees. Trees are masters at stashing carbon. They inhale carbon dioxide, a carbon-containing gas, from the air. They use the carbon to grow branches, leaves and roots. But the element moves around. If a tree falls down and rots — or is cut down and burned — its carbon goes back into the air as carbon dioxide. As a greenhouse gas, carbon dioxide traps heat in the atmosphere. Too much of the gas in the air can foster global warming.

In cold northern forests, like those in Sweden, fallen leaves and branches take a long time to rot. That means the carbon in those materials can stay put for a long time.

Karina Clemmensen, a fungal ecologist at the Swedish University of Agricultural Sciences in Uppsala, worked on the new study. She says that even though scientists know that trees send carbon to their roots underground, the process has not been well understood.

To learn more about what a tree does with its carbon, she and her coworkers collected soil from 30 forested Swedish islands. They then measured the amount and age of the carbon in the samples. The scientists found that deeper, fungi-filled soils contained more carbon than soils from the surface of the forest floor.

When the scientists dated the carbon in the samples, they got a surprise: They found “young” carbon in deeper soils. The researchers had expected to find young carbon only in soils closer to the surface of the forest floor. The only explanation, they say: The young carbon came from the underground fungi that slurp up carbon-containing sugars from tree roots.

With the new findings, earth scientists may better understand soil’s role in storing carbon. That information could help improve forecasts for global warming.

Power Words

fungus Any of a group of unicellular or multicellular, spore-producing organisms that feed on organic matter, both living and decaying. Molds, yeast and mushrooms are all types of fungi.

carbon A nonmetal chemical element that is a building block of all living things. Carbon has two main forms, diamond and graphite. It also makes up charcoal, soot and coal.

carbon dioxide A colorless, odorless gas produced by burning carbon and organic compounds and by respiration, or the breathing of animals.

organic Of or relating to living or once-living things.