Forests pump carbon into soil

By John Upton

Photosynthesis is easy enough to understand: Plants use the power of the sun to combine carbon dioxide and water into sugar. What’s perhaps less easy to understand is what happens to all of the carbon-rich sugar that it produces. New research shows that vast amounts of it are pumped down to fungi deep in the ground, keeping the carbon out of the atmosphere and keeping the climate cool.

Some of the energy-rich sugar is shipped around the plant to power cells, and then is often eaten by herbivorous animals or flutters to the ground with fallen leaves to be gobbled up by microscopic organisms. But some of the sugar is pumped down to the roots and traded with mycorrhizal fungi in exchange for nutrients.

The mycorrhizal fungi take the sugar from the plants, and in return they feed nutrients to the plants. Fungi send stretching tentacles, called mycelia, through the ground to forage for nitrogen, phosphorous and other nutrients that are valued by the plants. They use those nutrients as currency with which they buy sugar.

Illustrated by Perry Shirley.
Illustrated by Perry Shirley.

The sugar that’s passed from the plant to the fungi contains lots of carbon, which the plant originally sucked out of the air as carbon dioxide. Scientists have discovered that most of the carbon that’s stored in some forest floors is sequestered in the bodies of the dirt-dwelling fungi — not, as had been presumed, in the decomposing leaf litter.

Karina Clemmensen, a researcher at the Swedish University of Agricultural Scientists, led research that investigated where carbon was being stored in two forested Swedish islands. The researchers discovered that 50 to 70 percent of stored carbon in the forests was locked up in the root layer, where the mycorrhizal fungi thrive.

The research took place in boreal forests, but Clemmensen said other ecosystems might also push much of their carbon down into the soil.

“In agricultural fields, arbuscular mycorrhizal fungi are normally the dominant mycorrhizal type,” Clemmensen said in an email. “Our result though – as stands here – is valid for the boreal forest only.”