The largest fraction of carbon held in the soils of northern forests may derive from the living and the decomposing roots of trees and shrubs and the fungi that live on them.
By some estimates, the planet's soils contain more than twice the carbon in the atmosphere. Boreal forests cover about 11% of Earth’s land surface and contain around 16% of total soil carbon.
Until the last decade or so, most scientists had presumed that much of the decomposed organic matter, or humus, that makes up the soils in such forests came from fallen needles, twigs and branches, says Björn Lindahl, a fungal ecologist at the Swedish University of Agricultural Sciences in Uppsala. That was especially the case for soil on small islands in northern lakes, where wildfires rage only rarely and humus can accumulate to depths of more than 1 metre.
But when Lindahl and his colleagues carbon-dated samples taken at various depths throughout the soil on 30 islands in two Swedish lakes near the Arctic Circle, they found that accumulation of organic material on top of the ground alone could not explain the rates at which soil carbon built up. On islands larger than 1 hectare, each square metre of soil accumulated in the past 100 years holds about 6.2 kilograms of carbon. But on islands smaller than 0.1 hectare, the past century of soil contains a whopping 22.5 kilograms of carbon per square metre.
The difference in carbon-sequestration rates, the researchers report in Science1, can be explained entirely by carbon derived from the roots of trees and shrubs and their symbiotic fungi. These organisms, dubbed ectomycorrhizal fungi, colonize roots and gain nourishment from the plants while helping their hosts to absorb water and nutrients from the soil. Whereas about 47% of the soil carbon on the large islands came from roots and ectomycorrhizal fungi, almost 70% did so on the small islands, says Lindahl.
It is unclear why the small islands built up a larger fraction of root- and fungi-derived carbon in the past century, but it may be related to slower rates of decomposition in the soils there, the researchers speculate.
The team’s findings “are exciting, and quite a surprise”, says Sandra Holden, an ecosystems ecologist at the University of California, Irvine. Symbiotic fungi are a dominant component of a soil’s microbial community, she notes in an article accompanying the team’s research2. “We’ve known for quite a while that trees divert carbon to their ectomycorrhizal fungi, but having 70% of soil carbon derive from them is much more than we could have expected,” she says.
Benjamin Turner, a soil scientist at the Smithsonian Tropical Research Institute in Balboa, Panama, says that the findings are a “great example of how analyses of sequences of soils of different ages contribute to the understanding of processes that wouldn’t be possible through conventional experimentation”.
It is not clear how the results might affect estimates of how carbon sequestration in a warming climate, says Johan Bergh, a forest ecologist at the Swedish University of Agricultural Sciences who wasn’t involved in the study. An increase in temperature will probably boost rates of microbial activity in the soil, thereby triggering increased decomposition and a loss of soil carbon as carbon dioxide wafts into the atmosphere. But at the same time, he suggests, a warmer climate might lead to enhanced growth of the boreal forest’s trees and shrubs, as well as their roots and fungi, causing an ovrall increase in carbon sequestration.
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