Researchers working in Swedish boreal forests found that ectomycorrhizal fungi — symbiotic fungi that exchange nutrients with trees — are associated with a significant reduction in stored soil carbon, which could have implications for maintaining the important environmental services these forests provide, such as nutrient cycling.

While not establishing a causal relationship, the study, published May 2 in Ecology Letters, flies in the face of the prevailing view that ectomycorrhizal fungi increase the amount of carbon in the soil by suppressing other decomposers. It also highlights the major influence that individual microbe groups, including soil fungi, can exert on ecosystems, despite the tendency to lump them together.

"With microbial ecology, people from outside this field tend to compress all microbes to one single organism that always behaves the same," said first author Björn Lindahl, a soil biology professor at the Swedish University of Agricultural Sciences. "[It's] convenient to say that microbes are microbes and treat them as a black box, basically."

Forests are important global carbon sinks, areas of the environment that absorb more carbon than they release. Carbon sinks contribute to the overall carbon cycle by keeping the levels of carbon in the oxygen low and providing a source of carbon for decomposers, including fungi, which recycle this carbon and put it back into the ecosystem. 

The bulk of the carbon in forests, up to 69%, is stored in soil. Because plants' ability to absorb carbon is being compromised by climate change, this percentage may increase in the coming years. Carbon stored in soil is relatively inaccessible to plants and animals. Instead, decomposers in the soil take this stored carbon and convert it into carbon dioxide, which plants then reabsorb through photosynthesis. 

Boreal forests, those in cold, high-latitude environments, are the largest terrestrial biome, making them one of the most important carbon sinks. But because of the poor conditions found in these forests, they tend to be dominated by a small number of species.

"An ecosystem of the boreal zone is at the transition between ordinary forest and tundra in the north, which makes it quite challenging for the organisms," Lindahl said. "It's a marginal habitat from a global point of view. There's a lot of environmental constraints that reduce diversity."

Boreal forests store nearly twice as much carbon as their tropical counterparts. However, the limited diversity of boreal forests means that there are fewer decomposing species to convert this carbon into carbon dioxide, making the balance of carbon in the ecosystem "more sensitive," according to Lindahl.

While 92% of plant families are thought to have symbiotic relationships with fungi in the soil, the role that these fungi play in the wider ecosystem is not clearly understood. The prevailing hypothesis is that these soil fungi do not play a major role in decomposition of carbon and can even increase the amount of stored soil carbon by outcompeting decomposing bacteria. 

However, the team hypothesized that in the boreal forests, these fungi would reduce the levels of carbon in the forest soil because they have retained the genes to produce carbon-processing enzymes and have less competition in boreal forests. While this hypothesis is not widespread, it has been suggested by broader studies about the role of plants and soil in carbon storage. 

"We want to challenge the idea of enormous redundancy in microorganisms," Lindahl said, referring to the notion that if there are many microorganisms filling similar roles, individual organisms are less likely to have an impact on the ecosystem. 

To test the hypothesis, the team employed genetic methods to identify the presence of the fungi in the soil from across the Swedish boreal forests, sampling an area of some 240,000 square kilometers (92,665 square miles). They then compared how the presence of fungi relates to the thickness of the humus layer, the upper layer of the soil that contains the vast majority of its organic matter. 

They found that local carbon storage in the humus layer was 33% lower in the presence of Cortinarius fungi. They also found that younger forests that had been planted after clearcutting took almost 30 years to start rebuilding their populations of Cortinarius.

And while this relationship is only a correlation, the team also employed statistical control techniques to correct for sources of bias, such as the presence of other fungi and pH.

"We cannot claim a causal relationship. But you shouldn't overplay the uncertainty, because the mechanism we propose is highly plausible; everyone agrees on that," Lindahl said, referring to the reviewers of the study.

While more research is needed to definitively prove that the fungi are causing a decline in carbon storage in boreal forests, the fact that the results align so perfectly with the team's prior hypothesis, according to Lindahl, "makes a stronger case out of it."

The team is now working on extending its methodology. While the current analyses only showed the presence of the fungi in the soil, the team hopes to learn more about the role that these and other fungi play in the soil by looking at gene expression instead of DNA alone.

"We need to sketch the big picture of major differences in depth between different groups. We have some ideas, but we basically need to start off from scratch when it comes to fungi," Lindahl said. 

More broadly, Lindahl maintains that the findings are an "eye-opener" about the sensitivity of these important forests.

"It should make us think twice before breaking the continuity of trees in a forest," Lindahl said. "[Cortinarius] thrives in these quite undisturbed, very new forests, and when you start to mess around, you cut down trees, you fertilize, [you've] nitrogen deposition. More or less everything that a human does, this fungus doesn't like."

The study, "A group of ectomycorrhizal fungi restricts organic matter accumulation in boreal forest," published May 2 in Ecology Letters, was authored by Björn D. Lindahl, Julia Kyaschenko, Kerstin Varenius, Karina E. Clemmensen, Anders Dahlberg, Erik Karltun and Johan Stendahl, Swedish University of Agricultural Sciences.