Do plants and fungi exchange carbon for nutrients? Unlikely, researchers say

Every year, plants move 3.58 gigatons of carbon to mycorrhizal fungi, their underground partners – enough, in fact, that if it were ice, it would cover 112 million NHL hockey rinks. However, a prevailing scientific theory explaining a huge transfer as an economic bargain is likely incorrect, according to a new paper by a group of experts, including a University of Alberta researcher.

From the market perspective, carbon is exchanged for nutrients provided by the mushrooms – an exchange of resources between partners, governed by economic principles.

However, the new research suggests rethinking how these environmentally important systems work.

“Many researchers assume that the exchange of carbon for nutrients is directly coupled and that the quantities transferred are based on market economics. Markets are human constructs that do not seem to apply here,” explains Justine Karst , associate professor in the Faculty of Agricultural, Life and Environmental Sciences at the University of Alberta and co-author of the article. “We found no evidence of trade.”

The work is published in the journal New plant scientist.

Misapplication of economic models hinders the understanding of mycorrhizae – the mutual relationships between plants and subterranean fungi – which can in turn diminish our ability to fully understand how fungi work, including their role in plant growth and growth. carbon sequestration, she warns.

“This economic analogy may have closed our eyes to other possibilities regarding how mycorrhizas work,” says Karst.

In mycorrhizae, resources travel in opposite directions; mycorrhizal fungi receive carbon, in the form of lipids and sugars, while plants receive nutrients such as phosphorus and nitrogen.

This flow of resources has generally been framed by what are called biological market models, in order to understand how mycorrhizae maintain this mutual relationship over centuries, Karst explains.

At the other extreme is a recent alternative theory, called the “surplus C” hypothesis, which suggests that plants often produce more sugars than can be used for growth and that mycorrhizal fungi are a sink that receives this excess carbon.

“This means that the amount of carbon transferred to a fungus is independent of the supply of nutrients to the plant,” explains Karst.

After reviewing and analyzing evidence from a wide range of scientific studies, Karst and his co-authors did not find strong support for organic market models.

“We found no empirical evidence for direct regulation and reject the idea that ‘prices’ – the number of carbon units per nutrient unit – regulate carbon transfer to fungi. Instead “We found more support for the theory that carbon is attracted to the most powerful sink, which is mycorrhizal fungi,” she says.

Along with this, their study indicated that mycorrhizal plant growth was linked to nutrient uptake rather than carbon transfer, meaning that it is not the amount of carbon transferred to mycorrhizal fungi that reduces the plant growth, but it is the amount of nutrients delivered – or not. delivered – by the fungus. In this sense, the carbon transferred to mycorrhizal fungi does not cost the plant much.

Collectively, the results are more consistent with Surplus C theory than with market models, the researchers conclude.

The observations highlight the importance of going beyond the limitations of economic models to learn more about key interactions between plants and mycorrhizal fungi that could, for example, potentially benefit the environment, Karst adds. “A substantial amount of carbon flux from plants to fungi and mushrooms constitutes a large reservoir of carbon in the soil, so it is necessary to understand how this flux works to harness it in the sequestration of this carbon.”

Their review revealed the need for further research to more closely explore fundamental questions about what drives carbon transfer from plants to mycorrhizal fungi and how to fully measure the strength of its carbon sink, notes Karst.

And even if a mechanism directly linking the transfer of carbon and nutrients could one day be discovered, until then, researchers “urge caution” in characterizing the relationship in economic terms, she adds.

“We are familiar with the idea of ​​markets, but not the inner workings of mycorrhizae. So we need to remain open to other ways of understanding how these systems work.”