Thawing permaforst. Photo: Benjamin Jones via Wikimedia Commons

A key uncertainty in climate projections is the amount of carbon emitted by thawing permafrost in the Arctic. Plant roots in soil stimulate microbial decomposition, a mechanism called the priming effect. An international research team from Stockholm University, Umeå University and the French National Research Institute for Agriculture, Food and Environment (INRAE) shows that the priming effect alone can cause emission of 40 gigatons of carbon from thawing permafrost by 2100. The study was published in Nature Geoscience.

Permafrost is permanently frozen ground that stores about one-third of terrestrial soil carbon. Permafrost has an active layer, which thaws in summer, allowing plant and soil life to thrive. When microorganisms breathe, they emit greenhouse gases. On top of that, plant roots provide sugars to microorganisms in the soil, which the microbes can break down into new soil organic matter – i.e. the priming effect – resulting in even higher greenhouse gas emissions. By 2100, rapidly rising temperatures will drive the emission of 50-100 gigatons of permafrost carbon through thawing and increased microbial respiration.

“We have known about this process since the 1950s, but we did not know whether or not this small-scale ecological interaction had a significant impact on the global carbon cycle,” says Frida Keuper, Research Scientist at INRAE and Umeå University, Sweden.

The team combined maps of plant activity and data on soil carbon content from the Northern Circumpolar Soil Carbon Database with an extensive literature survey on priming and plant root properties, to estimate the priming effect in permafrost ecosystems and its influence on greenhouse gas emissions. They showed that the priming effect increased soil microbial respiration by 12 percent, which, in turn, caused the additional release of 40 gigatons of carbon by 2100. To put this estimate into perspective, the remaining ‘carbon budget’ for human activities for limiting global warming to 1.5°C currently stands at 200 gigatons of carbon.

“These new findings demonstrate how important it is to consider small-scale ecological interactions, such as the priming effect, in global greenhouse gas emission modelling,” says Birgit Wild, Assistant Professor at the Department of Environmental Science, Stockholm University.

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