As global temperature increases, one of the great uncertainties in climate projections is the amount of carbon that permafrost could thaw through microbial decomposition in the soil. Plant roots stimulate this microbial decomposition of organic matter in the soil, a process called the "priming effect." For the first time, an international team of researchers, coordinated by INRAE ​​and Stockholm University, shows that the priming effect would only cause the emission of 40 Gigatons of permafrost carbon by 2100. Their results are published on July 20, 2020 in Nature Geoscience.

Permafrost is a permanently frozen soil that is found mainly in high latitudes and covers just over 20% of the Earth's surface. It is mainly made up of organic matter and is believed to store about a third of the Earth's carbon. Permafrost has an active layer that thaws in the summer. This thawing is accompanied by the decomposition by microorganisms of the organic matter present in the soil. When breathing, these microorganisms will emit greenhouse gases from the carbon trapped in the soil. With global warming, the melting of permafrost is accelerating, and researchers currently estimate that it will be accompanied by the emission of 50 to 100 gigatons of carbon by the year 2100. But other factors influence permafrost's greenhouse gas emissions. , including the presence of plants.

The active layer of permafrost, which thaws in the summer, allows plants to grow that will develop their roots in the soil. These roots release sugars into the soil that will stimulate microorganisms. This can have the effect of accelerating the decomposition of organic matter in the soil and increasing the respiration of microorganisms and, therefore, greenhouse gas emissions - this is called the priming effect. "We have known about this process since the 1950s, and INRAE ​​researchers have played an important role in its international recognition. But we did not know if this small-scale ecological interaction had a significant impact on the global carbon cycle" . explains Frida Keuper, researcher at INRAE.

To develop their model, the researchers relied on plant activity maps and high-resolution in-depth data for the amounts of carbon in the soil. They also conducted two meta-analyzes to model, on the one hand, the relationship between the priming effect and plant activity, and on the other hand, the distribution of roots in the soil. These data allowed them to build a model to measure the priming effect on permafrost from field data and its influence on microbial respiration and greenhouse gas emissions.

Their results show that the priming effect could induce the emission of an additional 40 Gigatons of carbon by 2100 compared to current permafrost predictions. For comparison, the "remaining carbon budget" of human activities to limit global warming to 1.5 ° C is estimated at 200 Gigatons of carbon by 2100. These new data demonstrate the need to account for ecological interactions on a small scale. Scale, such as the root-induced priming effect, to have more accurate projections in global models of greenhouse gas emissions.