drought could destabilize terrestrial carbon sinks

2023-05-02 18:00:03

An outdoor structure built to reduce rainfall as part of the Loma Ridge climate change experiment in Irvine, California, USA, where researchers are studying the impact of drought on soil microbial communities and on carbon storage. STEVEN ALLISON

The adaptation of microorganisms to drier soil could worsen the warming of the atmosphere, according to an opinion piece published on April 12 in the Trends in Microbiology. According to the worrying hypothesis put forward by Steven Allison, an ecologist and professor at the University of California, the adaptation of microorganisms could destabilize the storage of carbon by the soil, which would lead to carbon losses. “greater than those currently anticipated under the effect of climate change”.

Based on available scientific knowledge, Steven Allison has drawn up a balance sheet of the responses that microbes are able to provide in the face of drought. They can modify their physiology to maintain the water level in their cells, by secreting more metabolites which form a biofilm for example, or turn into spores and remain dormant for decades before “resurrecting” when the habitat is moistened. Thanks to the speed of reproduction of microorganisms, the new communities most likely to survive dry conditions are established. Mutation and selection cause species to evolve.

According Pierre-Alain Maron, specialist in microbial ecology at the National Research Institute for Agriculture, Food and the Environment, “The soil is a veritable jungle for micro-organisms”, and microbes survive, despite their position at the bottom of the food chain, thanks to significant adaptive capacities. They are more resilient than plants to rebound after droughts, according to Steven Allison. The experiments that his team has been conducting since 2007 in Irvine, in southern California, have shown that microbes surviving droughts can resume their activities a year later, as if nothing had happened.

Thanks to organic matter, soils form a very powerful carbon sink in the Earth system. According to the sixth report of the Intergovernmental Panel on Climate Change (IPCC), they contain 1,700 gigatonnes of carbon – not counting the carbon stored in permafrost – a volume greater than all the carbon contained in plants and in the atmosphere.

See as well : Permafrost, the other climate threat

The balance of the carbon cycle linked to soils depends on living organisms. Plants absorb carbon dioxide from the atmosphere through photosynthesis and capture the carbon in their structure and root system. At the same time, the vast majority of microorganisms that inhabit the soil “breathe”. They use the molecules of organic matter to produce energy and emit carbon dioxide – CO₂, most of the time – in the atmosphere. And the cycle continues.

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