Soil life reacts surprisingly to warming

Soils in Iceland that have been warmed by geothermal activity have opened up special research opportunities for scientists. © C: Christina Kaiser

Climate change may be fueling the release of CO₂ from the soil in a different way than previously thought: warming activates soil bacteria in a special way, study results suggest. Until now, it was assumed that increasing temperatures generally increase the growth rates of microbes. But now it turns out that the division activity actually remains the same - but the warming leads to the activation of previously dormant bacterial species, which leads to an overall increase in growth. This discovery could lead to a better understanding of the microbial degradation processes in the soil and the climatic feedback mechanisms, say the researchers.

Soils literally form the basis of the earth's material and climate cycle. Because they are the largest reservoir of organic carbon on earth. The extent to which it remains stored there or enters the atmosphere in the form of carbon dioxide depends on the breakdown of plant biomass in the soil by microorganisms. In this context, concern is directed at a possible feedback process in the context of climate change: As temperatures rise, it can be assumed that the microbial communities in the soil will grow and thus release more carbon dioxide. This further fuels climate change, which again leads to more soil respiration.

The soil carbon-climate feedback in sight

“For decades it was assumed that this reaction was driven by increased growth rates of individual bacterial populations in a warmer climate,” explains senior author Andreas Richter from the University of Vienna. In their study, he and his colleagues wanted to gain more detailed insights into this system. To do this, they took advantage of a very special research opportunity: The team examined a subarctic grassland on Iceland that began to become increasingly warmer around 50 years ago due to a geothermal process in the volcanic region: The geological changes have now resulted in soil temperatures that are several degrees higher than in the surrounding areas.

The researchers have now conducted comparative studies to investigate how this warming has affected the microbial community underground. They carried out analyzes of the composition of the bacterial species and recorded the growth rates of the microbes in the various study areas using modern isotope labeling techniques.

First of all, it was confirmed: “The warming of the soil that has been ongoing for more than 50 years has increased microbial growth at the community level, as expected,” says first author Dennis Metze from the University of Vienna. However, the results of the analysis of the growth rates of the various types of bacteria seemed to contradict this result: In principle, they were indistinguishable in the warmer soils from those at normal temperatures. Analyzes of diversity – the presence of different types of bacteria in the soil – revealed how the overall increase had occurred. It turns out that warming appears to have activated previously dormant species. As a result, they expanded their stocks, which led to the increase on an overall level, the scientists say.

Instead of general growth, more diversity

“We confirm that soil warming has increased community-level microbial growth over more than 50 years. “But while this was previously thought to be caused by generally faster growth, we found that in bacteria this response is caused by a larger group of differently active species,” the authors write. As they emphasize, further studies must now show to what extent the finding could be a peculiarity of the specific area of ​​investigation.

However, if it turns out to be a fundamental mechanism, it would have significant implications for understanding soil microbial degradation processes and climatic feedback mechanisms. This is because soil bacterial species that become active under warmed conditions may differ functionally from those that are active at lower values. “More complex equations may be required to model the microbial warming response,” the researchers write.

Co-author Christina Kaiser from the University of Vienna concludes: “Predicting the response of the soil microbiome to climate change is a major challenge. Soil is therefore treated as a 'black box' in most climate and carbon cycle models. “This work creates the conditions for more accurate predictions of microbial behavior and the resulting effects on the carbon cycle in an evolving climate system,” said the researcher.

Source: University of Vienna, specialist article: Science Advances, doi: 10.1126/sciadv.adk6295

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