The vast reservoir of carbon stored in the soil is entering the Earth's atmosphere at an increasing rate, most likely as a result of warming temperatures, scientists say.
When bacteria chew on decaying leaves and fungi chow down on dead plants, they convert that storehouse of carbon into carbon dioxide that enters the atmosphere.
In a study published in the journal Nature, scientists show that this process is speeding up as Earth warms and is happening faster than plants are taking in carbon through photosynthesis.
Researchers from the Pacific Northwest National Laboratory (PNNL) in the US found that the rate at which microbes are transferring carbon from soil to the atmosphere has increased 1.2 per cent over a 25-year time period, from 1990 through 2014.
While that may not seem like a big change, such an increase on a global scale, in a relatively short period of time in Earth history, is massive.
The finding, based on thousands of observations made by scientists at hundreds of sites around the globe, is consistent with the predictions that scientists have made about how Earth might respond to warmer temperatures.
"It's important to note that this is a finding based on observations in the real world. This is not a tightly controlled lab experiment," said Ben Bond-Lamberty of the Joint Global Change Research Institute.
"Soils around the globe are responding to a warming climate, which in turn can convert more carbon into carbon dioxide which enters the atmosphere," Bond-Lamberty said.
"Depending on how other components of the carbon cycle might respond due to climate warming, these soil changes can potentially contribute to even higher temperatures due to a feedback loop," he added.
Globally, soil holds about twice as much carbon as Earth's atmosphere. In a forest where stored carbon is manifest in the trees above, even more carbon resides unseen underfoot.
The team relied heavily on two global science networks as well as a variety of satellite observations.
The Global Soil Respiration Database includes data on soil respiration from more than 1,500 studies around the globe.
FLUXNET draws data from more than 500 towers around the world that record information about temperature, rainfall and other factors.
"Most studies that address this question look at one individual site which we understand very well," said Vanessa Bailey, a soil scientist.
"This study asks the question on a global scale. We are talking about a huge quantity of carbon. Microbes exert an outsize influence on the world that is very hard to measure on such a large scale," said Bailey.
The study focused on a phenomenon known as "soil respiration," which describes how microbes and plants in the soil take in substances like carbon to survive, then give off carbon dioxide.
Soils don't exactly breathe, but as plants and microbes in soil take in carbon as food, they convert some of it to other gases which they give off - much like we do when we breathe.
Scientists have known that as temperatures rise, soil respiration increases.
The team sought to compare the roles of the two main contributors, increased plant growth and microbial action.
The researchers discovered a growing role for microbes, whose action is outstripping the ability of plants to absorb carbon.
In the 25-year span of the study, the proportion of soil respiration that is due to microbes increased from 54 to 63 per cent.
