As soil warms from climate change, it releases additional carbon into the atmosphere – but that effect diminishes over the long term, say scientists.
Soil micro-organisms release 10 times as much carbon dioxide as human activities, but historically have been kept in check by plants' uptake of CO2 from the atmosphere.
"While they're low on the charisma scale, soil microorganisms are so critically important to the carbon balance of the atmosphere," says Serita Frey, professor of natural resources at the University of New Hampshire.
"If we warm the soil due to climate warming, are we going to fundamentally alter the flux of carbon into the atmosphere in a way that is going to feed back to enhance climate change?"
The team examined just how efficiently soil micro-organisms utilize food sources to maintain their cellular machinery. In one short-term scenario, warming temperatures appeared to have little effect on soils' ability to use glucose, a simple food source released from the roots of plants.
For phenol, a more complex food source common in decomposing wood or leaves, soils showed a 60 percent drop in efficiency at higher temperatures.
"As you increase temperature, you decrease the efficiency – soil microorganisms release more carbon dioxide to the atmosphere – but only for the more complex food sources," says Frey. "You could infer that as the soil warms, more carbon dioxide will be released into the atmosphere, exacerbating the climate problem."
The effect diminishes, though, in the second scenario, in which soils were warmed to five degrees Celsius above the ambient temperature for 18 years.
"When the soil was heated to simulate climate warming, we saw a change in the community to be more efficient in the longer term," Frey says. "The positive feedback response may not be as strong as we originally predicted."