Sunlight and not bacteria is the key to triggering the production of carbon dioxide (CO2) from material released by Arctic soils, says a significant study.
The vast reservoir of carbon stored in Arctic permafrost (permanently frozen soil) is gradually being converted to carbon dioxide (CO2) after entering the freshwater system in a process which has so far been thought to be controlled largely by microbial activity.
"This represents a major change in thinking about how the carbon cycle works in the Arctic," said study co-author Byron Crump from Oregon State University in the US.
"It turns out, that in Arctic rivers and lakes, sunlight is faster than bacteria at turning organic carbon into CO2," noted lead author Rose Cory, an assistant professor of earth and environmental sciences at University of Michigan.
The research team measured the speed at which both bacteria and sunlight converted dissolved organic carbon into carbon dioxide in all types of rivers and lakes in the Alaskan Arctic, from glacial-fed rivers draining the Brooks Range to tannin-stained lakes on the coastal plain.
In virtually all of the freshwater systems they measured, however, sunlight was always faster than bacteria at converting the organic carbon into CO2.
"This is because most of the fresh water in the Arctic is shallow, meaning sunlight can reach the bottom of any river - and most lakes - so that no dissolved organic carbon is kept in the dark," Crump explained.
Another factor limiting the microbial contribution is that bacteria grow more slowly in these cold, nutrient-rich waters.
"Light, therefore, can have a tremendous effect on organic matter," Cory pointed out.
Arctic permafrost contains about half of all the organic carbon trapped in soil on the entire Earth - and equals an amount twice of that in the atmosphere.
"This new understanding is really critical because if we want to get the right answer about how the warming Arctic may feedback to influence the rest of the world, we have to understand the controls on carbon cycling," Cory said.
The study appeared in the journal Science.
