Lakes seep over the Arctic tundra Credit: Punchstock

In the largest experiment of its kind to date, ecologists have found that the wetter the Arctic tundra becomes, the more carbon dioxide it gives off.

If the tundra becomes increasingly warm and wet — which is anticipated as global temperatures rise — it might emit more carbon than expected, the work suggests.

"It's a big deal," says Walter Oechel, an ecologist at San Diego State University in California and principal investigator on the study. "These are aspects that haven't been recognized before."

His colleague Donatella Zona reported the findings in Albuquerque, New Mexico, on 3 August at the annual meeting of the Ecological Society of America.

The Arctic region locks up large amounts of carbon in its soil and permafrost, and researchers are working to tease out the complex relationships that might govern what happens to that carbon in a globally warmed world. So Oechel's team set out to deliberately manipulate the water table of a portion of the tundra.

The wetter it gets

Previous experiments to test the relationship between increasing water depth and carbon flux have all been on a much smaller scale, such as testing core soil samples in the laboratory or small field studies, says Zona. For the latest work, the team chose as their test site a 1.2-kilometre-long lake in the North Slope region of Alaska. It lies about 10 kilometres away from the coastal town of Barrow and is part of a long-term ecological study region called the Barrow Environmental Observatory.

Using plastic dikes, the researchers divided the lake into three parts. From the centre third they pumped water into the northern third, leaving the northern part extra soggy and the centre portion dry. They left the southern part untouched, as a control. Instrument towers, one above each portion, gathered data on the emission of trace gases such as methane and CO2.

The team thought that the higher water table would probably mean less CO2 entering the air. Rising water drowns both the vegetation and the 'aerobic' microbes that rely on oxygen to decompose the plant matter, giving off CO2 in the process. So the greater the depth of water above the surface, the less oxygen and light penetrates to stimulate microbes in the soil beneath, hence presumably resulting in the release of less CO2.

But measurements from the partitioned lake showed the opposite: the northern, flooded portion of the lake gave off more CO2, becoming a carbon source, whereas the drained portion and the control portion remained as carbon sinks.

Apparently, Zona says, anaerobic microbes in the soil — those that don't require oxygen but can still produce CO2 — thrived under the drowned land. This, she says, suggests that hydrological changes in the Arctic could have "unexpected results — huge amounts of carbon in the soil could be released to the atmosphere even under completely anaerobic conditions".

Permafrost under the flooded lake also thawed to greater depths than that under the drained lake, Zona says.

Jeffrey Welker, an ecologist at the University of Alaska in Anchorage, says that a deeper thaw means that older carbon — perhaps 1,000 years old rather than 100 years old — gets released from the soil. "We're beginning to see evidence that deeper, older carbon will be entering the atmosphere in the future," he says.

The implications of that are still unclear. At the moment, ecologists are monitoring changes in the Arctic as best as they can, trying to quantify differences from year to year. Early data from the lake manipulation have also revealed trends in methane emissions — as they reported earlier this year in Global Biogeochemical Cycles1. In that paper, Zona and colleagues show that methane emissions at the lake were at a maximum when the water level was just covering the ground surface.

Together, such work underscores the complexity of interactions in the Arctic system and shows just how much researchers have left to understand. For now, Zona and her colleagues are gathering another summer's worth of data to see what they reveal about both CO2 and methane fluxes. "We think it's interesting even if it's a short-term trend," she says.