May 17, 2013
Researchers Suprised By Arctic Resiliency In Carbon Storage
April Flowers for redOrbit.com - Your Universe Online
Certain assumptions were made by UC Santa Barbara doctoral student Seeta Sistla and her adviser, environmental studies professor Josh Schimel, when they traveled north recently to study the effects of long-term warming on the carbon storage of the Arctic.
"We expected that because of the long-term warming, we would have lost carbon stored in the soil to the atmosphere," said Schimel. He explained that the gradual warming would accelerate decomposition on the upper layers of what would have been frozen, or nearly frozen, soil. This would release the greenhouse gas into the atmosphere. Nearly half of all global soil carbon is contained in the permafrost of the high latitudes. Even a few degrees´ rise in temperature in these regions could be enough to release massive quantities of carbon, turning a carbon repository into a carbon emitter.
"The Arctic is the most rapidly warming biome on Earth, so understanding how permafrost soils are reacting to this change is of major concern globally," Sistla said in a statement.
The pair of scientists visited the longest running climate warming study in the tundra, the US Arctic Long-Term Ecological Research (ARC LTER) site at Toolik Lake in northern Alaska, to test their hypothesis. The ARC LTER was started in 1989 as an ecosystem-warming greenhouse experiment to observe the effects of sustained warming on the Arctic environment.
Their initial findings were typical of Arctic warming. Low-lying, shallow-rooted vegetation was giving way to taller plants with deeper roots, a greater dominance of wood shrub, and an increase thaw depth were all expected phenomenon. The surprise was that two decades of slow and steady warming had not changed the amounts of carbon in the soil, despite vegetation changes and even changes in the soil food web.
According to Sistla, the answer to this mystery might be found in the finer workings of the ecosystem. For instance, increased plant growth seems to have facilitated stabilizing feedbacks to soil carbon loss.
"We hypothesize that net soil carbon hasn't changed after 20 years because warming-accelerated decomposition has been offset by increased carbon inputs to the soil due to a combination of increased plant growth and changing soil conditions," Sistla said.
The warmer temperatures — on average 3.6 degrees Fahrenheit in the air and 1.8 degrees in the soil to the permafrost — has caused the increase plant productivity, which has increased plant litter inputs to the soil. The scientists found that the soils in the greenhouse experiment developed warmer temperatures in the winter, while the summer warming effect decreased.
"These changes reflect a complicated feedback," Sistla said. "Shrubs trap more snow than the lower-lying vegetation, creating warmer winter soil temperatures that further stimulate both decomposers and plant growth. Shrubs also increase summer shading, which appears to have reduced decomposer activity in the surface soil by reducing the greenhouse effect during the summer."
The increased carbon availability in the deeper mineral layer that overlies the permafrost might have been caused by the increased plant growth and deeper thaw, according to the study published in Nature. The research team found the strongest biological effects of warming at depth, a “biotic awakening.” The mineral soils decomposers showed more activity, as well as an increased carbon stock at that level.
"It's a surprising counterbalance," said Schimel. "It may be that those soil systems are not quite as vulnerable to warming as initially expected."
It is still unclear if this phenomenon — no net loss of soil carbon despite long-term warming — is a transient phase that will eventually give way to increased decomposition activity and more carbon release. The team is planning future studies to include investigation into the mineral soil to determine the age of the carbon, which may in turn yield clues into how the carbon cycle is changing at depth, where the majority of tundra soil carbon is stored.
Sistla and Schimel say this research paradigm validates the National Science Foundation LTER program´s commitment to supporting long-term experiments because it creates opportunities for younger scientists to observe effects and condition decades after experiments are established — results that could not have been foreseen when the experiments were started.