Carbon below the Surface

Penn State Biologist Eric Post is exploring the under-ground carbon cycle in Greenland's tundra.

Sean Cahoon flux chamber 2012
Sean Cahoon flux chamber 2012

Eric Post, professor of biology at Penn State University, has spent the past 20 years studying ecology on the tundra outside of Kangerlussuaq, Greenland. And until recently, his research concentrated on what he could see and test above the ground.

This included long-term observations on the timing of plant growth, also known as phenology, the overlap of caribou calving and plant growth, the impact of grazing on plant community composition, and more.

Underground carbon cycle

But in 2009, Post realized that a significant amount of activity that influenced the Arctic ecosystem was taking place underground.

Specifically, he and graduate student Sean Cahoon were working side by side, with Post recording plant growth in a study plot, and Cahoon measuring carbon flux. It was spring, and the plants were budding—signals that photosynthesis was underway. The scientists expected to find carbon uptake on the plots. Instead, the measurements indicated net carbon release.

This was Post’s “aha!” moment.

“I started wondering exactly what is going on below ground,” says Post. “What I’ve since learned is that when you look above ground you get less than half the picture of what’s going on in the system.”

Warming climate and plant root systems

Now in its second year, this four-year study,  is supported by the National Science Foundation. Post's research estimates and compares responses of plant phenology (both on the surface and underground) to warming and their respective contributions to the exchange of carbon between the atmosphere and tundra. Post also aims to determine through whether the root systems of shrubs or grasses are more affected by warming.

Above the ground, large, woody plants like shrubs have a higher carbon uptake capacity, which means they can potentially store more carbon in their tissues, than grasses.

Theoretically, tundra dominated by shrubs ought to have a larger carbon uptake during the growing season. Yet a great portion of an ecosystem’s carbon cycle occurs below ground.

Post and his team hope to clarify the connection between below ground carbon systems and above ground plant growth and carbon sequestration.

Experiment logistics

To conduct the experiment, researchers measure a series of plots—some in their natural vegetative state and others stripped of all organic matter. The latter offer the scientists a window into the soil’s role in the carbon cycle.

Scientists measure carbon on all of the plots by placing a two-foot-tall plastic chamber on the plot and evacuating the air within it to establish a consistent baseline. Then they allow the air to recalibrate to prime the chamber, which will measure carbon dioxide concentration, a function of how much carbon is being absorbed and released.

“The whole aspect of plant versus heterotrophic [carbon released from the soil] respiration is complex and difficult to resolve within total ecosystem carbon flux,” says Post.

How warming affects plant productivity

The results from this study will help Post and colleagues piece together a comprehensive picture of how warming affects above- and below-ground phenology and plant productivity in the tundra.

This information will help inform computer models of the impacts of climate change; in addition to anticipating how a warmer climate might impact plant growth on the surface, it can inform about potential changes in the carbon cycle—an important measure of ecosystem function as well as of biological feedbacks to climate change—that occur as a result of above- and below-ground growth dynamics.  —Rachel Walker