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Students look at soil samples

An essential relationship: Measuring carbon in soil

Note: This story is part of a special issue of The Warnell Log focused on carbon

How do you measure carbon in soil? Warnell researchers are doing the math.

Don’t let the “Soil Biogeochemistry Lab” name fool you. It’s all about carbon.

“Nearly everything in my lab is carbon related,” said Rebecca Abney, assistant professor of forest and disturbed soils at Warnell. The combination of soil and carbon can be powerful, she argues—the thing is, too many people discount this relationship.

“People just don’t think about soil,” she said with a laugh, “unless they’re gardeners or farmers­—and maybe foresters? But I think about this all day long, so I’m super biased.”

Abney’s lab is juggling several projects related to carbon. One of her main interests is pyrogenic carbon, or carbon created by the act of burning. The burnt landscape after a fire can have a very fertile, high-carbon soil. 

Over the years, scientists have been able to quantify the nutrients left by fire across a landscape, as well as the specific soil benefits—for example, the carbon left after a prescribed fire can increase soil pH, which is generally beneficial for Georgia’s clay soils.

But, it’s what happens to the carbon—left over from the fire and now distributed across the landscape—that she and others in her lab grapple with.

“Where is it going, how long is it in the soil, and how does it evolve in the soil” said Abney as she listed some of the topics she focuses on in her lab. “I’ve also been playing with terrestrial-to-aquatic-system connections and how it becomes dissolved in looser soil and how fast it decomposes—what are the conditions that are making that more or less likely?”

At Warnell’s Whitehall Forest, there is a spot where a prescribed burn got hot, burning through a stump in the ground.

Only you no longer see the stump. Rather, you see its outline in the clay soil, and black carbon left in the fire’s wake. Sometimes, that stump can even burn underground. What happens when these below-ground tree parts become carbon and are part of the soil?

All this ties into the questions Abney asks. How well is this carbon moving through the soil? Can roots and stumps left after a tree is harvested contribute to the soil’s carbon content? And are they even beneficial?

She and her students have taken soil samples from around the state, looking for carbon and dating it. Carbon is an essential part of soil and it provides a valuable amendment. But as they work to understand more about its role in feeding trees, she and her students are discovering more questions than answers.

For example, she has been examining carbon pools in water and soil, testing samples of soil using two methods to extract carbon and age it through radiocarbon dating. One method uses a relatively gentle technique called the water extraction method: you mix soil and water and shake it, then filter the particles out. In a second method, called the lysimeter method, a process collects water after forcing it through a tube.

“And, those are looking at very different organic matter pools; the carbon composition is really different,” she said. “And when we age them, and the age is very different.”

Abney is finalizing the results of that study in a new paper to be published later this year. When it comes out, she said, it may change how we view carbon composition in soil water.

Still, it’s just a drop in the bucket, so to speak.

“I still feel like we don’t know as much as we’d like to.”



Assistant Professor of Forest & Disturbed Soils

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