USDA Scientists Measure CO2 In Soil

Tom Sauer adjusts a Bowen ratio apparatus at the Neal Smith National Wildlife Refuge. (Credit: Kevin Jensen)

When most climate researchers consider the role of carbon dioxide around the globe in the current period of accelerated global climate change, they consider carbon dioxide in the atmosphere and how it contributes to warming the Earth. But scientists like Tom Sauer, supervisory research soil scientist with the United States Department of Agriculture-Agricultural Research Service, have been studying a different side of the carbon dioxide story: the CO2 in soil under our feet.

The carbon dioxide in soil, Sauer observes, is many times higher in concentration than the carbon dioxide above. The interplay among carbon dioxide, oxygen, organic matter and water is complex, and by using specially designed carbon dioxide sensors in the soil, Sauer and other researchers have discovered a great deal about the role of carbon dioxide in the soil environment, how soil dynamics influence global climate change and how important soil health is to the overall health of the globe.

Sauer uses a new soil monitoring method that involves Vaisala carbon dioxide sensors placed directly in the soil. The sensors use unique Carbocap technology to detect the amount of carbon dioxide in the air in spaces between soil particles. Sauer and his postdoc Tom DeSutter modified the sensors for soil use, as they were not originally built for that purpose.

“Later, Vaisala came out with sensors specifically for soil, which ironically we’ve never used ourselves,” Sauer says. “The modified ones have been working great, so we’ve continued to use them.” Sauer and colleagues Xinhua Xiao and Bob Horton at Iowa State University used the sensors at different depths in Midwestern crop lands.

Iowa State University undergrad Dawn Schroeder and undergrad student Karl Bear of Luther College install cylinders for the soil CO2 flux chambers at Iowa State University’s Agronomy and Agricultural Engineering Research Center. (Credit: Tom Sauer)

Sauer’s measurement of soil CO2 gradients was developed to avoid shortcomings of older methods that used a chamber placed on top of the soil surface to measure carbon dioxide emissions. This method had many drawbacks, for example the presence of the chamber changed the airflow and carbon dioxide transfer that actually occurred from the soil.

In addition, the chamber typically would sit on a PVC collar so it would not be placed directly on the soil. The presence of the collar changed soil properties within the collar, typically making it wetter than the soil surrounding it. This would also alter the organic matter decomposition processes compared to the surrounding soil. Chamber measurements were typically time intensive enough that they would only occur once a week, although the potential for hourly measurements existed with more expensive automated chamber systems.

Sauer’s new method involves carbon dioxide sensors placed directly in the soil, so there is no potential for a chamber to change the soil environment and alter the carbon dioxide measurements. Instead, the measurements accurately reflect those in the soil environment. The direct sensor method does not alter carbon dioxide flow as does the chamber method, nor does it change the surface soil profile. Making hourly measurements is not difficult, although the sensors do warm the soil slightly, so measurements more frequent than hourly would not be recommended by Sauer.

Interestingly, the tale of how Sauer happened upon the new soil carbon dioxide gradient measuring method begins with a story of lost luggage. “My luggage got lost and so did that of a Japanese researcher who was going to the same conference. We shrugged and decided to share a taxi to the hotel together. On the way there, he told me all about how they did soil research in Japan, a place with soil very different from here in the Midwest: It’s volcanic soil, highly porous, not as much water in it like we have here. He was the one who turned me on to the possibility of using these sensors directly in the soil. So it turned out some lost luggage changed my research direction,” Sauer laughs.

Sauer says a major challenge is calculating carbon dioxide flux with the gradient method is determining a diffusion coefficient that accurately estimates airflow through the soil. This coefficient is always changing due to the fact that only half of soil is solid and the other half is porous. “Big changes can happen just because the pores in soil can be dry or wet, or quickly change from one to the other,” said Sauer. “You need to figure out what the change is, yes, but also how fast it is. That can be quite complicated.”

Bowen ratio apparatus at the Shelton Vineyards in Dobson, North Carolina. (Credit: Tom Sauer)

But Sauer isn’t resting on the gains he’s made thus far. Even newer technology Sauer is getting into involves micro Bowen ratio systems, where air at 1 and 6 centimeters above the soil is drawn into an infrared gas analyzer. The technique, developed with colleagues in North Carolina and Israel has already been used successfully in North Carolina, Iowa, and California.

Although previous methods showed drawbacks to using a chamber to measure soil carbon dioxide emissions, Sauer admits that the sensors he places directly in the soil have drawbacks as well, causing some warming. He reasons that a better-built canopy chamber which disturbs the soil environment less might be the answer.

Sauer is currently working on a new flow-through canopy chamber using a LI-COR infrared gas analyzer that he hopes will counteract drawbacks in previous chamber work. The new method seeks to integrate the role of plants in the surface carbon dioxide measurements. Incorporating plants is an important next step because they take up some of the carbon dioxide just emitted from the soil. Sauer is working on an open-canopy chamber about 5 feet tall that will be also be able to measure evapotranspiration.

Lastly, Sauer’s research has given him some insight into how we might help the planet in terms of global climate change and also help ourselves: Stop tilling soil. “Soil has developed over thousands of years,” he says. “The slow decomposition of leaves, stems and roots is what builds stable soil organic matter. When you plow the soil, you add air and decompress the soil, accelerating decomposition and releasing more carbon dioxide to the atmosphere. The slower the decomposition, the less carbon dioxide is released and the better the soil quality.”

He estimates 50 percent of the native carbon in the soil has been lost through cultivation. “You can get more carbon into the soil by adding more organic matter. Or you can slow down the rate of decomposition,” said Sauer. “Current agricultural practices are still accelerating decomposition and reducing soil organic matter, whereas naturally the rate is quite slow and soil organic matter builds over time.”

Essentially, he says that what is good for us in terms of soil health is good for the planet as well. “If we can increase the organic matter and quality of the soil, it will be more resistant to drought and better able to withstand flooding. Also food production will be better,” he says. “Basically, we need to think about soil sustainability in the long term. As a scientist, my goal is to provide information for policymakers, farmers and others to make good decisions about how to treat, protect and maintain the health of our soil for years to come. That is my mission. If you want to keep the planet healthy, you can start by keeping the soil healthy.”

Top image: Tom Sauer adjusts a Bowen ratio apparatus at the Neal Smith National Wildlife Refuge. (Credit: Kevin Jensen)