For the sake of land and climate, coaxing soil to soak up carbon

Promising results of soil treatments to sequester carbon lead to field tests

In a novel approach to stalling global warming while reinvigorating nutrient-depleted farmland, chemists have found they can promote soil’s natural ability to soak up greenhouse-gas carbon dioxide from the surrounding air.

Experiments led by Jim Amonette at the Department of Energy’s Pacific Northwest National Laboratory in Richland, Wash., and reported today at the American Chemical Society national meeting, show that maintaining a proper alkalinity plus frequent wetting and drying cycles can coax soil to retain more carbon.

“Globally, soils contain four times as much carbon as the atmosphere, and half of the soil carbon is in the form of organic matter,” said Amonette, a PNNL senior research scientist. Until about 30 years ago, soil tillage released more carbon dioxide to the atmosphere than burning of fossil fuels. Some agricultural soils have lost a third of their carbon from tillage.

“These carbon-depleted soils are a tremendous potential reservoir for carbon that can help slow the increase in atmospheric carbon dioxide,” Amonette said. “The amount of carbon added to soil in a year is incredible. Today, 99 percent of it comes out the top as carbon dioxide. If we can increase the fraction that is retained in soil by even a small amount, it will make a huge difference.”

Amonette’s experiments promoted the activity of tyrosinase, a common enzyme that catalyzes soil’s natural “humification” process. This process involves the gradual incorporation of carbon from dead plants and microbes into stable organic matter called humus, which is responsible for the dark color in many soils. Tyrosinase increases the reaction rate between oxygen and humus precursors, such as phenols and hydroxybenzoic acids, to form quinones. The quinones react with amino acids released by soil microbes to form complex, durable molecules called humic polymers.

“Because humic polymers are less easily degraded by microbes than the precursor molecules, they survive to diffuse into small pores in soil aggregates where they are stabilized for decades, if not centuries,” Amonette said.

The humification rate depends on many factors: enzyme stability, moisture, alkalinity, oxygen availability, microbial population and the physical properties of different soils. Amonette’s experiments were designed to weigh the importance of these many factors and to learn ways they might be manipulated to increase humification.

In the lab, Amonette assembled 72 elaborate plastic-tube configurations he likens to “those Russian nesting dolls,” matrioshkas. The tubes allowed Amonette to control individual moisture levels and oxygen availability. Each soil sample was placed between the inner and outer walls of water-tight but gas-porous concentric cylinders. These were placed inside yet a larger “chimney” tube to control the humidity as well as the type of gas and its flow rate.

Amonette was particularly interested in identifying soil components and soil additives that might improve tyrosinase’s natural ability to promote humification. He found that an alkaline, porous material called “fly ash,” a byproduct of coal combustion, “speeds up the normal humification process by promoting the reaction of the quinones with the amino acids and providing small pores to protect humic polymers,” he said. “Frequent cycles of wetting and drying appear to be important, too, for fostering a rich microbial community that supplies many of the humic precursors and for aiding the formation of soil aggregates.”

Amonette is eager to put his results to the test where it matters most–in the field. He will get his wish in May, when he travels to a field outside of Charleston, S.C. There, he and collaborators from the U.S. Forest Service and Oak Ridge National Laboratory will plant 72 pots containing various controlled mixtures of soil and catalysts.

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PNNL is a DOE Office of Science laboratory that solves complex problems in energy, national security, the environment and life sciences by advancing the understanding of physics, chemistry, biology and computation. PNNL employs 3,800, has a $600 million annual budget, and has been managed by Ohio-based Battelle since the lab’s inception in 1965.