Soil's a natural for storing CO2
The soil carbon cycle shows the sources and sinks for atmospheric carbon dioxide, with an emphasis on the two-stage process of humification. Additives, such as fly ash, help catalyze the process.
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In a field outside Charleston, S.C., PNNL's Jim Amonette and his colleagues from the U.S. Forest Service and Oak Ridge National Laboratory have planted 72 pots with Sudan grass. They don't care much about the grass, however--it's the soil beneath that captures their attention.
The pots contain controlled mixtures of soil and additives, there to promote the conversion of carbon in plant residues to a more stable form known as humus. If the tactic works, Amonette will have found a promising way to tackle two problems at the same time: carbon depletion from soils and the relentless build-up of the greenhouse gas carbon dioxide in the atmosphere.
"Globally, soils contain four times as much carbon as the atmosphere, and half of the soil carbon is in the form of humus," 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. Tillage is responsible for the loss of as much as a third of the carbon originally present in soils before they were used for agriculture.
"These carbon-depleted soils are a tremendous potential reservoir for carbon that can help slow the increase in atmospheric carbon dioxide," Amonette said. "About a ton of carbon is added to an acre of a typical agricultural soil every year in the form of crop residues. Today, 99 percent of it comes out the top as carbon dioxide due to microbial processes. If we can increase the fraction that is retained in soil by even a small amount, it will make a huge difference."
Amonette's fieldwork is an extension of promising work in the lab, where his team has been able to promote a soil's natural ability to store carbon as humus (i.e., humification) by increasing the soil's alkalinity and the frequency of its wetting and drying cycles.
In the humification process, a common soil enzyme, tyrosinase, increases the reaction rate between oxygen and chemicals that are so-called humus precursors to form a class of compounds called quinones. The quinones further react with amino acids released by soil microbes to form humic polymers, complex and durable molecules that are crucial to a soil's ability to retain carbon.
"Because humic polymers are less easily degraded by microbes than the precursor molecules," Amonette said, "they survive to diffuse into small pores in soil aggregates where they are stabilized for decades, if not centuries."
The humification rate depends on many factors: enzyme stability, moisture, alkalinity, oxygen availability, microbial population and the physical roperties of different soils. Amonette's experiments are designed to weigh the importance of these many factors and to learn ways they might be manipulated to increase humification.
These experiments have shown that soil additives, such as fly ash, an alkaline porous byproduct of coal combustion, can promote humification. "Fly ash with a high unburned carbon content seems to be the best material, and this is fortunate as it has no other use and would otherwise be buried in a landfill," he notes.
By the end of the summer, Amonette hopes to have a handle on how his soil preparations play out in the less cooperative real world, where such things as humidity (considerably higher in the South) and the wetting and drying cycles that help promote humification are far less controllable and predictable.