Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
The Stories

Some soils better than others at holding carbon

By Owen Maurer

The carbon cycle plays a part in a mosaic of cycles that help to maintain the balance of the Earth’s overall ability to support life. It is also still being decoded by some scientists. Ongoing work by several scientists has added to the understanding of something a casual observer might not realize: It is not just the trees and the ocean but the earth itself – in the form of soil – that serves as storage tanks of carbon.

“In terms of the terrestrial systems, soils are the huge store of carbon on the surface of the Earth,” explained Craig Rasmussen, a soil scientist and director of the University of Arizona’s Center for Environmental Physics and Mineralogy. Soils store somewhere between 1,500 and 2,300 billion metric tons of organic carbon, he indicated. “That is roughly two to three times the amount of carbon in the atmosphere.”

In a recent interview, Rasmussen recalled how he originally became interested in studying what most people call dirt. As an undergraduate on a field trip for a soils class, he noticed two apparently different soils about a hundred yards from each other. The teaching assistant for the class was unable to answer his question satisfactorily regarding the anomaly, so he decided to begin the journey to uncover the answers for himself. Now an associate professor with the UA Department of Soil, Water and Environmental Science, Rasmussen does plenty of research to find his own answers to his questions, such as which processes control soil carbon storage.

The problem with carbon
Why is this even relevant?  When the soil is disturbed, such as in agricultural, industrial or recreational use, some of the carbon stored there can be released in the form of carbon dioxide, a greenhouse gas. The release of greenhouse gases is of great environmental concern because of their tendency to increase average global temperatures.

“Events are in motion that are going to be sustained for decades if not hundreds of years,” explained Steve Leavitt, a professor of dendrochronology and associate director of the Laboratory of Tree Ring Research with the University of Arizona. Temperatures are projected to increase between 3 and 7 degrees Fahrenheit by the end of this century compared to the 1980s, according to the Intergovernmental Panel on Climate Change.

Scientists link the ongoing temperature increase to emissions of greenhouse gases such as methane and carbon dioxide. For example, driving a gasoline-powered car releases carbon stored in the fossil fuels used to power it. The carbon wasn’t going anywhere as a fossil fuel – gas, oil, and coal – until someone pumped it up and refined it for vehicular use. The same could be said of the coal used to provide electricity in Tucson. This carbon dioxide released has now been put back into the everyday carbon cycle.

Even if carbon dioxide emissions were somehow miraculously sustained at their current levels, Leavitt said, the effects of a warmer climate means a drastic change in the carbon cycle as we know it. For example, a warmer global temperature means warmer ocean temperatures, Leavitt explained. A warmer ocean takes up less carbon dioxide. Less carbon being taken up into a regular cycle means more carbon in the atmospheric pool. More carbon in the atmosphere means a greater “greenhouse” effect that is causing the global warming in the first place.

A Global Problem
Photo by Joseph Robertson. Some rights reserved.
Deforestation releases carbon not only from trees, but also from soil.
Besides emissions from vehicles, carbon can be released from soil any time it is disturbed.  Disturbance in this case means the construction of roads, agricultural plowing, deforestation – anything that involves digging or moving the soil. On a local scale, this isn’t a big problem. When you take it into a global scale and include the various developing countries, such as China, India, and Brazil, for example, the amount of soil disturbed by deforestation and other types of land use change begins to accumulate and make a difference.

Annual emissions from fossil fuels accounted for about 6.4 billion metric tons of carbon, while emissions from land use change, mainly deforestation, accounted for about 1.6 billion metric tons, according to the IPCC’s 2007 report.

“Forest soils tend to hold a substantial amount of carbon,” Rasmussen said. Globally, soils hold more carbon than the trees themselves. He added that typical grassland soils can also have large carbon stores. Peat and bog soils can have several feet of dead organic matter, also high in carbon. Arctic soils and tundra have carbon reserves frozen inside, which can be released when thawed.

As Rasmussen and others have documented, different types of soils even within the same type of ecosystem can store carbon differently.

“Even the bedrock that makes up the parent material of a soil can have a huge impact on the behavior of a soil and its carbon interactions,” Rasmussen said. Soil forms over time from the underlying solid rock, known as bedrock. “Basalt tends to form clay and iron-oxide-rich soils with lots of mineral surface area which can store huge amounts of carbon. Granite based soils, on the other hand, don’t have a lot of reactive material. Carbon stocks in these soils tend to be lower, he indicated, and its cycling and turnover is much more rapid.

How does soil store carbon?
“There are three main mechanisms by which carbon is stored in soils,” Rasmussen explained.  Storing carbon in this case means taking it out of the everyday movement of the cycle.

Image courtesy of Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science.
In this graphic of the carbon cycle, arrows going up represent emissions of carbon into the atmosphere, while arrows pointing down indicate the taking up (sequestration) of carbon.

One way is “when carbon goes into the soil, it can associate with minerals or with aggregates and become protected simply by the physical association of that carbon with the mineral surface,” said Rasmussen, noting this mechanism is termed “physical protection.” Essentially, the soil itself acts like a kind of shield or protective bubble against outside forces such as bacteria and other microbes that would use the carbon as a food source.

Another mechanism is by the very nature of so-called chemical recalcitrance. As Rasmussen explained, generally the more complex the organic structure of an object, the longer it takes to decompose.

Picture a maple leaf and an oak branch. What are the chances that the same maple leaf will be around next season, assuming it doesn’t get raked up first? Almost none – the leaf has a relatively simple organic structure and decomposes quickly. The branch takes longer to decompose on the surface and the carbon in it has a larger chance of becoming part of the organic layer of the soil. Once it is in the soil, it continues to slowly decompose but any carbon that escapes decomposition by air becomes stored in the soil.

The last mechanism by which carbon can be stored relates to the combining of organic materials with metals such as iron and aluminum. These metals, Rasmussen explained, can have a chemical effect on the organic material and can prevent its natural decomposition by microbes.

“Aluminum in some cases can be toxic to microorganisms and so they won’t eat that carbon if it has a bunch of aluminum stuck to it,” he said, giving an example.

Some scientists and farmers are looking to soil as a way to store carbon instead of releasing it.  Agricultural processes such as no-till allow the soil to be left undisturbed.  Crops are planted into the leftovers of the crop before it without the conventional method of tilling in between. This method releases much less carbon into the air and seeks to introduce more carbon into the soil.

Like so many things in life, sometimes the small details can make a big difference.  Rasmussen, who once observed that one small patch of soil was different from a neighboring piece, now is producing work to inform policy makers on which types of soils take up the most carbon.


IPCC 2007 Summary for Policy Makers

IPCC 2007 Global Carbon Cycle

Department of Energy Global Carbon Cycle illustration

Earth Observatory: Carbon Cycle illustration

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