From ‘Dust to Dust'

December 13, 2006

BY Holly Jessen

Everything grown on Earth eventually returns to the earth. In a cornfield, the stalks, leaves and grain that fall to the ground after the combine passes over the field go through an involved decomposition process that returns carbon to the soil, providing fertility, according to Wally Wilhelm, a federal plant physiologist based at the University of Nebraska-Lincoln. On a microbial level, certain components of decomposing organic matter take months, years or even centuries to fully break down. Waxing a bit poetic, Wilhelm referenced from "dust to dust," a well-known phrase with biblical origins, as he discussed the process with EPM.

As the technology advances and the possibility of making ethanol from cellulosic sources, such as corn stover, on a commercial scale nears reality, federal researchers are now further exploring the carbon needs of soil. The goal is to find out how much crop residue has to remain on the ground to maintain soil productivity, Wilhelm says.

There are two important reasons for leaving some crop residue in the field, according to John Baker, a research leader with the USDA Agricultural Research Service (ARS) based at the University of Minnesota. First, crop residue keeps soil from eroding. The second factor, maintenance of soil organic matter, is the focus of the study.

"Just like an ethanol plant needs to have input—so does soil," Wilhelm says. Wilhelm and Baker are two of 14 ARS scientists working on the Renewable Energy Assessment Project (REAP). The ARS began working on the project this summer, Wilhelm says. The study will be conducted at nine research locations around the United States, largely in the Corn Belt.

In all, six in-field experiments will examine a corn monoculture, which is corn grown year after year with no crop rotation. Although the project was initially set up for five years, it may take longer to get the needed data on soil organic matter content, which changes slowly. "We think that it may take as long as 10 years," Wilhelm says.

With all the excitement surrounding cellulosic ethanol, the study is examining a critical question. Just as burning petroleum puts fossil carbon into the atmosphere, reducing soil organic matter also releases carbon, Wilhelm says. Over the short term, that would have a similar effect on the atmosphere but not at the same magnitude.

Corn stover has promise as a possible component of cellulosic ethanol. However, it's also important to protect soil resources "so that we don't create one problem while we are solving another," Baker says.

One thing researchers are looking at is planting winter cover crops. For example, winter rye could be planted as a companion crop to replace the carbon in soil. "It's a way to keep cover on the ground, all the time," Baker says, adding that it may be OK to remove all the corn stover from a field as long as the organic material is replaced in some way.

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