Farmers who are considering selling corn crop residue from their fields to produce cellulosic ethanol first should weigh a range of site-specific factors to their farming operations, according to new research from an Iowa State University agronomist.

Mahdi Al-Kaisi, a professor of agronomy and an ISU Extension agronomist who specializes in soil management, is urging farmers to take a thoughtful approach. He says they need to account for variables such as topography, tillage system, nitrogen application and amount of organic matter present in their soil to determine how much corn residue (the plant material left behind after harvest) they should remove from the field. 

HARVESTING RESIDUE: Crop residue removal to use to make cellulosic ethanol has some real environmental impacts on soil health and water quality, says an ISU study. Harvesting crop residue needs to be approached thoughtfully and on a site-specific basis.

Related: Effects of taking away residue

"Residue removal has some real environmental impacts on soil health and water quality," Al-Kaisi observes. "It needs to be approached thoughtfully and on a site-specific basis." His most recent publication, appearing in the Soil Science Society of America Journal, shows how a decrease in crop residue can lead to increases in greenhouse gas emissions from the soil. 

Loss of corn residue affects soil quality and environment
Al-Kaisi began the study in 2008. The research examines how loss of corn residue interacts with other variables to affect soil quality and environment. The need for the research arose from the continuing development of cellulosic ethanol, a biofuel made from corn crop residue. One cellulosic ethanol plant is up and running in northwest Iowa at Emmetsburg, another is under construction at Nevada in central Iowa.   

As the technology continues to mature, cellulosic ethanol plants will require more and more feedstock to fuel their production. Much of that feedstock likely will come from crop residue harvested from farmland. Al-Kaisi says research like this will help to balance the needs of the cellulosic ethanol industry with information that can help farmers safeguard their soil.

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The ISU research team conducted experiments at two locations, one in central Iowa and one in southwest Iowa, and monitored the effects of removing portions of corn residue from the test plots on yield, soil organic matter, greenhouse gas emissions and soil quality. The team also tested how various residue removal levels interact with different nitrogen rates and tillage systems such as no-till and conventional tillage.

Residue removal results in less organic matter in the soil
The research found a general increase in emissions of carbon dioxide and nitrous oxide from all residue removal plots as nitrogen application increased. They also found that conventional tillage led to an increase in carbon dioxide emissions regardless of residue removal level.

As residue is removed, black surface soil is exposed to direct sunlight. The dark surface absorbs heat and results in an increase in the oxidation of organic matter and the release of carbon dioxide, Al-Kaisi explains.

A previous paper published as a result of the study noted crop residue removal results in less organic matter in the soil, which Al-Kaisi says could lead to diminished productivity in the long term. Farmers should also exercise caution when considering removal of residue from fields prone to erosion. 

There are also soil erosion considerations, which will vary
"Residue protects soil from wind and water erosion, so removing it should be based on field-specific conditions and potential soil erosion," Al-Kaisi says. "Farmers who are going to remove corn residue should make sure they know how these different factors interact when making these decisions."

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Doug Karlen, soil scientist and research leader at USDA's Ag Research Service in Ames, says its unlikely farmers would remove as much crop residue as this research study explored. "It's a much higher removal rate in this study" than the cellulosic ethanol plants recommend or farmers would consider, he says. "In a research study you push the extremes. But this wouldn't happen in a production situation."

Farmers typically remove less than 25% of crop residue, while this study looked at different levels of crop residue removal: zero, 50% and 100% removal. Karlen is a member of the research team and has studied this issue extensively.

Farmers usually remove less than 25% of the crop residue
The research team considered effects on yield, soil organic matter, greenhouse gas emissions and soil quality. They also looked at how various residue removal levels interact with different nitrogen application rates and tillage systems such as no-till and conventional tillage. The research found a general increase in emissions of carbon dioxide from residue removal plots as nitrogen application increased. It also found conventional tillage led to an increase in carbon dioxide emissions regardless of the residue removal rate.

Al-Kaisi says this information will help farmers make decisions as more cellulosic ethanol plants are developed in the future. Karlen expects the growth of cellulosic ethanol production will lead to farmers planting energy crops such as switchgrass, especially on land that is environmentally vulnerable. "Not all of this material will come from corn crop residue," he says.