At the low end of the environmental impact scale, a no-till corn-soybean rotation where 38 percent of the stover was removed emitted 2.7 tons of carbon dioxide per acre and yielded 2 tons of stover.
"Combining no-till with continuous corn cultivation when stover is removed was capable of slightly lower sediment loss than the baseline today without any stover removal," Gramig said. "Introducing cover crops or replacing nitrogen that is removed with stover at lower rates was not considered in our study but should further reduce environmental impacts. These practices require additional study and would involve offsetting costs and savings."
Perhaps not surprisingly, researchers found that removing stover increased production costs over the predominant corn-soybean rotation in place today. Most of that cost is attributed to replacing nitrogen contained in stover. Stover removal was found to have the lowest cost when collected from corn grown in rotation with soybeans.
"For a given crop rotation and tillage system, as we simulated an increase in the rate of stover removal we found an increase in loss of sediment from crop fields, an increase in greenhouse gas flux to the atmosphere and a reduction in nitrate and total phosphorus delivered to waterways," Gramig said. "While optimizing production to maximize stover harvest at the lowest possible cost may lead to a reduction in nutrients delivered to rivers and streams, this comes at the expense of increased soil erosion and greenhouse gas emissions."
More study is needed to identify less environmentally harmful stover removal practices, Gramig said.
"In the meantime, farmers can use no-till to reduce the amount of sediment loss," he said. "Additional practices, such as the use of cover crops, are going to be necessary if we want to try to reduce greenhouse gas loss. We also need to determine what the correct nitrogen replacement rate is to maintain long-term soil productivity while minimizing nitrogen loss, whether to the atmosphere or to waterways."
"Environmental and Economic Trade-Offs in a Watershed When Using Corn Stover for Bioenergy" appears in the January 2013 issue of Environmental Science & Technology. It is available online at http://dx.doi.org/10.1021/es303459h.