A multi-year “Next Generation Feedstocks” project administered by the Institute for Sustainability, Energy, and Environment (iSEE) has identified economic and environmental considerations that make switchgrass a candidate for sustainable aviation fuel.
Two new studies from the project led by Professor D.K. Lee (pictured) of the University of Illinois Urbana-Champaign examined the benefits of new switchgrass cultivars in meeting federal targets for sustainable aviation fuel. iSEE was instrumental in helping the team land the original $5 million grant from the U.S. Department of Energy (DOE) for the five-year project, which will wrap up next fall.
The Sustainable Aviation Fuel Grand Challenge kicked off in 2021 with the goal of expanding SAF production to 35 billion gallons by 2050, while cutting greenhouse gas emissions in half. Forecasted to contribute up to 230 million dry tons annually, switchgrass is one of several purpose-grown bioenergy feedstocks that could help meet this challenge. Not only does the perennial species produce great quantities of biomass, switchgrass can be harvested annually for a decade or more without repeated planting, requires minimal nitrogen fertilizer compared to corn, and performs important ecosystem services.
Scientists know this because they have been studying switchgrass for its bioenergy potential for decades. But previous studies used less productive switchgrass cultivars, were conducted on smaller and less realistic plots of land, or overlooked the fertilizer inputs required for optimal productivity. In two new studies, U. of I. researchers grew modern “energy” cultivars at the field scale across the Midwest to determine which cultivars are most profitable where, and how they compare to corn in terms of ecosystem services.
“All the data that helps us estimate switchgrass suitability for SAF comes from small plot research or older forage-type switchgrass cultivars. We wanted to test high-yielding switchgrass cultivars on a larger scale to provide a more accurate picture of the benefits these new cultivars provide,” said Lee, senior author of both studies and Professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at U. of I.
Postdoctoral researcher Muhammad Umer Arshad led the effort to analyze switchgrass profitability. The team planted three newer energy-type cultivars — Independence, Liberty, and Carthage — alongside two forage cultivars — Shawnee and Sunburst — on low-productivity marginal land across four Midwestern states: Illinois, Iowa, Nebraska, and South Dakota. They also tested two nitrogen fertilizer rates, 28 and 56 kilograms per hectare; for comparison, corn typically gets about 200. After five years of growth, Arshad conducted economic analyses to calculate expenses and profits in each location.
“Our findings clearly show that Independence and Liberty are much more profitable than the forage cultivars on all the sites, but the most profitable nitrogen rate varied across locations,” Arshad said. “In most cases, 56 kilograms per hectare achieved higher yields, but in some sites, 28 kilograms performed better in terms of profit.”
Although Independence and Liberty outperformed the forage cultivars, these energy types of switchgrass did not perform equally across the sites. For example, depending on the nitrogen rate, Independence was most profitable in U.S. hardiness zone 6a, whereas Liberty showed the highest profit margins in zone 5b and Carthage, was most profitable in zone 4b.
“With these energy-type cultivars, farmers can put marginal lands to use and see returns after two years,” Arshad said. “Our results can help guide decision-makers to optimize input strategies for biomass production and meet renewable energy demands.”
During the decade or so that switchgrass is churning out biomass, it’s also busy providing ecosystem services — a win-win, according to postdoctoral fellow Nictor Namoi, who led a companion study in field-scale plots in Illinois.
Namoi assessed soil greenhouse gas emissions (carbon dioxide and nitrous oxide) and nitrate leaching in Independence switchgrass over three years, and compared these metrics to other fields planted in continuous corn under no-till management. The idea was to compare ecosystem services for these two cropping systems on equal footing.
“One of the many benefits of growing purpose-grown energy crops on marginal lands is ecosystem services associated with the perennial nature of energy crops. They can also potentially generate higher profits than conventional row crops on less productive land,” Namoi said. “Demonstrating ecosystem services of switchgrass, including reduction of greenhouse gas emissions and nutrient loss, will promote purpose-grown energy crops on marginal land.”
Nitrous oxide emissions and nitrate leaching were significantly reduced in switchgrass compared to corn, with 80% less nitrate leaching by the third year. Namoi says the nitrous oxide finding is straightforward: with switchgrass getting just 56 kilograms of nitrogen fertilizer per hectare and corn receiving 202 kilograms, there’s a lot more nitrogen available in corn fields to be emitted as the potent greenhouse gas.
Carbon dioxide emissions were less straightforward. After the second year, CO2 emissions were over 50% higher in switchgrass than in corn.
“I wasn’t expecting that,” Namoi said. “But there’s a lot more biomass belowground in switchgrass, about five times that of corn.”
More roots mean more respiration, the normal process in which roots convert oxygen and glucose into energy, with carbon dioxide as a byproduct. Lee says he’s seen this pattern with switchgrass and other purpose-grown bioenergy crops before, but he’s still convinced that the overall benefits of switchgrass outweigh this particular deficit.
“For one thing, more root biomass means more long-term carbon sequestration potential,” he said. “When we measure total biomass of switchgrass, there’s about 10 megagrams of carbon belowground. That’s huge.”
Another key advantage of switchgrass, Namoi added, is its ability to thrive on marginal land.
“By definition, marginal land is not profitable for commodity crops,” he said. “So switchgrass reduces competition with food crops and makes use of otherwise unproductive areas.”
With commodity and oil prices at a low point, the demand for purpose-grown bioenergy feedstocks is relatively weak at the moment. But that could all change rapidly as tariffs impact global economies. Namoi says when the market is ready, switchgrass will be, too.
“Our research ensures that we can feed productive cultivars into the SAF production system once the economy and the technology is ready to transition,” he said.
The first study, “Comparative Economic Analysis Between Bioenergy and Forage Types of Switchgrass for Sustainable Biofuel Feedstock Production: A Data Envelopment Analysis and Cost–Benefit Analysis Approach,” is published in GCB Bioenergy [DOI: 10.1111/gcbb.70020].
The second study, “Field-Scale Evaluation of Ecosystem Service Benefits of Bioenergy Switchgrass,” is published in the Journal of Environmental Quality [DOI: 10.1002/jeq2.70025].
Both studies were supported by DOE’s Bioenergy Technologies Office (DOE-BETO) under award number DE-EE0008521. The JEQ study had additional funding from the DOE Center for Advanced Bioenergy and Bioproducts Innovation (CABBI) under Award Number DE-SC0018420.
Lee is also affiliated with the Agroecosystem Sustainability Center, CABBI, the Center for Digital Agriculture, and the National Center for Supercomputing Applications at U. of I.
— News release by Lauren Quinn, Assistant Director for Research Communications for the College of ACES
