|In field trials in Illinois, researchers grew Miscanthus x giganteus and switchgrass in adjoining plots. Click to enlarge. Credit: University of Illinois|
Researchers at the University of Illinois have concluded that the perennial grass Miscanthus×giganteus could produce enough ethanol to offset 20% of current US gasoline use, while requiring 9.3% of current agricultural acreage. By contrast, using corn or switchgrass to produce the same amount would require 25% of current US cropland.
The findings come from side-by-side trials of Miscanthus and switchgrass established for the first time along a latitudinal gradient in Illinois. The results of the trials appear this month in the journal Global Change Biology.
Over 3 years of trials, Miscanthus×giganteus achieved average annual conversion efficiencies into harvestable biomass of 1.0% (30 t ha-1) and a maximum of 2.0% (61 t ha-1), with minimal agricultural inputs. The regionally adapted switchgrass variety Cave-in-Rock achieved somewhat lower yields, averaging 10 t ha-1. Given that there has been little attempt to improve the agronomy and genetics of these grasses compared with the major grain crops, these efficiencies are the minimum of what may be achieved. At this 1.0% efficiency, 12 million hectares, or 9.3% of current US cropland, would be sufficient to provide 133 × 109 L of ethanol, enough to offset one-fifth of the current US gasoline use. In contrast, maize grain from the same area of land would only provide 49 × 109 L, while requiring much higher nitrogen and fossil energy inputs in its cultivation.
In 2007, University of Illinois researchers presented the first direct comparisons of the biomass productivity of the two C4 perennial grasses switchgrass (Panicum virgatum) and Miscanthus. Results given at Plant Biology and Botany 2007 showed that Miscanthus is more than twice as productive as switchgrass. Its efficiency of conversion of sunlight into biomass is amongst the highest ever recorded. (Earlier post.)
(C4 refers to the type of photosynthesis used by the plant: in C4 photosynthesis, the CO2 is first incorporated into a four-carbon compound, as compared to the more common C3 photosynthesis and its three-carbon compound. Among their differences, C4 plants photosynthesize faster than C3 plants under high light intensity and high temperatures, and have better water use efficiency. Corn is also a C4 plant.)
The Miscanthus/switchgrass field trials study was led by U. of I. crop sciences professor Stephen P. Long. Long is the deputy director of the BP-sponsored Energy Biosciences Institute, a multi-year, multi-institutional initiative aimed at finding low-carbon or carbon-neutral alternatives to petroleum-based fuels. He also is the editor of Global Change Biology.
What we’ve found with Miscanthus is that the amount of biomass generated each year would allow us to produce about 2 1/2 times the amount of ethanol we can produce per acre of corn.—Stephen Long
In trials across Illinois, switchgrass produced only about as much ethanol feedstock per acre as corn, Long said. The yields for switchgrass were equal to the best yields that had been obtained elsewhere with switchgrass, Long said. The Miscanthus proved to be at least twice as productive as switchgrass. Miscanthus is also tolerant of poor soil quality.
Miscanthus begins producing green leaves about six weeks earlier than corn in the growing season and stays green until late October in Illinois while corn leaves wither at the end of August. The growing season for switchgrass is comparable to that of Miscanthus, but it is not nearly as efficient at converting sunlight to biomass as Miscanthus, Frank Dohleman, a graduate student and co-author on the study, found.
One of the criticisms of using any biomass as a biofuel source is it has been claimed that plants are not very efficient—about 0.1 percent efficiency of conversion of sunlight into biomass. What we show here is on average Miscanthus is in fact about 1 percent efficient, so about 1 percent of sunlight ends up as biomass.
Keep in mind that when we consider our energy use, a few hours of solar energy falling on the earth are equal to all the energy that people use over a whole year, so you don’t really need that high an efficiency to be able to capture that in plant material and make use of it as a biofuel source.—Stephen Long
Because Miscanthus is a perennial grass, it also accumulates much more carbon in the soil than an annual crop such as corn or soybeans.
Miscanthus is a sterile hybrid, and must be propagated by planting rhizomes. Mechanization allows the team to plant about 15 acres a day. In Europe, where Miscanthus has been grown for more than a decade, patented farm equipment can plant about 50 acres of Miscanthus rhizomes a day, Long said.
Once established, Miscanthus returns annually without need for replanting. If harvested in December or January, after nutrients have returned to the soil, it requires little fertilizer.
Keep in mind that this Miscanthus is completely unimproved, so if we were to do the sorts of things that we’ve managed to do with corn, where we’ve increased its yield threefold over the last 50 years, then it’s not unreal to think that we could use even less than 10 percent of the available agricultural land. And if you can actually grow it on non-cropland that would be even better.
Emily A. Heaton, Frank G. Dohleman, Stephen P. Long (2008) Meeting US biofuel goals with less land: the potential of Miscanthus. Global Change Biology doi: 10.1111/j.1365-2486.2008.01662.x