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Study: Miscanthus Guzzles More Water Than Corn but Soaks Up More Nitrates Too

Research emerging from the University of Illinois paints an ambiguous picture of the grass's biofuel potential

By Lisa Song

Oct 4, 2010

Miscanthus, a potential bioufuel crop, uses more water than corn or soybeans but is better for water quality, reported scientists from the University of Illinois.

This picture of Miscanthus’ pros and cons comes during a period of increasing interest in the U.S. in the perennial grass. Originally from Asia, Miscanthus has been studied in Europe for over 20 years but only gained popularity in the U.S. in the last decade. In particular, Miscanthus giganteus (often referred to as simply "Miscanthus") has been the focus of biofuel research. Field tests have shown that Miscanthus giganteus produces two to three times as much biomass as corn.

The researchers' findings, published in the September-October issue of the Journal of Environmental Quality, further revealed the pros and cons of Miscanthus as a biofuel.

The scientists planted Miscanthus alongside corn, soybeans and switchgrass (another perennial grass) in east-central Illinois, and studied its behavior for four years.

The study found that Miscanthus and switchgrass used more water early in the growing season. This is a potential advantage, since the plants can soak up extra water during spring floods, absorbing nitrates in the process, which can reduce the amount that drains into rivers and streams.

Overall, it took about 870,000 gallons of water per year to grow one acre of Miscanthus, versus 760,000 gallons for corn.

"How that's going to be a disadvantage [when scaled up] will depend on how extensively [Miscanthus] gets planted," said Gregory McIsaac, lead author of the paper and an associate professor emeritus at the University of Illinois. "If we planted Miscanthus everywhere in Illinois—just as a thought experiment—we'd probably reduce stream flow by about a third."

On a more realistic level, if Miscanthus were planted over 10% of the land surface, stream flow would only decrease by 3%.

"That sounds a lot better, but there are still complications," McIsaac explained. In late summer and early fall, when stream levels are at their lowest, Miscanthus continued to use more water than the other three crops. This means that during the months when water supply is lowest, more rain will be soaked up by plants instead of replenishing streams. It's also a "critical period" for aquatic organisms.

Scale, too, is an issue. "For biofuels to be economically feasible…you need to plant a lot of it [in one area]," said Mark David, a professor at the University of Illinois and co-author of the paper. It's not cost-efficient to truck biofuels from faraway fields to a central processing plant. But dense plantings would exacerbate the water-consumption problem.

It helps that Miscanthus will probably be planted on nonirrigated land, said McIsaac. If the Miscanthus biofuel industry ever takes off, he expects the crop to be concentrated in regions with high rainfall and relatively inexpensive land, such as the southeastern U.S. or southern Illinois.

In terms of water quality, corn requires high levels of nitrate fertilizers. Nitrates not absorbed by the corn can be leached into local groundwater and washed into rivers. Excess nutrients from agrochemical fertilizers have already created a 7,000-acre "dead zone" in the Gulf of Mexico.

Corn has a very short growing season; it's planted in June and harvested in September. That leaves the soil bare for most of the year, further facilitating nitrate drainage into the watershed.

Miscanthus, on the other hand, requires little, if any, fertilizer. It has a longer growing season, so the roots spend more time pulling out nitrates and slowing soil erosion. As a perennial, Miscanthus undergoes something called retranslocation: every fall when plant growth slows down, nitrogen moves from the Miscanthus' leaves into the roots. When spring comes again, the nitrogen moves back into the shoots to fuel plant growth.

"If we got rid of corn ethanol and used Miscanthus, that would really help reduce nitrate losses," said David.

Not Commercially Viable Yet

Despite growing interest in using Miscanthus as a biofuel, so far the crop remains commercially unviable.

There are scattered cases where Miscanthus is burned in power plants, but the practice isn't cost-effective because coal is so cheap, said McIsaac.

A more promising method involves converting plants into liquid fuel. In corn-based biofuels, bacteria can ferment the starch into ethanol. But Miscanthus is a cellulosic biofuel that requires extra pre-treatment: the cellulose must be broken down into simple sugars before fermentation.

"The science isn't all worked out, and (there's) still much work to do," said Emily Heaton, an assistant professor of agronomy at Iowa State University. Heaton was involved in an early part of the water-use research but is not a co-author of the paper.

For Miscanthus to be economically feasible in the long run, the crop would need broad economic and political support. In 2007 Congress passed a major energy bill that included a goal to produce 36 billion gallons of biofuels per year by 2022, tripling the 2009 national yield of 12 billion gallons.

"I think we could reach that [2022] goal, [but] I don't think we're on track to do so," said Heaton. "We spend a lot of time talking about a secure fuel supply," said Heaton, "so if you're getting it from biofuels, [you need] farmers who are willing to plant Miscanthus year after year." That means offering federal support for Miscanthus crops, or helping farmers obtain long-term contracts with utilities, she said.

There's also the problem of storage. A biofuel processing plant must operate year-round, while Miscanthus is harvested only once a year.

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