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The gas you put in your car is 10 percent ethanol, which comes from corn, but it doesn't have to. A federal law from 2007 is pushing gas refiners to start using ethanol from sources that aren't grain crops. The government wants us to import less oil but also to reduce carbon emissions. A study in Nature magazine suggests that vegetation on marginal farmland could play a role.

As NPR's Richard Harris reports, it's at best a small part of the solution.

RICHARD HARRIS, BYLINE: Biofuels sound like a great idea. Why not grow fuel instead of importing oil? But that's more easily said than done. For one thing, if you tear up crops used for food and instead plant crops for fuel, you reduce food supplies and drive up prices. So Phil Robertson and colleagues at Michigan State University have been looking at plants that don't require farm fields.

PHIL ROBERTSON: First we discovered that the grasses and flowers that take over fields once you stop farming produce a fair amount of biomass, especially if you provide them a little bit of fertilizer.

HARRIS: That sounds promising. The next question is about climate change. Would using these crops help reduce the amount of carbon dioxide that ends up in our atmosphere? After all, if you leave those lands alone, the plants will soak up carbon dioxide and store it in the soil.

ROBERTSON: Is there truly a climate benefit to doing this or are we just robbing Peter to pay Paul?

HARRIS: The answer in his study is using these crops for fuel is better for the atmosphere than burning gasoline. Robertson and his colleagues surveyed the Midwest and identified 27 million acres of marginal farmland, where these plants could grow and where the acreage falls into a compact enough area that someone might want to build a refinery to produce biofuel.

ROBERTSON: At the end of the day, we discovered we could produce enough biomass to supply 30 or so of these potential bio-refineries.

HARRIS: Robertson likes this idea because the land could be harvested once a year but would otherwise be left as wildlife habitat. Sounds pretty good, but some people in the field aren't convinced the resource is nearly that big.

Adam Liska, at the University of Nebraska in Lincoln, says a lot of this acreage is in the Great Plains, which wouldn't produce a reliable crop year after year.

ADAM LISKA: One year, you may have high rainfall and high crop yields and be able to sustain your facility. The next year you may have a drought, low crop yields, and your profit margins will go down or be negative.

HARRIS: So Liska doubts that anyone would ever build a biofuels refinery to reap this unpredictable harvest.

Indeed, Brent Erickson at the Biotechnology Industry Organization says nobody has plans just yet to use this kind of plant material to make bio-fuel. Instead, the young industry is eyeing other sources of material to use as a feedstock.

BRENT ERICKSON: Every region of the country has some form of biomass. So the Northwest would have sawdust and wood waste, the California area might have rice straw or wheat straw.

HARRIS: Refiners in the Midwest are looking at corn cobs. And there's a plant that's actually operating in Florida that uses dead citrus trees.

ERICKSON: As this technology progresses we're going to see a great diversification of biomass supply.

HARRIS: But Tim Searchinger, at Princeton University, says biofuels could at best provide only a tiny fraction of our energy needs. Plants are very inefficient when it comes to capturing solar energy.

TIM SEARCHINGER: If you were to take every gram of crops produced anywhere in the world for all purposes - and that includes every grape, every ton of wheat, every ton of soybeans and corn - and you were to use that for biofuels and essentially stop eating, those crops would produce about 14 percent of world energy.

HARRIS: The 27 million acres identified in the latest study would provide less than half of one percent of our national energy demand, he says. And the more we try to expand biofuels, the more we risk displacing crops for food, or chopping down forests, which store a huge amount of carbon.

Searchinger says Europe has recently recognized those potential hazards and is scaling back its biofuels ambitions.

SEARCHINGER: They realize that it was a mistake. And their compromise for the moment is essentially to cap what they're doing. And then, they promise by 2020 to phase out all government support for biofuels.

HARRIS: But the U.S. government continues to push ahead. It may be creating a new industry, but Searchinger says it's also creating new risks.

Richard Harris, NPR News.

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