Farm-fuel feedback: soybeans have advantages over corn.

A new analysis of two commercial biofuels finds that while both provide more energy than they consume, soybean biodiesel gives more bang for the buck than ethanol made from corn does.

Corn-grain ethanol and soybean biodiesel are the two major alternative transportation fuels in the United States. The biofuels can replace gasoline and diesel, respectively. A team of Minnesota ecologists and economists set out to add up all the energy and environmental costs and benefits of the two food-based biofuels.

The researchers included the energy required to grow the crops, run farm machinery, manufacture fertilizers and pesticides, transport the crops, and transform the raw material into fuel. They also considered the environmental impact of the added fertilizers and pesticides.

Both biofuels yield energy, but with corn-based ethanol, "it takes so much energy to grow the corn and convert it into a fuel, you don't gain very much energy in the overall process," says ecologist David Tilman at the University of Minnesota in St. Paul. While ethanol provides 25 percent more energy than it consumes, the energy gain for soybean biodiesel is 93 percent. Various steps in making ethanol, such as distillation, are energy intensive.

Corn also needs more fertilizer and pesticides than soybeans do. Per unit of energy gained, biodiesel requires 1 percent of the nitrogen, 8.3 percent of the phosphorus, and 13 percent of the pesticides that corn-derived ethanol does. Meanwhile, producing and using ethanol from corn decreases greenhouse gas emissions by 12 percent, compared with making and burning gasoline. But soybean biodiesel results in 41 percent less of those emissions than diesel does, the researchers report in an upcoming Proceedings of the National Academy of Sciences.

"The goal wasn't to pick a winner or loser," says Tilman. Instead, the team intends the new information to be a guide "to help formulate better biofuels for the future."

The new analysis points out that even if all the U.S. corn and soybean crops became biofuels, they would still satisfy only 12 percent of the country's gasoline demand and 6 percent of its diesel demand. "Using foods for biofuels has been a very good way to demonstrate that biofuels are a viable product," says Tilman, but to meet energy needs in the long term, "we need non-food-based crops."

For example, converting prairie grasses to ethanol could provide a larger energy gain than corn does and would cost less environmentally, he says. The grasses can be grown on abandoned agricultural lands and need little or no fertilizer or pesticides.

Daniel M. Kammen of the University of California, Berkeley, who studies energy resources, says that the Minnesota team has done "neat work." He agrees that ethanol from nonfood sources is more promising. However, he says that "the U.S. is awash in corn, and some of that could and should be used" as biofuel.