In efforts to curb our use of greenhouse gas-generating fossil fuels, plant-based biofuels are among the top contenders as alternative liquid energy sources for transportation. However, strategies to produce high yields of biomass for fuels are not a one-size-fits-all proposition, according to a study led by UC Santa Barbara professor of ecology David Tilman.
"It is difficult to make a biofuel that actually has environmental benefits," said Tilman, a faculty member in the Bren School of Environmental Science & Management, and co-author of a paper published in the journal Nature Sustainability. "When a food crop is used to make a biofuel, this, in essence, takes food away from poor people around the world, and, it turns out, offers little, if any, greenhouse gas reductions."
Whereas conventional production of biofuels has largely used food crops such as corn, soybeans, oil palm and sugarcane, these practices have their pitfalls, such as intensive use of nitrogen fertilizers, and competition for fertile croplands that had been growing food. In Tilman's 10-year experiment, the researchers explored alternative ways to generate biomass, but with fewer environmental and economic side effects.
"We wanted to see if prairie grasses might prove to be a better crop," he said. In contrast to relatively shallower-rooted annual crops, the deep roots of the perennial grasses of the Midwest are better able to store carbon in the earth -- an additional environmental benefit. Moreover, according to the study, growing a diversity of perennial grasses on lands so infertile that they had been abandoned from agriculture "could minimize competition with food and the greenhouse gas emissions associated with potential direct or indirect land clearing, enhance recovery of ecosystem services and provide wildlife habitat."
However, because the land was depleted of nutrients, some agricultural treatment might have value. And so the researchers, working with 36 plots and 32 native grassland species, set out to find the optimal amount of fertilizer and irrigation that would yield the greatest -- and most diverse -- amounts of biomass, while also resulting in underground carbon storage and minimal nitrate leaching.
The result, after a decade of observation and analysis? More is not necessarily better.
"Our results indicate that different intensification levels have different environmental benefits and costs," said lead author Yi Yang, who is now a postdoctoral researcher at the University of Minnesota.
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