BioEd Online

Powering cars with biodiesel from soyabeans might be the cool way for earth-friendly hipsters to get around town now. But another source prairie plants could turn out to make the greenest fuel.

A report in Science today concludes that fuel made from mixtures of prairie plants would save more greenhouse gases than the alternatives proposed so far, including soyabean biodiesel and corn ethanol. These mixtures suck up a lot of carbon dioxide into roots and soil so much that this should far exceed the amount of CO₂ produced by making fuels from the plants and burning them. The result: a net decline in greenhouse gases in the atmosphere.

For now this idea is only theoretical: conversion of biomass from grasses into fuels such as ethanol only occurs in pilot plants. But there are plans for larger facilities in progress.

Other researchers have focussed on the idea of growing monocultures of fast-growing, high-biomass plants, such as switchgrass and poplar trees, to power such facilities. But David Tilman, an ecologist at the University of Minnesota, St. Paul, says that purpose-grown monocultures wouldn't be as good for the planet as old-fashioned prairie.

Fields of dreams

Tilman has been cultivating experimental plots of prairie for years. His work is widely credited with helping to revive the notion, in the mid-1990s, that high biodiversity keeps an ecosystem stable and productive.

He has previously shown that mixed plots produce more plant mass in a given area than monocultures. In agreement with that finding, Tilman and his colleagues now report that mature prairie plots, planted in what was originally a nutrient-poor soil, can produce more than twice as much bioenergy as a monoculture field.

If you take into account the greenhouse gas emissions produced by growing, harvesting, transporting and converting plants into fuel along with the carbon dioxide produced by eventually burning that fuel and weigh this against the amount of carbon dioxide sucked up by plants during growth, prairie comes out 6-16 times better than corn grain ethanol or biodiesel.

The magic behind the mixtures, says Tilman, is "niche complementarity". Plants filling one niche such as spring growth complement plants in another niche, such as summer growth.

And by starting on poor soils, the prairie grasses have a chance to squirrel away more and more carbon below ground, just by growing. Over the years (especially over the first decade) the plant mixtures become more productive: their root systems grow deeper and more intricate, adding nutrients and humus to the soil.

Working the margins

Another advantage to mixtures is that they can grow on land not suitable for agriculture, says John Sheehan, at the National Renewable Energy Laboratory in Golden, Colorado.

"The study will send a message to the environmental community that technologists and others interested in biofuels are actually taking a cautious and intelligent approach to sustainably using marginal land," says Sheehan.

Growing mixtures of plants on 500,000,000 hectares of degraded land worldwide could displace about 13% of global petroleum consumption, and sequester about 15% of carbon dioxide emissions, estimate Tilman and his colleagues.

Some experts are skeptical of Tilman's stronger claims, and argue that the hipsters of tomorrow and maybe the rest of us will be using fuel from multiple plant sources, not just prairie mixtures.

But that doesn't stop Tilman from dreaming. In one scenario, he imagines that food crops like corn could be surrounded by a buffer of prairie yielding fuel while sponging up agricultural runoff and preventing erosion.

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