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Mixed Prairie Grasses Are Better Biofuel Source

8 December 2006

A new study led by David Tilman, Regents Professor of Ecology in the University of Minnesota’s College of Biological Sciences, indicates that mixtures of native perennial grasses and other flowering plants provide more usable energy per acre than corn grain ethanol or soybean biodiesel and are far better for the environment.

The findings are published in the Dec. 8 issue of the journal Science and featured on the cover.

Biofuels made from high-diversity mixtures of prairie plants can reduce global warming by removing carbon dioxide from the atmosphere. Even when grown on infertile soils, they can provide a substantial portion of global energy needs, and leave fertile land for food production.

—David Tilman, Regents Professor of Ecology in the University of Minnesota's College of Biological Sciences

Based on 10 years of research at Cedar Creek Natural History Area, the study shows that degraded agricultural land planted with highly diverse mixtures of prairie grasses and other flowering plants produces 238% more bioenergy on average, than the same land planted with various single prairie plant species, including monocultures of switchgrass.

Tilman and two colleagues, postdoctoral researcher Jason Hill and research associate Clarence Lehman, estimate that fuel made from this prairie biomass would yield 51% more energy per acre than ethanol from corn grown on fertile land. This is because perennial prairie plants require little energy to grow and because all parts of the plant above ground are usable.

Fuels made from prairie biomass are carbon negative—producing and using them actually reduces the amount of carbon dioxide (a greenhouse gas) in the atmosphere. This is because prairie plants store more carbon in their roots and soil than is released by the fossil fuels needed to grow and convert them into biofuels. Using prairie biomass to make fuel would lead to the long-term removal and storage of from 1.2 to 1.8 US tons of carbon dioxide per acre per year. This net removal of atmospheric carbon dioxide could continue for about 100 years, the researchers estimate.

In contrast, corn ethanol and soybean biodiesel are carbon positive, the researchers contend, meaning they add carbon dioxide to the atmosphere, although less than fossil fuels.

Switchgrass, which is being developed as a perennial bioenergy crop, was one of 16 species in the study. When grown by itself in poor soil, it did not perform better than other single species and gave less than a third of the bioenergy of high-diversity plots.

Switchgrass is very productive when it’s grown like corn in fertile soil with lots of fertilizer, pesticide and energy inputs, but this approach doesn’t yield as much energy gain as mixed species in poor soil, nor does it have the same environmental benefits.

—Jason Hill

To date, all biofuels, including cutting-edge nonfood energy crops such as switchgrass, elephant grass, hybrid poplar and hybrid willow, have been produced as monocultures grown primarily in fertile soils.

The researchers estimate that growing mixed prairie grasses on all of the world’s degraded land could produce enough bioenergy to replace 13% of global petroleum consumption and 19% of global electricity consumption.

The practice of using degraded land to grow mixed prairie grasses for biofuels could provide stable production of energy and have additional benefits, such as renewed soil fertility, cleaner ground and surface waters, preservation of wildlife habitats, and recreational opportunities.

There are 30 million acres of grasslands in the US Conservation Reserve Program (CRP), which pays farmers to manage land to benefit the environment. Current CRP regulations do not allow prairie grasses grown on this land to be used for renewable energy, but the US Farm Bill could be revised to accommodate this practice.

The research was supported by the University of Minnesota Initiative for Renewable Energy and the Environment and by the National Science Foundation (NSF).

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December 8, 2006 in Cellulosic ethanol | Permalink | Comments (11) | TrackBack (0)

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Comments

I wonder how farm subsidies impact land use for biofuel growth...

but this is excellent news. I'd think that the land should be cheaper and its producing more usable fuelstock per acre -- it should be a win-win. At the same time, areas with lower agricultural value may not have the transportation infrastructure necessary to transport the product to market.

Absolutely, the main thing for biofuels is to stop thinking about food production and take a blue skies approach to fuel crops, whatever they may be. Mind you, it has to be worth the farmers' time growing them.

Mixing plant species in a prairie would almost certainly reduce pesticide requirements as well, as it would likely better support natural predators.

Monocultures are the result of market demand for pure product, especially for foodstuffs. In the case of biofuels, there could well be a market for mixed feedstock provided the statistical composition can be maintained within tolerance. That might prove tricker than it sounds. Another problem is that certain crops (e.g. corn) enjoy subsidies, skewing the market against the wider public interest.

Wow, so you mean natural prairie grass mixed in with other plants that naturally grow on the prairie grow better and produce more energy than artificially tilled single crops pesticided and fertilized to no end? Wow... that's just ... so obvious it's almost painful that they took this long figuring this out. We humans think so highly of ourselves and our advanced intellect we really miss the grass under our feet (in this case).

Oorgo:

On the contrary, it's clear that we're smart enough that haven't missed it. We just needed the scientific evidence to show it. I doubt it was the foregone conclusion you seem to think.

These species have evolved together over time, competing and benefiting from each other. Nitro fertilization nay be done by one group of species, while others provide nesting locations for insect eating birds. We do this, somewhat, via crop rotation, multicropping, cover crops, and other arrangements.

I think that we pay farmers to keep 50 million acres of switchgrass on land for soil protection. If they can harvest it, then maybe we do not need the subsidies.

i may be wrong, but it appears the above says mixed species on degraded land produced 238% more bioenergy than any single species ON THE SAME LAND.

and 51% more NET bioenergy than CORN ethanol on FERTILE LAND.

so, no comparison to intensive switch grass monoculture on fertile land, which is likely the most energy positive option.

and no comparison of GROSS fuel production, which is what you sell. even if your net energy production is negative, you could still be profitable because of cost differentials between petrol and ethanol and because of subsidies.

Swtchgrass can grow on poor land with little water like sugarcane and sorghum. The one thing I hear about grass though is it is actually CO2 negative. The roots go 10 feet down and store carbon. If it grows 10 feet per year and you have 50 million acres now to preserve the soil, just mow it down and gasify it.

I need to get hold of this article. Because one thing the summaries don't answer is what the net energy production is. Because they are comparing production on degraded land, the percentage is not easy to translate into an absolute number, because I don't know what net energy per acre for corn would be on the same land.

OK - found a copy on-line:

http://www.ce.cmu.edu/~gdrg/readings/2006/12/12/Tilman_CNegativeBiofuelsFromLowInputHighDiversityGrasslandBiomass.pdf

"Conversion into gasoline and diesel synfuels and
electricity via integrated gasification and combined
cycle technology with Fischer-Tropsch hydrocarbon synthesis (IGCC-FT) is estimated to net 28.4 GJ ha−1 "

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