Carbon-Negative Biofuels

A study described in the December 8 Science magazine found that a low-input (you water it when you plant) high-diversity mixture of native grassland perennials can produce net greenhouse gas savings.

Biofuels, plants used for fuels such as ethanol or biodiesel, are generally considered greenhouse gas neutral if the carbon dioxide taken in equals the carbon dioxide released when the fuel is burned, or more commonly, as a greenhouse gas source because fuels require energy for planting, production, etc. Even under the best circumstances, some energy (which could come from the plants) is needed to produce fuels, and some energy to transport them.

Worldwide, there are some 1,200 million acres (500 million hectares) in abandoned degraded farmland. Tilman, et al, grew from 1 – 16 species/plot on “agriculturally degraded and abandoned nitrogen-poor sandy soil” and examined energy produced/acre and carbon dioxide stored in the soil/acre. Increasing the number of species/plot (up to 16) increased both the energy produced and the rate carbon dioxide was stored. For 16 species, the amount of carbon stored is greater than the amount produced in making and transporting the biofuels.

This method could produce fuel equivalent to 13% of today’s petroleum, plus 19% of today’s electricity (above that needed to produce the fuels?) The use of degraded land would also initially store about 4 metric tonnes carbon per hectare per year for the first decade, 2.7 – 3 metric tonnes carbon for subsequent decades. [Note: The amount of carbon the soil can hold is expected to go down as temperatures increase.]

There are serious questions about using good farmland for fuels when the need for food and fiber are increasing along with the population and per capita consumption. But biofuels grown this way could be a financial incentive to restore damaged land, and reduce greenhouse gas emissions. Sounds like a win-win.

Grass mixture including switchgrass
Grass mixture including switchgrass

5 Responses to “Carbon-Negative Biofuels”

  1. Jon Wharf says:

    Sounds like a good deal, and I can see that a certain amount of carbon would be incorporated into poor soil using the right farming techniques. I’m not sure how the soil can carry on storing more and more carbon though; I would have thought there was a limit. On the other hand I guess a peat bog would be an example of land continuous accumulating carbon.

  2. Karen Street says:

    Not forever, but apparently for a few decades! A temporary free lunch.

    I’m curious, though, where the abandoned farmland is — which countries get to take most advantage of this technique?

  3. Judy Tart says:

    Using biomass for fuel seems like a good idea – but the consequences are already being felt. Today’s Contra Costa Times has a story on the rising cost of tortillas in Mexico due to the increased use of US corn for biofuels. Poor people in Mexico are not able to afford their staple food.

  4. Grist recently published an interview with David Pimentel (Cornell U) in which he seriously questions the energy efficiency of cellulosic ethanol. He doesnt’ think it’s much better than corn in terms of efficiency. Does anyone have a counter to his argument? (See Grist, 12/05/06; their entire series on biofuels is very helpful).

  5. Karen Street says:

    Here is the Grist interview with Pimentel.

    The analysis I’ve seen of Pimentel’s work on ethanol comes from a group at UC, Berkeley, published in the January 27, 2006 Science (subscription required). Here they are referring to two of the six studies they evaluated, one by Pimentel and Patzek, one by Patzek:

    Two of the studies stand out from the others because they report negative net energy values and imply relatively high GHG emissions and petroleum inputs. The close evaluation required to replicate the net energy results showed that these two studies also stand apart from the others by incorrectly assuming that ethanol coproducts (materials inevitably generated when ethanol is made, such as dried distiller grains with solubles, corn gluten feed, and corn oil) should not be credited with any of the input energy and by including some input data that are old and unrepresentative of current processes, or so poorly documented that their quality cannot be evaluated.

    I do have a question about current ethanol plans in California. While it is good to pay attention to how ethanol is made, whether it is made with natural gas or coal, won’t using more natural gas to make fuels lead to using more coal to make electricity?