Poplar trees, seen here, are being considered as a source for biofuels. Scientists were able to increase the growth of these trees on marginal land by adding specific bacteria. [Credit: Robert Crum, flickr.com].
With biofuel production on the rise, researchers are looking for ways to grow plants for use as energy sources without taking over fertile land currently used for food crops. One possible solution is to move biofuel feedstocks onto poor quality soil that is not ideal for agriculture, such as soils that lack nutrients, receive little rain or have been contaminated from previous industrial use. Now, scientists have found a unique way to increase the growth of one promising biofuel source on marginal land: just add bacteria.
“If we have bacteria that can help plants to grow better, then these plants will be able to get established [on marginal land], and we can then use these soils for the economic production of biofuels,” says Daniel van der Lelie, a microbiologist at Brookhaven National Laboratory on Long Island, NY and lead author of a study published in the February 1 issue of the journal Applied and Environmental Microbiology.
In the study, the researchers focused on improving the growth of poplar trees. These trees are known for their rapid growth and ability to survive in many different types of climates, both ideal traits for biofuel production. The Brookhaven group found that adding the right kinds of naturally occurring bacteria to the roots of poplar trees increased their biomass production by up to 80 percent over ten weeks, according to van der Lelie.
Biofuels have recently become an active area of alternative energy research. In 2007, Congress passed the Energy Independence and Security Act, which requires that biofuel production increase from 4.7 billion gallons per year (about 1.5 percent of current liquid fuel consumption) to 36 billion gallons (about 12 percent of the projected consumption) by 2022. To meet this mandate without infringing upon land used for growing food, many scientists believe it is important to explore ways to grow fuel crops on marginal soils.
“Land is a finite resource,” says M. Cristina Negri, a soil scientist at Argonne National Laboratory in Illinois who was not involved in this study. “If you start growing biofuels on land that was previously used to grow food, then you have a conflict.”
Perhaps the best example of the food-fuel conflict is the U.S. corn crop. Some experts believe that the recent increase in corn prices, which peaked in the summer of 2008, is at least partly caused by the fact that almost a third of the crop is being used to make ethanol fuel. “Corn is used to feed animals, it’s used to make high fructose corn syrup and it’s used for a few other things, but now about 30 percent of it is used to make ethanol. And that additional demand is going to mean higher prices [than] we’ve had historically,” says Wallace Tyner, an agricultural economist at Purdue University. Researchers want to alleviate this problem by growing non-food crops such as poplar trees on non-agricultural land.
To get the most out of marginal land, the Brookhaven researchers turned to specific bacteria that are essential to plant life, called endophytic bacteria. Just as humans have microbes in their gut, plants naturally have bacteria that live inside their tissues and do not cause disease, says van der Lelie. While scientists have long known about the existence of these microorganisms, they are only just beginning to isolate and characterize the many types of bacteria that are beneficial to plant growth for potential biofuel crops.
The Brookhaven scientists identified 78 strains of bacteria from the roots and stems of poplar trees, and they tested the ability of certain strains to help poplars grow in a greenhouse. The researchers first grew poplar tree cuttings in beakers containing a nutrient solution, then added specific bacteria to the beakers. After a few days, the plants were transferred to sandy soil in a greenhouse. Once ten weeks were up, the scientists harvested the trees and determined their total biomass. They found that two bacteria species, Burkholderia cepacia and a species of Enterobacter, had a large growth-promoting effect.
The researchers plan to carry out more experiments to decipher the mechanisms behind the growth-boosting effect and look for ways they can use microbes to enhance the overall health of poplar trees, including helping them ward off plant diseases. They will also examine whether the same result can be achieved in fields, not just in greenhouses.
One way to apply this idea to marginal lands would be to use “root trainers” for the poplar trees. Agriculturalists would first grow poplar tree cuttings in soil pots supplemented with endophytic bacteria. This method would allow the bacteria to properly colonize the plant roots, says van der Lelie. Then the trees would be transplanted into the fields.
Crop growers may want to consider adding these bacteria supplements to their plants, according to Kurt Thelen, a soil scientist at Michigan State University. However, he adds, each of the proposed products should be evaluated for cost effectiveness.
Supplying beneficial bacteria to poplar tree feedstocks would cost about as much as adding Miracle-Gro fertilizer to the soil, according to van der Lelie.
However, even if researchers are able to improve poplar growth on poor soils, there are still obstacles to overcome before these trees can be used as biofuel. One problem is that the current methods for converting plant matter, called cellulose, into ethanol fuel are very expensive. There are presently no commercial production facilities for cellulosic ethanol in the United States, but in 2007 the U.S. Department of Energy announced its plan to invest up to $385 million through 2011 in biorefineries that will look for more cost effective ways to refine cellulose.
If the conversion problems are solved, cellulosic biofuels could not only become crucial energy sources, they could also provide a way to put otherwise unproductive land to good use. “I think it will offer some opportunities for those lands that have been on the fringe of the Corn Belt that really weren’t productive enough for supporting corn cropping systems,” says Michigan State’s Thelen.
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