Biomass for Biofuels

Growing Bioenergy Crops without Chemical Fertilizer

Biomass is a type of renewable resource that includes plants and organic wastes.  Researchers at SFR are studying how to convert biomass to liquid fuels for transportation, called biofuels, as a means of reducing pollution and our dependence on non-renewable oil.  Trying to reach this goal by growing plants that require a heavy dose of chemical fertilizer doesn’t make sense, because the technology used to make the fertilizer itself uses high amounts of fossil fuels and because these fertilizers produce harmful environmental impacts.  To produce enough plant biomass to generate the vast amount of biofuels needed to reduce our fossil fuel dependency, we need to grow plants at less cost and without doing harm to the environment. So how do we get around this problem? 
Combined nitrogen, one of the main components of fertilizer, can also be produced naturally by certain “nitrogen-fixing” microbes.  Associate Professor Sharon Doty, with the help of PhD student Jenny Knoth and Research Scientist Zareen Khan, is researching the use of nitrogen-fixing endophytes for improved growth of bioenergy crops without the use of environmentally-harmful chemical fertilizers.  

Says Doty, “The air we breathe is about 80 percent dinitrogen gas, but it is only available to organisms that can take nitrogen gas from air and convert it (‘fix it’) to a more user-friendly version, ammonia.  A large group of plants, the legumes, have a close relationship with rhizobia, a soil bacteria that can fix nitrogen through a complicated interaction resulting in the familiar root nodules of these plants. There are also some woody plants that associate with a nitrogen-fixing microbe, Frankia, and these microbes also live in specialized root nodules. The plant trades the abundant sugars that it gets through photosynthesis for the combined nitrogen from its root nodule inhabitants.  It was long believed that the only nitrogen-fixing symbioses with plants involved root nodules and, thus, if a plant species had no root nodules, the plant could not take advantage of nitrogen-fixers.”

But what about those plants that grow rapidly and seem perfectly healthy yet are not fertilized, are not grown in rich soil, and do not have root nodules?  How do they get that combined nitrogen so essential for life?  An answer came in the 1990s when it was clearly demonstrated that sugarcane and other tropical area crop plants had nitrogen-fixing bacteria growing inside the plant stems.  These so-called endophytes (microbes within plants) were not causing disease by living inside the plant but rather were benefiting the plants by providing substantial amounts of combined nitrogen for plant growth.

Do higher biomass plants, such as trees that could be used for biofuel production, also have nitrogen-fixing endophytes?  Says Doty, “Poplar (Populus sp.), a fast-growing pioneer tree, is on the U.S. Department of Energy’s short list of potential biofuel crop species.  I have been studying the endophytes of poplar trees for nearly a decade and have found microbial species that are capable of nitrogen-fixation.  Black cottonwood (Populus trichocarpa) and its family member, Sitka willow, grow in riparian zones in western Washington.  In these areas, the plants grow vigorously in rocky or sandy substrate with no soil or any other substantial source of combined nitrogen.  Finding nitrogen-fixing microbes within poplar and willow gave us the idea that endophytes could be the reason for this growth.”  With funding from the National Science Foundation through the American Recovery and Reinvestment Act, Doty and her colleagues are investigating which endophytes provide the highest growth benefit to plants, and if nitrogen fixation is indeed occurring within the plants.  Knoth's research demonstrated that adding the natural endophytes from wild poplar to greenhouse-grown poplar led to a doubling of the root mass.  Results from the first growing season of Knoth’s field trial at the UW Center for Sustainable Forestry at Pack Forest showed clear increases in growth in one of the groups of inoculated poplar.  This study will be the first longer-term study of the beneficial impact of endophytes on poplar trees.

Adds Doty, “Not only does this work help explain how this important colonizing species is able to naturally live in nutrient-poor environments, it could potentially impact the biofuel industry and agriculture in general by providing an environmentally-friendly and less expensive method for improving plant growth.”  The poplar-endophyte research will continue and be put to the test at commercial poplar plantations with new funding from the U.S. Department of Agriculture's Agriculture and Food Research Initiative.  In this $40 million project led by SFR Professor Rick Gustafson and involving multiple researchers from UW, WSU, UC Davis, Greenwood Resources, and ZeaChem, poplar trees will be harnessed for biofuel production with every effort being made to keep the overall process environmentally-friendly and sustainable.  Doty’s graduate student, Amy Baum, and Research Scientist, Zareen Khan, will study the colonization of the plantation poplar with the nitrogen-fixing endophytes and monitor the growth effects compared to uninoculated control plants.  By returning the symbiotic microbial partners naturally found in poplar trees in their native environments back into plantation-grown poplar, the researchers hope to develop a much better method for producing biomass for bioenergy without relying heavily on chemical fertilizers.

PhD student Jenny Knoth and Research Scientist Zareen Khan are propagating poplar plants that are free of endophytes to test individually which nitrogen-fixing endophytes are the most effective for improving plant growth with limited or no fertilizer.

Jenny Knoth checks poplar plant leaves with a hand held chlorophyll meter.  The data from these plants will be used to measure changes in nitrogen allocation to the top leaves in the plants over the growing season. Photo: Conner Knoth.