Biomass for Biofuels Without Chemical Fertilizers?
By Associate Professor Sharon Doty, University of Washington
To produce enough plant biomass to generate the vast amount of biofuels we would need to
reduce our dependency on fossil fuels, we need a way to grow plants at less cost and without
doing harm to the environment. It does not make sense to try to reach this goal by growing
plants with a heavy requirement for chemical fertilizers when the technology used to make the
fertilizer itself uses high amounts of fossil fuels. In addition, synthetic fertilizers have a number
of harmful environmental impacts due to leaching and formation of greenhouse gases. 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. The air we breath is about
80% dinitrogen gas, but it is only available to these 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 kind of soil bacteria that can fix nitrogen.
Through a complicated interaction, a specific bacteria communicates with a specific legume,
resulting in the familiar root nodules in which nitrogen fixation occurs. In addition to peas,
beans, clover, and other common legumes, there are some woody plants that associate with
another nitrogen-fixing microbe, Frankia, and these too live in specialized root nodules. The
plant trades its abundant sugars that it gets through photosynthesis for the combined nitrogen
from its root nodule inhabitants. And so it was long thought that the only nitrogen-fixing
symbioses with plants involved root nodules, leading to the belief that if a plant species had no
root nodules, then the plant was not able to take advantage of nitrogen-fixers.
But what about those crop 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 1990's when it was clearly
demonstrated that sugarcane 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 providing a great benefit to the plant by providing substantial amounts
of combined nitrogen for plant growth (Urquiga et al., 1992; Reinhold-Hurek & Hurek, 1998;
Sevilla et al., 2001). Researchers have identified nitrogen-fixing endophytes in other tropical
area crop species including rice, maize, coffee, and sweet potato (Reinhold-Hurek & Hurek,
1998; Doty, 2010).
Are nitrogen-fixing endophytes also associated with higher biomass plants such as trees that can
be used for biofuel production? Poplar (Populus sp.), a fast-growing pioneer tree, is on the
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 we have found a number of different
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 State. 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 the poplar and
willow gave us the idea that endophytes could be the reason for this growth. With new funding
from NSF through the American Recovery and Reinvestment Act (ARRA), we are investigating
which endophytes provide the highest growth benefit to plants, and we will determine if nitrogen
fixation is indeed occurring within the plants. Not only will 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.
Figures
1. Black cottonwood (Populus trichocarpa) and Sitka willow (Salix sitchensis) naturally grow in
low-nutrient substrates such as this rocky riverside in Western Washington state. We found
nitrogen-fixing microbes within the stems of these plants (Doty et al., 2005; Doty et al., 2009).
2. Graduate student, Jenny Knoth, and Research Scientist, Zareen Khan, are propagating poplar
plants that are free of endophytes in order to test individually which of the nitrogen-fixing
endophytes are the most effective for improving plant growth with limited or no fertilizer.
References
Doty SL. 2010. Nitrogen-fixing endophytic bacteria for improved plant growth. In: Maheshwari
DK, ed. Bacteria in Agrobiology. Springer (Germany). In press.
Doty SL, Dosher MR, Singleton GL, Moore AL, van Aken B, Stettler RF, Strand SE,
Gordon MP. 2005. Identification of an endophytic Rhizobium in stems of Populus. Symbiosis
39: 27-36.
Doty SL, Oakely B, Xin G, Kang JW, Singleton G, Khan Z, Vajzovic A, Staley JT. 2009.
Diazotrophic endophytes of native black cottonwood and willow. Symbiosis 47: 23-33.
Reinhold-Hurek B, Hurek T. 1998. Life in grasses: diazotrophic endophytes. Trends in
Microbiology 6: 139-144.
Sevilla M, Burris RH, Gunapala N, Kennedy C. 2001. Comparison of benefit to sugarcane
plant growth and 15N2 incorporation following inoculation of sterile plants with Acetobacter
diazotrophicus wild-type and Nif- mutant strains. Molecular Plant Microbe Interactions 14: 358366.
Urquiga S, Cruz KHS, Boddey RM. 1992. Contribution of nitrogen fixation to sugar cane:
Nitrogen-15 and nitrogen balance estimates. Soil Sci.Soc.Am.J. 56: 105-114.