EDITORS FILE COPY A Masterful Scheme Symbiotic Nitrogen-Fixing Plants of the Pacific Northwest BERNARD T. BORMANN Mr. Bonnann is the Principal Plant Physiologist with the USDA Forest Service at the Pacific Northwest Research Station in Juneau, Alaska. His current research centers on understanding forest ecosystems and deter- mining the effect of forest management on the long-term productivity of the forests of the Pacific Northwest and southeast Alaska. A masterful scheme has evolved in a few higher plants that is a type of symbiosis. In this case, it is a dose, mutually beneficial association between higher plants that produce energy, and nitrogen-fixing bacteria. The root hairs of these plants form into nodules when infected by these specific nitrogen-fixing bacteria. Photosynthetic sugars produced in the foliage are transported to the nodules, supplying the energy to support the activity of a bacterial enzyme, which in turn produces nitrogenous compounds useful to the plant and ultimately, to people. Why is nitrogen fixation important? Nitrogen is thought to be the most impor- tant nutrient element limiting plant growth in the Pacific Northwest. Nitrogen is a major component of all amino acids, proteins, en- zymes and chlorophyll. Plant growth slows when the demand for nitrogen exceeds the abil- ity of the soil to supply it. While other nutrients such as phosphorus and potassium can be derived from the breakdown or weather- ing of rocks and minerals, nitrogen is not de- rived in significant quantities from this source. Pacific Northwest rain, although usually plen- tiful, contains only trace amounts of nitrogen (adding only about 0.5-5 kig of nitrogen per hectare every year). To make matters worse, nitrogen is more easily lost than many nutrient elements. Nitrogen in organic matter is released as a gas when combusted; thus intense wildfires and slashburning can greatly deplete nitrogen reserves from a forested area. Further, as plant residues decompose, nitrate (NO 3 ) can be produced and sometimes lost by leaching below the rooting zone. Nitrates can also be reconverted to nitrogen gas by denitrifying bacteria, such as Pseudomonas spp. How is nitrogen "fixed"? Paradoxically, nitrogen is abundant in gaseous form; it makes up 79 percent of the air we breathe. But the gas is unavailable to higher plants and most microorganisms because of its great molecular stability. Gaseous dinitrogen (N2 ) consists of two nitrogen atoms held very tightly by a strong triple bond. A few micro- organisms have developed a very special en- zyme, called nitrogenase, that has the ability to break this bond, adding hydrogen to form ammo- nium (NH4 ) that can then be used by plants and microorganisms. This process is called nitrogen fixation. This complicated reaction requires the 10 UW Arboretum Bulletin
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EDITORS FILE COPY
A Masterful SchemeSymbiotic Nitrogen-Fixing Plants
of the Pacific Northwest
BERNARD T. BORMANN
Mr. Bonnann is the Principal Plant Physiologist with the USDA ForestService at the Pacific Northwest Research Station in Juneau, Alaska. Hiscurrent research centers on understanding forest ecosystems and deter-mining the effect of forest management on the long-term productivity ofthe forests of the Pacific Northwest and southeast Alaska.
A masterful scheme has evolved in a fewhigher plants that is a type of symbiosis. In thiscase, it is a dose, mutually beneficial associationbetween higher plants that produce energy, andnitrogen-fixing bacteria. The root hairs of theseplants form into nodules when infected by thesespecific nitrogen-fixing bacteria. Photosyntheticsugars produced in the foliage are transportedto the nodules, supplying the energy to supportthe activity of a bacterial enzyme, which inturn produces nitrogenous compounds useful tothe plant and ultimately, to people.
Why is nitrogen fixation important?Nitrogen is thought to be the most impor-
tant nutrient element limiting plant growth inthe Pacific Northwest. Nitrogen is a majorcomponent of all amino acids, proteins, en-zymes and chlorophyll. Plant growth slowswhen the demand for nitrogen exceeds the abil-ity of the soil to supply it. While othernutrients such as phosphorus and potassiumcan be derived from the breakdown or weather-ing of rocks and minerals, nitrogen is not de-rived in significant quantities from this source.Pacific Northwest rain, although usually plen-tiful, contains only trace amounts of nitrogen
(adding only about 0.5-5 kig of nitrogen perhectare every year). To make matters worse,nitrogen is more easily lost than many nutrientelements. Nitrogen in organic matter is releasedas a gas when combusted; thus intensewildfires and slashburning can greatly depletenitrogen reserves from a forested area. Further,as plant residues decompose, nitrate (NO 3) canbe produced and sometimes lost by leachingbelow the rooting zone. Nitrates can also bereconverted to nitrogen gas by denitrifyingbacteria, such as Pseudomonas spp.
How is nitrogen "fixed"?Paradoxically, nitrogen is abundant in
gaseous form; it makes up 79 percent of the airwe breathe. But the gas is unavailable to higherplants and most microorganisms because of itsgreat molecular stability. Gaseous dinitrogen(N2) consists of two nitrogen atoms held verytightly by a strong triple bond. A few micro-organisms have developed a very special en-zyme, called nitrogenase, that has the ability tobreak this bond, adding hydrogen to form ammo-nium (NH4) that can then be used by plants andmicroorganisms. This process is called nitrogenfixation. This complicated reaction requires the
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UW Arboretum Bulletin
Flowers of Purshia tridentata, Kitthas County.
rare heavy-metal element molybdenum as acatalyst. Thus, molybdenum deficiency cancause an indirect nitrogen deficiency innitrogen-fixing microorganisms.
Why do these nitrogen-fixing microorganismsnot proliferate to the point that nitrogen is nolonger limiting? Part of the answer lies with theamount of energy needed to support the pro-cess. To fix 1 gram of nitrogen, an estimated10-100 grams of glucose is required.' Mostmicroorganisms rarely have energy resources toproduce large amounts of fixed nitrogen.Higher plants can produce excess energythrough photosynthesis, but they cannot pro-duce the necessary enzyme.
Which plants fix nitrogen?Two groups of symbiotic nitrogen-fixing
plants are recognized. Many members of thepea or legume family form a symbiotic associa-
Gutschick, V.P. 1978. Energy and nitrogen fixa-tion. Bioscience 28(9):571-575.
Summer 1988 (51:2)
photo: B.O. Mulligan
tion with Rhizobium spp. bacteria. Severalother genera of woody perennials associate withactinomycete bacteria. This second group isknown as actinorrhizal plants. Some lichens (forexample the common foliose lichen Lobariaoregano) are formed through a symbiotic rela-tion between a fungus and a nitrogen-fixingblue-green alga (Nostoc spp.). Some of the moreimportant nitrogen-fixing plants in this regionare listed in Table 1.
Role of nitrogen-fixing plants inPacific Northwest ecosystems
Nitrogen-fixing plants have played a key rolein developing and maintaining Pacific North-west ecosystems. Red and Sitka alder (Alnusrubra and A. sinuata) pollen is found in greatabundance in sediments corresponding to aperiod of early plant development after theWisconsin glaciers left the Puget Sound areasome 12,000 years ago. If we assume thatthroughout the post-glacial period precipitationwas low in nitrogen, as today, and that there
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were at least a few wildfires, much of the5,000+ kg of nitrogen per hectare often foundon forested sites probably originated throughnitrogen fixation by alder. Sediments in LakeWashington have shown a resurgence of alderpollen corresponding with land clearing and log-ging during the early development of Seattle.2Most nitrogen-fixing plants are adapted todisturbance. Ceanothus spp. dominates manysites in the Oregon Cascade Range after burn-ing. Many introduced legumes like Scotchbroom (Cytisus scoparius) come in after soildisturbance from logging or along roadsides.The ability to fix nitrogen gives these plants acompetitive advantage on nitrogen-deficientsoils, although many also do well on non-nitrogen-deficient soils.
Important uses for nitrogen-fixingplants in agriculture and forestry
Currently, two percent of world fossil-fuelproduction is used in direct manufacture ofnitrogen fertilizer. Increased use of nitrogen-
2 Davis, M.D. 1973. Pollen evidence of changingland-use around the shores of Lake Washington.Northwest Science 47(3): 133-148.
Red alder, Alnus rubra
Foliage of red alder. photo: B.O. Mulligan
fixing plants in agriculture and forestry couldreduce this dependency. Because the cost offossil-fuel-derived nitrogen fertilizer has de-creased in recent years, it is important to findnitrogen-fixers that have economic value inaddition to the value of the nitrogen they fix.This will be less important when energy pricesincrease again.
AgricultUral legumes such as soybeans,alfalfa, lentils, and clover require little or nonitrogen fertilizer and provide valuable protein-rich foodstuffs. Rangeland management to pro-
Red alder, Alnus rubra, can form large nodule clusters,some as large as a baseball.
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Scientific Name Common Name Range Native/Introduced
Table L) Common nitrogen-fixing plants of the Pacific Northwest (from Hitchcock, C.L and A. Cronquist. 1974.Flora of the Pacific Northwest).
Summer 1988 (51:2) 13
Mountain balm, Ceanothus velutinus near Leavenworth, Chelan County.!It
photo: B.O. Mulligan
mote nitrogen-fixers such as bitterbrush (Pur-shia tridentata Pursh) could increase soil fertilityand provide protein-rich forage for wildlife andcattle of the Great Basin. Native legumes couldbe established after logging to improve foragequality for coastal wildlife species. Red aldermay become a more important part of forestmanagement on the west side of the CascadeRange because it has one of the highest nitro-gen-fixation rates known (50-150 kg/ha an-minIly) and because of the moderately valuablewood it produces.
SummaryNatural sources of nitrogen in rainfall are
usually inadequate to meet the demand of grow-
ing plants and thus limit their productivity. Asa consequence, nitrogen-fixing plants can play avital role in maintaining the fertility of PacificNorthwest ecosystems. Understanding moreabout the process of nitrogen fixation and hownitrogen-fixing plants interact with other plantswill help to develop sustainable management offorest and agricultural ecosystems.
Editor's Note:Refer to the Winter, 1987, issue of theArboretum Bulletin, page 4, for more informa-tion on nitrogen-fixation in connection with theLeguminosae.