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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UWArbcirtum Bulletin

Scientific Name Common Name Range Native/Introduced

Leguminous plantsCytlsus scoparius (L) Unk Scotch broom CW ILathyrus latifolius L REG ILathyrus nevadensIs Wats. CE NLathyrus polyphyllus Nutt. REG NLotus crassifolius (Benth.) Greene Pink trefoil CW NLotus micranthus Benth. Slender trefoil CW NLupinus albicaulis Dougl. Silky-stemmed lupine CW NLupinus albifrons Benth. White-leaved lupine CW NLupinus latifolius Agardh REG NLupinus laxiflorus Dougl. Spurred lupine CW NLupinus lepidus Dougl. CW NLupinus micranthus Dougl. Small lupine REG NLupinus polyphyllus Undl. Large lupine REG NLupinus rivularis Dougl. Riverbank lupine REG NLupinus su/phureus Dougl. Oregon lupine CW NMedicago hispida Gaertn. Bur clover REG IMedicago fupulina L Black medic REG IPsoralea physodes Dougl. Califomia tea REG NTrifolium bifidum Gray REG NTrifolium ciliolaturn Benth. CW NTrifolium dub/um Sibth. Small ho pclover REG ITrifolium eriocephalum Nutt. REG NTrifolium gracilentum TAG. CW NTrifolium hybridum L. Alsike clover REG ITrifolium longipes Nutt. CW NTrifolium microcephalum Pursh Small-headed clover REG NTrifolium microdon H.&A. Cup clover CW NTrifolium oliganthum Steud. CW NTrifolium pratense L Red clover REG ITrifolium procumbens L Hop-clover REG ITrifolium repens L White clover REG ITrifolium tridentatum Undi. 3-toothed clover CW NTrifolium variegaturn Nutt. CW NUlex europaeus L REG IVicia amerlcana Muhl. Wild pea REG NVicia gigantea Hook. Giant vetch CW N

Actinomycete-nodulated (actInorrhizal) plantsAlnus /ncana (L) Moench Mountain alder CE NAlnus rhomblfolia Nutt. White alder CE NAlnus rubra Bong. Red alder CW NAlnus slnuata (Regef) Rydb. Sitka alder CW NCeanothus cuneatus (Hook.) TAG. Common buckbrush CW NCeanothus Integerrimus H.&A. Deerbrush CE NCeanothus prostratus Benth. CE NCeanothus sangulneus Pursh Buckthom REG NCeanothus ve/utinus Dougl. Mountain balm REG NCercocarpus montanus Raf. Mountain-mahogany CE NDryas drummondll Richards. Dryas REG NDryas octopetala L REG NMyrIca califomIca Cham. Pacific wax-myrtle CW NMyrIca gale L Sweet gale CW NPurshla fridentata (Pursh) DC. Bitterbrush CE NShepherdla argentea (Pursh) Nutt. Thorny buffalo-berry CE NShepherdia canadensIs (L) Nutt. Soapberry REG N

Ranges are: CW—Cascades westward; CE—Cascades eastward; REG—reglonal

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.

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UTV Arboretum Bulletin