Nitrogen Fixation

Part I. The range of organisms that can fix nitrogen

Section A: Everything but the legumes
Section B: The legumes

(Note: If you click on any of the pictures displayed on this page, you will be shown a larger, higher-quality version of the same image.)

This table shows some representative plants in the legume family (Fabaceae or Leguminosae) that fix nitrogen. Most legumes can be identified by their "papilionaceous" flower (literally - butterfly-like) which consists of a large, uppermost (banner) petal , 2 lateral (wing) petals, and 2 lower, fused (keel) petals. Other distinguishing factors include: 1) leaves that are alternate, stipulate, and pinnately (or palmately) compound and 2) fruits that are a "legume" or pod which consist of a single carpel that dehisces along 2 sutures.
Figure L1. Root system of soybean (Glycine max), a typical plant in the legume family. Nitrogen fixation occurs in the root nodules that contain bacteria (Bradyrhizobium for soybean, Rhizobium for most other legumes). Almost all legumes can fix nitrogen. The legume family (Leguminosae or Fabaceae) includes many important crop species such as pea, alfalfa, clover, common bean, peanut, and lentil.
Figure L2. Roots of pea showing numerous N-fixing nodules.
Figure L3. Tree lupine (Lupinus arboreus), a common wild legume native to coastal California.
Figure L4. Scot's broom (Cytisus scoparius), a widespread pest of the Pacific coast where it was introduced as an ornamental. This aggressive shrub is a problem in pastures, forests, and wasteland.
Figure L5. Big-head clover (Trifolium macrocephalum), a common attractive legume found in sagebrush deserts of the western U.S.
Figure L6. Bird's foot trefoil (Lotus corniculatus) a European weed (widely escaped in the U.S.) that is a favorite subject of molecular biologists and geneticists who work on nitrogen fixation.
Figure L7. Gorse (Ulex europaeus), a native of Europe, now a serious pest along the coasts of Oregon, Washington and California. Gorse forms impenetrable stands due to its dense, thorny growth.
Figure L8. The genus Oxytropis (many common names such as vetch, crazyweed, stemless-loco weed) is common in many arid parts of the western U.S.
Figure L9. This is Astragalus filipes (also called loco weed) from eastern Oregon. There are many species of Astragalus, some of which are serious problems for livestock because of production of toxic alkaloids or accumulation of selenium from the soil.
Figure L10. A few legumes (such as Sesbania rostrata) have stem nodules as well as root nodules. Stem nodules (arrows) are capable of photosynthesis as well as nitrogen fixation.
Figure L11. Close-up view of stem nodules of Sesbania.
Figure L12. The Leguminosae is a very large family that is especially abundant in the tropics. Some of the flower types of tropical legumes (such as with this Mimosa) appear very different from the "banner and keel" (papilonaceous) flowers of temperate legumes shown in Figs. L3-L9, but most of the Mimosoideae are also effective nitrogen-fixers.

Photo Credits
Figures L1, L2Harold Evans, Oregon State Univ.
Figures L4, L5, L6, L7, L8, L9, L12Keith Karoly, Reed College


Nitrogen Fixation Home Page

Part I. The range of organisms that can fix nitrogen

Part II. Physiology and anatomy of nitrogen fixation

Part III. Ecology of nitrogen fixation

This page was created for David Dalton, a faculty member in the Biology Department at Reed College.

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Created July 31, 1997. Last modified August 20, 2007.
Copyright ©2007 David Dalton