What is nitrogen fixation?

By Chelsie Vandaveer

July 10, 2003

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White or Dutch clover (Trifolium repens Linnaeus) is in the Fabaceae (Leguminosae), the family of peas and beans. It was once a common plant mixed with various grasses for lawns. Clover lost favor in home lawn use, but can still be found in parks, ball fields, and pastures.

Like other members of the genus Trifolium, white clover is known for improving soil. Clover root systems have a symbiotic relationship with bacteria, Rhizobium trifolii. Rhizobium bacteria live in the soil. The bacteria find clover roots by chemotaxis--'recognizing' their presence by the phenolic (aromatic 6-carbon ring) compounds secreted from the roots. The bacteria move toward the seedling clover roots.

Plant of the Week 07/07/2003

Trifolium repens Linnaeus var. 'Good Luck'
Plant of the Week 07/07/2003

The root cells also secrete trifolin, a chemical classified as a lectin. (See Herbal Folklore, September 17, 2001) Lectins are carbohydrate-binding proteins. Trifolin has a specific affinity for the carbohydrate, 2-desoxyglucose, found in the cell wall of the Rhizobium bacterium. The bacteria attach to emerging root-hairs. Once bound together, specific genes called Nod genes in the bacteria are switched on. Nod genes somehow induce the root-hair to change--the hair bends around the bacteria and grows into a nodule. ("Nitrogen Fixation", Peter v. Sengbusch, Department of Botany, University of Hamburg)

The enclosed bacteria change. Sheltered inside the nodule from rapid or drastic changes in the soil and receiving food (sugars, carbohydrates) from the clover, the bacteria become bacteroids. Bacteroids have lost their outer membrane. The bacteroids get room and board in exchange for nitrogen. ("Nitrogen-Fixing Symbiosis", Biology of Plants, 5th Edition, P. Raven, R. Evert, and S. Eichhorn, Worth Publishers, 1992)

Rhizobium pulls nitrogen out of the atmosphere and converts or 'fixes' it as ammonia (NH₃). The ammonia is further converted into nitrite and nitrate ions. This is the state that the plant can use the nitrogen for building amino acids and proteins. The excess nitrates the clover doesn't use or when the clover plant dies, go into the soil improving it for other plants. ("Nitrogen Cycle", CHM110-Chemistry and Issues in the Environment, Department of Chemistry, Elmhurst College)

The Samuel Roberts Noble Foundation has posted photographs of 'Ladino', a tall growing variety of white clover used in pastures for animal forage and to improve soils. To view the photographs, click on the link:

http://www.noble.org/imagegallery/Forbhtml/WhiteClover.html

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