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Assessment of Acetylene Reduction by Sainfoin and Alfalfa over Three Growing Seasons¹

Sainfoin (Onobrychis viciifolla Scop.) has many positive characteristics as a forage legume, but frequently exhibits poor stand persistence. Insufficient nitrogen (N₂) fixation has been suspected contributing to the stand persistence problem in sainfoin. Nitrogen fixation (acetylene reduction) rates of sainfoin have been reported for seedling plants, however, there is no information on fixation rates for established stands.

A root box technique which allowed for the measurement of acetylene reduction on intact perennial plants for several growing seasons was used. Acetylene reduction of sainfoln and alfalfa (Medicago sativa L.) grown in soils collected from six existing sainfoin fields was measured at various stages of plant growth over three growing seasons. Sainfoin acetylene reduction rates were 36 to 75 % greater than alfalfa with the difference increasing with plant age.

Percent non-structural carbohydrates (NSC) were 10 % higher alfalfa taproots compared to sainfoin after the third growing season. Total NSC of sainfoin and alfalfa taproots was 1.8 and 6.9 g, respectively. Visual observation indicated that root and crown diseases resulted in significant mortality of sainfoin.

Results indicated that acetylene reduction of sainfoin was superior to alfalfa while a consistent trend favoring a particular soil was not found. Carbohydrate storage and disease susceptibility may contribute more to the stand persistence problem of sainfoin than poor N₂ fixation.

Key Words: Symbiotic nitrogen fixation • Onobrychis viccifolia Scop. • Medicago sativa L. • Carbohydrate storage • Stand persistence • Root box technique • Crude protein

² Former research associate (presently assistant professor of agronomy, Univ. of Nevada, Reno) and professor of crop physiology, Univ. of Wyoming, Laramine.

Received for publication April 17, 1981.