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Dinitrogen Fixation (C₂H₂) by Established Forage Grasses in Texas¹

Efficient forage production by grasses requires an input of fertilizer nitrogen. Fertilizer N applications substantially increase the cost of forage production. Development and management of a biological N fixation system for forage grasses would greatly increase forage production efficiency. As a first step toward development of such a system, a survey was undertaken in the subtropical region of Texas to determine the rates of biological N fixation naturally occurring in the rhizosphere of native and introduced forage grasses. The acetylene reduction method was used to estimate potential rates of biological N fixation. During 3 years, 463 soil cores were collected, of which 13% produced ethylene when exposed to acetylene. In the third year of the survey, 216 cores were collected, of which 21% reduced acetylene to ethylene when tested immediately after sampling, and 70% showed activity after a period of regrowth in the greenhouse. Only samples extrapolated to fix N at rates exceeding 0.5 kg/ha/100 days were classified as active. The most active rhizosphere samples were extrapolated to fix N at rates of 33, 26, 20, and 20 kg/ha/100 days for Cynodon dactylon (L.) Pers., Paspalum urvillei Steud., Brachiaria sp., and Andropon gerardi Vitman respectively. These high rates indicate that under the proper environmental conditions a potential exists for agronomically significant rates of biological N fixation to occur in the rhizosphere of forage grasses.

Key Words: Biological N fixation • Associative N fixation • Diazotrophs N • Grass-N fixation • Acetylene reduction

² Associate professor, graduate research assistant, research associate, research scientist, and professor, respectively, Dep. of Soil & Crop Sciences, Texas A&M University, College Station, Texas 77843.

Received for publication August 19, 1966.