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Dinitrogen Fixation (C2H2) by Established Forage Grasses in Texas

Weaver, R. W., Wright, Sara F., Varanka, M. W., Smith, O. E., Holt, E. C.
Agronomy journal 1980 v.72 no.6 pp. 965-968
Andropogon gerardii, Brachiaria, Cynodon dactylon, Paspalum urvillei, acetylene, acetylene reduction, environmental factors, ethylene, ethylene production, forage grasses, forage production, greenhouses, indigenous species, introduced species, nitrogen fertilizers, nitrogen fixation, regrowth, rhizosphere, soil, subtropics, surveys, Texas
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.