Dinitrogen Fixation (C2H2) by Established Forage Grasses in Texas1

Abstract

AbstractEfficient 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.