Isolation and Screening of Effective Rhizobium Species Nodulating and Fixing Atmospheric Nitrogen on Groundnut (Arachis hypogaea) Grown in Some Important Regions of Ethiopia

Groundnut cultivation can benefit from establishing effective nodules through rhizobia inoculation, enhancing nitrogen fixation, promoting crop growth and soil fertility. However, little is known about the root nodule bacteria associated with groundnut cultivation in Ethiopia. To address this, soil samples were collected from three regions, and 72 bacterialstrains were isolated using a groundnut host variety. These isolates were classified and evaluated based on their characteristics and symbiotic effectiveness. The study found that 86% of the isolates belongs to alkali-producing and slow-growing Bradyrhizobium species, while the remaining 14% were fast-growing acid-producing bacteria. The isolates showed variations in carbon and nitrogen source utilization, stress tolerance, and profiles of antibiotic and heavy metal resistance. All isolates were capable of nodulating groundnut, but their effectiveness in nitrogen fixation varied. Nodule numbers, nodule dry weight, and shoot dry weight differed significantly among the isolates. Thirteen percent exhibited high effectiveness (SE > 80%), while 66% were classified as effective (SE ranging from 50 to 80%) on groundnut. Based on their competence, tolerance, and symbiotic effectiveness, three isolatesGNR-37, GNR-19, and GNR-28 performed best and are recommended for further field trials. This study sheds light on the diversity of rhizobial isolates and their varying symbiotic effectiveness, providing valuable insights for groundnut cultivation in Ethiopia. The effective isolates identified show promise for improving groundnut production through enhanced nitrogen fixation. Further field experiments are necessary to validate their potential impact.