Asymbiotic nitrogen-fixing bacteria from the surfaces of nodules and roots of legumes

Facultative anaerobic asymbiotic N 2 -fixing bacteria have been identified consistently on the surfaces of roots and nodules of soybean plants cultured under laboratory and greenhouse conditions. An examination of a limited number of samples of alfalfa and clover plants from natural soils also has revealed similar types of bacteria. Most of the isolates investigated were members of the Klebsiella–Aerobacter group. Serological tests of 11 isolates indicated different degrees of relationship to Klebsiella pneumoniae. Rates of growth and N 2 fixation in anaerobic cultures were markedly stimulated by a sterile extract of soybean nodules. Bacteria of the type identified on root surfaces of legumes were found often as contaminants of legume seeds and in most cases were not removed by surface sterilization treatments.