Abstract
Renodulation and nitrogen fixation potential of indigenous and exotic isolates of Rhizobium were studied in a field experiment with Acacia nilotica from July 1990 to June 1991. The inoculum isolates belonged to different serotypes and did not show cross-reaction with the native population of Rhizobium. Nitrogenase activity of root nodules observed at 4-month intervals covering three seasons snowed a decline during winter months, which corresponded with the senescence of the nodules. Maximal renodulation as checked by serotyping and antibiotic resistance pattern was observed with isolate AB 3 followed by AD 4 and USDA 3325. The highest nitrogenase activity was exhibited in plants inoculated with AD 4 at 12 months. Plants inoculated with Rhizobium isolate USDA 3325 showed the highest increase in dry-matter yield at 12 months. Except for AB 3, dual inoculation with Rhizobium isolates and Glomus fasciculatum did not enhance dry matter production as compared with uninoculated controls or single inoculation of either Rhizobium isolate or G. fasciculatum.Key words: Rhizobium, Acacia nilotica, nitrogenase activity, renodulation.

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