Asian Journal of Advances in Agricultural Research

Field experiments were conducted at Kericho East (0°22' S, 35°17' E) and Bomet central (0°47' S, 35°21' E) in South Western Kenya between July 2012 and March 2014, to determine the effects of rhizobia inoculation on nodulation, growth, yield, nitrogen (N) fixation and nodule occupancy of cowpea. Four cowpea varieties (KVU 27-1, K80, M66 and Ngor) received each of the following treatments: inoculation with Bradyrhizobiumsp. USDA 3456, 20 kg N ha^-1, 40 kg N ha^-1, or untreated (absolute control). The experimental design used was randomised complete block design in a 4 x 4 factorial arrangement. N-fixed by cowpea plants in inoculated and untreated plots was determined using ¹⁵N natural abundance technique. Nodule occupancy was done by sequence analyses of 16S rRNA gene in bacterial isolates from cowpea nodules. Data collected were: nodule number and weight, leaf area index, shoot dry weight, shoots and grain nitrogen content, nitrogen fixed and grain yield. Results showed that rhizobial inoculation significantly (P≤.05) increased cowpea nodulation twice out of five sampling times in moderately acidic soils of Bomet central, but increased cowpea nodules only once in strongly acidic soils of Kericho East. Bradyrhizobium inoculation had no significant (P≤.05) effects on growth, tissue N or on the amount of N fixed at the experimental sites. The quantity of N-fixed by the four cowpea varieties in the acid soils was between 9.8 - 19.8 mg N plant^-1, which was less than 2 kg N ha^-1. None of the bacterial isolates from cowpea nodules had similarity to inoculated Bradyrhizobium sp. or any species in Rhizobiaceae family, possibly due to antagonistic effects of nodule endophytes on rhizobia. Nodules were dominated by two species of endophytic plant growth promoting bacteria (PGPB): Bacillus megaterium and Bacillus aryabhattai. It is concluded that under the prevailing soil conditions in South Western Kenya, cowpea plants do not respond to Bradyrhizobium inoculation, and amount of N-fixed by both indigenous and commercial rhizobia is low. Two species of endophytic PGPB are predominant in the acid soils, and their role in cowpea production need to be determined.