A study of biological nitrogen fixation in maize-legume intercrop
Muthini, Maingi John
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Nodulation studies on the two major legumes (green grams and common beans) grown in Kiboko were carried out in the green house of the Botany Department , Kenyatta University with soils samples that had been obtain ed from the study area. Indigenous rhizobia were isolated from the two legumes and screened for ability to fix nitrogen in comparison with commercial strains used in inoculant production at MIRCEN, university of Nairobi. The population of indigenous rhizobia specific to the two legumes was determined using the most probable number (MPN) plant infection technique. Field investigations were carried out to assess the effect of intercropping of local maize variety with local bean variety and green grams in the field at KARI/CRISAT,Kiboko experimental field site , S.E Kenya. The results of the study showed that nodulation in common beans was better (80 nodules per plant) than in green grams (18 nodules per plant) However , 68% of the nodules in common beans were white in colour, while only 23% of those of green grams were white. The results of MPN counts indicated that the numbers of indigenous rhizobia resident in Kiboko soils, and specific to green grams and common beans was 519-3, 780, and 2,037-14,850 rhizobia cells per gram of soil, respectively. Two different isolates of rhizobia for common beans, and one for green grams, were isolted. Presumptive and authentication tests confirmed these isolates as rhizobia. Laboratory trials showed that the isolates of beans were not effective in nitrogen fixation, and that Rhizobium leguminosarum biovar. phaseoli strain 446 from MIRCEN was superior in fixation to the two isolates. It was also noted that the isolate from green grams was not as effective in nitrogen fixation as the commercial strain, Bradyrhizobium sp.strain CB-1015. The field experiments set to investigate nitrogen fixation, and to assess the effect of intercropping of maize with green grams and beans revealed that at final harvest of common beans, the monocrop common bean plant inoculated with strain 446 and N-fertilizer treated plants had significantly higher (P=0.05) total plant dry weight than the other common bean treatments. The inoculated monocrop common bean plants had the highest total seed dry weight. The intercrop bean plants were statistically inferior to pure bean plants in both total plant dry weight and total seed dry weight. It was however noted that the nitrogen content in all greem gram treatments. Total plant dry weight was higher in monocrop maize treatments than in the intercrop maize. The maize-common bean intercrop maize performed better than the maize-green gram intercrop maize in total plant dry weight. The performance of monocrop maize treatments and the maize-common bean intercrop was better than that of maize-green gram intercrop maize total grain dry weight. Nitrogen content was however similar in all the treatments.