Improving biological nitrogen fixation in common bean (phaseolus vulgaris) varieties grown within eastern Kenya

Abstract

Common bean (Phaseolus vulgaris L.) is the third most valuable food crop in Kenya and the main source of proteins to smallholder farmers. However, its production is constrained by insufficient nitrogen (N) in the soil exacerbated by acidic soil conditions and phosphorus (P) fixation. Majority of smallholder farmers are resource limited hence apply negligible amount of inorganic fertilizers. Therefore, enhancing biological nitrogen fixation (BNF) by bean germplasms grown by smallholder farmers using low-cost native rhizobia adapted to local agro-climatic conditions is imperative. Thus, this study was aimed at assessing the efficiency of biological nitrogen fixation in selected common bean varieties grown in Eastern Kenya. The questionnaires were used to identify the common bean varieties grown in Eastern Kenya. The native rhizobia were isolated from the root nodules of (MAC 13 and Mac 64) bean varieties grown as trap cultures in Eastern Kenya. Isolation of native rhizobia was done on Yeast extract Mannitol agar (YEMA) supplemented with Congo red dye and Bromothymol blue (BTB) for characterization. A greenhouse bioassay was carried out at Kenyatta University where ten common bean varieties widely cultivated in Eastern Kenya were grown and either inoculated with a consortium of native rhizobia, exotic Rhizobium, a mix of native consortium and exotic rhizobia, or left without inoculation (control). The experiment was set up as a completely randomized design with three replications per treatment. The crop was sampled after four weeks and examined for nodule number (NN), nodule dry weight (NDW), shoot dry weight (SDW), root dry weight (RDW) and shoot nutrients namely; nitrogen (N), P, and potassium (K). The inoculated bean plants produced a significantly higher nodule number (NN), nodule dry weight (NDW), shoot dry weight (SDW), as well as shoot N and P contents than non-inoculated bean plants. The highest significant SDW and N content were achieved in bean plants inoculated with a mix of native and exotic rhizobia, while the highest significant NN, NDW, and P content were realized in bean plants inoculated with native rhizobia. Among the ten common bean varieties, Kabuu produced the highest significant NDW, SDW, N and P content as compared to other varieties. These results demonstrate a key potential of native rhizobia inoculants in promoting BNF and form an important step towards the development of cost effective rhizobial cultures. Further studies should elucidate the performance of the native rhizobia inoculants used here under field conditions.