Assessing the Effectiveness of Native Rhizobia as Potential Strains for Local Inoculant Production for Enhanced Cowpea and Groundnut Yields in Northern Ghana

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October, 2018
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Efficient exploration of the legume-rhizobium symbiosis via inoculation with rhizobia is constrained by the limited knowledge on the fate of introduced strains under field conditions and the unavailability of effective native strains for use as local inoculants. This research therefore sought to address these challenges in four studies: i) nodule occupancy determination ii) identification of effective native isolates iii) symbiotic performance evaluation of elite native isolates under field conditions and iv) assessment of the persistence of elite isolates following field inoculation. Significant differences (p< 0.05) in shoot biomass of cowpea following inoculation was observed. The differences were explained via nodule occupancy studies by designing specific primers for the test strain Bradyrhizobium pachyrhizi BR 3262 (a recommended cowpea strain in Brazil) using the comparative genomics approach. Out of eleven specific primer pairs designed, the primers 2645 and 2736 were observed to be highly sensitive and reliably detected the target strains in nodules extracts of gnotobiotic system and potted soil grown cowpea. These two primers are thus novel tools for determining the fate of BR 3262 in field inoculation studies. The approach for designing specific primers in this study should be applicable to other bacteria/rhizobium strains whose genomic sequences are available. Bioprospecting for nodules of groundnut and cowpea cultivated in farmers’ fields without inoculation was done followed by isolation of rhizobium in the laboratory. Authentication and symbiotic effectiveness evaluation of the isolated rhizobium strains led to the selection of seven effective isolates. The nitrogen accumulated via BNF of the selected effective isolates was significantly higher (p< 0.05) compared to the –N control treatment. Treatment with isolate KNUST 1002 resulted in a total N accumulation that was comparable to the reference strain 32H1 on groundnut (BR 1 variety) grown in potted soils. Genetic characterization of the seven effective isolates revealed them as diverse with isolates KNUST 1003 and KNUST 1007 belonging to the Rhizobium tropici speices. The five remaining isolates were identified to belong to the Bradyrhizobium yuanmingense species. The two most effective isolates were evaluated in the field alongside a positive (with nitrogen) and negative (without nitrogen or inoculation) control on cowpea (Songotra variety) and groundnut (Chinese variety) in multi-locational trials in the Northern region of Ghana using the randomized complete block design. On the average, cowpea grain yields produced by the inoculated treatments were significantly (p< 0.05) larger than the –N treatment. Isolate KNUST 1002 produced the best average yield (1056 kg ha-1 ) which was significantly different from the other treatments. Groundnut yields on the other hand were significantly larger with isolate KNUST 1006 (1234 kg ha-1 ). The chemical properties of soils in the study locations generally showed low soil fertility. The different soil types significantly influenced the performance of treatments across the study locations. Twenty-six and 23% of the variability in grain yields of cowpea and groundnut respectively, was explained by the soil type and treatment interaction. These results imply that effective native isolates in this study can improve grain yields of cowpea and groundnut based on location specific recommendations. The persistence of effective isolates following previous inoculation on cowpea and groundnut was assessed in the 2017 cropping season on ten of the sites considered in 2016. Nodulation in both previously inoculated and re-inoculated plots were significantly higher than the un-inoculated plots (+N and –N). Nodule dry mass recorded for isolate KNUST 1006 in previously inoculated plots did not differ significantly from that of re-inoculated plots on both target host. However, the nodule dry mass recorded for isolate KNUST 1002 on cowpea in previously inoculated plots was significantly lower than in re-inoculated plots. Increases in grain yield of groundnut were significantly larger for treatment with isolate KNUST 1006 than the other treatments for previously inoculated and re-inoculated plots. Isolate KNUST 1002 on cowpea produced significantly higher yields than the other treatments in previously inoculated and re-inoculated plots. The type of legume host in each case influenced the performance of the isolates. There were no significant differences in the yields produced by either of the isolates in previously inoculated and re-inoculated plots. These results indicate that effective native isolates persisted and effectively nodulated the target legumes obviating the need for re-inoculation in subsequent cropping season. The outcomes of these studies have important implications for the use of effective isolates from this study as inoculants to improve cowpea and groundnut yields particularly in Northern Ghana.
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, in the partial fulfillment of the requirements for the award degree of Doctor of Philosophy in Soil Science.