An assessment of biological nitrogen fixation on legumes growing on farmers’ fields and in natural systems and authentication of associated root nodule bacteria
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The low inherent soil fertility, especially nitrogen (N) constrains arable agriculture in Botswana. Nitrogen is usually added to soil through inorganic fertilizer application. In this dissertation, biological N? fixation by legumes is explored as an alternative source of N. Tire objectives of this study were i) to measure levels of N2 fixation by grain legumes such as cowpea, Bambara groundnut and groundnut in fanners’ fields as well as to estimate N; fixation by indigenous herbaceous legumes growing in the Okavango Delta; ii) to isolate root nodule bacteria, characterize and authenticate them on their homologous hosts. Flowering plants were sampled from the panhandle part of the Okavango Delta, Tswapong area, Kgalagadi and Gantsi regions. N2 fixation was measured using the I5N stable isotope natural abundance technique. Root nodule bacteria were isolated, characterized and authenticated under sterile conditions. The 8I5N values of indigenous herbaceous legumes indicated that they fixed Na (-1.88 to +1.35 %o) with the lowest value measured in Chamaecrista absus growing in Ngarange (Okavango Delta). The 815N values of grain legumes growing on farmers’ fields ranged from -12 %o to +3.3 %o. For most farms, %Ndfa was above 50% indicating that they largely depended on symbiotic fixation for their N nutrition. With optimal planting density, Bambara groundnuts on farmers’ fields could potentially fix about 100 kg N/ha in some parts of Tswapong area and about 60 kg N/ha in areas around the Okavango Delta. Root nodule bacteria exhibited various morphological characteristics. Not all of the authenticated strains fixed Ni, for example, seventeen out of the twenty-six nodulating strains were fixing N2. Some rhizobial strains such as cowpea strain BUAN316/QAB-Vu48 and Crotalaria sphaerocarpa strain BUAN316/XAU-Cs70B induced high shoot and root biomass comparable to 5 mM KNO3 positive control. Taken together, results from this thesis have shown that herbaceous indigenous legumes and cultivated legumes play an important role in the cycling of N in the soil. It has also been shown that biological N2 in farmer’s field could potentially supply the much needed N for the legumes and the subsequent cereal crops if plant densities are optimized. This desertion has also shown that some of the isolated rhizobial strains have potential for development of elite rhizobial strains as well as Plant Growth Promoting Bacteria (PGPBs) with the potential to increase food security and mitigate climate change.
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