Graduate Thesis Or Dissertation
 

Evaluation of Rhizobium strains with three Mediterranean forage legumes for biological nitrogen fixation

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  • Centuries of continuous grain cropping in northern Africa have reduced N levels in soils to a degree such that agricultural production is now largely dependent upon nitrogen fertilizer. Through the identification of highly effective legume and Rhizobium strain combinations, the production of protein-rich livestock forage and green manure can be substantially increased. Three common Mediterranean legumes, Lupinus albus, Medicago truncatula, and Trifolium alexandrium were chosen to be tested with various strains of Rhizobium. Greenhouse and growth chamber experiments were conducted to evaluate these forage legumes with commercially available Rhizobium inoculum, nodule isolates collected in Tunisia, and nodule isolates from other sources. Strain effectiveness was determined following 6 weeks of growth in plant tubes containing nutrient agar or modified Leonard jars containing nutrient solution. Each symbiotic system was evaluated for shoot, root, and nodule dry weight, nodule number, nodule acetylene reduction activity, and total plant N. For L. albus, four single strains, 96A5, 96A19, 96B15, and 96B23, and one commercially available multiple strain 'H', produced acetylene reduction activity from 100 to 190 u moles ethylene evolved.h⁻¹.g⁻¹ dw of nodules. Plant dry weights and total plant N values were comparable to the +N treatment. All five strains were therefore recommended for further evaluation under field conditions. Two single strains for M. truncatula, 102D6 and 102B11, produced acetylene reduction values from 800 to 900 u moles ethylene evolved. h⁻¹.g⁻¹ dw of nodules (1.5-5.1 umoles.h⁻¹.plant⁻¹). Plant dry weights and total plant N values were higher than the +N treatment. Both strains were recommended for further evaluation under field conditions. Strains WCI-1 and 162X95 for T. alexandrium produced acetylene reduction activity from 900 to 1000 u moles ethylene evolved.h⁻¹.g⁻¹ dw of nodules (1.9-11.4 umoles.h⁻¹.plant⁻¹). Plant dry weights were 50%, and total plant N 80%, of the +N treatment. Both strains were recommended for further evaluation under field conditions. Further evaluation of these strains in field trials is recommended. These trials should be performed in the area of eventual use or with soil and climatic conditions typical of the production area.
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