The legume-rhizobia symbiosis is an interaction in which nitrogen fixing bacteria called rhizobia colonize plant roots and supply fixed nitrogen to the plant in exchange for photosynthetically fixed carbon. This interaction has global impacts on the nitrogen cycle and offers an alternative to environmentally damaging synthetic nitrogen fertilizers. Resource allocation between plants and rhizobia is shaped by several factors, including the abiotic environment and various levels of host-symbiont specificity. The first two chapters of this dissertation explore the effects of the abiotic environment. First, a theoretical ecological model was adapted to the legume-rhizobia symbiosis and parameterized with a series of detailed measurements of plant and nodule biomass, carbon and nitrogen content, and plant photosynthesis. These results were compared to model predictions, which illustrated that the model assumption of fair trade was invalid, that plants have more bargaining power than rhizobia, and that plant bargaining power is highest when soil nitrogen is lowest. In the second chapter, the effects of factorial manipulation of soil nitrogen and light availability on resource trade between legumes and rhizobia was assessed. The results revealed that plants adjusted their resource acquisition strategy to take up the most limiting resource, and that both nitrogen and light affect allocation to rhizobia, but not their symbiotic effectiveness per unit of resource received. Finally, the third project assessed the level of specificity between host plant and rhizobial symbiont by comparing the effectiveness of rhizobia isolated from the same plant species to that of rhizobia isolated from different plant species. The results were contrary to ecological theory predicting positive plant-soil feedbacks between legumes and rhizobia. The effects of environmental context dependence and plant-rhizobia specificity are vital for understanding the role of rhizobia in natural and agricultural ecosystems as well as the future development of effective rhizobial crop inoculants. -- Abstract.
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