Legumes and nitrogen fixation in an annual grassland: Responses to herbivory and climate change

Nitrogen's availability as a nutrient often limits the function of terrestrial ecosystems. In turn, ecosystem nitrogen availability may be constrained by factors that limit nitrogen fixation (the process whereby nitrogen gas in the atmosphere is converted to a form that is useful to most organisms). This study examines some of the factors that may regulate nitrogen fixing plants (legumes) in an annual California grassland, and quantifies their impacts on the rate at which nitrogen is fixed in this ecosystem.; In Chapter 1, I describe a method for quantifying nitrogen fixation rates using the natural abundances of stable nitrogen isotopes. I analyzed plant samples from the field and the greenhouse to address the important criticisms aimed at this method. I demonstrate that the method is valid in this ecosystem with respect to Vicia sativa, the dominant legume, and that the rates of symbiotic nitrogen fixation in this grassland are significant in the context of other ecosystem nitrogen fluxes.; In Chapter 2, I examine the hypothesis that herbivores can constrain the activity of nitrogen fixing plants. In a mechanical exclosure experiment, I demonstrate that foliar herbivory can have a substantial impact on legume growth, reproductive output, and nitrogen fixation rates. However, the impacts of herbivory are species-specific: the dominant legume (Vicia sativa ) showed strong responses to herbivory, whereas a less common legume (Medicago polymorpha) showed little or no responses. I conclude that herbivory may account for a 75% decrease in ecosystem scale nitrogen fixation by Vicia.; In Chapter 3, I quantify the responses of Vicia sativa to a factorial set of simulated global changes expected to occur over the next century (increases in atmospheric carbon dioxide concentration, temperature, precipitation, and nitrogen deposition). In contrast to previous reports of strong legume responses to increased carbon dioxide, I find that Vicia's responses are inconsistent and weak. As expected, Vicia was negatively impacted by nitrogen additions, and largely unaffected by precipitation increases. Vicia showed positive responses to increased temperature, and I present evidence that this response is linked to temperature-driven changes in both herbivory and the phenology of the dominant grasses that compete with Vicia.