| In this dissertation I present four projects that focus on the role of belowground processes in ecosystem responses to global change. First, I describe how size and sequence variation in the internal transcribed spacer region of nuclear ribosomal DNA can be used to identify the presence or absence of species in root samples with more than one species. This method overcomes some of the difficulties inherent in studying belowground processes. Next, I show that elevated carbon dioxide (CO2) can interact with plant species composition to affect plant water uptake and vertical soil moisture profiles in an annual grassland. In a microcosm experiment, I grew monocultures and mixtures of Avena barbata and Hemizonia congesta under ambient and elevated CO2. Soil moisture increased under elevated CO2 in communities with A. barbata, the dominant early-season annual grass. In contrast, monocultures of H. congesta, a late-season annual forb, did not conserve water under elevated CO2, and these plants began extracting water from deep soils two weeks earlier than plants in ambient CO2. In a third chapter, I synthesize six years of data from a global change experiment to infer the likely effects of global change on the carbon balance of an annual grassland. In an elevated CO2 background, increased precipitation, nitrogen deposition, and warming had competing effects on carbon dynamics. The ecosystem appeared to lose carbon when rainfall increased, store carbon when nitrogen was added, and maintain a tenuous equilibrium when temperatures were increased. In my final chapter, I use the process-based ecosys model to explore the long-term effects of elevated CO2 and nitrogen deposition on grassland carbon storage. Model performance improved as the simulated ecosystem transitioned from nitrogen to phosphorus limitation, matching a number of field studies suggesting that the ecosystem is simultaneously or sequentially limited by nitrogen and phosphorus. Simulations also showed that carbon storage increases under nitrogen deposition, but that this response is not sustainable after the onset of phosphorus limitation. |
