UC Riverside

Disruptions of the hydrological and nitrogen cycles in arid regions can seriously impact desert ecosystems by altering the fire cycle and exacerbating shifts in vegetation. With the potential for nitrogen deposition, spread of invasive species, and climate change to alter the hydrological and nitrogen cycles, understanding these cycles is necessary for assessing the threats to desert ecosystems. This dissertation explores the connections between the hydrological and nitrogen cycles in sandy soils in arid ecosystems. An analysis of the hydrological cycle suggested that the water flux at highly negative potentials during the dry summers contributes significantly to the total annual water flux at two study sites in the Sonoran desert. Because commonly-used soil hydraulic models do not accurately approximate the retention curve at highly negative potentials, predictions of liquid water flow at moderate potentials during the growing season were inaccurate due to very dry initial conditions. An examination of how monthly nitrate measurements throughout the top 100cm of soil varied with precipitation at the same study sites showed how strongly the hydrological cycle affects the nitrogen cycle in the desert. After rapid nitrate creation followed the first rains of the growing season, the leaching of nitrate with subsequent rainfalls was likely a major factor causing nitrogen limitation in annual plants. Significantly greater nitrate in the soil in plots fertilized with ammonium nitrate relative to unfertilized plots throughout the dry summer suggested that a considerable amount of nitrate in the soil was not taken up by plants. Variations in ammonium measurements with precipitation in the soil at the sites were similarly analyzed. Results suggest that the adsorption of ammonium is likely an important factor in increasing nitrogen availability to short-rooted plants through inhibiting leaching. Identifying how the hydrological cycle influences the nitrogen cycle in arid ecosystems is critical for predicting how deserts will change in the future.