| The goal of this research project was to characterize coupled hydrological and biogeochemical processes in a cold desert ecosystem and determine how they vary with space and time. My research questions included: 1) How does soil moisture influence soil biogeochemical processes and change stores of carbon and nutrients through space and time? 2) How does soil thickness exert control on hydrological and biogeochemical cycling? How might those properties influence landscape patterns in ecosystem function? 3) How do unique temporal and spatial characteristics of N cycling and plant communities exert control on soil biogeochemistry? Specifically, we examined the response of water, C, and N cycling processes to long-term experimental manipulations of precipitation seasonality and magnitude, native vs. invasive plant communities, and varying soil profile thickness in cold desert soils. In contrast to predictions, my results indicate strong distinctions in water, C, and N cycling between native sagebrush and exotic bunchgrass under predicted increases in spring and fall precipitation. Those differences may equate to shifts in cold desert C sequestration and N losses, particularly for the poorly buffered inter-plant spaces. Surprisingly, variability in year-to-year antecedent soil moisture, not long-term increase in mean annual precipitation, was the primary factor controlling temporal fluctuations in surface soil N cycling. The long-term experimental findings presented here strongly support the posited hierarchical pulse-dynamics framework, highlighting pulse-reserve characteristics within and between seasonal timeframes. Furthermore, soil thickness, in addition to plant patchiness, can greatly vary the spatial distribution of soil biogeochemical activity through controls on soil water and material storage capacity. The functional behavior of soil thickness is similar to the inverse texture hypothesis in its control on water loss and storage; both texture and soil thickness control soil storage and process rates in a way that shifts along a precipitation gradient. In dryland ecosystems, predictions of landscape water, C, and nutrient storage and fluxes would greatly benefit from surveys of soil thickness. Soil thickness is a strong regulator of biogeochemical activity, integrating spatiotemporal interactions, and should be a key component of control point theory. |