Nitrogen and Phosphorus Biogeochemistry of Watersheds Along the Western Slope of the Sierra Nevada

Human activities have more than tripled the amount of N and P available to ecosystems, resulting in complex effects on the environment. The objectives of this study are to evaluate the N saturation status of chaparral ecosystems exposed to elevated atmospheric N deposition and identify mechanisms for the destabilization of P in soils and lacustrine sediments that may be contributing to the eutrophication of high-elevation Sierra Nevada lakes. The N saturation status of a chaparral catchment was assessed through an N budget including atmospheric N inputs, hydrologic and gaseous N outputs, belowground N dynamics, and isotopic separation of streamwater nitrate sources. Phosphorus pools in soil and lake sediment were identified and monitored seasonally to understand how changes in physicochemical conditions and hydrology affect the transport of P to Sierran lakes. In chaparral, seasonal transitions, microbial C limitation, and the asynchrony between N availability and plant N demand modulated the kinetic N saturation of the catchment. The transition from dry to wet soil conditions induced rapid nitrification of ammonium leading to large hydrologic and gaseous N losses. However, during the growing season, N losses were minimal highlighting the limitations of applying standard N-saturation theory to xeric landscapes. In high elevation catchments, P is tightly bound by Al and atmospheric P inputs and rock weathering during the Holocene are the primary sources of present day soil and sediment P. Lake sediments represent a strong sink for P in high elevation lakes even during periods of anoxia. During snowmelt, 27% of the soil P cycles between the inorganic and organic pools raising the possibility that changes in snowpack dynamics could potentially explain long-term trends in P supply to lakes. Although intrinsic differences exist between chaparral and subalpine catchments, they respond similarly to the Mediterranean climate of California. The loss of tight biological control of N and P during seasonal transitions in the Sierra Nevada is similar to the response of ecosystems to periodic disturbances like fire and logging. Thus, nutrient limitation of these terrestrial and aquatic ecosystems and their sensitivity to anthropogenic pollution is tightly linked to interannual and long-term changes in climate