Peter M. Homyak, University of California, Riverside, James O. Sickman, University of California, Riverside, Amy E. Miller, National Park Service, John Melack, University of California, Santa Barbara, and Kevin Skeen, University of California, Santa Barbara.
Background/Question/Methods Chaparral ecosystems in the Sierra Nevada receive N deposition rates of 10-15 kg-N ha-1 yr-1 and exhibit high concentrations of stream nitrate during rainfall events in the autumn. Excess N in Southern California chaparral ecosystems has resulted in changes to plant community composition. However, few studies of the N-saturation status of chaparral have been conducted in the Sierra Nevada. We studied N dynamics in a 4.3-hectare chamise dominated watershed located in the foothills of the western Sierra Nevada to document the response of seasonally dry Mediterranean catchments to elevated rates of N deposition. During our study, we measured atmospheric deposition of N, stream discharge and stream water inorganic and organic nitrogen, nitrogen pools in vegetation and soils, and NO and N2O gas flux from soils.
Results/Conclusions Our results suggest that during a wet year, stream N export can be in excess of atmospheric N inputs, but for a dry year, N retention can be significant. DIN export patterns were typical of Mediterranean ecosystems exhibiting a strong nitrate flush during storms at the onset of the wet season, but with DIN concentrations remaining elevated regardless of decreases in stream discharge. Gas flux measurements in watershed soils will be used to understand current budget imbalances that show greater N inputs than N outputs and to address whether the watershed is N saturated.
