Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation

To evaluate nitrogen (N) saturation in xeric environments, we measured hydrologic N losses, soil N pools, and microbial processes, and developed an N-budget for a chaparral catchment (Sierra Nevada, California) exposed to atmospheric N inputs of approximately 8.5 kg N ha⁻¹ y⁻¹. Dual-isotopic techniques were used to trace the sources and processes controlling nitrate (NO₃ ⁻) losses. The majority of N inputs occurred as ammonium. At the onset of the wet season (November to April), we observed elevated streamwater NO₃ ⁻ concentrations (up to 520 µmol l⁻¹), concomitant with the period of highest gaseous N-loss (up to 500 ng N m⁻² s⁻¹) and suggesting N-saturation. Stream NO₃ ⁻ δ¹⁵N and δ¹⁸O and soil N measurements indicate that nitrification controlled NO₃ ⁻ losses and that less than 1% of the loss was of atmospheric origin. During the late wet season, stream NO₃ ⁻ concentrations decreased (to