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

  • Author(s): Homyak, Peter M
  • Sickman, James O
  • Miller, Amy E
  • Melack, John M
  • Meixner, Thomas
  • Schimel, Joshua P
  • et al.

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 <2 µmol l⁻¹) as did gaseous N emissions, together suggesting conditions no longer indicative of N-saturation. We propose that chaparral catchments are temporarily N-saturated at ≤8.5 kg N ha⁻¹ y⁻¹, but that N-saturation may be difficult to reach in ecosystems that inherently leak N, thereby confounding the application of N-saturation indicators and annual N-budgets. We propose that activation of N sinks during the typically rainy winter growing season should be incorporated into the assessment of ecosystem response to N deposition. Specifically, the N-saturation status of chaparral may be better assessed by how rapidly catchments transition from N-loss to N-retention.

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