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

Abstract

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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Acknowledgments

We thank the National Park Service and Annie Esperanza for facilitating access to the study sites and for other logistical support. We are indebted to Kevin Skeen, Delores Lucero, Jennifer Quach, and Amanda James for help in the field and laboratory and thank Gary Lovett and three anonymous reviewers for their insightful comments. This research was funded by the National Science Foundation (DEB-0089839, DEB-0614207, and DBI-1202894 post-doctoral research award) and fellowship support from a Graduate Mentorship award at UCR.

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Correspondence to Peter M. Homyak.

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PMH wrote the paper, analyzed data, collected and processed samples; JOS conceived the study, obtained funding, and collected and processed samples; AEM led field work and collected and processed samples; JMM conceived the study and obtained funding; TM conceived the study, obtained funding, and collected and processed samples; JPS conceived the study and obtained funding. All authors reviewed and edited the manuscript.

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Homyak, P.M., Sickman, J.O., Miller, A.E. et al. Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation. Ecosystems 17, 1286–1305 (2014). https://doi.org/10.1007/s10021-014-9792-2

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Keywords

  • N-saturation
  • drylands
  • chaparral
  • N deposition
  • N-budgets
  • xeric landscapes
  • Adenostoma fasciculatum