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Evidence for a uniformly small isotope effect of nitrogen leaching loss: results from disturbed ecosystems in seasonally dry climates.

Authors
  • Mnich, Meagan E1
  • Houlton, Benjamin Z2
  • 1 University of California, Davis, USA. [email protected]
  • 2 University of California, Davis, USA.
Type
Published Article
Journal
Oecologia
Publisher
Springer-Verlag
Publication Date
June 2016
Volume
181
Issue
2
Pages
323–333
Identifiers
DOI: 10.1007/s00442-015-3433-0
PMID: 26343040
Source
Medline
Keywords
License
Unknown

Abstract

Nitrogen (N) losses constrain rates of plant carbon dioxide (CO2) uptake and storage in many ecosystems globally. N isotope models have been used to infer that ~30 % of terrestrial N losses occur via microbial denitrification; however, this approach assumes a small isotope effect associated with N leaching losses. Past work across tropical/sub-tropical forest sites has confirmed this expectation; however, the stable N isotope ratio (δ(15)N) of ecosystem leaching has yet to be systematically evaluated in seasonally dry climates or across major ecosystem disturbances. We here present new measurements of the δ(15)N of total dissolved N (TDN) in small streams, bulk deposition, and soil pools across eight watershed sites in California, including grassland, chaparral, and coastal redwood forest ecosystems, with and without fire, grazing, and forest harvesting. Regardless of the dominant vegetation type or disturbance regime, average δ(15)N of TDN in stream water differed only slightly (<~1 ‰) from that of bulk soil δ(15)N, revealing a uniformly small isotope effect associated with N leaching losses even under non-steady state conditions. Rather, lower input δ(15)N compared to TDN δ(15)N in streams pointed to fractionations via gaseous loss pathways as the dominant mechanism behind soil δ(15)N enrichment. We conclude that N leaching does not impart a major isotope effect across a broad range of ecosystems and conditions examined, thereby advancing the N gas-loss hypothesis as the principal explanation for variation in bulk soil δ(15)N.

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