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Volume 5, issue 6
Biogeosciences, 5, 1693–1704, 2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 5, 1693–1704, 2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

  11 Dec 2008

11 Dec 2008

Unusually negative nitrogen isotopic compositions (δ15N) of mangroves and lichens in an oligotrophic, microbially-influenced ecosystem

M. L. Fogel1, M. J. Wooller1,2, J. Cheeseman3, B. J. Smallwood4,5, Q. Roberts1,5, I. Romero5, and M. J. Meyers5 M. L. Fogel et al.
  • 1Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., NW, Washington, DC 20015, USA
  • 2Alaska Stable Isotope Facility: Water and Environmental Research Center, Institute of Marine Science and School of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, Alaska, 99775, USA
  • 3Department of Plant Biology, University of Illinois, Urbana, Illinois, 61801, USA
  • 4Department of Oceanography, Texas A&M, College Station, Texas, 77843-3146, USA
  • 5University of Southern California, 3616 Trousdale Parkway, Los Angeles, California, 90089, USA

Abstract. Extremes in δ15N values in mangrove tissues and lichens (range =+4 to −22‰) were measured from a mangrove forest ecosystem located on Twin Cays, offshore islands in Belize, Central America. The N isotopic compositions and concentrations of NH4+/NH3 in porewater, rainwater, and atmospheric ammonia, and the δ15N of lichens, mangrove leaves, roots, stems, and wood were examined to study the biogeochemical processes important for establishing these unusual N isotopic ratios. Dwarfed Rhizophora mangle trees had the most negative δ15N, whereas fringing Rhizophora trees, the most positive δ15N values. Porewater ammonium concentrations had little relationship to N isotopic fractionation in mangrove tissues. In dwarfed mangroves, the δ15N of fine and coarse roots were 6–9‰ more positive than leaf tissue from the same tree, indicating different sources of N for root and leaf tissues. When P was added to dwarfed mangrove trees without added N, δ15N increased within one year from −12‰ to −2‰, approaching the δ15N of porewater ammonium (δ15N=+4‰). Isotopically depleted ammonia in the atmosphere (δ15N=−19‰) and in rainwater (δ15N=−10‰) were found on Twin Cays. We propose that foliar uptake of these atmospheric sources by P-stressed, dwarfed mangrove trees and lichens can explain their very negative δ15N values. In environments where P is limiting for growth, uptake of atmospheric N by Rhizophora mangle may be an important adaptive strategy.

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