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

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Fogel, M. L. ; Wooller, M. J. ; Cheeseman, J. ; Smallwood, B. J. ; Roberts, Q. ; Romero, I. ; Meyers, M. J. (2008)

Extremes in &delta;<sup>15</sup>N values in mangrove tissues and lichens (range =+4 to &minus;22&permil;) were measured from a mangrove forest ecosystem located on Twin Cays, offshore islands in Belize, Central America. The N isotopic compositions and concentrations of NH<sub>4</sub><sup>+</sup>/NH<sub>3</sub> in porewater, rainwater, and atmospheric ammonia, and the &delta;<sup>15</sup>N of lichens, mangrove leaves, roots, stems, and wood were examined to study the biogeochemical processes important for establishing these unusual N isotopic ratios. Dwarfed <i>Rhizophora</i> <i>mangle</i> trees had the most negative &delta;<sup>15</sup>N, whereas fringing <i>Rhizophora</i> trees, the most positive &delta;<sup>15</sup>N values. Porewater ammonium concentrations had little relationship to N isotopic fractionation in mangrove tissues. In dwarfed mangroves, the &delta;<sup>15</sup>N of fine and coarse roots were 6–9&permil; 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, &delta;<sup>15</sup>N increased within one year from &minus;12&permil; to &minus;2&permil;, approaching the &delta;<sup>15</sup>N of porewater ammonium (&delta;<sup>15</sup>N=+4&permil;). Isotopically depleted ammonia in the atmosphere (&delta;<sup>15</sup>N=&minus;19&permil;) and in rainwater (&delta;<sup>15</sup>N=&minus;10&permil;) 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 &delta;<sup>15</sup>N values. In environments where P is limiting for growth, uptake of atmospheric N by <i>Rhizophora mangle</i> may be an important adaptive strategy.
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