Unusually negative nitrogen isotopic compositions (δ15N) of mangroves and lichens in an oligotrophic, microbially-influenced ecosystem
Other literature type
Fogel, M. L.
Wooller, M. J.
Smallwood, B. J.
Meyers, M. J.
(issn: 1726-4189, eissn: 1726-4189)
Extremes in δ<sup>15</sup>N 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 NH<sub>4</sub><sup>+</sup>/NH<sub>3</sub> in porewater,
rainwater, and atmospheric ammonia, and the δ<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 δ<sup>15</sup>N, whereas
fringing <i>Rhizophora</i> trees, the most positive δ<sup>15</sup>N values. Porewater
ammonium concentrations had little relationship to N isotopic fractionation
in mangrove tissues. In dwarfed mangroves, the δ<sup>15</sup>N 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, δ<sup>15</sup>N increased
within one year from −12‰ to −2‰, approaching the δ<sup>15</sup>N of
porewater ammonium (δ<sup>15</sup>N=+4‰). Isotopically depleted ammonia
in the atmosphere (δ<sup>15</sup>N=−19‰) and in rainwater (δ<sup>15</sup>N=−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 δ<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.