
TY  - JOUR
A1  - Schmidt, Susanne 
A1  - Dennison, William C. 
A1  - Moss, Gordon J. 
A1  - Stewart, George R. 
T1  - Nitrogen ecophysiology of Heron Island, a subtropical coral cay of the Great Barrier Reef, Australia
JF  - Functional Plant Biology
JO  - Functional Plant Biol.
Y1  - 2004
VL  - 31
IS  - 5
SP  - 517
EP  - 528
KW  - ammonia, ammonium, nitrate, nitrate reductase, 15N, &#948;15N, uric acid, xylem sap.
UR  - https://doi.org/10.1071/FP04024 
N2  - Coral cays form part of the Australian Great Barrier Reef. Coral cays with high densities of seabirds are areas of extreme nitrogen (N) enrichment with deposition rates of up to 1000 kg N ha&#8211;1 y&#8211;1. The ways in which N sources are utilised by coral cay plants, N is distributed within the cay, and whether or not seabird-derived N moves from cay to surrounding marine environments were investigated. We used N metabolite analysis, 15N labelling and 15N natural abundance (&#948;15N) techniques. Deposited guano-derived uric acid is hydrolysed to ammonium (NH4+) and gaseous ammonia (NH3). Ammonium undergoes nitrification, and nitrate (NO3&#8211;) and NH4+ were the main forms of soluble N in the soil. Plants from seabird rookeries have a high capacity to take up and assimilate NH4+, are able to metabolise uric acid, but have low rates of NO3&#8211; uptake and assimilation. We concluded that NH4+ is the principal source of N for plants growing at seabird rookeries, and that the presence of NH4+ in soil and gaseous NH3 in the atmosphere inhibits assimilation of NO3&#8211;, although NO3&#8211; is taken up and stored. Seabird guano, Pisonia forest soil and vegetation were similarly enriched in 15N suggesting that the isotopic enrichment of guano (&#948;15N 9.9&#8240;) carries through the forest ecosystem. Soil and plants from woodland and beach environments had lower &#948;15N (average 6.5&#8240;) indicating a lower contribution of bird-derived N to the N nutrition of plants at these sites. The aquifer under the cay receives seabird-derived N leached from the cay and has high concentrations of 15N-enriched NO3&#8211; (&#948;15N 7.9&#8240;). Macroalgae from reefs with and without seabirds had similar &#948;15N values of 2.0&#8211;3.9&#8240; suggesting that reef macroalgae do not utilise 15N-enriched seabird-derived N as a main source of N. At a site beyond the Heron Reef Crest, macroalgae had elevated &#948;15N of 5.2&#8240;, possibly indicating that there are locations where macroalgae access isotopically enriched aquifer-derived N. Nitrogen relations of Heron Island vegetation are compared with other reef islands and a conceptual model is presented. 
ER  -

