Schmidt Susanne, Dennison William C, Moss Gordon J, Stewart George R
Department of Botany, The University of Queensland, Brisbane, Qld 4072, Australia. Corresponding author; email:
Centre for Environmental Science, The University of Maryland, Cambridge MD 21613, USA.
Funct Plant Biol. 2004 Jun;31(5):517-528. doi: 10.1071/FP04024.
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 y. 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, N labelling and N natural abundance (δN) techniques. Deposited guano-derived uric acid is hydrolysed to ammonium (NH) and gaseous ammonia (NH). Ammonium undergoes nitrification, and nitrate (NO) and NH were the main forms of soluble N in the soil. Plants from seabird rookeries have a high capacity to take up and assimilate NH, are able to metabolise uric acid, but have low rates of NO uptake and assimilation. We concluded that NH is the principal source of N for plants growing at seabird rookeries, and that the presence of NH in soil and gaseous NH in the atmosphere inhibits assimilation of NO, although NO is taken up and stored. Seabird guano, Pisonia forest soil and vegetation were similarly enriched in N suggesting that the isotopic enrichment of guano (δN 9.9‰) carries through the forest ecosystem. Soil and plants from woodland and beach environments had lower δN (average 6.5‰) 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 N-enriched NO (δN7.9‰). Macroalgae from reefs with and without seabirds had similar δN values of 2.0-3.9‰ suggesting that reef macroalgae do not utilise N-enriched seabird-derived N as a main source of N. At a site beyond the Heron Reef Crest, macroalgae had elevated δN of 5.2‰, 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.
珊瑚礁岛是澳大利亚大堡礁的一部分。海鸟密度高的珊瑚礁岛是氮（N）极度富集的区域，沉积速率高达1000千克氮/公顷·年。研究了珊瑚礁岛植物利用氮源的方式、氮在珊瑚礁岛内的分布情况，以及海鸟源氮是否从珊瑚礁岛转移到周围海洋环境中。我们使用了氮代谢物分析、氮标记和氮自然丰度（δN）技术。沉积的鸟粪衍生尿酸被水解为铵（NH₄⁺）和气态氨（NH₃）。铵进行硝化作用，硝酸盐（NO₃⁻）和NH₄⁺是土壤中可溶性氮的主要形式。来自海鸟繁殖地的植物具有很高的吸收和同化NH₄⁺的能力，能够代谢尿酸，但对NO₃⁻的吸收和同化速率较低。我们得出结论，NH₄⁺是海鸟繁殖地植物的主要氮源，尽管土壤中的NO₃⁻被吸收并储存，但土壤中的NH₄⁺和大气中的气态NH₃的存在会抑制NO₃⁻的同化。海鸟鸟粪、滨刺林土壤和植被的氮含量同样较高，这表明鸟粪的同位素富集（δN 9.9‰）贯穿了森林生态系统。林地和海滩环境中的土壤和植物的δN值较低（平均6.5‰），表明这些地点鸟类源氮对植物氮营养的贡献较低。珊瑚礁岛下方的含水层接收从珊瑚礁岛淋滤而来的海鸟源氮，并且含有高浓度的富氮NO₃⁻（δN 7.9‰）。有海鸟和没有海鸟的珊瑚礁上的大型藻类的δN值相似，为2.0 - 3.9‰，这表明珊瑚礁大型藻类不以富氮的海鸟源氮作为主要氮源。在鹭礁礁顶以外的一个地点，大型藻类的δN值升高至5.2‰，这可能表明在某些地点大型藻类获取了同位素富集的含水层源氮。将鹭岛植被的氮关系与其他珊瑚礁岛进行了比较，并提出了一个概念模型。
