Longitudinal distribution of nitrate δ15N and δ18O in two contrasting tropical rivers: implications for instream nitrogen cycling
Longitudinal distribution of nitrate δ15N and δ18O in two contrasting tropical rivers: implications for instream nitrogen cycling
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DOI：
10.1007/s10533-009-9334-8
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发表时间：
2009-06
期刊：
影响因子：
4
通讯作者：
T. Miyajima;C. Yoshimizu;Y. Tsuboi;Yoshiyuki Tanaka;I. Tayasu;T. Nagata;I. Koike
中科院分区：
文献类型：
--
作者：
T. Miyajima;C. Yoshimizu;Y. Tsuboi;Yoshiyuki Tanaka;I. Tayasu;T. Nagata;I. Koike
通过对东南亚两条河流的流域地质和土地利用的对比，测定了其硝酸盐氮(δ15N)和氧(δ18O)同位素组成、溶解无机碳(−)的碳同位素组成(δ13C)和颗粒态有机质的δ13C和δ15N的纵向变化，以了解其内部氮素循环过程。在两条河流的淡水河段，水位在纵向上变得更高。在纵向坡度较大、流域开垦较少的河流中，纵向上也有所增加，而在另一条河流中，纵向上则逐渐减小。同时输入和去除NO_3-−的简单模型表明，前者的NO_3-−的动态受硝化和反硝化作用的内生产物控制，而后者的NO_3-−除受反硝化和/或同化作用的影响外，还受人为的NO_3-DNA负荷的影响。在淡水-微咸水过渡带，以最小溶解氧、最小δ13CDIC和最大PCO2为指标，河水异养活性明显增强。悬浮颗粒氮(Pn)的δ15N与之平行变化，表明生物化学循环过程(Pn再矿化与硝化耦合，NO3-−-N同化回到Pn)在内河氮素转化中起主导作用。在两条河流的咸水区，水位呈下降趋势，而水位急剧上升。NO3−/NO2−的氧化还原循环和/或大气中氮氧化物的沉积可能是河口的主要控制因素，然而，目前观察到的趋势背后的确切机制尚不清楚。
The longitudinal variations in the nitrogen (δ15N) and oxygen (δ18O) isotopic compositions of nitrate (NO3−), the carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) and the δ13C and δ15N of particulate organic matter were determined in two Southeast Asian rivers contrasting in the watershed geology and land use to understand internal nitrogen cycling processes. Thebecame higher longitudinally in the freshwater reach of both rivers. Thealso increased longitudinally in the river with a relatively steeper longitudinal gradient and a less cultivated watershed, while thegradually decreased in the other river. A simple model for theand thethat accounts for simultaneous input and removal of NO3−suggested that the dynamics of NO3−in the former river were controlled by the internal production by nitrification and the removal by denitrification, whereas that in the latter river was significantly affected by the anthropogenic NO3−loading in addition to the denitrification and/or assimilation. In the freshwater-brackish transition zone, heterotrophic activities in the river water were apparently elevated as indicated by minimal dissolved oxygen, minimal δ13CDICand maximalpCO2. The δ15N of suspended particulate nitrogen (PN) varied in parallel to thethere, suggesting that the biochemical recycling processes (remineralization of PN coupled to nitrification, and assimilation of NO3−-N back to PN) played dominant roles in the instream nitrogen transformation. In the brackish zone of both rivers, thedisplayed a declining trend while theincreased sharply. The redox cycling of NO3−/NO2−and/or deposition of atmospheric nitrogen oxides may have been the major controlling factor for the estuarineand, however, the exact mechanism behind the observed trends is currently unresolved.