汉江上游金水河悬浮物及水体碳氮稳定同位素组成特征

Stable isotope composition of particulate organic matters and dissolved nitrate in the Jinshui River,Upper Han River Basin

金水河位于南水北调中线工程水源地的汉江上游,研究其污染物来源及分布规律对水源地水资源保护尤为重要。研究了不同水文季节金水河中悬浮颗粒物C和N稳定同位素值、水体硝酸盐与铵盐含量及其N稳定同位素特征。结果表明:金水河流域可溶性氮素与悬浮颗粒物的来源具有明显的空间性和季节性差异,并且流域内叶绿素浓度、水体浊度、悬浮物浓度都会对河流碳氮素稳定同位素值造成影响,主要体现在环境因子的变化制约着水体中硝化和反硝化生物对氮素的可利用性。结果显示:1)水体中悬浮颗粒物的碳稳定同位素为-8.03‰-14.57‰,平均值为2.59‰;氮稳定同位素范围为-7.50‰-7.34‰,平均值为:4.33‰,表明悬浮颗粒物的来源主要为外源性土壤有机质与内源性水生植物残体的混合;2)河流水体中铵盐与硝酸盐N-稳定同位素范围分别为-5.86‰-17.20‰,平均值为5.02‰及-1.48‰-15.86‰,平均值为5.75‰;水体可溶性氮素主要来源为大气沉降、河流水生生物以及地表径流所带入的化肥农药等;3)悬浮颗粒物含量不仅随着河流径流量的季节性变化而变化,还随着人为干扰强度的加强而呈递增的趋势,水体悬浮颗粒物含量最高达到(9.883±3.45)mg/L。而NH_4^+及NO_3^-的浓度也呈现出相同的趋势,含量分别为0.07-0.45 mg/L,平均值为0.25 mg/L;0.08-0.44 mg/L,平均值为0.37 mg/L。稳定同位素测定为河流生态系统提供了一个整合时空氮素来源和转移循环过程的综合指标,揭示了环境因子对河流生态系统氮循环的影响过程与机制。

Nitrogen and carbon are important elements regulating the structure and function of river ecosystems; therefore, determining their sources has important implications for integrated river basin management. Since physical processes and human activities can lead to different isotopic signatures in organic matter, the characteristics of the stable isotopes of carbon and nitrogen could be used to determine carbon and nitrogen sources in riverine systems. The upper Han River is the water source area for the Middle Route of the South-to-North Water Diversion Project （MR-SNWTP）; hence, identifying pollutant origins in the riverine system is critically important for the interbasin water transfer project. In this study, we investigated the δ13C and δ15N values of particulate organic matter （POM） and δ15N values of nitrogen including nitrate-nitrogen （ N-NO3- ） and ammonium nitrogen （ N-NH4＋ ） to trace the sources of organic matter in the river ecosystem. The. study site was the Jinshui River with a length of 87 km and drainage area of 730 km2 in the upper Han River during 2012- 2013. Furthermore, we measured chlorophyll content, turbidity, and the concentrations of N-NH4 and N-NO3 in the river system using YSI 6920. The results demonstrated spatial variability in POM, with higher content in the moderately disturbed zone and intensely disturbed zone than in the slightly disturbed zone. The concentration of ammonium showed seasonality with minimal concentrations （0.07 mg/L） in the spring and maximum concentrations （0.45 mg/L） in winter, whereas the maximum concentration of nitrate was 0.44 mg/L in winter and the minimal value was 0.08 mg/L in summer. The carbon isotope value of POM also showed an obvious seasonality, in decreasing order of spring, summer, winter, and fall. The turbidity reached its maximum in summer from the heavily rainfall in the same season due to the monsoon climate. The Chl/ turbidity ratio could be used to identify the contribution of algae to the total particulate organic materials in the river. Algae, which have a higher carbon isotope value （-21.18‰-27.14‰） than terrestrial organic matter （-27.34‰-28.83‰）, were the main source of POM. POM with an N isotope range of -8.03‰-14.5‰ primarily originated from soil organic matter and plankton. The dissolved nitrate with isotope ranges of -5.86‰ --17.20‰ for ammonium and -1.48‰-15.86‰ for nitrogen in the riverine system primarily originated from precipitation, plankton, and fertilizer. Also, larger values for δ15 N-NH4＋ and δ15N-NO3- were observed in the growing season （i.e., spring/summer）, which was possibly due to the seasonally higher chlorophyll concentration （i.e., high algae density in the river）. Additionally, the low nitrate stable isotope value and low concentration of ammonium and nitrate in summer were possibly due to nitrification and alga uptake in the riverine system. Moreover, microbial denitrification could increase the nitrate stable isotope value and decrease nitrate concentrations; our results showed that the maximum δ15N value appeared in winter due to the low temperatures. The research implies that N-loading from the catchment reflects the upland land use pattern along the riverine system, and the isotopic technique could be an effective approach for quantifying pollutant sources in river systems.