| 摘要: |
| 近年来，河流氮污染一直是生物地球化学领域研究的热点问题。然而，识别水体硝酸盐来源、端元贡献比例及其在水体中存在生物转化（硝化、反硝化）过程，仍旧是氮循环研究的难点问题。本研究选取流经西安市的两条河流——浐河和灞河，测定其河水溶解态硝酸盐氮、氧同位素组成，并结合Bayesian同位素混合模型，有效识别了两条河流从源头到汇入渭河河口处，氮素来源的变化，同时，定量分析了其贡献比例的变化。结果显示，河流源头附近，土壤有机氮是河流硝酸盐主要来源，其贡献比例接近30%；河流中游，由于沿河农业活动的增加，同位素指示河流硝酸盐主要来源转化为化学肥料，其贡献比例接近25%；河流下游，由于城市用水的汇入，硝酸盐氮、氧同位素值偏正，主要位于污水及粪肥区间，指示硝酸盐含量较高的生活污水及工业废水的输入，其贡献比例能达到30%以上。通过本研究，研究者定性及半定量的区分和浐河、灞河氮素来源，为今后有效控制氮污染提供了理论基础。 |
| 关键词: 河水 硝酸盐 氮同位素 氧同位素 Bayesian |
| DOI：10.7515/JEE201601004 |
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| 基金项目:国家自然科学基金项目（41303011）；中国科学院重点部署项目（KZZD-EW-04-06）；中国科学院西部之光西部博士资助项目 |
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| Nitrate source proportional contributions in the Chanhe and Bahe rivers— Using its isotopic ratios in combination with a Bayesian isotope mixing mode |
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XING Meng, LIU Weiguo1,2
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. School of Human Settlement and Civil Engineering,
Xi’an Jiaotong University, Xi’an 710049, China
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| Abstract: |
| Background, aim, and scope In recent years, nitrogen pollution in rivers is a research hotspot in the field of biogeochemistry. However, the types and sources of pollution have historically been poorly understood in the water catchments of the Loess Plateau in China. This study had chosen two rivers, Chanhe and Bahe rivers, which flowed through the Xi’an city. By using the dual nitrate isotopic composition, the nitrate sources were well identified from its sources to the site where they entered the Weihe River. Materials and methods Waters from the two river catchments were sampled along the reaches from their sources to the site where they entered the Weihe River. 5 water samples from Chanhe River, and 7 water samples from Bahe River were collected during November 2011. The Cl-,SO42-, NO2--N, and NO3--N concentrations were measured using ion chromatography (Dionex ICS-1000), NH4+-N concentrations were determined by spectrophotometry using the Nessler method. Isotopic measurements of δ15N-NO3- and δ18O-NO3- were performed using the improved ion exchange method. In addition, the nitrate sources contribution proportions were quantified by using the Bayesian isotope mixing mode. Results The NO3--N concentrations of Chanhe and Bahe rivers ranged from 1.8 mg∙L-1 to 6.0 mg∙L-1 and 2.0 mg∙L-1 to 4.5 mg∙L-1, respectively. The study found that NO3--N was the primary nitrogen species in the rivers. The NO2--N and NH4+-N concentrations were lower, such that NO2--N could not be detected in most samples. The δ15N-NO3- values of Chanhe and Bahe rivers from upper to lower stream were from 1.4‰ to 7.8‰ and 2.9‰ to 8.3‰, respectively. Discussion The NO3- isotope results in the studied river water samples were mainly distributed in three sections: soil organic nitrogen, manure and sewage, and synthetic NO3- fertilizer source pool, indicating that these might be the sources of river NO3-. All of the samples had δ18O-NO3- value above theoretical nitrification values. This indicated that NO3- concentrations and isotopic compositions were less affected by nitrification in these rivers. In the present study, no positive interaction was found between δ15N-NO3- and δ18O-NO3- in spatial change of these rivers. This indicated that no significant denitrification was found to impact on NO3- distribution in the river waters. The isotopic results show that the δ15O-NO3- values in the headstream of the Chanhe and Bahe rivers are the lowest and attributed to the organic nitrogen from the natural soil. The contribution of soil organic nitrogen can reach approximately 30%. The nitrate isotopic compositions indicate that the nitrate sources change into NO3- and NH4+ fertilizer with the increasing agricultural activities in the middle reaches, and the proportions can reach approximately 25%. The highest δ15N-NO3- values in the lower reaches of the two rivers result mainly from industrial wastewater, sewage and manure in this area. The industrial wastewater, sewage and manure input can reach above 30%. Conclusions In this study, we observed the spatial variability of dissolved nitrogen and the isotopic composition of nitrate in water from two rivers. The result of the present study demonstrated that NO3--N was the dominant species of dissolved inorganic nitrogen in the rivers. By using the dual nitrate isotope, the study found that there was little nitrification or denitrification in the river waters, and the spatial variation of isotopic composition in rivers reflected the nitrogen sources change along the rivers. Further, the nitrogen sources change mainly was controlled by the land use types around the rivers. The contributions of the NO3- sources were quantified and estimated using the SIAR model given isotopic data in the two rivers and NO3- sources. The results showed that source contributions were manure and sewage > soil organic nitrogen > synthetic fertilizer > atmospheric deposition. The results suggest that the more anthropogenic impacted river water had higher nitrate concentration and enriched dual isotopes imprinting. Recommendations and perspectives This study quantitative and semi-qualitative proved the nitrogen source in the Chanhe and Bahe rivers, and better agricultural management practices and sewage disposal programs can be implemented to protect water quality in this watershed. |
| Key words: river water nitrate nitrogen isotope oxygen isotope Bayesian |
