Isotopic Constraint on the Sources and Biogeochemical Cycling of Nitrate in the Jiulong River Estuary
Isotopic Constraint on the Sources and Biogeochemical Cycling of Nitrate in the Jiulong River Estuary
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DOI：
10.1029/2020jg005850
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发表时间：
2021-02
期刊：
影响因子：
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通讯作者：
Xin Wang;Xiaoyan Wu;Min Chen;Hua Cheng;Nengwang Chen;Weifeng Yang;Yihua Cai
中科院分区：
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作者：
Xin Wang;Xiaoyan Wu;Min Chen;Hua Cheng;Nengwang Chen;Weifeng Yang;Yihua Cai
为了更好地了解九龙江口硝酸盐的转化和来源贡献，于2014年对九龙江口硝酸盐浓度和同位素组成(δ15N-NO3−和δ18O-NO3−)进行了测定。与硝酸盐浓度通常遵循保守的二元混合模式相反，在所有采样航次中，δ15N-NO3−和δ18O-NO3−都显著偏离二元同位素混合线，表明硝酸盐在JRE中的动态转化。硝酸盐同位素特征与保守混合线的偏离以及氮、氧同位素回归的斜率(δ15N-NO3−与δ18O-NO3−)具有显著的季节变化，反映了硝化作用和沉积反硝化作用的季节变化。此外，旱季JRE下部硝酸盐浓度和同位素组成的升高，以及1.16的同位素回归斜率，表明当地有显著的硝酸盐输入。通过R模型中的稳定同位素分析，结合JRE中硝化和反硝化的硝酸盐同位素分馏因子，进行了硝态氮来源解析。模型结果表明，肥料是土壤硝态氮的最大来源，在整个采样期内，肥料对土壤硝态氮的贡献率为33%~55%。粪便是另一个主要来源，为JRE贡献了32%-39%的硝酸盐。在4月和6月期间，污水硝酸盐对JRE的贡献率为7%，但在雨季大幅上升至18%，这可能是由于郊区和农村未经处理的生活污水迅速流入河流所致。
Nitrate concentrations and isotopic compositions (δ15N‐NO3− and δ18O‐NO3−) in the Jiulong River Estuary (JRE) were determined in 2014 to better understand the transformation and source contributions of nitrate. In contrast to nitrate concentrations, which generally followed a conservative binary mixing pattern, both δ15N‐NO3− and δ18O‐NO3− deviated significantly from the binary isotopic mixing lines during all sampling cruises, indicating a dynamic transformation of nitrate in the JRE. The deviation of nitrate isotopic signatures from conservative mixing lines as well as the slope of the regression between N and O isotopes (δ15N‐NO3− vs. δ18O‐NO3−) had significant seasonal variability, showing the seasonal shifting of the dominant nitrification and sedimentary denitrification in the JRE. Furthermore, the elevated nitrate concentrations and isotopic compositions in the lower JRE during the dry season, along with an isotope regression slope of 1.16, indicated a significant local input of nitrate. Nitrate source apportionment is carried out through the Stable Isotope Analyses in R model with the incorporation of nitrate isotopic fractionation factors for nitrification and denitrification in the JRE. The model results revealed that fertilizer is the largest nitrate source to the JRE, contributing 33%–55% of the nitrate throughout the sampling period. Manure is another major source and contributed 32%–39% of the nitrate to the JRE. Sewage nitrate contributions to the JRE were 7% during April and June but dramatically increased to 18% during the wet season, probably as a result of the rapid flushing of suburban and rural untreated domestic sewage into the river.