營養鹽來源追蹤 – 水體硝酸鹽

硝酸是一種天然存在的離子,中心氮原子周圍有3個氧原子。硝酸鹽是由細菌固氮形成的,存在於土壤、廢水和地表水中。硝酸鹽也可以經由人為的商業規模生產,在醃製肉類時當做防腐劑使用,由於其高溶解性和生物降解性,還可以做為肥料中其他營養素的載體。

BETA實驗室硝酸鹽測試服務

ISO 17025認證的BETA實驗室提供硝酸鹽濃度測試和穩定同位素測試,可用於研究環境中硝酸鹽的來源、跟隨環境的變化與硝酸鹽演變等課題。

硝酸鹽樣品首先被還原製備成一氧化二氮,然後連續流動至穩定同位素比質譜儀(IRMS)進行測試 (Casciotti et al., 2002; Foreman et al., 2016; and Altabet and Wassenaar, 2017)。同位素比值資料報告為δ值,單位為千分比 (‰) (Coplen, 2011)。

氮同位素比值的參考標準是空氣中的氮氣 (Mariotti, 1983) ,氧同位素比值得參考標準是VSMOW標準水樣,以 δ18OSLAP = -55.5‰ 表示 (Coplen, 1994; IAEA, 2017)。

測試結果將標示在直角座標圖中,不同的同位素值組成(δ18O與δ15N)代表不同硝酸鹽來源(Kendall et al, 2007; Hastings et al., 2013)。

nitrogen isotopes plot

來源

硝酸離子含有不同的氧和氮同位素,最常見的是氧-16和氮-14,罕見的是氧-18和氮-15。因為硝酸鹽可以由天然形成或人工生產,所以形成質量不同的硝酸離子

這種差異以δ(delta)值表示,並與國際公認的標準進行比較。

例如:

變化

自然界中的硝酸離子可能會發生變化,例如透過反硝化作用,即細菌利用 NO3 的過程。在反硝化過程中,微生物優先利用含氮-14和含氧-16的硝酸離子,留下富含氧-18和氮-15同位素的剩餘硝酸鹽。而硝化作用則正好相反。

影響因素

來源、氧的利用率、酸鹼度和土地利用率都與硝酸鹽的同位素值有關,在分析同位素資料時這些因素都應考慮在內。硝酸鹽穩定同位素測試是瞭解硝酸鹽來源和其離子所經歷變化過程的有用工具。

Beta實驗室使用同位素比質譜儀(IRMS)進行快速、ISO 17025認證的氧與氮同位素測量。14個工作天內完成測試並出具報告.

為什麼過量的硝酸鹽是一個問題?

在過去一個世紀裡,天然和人造硝酸鹽肥料的過度使用導致表層土壤中含有大量的硝酸鹽,然後又滲透進入地下水中。飲用水中高濃度的硝酸鹽可能有毒(10毫克/升或更高,EPA,2018年),造成高鐵血紅蛋白血症從而使血液缺氧。

此外,地表水中高濃度的硝酸鹽會導致藻類的過度生長。水體中的藻華阻擋陽光,會在水體中形成缺氧區(極低的溶解氧濃度),不利於海洋生物生存的環境。水體優氧化會對海洋和漁業產生巨大的負面影響。


參考文獻:

Altabet, M.A. and L. I. Wassenaar. New Chemical methods for the precise determination of nitrate isotopic composition Presented at Chemical Oceanography at the 86th annual Gordon Research Conference; 2017 July, 23-28; Colby-Sawyer College, New London, NH.

Aravena, R., Evans, M. L., & Cherry, J. A. (1993). Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems. Groundwater, 31(2), 180-186.

Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., & Hilkert, A. (2002). Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Anal. Chem, 74(19), 4905-4912.

Coplen, T. B., 1994. Reporting of Stable Hydrogen, Carbon, and Oxygen Isotopic Abundances, Pure and Applied Chemistry, v. 66, p. 273-276.

Coplen, T. B., 2011, Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results. Rapid Communications in Mass Spectrometry, v. 25, 2538–2560. 

Florida Department of Health Fact Sheet – Chemicals in Private Drinking Water Wells

Foreman, R. K., Segura-Noguera, M., & Karl, D. M. (2016). Validation of Ti (III) as a reducing agent in the chemiluminescent determination of nitrate and nitrite in seawater. Marine Chemistry, 186, 83- 89.

Hastings, et al., 2013 “Stable Isotopes as Tracers of Anthropogenic Nitrogen Sources, Deposition, and Impacts”, Elements, 9(5), 339-344.

Hosono, T., Tokunaga, T., Kagabu, M., Nakata, H., Orishikida, T., Lin, I. T., & Shimada, J. (2013). The use of δ15N and δ18O tracers with an understanding of groundwater flow dynamics for evaluating the origins and attenuation mechanisms of nitrate pollution. Water Research, 47(8), 2661-2675.

International Atomic Energy Agency (IAEA), Reference Sheet for International Measurement Standards, 2017

Isotope Tracers in Catchment Hydrology (1998), C. Kendall and J. J. McDonnell (Eds.). Elsevier Science B.V., Amsterdam. pp. 519-576.

Kendall, C., Elliott, E.M., and Wankel, S.D. (2007), “Tracing anthropogenic inputs of nitrogen to ecosystems.” In Stable Isotopes in Ecology and Environmental Science, 2nd ed., Ch. 12.

Li, S. L., Liu, C. Q., Li, J., Xue, Z., Guan, J., Lang, Y., … & Li, L. (2013). Evaluation of nitrate source in surface water of southwestern China based on stable isotopes. Environmental Earth Sciences, 68(1), 219-228.

Mariotti, A., 1983, Atmospheric nitrogen is a reliable standard for natural 15N abundance measurements: Nature, v. 303, p. 685-687.

US EPA Fact Sheet – Preventing Eutrophication: Scientific Support for Dual Nutrient Criteria

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