Sample preservation methods for nitrous oxide concentration and isotope ratio measurements in aquatic environments

IF 2.1 3区 地球科学 Q2 LIMNOLOGY
Claudia Frey, Weiyi Tang, Bess B. Ward, Moritz F. Lehmann
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Abstract

The nitrogen (N) and oxygen (O) stable isotope analysis of dissolved nitrous oxide (N2O) can provide important constraints on the sources and cycling of N2O in aquatic environments. The isotopic composition of aqueous N2O, both in field (natural abundance) or experimental (15N-labeling) samples, however, may be altered by abiotic reactions involving nitrite ( NO 2 ) or hydroxylamine (NH2OH) and microbial activity during sample storage, if samples are not adequately preserved. Here we tested five different preservatives, mercuric chloride (HgCl2), copper sulfate (CuSO4), zinc chloride (ZnCl2), hydrochloric acid (HCl) mixed with sulfamic acid (SFA), and sodium hydroxide (NaOH), for fixing natural water samples from an estuary and a lake with different NO 2 concentrations over a range of different storage times for N2O analyses. ZnCl2 and CuSO4 decreased the pH, and led to abiotic N2O production from 15 N O 2 , shifting the N2O isotopic composition significantly. Removal of NO 2 with a mixture of SFA and HCl did not always prevent the alteration of the original N and O isotope composition of N2O, confirming the requirement for complete NO 2 removal, and underscoring the biasing effects of NO 2 at very low pH, even at trace levels. At low NO 2 background, HgCl2 preservation led to robust and accurate results. However, in light of its toxicity and bioaccumulation potential in food webs, this fixative should be avoided. The addition of NaOH reduced abiotic side reactions involving NO 2 , and yielded the most reliable and reproducible results for N2O concentration and isotope ratio measurements across tested settings and storage times.

Abstract Image

水生环境中氧化亚氮浓度和同位素比测量的样品保存方法
对溶解的一氧化二氮(N2O)进行氮(N)和氧(O)稳定同位素分析,可以对水生环境中一氧化二氮的来源和循环提供重要的制约因素。然而,无论是野外样本(自然丰度)还是实验样本(15N 标记),如果样本保存不当,其同位素组成可能会因涉及亚硝酸盐()或羟胺(NH2OH)的非生物反应以及样本储存过程中的微生物活动而发生改变。在此,我们测试了五种不同的防腐剂,即氯化汞(HgCl2)、硫酸铜(CuSO4)、氯化锌(ZnCl2)、盐酸(HCl)与氨基磺酸(SFA)混合液以及氢氧化钠(NaOH),用于固定来自河口和湖泊的不同浓度的天然水样,并在一系列不同的储存时间内进行氧化亚氮分析。ZnCl2 和 CuSO4 会降低 pH 值,并导致 N2O 的非生物生成,从而显著改变 N2O 的同位素组成。用 SFA 和 HCl 的混合物去除 N2O 并不总能阻止 N2O 原始 N 和 O 同位素组成的改变,这证实了完全去除的要求,并强调了在极低的 pH 值下,即使是痕量水平,也会产生偏差效应。在低本底条件下,氯化汞的保存会带来可靠而准确的结果。不过,鉴于氯化汞的毒性和在食物网中的生物累积潜力,应避免使用这种固定剂。加入 NaOH 可减少涉及Ⅴ的非生物副反应,在不同的测试环境和储存时间下,N2O 浓度和同位素比值测量的结果最可靠,可重复性最高。
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来源期刊
CiteScore
4.80
自引率
3.70%
发文量
56
审稿时长
3 months
期刊介绍: Limnology and Oceanography: Methods (ISSN 1541-5856) is a companion to ASLO''s top-rated journal Limnology and Oceanography, and articles are held to the same high standards. In order to provide the most rapid publication consistent with high standards, Limnology and Oceanography: Methods appears in electronic format only, and the entire submission and review system is online. Articles are posted as soon as they are accepted and formatted for publication. Limnology and Oceanography: Methods will consider manuscripts whose primary focus is methodological, and that deal with problems in the aquatic sciences. Manuscripts may present new measurement equipment, techniques for analyzing observations or samples, methods for understanding and interpreting information, analyses of metadata to examine the effectiveness of approaches, invited and contributed reviews and syntheses, and techniques for communicating and teaching in the aquatic sciences.
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