Clumped isotope signatures of nitrous oxide formed by bacterial denitrification,Geochimica et Cosmochimica Acta

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Clumped isotope signatures of nitrous oxide formed by bacterial denitrification
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2022-05-13 , DOI: 10.1016/j.gca.2022.05.006
Kristýna Kantnerová , Shohei Hattori , Sakae Toyoda , Naohiro Yoshida , Lukas Emmenegger , Stefano M. Bernasconi , Joachim Mohn

Multiply substituted isotopic species of nitrous oxide (N₂O), referred to as clumped isotopes, represent a promising new tool for distinguishing production pathways of this potent greenhouse gas. This work presents the first determination of enrichment factors of N₂O clumped isotopes during bacterial denitrification. Samples of N₂O obtained after 1-, 3-, and 7-day incubations of a pure culture of the denitrifier Pseudomonas aureofaciens at 20 °C and 30 °C were analysed by the recently developed quantum cascade laser absorption spectroscopy (QCLAS) method. Enrichment factors ε_p/s of the cumulative product (p) relative to the substrate (s) were determined using a Rayleigh model for the seven most abundant isotopically substituted molecules (isotopocules) of N₂O. Values of the enrichment factors ε_p/s (with uncertainty expressed as expanded standard uncertainty at the 95% confidence interval) at the two incubation temperatures (20 °C/30 °C) are:

¹⁴N¹⁵N¹⁶O (456): ε₄₅₆ = (−40.3 ± 2.6)‰/(−35.1 ± 0.7)‰

¹⁵N¹⁴N¹⁶O (546): ε₅₄₆ = (−38.1 ± 3.4)‰/(−31.2 ± 0.6)‰

¹⁴N¹⁴N¹⁷O (447): ε₄₄₇ = (21.3 ± 1.2)‰/(24.5 ± 0.5)‰

¹⁴N¹⁴N¹⁸O (448): ε₄₄₈ = (38.8 ± 1.5)‰/(46.4 ± 1.2)‰

¹⁴N¹⁵N¹⁸O (458): ε₄₅₈ = (−8.9 ± 2.0)‰/(−11.7 ± 0.6)‰

¹⁵N¹⁴N¹⁸O (548): ε₅₄₈ = (−3.4 ± 1.1)‰/(−1.8 ± 0.5)‰

¹⁵N¹⁵N¹⁶O (556): ε₅₅₆ = (−85.9 ± 1.5)‰/(−63.9 ± 1.4)‰

Temporal evolutions of the abundances of singly substituted N₂O isotopocules during nitrate reduction agree with previously published experiments: there is normal isotope effect associated with the production of ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O; i.e., intermediates leading to ¹⁴N¹⁴N¹⁶O react faster than intermediates leading to ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O. However, the production of ¹⁴N¹⁴N¹⁷O and ¹⁴N¹⁴N¹⁸O is associated with inverse isotope effect; i.e., intermediates leading to ¹⁴N¹⁴N¹⁶O react slower than intermediates leading to ¹⁴N¹⁴N¹⁷O and ¹⁴N¹⁴N¹⁸O due to preferential cleavage of ¹⁶O during nitrate reduction to N₂O. Isotopic fractionation at the incubation temperature of 30 °C was significantly lower compared to 20 °C. We observed a large kinetic isotope effect of the ¹⁵N site preference (SP) and the ¹⁵N–¹⁸O site preference (SP¹⁸) at the onset of the reaction. SP¹⁸ was found to be closer to 0‰ than SP, which is thought to arise from similar rates of breakage of the ¹⁵N–O and ¹⁴N–O bonds in the reaction intermediates. The ¹⁵N–¹⁸O clumped isotope anomalies in two isotopic isomers (isotopomers) ¹⁴N¹⁵N¹⁸O and ¹⁵N¹⁴N¹⁸O (Δ^458+548_avg) follow a temporal trend similar to those of SP and SP¹⁸. The ¹⁵N–¹⁵N clumped isotope anomalies in ¹⁵N¹⁵N¹⁶O are greater than 0‰ and show no clear temporal trend or influence of incubation temperature, suggesting no strong combinatorial effects involved during the N–N bond formation. Overall, our data illustrate that clumped N₂O isotopes may be used as independent tracers for reaction mechanisms of N₂O conversion and may establish themselves as a worthwhile tool to study the biogeochemical cycle of N₂O.

中文翻译：

细菌反硝化形成的一氧化二氮的聚集同位素特征

一氧化二氮 (N ₂ O) 的多重取代同位素物种，称为丛集同位素，代表了一种有前途的新工具，可用于区分这种强效温室气体的生产途径。这项工作首次确定了细菌反硝化过程中 N _{2 O 聚集同位素的富集因子。}通过最近开发的量子级联激光吸收光谱 (QCLAS) 方法分析反硝化菌金黄色假单胞菌纯培养物在 20 °C 和 30 °C 培养 1、3 和 7 天后获得的 N ₂ O 样品. 富集因子ε _p/s相对于底物 (s) 的累积产物 (p) 使用 Rayleigh 模型确定了七种最丰富的 N ₂ O 同位素取代分子（同位素体）。富集因子ε _{p/s 的}值（不确定性表示为两个孵育温度 (20 °C/30 °C) 下 95% 置信区间的扩展标准不确定度为：

¹⁴ N ¹⁵ N ¹⁶ O (456): ε ₄₅₆ = (-40.3 ± 2.6)‰/(-35.1 ± 0.7)‰

¹⁵ N ¹⁴ N ¹⁶ O (546): ε ₅₄₆ = (-38.1 ± 3.4)‰/(-31.2 ± 0.6)‰

¹⁴ N ¹⁴ N ¹⁷ O (447): ε ₄₄₇ = (21.3 ± 1.2)‰/(24.5 ± 0.5)‰

¹⁴ N ¹⁴ N ¹⁸ O (448): ε ₄₄₈ = (38.8 ± 1.5)‰/(46.4 ± 1.2)‰

¹⁴ N ¹⁵ N ¹⁸ O (458): ε ₄₅₈ = (-8.9 ± 2.0)‰/(-11.7 ± 0.6)‰

¹⁵ N ¹⁴ N ¹⁸ O (548): ε ₅₄₈ = (-3.4 ± 1.1)‰/(-1.8 ± 0.5)‰

¹⁵ N ¹⁵ N ¹⁶ O (556): ε ₅₅₆ = (-85.9 ± 1.5)‰/(-63.9 ± 1.4)‰

_{硝酸盐还原过程中单一取代的 N 2} O 同位素丰度的时间演变与先前发表的实验一致：正常同位素效应与¹⁴ N ¹⁵ N ¹⁶ O 和¹⁵ N ¹⁴ N ¹⁶ O 的产生相关；即，生成¹⁴ N ¹⁴ N ¹⁶ O 的中间体比生成¹⁴ N ¹⁵ N ¹⁶ O 和¹⁵ N ¹⁴ N ¹⁶ O 的中间体反应更快。然而，生成¹⁴ N ¹⁴ N¹⁷ O 和¹⁴ N ¹⁴ N ¹⁸ O 与反同位素效应有关；即，产生¹⁴ N ¹⁴ N ¹⁶ O 的中间体比产生¹⁴ N ¹⁴ N ¹⁷ O 和¹⁴ N ¹⁴ N ¹⁸ O 的中间体反应慢，这是因为在硝酸盐还原成 N _{2 O 的过程中}¹⁶ O优先裂解。孵育时的同位素分馏与 20°C 相比，30°C 的温度显着降低。我们观察到¹⁵ N 位点偏好 (SP) 和¹⁵反应开始时的N– ¹⁸ O 位点偏好 (SP ¹⁸ )。发现SP ¹⁸比 SP 更接近 0‰，这被认为是由于反应中间体中¹⁵ N-O 和^{14 N-O 键的断裂率相似。}^{两种同位素异构体（同位素异构体） 14} N ¹⁵ N ¹⁸ O 和¹⁵ N ¹⁴ N ¹⁸ O（Δ ^458+548_avg ）中的¹⁵ N– ¹⁸ O 成簇同位素异常遵循类似于 SP 和 SP ¹⁸的时间趋势。¹⁵ N– ¹⁵ _¹⁵ N ¹⁵ N ¹⁶ O中的N团簇同位素异常大于0‰，并且没有显示出明显的时间趋势或孵化温度的影响，表明在N-N键形成过程中不涉及强烈的组合效应。总体而言，我们的数据表明，团簇 N ₂ O 同位素可用作 N ₂ O 转化反应机制的独立示踪剂，并可能成为研究 N ₂ O生物地球化学循环的有价值的工具。

更新日期：2022-05-13

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