Isotopic enrichment factors are key to using stable isotope signatures in biogeochemical studies. However, these are typically determined in laboratory experiments and their applicability to environmental conditions is difficult to test. Here, we analyzed nitrogen stable isotope changes associated with nitrification in a coastal basin using weekly time-series measurements of δ¹⁵N in particulate nitrogen, ammonium, nitrite, and nitrate. Two year-long time series were selected as contrasting natural experiments in the ammonium-rich, aphotic bottom water of Bedford Basin, Nova Scotia, Canada. In 2014, ammonia oxidation (AO) was associated with Thaumarchaeota and nitrite concentrations remained low (< 0.5 μmol kg⁻¹). In contrast, transient nitrite accumulation (~ 8 μmol kg⁻¹) and a more rapid δ¹⁵N_NH4 increase in the fall of 2017 were likely caused by ammonia-oxidizing bacteria, associated with higher AO rates and, possibly, stronger nitrogen-isotope enrichment (¹⁵ε_AO). Estimates of ¹⁵ε_AO (21.8 ± 2.2‰, 24.1 ± 1.1‰) were derived empirically using Rayleigh models applied to field data from restricted periods during which the bottom waters approximated a closed system and influence on ¹⁵ε_AO from other processes was demonstrably insignificant. Using a numerical reactive-transport model, we found that the best fit for the δ¹⁵N data was obtained with ¹⁵ε_AO values (18.9‰, 25.1‰) close to those determined by the Rayleigh models. The time series also revealed substantial (~ 7‰) ¹⁵N-enrichment of the particulate nitrogen due to light-independent assimilation of partially nitrified ammonium. Consistent with previous studies, these field-based nitrogen isotope fractionation experiments suggest that the range of ¹⁵ε_AO values relevant for marine systems may be narrower than determined in laboratory studies.

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基于海岸时间序列的硝化过程中氮同位素分馏表征

同位素富集因子是在生物地球化学研究中使用稳定同位素特征的关键。然而，这些通常是在实验室实验中确定的，它们对环境条件的适用性很难测试。在这里，我们使用颗粒状氮、铵、亚硝酸盐和硝酸盐中 δ ¹⁵ N 的每周时间序列测量值分析了与沿海盆地硝化作用相关的氮稳定同位素变化。在加拿大新斯科舍省贝德福德盆地富含铵的无光底水中选择了为期两年的时间序列作为对比自然实验。2014 年，氨氧化 (AO) 与Thaumarchaeota相关，亚硝酸盐浓度仍然很低 (< 0.5  μ mol kg ^-1）。相比之下，2017 年秋季短暂的亚硝酸盐积累（~ 8  μ mol kg ^-1）和更快速的 δ ¹⁵ N _NH4增加可能是由氨氧化细菌引起的，与更高的 AO 率和可能更强的氮-同位素富集 ( ¹⁵ ε _AO )。¹⁵ ε _{AO ( 21.8} ± 2.2‰, 24.1 ± 1.1‰) 的估计值是使用瑞利模型经验^得出的来自其他过程显然是微不足道的。使用数值反应传输模型，我们发现最适合 δ ¹⁵ N 数据的¹⁵ ε _AO值（18.9‰，25.1‰）接近由瑞利模型确定的值。时间序列还显示，由于部分硝化铵的光独立同化，颗粒氮的大量（~7‰）^{15 N 富集。}与以前的研究一致，这些基于现场的氮同位素分馏实验表明，与海洋系统相关的¹⁵ ε _AO值范围可能比实验室研究中确定的范围更窄。