The role of permeable sediments and subterranean estuaries as coastal nutrient filters is a question of key interest, particularly in areas with high nitrogen loadings. Here, we evaluated the effectiveness of a sandy subterranean estuary in cycling and removing nitrate using stable isotopes of N and O at natural (δ¹⁵N‐NO₃⁻ and δ¹⁸O‐NO₃⁻) and enriched levels (¹⁵N). Isotopes were used in conjunction with flow through reactors under anoxic conditions to quantify (1) the overall enrichment factor (¹⁵ε) of nitrate removal processes which was then applied to estimate the in situ percentage of nitrate removal within the subterranean estuary and (2) the potential rates of denitrification, dissimilatory nitrate reduction to ammonium, and anammox. We found that ¹⁵ε varied between −24 and −34‰ and were positively correlated with nitrate concentrations and the percentage of organic carbon added to the sediments. Using these ¹⁵ε values in a Rayleigh distillation model resulted in an estimated average of 34% ± 14% nitrate removal within the subterranean estuary, less than half of the percentage estimated using the nitrate‐salinity mixing model (66% ± 28%). Denitrification was the most dominant nitrate removal pathway within the subterranean estuary with potential rates among the highest denitrification rates reported for both permeable and cohesive sediments. The contribution of dissimilatory nitrate reduction to ammonium showed significant seasonal variation while the rates of anammox were consistent throughout the study. We suggest that the spatial shift of the subterranean estuary is the most likely explanation for the seasonal differences in the rates of denitrification and dissimilatory nitrate reduction to ammonium.

中文翻译：

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砂质地下河口中硝酸盐的生物地球化学衰减：两种稳定同位素方法的见解

渗透性沉积物和地下河口作为沿海营养物过滤器的作用是一个关键问题，特别是在氮含量高的地区。在这里，我们评估了沙地下河口的有效性在循环和在自然去除使用的N和O的稳定同位素硝酸盐（δ ¹⁵ N-NO ₃ ^-和δ ¹⁸ O型NO ₃ ^{- ）}和富集水平（¹⁵ N）。在缺氧条件下将同位素与流过反应器结合使用以量化（1）总体富集因子（¹⁵ε）硝酸盐去除过程，然后将其用于估算地下河口内硝酸盐去除的原位百分比，以及（2）潜在的反硝化速率，硝酸盐异化还原为铵和厌氧氨氧化的潜在速率。我们发现，¹⁵ ε-24和之间变化-34‰和与硝酸浓度和有机碳添加到沉积物百分数呈正相关。使用这¹⁵瑞利蒸馏模型中的ε值导致地下河口的硝酸盐去除平均值估计为34％±14％，不到使用硝酸盐-盐度混合模型估算的百分比的一半（66％±28％）。反硝化作用是地下河口中最主要的硝酸盐去除途径，在渗透性和粘性沉积物中，反硝化率最高。异化硝酸盐还原对铵的贡献表现出明显的季节性变化，而整个研究中的厌氧氨氧化速率保持一致。我们建议地下河口的空间移动是反硝化和硝化硝酸盐还原成铵的速率季节性差异的最可能解释。