Abstract Denitrification is a major biological source or sink of N2O, an important greenhouse gas, which is a multi-step respiratory process that converts nitrate (NO3−) to gaseous forms of nitrogen (N2 or N2O). In aquatic ecosystems, the residence time and flow length within the streambed can modulate denitrification, which is highly affected by micro-topography of streambed. For the reason, various restoration projects targeting streams and rivers incorporate the installment of dune-shaped streambed, but its role in ecosystem functions and key controlling variables are still elusive. In this study, N2O fluxes according to streambed micro-topography were measured at four locations in two streams with dune-shaped streambeds in Korea. We found that micro-topographical variations influenced N2O fluxes, chemical properties, and denitrifier abundances, but that this effect was highly constrained by carbon availability. In sites with relatively higher DOC, N2O fluxes increased along the flow direction, and the highest flux of each site (19.1 ± 5.67 μg N2O-N m−2 h−1 and 16.2 ± 0.896 μg N2O-N m−2 h−1, respectively) was observed in the slope on the back side of the dune, followed by decreases afterward. In contrast, streams with low DOC concentration did not exhibit micro-topographical variations. This study suggests that the presence of small dunes in streams can substantially enhance N2O fluxes when carbon availability was appropriate. The variations of N2O flux highlight the importance of micro-topography and DOC concentration of sediment for the accurate estimation of uncertainty of N2O flux. This has further implication for stream restoration where installment or management of dune-shaped streambed is of importance in the removal of inorganic N in water.

中文翻译：

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微地形对温带河流沉积物中N2O排放的影响

摘要 反硝化作用是重要的温室气体 N2O 的主要生物源或汇，N2O 是将硝酸盐 (NO3−) 转化为气态氮 (N2 或 N2O) 的多步骤呼吸过程。在水生生态系统中，河床内的停留时间和流长可以调节反硝化作用，反硝化作用受河床微地形的影响很大。因此，各种针对溪流和河流的修复工程都包括设置沙丘状河床，但其在生态系统功能和关键控制变量中的作用仍然难以捉摸。在这项研究中，在韩国具有沙丘形河床的两条河流中的四个位置测量了根据河床微地形的 N2O 通量。我们发现微地形变化影响 N2O 通量、化学性质和反硝化剂丰度，但这种影响受到碳供应量的高度限制。在 DOC 相对较高的站点，N2O 通量沿流动方向增加，每个站点的最高通量（19.1 ± 5.67 μg N2O-N m-2 h-1 和 16.2 ± 0.896 μg N2O-N m-2 h-1 , 分别) 在沙丘背面的斜坡中观察到，然后下降。相比之下，具有低 DOC 浓度的流没有表现出微地形变化。这项研究表明，当碳供应合适时，溪流中小沙丘的存在可以显着提高 N2O 通量。N2O 通量的变化突出了沉积物的微地形和 DOC 浓度对于准确估计 N2O 通量不确定性的重要性。