The sediment–water interfaces (SWI) of streams serve as important biogeochemical hotspots in watersheds and contribute to whole-catchment reactive nitrogen budgets and water-quality conditions. Recently, the SWI has been identified as an important source of nitrous oxide (N 2 O) produced in streams, with SWI residence time among the principal controls on its production. Here, we conducted a series of controlled manipulations of SWI exchange in an urban stream that has high dissolved N 2 O concentrations and where we concurrently evaluated less-mobile porosity dynamics. Our experiments took place within isolated portions of two sediment types: a coarse sandy stream bed resulting from excess road-sand application in the watershed, and a coarse sand mixed with clay and organic particles. In these manipulation experiments we systematically varied SWI vertical-flux rates and residence times to evaluate their effect on the fate of nitrate and production rates of N 2 O. Our experiments demonstrate that the fate and transport of nitrate and N 2 O production are influenced by hydrologic flux rates through SWI sediments and associated residence times. Specifically, we show that manipulations of hydrologic flux systematically shifted the depth of the bulk oxic–anoxic interface in the sediments, and that nitrate removal increased with residence time. Our results also support the emerging hypothesis of a ‘Goldilocks’ timescale for the production of nitrous oxide, when transport and reaction timescales favor incomplete denitrification. Areal N 2 O production rates were up to threefold higher during an intermediate residence-time experiment, compared to shorter or longer residence times. In our companion study we documented that the studied sediments were dominated by a long-residence-time less-mobile porosity domain, which could explain why we observed N 2 O production even in bulk-oxic sediments. Overall, we have experimentally demonstrated that changes to SWI hydrologic residence times and SWI substrate associated with urbanization can change the biogeochemical function of the river corridor.

中文翻译：

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沙质城市河流沉积物-水界面中水文停留时间的实验变化改变硝酸盐去除和一氧化二氮通量

河流的沉积物-水界面 (SWI) 是流域中重要的生物地球化学热点，有助于整个流域的活性氮收支和水质条件。最近，SWI 已被确定为溪流中产生的一氧化二氮 (N 2 O) 的重要来源，SWI 停留时间是其生产的主要控制因素之一。在这里，我们在具有高溶解 N 2 O 浓度的城市河流中对 SWI 交换进行了一系列受控操作，并且我们同时评估了流动性较差的孔隙度动力学。我们的实验发生在两种沉积类型的隔离部分：流域中过量使用道路沙子导致的粗沙河床，以及混合有粘土和有机颗粒的粗沙。在这些操作实验中，我们系统地改变了 SWI 垂直通量率和停留时间，以评估它们对硝酸盐的归宿和 N 2 O 生成率的影响。我们的实验表明，硝酸盐的归宿和运输以及 N 2 O 的生成受以下因素的影响：通过 SWI 沉积物的水文通量率和相关的停留时间。具体来说，我们表明水文通量的操纵系统地改变了沉积物中大量氧-缺氧界面的深度，并且硝酸盐去除率随着停留时间而增加。我们的研究结果还支持了新出现的关于一氧化二氮产生的“金发姑娘”时间尺度的假设，当运输和反应时间尺度有利于不完全反硝化时。在中间停留时间实验期间，N 2 O 的面积产率高达三倍，与较短或较长的停留时间相比。在我们的配套研究中，我们记录了所研究的沉积物主要是一个长驻留时间较不流动的孔隙度域，这可以解释为什么我们甚至在大量含氧沉积物中观察到 N 2 O 产生。总体而言，我们已经通过实验证明，与城市化相关的 SWI 水文停留时间和 SWI 基质的变化可以改变河流走廊的生物地球化学功能。