Coastal waterways can be significant sources of the potent greenhouse gas nitrous oxide (N₂O) due to nitrogen inputs and eutrophication. Here, we quantify groundwater derived N₂O inputs and atmospheric emissions within a modified urban embayment (Sydney Harbour, Australia). Overall, we found low N₂O saturation (91–171%) and air–water fluxes (−2.2 to 24.6 μmol m⁻² d⁻¹). Concentrations were highest in upstream brackish areas and a commercial/industrial subembayment. Dissolved inorganic nitrogen concentrations were low and inversely correlated to N₂O throughout the harbor. N₂O surface water dynamics were apparently driven by saline submarine groundwater discharge, as quantified by the radioisotope tracer radon-222. Groundwater discharge was highest within the embayments and mangrove-lined upper estuary. While groundwater was a net N₂O source to surface waters, two upstream sub-embayments featured groundwater N₂O concentrations lower than surface water, suggesting a sink driven by surface waters recirculating in intertidal sediments. Surface-water N₂O was undersaturated within one upstream embayment, likely due to N₂O consumption within sediments. Contrastingly, the downstream embayments featured higher groundwater N₂O and accounted for 45% ± 21% of the groundwater N₂O flux. Sydney Harbour was a net source of N₂O to the atmosphere (mean 0.6 ± 0.3 μmol m⁻² d⁻¹) with larger N₂O fluxes occurring from relatively small areas. N₂O emissions (expressed in CO₂ equilivents) were equivalent to 17% of CO₂ emission estimates from previous studies. The low N₂O emissions in Sydney Harbour contrast with other modified estuaries which often emit higher N₂O fluxes due to larger nitrogen inputs.

中文翻译：

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海底地下水排放驱动大都市河口中的一氧化二氮源/汇动态

由于氮的输入和富营养化，沿海水道可能是强大的温室气体一氧化二氮（N ₂ O）的重要来源。在这里，我们量化了经过改进的城市环境（澳大利亚悉尼港）中地下水衍生的N ₂ O投入量和大气排放量。总的来说，我们发现低氮₂ ö饱和度（91-171％）和空气-水通量（-2.2至24.6 μ摩尔米^-2  d ^-1）。上游咸淡地区和商业/工业子堤坝的浓度最高。溶解的无机氮浓度很低，并且与整个港口的N ₂ O成反比。N ₂O地表水动力学显然是由盐水盐下的地下水排放驱动的，如放射性同位素示踪剂radon-222所定量的。在隔离带和衬有红树林的河口内，地下水排放量最高。地下水是地表水的净N ₂ O来源，但两个上游子河床的地下水N ₂ O浓度低于地表水，表明潮汐沉积物中再循环的地表水驱动了水槽。在一个上游河道内，地表水N ₂ O饱和度低，这可能是由于沉积物中的N ₂ O消耗所致。相反，下游河床的地下水N ₂ O较高，占地下水N的45％±2​​1％₂ O通量。悉尼港是大气中N ₂ O的净来源（平均0.6± 0.3μmol m ^-2  d ^-1），其中N ₂ O的通量较大，而面积相对较小。N ₂ O排放量（以CO ₂平衡当量表示）相当于先前研究估计的CO ₂排放量的17％。悉尼港的N ₂ O排放量低，与其他改良河口形成对比，这些河口由于氮输入量较大而经常排放较高的N ₂ O通量。