Abstract Streams and rivers emit significant amounts of CO 2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. However, the estimation of CO 2 degassing based on the water-air CO 2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. Furthermore, the stable isotope signature of dissolved inorganic carbon (δ 13 C-DIC) in streams is strongly impacted by gas exchange, which makes it a useful tracer of CO 2 degassing under specific conditions. For this study, we characterized the annual transfers of dissolved inorganic carbon (DIC) along the groundwater-stream-river continuum based on DIC concentrations, stable isotope composition and measurements of stream discharges. We selected a homogeneous, forested and sandy lowland watershed as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). We observed the first general spatial pattern of decreases in pCO 2 and DIC and an increase in δ 13 C-DIC from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater 12 C-DIC compared to 13 C-DIC. This downstream enrichment in 13 C-DIC could be modelled by simply considering the isotopic equilibration of groundwater-derived DIC with the atmosphere during CO 2 degassing. A second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the DIC accompanied by the enrichment of 13 C in the stream DIC, which was due to the occurrence of carbonate rock weathering downstream. We could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. Thereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. After combining with the dissolved CO 2 concentrations, we quantified CO 2 degassing for each stream order for the whole watershed. Approximately 75% of the total CO 2 degassing from the watershed occurred in first- and second-order streams. Furthermore, from stream order 2–4, our CO 2 degassing fluxes compared well with those based on stream hydraulic geometry, water pCO 2 , gas transfer velocity, and stream surface area. In first-order streams, however, our approach showed CO 2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.

中文翻译：

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地下水-河流-大气界面处的二氧化碳脱气：沙质流域中的同位素平衡和水文质量平衡

摘要 溪流和河流会排放大量的 CO 2 并构成碳从陆地生态系统到大气的优先传输途径。然而，基于水-空气CO 2 梯度、气体传输速度和流表面积的CO 2 脱气估计存在很大的不确定性。此外，气流中溶解的无机碳 (δ 13 C-DIC) 的稳定同位素特征受到气体交换的强烈影响，这使其成为特定条件下 CO 2 脱气的有用示踪剂。在本研究中，我们根据 DIC 浓度、稳定同位素组成和河流排放测量，表征了沿地下水-河流-河流连续体的溶解无机碳 (DIC) 的年度转移。我们选择了一个均质的、森林覆盖的沙质低地流域作为研究地点，水文几乎完全通过浅层地下水的排水（无地表径流）发生。我们观察到 pCO 2 和 DIC 减少以及 δ 13 C-DIC 从地下水到流阶 1 和 2 的第一个总体空间模式，这是由于与 13 相比，经过实验验证，地下水 12 C-DIC 脱气速度更快。 C-DIC。可以通过简单地考虑在 CO 2 脱气过程中地下水衍生的 DIC 与大气的同位素平衡来模拟 13 C-DIC 中的这种下游富集。第二种空间模式发生在 2 级和 4 级河流之间，包括碳酸盐碱度与 DIC 的比例增加，伴随着河流 DIC 中 13 C 的富集，这是由于下游碳酸盐岩风化的发生。我们可以将这两个过程（气体交换和碳酸盐风化）对河网稳定同位素收支的贡献分开。此后，我们根据排水量和地下水在递增顺序中的相对贡献建立了水文质量平衡。在与溶解的 CO 2 浓度结合后，我们量化了整个流域的每个河流顺序的 CO 2 脱气。大约 75% 的从流域脱气的 CO 2 发生在一级和二级流中。此外，从流阶 2-4 开始，我们的 CO 2 脱气通量与基于流水力几何学、水 pCO 2 、气体传输速度和流表面积的那些相比较。然而，在一级流中，我们的方法显示出两倍大的 CO 2 通量，