Coupling multiscale observations to evaluate hyporheic nitrate removal at the reach scale
Coupling multiscale observations to evaluate hyporheic nitrate removal at the reach scale
复制标题
耦合多尺度观测以评估可达范围内的次流硝酸盐去除
作者：
J. Zarnetske;R. Haggerty;S. Wondzell
河流中过量的NO3 -是内陆和沿海生态系统中一个日益严重和持续存在的问题，而反硝化是NO3 -的主要去除过程。低渗带可能具有很高的反硝化潜力，但它们在河段和网络尺度NO3 -去除中的作用尚不清楚，因为很难估计。我们使用独立和互补的反硝化和总NO3 -吸收的多尺度测量来量化美国俄勒冈州西部三级农业河流303米河段中低渗NO3 -去除的作用。通过稳态15N-NO3 -示踪剂添加实验和溶质输运模拟表征了达尺度NO3 -动力学，并通过原位生物地球化学和地下水模拟测量了亚循环条件。我们还开发了一种方法来连接这些独立的多尺度测量。潜流NO3 -去除率（速率系数λHZ = 0.007/h）占15N实验中观察到的总河段NO3 -吸收量的17%和河段反硝化的32%。在河段尺度上，抑制潜流反硝化作用的主要因素是易溶有机碳的有效性和人为河道化造成的潜流带大小的限制（沉积物厚度≤1.5 m）。连接多尺度方法使估计低循环对流NO3 -动力学的影响成为可能。然而，这也表明传统的河尺度示踪剂实验设计和随后的输运模型不能单独用于直接研究潜流带对河尺度水和溶质动力学的作用。
Excess NO3– in streams is a growing and persistent problem for both inland and coastal ecosystems, and denitrification is the primary removal process for NO3–. Hyporheic zones can have high denitrification potentials, but their role in reach- and network-scale NO3– removal is unknown because it is difficult to estimate. We used independent and complementary multiscale measurements of denitrification and total NO3– uptake to quantify the role of hyporheic NO3– removal in a 303-m reach of a 3rd-order agricultural stream in western Oregon, USA. We characterized the reach-scale NO3– dynamics with steady-state 15N-NO3– tracer-addition experiments and solute-transport modeling, and measured the hyporheic conditions via in-situ biogeochemical and groundwater modeling. We also developed a method to link these independent multiscale measurements. Hyporheic NO3– removal (rate coefficient λHZ = 0.007/h) accounted for 17% of the observed total reach NO3– uptake and 32% of the reach denitrification estimated from the 15N experiments. The primary limitations on hyporheic denitrification at the reach scale were availability of labile dissolved organic C and the restricted size of the hyporheic zone caused by anthropogenic channelization (sediment thickness ≤1.5 m). Linking multiscale methods made estimates possible for hyporheic influence on stream NO3– dynamics. However, it also demonstrated that the traditional reach-scale tracer experimental designs and subsequent transport modeling cannot be used alone to directly investigate the role of the hyporheic zone on reach-scale water and solute dynamics.