Department of Civil, Environmental and Geodetic Engineering, Ohio State University, 470 Hitchcock Hall, 2070 Neil Avenue, Columbus, OH 43210, USA; School of Geosciences, University of Louisiana at Lafayette, 323 Hamilton Hall, 611 McKinely Street, Lafayette, LA 70504, USA.
Department of Microbiology, Ohio State University, 105 Biological Sciences Building, 484 W. 12 Ave., Columbus, OH 43210, USA.
Sci Total Environ. 2020 May 1;715:136920. doi: 10.1016/j.scitotenv.2020.136920. Epub 2020 Jan 27.
Greenhouse gas (GHG) emissions from rivers are a critical missing component of current global GHG models. Their exclusion is mainly due to a lack of in-situ measurements and a poor understanding of the spatiotemporal dynamics of GHG production and emissions, which prevents optimal model parametrization. We combined simultaneous observations of porewater concentrations along different beach positions and depths, and surface fluxes of methane and nitrous oxide at a plot scale in a large regulated river during three water stages: rising, falling, and low. Our goal was to gain insights into the interactions between hydrological exchanges and GHG emissions and elucidate possible hypotheses that could guide future research on the mechanisms of GHG production, consumption, and transport in the hyporheic zone (HZ). Results indicate that the site functioned as a net source of methane. Surface fluxes of methane during river water stages at three beach positions (shallow, intermediate and deep) correlated with porewater concentrations of methane. However, fluxes were significantly higher in the intermediate position during the low water stage, suggesting that low residence time increased methane emissions. Vertical profiles of methane peaked at different depths, indicating an influence of the magnitude and direction of the hyporheic mixing during the different river water stages on methane production and consumption. The site acted as either a sink or a source of nitrous oxide depending on the elevation of the water column. Nitrous oxide porewater concentrations peaked at the upper layers of the sediment throughout the different water stages. River hydrological stages significantly influenced porewater concentrations and fluxes of GHG, probably by influencing heterotrophic respiration (production and consumption processes) and transport to and from the HZ. Our results highlight the importance of including dynamic hydrological exchanges when studying and modeling GHG production and consumption in the HZ of large rivers.
温室气体（GHG）排放主要来自河流，而目前的全球 GHG 模型往往忽略了这一重要组成部分。其主要原因是缺乏原位测量数据，以及对 GHG 产生和排放的时空动态缺乏了解，这使得模型的最佳参数化难以实现。本研究在一个大型调节河流的三个水期（涨水期、退水期和枯水期），通过同时观测不同滩位和深度的孔隙水浓度以及甲烷和氧化亚氮的表面通量，来研究河滨带（HZ）水文交换与 GHG 排放之间的相互作用，并提出可能的假说来指导未来 GHG 产生、消耗和输移机制的研究。结果表明，该研究区域是甲烷的净源。在三个滩位（浅滩、中滩和深滩）的河水阶段，甲烷的表面通量与孔隙水中的甲烷浓度相关。然而，在低水位阶段，中滩的甲烷通量显著增加，这表明低停留时间会增加甲烷排放。在不同的河水阶段，甲烷的垂直分布在不同的深度达到峰值，这表明在不同的河水阶段，HZ 中地下水混合的幅度和方向对甲烷的产生和消耗有影响。该区域是硝酸盐的汇或源取决于水柱的高度。在整个不同的水阶段，硝酸盐的孔隙水浓度在沉积物的上层达到峰值。河流水文阶段显著影响 GHG 的孔隙水浓度和通量，这可能是通过影响异养呼吸（产生和消耗过程）以及向 HZ 和从 HZ 的输移来实现的。本研究结果强调了在研究和模拟大河 HZ 中 GHG 产生和消耗时，纳入动态水文交换的重要性。
