Liu Ting-Ting, Wang Xiao-Feng, Yuan Xing-Zhong, Gong Xiao-Jie, Hou Chun-Li
Chongqing Key Laboratory of Wetland Science Research of the Upper Yangtze River, Chongqing 401331, China.
College of Geography and Tourism, Chongqing Normal University, Chongqing 401331, China.
Huan Jing Ke Xue. 2019 Jun 8;40(6):2827-2839. doi: 10.13227/j.hjkx.201810213.
Rivers play an important role in greenhouse gas emissions. Over the past decade, because of global urbanization trends, rapid land use changes have led to changes in river ecosystems that have had a stimulating effect on the greenhouse gas production and emissions. Presently, there is an urgent need for assessments of the greenhouse gas concentrations and emissions in watersheds. Therefore, this study was designed to evaluate river-based greenhouse gas emissions and their spatial-temporal features as well as possible impact factors in a rapidly urbanizing area. The specific objectives were to investigate how river greenhouse gas concentrations and emission fluxes are responding to urbanization in the Liangtan River, which is not only the largest sub-basin but also the most polluted one in Chongqing City. The thin layer diffusion model method was used to monitor year-round concentrations of CO, CH, and NO in September and December 2014, and March and June 2015. The CO range was (23.38±34.89)-(1395.33±55.45) Pa, and the concentration ranges of CH and NO were (65.09±28.09)-(6021.36±94.36) nmol·L and (29.47±5.16)-(510.28±18.34) nmol·L, respectively. The emission fluxes of CO, CH, and NO, which were calculated based on the method of wind speed model estimations, were -6.1-786.9, 0.31-27.62, and 0.06-1.08 mmol·(m·d), respectively. Moreover, the CO and CH emissions displayed significant spatial differences, and these were roughly consistent with the pollution load gradient. The greenhouse gas concentrations and fluxes of trunk streams increased and then decreased from upstream to downstream, and the highest value was detected at the middle reaches where the urbanization rate is higher than in other areas and the river is seriously polluted. As for branches, the greenhouse gas concentrations and fluxes increased significantly from the upstream agricultural areas to the downstream urban areas. The CO fluxes followed a seasonal pattern, with the highest CO emission values observed in autumn, then successively winter, summer, and spring. The CH fluxes were the highest in spring and the lowest in summer, while NO flux seasonal patterns were not significant. Because of the high carbon and nitrogen loads in the basin, the CO products and emissions were not restricted by biogenic elements, but levels were found to be related to important biological metabolic factors such as the water temperature, pH, DO, and chlorophyll a. The carbon, nitrogen, and phosphorus content of the water combined with sewage input influenced the CH products and emissions. Meanwhile, NO production and emissions were mainly found to be driven by urban sewage discharge with high NO concentrations. Rapid urbanization accelerated greenhouse gas emissions from the urban rivers, so that in the urban reaches, CO/CH fluxes were twice those of the non-urban reaches, and all over the basin NO fluxes were at a high level. These findings illustrate how river basin urbanization can change aquatic environments and aggravate allochthonous pollution inputs such as carbon, nitrogen, and phosphorus, which in turn can dramatically stimulate river-based greenhouse gas production and emissions; meanwhile, spatial and temporal differences in greenhouse gas emissions in rivers can lead to the formation of emission hotspots.
河流在温室气体排放中起着重要作用。在过去十年中，由于全球城市化趋势，快速的土地利用变化导致河流生态系统发生改变，对温室气体的产生和排放起到了刺激作用。目前，迫切需要对流域内的温室气体浓度和排放进行评估。因此，本研究旨在评估快速城市化地区基于河流的温室气体排放及其时空特征以及可能的影响因素。具体目标是调查梁滩河（它不仅是重庆市最大的子流域，也是污染最严重的子流域）中河流温室气体浓度和排放通量如何响应城市化。采用薄层扩散模型方法在2014年9月、12月以及2015年3月和6月对CO、CH₄和N₂O进行全年浓度监测。CO的浓度范围为(23.38±34.89)-(1395.33±55.45) Pa，CH₄和N₂O的浓度范围分别为(65.09±28.09)-(6021.36±94.36) nmol·L⁻¹和(29.47±5.16)-(510.28±18.34) nmol·L⁻¹。基于风速模型估算方法计算得出的CO、CH₄和N₂O的排放通量分别为-6.1 - 786.9、0.31 - 27.62和0.06 - 1.08 mmol·(m²·d)。此外，CO和CH₄排放呈现出显著的空间差异，且这些差异与污染负荷梯度大致一致。干流的温室气体浓度和通量从上游到下游先增加后减少，在城市化率高于其他地区且河流污染严重的中游检测到最高值。至于支流，温室气体浓度和通量从上游农业区到下游城市区显著增加。CO通量呈现季节性模式，秋季观测到的CO排放值最高，其次依次是冬季、夏季和春季。CH₄通量在春季最高，夏季最低，而N₂O通量的季节性模式不显著。由于流域内碳和氮负荷较高，CO的产生和排放不受生源要素限制，但发现其水平与水温、pH值、溶解氧和叶绿素a等重要生物代谢因素有关。水中的碳、氮和磷含量以及污水输入影响了CH₄的产生和排放。同时，N₂O的产生和排放主要由高浓度N₂O的城市污水排放驱动。快速城市化加速了城市河流的温室气体排放，使得在城市河段，CO/CH₄通量是非城市河段的两倍，并且整个流域的N₂O通量都处于较高水平。这些发现说明了流域城市化如何改变水生环境并加剧碳、氮和磷等外源污染输入，进而极大地刺激基于河流的温室气体产生和排放；与此同时，河流中温室气体排放的时空差异会导致排放热点的形成。
