Abstract:

Global warming, caused by an increase in the concentration of greenhouse gases (GHGs), has gained significant international attention. Nitrous oxide (N₂O) is a trace greenhouse gas that contributes significantly to global warming. While rivers and reservoirs are considered significant sources of N₂O emissions, current research primarily focuses on monitoring individual water bodies, leaving a gap in the study of N₂O across large regions such as the Lancang River (Yunnan section). To address this gap, gas samples were collected at 24 locations along the water-air interface of the river using floating chambers in April and August 2022. Gas chromatography was used to determine nitrous oxide concentrations and examine the spatiotemporal distribution characteristics of N₂O flux in the main and tributary streams of this basin. Simultaneously, the water environmental parameters were measured to evaluate the environmental factors that affect spatiotemporal differences. The results indicate that 1) on a temporal scale, the N₂O flux during the dry season in the Lancang River (Yunnan section) is lower than that in the rainy season, with average values of 0.10 mg·m⁻²·d⁻¹ and 0.18 mg·m⁻²·d⁻¹, respectively. The dry season mean is 0.11 mg·m⁻²·d⁻¹ for the main stream and 0.05 mg·m⁻²·d⁻¹ for the tributary; the rainy season mean is 0.18 mg·m⁻²·d⁻¹ for the main stream, and 0.15 mg·m⁻²·d⁻¹ for the tributary. 2) Spatially, N₂O flux along the river showed an overall increasing trend. The main stream’s N₂O flux mean is 0.15 mg·m⁻²·d⁻¹, higher than the tributary’s 0.10 mg·m⁻²·d⁻¹. The N₂O flux above the dam of the reservoir is 0.02 mg·m⁻²·d⁻¹, while it is 0.31 mg·m⁻²·d⁻¹ below the dam, indicating that the N₂O flux below the dam is 15.5 times higher than above, suggesting that the accelerated water flow during reservoir power generation enhances the water-air interface N₂O release. 3) The relationship between N₂O flux at the Lancang River water-air interface and various factors was analyzed, revealing a positive correlation with water temperature (WT) (r=0.561, P=0.000), ammonia nitrogen (NH₄⁺) (r=0.377, P=0.015), and flow velocity (r=0.331, P=0.026) and a negative correlation with dissolved oxygen (DO) (r= −0.507, P=0.000). These findings suggest that N₂O production and release are favored under conditions of high WT, low DO, high NH₄⁺, and high flow velocity, and provide essential information on the spatiotemporal distribution characteristics and influencing factors of nitrous oxide release flux in the Lancang River (Yunnan section) river-reservoir system. This information is crucial for assessing greenhouse gas emissions in watersheds.

Key words: Lancang River, nitrous oxide (N₂O), emission flux, spatial and temporal distribution, influencing factors, water-gas interface

摘要：

温室气体浓度上升导致全球气候变暖已成为国际社会关注的焦点。氧化亚氮（N₂O）作为痕量温室气体，在全球气候变暖过程中扮演着重要的角色。河流和水库被认为是N₂O释放的活跃区域，然而目前研究多集中在对单一河流或水库的监测，对大范围河库系统的N₂O研究仍相对欠缺。选取澜沧江（云南段）流域作为考察对象，在2022年4月、8月采用悬浮箱采集24个点位的水-气界面的气体样品，并于实验室通过气象色谱仪检测气样氧化亚氮浓度，探究该流域干支流N₂O释放通量的时空分布特征；同时，测定了水体水环境参数，解析其时空差异的环境影响因素。结果表明，1）在时间尺度上，澜沧江（云南段）旱季N₂O通量均值低于雨季。旱季N₂O通量均值为0.10 mg·m⁻²·d⁻¹，雨季为0.18 mg·m⁻²·d⁻¹，其中旱季干流均值为0.11 mg·m⁻²·d⁻¹，支流为0.05 mg·m⁻²·d⁻¹；雨季干流均值为0.18 mg·m⁻²·d⁻¹，支流为0.15 mg·m⁻²·d⁻¹。2）在空间尺度上，N₂O通量沿程整体呈现释放量递增的趋势。干流N₂O通量均值为0.15 mg·m⁻²·d⁻¹，支流为0.10 mg·m⁻²·d⁻¹，干流N₂O通量高于支流。水库坝上N₂O通量均值为0.02 mg·m⁻²·d⁻¹，坝下为0.31 mg·m⁻²·d⁻¹，水库坝下N₂O通量值是坝上的15.5倍，水库发电下泄高速水流加速了水-气界面N₂O释放。3）澜沧江水-气界面N₂O通量与水温（WT）（r=0.561，P=0.000）、氨氮（NH₄⁺）（r=0.377，P=0.015）和流速（r=0.331，P=0.026）呈显著正相关关系，与溶解氧（DO）（r= −0.507，P=0.000）呈显著负相关关系，表明在高WT、低DO、高NH₄⁺和高流速条件下有利于N₂O产生与释放。研究结果揭示澜沧江（云南段）河库系统氧化亚氮释放通量时空分布特征以及影响因素，为流域温室气体排放评估提供数据支持和科学依据。

关键词: 澜沧江, 氧化亚氮（N₂O）, 释放通量, 时空分布, 影响因素, 水-气界面