Global methane and nitrous oxide emissions from inland waters and estuaries

Inland waters (rivers, reservoirs, lakes, ponds, streams) and estuaries are globally significant emitters of methane (CH4) and nitrous oxide (N2O) to the atmosphere, while global estimates of these emissions have been hampered due to the lack of a worldwide comprehensive dataset with the collection of complete CH4and N2O flux components. Here, we synthesize 2,997 in-situ flux or concentration measurements of CH4 and N2O from 277 peer-reviewed publications to explore the role of inland waters and estuaries in shaping climate change. We estimate that inland waters including rivers, reservoirs, lakes, and streams together release 95.18 Tg CH4 yr-1 (ebullition plus diffusion) and 1.48 Tg N2O yr-1 (diffusion) to the atmosphere, yielding an overall CO2-equivalent emission total of 3.06 Pg CO2 yr-1, representing roughly 60% of CO2 emissions (5.13 Pg CO2 yr-1) from these four inland aquatic systems, among which lakes act as the largest emitter for both CH4and N2O. Ebullition is noticed as a dominant flux component of CH4, contributing up to 62–84% of total CH4fluxes across all inland waters. Chamber-derived CH4 emission rates are significantly greater than those determined by diffusion model-based methods for commonly capturing both diffusive and ebullitive fluxes. Water dissolved oxygen (DO) showed as a dominant factor among all variables to influence both CH4(diffusive and ebullitive) and N2O fluxes from inland waters. Our study reveals a major oversight in regional and global CH4budgets from inland waters, caused by neglect of the dominant role of ebullition pathways in those emissions. The indirect N2O EF5 values established in this study generally suggest a downward revision is required in current IPCC default EF5 values for inland waters and estuaries. Our findings further indicate that a comprehensive understanding of the magnitude and patterns of CH4 and N2O emissions from inland waters and estuaries is essential in defining how these aquatic systems will shape our climate.

内陆水域（河流、水库、湖泊、池塘、溪流）与河口是全球范围内向大气排放甲烷（CH4）和一氧化二氮（N2O）的重要源区。然而，由于缺乏涵盖完整甲烷、一氧化二氮通量组分的全球综合数据集，这类排放的全球估算长期受阻。
本研究整合了来自277篇同行评议论文的2997组甲烷与一氧化二氮原位通量或浓度实测数据，以探究内陆水域与河口在气候变化塑造中的作用。
我们估算得到，包括河流、水库、湖泊与溪流在内的内陆水域整体向大气排放95.18 Tg CH4 yr-1（含冒泡释放与扩散通量）以及1.48 Tg N2O yr-1（仅扩散通量），总二氧化碳当量排放量达3.06 Pg CO2 yr-1，约占这四类内陆水生系统二氧化碳排放量（5.13 Pg CO2 yr-1）的60%；其中湖泊是甲烷与一氧化二氮的最大排放源。
冒泡释放是甲烷通量的主要组分，在所有内陆水域的总甲烷通量中占比可达62%~84%。针对同时捕捉扩散与冒泡通量的常规监测方法，箱式法测得的甲烷排放速率显著高于基于扩散模型的估算结果。
在所有影响内陆水域甲烷（扩散与冒泡通量）及一氧化二氮通量的变量中，水体溶解氧（DO）被证实为首要控制因子。
本研究揭示，现有内陆水域甲烷收支研究存在重大疏漏——该疏漏源于忽略了冒泡释放路径在甲烷排放中的主导作用。本研究确立的间接一氧化二氮排放因子EF5值整体表明，当前针对内陆水域与河口的政府间气候变化专门委员会（Intergovernmental Panel on Climate Change, IPCC）默认EF5值需要向下修正。
我们的研究结果进一步表明，全面理解内陆水域与河口的甲烷、一氧化二氮排放规模与模式，是明确这类水生系统如何影响全球气候的关键所在。