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Abstract Methane (CH₄) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH₄emissions from freshwater ecosystems^1,2, providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH₄emissions remain highly uncertain^3,4. Here we report a spatially explicit global estimate of CH₄emissions from running waters, accounting for 27.9 (16.7–39.7) Tg CH₄ per year and roughly equal in magnitude to those of other freshwater systems^5,6. Riverine CH₄emissions are not strongly temperature dependent, with low average activation energy (E_M = 0.14 eV) compared with that of lakes and wetlands (E_M = 0.96 eV)¹. By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land–water connections in regulating CH₄supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land. 

The magnitude of stream and river carbon dioxide (CO 2 ) emission is affected by seasonal changes in watershed biogeochemistry and hydrology. Global estimates of this flux are, however, uncertain, relying on calculated values for CO 2 and lacking spatial accuracy or seasonal variations critical for understanding macroecosystem controls of the flux. Here, we compiled 5,910 direct measurements of fluvial CO 2 partial pressure and modeled them against watershed properties to resolve reach-scale monthly variations of the flux. The direct measurements were then combined with seasonally resolved gas transfer velocity and river surface area estimates from a recent global hydrography dataset to constrain the flux at the monthly scale. Globally, fluvial CO 2 emission varies between 112 and 209 Tg of carbon per month. The monthly flux varies much more in Arctic and northern temperate rivers than in tropical and southern temperate rivers (coefficient of variation: 46 to 95 vs. 6 to 12%). Annual fluvial CO 2 emission to terrestrial gross primary production (GPP) ratio is highly variable across regions, ranging from negligible (<0.2%) to 18%. Nonlinear regressions suggest a saturating increase in GPP and a nonsaturating, steeper increase in fluvial CO 2 emission with discharge across regions, which leads to higher percentages of GPP being shunted into rivers for evasion in wetter regions. This highlights the importance of hydrology, in particular water throughput, in routing terrestrial carbon to the atmosphere via the global drainage networks. Our results suggest the need to account for the differential hydrological responses of terrestrial–atmospheric vs. fluvial–atmospheric carbon exchanges in plumbing the terrestrial carbon budget. 

ABSTRACT Nitrous oxide (N₂O) reductase, the sole natural microbial sink for N₂O, exists in two microbial clades:nosZI andnosZII. Although previous studies have explored inter‐clade ecological differentiation, the intra‐clade variations and their implications for N₂O dynamics remain understudied. This study investigated both inter‐ and intra‐clade ecological differentiation among N₂O reducers, the drivers influencing these patterns, and their effects on N₂O emissions across continental‐scale river systems. The results showed that bothnosZI andnosZII community turnovers were associated with similar key environmental factors, particularly total phosphorus (TP), but these variables explained a larger proportion of variation in thenosZI community. The influence of mean annual temperature (MAT) on community composition increased for more widespread N₂O‐reducing taxa. We identified distinct ecological clusters within each clade of N₂O reducers and observed identical ecological clustering patterns across both clades. These clusters were primarily characterized by distinct MAT regimes, coarse sediment texture as well as low TP levels, and high abundance of N₂O producers, with MAT‐related clusters constituting predominant proportions. Intra‐clade ecological differentiation was a crucial predictor of N₂O flux and reduction efficiency. Although different ecological clusters showed varying or even contrasting associations with N₂O dynamics, the shared ecological clusters across clades exhibited similar trends. Low‐MAT clusters in both thenosZI andnosZII communities were negatively correlated with denitrification‐normalized N₂O flux and the N₂O:(N₂O + N₂) ratio, whereas high‐MAT clusters showed positive correlations. This contrasting pattern likely stems from low‐MAT clusters being better adapted to eutrophic conditions and their more frequent co‐occurrence with N₂O‐producing genes. These findings advance our understanding of the distribution and ecological functions of N₂O reducers in natural ecosystems, suggesting that warming rivers may have decreased N₂O reduction efficiency and thereby amplify temperature‐driven emissions. 

Abstract The relative capacity for watersheds to eliminate or export reactive constituents has important implications on aquatic ecosystem ecology and biogeochemistry. Removal efficiency depends on factors that affect either the reactivity or advection of a constituent within river networks. Here, we characterized Damköhler number (Da) for dissolved organic carbon (DOC) uptake in global river networks. Da equals the advection to reaction timescale ratio and thus provides a unitless indicator for DOC reaction intensity during transport within river networks. We aim to demonstrate the spatial and temporal patterns and interplays among factors that determine DOC uptake across global river networks. We show that watershed size imposes a primary control on river network DOC uptake due to a three orders of magnitude difference in water residence time (WRT) between the smallest and largest river networks. DOC uptake capacity in tropical river networks is 2–6 times that in temperate and the Arctic river networks, coinciding with larger DOC removals in warm than in cold watersheds. River damming has a profound impact on DOC uptake due to significantly extended WRTs, particularly in temperate watersheds where most constructed dams are situated. Global warming is projected to increase river network DOC uptake by ca. 19% until year 2100 under the RCP4.5 scenario. 

Abstract Inland waters are important sources of the greenhouse gasses (GHGs) carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) to the atmosphere. In the framework of the second phase of the REgional Carbon Cycle Assessment and Processes (RECCAP‐2) initiative, we synthesize existing estimates of GHG emissions from streams, rivers, lakes and reservoirs, and homogenize them with regard to underlying global maps of water surface area distribution and the effects of seasonal ice cover. We then produce regionalized estimates of GHG emissions over 10 extensive land regions. According to our synthesis, inland water GHG emissions have a global warming potential of an equivalent emission of 13.5 (9.9–20.1) and 8.3 (5.7–12.7) Pg CO₂‐eq. yr⁻¹at a 20 and 100 years horizon (GWP₂₀and GWP₁₀₀), respectively. Contributions of CO₂dominate GWP₁₀₀, with rivers being the largest emitter. For GWP₂₀, lakes and rivers are equally important emitters, and the warming potential of CH₄is more important than that of CO₂. Contributions from N₂O are about two orders of magnitude lower. Normalized to the area of RECCAP‐2 regions, S‐America and SE‐Asia show the highest emission rates, dominated by riverine CO₂emissions.