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River damming can significantly alter the hydrology and nutrient levels of river water, resulting in substantial greenhouse gas (GHG) emissions to the atmosphere. However, the dynamics of greenhouse gases in the discharged water downstream of dams remain poorly understood, despite being recognized as a crucial source of GHG emissions in river‐reservoir systems. In this study, we conducted comprehensive measurements of GHG concentrations and water chemistry in a large subtropical reservoir and its upstream and downstream rivers to investigate the spatiotemporal patterns of GHG concentrations and fluxes and to identify their governing mechanisms, with a primary focus on downstream GHG dynamics. Our analysis revealed that the distribution of pCO2 among the reservoir and its upstream and downstream rivers was predominantly controlled by aquatic metabolism and atmospheric CO2 exchange. Conversely, the distribution of CH4 and N2O levels was largely influenced by anaerobic metabolism. Seasonal fluctuations in GHG dynamics were linked to hydroclimatic conditions, including water temperature, hydrologic connectivity between land and rivers, and reservoir thermal stratification. Anthropogenic activities (e.g., agricultural land use) were found to affect the downstream trend of GHG concentrations. Higher GHG fluxes in the downstream rivers compared to reservoir were attributed to the anaerobic production of CH4 in the reservoir and increased gas transfer velocity in the downstream rivers. These findings underscore the critical influence of anthropogenic activities on downstream GHG dynamics and emphasize the necessity of integrating anthropogenic impacts and seasonal variability in downstream GHG emissions to enhance our understanding of the carbon budget in river‐reservoir systems. Plain Language Summary: The discharged water from dams is a key source of greenhouse gases (GHGs) in river‐reservoir systems; however, we still lack a comprehensive understanding of the gases' behavior downstream. In our study, we measured GHG levels and water quality in a large subtropical reservoir and its surrounding rivers to investigate how these gases move over time and space. We focused on understanding how GHGs behave downstream. Our analysis showed that the distribution of CO2 in these water bodies is mostly influenced by how microbes and the atmosphere interact with the water, while the levels of CH4 and N2O are largely controlled by production in low‐oxygen conditions. Seasonal changes in GHG levels were tied to shifts in weather and water conditions. Human activities such as agriculture were seen to impact GHG concentrations downstream. We found that higher GHG levels in downstream rivers were due to the production of CH4 in the reservoir and the rate of gas emit to the air. Our study highlights how human actions affect GHG levels in downstream rivers and stresses the need to consider these impacts and seasonal variations when studying GHG emissions in river‐reservoir systems. Key Points: Aquatic metabolism and hydroclimatic conditions shape the spatiotemporal patterns of GHG dynamicsAnthropogenic activities affect the downstream trend of GHG concentrationsDownstream river water acts as a significant emission hotspot for CO2 and CH4 [ABSTRACT FROM AUTHOR]

Mountainous rivers are critical in transporting dissolved organic carbon (DOC) from terrestrial environments to downstream ecosystems. However, how geomorphologic factors and anthropogenic impacts control the composition and export of DOC in mountainous rivers remains largely unclear. Here, we explore DOC dynamics in three subtropical mountainous catchments (i.e., the Yinjiang, Shiqian, and Yuqing catchments) in southwest China, which are heavily influenced by anthropogenic activities. Water chemistry, stable and radioactive carbon isotopes of DOC (δ13CDOC and Δ14CDOC), and optical properties (UV absorbance and fluorescence spectra) were employed to assess the biogeochemical processes and controlling factors on riverine DOC. The radiocarbon ages of DOC in the Yinjiang River varied widely from 928 years BP to the present. Stepwise multiple regression analyses and partial least square path models revealed that geomorphology and anthropogenic activities were the major drivers controlling DOC concentrations and DOM characteristics. Catchments with higher catchment slope gradients were characterized by lower DOC concentrations, enriched δ13CDOC and Δ14CDOC , and more aromatic dissolved organic matter (DOM), which were opposite to catchments with gentle catchment slopes. Variabilities in DOC concentrations were also regulated by land use, with higher DOC concentrations in urban and agricultural areas. Furthermore, DOM in catchments with a higher proportion of urban and agricultural land uses was less aromatic, less recently produced, and exhibited a higher degree of humification and more autochthonous humic-like DOM. This research highlights the significance of incorporating geomorphologic controls on DOC sources and anthropogenic impacts on DOM composition into the understanding of DOC dynamics and the quality of DOM in mountainous rivers, which are globally abundant. [ABSTRACT FROM AUTHOR]

Volcanic activities have great implications on the geological carbon cycle, and ascertaining the deep carbon contribution in the Earth's surface that runs along the volcanic edifices is important to understand the relationship between solid Earth degassing and global climate change. This study reports analytical results of major dissolved ions concentrations, carbon isotopic compositions (δ13CDIC and Δ14CDIC) of dissolved inorganic carbon (DIC) of rivers, cold springs, and hot springs from Changbaishan volcanic area, Northeast China. The hydrothermal fluids had a significant impact on solutes budgets, as well as carbon isotopes for the rivers. The changes in concentrations of major ions are mainly controlled by mixing of high‐temperature water/rock interaction and low‐temperature water/rock interaction, and low‐temperature water/rock interaction can be explained by the change of chemical composition between volcanic cone (trachyte) and basaltic shield. Because Δ14CDIC is conservative to CO2 outgassing, we used Δ14CDIC to figure out the contributions of deep carbon and surface carbon. While δ13CDIC is sensitive to CO2 outgassing, we thus estimated the minimum deep CO2 outgassing yield (1.24 × 104 t C yr−1) based on DIC flux corrected for outgassing by a Rayleigh model. In the Changbaishan volcanic area, deep carbon release flux was higher than CO2 consumption flux by silicate weathering, while the deep CO2 outgassing flux was an underestimate, consistent with the hypothesis that deep CO2 release regulates climate on geological timescales. This study calls for a better understanding of the effects of volcanic activities on Earth's surface carbon cycling, which has great implications on studying global climate change. Plain Language Summary: The balance between volcanic degassing and silicate weathering may control the atmospheric CO2, regulating long‐term global climate. Volcanic areas have attracted large amounts of attentions, because of its intricate effects on atmospheric CO2. We investigated water chemistry, δ13CDIC and Δ14CDIC in Changbaishan volcanic area to understand carbon biogeochemical processes in volcanic areas. In most previous studies, δ13CDIC was used to trace the sources of DIC in volcanic areas, but this study evidenced that δ13CDIC is highly controlled by CO2 outgassing, bringing great uncertainties on source discrimination. Δ14CDIC is non‐sensitive to CO2 outgassing, so we used Δ14CDIC to trace the sources of DIC. Based on the Rayleigh fractionation model, δ13CDIC was used to estimate the minimum CO2 outgassing fluxes in this study area. At last, we evaluated the net carbon budget concerning deep carbon release and silicate weathering, and found that deep carbon release flux was higher than CO2 consumption flux by silicate weathering. This study highlights the effects of deep carbon release on Earth's surface and provides mechanistic insights into carbon biogeochemical processes in volcanic areas. Key Points: The water chemical compositions and isotopic compositions of DIC varied in a wide range in Changbaishan volcanic riversThe hydrothermal fluids had a significant impact on solutes budgets, as well as carbon isotopes for the volcanic riversDeep carbon release flux was higher than CO2 consumption flux by silicate weathering in Changbaishan volcanic area [ABSTRACT FROM AUTHOR]

Green development is not only significant for high-quality Belt and Road construction, but also for the new development paradigm of domestic and international circulations reinforcing each other. As global green governance steps up, the green BRI may correct the current global governance deficit and boost a green recovery in the post COVID-19 era, making up for the lack of sufficient green public goods while facilitating the establishment of a global ecological civilization. [ABSTRACT FROM AUTHOR]