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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]

Riverine transport of carbon from the land to the oceans plays a significant role in global carbon cycle. However, multiple processes can affect aquatic carbon cycling, and the carbon sources and processing in river systems are still elusive. Here, we analysed the water chemistry and dual carbon isotopes (δ 13 C and Δ 14 C) of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) from mixed karst and non-karst subtropical monsoonal catchments, southwest China. The water chemistry of the river water showed that DIC concentrations were mainly controlled by carbonate weathering and modulated by agricultural activities and geomorphic characteristics (i.e. elevation and slope), but the stable isotope of DIC (δ 13 C DIC ) was highly affected by CO 2 outgassing and in-stream photosynthesis. The C/N ratios and stable isotope of POC (δ 13 C POC ) indicated that the composition of riverine POC derived from a mixture of terrestrial sources and algae/microbial sources. Based on the δ 13 C and Δ 14 C of POC, we used a Bayesian mixing model to constrain the POC sources, which showed that aquatic photosynthesis was the main source for POC. Our findings suggest that carbon dynamics in subtropical rivers are highly affected by aquatic photosynthesis, which has significant implications on carbon cycling within river systems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)