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Reservoirs around the world have significantly altered the natural transport of nutrients in rivers. However, the specific effects of the cascade damming on the migration, transformation, and environmental consequences of these nutrients remain unclear. To address this knowledge gap, we analyzed spatiotemporal variations in water chemistry, nutrient concentrations, stable isotope of dissolved inorganic carbon (δ 13 C DIC ) and nitrate isotope (δ 15 N-NO 3 - ) in seven cascade reservoirs along the Wujiang River, each characterized by different regulatory regimes. Our findings reveal that the average absolute changes in concentrations of total nitrogen (TN), total phosphorus (TP), and silicon dioxide (SiO 2 ) during the wet season (WS, spring and summer) were 2.4, 1.4, and 1.1 times higher than those observed in the dry season (DS, autumn and winter). During the WS, the average apparent retention efficiency (*RET f ) values in the Hongjiadu reservoir were 97 % for TN, 98 % for TP, and 95 % for SiO 2 , indicating substantial nutrient consumption in the cascading reservoirs. Conversely, during the DS, the *RET f values for TN, TP, and SiO 2 were negative, suggesting notable nutrient accumulation within the reservoirs. The nutrient fluxes released downstream from the cascade reservoirs in the Wujiang River were significantly greater than the upstream inflow fluxes. These findings help demonstrate how downstream discharge across cascade reservoirs amplifies nutrient flux disparities due to dam construction. Our study enhances the understanding of how cascade dam construction impacts nutrient dynamics, supporting the optimization of reservoir operation models and advancing scientific water resource management and conservation efforts., 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 © 2024 Elsevier Ltd. All rights reserved.)

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]

• Unveiling the dissolved inorganic carbon cycle in a karst reservoir through hydrodynamic modeling and δ13C. • The DIC retention capacity is more pronounced under weak hydraulic conditions in HJD. • Significant CO 2 emissions during periods of intense hydrodynamics necessitate close monitoring. • Quantification of DIC flux across diverse hydrodynamic scenarios in a deep subtropical reservoir. • The method (hydrodynamic model and δ13C) is applicable to karst canyon-type reservoir worldwide. Dam construction significantly impacts river hydrodynamics, subsequently influencing carbon biogeochemical processes. However, the influence of hydrodynamic conditions on the migration and transformation of Dissolved Inorganic Carbon (DIC) remains uncertain. To bridge this knowledge gap, we integrated hydrochemistry, isotopic composition (δ13C DIC), and a hydrodynamic model (CE-QUAL-W2) to examine the distinctions, control mechanisms, and environmental effects of DIC biogeochemical processes in a typical large and deep reservoir (Hongjiadu Reservoir) under different hydrodynamic conditions. We evaluated hydrodynamic alterations through the Schmidt stability index and relative water column stability. The analysis disclosed that during weak hydrodynamics periods, the energy necessary for complete mixing the surface and deep water was 34 times higher (3615.32 J/m2 vs.106.86 J/m2), and stability was 13 times greater (312.96 vs. 24.69) compared to periods of strong hydrodynamics. Additionally, the spatiotemporal heterogeneity of DIC concentrations (1.4 % to -9.1 %) and δ13C DIC (-1.7 % to -19.5 %) from the dry to wet seasons reflected disparities in DIC control mechanisms under varied hydrodynamic conditions. Based on model simulations, our calculations indicate that during weak hydrodynamics periods, the enhancement of the biological carbon pump effect resulted in substantial sequestration of DIC, reaching up to 379.6 t-DIC·d−1 in the water. Conversely, during strong hydrodynamics periods, DIC retention capacity decreased by 69.2 t·d−1, resulting in reservoir CO 2 emissions of 22.7 × 104 t, which were more than 7 times higher than during weak hydrodynamics periods (3.2 × 104 t). Our findings emphasize the discernible impact of hydrodynamic conditions on reservoir biogeochemical processes related to DIC. Considering the increasing construction of reservoirs globally, understanding and controlling hydrodynamic conditions are crucial for mitigating CO 2 emissions and optimizing reservoir management. [Display omitted] [ABSTRACT FROM AUTHOR]

• First measurements of PIC radiocarbon age in a karst reservoir system. • Photosynthesis-induced PIC precipitation is a source of PIC sink in the lentic zones. • The dissolution of autochthonous PIC is responsible for the build-up of DIC in the hypolimnion. • Discharge of the DIC-rich hypolimnion water through the dam causes downstream CO 2 emission to the air. Damming alters the biogeochemical cycling of carbon along river systems. However, the effects of river damming on the fate and transport of particulate and dissolved inorganic carbon (PIC and DIC) remain unclear. Here, we use dual isotope (13C and 14C) compositions, water chemistry, and electron microscopy to unravel the processes controlling the in-reservoir cycling of inorganic carbon in a river-reservoir system of a karst region of southwest China. From the reservoir's fluvial to lentic zone, δ13C PIC decreases from −2.2 ‰ to −7.6 ‰, whereas the PIC radiocarbon age drops from 11,512 to 2,027 years B.P. Electron microscopy images and water chemistry provide further evidence of in-reservoir photosynthesis-induced carbonate precipitation of autochthonous PIC in the water column. Although part of PIC is dissolved into DIC in the reservoir's hypolimnion, the contribution of carbonate minerals (allochthonous PIC) accounted for 41 % of PIC based on an isotopic mass balance model. This indicates that the precipitation rate of autochthonous PIC was greater part of carbon cycle in the reservoir. Moreover, the autochthonous PIC accounted for 61 % of the PIC and its dissolution accounted for 50 % of the DIC and the high CO 2 efflux in the water discharging from the reservoir. The results shed light on the important role of autochthonous PIC in carbon cycling in karst river-reservoir systems. With continued dam construction in karst areas, the role of in-reservoir PIC production and dissolution on the riverine carbon cycle deserves more attention. [ABSTRACT FROM AUTHOR]

• Evaluating the carbon transformation intensity and amounts in the reservoir. • The ultimate reservoir impacts consume river carbon in most of the seasons. • Evaluating the influence of hydrological management and seasonal variation. • Providing the scientific basis for weakening the influence of reservoir impacts. Reservoirs are widely established worldwide with considerable environmental impacts, especially on the riverine carbon cycle. However, the influence of reservoirs on the cycling of different forms of carbon and its regulation factors (e.g., seasonal variations versus hydrological management) have not been simultaneously studied. To fill this knowledge gap, seasonal water samples from the deep subtropical reservoir (Longtan reservoir) in the Pearl River were collected, and the concentrations and stable carbon isotopes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were determined. The variations in stable carbon isotopes of DIC (-11.4‰ to -5.2‰), DOC (-32.2‰ to -26.2‰), and POC (-38.9‰ to -25.3‰) in the river-reservoir system indicated active production and degradation processes in different layers. We estimated that up to 23.0% of DIC, 20.5% of DOC, and most POC were intercepted or degraded within the reservoir. Our results further illustrated that hydrological management (water storage regulation) and seasonal variations from different perspectives controlled the cycling of different forms of carbon in the reservoir. In addition, with the gradual increase in the number of reservoirs, hydrological management can be considered as a potentially effective strategy to adjust the carbon biogeochemical cycling of reservoirs in the future. [Display omitted] [ABSTRACT FROM AUTHOR]