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• Carbon transport and transformation varies in two reservoirs with different geological settings. • Karst reservoirs exhibit a higher Revelle factor (RF) and low CO 2 emissions. • Attention should be paid to the high CO 2 emission effect of non-karst reservoirs. • Consider geological settings when estimating the global budget for reservoir CO 2 emissions. Carbon migration, transformation, and emissions as CO 2 in reservoir and lake systems have been extensively studied. However, uncertainties persist regarding carbon cycling variations in both karst and non-karst regions within large thermal stratified river-reservoir systems. To address this knowledge gap, we combined measurements of water chemistry, isotopic compositions (δ13C), partial pressure of CO 2 (p CO 2), and CO 2 fluxes (F CO2) to elucidate the differences, control mechanisms, and environmental effects of various carbon biogeochemical processes in the Hongjiadu reservoir (HJD, karst reservoir) and Shangyoujiang reservoir (SYJ, non-karst reservoir), Yangtze River basin, China. Our results demonstrate that key biogeochemical processes are associated with CO 2 production and emissions. The stronger biological carbon pump (BCP) effect in the lentic area of the HJD reservoir limited CO 2 emission (0.9 ± 5.0 mmol m−2 d−1) compared to the SYJ reservoir (50.9 ± 33.4 mmol m−2 d−1), leading to low dissolved inorganic carbon (DIC) and high saturation state of carbonate minerals in the lentic epilimnion. Although the released water from both reservoirs acted as hotspots for CO 2 emissions, the results of higher Revelle factor (RF) in the HJD reservoir than SYJ reservoir suggest the greater buffer capacity and lower sensitivity of OC degradation changes in DIC. Despite the shorter hydraulic retention time (HRT) of SYJ reservoir (120 d) compared to HJD reservoir (368 d), F CO2 was 3.5 times higher than that of the HJD reservoir (652.9 mmol m−2 d−1). Moreover, upon analyzing selected reservoirs worldwide, it becomes apparent that karst reservoirs demonstrate a reduced potential for CO 2 emissions. Our results indicate that the impact of varying geologic settings should be considered to minimize errors in regional and global CO 2 emission estimates. [Display omitted] [ABSTRACT FROM AUTHOR]

• Using δ13C DIC and water chemistry to refine the sources and multiple processes in the waters. • A new model to understand migration and transformation of DIC in a karst river-reservoir system. • HRT is a key factor for carbon transportation and transformation in the reservoir. In order to advance our understanding of the driving factors controlling carbon dynamics and evolution of water quality in river-reservoir systems for the generation of hydropower, we conducted a study in a karst deep-water reservoir (Wujiangdu Reservoir), southwest China. Water samples were collected from the inflow/outflow of the Wujiangdu Reservoir, four vertical columns along the reservoir, and from three tributaries to the reservoir in January, April, July, and October 2017. The dissolved inorganic carbon (DIC) concentrations and carbon isotope composition (δ13C DIC) varied greatly (1.99–3.45 mmol/L and −10.7‰ to −6.0‰, respectively) and were controlled by multiple processes including CO 2 outgassing, primary production, and organic matter degradation. In the four vertical profiles, the difference between the values of samples with those at 15 m of Δ[DIC] and Δ[δ13C DIC ], Δ[DIC] and ΔCa2+ in the same water column had positive correlations, and the variation in dissolved O 2 , partial pressure of CO 2 , and flux of CO 2 suggested that primary production dominated above the epilimnion and degradation of OM dominated below the epilimnion during the warm season. Continuous CO 2 outgassing was found from riverine water to surface water of the reservoir before the dam based on carbon isotopic compositions and water chemistry. These processes would cause isotopic fractionation between the residual DIC and CO 2 (aqueous), the degree of which varied among different seasons (July > April > October > January). Carbon transport and biogeochemical processes were highly controlled by the hydraulic retention time (HRT) in the river-reservoir system. It was estimated that approximately 71.5% of the annual DIC flux was produced in the water column below 15 m depth during the study year, suggesting that the processes of DIC generation and consumption occurred at the same time. The results highlight that carbon behavior in the impounded rivers is influenced by multiple processes. The carbon transformation processes should be taken into account for improving the estimation accuracy of the carbon budget calculations and the management of water quality for river-reservoir systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • There is an active SIC-CO 2 -SOC system and a fast C cycle in karst soils. • The impact of biological factors on SIC in karst soils exceeds expectations. • Both SOC and SIC are labile and susceptible to land use change in karst regions. • Ceasing tillage benefits SOC restoration which mainly occurs within topsoil. Soil is a key carbon reservoir balancing global carbon budget and regulating climate change. Barren soils in karst zones have weak capacity for soil and water conservation and are readily erodible, making the carbon biogeochemical processes within karst soils potentially rapid and complex. To explore the vertical variation of soil carbon under karst conditions and its response to land-use change, this study investigated concentrations and isotopic compositions of both soil organic and inorganic carbon (SOC and SIC) in different depths of four land-use types in a typical karst region, SW China. Results show that as soil depth increases, SOC concentrations decrease, and its δ13C ratios (from −27.3‰ to −19.4‰) increase in 0–20 cm depth but decrease at depths below 20 cm. The fresh SOC is mostly sequestered and cycled within the topsoil and subject to different controls than that of the subsoil. The turnover rate of SOC in karst soils does not directly co-vary with isotopic fractionation among different land uses. Long-term cultivation causes SOC loss from karst soils, which can be alleviated or even partially restored after farming cessation. By contrast, SIC represents < 10% of soil total carbon. The vertical heterogeneity of SIC variation and the direct influence of biological factors on SIC are both weaker relative to SOC. The low δ13C ratios of SIC (from −20.4‰ to −3.0‰) indicate there is intense dissolution and reprecipitation of pedogenic carbonate within karst soil, especially in the upper cultivated layers. These results highlight that both SOC and SIC are labile and susceptible to land-use change in karst zones, which need to be considered in estimating karst carbon sink and its role in balancing global carbon budget on variable temporal scales. [ABSTRACT FROM AUTHOR]

Abstract: The sources of dissolved inorganic carbon (DIC) and rock weathering processes were studied in the Houzhai catchment, a typical karstic catchment in the Changjiang River Basin, Southwest China. The carbon isotopic compositions (δ 13CDIC) of DIC in the samples collected from the catchment vary from −13.5‰ to −6.9‰, with a mean value of −9.8‰. The DIC in the catchment is thereby considered to be mainly derived from soil CO2 and weathering of carbonate rocks. The DIC concentrations and the δ 13CDIC values show pronounced seasonal variations, with the lowest values being observed during the high flow season (from May to October). The logPCO2 values in the waters were positively correlated with the DIC contents and negatively correlated to the saturation index of calcite (SIc) and δ 13CDIC values. These observations indicate that CO2 derived from organic matter oxidation plays an important role in the dissolution of carbonate for this typical karstic environment. Based on a chemical mass balance, the weathering rate of carbonate rocks in the Houzhai catchment was estimated to be approximately 133t/km2/year or 584×103 mol/km2/year in terms of CO2 consumption rate, which is higher than the reported values for other rivers in Southwest China. [ABSTRACT FROM AUTHOR]

[Display omitted] • Significant temporal variations of carbon concentrations and δ13C DIC were observed. • Short-term climate variability plays an important role in carbon dynamics. • Biological carbon influxes should be responsible for the seasonal carbon dynamics. • The carbon-climate feedback is sensitive for the two rivers. Carbon-climate feedback is sensitive in the Qinghai-Tibet Plateau. A series of temporal measurements from Jinsha River and Yalong River, in conjunction with flow information, were used to study the carbon dynamics and predict future carbon fluxes under ongoing climate change. DIC and DOC concentrations showed considerable temporal variations, with low DIC and high DOC concentrations in the high-flow season, and vice versa. DIC and DOC concentrations had negative and positive relationships with runoff changes, respectively, showing the hydro-biogeochemical controls on carbon dynamics. With the increase of runoff, the accelerated chemical weathering and the high carbonate buffering capacity should be responsible for the chemostatic behaviors of DIC. Meanwhile, warm weather would enhance organic carbon degradation, and also thicken the active layer of permafrost in the source area, both of which would produce DOC. In addition, organic carbon degradation in the high-flow season would produce DIC with 13C-depleted values. δ13C DIC also had significant temporal variations, synchronous with runoff changes (i.e., light values under high runoff conditions), supporting that biological carbon plays an important role in carbon dynamics during the warm season. Based on the clear positive correlations between carbon fluxes and runoff, we predicted that the sensitivities of DOC fluxes to temperature changes are 12.2%/°C and 8.3%/°C for the Jinsha River and Yalong River, respectively. The sensitivities of DIC fluxes to temperature changes are much lower, which are 5.5%/°C and 6.1%/°C for the Jinsha River and Yalong River, respectively. This study sheds lights on the alpine riverine carbon cycling based on runoff-shifting concentration-isotope (q - C - I) relationships in the Qinghai-Tibet Plateau, which has implications on the understanding of climate forcing on carbon fluxes in alpine areas. [ABSTRACT FROM AUTHOR]

The effect of dams on dissolved inorganic carbon (DIC) transport and riverine ecosystems is unclear in karst cascade reservoirs. Here, we analyzed water samples from a karst river system with seven cascade reservoirs along the Wujiang River, southwestern China, during one hydrological year. From upstream to downstream, the average concentration of DIC increased from 2.2 to 2.6 mmol/L and its carbon isotope composition (δ13C DIC) decreased from −8.0 to −10‰. Meanwhile, the air temperature (Ta) increased from 20.3 °C to 26.7 °C and 10 °C to 13.7 °C in the warm and cold seasons, respectively. The results suggest that a cascade of dams has a stronger effect on DIC dynamics and retention than a single dam. The good correlation between Ta/HRT (hydraulic retention time) and Δ[DIC] as well as Δ[δ13C DIC ] mean that Ta and HRT affected the magnitude of the damming effect by altering changes in concentration of DIC and δ13C DIC in the reservoir compared to the inflowing water. In particular, daily regulated reservoirs with short retention times acted more like river corridors and had a smaller effect on carbon dynamics, so modulating retention time might be used reduce the effect of dams on the riverine ecosystem. Unlabelled Image • DIC concentrations and δ13C DIC trends were investigated across cascaded reservoirs. • Carbon dynamics in the reservoirs were mainly impacted by biological processes. • Damming effect is controlled by both hydraulic retention time and air temperature. • The damming effect can be weakened by regulating the hydraulic retention time. [ABSTRACT FROM AUTHOR]

• Water chemistry, δ13C DIC and δ34S SO4 were investigated in the Jialing River. • Sulfide oxidation played an important role in carbon cycling. • Hydrologic variability impacts chemical weathering and carbon cycling. The negative feedbacks between chemical weathering and climate change were hypothesized to regulate the atmospheric CO 2 over short-term and geologic timescales. In this study, we collected temporal water samples from the Jialing River, and analyzed water chemistry, stable isotopic compositions of dissolved inorganic carbon (δ13C DIC) and sulfur isotope of sulfate (δ34S SO4) to investigate the effects of hydrological changes on chemical weathering. The results of the inverse model based on the Monte Carlo method showed that major cations were mainly derived from carbonate (63.9%) and silicate (12.0%) weathering, and sulfuric acid plays an important role in carbon biogeochemical processes. Based on water chemistry, δ34S SO4 and δ13C DIC , the carbon biogeochemical processes were investigated, showing the influence of secondary processes. We estimated the weathering rates of carbonate (19.7 t/km2yr) and silicate (3.9 t/km2yr) in the Jialing River, and then estimated CO 2 consumption flux by carbonate weathering (24.5 t/km2yr), silicate weathering (8.1 t/km2yr), and total CO 2 consumption flux (27.5 t/km2yr) deducting the influence of pyrite oxidation. Our research showed the significant role of pyrite oxidation in carbon cycling. This study highlighted the impacts of hydrological variabilities on the generation and transport of solutes, which have significant implications for understanding carbon cycling under ongoing climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] • δ13C, δ15N, and δ18O were analyzed to reveal carbon–nitrogen coupled absorption. • Stability regulation mechanism of karst reservoir was indicated by diel monitoring. • The stability promotes the biological carbon pump development of karst reservoir. • Biological carbon pump caused carbon burial in karst reservoir. While river damming has been extensively investigated, there remains unclear in the diel biogeochemical processes and carbon control mechanisms in karst reservoirs, under the influence of the biological carbon pump (BCP) effect. We investigated the diel variation of Hongjiadu (HJD) and Wujiangdu (WJD) reservoirs, which exhibit typical periodic thermal stratification, in the Wujiang River located in Southwest China. We found that diel variation of dissolved inorganic carbon (DIC) (HJD: 1668.6 ± 10.4 µmol/L; WJD: 1777.9 ± 31.5 µmol/L), δ13C DIC (HJD: −3.1 ± 0.1 ‰; WJD: −4.6 ± 0.4 ‰), and other parameters in the lentic zones were relatively stable. Moreover, the Revelle factor in the lentic zones (HJD: 44.3-49.0; WJD: 44.3-53.9) were substantially higher than those of the fluvial zones (HJD: 24.4-33.4; WJD: 14.2-35.1). The stable diel variations indicate that the HJD and WJD reservoirs may have distinct stability regulatory mechanisms. In the lentic zones, δ13C DIC , nitrogen–oxygen isotope (δ15N and δ18O), and the correlation of C-N indicated that the conversion of DIC and NO 3 – is primarily influenced by primary production, and there was coupled absorption of carbon and nitrogen nutrients during photosynthesis. Meanwhile, the C/N weight ratio (6.0-10.1) and the δ13C POC (−30.0 ‰ to −24.7 ‰) also revealed a gradual transition from soil organic matter (OM) in the fluvial zone to phytoplankton source in the intermediate-lentic zone. Thus, two reservoirs had a significant BCP effect and converted a significant amount of DIC to autochthonous organic carbon (AOC) burial in the lentic zones. The BCP effect in subtropical deep karst reservoirs buried lots of carbon, increased carbon sequestration and decreased CO 2 emissions in the lentic zone. Simultaneously, the stability of reservoirs promotes BCP effect and intensifies carbon sequestration, which is of great significance for the study of the global carbon sources-sinks pattern. [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]

• Biological carbon influxes control the temporal dissolved inorganic carbon dynamics. • Dissolved organic carbon concentrations are highly affected by flushing effect. • Dissolved inorganic carbon dynamics in the Tibetan Plateau rivers play an important role in global carbon cycles. • Carbon concentrations and fluxes of the Tibetan Plateau rivers show high sensitivity to future global climate changes. Studying the temporal dynamics and fluxes of riverine dissolved carbon is crucial in understanding the regional and global carbon cycles under various climatic conditions. Here, we studied the behaviors and fluxes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) responding to various hydrologic conditions in the Min River, originated from Tibetan Plateau. The DIC concentrations decreased with increasing runoff, partially reflecting dilution effect, which may be ascribed to the shortened fluid transit time and then the reduced contact time with rocks. Nevertheless, DOC concentrations were positively correlated with runoff, which can be attributed to that a large amount of soil organic carbon flowed into the river as a result of the strong flushing effect. The negative relationship between δ13C DIC and runoff could be explained by soil CO 2 influx and organic matter degradation during the high flow season. ΔDIC (the production of DIC with changing hydrologic conditions) had a strong positive correlation with water temperature due to the accelerated DIC production rates by high temperature, which always co-varied with intense precipitation in Asian monsoonal regions. The mean DIC/DOC ratio in the Min River was 15.09, and the DOC and DIC fluxes were 1.1 and 15.2 t C km−2 yr−1, respectively, for the studied year. And the DOC and DIC fluxes varied dramatically with runoff changes, suggesting that hydrologic conditions were critical factors for the variations in dissolved carbon export. This study shows that carbon dynamics of rivers draining from the Tibetan Plateau are greatly affected by short-term climatic variabilities, which has implications for understanding global carbon cycle under future climate change. [ABSTRACT FROM AUTHOR]

• DIC and δ13C DIC showed strong spatiotemporal heterogeneities in karst river-reservoir system. • After damming, water chemical characteristics varied markedly in reservoir profiles. • The reservoir effect index (R i) can be used to reveal relevant biogeochemical processes. • Reservoir regulation can be optimized to reduce the damming effect based on R i. Cascade damming has seriously affected the hydrologic regimes and solutes transport of the river systems. However, the effect of damming on carbon cycle and the underlying mechanisms are still unclear. In this study, to reveal the effects of damming on riverine dissolved inorganic carbon (DIC) transport, the water chemistry and stable isotopic signature of dissolved inorganic carbon (δ13C DIC) were analyzed in the tributaries and four subtropical karst reservoirs on the Pearl River in southern China. From reservoir epilimnion to hypolimnion, DIC concentrations increased from 183.2 to 229.4 mg L−1, 167.6 to 192.0 mg L−1, 179.1 to 181.5 mg L−1, and 182.4 to 183.8 mg L−1 while δ13C DIC decreased from −6.6‰ to −9.7‰, −7.8‰ to −10.4‰, −9.7‰ to −10.0‰ and −9.9‰ to −10.0‰ in the Chaishitan (CST), Longtan (LT), Yantan (YT), and Dahua (DH) reservoirs, respectively. Our analyses suggest that primary production and organic matter degradation were two dominant processes in carbon evolution in the epilimnion and hypolimnion, respectively. However, there was no obvious spatial heterogeneity of solutes' concentration in the DH reservoir. Our results also suggested that cascade reservoirs had a regional damming effect and could be significantly weakened without damming or in daily regulated reservoirs. Furthermore, δ13C DIC had a significant positive relationship with the reservoir effect index (R i), which was affected by hydraulic retention time and water temperature in the water column of reservoirs and is useful to evaluate the thermal stratification strength. This study highlights that dam constructions affected carbon transport and transformation significantly, and the degree of the damming effect on the carbon cycle can be expressed by R i. [ABSTRACT FROM AUTHOR]