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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]

• 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]

Understanding the effects of hydrological processes on solute dynamics is critical to interpret biogeochemical processes. Water chemistry and isotopic compositions of surface water (δ18Ow and δDw) were investigated in rivers from Southwest China to study the effects of hydrological variability on biogeochemical processes. The inverse relationship between deuterium excess (d‐excess) and δ18Ow could be ascribed to non‐equilibrium fractionation processes, and the slope of the Local River Water Line was much lower than the Local Meteoric Water Line, suggesting the post‐precipitation evaporation pattern. The evaporation fraction (1–f) was estimated by the d‐excess method, varying from 0.01 to 0.18. (1–f), was a function of water temperature and drainage mean elevation, indicating that evaporation easily occurs at high temperatures in low‐elevation regions. The hydrological processes co‐varied with solute dynamics in the river network, and fluid transit time and temperature were likely responsible for the co‐variations. Also, we found that hydrological processes played an important role in solute dynamics through shifting the geochemical processes (e.g., enrichment, water‐rock reaction, photosynthesis, and secondary mineral precipitation). This study highlights that biogeochemical processes co‐vary with hydrological processes, and we suggest that investigating hydrological processes can help to understand biogeochemical processes. [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]

The occurrence, spatiotemporal distribution and ecological risks of 27 antibiotics in water and sediments from rivers and coastal area of Zhuhai, Pearl River estuary, south China were investigated. Higher concentrations of antibiotics were found in river water in dry season than those in wet season ( p  < 0.01), especially for quinolones (QNs) (6.36–463 ng/L) and aminoglycosides (AGs) (94.9–458 ng/L). In coastal water samples, the concentrations of antibiotics were up to 419 ng/L and 357 ng/L in dry season and wet season, respectively. Higher concentrations of antibiotics in coastal sediment samples were observed in wet season compared with those in dry season ( p  < 0.01). This may be ascribed to the greater discharge of antibiotics from mariculture and surface sediment flushing in wet season, leading to the accumulation of polluted sediments in the estuary. Redundancy analysis showed that the concentrations of antibiotics in water were correlated with biological/chemical oxygen demand, ammonia nitrogen, and/or total nitrogen (TN). In addition, sediment organic matter (SOC) and TN strongly affected the distribution of antibiotics in sediments. Ecological risk assessment based on risk quotients (RQs) indicated that most antibiotics in water samples posed insignificant risk to fish and green algae, and insignificant to medium risk to daphnid. [ABSTRACT FROM AUTHOR]

Feedback between hydrologic variations and chemical weathering is thought to play a crucial role in modulating global carbon cycling. The mechanisms associated with the impacts of hydrologic variations on solute sources and chemical weathering were evaluated by examining the relationships between river discharge and hydrochemistry based on high-frequency sampling of the Min River, which originates in the Himalayan-Tibetan region. Fluid transit times and flow pathways vary with changes in discharge, thereby affecting various biogeochemical processes. Although shorter transit times occur during the high-flow season than during the low-flow season, concentrations of chemical weathering products exhibit chemostatic behaviour (less variation than changes in discharge) in response to increasing discharge due to hydrologic flushing of minerals, which increases the amount of reactive mineral surface area. The contributions of various sources to dissolved loads in the Min River were estimated using a forward model. The calculated annual carbonate and silicate weathering fluxes are 24.1 and 9.6 t/km year, respectively. Atmospheric contributions increase with increasing discharge, whereas the contributions of silicate weathering decrease with increasing discharge. Both the carbonate weathering flux (F) and silicate weathering flux (F) are positively correlated with the discharge, indicating that temporal variations in chemical weathering fluxes in the Min River are highly affected by hydrologic variations. The slope of the relationship between F and discharge is much greater than that between F and discharge due to the rapid dissolution of carbonate minerals, suggesting that carbonate weathering is more sensitive than silicate weathering to hydrologic variations. This study demonstrates that high-frequency sampling is necessary when investigating solute sources and chemical weathering processes in river basins influenced by a monsoon climate. [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]

Highlights: [•] Water chemistry and isotopes (δ34S, δ18O, δ13C) reveal the weathering process. [•] 72% of sulphate derived from sulphide oxidation in the Yalong River. [•] Dynamics of DIC affected by carbonate weathering and equilibration processes. [•] Sulphuric acid dominate carbonate weathering in the cold and dry zone with high elevation. [ABSTRACT FROM AUTHOR]

Hydrochemical behavior and dissolved carbon dynamics are highly-sensitive to hydrological variations in the monsoon-influenced karstic critical zone which has high chemical weathering rates and experiences strong anthropogenic impact. Continuous high-frequency monitoring in the spring outlet of a karstic catchment in Southwestern China revealed that most hydrochemical variables changed distinctively in response to hydrologic variations, influenced by mixing of different sources and miscellaneous biogeochemical processes. Na+, K+ and SO 4 2− varied significantly with hydrology, showing weak chemostatic behavior controlled by dilution. The flushing effect and random behavior of NO 3 − and Cl− likely reflect agricultural inputs from high throughflow. Soil CO 2 in infiltrated water supports carbonate weathering, enabling DIC (dissolved inorganic carbon) and weathering products (e.g., Ca2+ and Mg2+) to maintain chemostatic behavior. Biogenic DIC exhibited a stronger chemostatic response than carbonate sources and was the foremost control in DIC behavior. Carbon exchange between DIC and DOC (dissolved organic carbon) did not significantly influence DIC concentration and δ13C due to very low DOC concentration. More DOC was exported by flushing from increasing discharge. Hysteretic analysis indicated that the transport processes were controlled by proximal sources mixing and diverse mobilization in various periods responding to rainstorms. NO 3 − and Cl− presented different hysteresis behavior as sourced from agricultural activities. DOC increased on the hydrograph rising limb and was controlled by a transport-limited regime. However, the hysteresis behavior of most weathering products and DIC were regulated by a process-limited regime in the karstic critical zone. Overall, biogeochemical processes, hydrogeological properties, storm intensity/magnitude and the timing of storms (antecedent conditions) are main factors influencing the response of hydrochemical variables and dissolved carbon to storm events. Unlabelled Image • High-resolution data reveal the chemostatic behavior of HCO 3 −, Ca2+ and Mg2+. • NO 3 − and Cl− are affected by throughflow flushing and show randomness behavior. • Biologically-derived DIC controls DIC behavior during high discharge condition. • DOC and DIC are regulated by transport-limited and process-limited, respectively. • Hydrogeology, storm intensity and antecedent conditions affect solutes behavior. [ABSTRACT FROM AUTHOR]

• Nitrogen accumulated in dry condition and was flushed into river by rainfall events. • Nutrient dynamics are dependent on interaction of hydrology with source availability. • Nitrogen transformation processes are more active under the wet conditions. • Scientific fertilization and planning of cropping are advised for water NO 3 − control. High discharge associated with rainfall events causes rapid transport of nutrients to drainage systems in karst terrain. The objective of this study was to understand nitrate dynamics and transport mechanisms during rainfall events in a typical mixed land-use karst catchment in Southwest China. High frequency (hourly) water sampling, discharge and hydrochemistry sensor data at a 15-minute interval were collected from a karst agricultural surface river over four rainfall events, spanning 40 days at the commencement of the wet season. The results showed that high [NH 4 +–N] (up to 3.12 mg L−1) and [PO 4 3−– P] (up to 1037 μg L−1) were observed in the first rainfall event of the wet season, after a long preceding dry period. Increased [NO 3 −–N] also occurred during the high discharge associated with early event flows, but the highest [NO 3 −–N] concentrations (up to 14.9 mg L−1) were observed later in the season during the highest discharge events. All minima for dual nitrate isotopes were found during high discharge conditions and increased through the event time series. This indicates that nitrogen accumulated in the catchment prior to the commencement of the wet season and was flushed by the early rainfall events into the drainage system. Analysis of [NO 3 −–N], δ 15N and δ 18O-NO 3 − values indicate that denitrification, not assimilation, resulted in the high nitrate isotope values before and after rainfall events. According to source apportionment, chemical fertilizer, soil organic nitrogen, and manure and sewage, which varied with discharge, were the main sources of nitrate during rainfall events. The high proportion of nitrate from soil organic nitrogen suggested that soil loss from peak clusters and lowland farmland during heavy rainfall events should be paid more attention in karst area. [ABSTRACT FROM AUTHOR]

Investigating weathering processes in landslide-prone areas is important for understanding the carbon budget due to weathering during mountain building. Here, we present the monthly hydrochemistry and sulfur isotopic compositions of SO 4 2− (δ34S–SO 4 2−) in the Bailong River which drained landslide-prone areas in the northeastern Qinghai-Tibet Plateau. Ca2+ and HCO 3 − were dominant in cations and anions respectively, and δ34S–SO 4 2− values varied over a narrow range, from 5.1‰ to 6.7‰. We constrained the sources of major cations and SO 4 2− through an inverse model and the mass balance model of δ34S–SO 4 2−. The results showed that cations in the Bailong River were mainly from carbonate weathering, and sulfide oxidation was an important source of SO 4 2−. Weathering in the Bailong River catchment showed an CO 2 source over geologic timescales due to the reaction of H 2 SO 4. The relationship between weathering fluxes and physical erosion rate indicated that high runoff facilitated physical erosion, and in turn accelerated chemical weathering. However, under the conditions of extreme discharge and physical erosion, SO 4 2− fluxes from sulfide oxidation were not as high as expected, which was ascribed to the transport of old landslide deposits where most sulfides had been exhausted. Therefore, in tectonically active areas with massive landslides, the high erosion rate caused by landslides promoted chemical weathering, but sulfide oxidation did not respond sensitively to strong physical erosion under high runoff conditions. This study helped to understand the relationship among landslides, chemical weathering, and geologic carbon cycle. • Carbonate weathering is the main contributor to dissolved cations in the Bailong River. • Sulfide oxidation is the main source of SO 4 2− in the Bailong River. • High runoff facilitated physical erosion, and in turn accelerated chemical weathering. [ABSTRACT FROM AUTHOR]

Abstract: The dual isotopic compositions of dissolved in aquatic systems are commonly used to ascertain sources and possible biogeochemical processes. In this study, the physical parameters, major anions and isotopic compositions of in water samples from Changjiang River (Nanjin) to the East Sea in Changjiang Estuary were determined. The salinity ranged from 0‰ to 32.3‰ in the estuary water samples. The concentrations and δ18O–H2O values followed the salinity variations from freshwater to seawater, which indicated that mixing processes might be a major factor involved in the distribution of water and solutes. The contents and isotopic compositions of suggested that atmospheric deposition, evaporite dissolution and sulphide oxidation were the major sources of dissolved in the freshwater of Changjiang River. In addition, the mixing model calculated by contents and isotopic compositions of indicated that the mixing of freshwater and sea water was the major factor involved in distribution in Changjiang Estuary. However, slightly elevated δ18O–SO4 values were observed in the turbidity maximum zone, which suggested that biological processes might affect the O isotopic compositions of there. [Copyright &y& Elsevier]

• Seawater intrusion and weathering determined water quality of coastal zone (CZ). • Dam regulation in CZ rivers affected water chemical components and nitrogen cycle. • N fate in the weak coastal hydrological zone rivers was driven by microorganisms. • Sewage and manure sources were major nitrate contributors in the studied CZ. Coastal zones (CZ) are ecologically and environmentally significant, and it is thus critical to study the interactions among different types of natural CZ waters for better understanding biogeochemical processes. Water samples (n = 101) were analyzed for multiple stable isotopes (δ18O-H 2 O, δD-H 2 O, δ15N-PON, δ15N-NO 3 −, and δ18O-NO 3 −) in seawater, river water, reservoir water and groundwater at Tianjin CZ to demonstrate the spatial variations in the CZ aqueous environment and to identify key influencing processes factors of the N geochemical cycle. This study confirmed that weathering dominated upstream water properties, whereas seawater intrusion determined the downstream and midstream water chemical components, where the seawater fraction was 44% and 11%, respectively. Evaporation and precipitation processes were active across the whole CZ area, but their impacts on water chemical components were less significant than seawater intrusion and weathering. Dam regulation in CZ rivers has significantly affected water chemical components and the nitrogen cycle. The isotopic evidence indicated that bacterioplankton and phytoplankton were the primary forms of PON. The dual nitrate isotopes revealed that animal manure and industrial sewage contributed the leading nitrate to river water. The spatial variation of their contribution was quantitated (from upstream to midstream and downstream: 62%, 91% and 83%, respectively). Considering the potential isotopic fractionation of nitrification reduction from upstream to downstream, 39%, 61% and 57%, respectively. This study proposes a quantitative framework for detecting CZ areas with similar hydrodynamic conditions and climate characteristics found at the Tianjin CZ, which has important implications for CZ water management and environmental protection policies. [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]

Microcystins are frequently detected in cyanobacterial bloom-impacted sites; however, their mobility potential in soils is poorly understood. This study aimed to elucidate the sorption behaviors of microcystin-RR (MC-RR) in heterogeneous soils and evaluate critical affecting factors. MC-RR sorption followed the pseudo-second-order kinetics and Freundlich model. All isotherms (n = 0.83–1.03) had no or minor deviations from linearity. The linear distribution coefficients (K d) varied from 2.64 to 15.2 across soils, depending mainly on OM and CEC. Stepwise regression analysis indicated that the K d was predictable by the fitting formula of: K d = 2.56 + 0.15OM + 0.28CEC (R 2 = 0.45). The sorption was an endothermic physisorption process, involving electrostatic forces, cation exchange and bridging, H-bonding, ligand exchange, and van der Waals forces. The sorption of MC-RR (dominantly behaved as electroneutral zwitterions) at pH > 5 was insensitive to pH change, while more MC-RR (anionic species) was adsorbed at lower pH and in the presence of Ca2+. The study provides insights into the sorption of MC-RR across a range of soil properties and water chemistry for the first time, which is of importance for a better understanding of the mobility potential of microcystins in the terrestrial systems. [Display omitted] • MC-RR sorption onto soils follows the pseudo-second-order kinetics and Freundlich isotherms models. • Distribution coefficients for MC-RR varied across soils, depending mainly on OM and CEC. • Water chemistry complicates the sorption via influencing the chemical species and confirmation of MC-RR. • The favorable sorption occurs at lower pH and in the presence of Ca2+. [ABSTRACT FROM AUTHOR]

Summary To understand the sources and transformations of nitrate in the Songhua River basin, which is one of seven largest river basins in China, the concentration of dissolved nitrogenous species, nitrogen and oxygen isotopes of NO 3 − , nitrogen isotopes of NH 4 + , and stable isotopes of water were determined in this study. Low NO 3 − concentrations and a high dissolved organic nitrogen/total dissolved nitrogen ratio (DON/TDN) were observed in the Nen River and other rivers originating from the mountains, which are covered by forest. NO 3 − and DON were the major nitrogenous compounds in aquatic systems, accounting for the TDN being about 90% during high flow season and about 85% during low flow season, respectively. The nitrogen efflux for the entire basin was estimated to be approximately 1.17 × 10 5 tons/yr, which represents an annual N output of 0.21 ton/(km 2 yr). The majority of the δ 18 O-NO 3 − values were between −4‰ and 4‰, reflecting nitrification. During the high flow season, the isotopic compositions of NO 3 − and the water chemistry suggest that NO 3 − in the Nen River was mainly derived from soil organic nitrogen (SON), whereas NO 3 − in the Songhua River originated from organic nitrogen, nitrogenous fertilizers and sewage waters. NO 3 − in the low flow season samples generally originates from SON and sewage waters. Moreover, the calculated loss of nitrate via the mass budget in rivers, together with isotopic values and water chemistry confirm that denitrification occurs during the high flow season, especially in the Songhua River. This study suggests that the mass calculation and isotopic proof provide a better understanding for riverine N budget and biogeochemical processes. [ABSTRACT FROM AUTHOR]

Abstract: For the first time, the river water geochemistry of the Songhuajiang Basin, the third largest river system in China, has been studied in order to understand chemical weathering and associated CO2 consumption rate in a mid- to cool temperate and monsoon-controlled climatic zone. The major ion compositions of the river waters are characterized by the dominance of Ca2+ and , accounting for 46% and 74% of major cations and anions, respectively. The average total dissolved solids (TDS, 116mgL−1) and total cation concentration (TZ+, 1388μEqL−1) of the river waters are similar to those of global major rivers. The chemical weathering rates of carbonate, silicate and evaporites in the whole Songhuajiang Basin are estimated to be approximately 5.15, 2.23 and 0.40tkm−2 a−1, respectively. The total rock weathering rate for the whole Songhuajiang Basin is approximately 7.78tkm−2 a−1, which is at the lower end of the spectrum for global major rivers, and is comparable with that of the Amur and the Congo-Zaire River. The estimated CO2 consumption rates for the whole Songhuajiang Basin are 53.4×103 molkm−2 a−1 and 66.6×103 molkm−2 a−1 by carbonate and silicate weathering, respectively. As a sub-basin, the 2nd Songhuajiang has the highest (18.9tkm−2 a−1) while the Nenjiang River Basin has the lowest total rock weathering rate (5.03tkm−2 a−1), which indicates important controls of regional climate and lithology. Compared to the large rivers of China, total rock weathering rates increase from north to south, supporting the idea that the climate acts as major control on global chemical weathering. The CO2 consumption rate by silicate weathering within the whole Songhuajiang River Basin, though under mid- to cool temperate climate, is in the same order of magnitude as that of Huanghe (82.4×103 molkm−2 a−1) and is not much lower than that of Changjiang (112×103 molkm−2 a−1), which suggests that the role of Songhuajiang River weathering in long-term climate change cannot be neglected compared to those of the large rivers of China. [Copyright &y& Elsevier]

Abstract: This paper presents hydrogeochemical and S isotopic characteristics of karstic surface and ground water in the karstic aquifers of Zunyi city of Guizhou Province, SW China. The average δ34S value of SO4 is +2.2‰ (n =28) in ground waters and +7.0‰ (n =15) in surface waters in the low flow season, while in the high flow season it is +1.0‰ in ground waters and +2.8‰ in surface waters. Natural and anthropogenic sources of the dissolved can be discriminated by a combined approach using water chemistry and stable S isotope data. A pollutant source of SO4 is estimated to have very high TDS contents and a narrow range of δ34S values (0‰ to +5.0‰). The water with a distinctively high δ34S value of +30.5‰ originates from an evaporite (gypsum and anhydrite)-bearing aquifer, and the δ34S values lower than −10‰ suggest an origin by oxidation of sulfide minerals in coal seams with intermediate TDS contents. [Copyright &y& Elsevier]

Abstract: Rapidly increasing populations, and associated intensification of agriculture, urbanization and industrialization, place increased demands on water resources and increased likelihood of pollution in many areas of the world. The Shuicheng Basin, southwestern China, is one such area and in order to understand water–rock interactions (carbonate dissolution) and anthropogenic impacts on groundwater quality in this karstic landform-dominated area, chemical as well as C, Sr and S isotopic compositions of groundwater (spring and well) and surface water (river) were measured during and following rainy seasons. The concentrations of various ions in groundwater were characterized by the dominant cations (Ca2+, Mg2+) and anions (, ), which account for more than 80% of the total ion concentration. Strontium isotope ratios (87Sr/86Sr, 0.70760–0.70918, mean 0.70831) and δ 13CDIC (−14.2‰ to −8.4‰, mean −10.7‰) indicate that the weathering dissolution of limestone controls Ca2+ and . The decrease in δ 13CDIC values with increasing concentrations of anthropogenic species (Cl−, and Na+) shows that the C isotopic composition of DIC can be a useful tracer of contaminants. Chemical compositions, hydrogeological conditions and δ 34S analyses showed increasing concentration, resulting from domestic wastewater, fertilizer applications, atmospheric inputs through coal combustion, and oxidation of sulfide minerals, which typically are abundant in coal formations in the basin. Groundwater from the old downtown, industrial and agricultural areas exhibited high concentrations of Cl−, , and Na+, suggesting the groundwater is impacted by significant levels of contamination from human activities. [ABSTRACT FROM AUTHOR]

• Large amounts of water and dissolved inorganic carbon were lost in the rivers of Hainan Island. • Significant evaporation and primary production co-existed in these tropical rivers. • Intensive in-stream hydrological and biogeochemical processes mainly occurred in headwater segments. Rivers play an important role in global water and carbon cycling, but there are still large uncertainties concerning evaporation and aquatic photosynthesis. Here we combined measurements of water chemistry, isotopic compositions (i.e., δ D w , δ 18 O w , δ 13 C DIC and ▵ 14 C DIC) and geographic characteristics (i.e., river width) to elucidate in-stream hydrological and biogeochemical processes across rivers in Hainan Island, China. The results showed that dissolved inorganic carbon (DIC) in river waters was largely of modern origin, with about 95% from contemporary biogenic sources based on an isotopic mass balance of ▵ 14 C DIC. Significant evaporation and aquatic primary production co-occurred in these tropical rivers with large amounts of water and DIC being rapidly turned over in the water column, altering the water cycle and the carbon balance. High rates of evaporation and aquatic primary production were observed in the headwater segments, with narrow river width but broad available reactive surface area at the air-water interface. The asymmetric aquatic photosynthesis at different river segments caused the spatial heterogeneities of dissolved solutes. The results suggest that the available reactive area at the water-air interface is responsible for synchronous water loss and dissolved carbon evolution in flat tropical rivers. This study provides evidence that intense evaporation and aquatic photosynthesis mainly occurred in headwater segments, which has implications for understanding global carbon cycling. [Display omitted] [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]