High-frequency dynamics of CO2 emission flux and its influencing factors in a subtropical karst groundwater-fed reservoir, south China.

Revealing the magnitude, dynamics, and influencing factors of CO2 emissions across the water-air interface in karst water with high frequency is crucial for accurately assessing the carbon budget in a karst environment. Due to the limitations of observation methods, the current research is still very insufficient. To solve the above problems and clarify the main influencing factors of CO2 emission in karst water, this study selected Dalongdong (DLD) Reservoir, located in the typical karst peak and valley area in southwest China, to carry out a multi-parameter high-frequency monitoring study from January to December 2021, and used the thin boundary model method to estimate the CO2 flux across the water-air interface (CF). The average annual flux of DLD reservoir is 84.48 mmol·(m2·h)-1, which represents a CO2 source overall. However, during the stratification period in August, there is a transient carbon sink due to negative CO2 emission. The alteration of thermal stratification in water is crucial in regulating the seasonal variation of CF. Meanwhile, the diurnal variation is significantly influenced by changes in hydrochemical parameters during the thermal stratification stage. Compared to low wind speeds (<3 m/s), high wind speeds (≥3 m/s) have a greater impact on the CO2 flux. Furthermore, high-frequency continuous data revealed that the reservoir triggered a CO2 pulse emission during the turnover process, primarily at night, leading to unusually high CO2 flux values. It is of great significance to monitor and reveal the process, flux, and control factors of CO2 flux in land water at a high-frequency strategy. They will help improve the accuracy of regional or watershed carbon budgets and clarify the role of global land water in the global carbon budget.

Aquatic photosynthetic carbon pump driven by periodic temperature variations affects dissolved inorganic carbon and hydrochemical characteristics in a groundwater-fed reservoir.

Under the influence of periodic temperature variations, biogeochemical cycling in water bodies is markedly affected by the periodic thermal stratification processes in subtropical reservoirs or lakes. In current studies, there is insufficient research on the influence and mechanism of dissolved inorganic carbon (DIC) distribution in karst carbon-rich groundwater-fed reservoirs under the coupled effects of thermal structure stratification and the biological carbon pump (BCP) effect. To address this issue, the Dalongdong (DLD) reservoir in the subtropical region of southern China was chosen as the site for long-term monitoring and research on relevant physicochemical parameters of water, DIC, and its stable carbon isotope (δ13CDIC), CO2 emission flux, as well as the reservoir's thermal stratification index. The results show that: (1) the DLD reservoir is a typical warm monomictic reservoir, which exhibits regular variations of mixing period-stratification period-mixing period on a yearly scale due to thermal structure changes; (2) DIC was consumed by aquatic photosynthetic organisms in the epilimnion during the stratification period, leading to a decrease in DIC concentration, partial pressure of CO2 (pCO2) and CO2 emission flux, and an increase in stable carbon isotope (δ13CDIC). During the mixing period, the trend was reversed; (3) During the thermal stratification, aquatic photosynthesis and water temperature were the primary factors controlling DIC variations in both the epilimnion and thermocline. Regarding the hypolimnion, calcite dissolution, organic matter decomposition, and water temperature were the dominant controlling factors. These results indicate that although carbon-rich karst groundwater provides a plentiful supply of DIC in the DLD reservoir, its availability is still influenced by variations in the reservoir's thermal structure and the metabolic processes of aquatic photosynthetic organisms. Therefore, to better estimate the regional carbon budget in a reservoir or lake, future studies should especially consider the combined effects of BCP and thermal structure variations on carbon variations.

Accumulation in nutrient acquisition strategies of arbuscular mycorrhizal fungi and plant roots in poor and heterogeneous soils of karst shrub ecosystems.

BACKGROUND: Arbuscular mycorrhizal (AM) fungi and roots play important roles in plant nutrient acquisition, especially in nutrient poor and heterogeneous soils. However, whether an accumulation strategy of AM fungi and root exists in such soils of karst shrubland ecosystems remains unclear. Root traits related to nutrient acquisition (root biomass, AM colonisation, root acid phosphatase activity and N2 fixation) were measured in two N2-fixing plants (i.e. Albizia odoratissima (Linn. f.) Benth. and Cajanus cajan (Linn.) Millsp.) that were grown in heterogeneous or homogeneous nutrient (ammonium) soil with and without AM fungi inoculation. RESULTS: Both of these plants had higher AM colonisation, root biomass and relative growth rate (RGR), but lower N2 fixation and root acid phosphatase activity in the rhizosphere in the heterogeneous soil environment, than that in the homogeneous soil environment. Plants grown in the AM fungi-inoculated heterogeneous soil environment had increased root biomass and root acid phosphatase activity compared with those grown in soil without inoculation. AM colonisation was negatively correlated with the N2 fixation rate of A. odoratissima, while it was not significantly correlated with the root phosphatase activity. CONCLUSIONS: Our results indicated that enhanced AM symbiosis and root biomass increased the absorptive surfaces for nutrient acquisition, highlighting the accumulation strategies of AM and root traits for plant nutrient acquisition in nutrient poor and heterogeneous soils of the karst shrubland ecosystem.

A comprehensive conceptual framework for signaling in-lake CO2 through dissolved organic matter.

Organic carbon (C) and CO2 pools are closely interactive in aquatic environments. While there are strong indications linking freshwater CO2 to dissolved organic matter (DOM), the specific mechanisms underlying their common pathways remain unclear. Here, we present an extensive investigation from 20 subtropical lakes in China, establishing a comprehensive conceptual framework for identifying CO2 drivers and retrieving CO2 magnitude through co-trajectories of DOM evolution. Based on this framework, we show that lake CO2 during wet period is constrained by a combination of biogeochemical processes, while photo-mineralization of activated aromatic compounds fuels CO2 during dry period. We clearly determine that biological degradation of DOM governs temporal variations in CO2 rather than terrestrial C inputs within the subtropical lakes. Specifically, our results identify a shared route for the uptake of atmospheric polycyclic aromatic compounds and CO2 by lakes. Using machine learning, in-lake CO2 levels are well modelled through DOM signaling regardless of varying CO2 mechanisms. This study unravels the mechanistic underpinnings of causal links between lake CO2 and DOM, with important implications for understanding obscure aquatic CO2 drivers amidst the ongoing impacts of global climate change.

Transport and transformation of dissolved inorganic carbon in a subtropical groundwater­fed reservoir, south China.

Most reservoirs in subtropical areas experience periodic variations in the thermal structure of their water columns, with times of strong thermal stratification being succeeded by periods of mixing, over the course of the year. Understanding of the transport and transformation of dissolved inorganic carbon over such thermal cycles in artificial reservoirs remains poor. To address this problem, this study examined the spatiotemporal behavior of dissolved inorganic carbon (DIC), the partial pressure of CO2 (pCO2), carbon isotope ratios (δ13CDIC), and CO2 emission (FCO2), from 2014 to 2018 in a subtropical, groundwater-fed reservoir in southern China. It was found that CO2 emissions during mixing periods are much higher than in thermally stratified periods (particularly during transition from stratified to mixing) as a result of upwelling and release of dissolved CO2 (CO2aq) accumulated in the hypolimnion. CO2 emission fluxes at the water-gas interface accounted for only a small proportion of the DIC in the reservoir. The relationships between of DIC and δ13CDIC displayed two distinct modes, due to spatial differences in water depths and to strong thermal stratification during warmer seasons: (1) DIC concentrations increase and δ13CDIC values decrease from epilimnion to hypolimnion, and (2) δ13CDIC values decrease with increasing DIC concentrations but δ13CDIC is progressively enriched near the bottom during periods of thermal stratification. In addition, this study found three distinct processes of DIC accumulation and consumption in the reservoir: (1) DIC accumulated in the hypolimnion during thermal stratification periods, due to carbon retention but (2) DIC was substantially consumed in the epilimnion during such periods, and (3) average DIC concentrations and pCO2 increased significantly from upstream to downstream along the reservoir, while average δ13CDIC values became lighter. These results highlight that carbon behavior in groundwater­fed reservoirs is often controlled by a combination of biogeochemical processes and seasonal variations in thermal structure. Sampling and monitoring strategies should consider these factors in order to accurately estimate carbon budgets in reservoirs, lakes or ponds.

Physical and chemical control on CO2 gas transfer velocities from a low-gradient subtropical stream.

CO2 exchanges across the water-air interface in rivers and lakes are currently believed to be responsible for the dominant share of global aquatic CO2 emissions. The gas transfer velocity (k600) is the key factor that constrains the CO2 fluxes. It is also the most problematic to establish because of its high spatial and temporal variability. Here, we have evaluated the seasonal and spatial dynamics in k600 values and their physical and chemical controlling processes by gas tracer and floating chamber (FC) methods in three reaches of a low-gradient stream channel (Guancun surface stream, 'GSS') in a karst terrain in subtropical southwestern China in December 2016 and March, July and September 2017. The k600 values were highly variable in space and time in this small stream. Physical processes, including the velocity of the stream and its slope, were found to control the variations of k600. The k600 values recorded in the dry season (March and December) were at minimal levels due to very slow flow and gentle slope, and were also affected by complexation in the solute-enriched waters. The characteristics high pH and low turbulence of gentle streams in carbonate karst areas are conducive to such complexation, which is of great significance in the limiting CO2 degassing in such regions. We have obtained the first k600 prediction model for small streams in subtropical karst regions. In conclusion, we present a comprehensive approach for predicting the k600 values in small channels by comparison of independent SF6 gas tracer and floating chamber methods.

Dynamics in riverine inorganic and organic carbon based on carbonate weathering coupled with aquatic photosynthesis in a karst catchment, Southwest China.

Carbonate mineral weathering coupled with aquatic photosynthesis, herein termed 'coupled carbonate weathering' (CCW), represents a significant carbon sink which is determined by riverine hydrochemical variations. The magnitudes, variations and mechanisms responsible for the carbon sink produced by CCW are still unclear. In this study, major ions, TOC and discharge data at the Darongjiang, Lingqu, Guilin and Yangshuo hydrologic stations in Li River basin, a karst catchment typical of this geographic region, were analysed from January 2012 to December 2015 to elucidate the temporal variations in riverine inorganic and organic carbon and their controlling mechanisms. The results show that (1) HCO3- was sourced from carbonate weathering and silicate weathering, carbonate weathering by carbonic acid being predominant; (2) TOC was created chiefly by the transformation of bicarbonate to organic carbon by aquatic phototrophs during the non-flood period; (3) The carbon sink produced by coupled carbonate weathering in the Li River basin was calculated to be 14.41 tC·km-2·yr-1, comprised of the sink attributable to carbonate weathering (12.17 tC·km-2·yr-1) and sink due to the "biological carbon pump" (SBCP) (2.24 tC·km-2·yr-1). The SBCP thus accounted for approximately 15.54% of the total carbon sink, indicating that the proportion of riverine TOC sourced by the transformation from bicarbonate to organic carbon by aquatic phototrophs may be high and must be considered in the estimation of carbonate weathering-related carbon sinks elsewhere.

Varying thermal structure controls the dynamics of CO2 emissions from a subtropical reservoir, south China.

Thermal stratification and mixing are important to the physicochemical composition of reservoirs and lakes and impact their water quality and biogeochemical cycles. However, it remains unclear how thermal stratification and mixing process control the exchange of CO2 between surface water and the Earth's atmosphere. To address this issue, we examine the temporal characteristics of some physicochemical parameters, partial pressure of CO2 (pCO2), the δ13CDIC, and CO2 emission from a typical karst groundwater-fed reservoir (Dalongdong reservoir). During the 23 month study (2016-2018) thermal stratification limited CO2 emission, in part from photosynthetic uptake of CO2, from early April to late October, while mixing processes stimulated CO2 emission of CO2 generated from organic matter remineralization in bottom water from October to April. The Dalongdong reservoir is an atmospheric source of CO2 for most of the study period; however, during periods of stratification, approximately 0.37 ± 0.44 Gg CO2 (1 Gg = 109g) dissolved into the water from the atmosphere, while approximately 6.24 ± 3.73 Gg CO2 was lost to the atmosphere during periods lacking stratification. Limited emissions during stratified period may thus represent a negative feedback to CO2 contributions to global warming, which has increased lengths of stratified periods. These study results are important to optimize sampling monitoring strategies to reduce errors of regional CO2 emission estimation.

Integrated understanding of the Critical Zone processes in a subtropical karst watershed (Qingmuguan, Southwestern China): Hydrochemical and isotopic constraints.

Improving the management and protection of karst groundwater resources and addressing karst-related environmental and ecological problems still face challenges raised from the limited knowledge on the entire karstic Critical Zone (K-CZ), including soil, epikarst, the vadose and saturated zones. Particularly, there is still a lack of integrated understanding of K-CZ properties and major CZ processes across space and time. In this study, we measured and analyzed the hydrochemical and multiple stable isotopic compositions of soil water, surface- and groundwaters from various compartments of the K-CZ in a typical subtropical karst watershed - Qingmuguan (QKW), Southwestern China, in order to explore the source and spatiotemporal variations of water and solutes (C, N, S) within the K-CZ; thereby elucidating the hydrological and biogeochemical processes and their affecting factors. The results show that (i) the K-CZ of QKW is characterized by high heterogeneity and permeability, with fast and strong hydrologic variations in response to rainfall variability; (ii) water-CO2­carbonate interactions (i.e. carbonate weathering) are remarkably active in different zones and are significantly modulated by hydrologic dynamics and seasonal change in biological activities; (iii) efficient migration of nitrate and sulfate occurs across the surface toward the saturated aquifer zone, which is affected by the source availability, elemental transformation and flow transport processes; (iv) human activities have clearly influenced groundwater quality and the natural K-CZ processes, for example, exogenic acids of anthropogenic origin (e.g. acid precipitation and nitrogenous fertilizers from crop lands) have been proven to be involved in the carbonate weathering, with a contribution of ~20%-30%. Our study highlights the strong coupling of hydrological and various biogeochemical processes and the interactive connection among various layers of K-CZ. Thus, systematical monitoring along the CZ profile and a process-based dynamic approach to elucidating climatic and anthropogenic forcing are necessary to better understand the K-CZ properties and functions.

High spatial and seasonal heterogeneity of pCO2 and CO2 emissions in a karst groundwater-stream continuum, southern China.

Accurate quantification of the emission of CO2 from streams and rivers is one of the primary challenges in determining the global carbon budget because our knowledge of the spatial and seasonal heterogeneity on these CO2 emissions is limited. In karst areas, the groundwater-stream continuum is likely ubiquitous because the carbon-rich groundwater discharges into some of the streams through springs or subterranean streams, which results in more complex spatial and seasonal variations in the CO2 emissions. To address this issue, the spatial and seasonal characteristics of partial pressure of CO2 (pCO2), the δ13CDIC, and the CO2 emission flux of the Guancun surface stream (GSS) karst groundwater-stream continuum in southern China were investigated from the stream head (groundwater outlet) to the downstream mouth during the 2014-2017 period. Our results reveal that the pCO2 and CO2 emissions exhibit high spatial and seasonal heterogeneities over ~ 1300 m in the GSS. Spatially, the pCO2 and CO2 emissions decrease sharply from the stream head (mean 8818.4 µatm for pCO2 and mean 423.4 mg m-2 h-1 for CO2 emission) to the site farthest downstream (mean 2752.7 µatm for pCO2 and 257.0 mg m-2 h-1 for CO2 emission). Except for the dates when extreme rainfall occurred, the pCO2 and CO2 emission values were higher in the rainy season than in the dry season. This suggests that in a groundwater-stream continuum, CO2 emission occurs very soon after the water is transferred from the karst groundwater to the surface water. We estimate that the total amount of CO2 released to the atmosphere from the GSS is 21.75 t CO2/year, which is only 1.71-5.62% of the dissolved inorganic carbon loss flux in the GSS during the study period. It is important to note that the measured CO2 emission and pCO2 levels decrease farther downstream, so carbon loss is underestimated when it is calculated using downstream sampling points. Therefore, accurate assessments of the CO2 emission flux need to take into consideration the high spatio-temporal heterogeneity in order to reduce the bias of the entire CO2 emission flux.

Diel-scale variation of dissolved inorganic carbon during a rainfall event in a small karst stream in southern China.

Metabolic processes of the submerged aquatic community (photosynthesis and respiration) play important roles in regulating diel cycles of dissolved inorganic carbon (DIC) and sequestering carbon in a karst stream. However, little is known of whether diel DIC cycling occurs during rainfall in a karst groundwater-fed stream, even though this question is critical for the accurate estimation of what may be a major terrestrial carbon sink. Here, we measured diel variations of water chemical composition in a small karst groundwater-fed stream in southwest China during a rainfall event to assess the influences of rainfall and rising discharge on DIC diel cycling and the potential carbon sink produced by in-stream metabolism. Our results show that water chemical composition at the source spring (CK site) is relatively stable due to chemostatic behavior during rising discharge after a rainfall period. This site lacked submerged aquatic vegetation and, thus, had no diel variations in water chemistry. However, diel cycles of all hydrochemical parameters occurred at a site 1.3 km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+, and pCO2. These results indicated that diel cycling of DIC due to in-stream metabolism of submerged aquatic community was still occurring during elevated discharge from rainfall. We estimate the carbon sink through the in-stream metabolism of the submerged aquatic community to be 5.6 kg C/day during the studied rainfall event. These results imply that submerged aquatic communities in a karst stream can significantly stabilize carbon originating from the carbonate rock weathering processes in karst areas.

Vertical migration from surface soils to groundwater and source appointment of polycyclic aromatic hydrocarbons in epikarst spring systems, southwest China.

Understanding the transfer process of polycyclic aromatic hydrocarbons (PAHs) in the karst terrain is of great importance to their ecological risk assessments, however, the impact of the vertical transfer of the soil PAHs on the underground water is largely unknown in the karst system. Here, the vertical distribution and the seasonal variation of 16 PAHs in the soils and the water of 4 epikarst spring catchments in Southwest China were investigated. The total concentration of the PAHs ranged within 61-3285 ng g-1 in the soils, and 341-4969 ng L-1 in the spring water. The vertical distribution of the PAHs in soils varied with ring numbers and altitude of the catchment. PAHs concentrations were linearly related with the total organic carbon (TOC) at different depths in the catchments 563-783 m above the sea level (A.S.L.). However, no correlation with TOC was observed in the catchment of a high altitude (2090 m A.S.L.), because the large water flux led to the fast migration of the 2-3 rings PAHs in soils. The PAHs in soils and springs were mainly derived from the combustion of grass/wood/coal, closely related with the primary fossil fuels used in this area. This study demonstrate that the groundwater was heavily polluted by PAHs in the karst terrains of Southwest China, due to the vertical transfer of PAHs from the surface soils, and effective protection was urgently needed.

Origin and effect factors of sedimentary organic carbon in a karst groundwater-fed reservoir, South China.

Reservoirs are commonly recharged by groundwater that is rich in bicarbonate ions in karst regions of South China, and the recharge of this groundwater to the reservoir can affect the biogeochemical processes of carbon sedimentation at the reservoir bottom. In this study, Dalongdong Reservoir, which is mainly recharged by two subterranean streams, was investigated based on a 42-cm-thick sedimentary core and the 210Pb/137Cs dating technique and isotope analyses to understand the sedimentary history and identify the carbon sources. The 210Pb/137Cs age model showed that the sediments were accumulated over the last 60 years. The annual increase precipitation and temperature showed no obvious change compared with trends of δ13C in total organic carbon (δ13Corg), δ15N values in total nitrogen, and the carbon and nitrogen ratio (C/N). This shows that climate was not the main control of the variation in sediment factors. Based on δ13Corg, δ15N, C/N, and isotopic mixing modeling, sources of organic carbon in the sediments were derived from plankton (60.84%), soil (22.93%), waste water (14.56%), and terrestrial plants (1.67%). From 1958 to 1978, reservoir establishment and leakage affected the contribution of the four sources. The contribution of the plankton source increased from 1978 to 2015, resulting from change of water level and continued input of external nitrogen. However, because of the revegetation supplied by an economic aid project the contribution of soil showed a considerable decreasing trend from 1978 to 2002. After 2002, For "Grain for Green" project, the contribution from soil further decreased. After reservoir construction, the contribution of waste water stabilized. The contribution of terrestrial plants started increased rapidly after 2002. Karst groundwater, which contains more dissolved inorganic carbon containing lower δ13CDIC than the water sources of other lakes or reservoirs, makes the δ13Corg value of sediment more negative by phytoplankton photosynthesis in the reservoir.

[Temporal and Spatial Variations of Dissolved Inorganic Carbon and Its Stable Isotopic Composition in the Surface Stream of Karst Groundwater Recharge].

Stable carbon isotope of dissolved inorganic carbon (δ13CDIC), which is mainly constituted by HCO3- in karst water, is widely used to trace the different sources and influential factors of dissolved inorganic carbon (DIC). In order to understand the distribution of DIC and δ13CDIC in subtropical karst area, this paper researched the water chemistry and δ13CDIC in a karst surface stream in detail, which is fed by Guancun subterranean stream in Liuzhou City, Guangxi Province, in the southwest of China. The results showed that the contents of DIC in subterranean stream outlet (G1 site) ranged from 4.60 to 4.90 mmol·L-1 with an average of 4.73 mmol·L-1 in dry season, and from 2.80 to 4.70 mmol·L-1 with an average of 4.23 mmol·L-1 in rainy season. The contents of DIC in 1.35 km downstream site (G2 site) ranged from 4.30 to 4.90 mmol·L-1 with an average of 4.56 mmol·L-1 in dry season, and from 3.00 to 4.70 mmol·L-1 with an average of 4.20 mmol·L-1 in rainy season. The δ13CDIC of subterranean stream outlet (G1 site) varied from -12.8‰ to -11.53‰ with an average of -12.22‰ in dry season, and from -13.12‰ to -11.01‰ with an average of -12.28‰ in rainy season. The δ13CDIC of stream downstream site (G2 site) ranged from -11.71‰ to -9.55‰ with an average of -10.73‰ in dry season, and ranged from -12.18‰ to -9.85‰ with an average of -11.10‰ in rainy season. The contents of DIC of G1 site were higher than those of G2 site. The DIC contents in dry season in both G1 and G2 site were higher than those in rainy season. The values of δ13CDICof G1 and G2 site in dry season were more positive than those in rainy season. The δ13CDICvalue of G1 site was more negative than that of G2 site. The main sources of DIC in underground river and surface stream were soil CO2and carbonate dissolution. However, the differences of DIC and δ13CDICbetween G1 and G2 site showed that CO2degassing and photosynthesis of aquatic plants had significant influence on water DIC and δ13CDIC value. This study is helpful to understand the dynamic change and distribution of DIC and δ13CDIC in karst surface stream.

In-stream metabolism and atmospheric carbon sequestration in a groundwater-fed karst stream.

Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of dissolved carbonate minerals emits CO2 back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2+, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+ and pCO2. This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2+ and δ13CDIC, with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2 degassing from stream to the atmosphere, which is estimated to be 3-5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73tCkm-2a-1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2 evasion.

[Spatial Variations of CO2 Degassing Across Water-air Interface and Its Impact Factors in Summer in Guijiang River, China].

River is characterized by obvious spatial heterogeneity in catchment, which is exacerbated by special environment features of calcium-rich, alkaline and DIC-rich(dissolved inorganic carbon) in karst river. Thus, it also leads to significant spatial variation in the CO2 degassing across water-air interface. Main ions, physicochemical parameters, δ13CDIC value and two common approaches(floating chamber(FC) and thin boundary layer models(TBL) were used to analyze the CO2 degassing characteristics in Guijiang River, a karst river, China. The results were as follows:1 Hydrochemistry in Guijiang River basin showed a significant spatial change. All of HCO3-, Ca2+, specific conductivity, total dissolved solids(TDS), SIc and pCO2 showed similar distribution characteristics in the following order:tributaries in the middle reaches > middle reaches > Downstream > Upstream of Guijiang River. 2 During the monitoring period, CO2 degassing occurred in all the sampling sites and it was the CO2 source for the atmosphere. The mean CO2 evasion was 237 mg·(m2·h)-1 in Guijiang River, which located in the range of average CO2 evasion of global river. However, significant spatial variations also occurred along Guijiang River. The CO2 degassing flux in tributaries of the middle reaches and middle reaches of the mainstream were obviously larger than those in downstream and upstream of the mainstream. 3 CO2 degassing was mainly affected by carbonate equilibrium system in tributaries in the middle reaches and middle reaches in the mainstream of the Guijiang River basin, which resulted in obviously larger CO2 degassing than those in downstream and upstream of mainstream. However, the CO2 degassing flux in tributaries of the middle reaches was also simultaneously affected by biological photosynthesis, and the minimum CO2 degassing flux[6.38 mg·(m2·h)-1] appeared in tributaries of the middle reaches. In addition, the CO2 degassing flux in mainstream upstream was mainly affected by atmospheric environmental factors, while it was synergetically influenced by many factors in mainstream downstream.

[Impacts of Thermal Stratification on the Hydrochemistry and Dissolved Inorganic Carbon in a Typical Karst Reservoir in Summer].

Thermal stratification leads to significant stratification characteristics of hydrochemistry and aquatic organisms in reservoirs, and thus affects the biogeochemical cycle in the reservoir. This study aims to understand physico-chemical properties and dissolved inorganic carbon change processes and its factors in a karst groundwater-fed reservoir, Dalongdong Reservoir, located in Shanglin County, Guangxi Zhuang Autonomous Region, China. The eight sampling points were placed along the direction of the water flow on June 19-21, 2015. The results show that: ① The reservoir exhibited obvious thermal stratification in the summer. There were significant differences in physical and chemical parameters, such as pH and conductivity (Spc) between the epilimnion and thermocline; ② The dissolved oxygen (DO) and chlorophyll a (Chl-a) content from the surface to the bottom did not show a single decreasing trend, but the maximum value occurred 2.5 m or 5 m below the surface; ③ From the surface to the bottom, dissolved inorganic carbon (DIC) concentrations showed an increasing trend with the average DIC concentration of 2.03 mmol·L-1 in the epilimnion and the average DIC concentration of 4.18 mmol·L-1 at the bottom of the thermocline. The value of stable carbon isotope (δ13CDIC) was more positive in the epilimnion than in the thermocline, where δ13CDIC gradually became partially negative with water depth. Possible reasons of these results include: ① The significant differences in temperature, distribution of aquatic organisms, and strength and direction of metabolisms in different water layers due to thermal stratification; ② The DIC variations in the epilimnion were mainly affected by the carbonate precipitation process and phytoplankton photosynthesis, thereby affecting the DIC stable isotope fractionation. DIC was mainly controlled by biological respiration and the organic matter decomposition process in the thermocline.

River sequesters atmospheric carbon and limits the CO2 degassing in karst area, southwest China.

CO2 fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO2 fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca2+ concentrations) and CO2 fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO2 to and absorbs CO2 from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO2 fluxes evolve through diurnal cycles. At DM, the river evaded CO2 from early morning through noon and absorbed CO2 from afternoon through early morning. At PY in summer, the CO2 evasion decreased during the daytime and increased at night while in winter at night, CO2 uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~15mMm-2day-1), the evasion rate is the smallest of all reported world's inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO2 fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO2 emission.

[Variations of CO2 Exchange Fluxes Across Water-air Interface and Environmental Meaning in a Surface Stream in Subtropical Karst Area, SW China].

CO2 cycle process or sources/sinks are not only the basis of understanding and responding to global climate change, but also the core of the current global climate change research. Gas exchange across water-air interface in terrestrial surface water is an important way of nutrient elements (carbon, nitrogen) exchange between aquatic ecosystems and ambient air. Escaping CO2 gas from surface water is also actively involved in the modern carbon cycle. In the material cycle in karst regions, CO2 plays a key role in karst processes, driving the formation of karst features. Karst groundwater with high water CO2 partial pressure (pCO2) often shows highly positive CO2 concentration gradient to atmosphere after it is discharged to surface, so the evaluation of CO2 exchange fluxes across karst water-air interface is important for karst carbon cycle research. This paper researched CO2 exchange fluxes across water-air interface in the karst surface stream in detail which was fed by Guancun subterranean stream in Liuzhou city, Guangxi province. Closed static chamber method and portable hand-holding CO2 sensor (GM70) were both employed in CO2 exchange fluxes monitoring. The results showed that CO2 degassing was the mainly form of CO2 exchange across the steam water-air interface. CO2 degassing flux in subterranean stream outlet (G1 site) ranged from 139.48 to 890.84 mg·(m2·h)-1 with an average of 445.72 mg·(m2·h)-1. CO2 degassing flux in stream downstream site (G2 site) ranged from 16.54 to 844.18 mg·(m2·h)-1 with an average of 159.81 mg·(m2·h)-1. The CO2 degassing flux in G1site was higher than that in G2 site. CO2 degassing fluxes in rainy season in both G1 and G2 site were higher than those in dry season. Stable carbon analysis of CO2 gas (δ13C-CO2) found that CO2 degassing from karst stream might influence air CO2 carbon isotope near water surface, which resulted in the more negative δ13C-CO2 value with the increase of CO2 degassing flux. Significant spatio-temporal variations of δ13C-CO2 were found, and the δ13C-CO2 in the rainy season was more negative than that in dry season and δ13C-CO2 in G1 site was more negative than that in G2 site. As a result of stream CO2 degassing, the hydrochemical characteristics of steam varied along stream running, which resulted in decrease of HCO3-, EC and pCO2 and increase of pH, SIc and δ13C-DIC in the stream.

[Summer Greenhouse Gases Exchange Flux Across Water-air Interface in Three Water Reservoirs Located in Different Geologic Setting in Guangxi, China].

Due to special hydrogeochemical characteristics of calcium-rich, alkaline and DIC-rich ( dissolved inorganic carbon) environment controlled by the weathering products from carbonate rock, the exchange characteristics, processes and controlling factors of greenhouse gas (CO2 and CH4) across water-air interface in karst water reservoir show obvious differences from those of non-karst water reservoir. Three water reservoirs (Dalongdong reservoir-karst reservoir, Wulixia reservoir--semi karst reservoir, Si'anjiang reservoir-non-karst reservoir) located in different geologic setting in Guangxi Zhuang Autonomous Region, China were chosen to reveal characteristics and controlling factors of greenhouse gas exchange flux across water-air interface. Two common approaches, floating chamber (FC) and thin boundary layer models (TBL), were employed to research and contrast greenhouse gas exchange flux across water-air interface from three reservoirs. The results showed that: (1) surface-layer water in reservoir area and discharging water under dam in Dalongdong water reservoir were the source of atmospheric CO2 and CH4. Surface-layer water in reservoir area in Wulixia water reservoir was the sink of atmospheric CO2 and the source of atmospheric CH4, while discharging water under dam was the source of atmospheric CO2 and CH4. Surface-layer water in Si'anjiang water reservoir was the sink of atmospheric CO2 and source of atmospheric CH4. (2) CO2 and CH4 effluxes in discharging water under dam were much more than those in surface-layer water in reservoir area regardless of karst reservoir or non karst reservoir. Accordingly, more attention should be paid to the CO2 and CH4 emission from discharging water under dam. (3) In the absence of submerged soil organic matters and plants, the difference of CH4 effluxes between karst groundwater-fed reservoir ( Dalongdong water reservoir) and non-karst area ( Wulixia water reservoir and Si'anjiang water reservoir) was less. However, CO2 efflux in karst groundwater-fed reservoir was much higher than that of reservoir in non-karst area due to groundwater of DIC-rich input from karst aquifer and thermal stratification.