Liu et al. Environ Sci Eur (2021) 33:53 https://doi.org/10.1186/s12302-021-00495-x RESEARCH Open Access Hydrological and biogeochemical controls on temporal variations of dissolved carbon and solutes in a karst river, South China Jing Liu1, Jun Zhong2* , Shuai Chen3, Sen Xu2 and Si‑Liang Li2 Abstract Background: Understanding the responses of riverine dissolved carbon dynamics and chemical weathering processes to short‑term climatic variabilities is important to understand the Surface‑Earth processes under ongoing 13 climate change. Temporal variations of solutes and stable carbon isotope of dissolved inorganic carbon (δ CDIC) were analysed during a hydrological year in the Guijiang River, South China. We aimed to unravel the chemical weathering processes and carbon dynamics in karst areas under ongoing climate changes. Results: Signifcant positive relationships were found between weathering rates and climatic factors (i.e. temperature and discharge) over the hydrological year. The total fux of CO consumption (760.4 ­103 mol/km2/year) in the Guiji‑ 2 × ang River was much higher than the global mean fux, with a higher CO2 consumption capacity in the Guijiang River relative to most other global rivers. Chemical weathering fuxes in this karst area showed high sensitivity to global climate change. CO2 evasion during the warm–wet seasons was much lower than those during cold–dry seasons. 13 Light δ CDIC values occurred under high‑fow conditions, corresponding with the high temperatures in high‑fow seasons. IsoSource modelling revealed that biological carbon could account for 53% of all dissolved inorganic carbon (DIC), controlling the temporal carbon variabilities. Conclusion: This study quantitatively evaluated the temporal variations in CO2 fuxes and carbon cycling of karstic river systems and demonstrated that riverine carbon cycling will have a higher sensibility to ongoing global climate change. High discharges accelerate solutes transport, with relatively large quantities of 13C‑depleted carbon being fushed into rivers. Meanwhile, high temperatures also accelerate organic carbon mineralisation, producing high 13 content of soil CO2, whose infux can shift the C‑depleted values in the high‑fow seasons. Taken together, biological carbon infux should be responsible for the temporal carbon dynamics. 13 Keywords: Dissolved carbon, Karst landscape, δ CDIC, CO2 outgassing, Carbon cycling Background river system is the key channel, transporting dissolved Understanding the responses of carbon cycling to short- carbon from the continents to the oceans [3]. Rock term climate variabilities (i.e. temperature and discharge) weathering is one of the main sources of dissolved carbon is necessary to investigate the efects of future global cli- in river systems, which could consume atmospheric CO2 mate change on Surface-Earth processes [1, 2]. Of the by converting it into dissolved inorganic carbon (DIC). diferent components within the global carbon cycle, the Terefore, chemical weathering processes have a signif- cant efect on modulating atmospheric CO2, and thereby *Correspondence: [email protected] afect global climate change. 2 Institute of Surface‑Earth System Science, School of Earth System Silicate weathering involves carbonic acid and leads Science, Tianjin University, Tianjin 300072, China to the net sequestration of CO2, which can be defned Full list of author information is available at the end of the article by Eq. (1). Carbonate rocks cover approximately 12% © The Author(s) 2021. 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Environ Sci Eur (2021) 33:53 Page 2 of 15 of the Earth’s ice-free land area [4], with thin soil cover Chemical weathering and related CO2 consumption and well-developed conduit networks forming large fuxes are highly afected by hydrological conditions. underground drainage systems [5]. Te dissolution of However, the coupled efects of hydrological and bio- carbonate rocks consumes ~ 0.15 Pg of atmospheric geochemical processes on CO2 fuxes in river systems CO2 each year, based on the geochemical data from remain unclear. For example, the fuxes of CO2 consump- large rivers worldwide [6]. And the produced DIC is tion are calculated by discharge and the dissolved solute then transported to the ocean, which plays an impor- content, yet the covariation between the above two vari- tant role as a carbon sink. On the other hand, as car- ables could induce great uncertainty [21]. Additionally, bonate weathering can be consumed by other acids (e.g. seasonal and diurnal variations in CO2 degassing may nitric, sulfuric, and organic acids), it can lead to CO2 result from diferences in solar radiation and tempera- release rather than CO2 consumption [7], which can be ture, causing changes in the content of DIC and CO 2 [22]. expressed as Eqs. (3 and 4). Tus, to reduce the overall uncertainty, high-frequency (monthly or even daily scales) sampling and analyses of CaSiO3 + CO2 → CaCO3 ↓+SiO2 (1) hydrochemistry, isotopes, synchronous hydrology and 2+ − CaCO3 + CO2 + H2O → Ca + HCO3 → CaCO3 ↓+CO2 ↑+H2O (2) + → ↓+ ↑+ + − meteorology of river systems are required. CaCO3 2HNO3 CaCO3 CO2 H2O 2NO(3)3 Te Guijiang River catchment is located in Southwest 2+ − 2− 2− 2+ 2CaCO3+ H2SO4 → 2Ca +2HCO3 +SO4 → CaCO3 ↓+CO2 ↑+SO4 + Ca (4) In addition to delivering a large amount of DIC to the China, which is one of the largest karst regional cent- oceans, rivers also release CO2 into the air through the ers in the world, exhibiting a high sensibility respond- water–air interface [8]. Raymond et al. [9] reported that ing to chemical weathering processes under changing global CO2 emissions from inland waters to the atmos- climate conditions. It is an ideal study object to under- phere is approximately equal to 2.1 Pg of C per year. stand carbon dynamics and weathering processes in Hence, it is important to conduct extensive research on response to various climatic conditions in karst rivers. both the vertical and lateral transport of DIC in river sys- Here, the Guijiang River catchment was selected to ana- tems, especially the transference of carbon in the litho- lyse the high-frequency contents of dissolved solutes and 13 sphere, hydrosphere, and atmosphere [10–13]. δ CDIC, aiming to (1) depict the temporal variations of Te riverine DIC is generally afected as follows: (1) the solute dynamics in a typical karstic catchment; (2) obtain weathering of carbonate/silicate rocks, (2) soil CO 2 pro- information on the fuxes of chemical weathering and duced by the organic matter decomposition, and (3) the CO2 consumption in response to climatic variabilities; exchange of CO2 at the water–air interface [2, 14]. Each (3) quantify the potential sources of DIC; and (4) esti- source/process is involved in the carbon cycle, which mate the vertical CO 2 fuxes from surface waters into the 13 could be traced using δ CDIC combined with the dis- atmosphere. solved carbon contents [15, 16]. For marine carbonate bedrock sources, δ13C values are typically 0‰ (Vienna Materials and methods Pee Dee Belemnite) [17]. Te δ13C values of plants are Site description and data source controlled by specifc photosynthetic paths, involving the Te Guijiang River catchment (23° 28′–23° 55′ N, 110° ′ ′ pathways of the Calvin cycle (C 3) and the Hatch–Slack 05 –110° 29 E) is located in the northeastern Guangxi (C4). Tese C 3 and C 4 plants induce diferent carbon Province of South China. Te study catchment has a 13 isotopic fractionation, with δ C of the CO 2 are 27‰ length of 426 km from north to south, with a drainage − 2 and − 13‰ (VPDB), respectively [17]. Due to the rela- area of 19,288 km . It originates from the Maoer Moun- tive diference in the partial pressure of CO 2 (pCO2) of tain, with an elevation of approximately 2141 m, and then the river and overlying atmosphere, CO 2 evasion causes fows southward through Guilin, where a typical karstic 13C-enriched in the residual DIC. Related studies have landscape is developed, fnally entering the Xijiang River. reported that rivers are usually supersaturated to atmos- Its landscape contains mainly hills and mountains with pheric CO2, leading to vertical CO2 degassing to the signifcant topographic relief. Karstic topography in the atmosphere [18, 19]. For example, African inland rivers study catchment is well-developed (Fig. 1). Specifcally, degassed 0.27–0.37 Pg C/year into the atmosphere [20]. the upper–middle catchment is dominated by Permian Liu et al. Environ Sci Eur (2021) 33:53 Page 3 of 15 Fig. 1 Map showing the sampling location and geological setting of the Guijiang River catchment, which is [16] modifed from Zhong et al. and Triassic carbonate rocks including sulfde-rich lime- Te sampling site was situated at the outlet of the stones and dolomites. Te bedrocks distributed in the Guijiang River (Fig. 1), which is ~ 39 km away from the middle–lower catchment area are mainly Precambrian mainstream of the Xijiang River. We collected 8 river metamorphic rocks (gneiss) and igneous rocks (primar- water samples from October of 2013 to May of 2014 ily granite).