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One of the most important questions in the science of global carbon cycle is how to balance the atmospheric CO2 budget. However, there is a large terrestrial carbon sink is missing. The locations, magnitudes, variations, and mechanisms responsible for this terrestrial missing carbon sink are uncertain and the focus of much continuing debate. In order to provide a basis for the assessment of carbonate weathering-related carbon sink, this study investigated the spatial and temporal variations of dissolved inorganic carbon (DIC) and particulate organic carbon (POC), and used stable carbon isotopes and hydrochemical data to quantitatively estimate the proportion of DIC transformation to POC and analyze the potential influence of the transformation of DIC into OC on the carbon isotope composition in the Lijiang River (typical karst surface water), Southwest China. The results showed that DIC concentrations are high in winter and low in summer and increase from upstream to downstream. POC concentrations are high in summer and low in spring. δ13CPOC and δ13CDIC are negatively correlated in the Lijiang River. The δ13CDIC values are high in spring and low in summer, compared with the estimated δ13CDIC(cal.); the observed δ13CDIC values have a maximum positive shift in spring and similar in the summer; the δ13CPOC values decrease from upstream to downstream, and are high in summer and low in spring. These observations indicate that the isotopic compositions of both DIC and POC are significantly affected by aquatic photosynthesis that transforms DIC into OC. In-river primary production contribute 12–54% of POC, and the contributions of carbonate rock-sourced DIC to the riverine POC range from 7 to 30%, which suggests an important sink of atmospheric CO2 in river systems and should be taken into account in global carbon cycle. [ABSTRACT FROM AUTHOR]

The cycling of carbon (C) and nitrogen (N) in karst aquatic systems has been shown to be closely related, with coupled control of dissolved organic carbon (DOC) concentrations through the metabolic pathways of subaquatic communities. However, the coupled C N cycling involving in the transformation of dissolved inorganic carbon (DIC) into DOC has not been well-explored. In this study, we chose the Lijiang River, a typical karst aquatic system in Southwest China as our study area and documented its diurnal and seasonal variations in terms of several hydrochemical and isotopic parameters to identify how to couple cycling for C and N. The results of the Bayesian stable isotope-mixing model showed that approximately 50% and 72% of the total DOC formed in summer and winter, respectively, represented autochthonous organic carbon in the Lijiang River. Diurnal monitoring results revealed that DIC and NO 3 − transformations were primarily controlled by metabolic processes (photosynthesis and respiration) of subaquatic communities, accompanying DOC formation, in the Lijiang River. The consumption of DIC and NO 3 − by aquatic photosynthesis was in the ratio of 9:1 (mol/mol) to produce autochthonous DOC, accompanying the enriched δ13C DIC , δ15N-NO 3 − and δ18O-NO 3 −, with a daily variation of 7.9‰, 10.6‰ and 11.2‰, respectively. On the diurnal scale, 6.2% of the total DIC and 7.1% of the total NO 3 − were consumed by metabolic processes of subaquatic communities and these values were consistent with their corresponding values on the interannual scale. However, the proportions of DIC and NO 3 − utilized in the dry season were higher than those in the wet season. Approximately 1.18 × 107 kg C/yr of DIC and 1.64 × 106 kg N/yr of NO 3 − were converted into organic matter by the aquatic photosynthesis, with 80% and 79% of the total DIC and NO 3 − consumption respectively occurring in the wet season. Furthermore, the coupled C N cycling involving DIC and NO 3 − can promote the production of autochthonous DOC, constituting a relatively long-term natural C and N sinks in karst aquatic systems. • Coupled C N cycling in a karst aquatic system is controlled by metabolic processes. • Most DOC was autochthonous OC formed in the coupled C N cycling. • The ratio of consumption of DIC and NO 3 − by aquatic photosynthesis was 9:1. [ABSTRACT FROM AUTHOR]

Soil carbon is an important component of the global carbon cycle and consists of organic carbon (SOC) and inorganic carbon (SIC) pools. Although SIC has been recognized as a large pool, little is known about how it has been influenced by climate change and land degradation on a decadal timescale. Here, we evaluated the magnitude of the SIC stock and detected its changes under different land cover types across the Tibetan Plateau over the past decades based on repeated SIC measurements in the 1980s and 2020s at the same sites. In the soils of the Tibetan Plateau, SIC accounts for 45% and 49% of the total (organic plus inorganic) soil carbon at depths of 0–50 cm and 0–100 cm, with estimated SIC stocks of approximately 15.6 and 20.9 Pg C, and average SIC densities of 7.2 and 9.4 kg C m−2, respectively. Partial correlation and variation partitioning analyses revealed that the spatial distribution of SIC was predominantly influenced by soil properties (soil type, texture, and pH) and climate, explaining 12.1% and 7.5% of the SIC variance, respectively. Between the 1980s and the 2020s, an overall accumulation of SIC was observed for all land cover types, with an average increase of 2.3 and 2.0 g C kg−1 for the topsoil (notably 0–20 cm) and subsoil (notably 20–50 cm), respectively. However, the response of SIC to a changing climate is more nuanced; increased precipitation may reduce SIC stock in arid areas by enhancing leaching, whereas warming temperatures may contribute to SIC accumulation in humid regions. Overall, these findings have important implications for understanding the role of SIC in the global carbon cycle under future climate change events. [Display omitted] • Decadal changes in SIC were detected based on field resampling. • Overall SIC accumulation was observed across the Tibetan Plateau over the past decades. • SIC in arid and humid areas have different responses to climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] • EEM-PARAFAC CDOM compounds were used to illustrate DOC composition and origination. • Soil-, aquatic plant- and microbial sourced organic matter existed in Lijiang river. • Aquatic plant- and microbial sourced Autochthonous DOC dominated in downstream. • Precipitation and T induced seasonal variation of soil- and aquatic plant sourced OC. • DIC fertilization effects induced spatial increase of autochthonous OC. Aquatic photosynthesis transforms inorganic carbon to organic carbon (OC), which contributes to autochthonous organic carbon (AOC) in sediment and particulate organic carbon in surface karst aquatic systems. Aquatic plant and microbes are participated in autochthonous dissolved organic carbon (ADOC) formation in surface karst aquatic systems, but the composition and formation of ADOC remains little known, which leaves problem on calculation of organic carbon flux. In this study, the Lijiang River was chosen as typical surface karst river to identify the DOC composition and its origin, and to explore the environmental influencing factors. Samples were collected seasonally from the upper to lower reaches of the river to analyze hydrochemical parameters and the excitation-emission matrix (EEM) spectrum of chromophoric dissolved organic matter (CDOM). Three CDOM components were calculated by parallel factor analysis (PARAFAC) from EEM spectra, which were allochthonous soil-sourced DOC (SDOC), autochthonous aquatic plant-sourced DOC (APDOC), and microbial-sourced DOC (MDOC). Based on the DOC component concentrations, SDOC is induced by large amounts of precipitation causing soil erosion in summer. APDOC formation is encouraged by moderate water temperatures in spring and fall restricted by high water turbidity in summer. The significant positive linear correlations between APDOC and dissolved inorganic carbon (DIC) and MDOC indicate DIC fertilization effect in karst aquatic systems directly promotes APDOC formation and indirectly promotes MDOC. Our study develops a relatively simple way to calculate composition of karst aquatic DOC, which demonstrates the participation of aquatic plants and microbes in APDOC production and reiterate that autochthonous DOC should be considered when calculating the carbon sink in surface karst aquatic systems. [ABSTRACT FROM AUTHOR]

We analysed precipitation and river water samples from the Lijiang River basin with the aim of understanding the important role of acid rain components, especially NH 4 , in chemical weathering of karst areas. First, analysis of the hydrochemical characteristics revealed the following: In the precipitation samples, the volume-weighted mean pH was 4.91, and the acid rain rate was 65%. The major ion composition was characterized by a dominance of NH 4 , Ca, SO 4 and NO 3 , accounting for 40.0%, 35.3%, 46.3% and 30.5%, respectively, of the total cation and anion concentrations. In the river water samples, the pH ranged from 6.75 to 9.16, with an average of 7.55. The major hydrogeochemical species in the river water samples were Ca–HCO 3 and Mg–HCO 3. Second, by employing chloride-normalized ratios of the other major ions in precipitation, we estimated that the contributions of SO 4 and NO 3 from the atmosphere to the river were53.6% and 35.0%, respectively. Third, combining the hydrochemical index with the value of δ13C DIC and using the hydrochemical composition method showed that the proportion of carbonate weathered (R) by NH 4 , H 2 SO 4 and HNO 3 from acid rain accounted for 9.3%, 9.4% and 3.1% of the total weathering rate, respectively, and the proportions in the upper reaches were significantly higher than those in the lower reaches of the river. The average values of the carbonate weathering proportion (R) and the carbonate weathering rate (CWR) due to acid rain were approximately 22.0% and 2.1 t/km2/yr, respectively. Taken together, our results revealed that carbonate weathering by acid rain can significantly increase the carbonate weathering rate, which is essential information for accurate estimation of the carbon cycle (namely, inorganic carbon sources and sinks). Image 1 • Analysis of hydrochemical characteristics of precipitation and river water. • NH 4 (H), H 2 SO 4 and HNO 3 may provide the required protons for weathering reactions. • Carbonate weathering by acid rain can significantly increase weathering rate. [ABSTRACT FROM AUTHOR]