The high-intensity partial pressure of CO₂ (pCO₂) in karst underground rivers rapidly releases in open-flow sections. This is an important process affecting the global karst carbon cycle. This study focuses on the diurnal variation patterns and driving factors of CO₂ exchange flux at the water–air interface in the open-flow sections of typical karst underground rivers in southwestern China. The inorganic carbon in water and water–air interface exchange fluxes are observed. Three representative survey stations, i.e., the outlet of the underground river (Q1), the river sections without submerged plants (H1), and the river sections with submerged plants (H2), are selected to study the CO₂ exchange process and its influencing factors. The results show that the CO₂ release flux at Q1 exhibits high pressure in the daytime and low pressure in the nighttime, while H1 and H2 exhibit the opposite pattern. The photosynthesis of submerged plants significantly inhibits the carbon release flux of the river, and in the river sections where submerged plants are distributed, their biological effects have inhibited approximately 0.131 Tg C/yr of carbon emissions. This study emphasizes the significant contribution of submerged plants in restraining the release of CO₂, thereby promoting carbon sequestration and storage in karst water systems.

Cascade hydropower reshapes land use and interactions in reservoir regions, potentially altering soil heavy metal risks. Eight metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) were quantified in 20 surface soils from the head, middle and tail resettlement zones of the Wudongde reservoir to characterize spatial patterns, apportion sources and evaluate ecological risk. Principal component analysis (PCA), positive matrix factorization (PMF) and GeoDetector, combined with region-specific geochemical baselines, were applied to disentangle mixed sources and environmental drivers. PMF resolved four sources: mining-related tailings leachate and effluents (38.7 %), traffic and construction dust (27.5 %), agricultural and domestic inputs (21.8 %) and geogenic background (12.0 %). GeoDetector identified population density, soil organic carbon and pH as drivers of spatial heterogeneity and revealed non-linear enhancement between population density and SOC (q=0.62) and between population density and pH (q=0.58). Baseline-based geo-accumulation and ecological risk indices indicated low ecological risk (all ERI_total within the low-risk class) but revealed localized Cd-Pb-As-Cu hotspots in mining- and agriculture-dominated areas. These results indicate that cascade hydropower-induced land use change reshapes the source-risk pattern of soil heavy metals and provide a quantitative basis for targeted monitoring and source-specific mitigation in reservoir resettlement zones.

The high-intensity pCO₂ in karst underground river water is rapidly released or absorbed by aquatic organisms in open flow sections and is an important factor affecting the karst carbon cycle. This article takes a typical karst underground river basin in southwestern China as the research object. By observing the water carbon and water‒air interface exchange fluxes at the outlet of the underground river (Q1), the river section without submerged plants (H1), and the river section with submerged plants (H2), the diurnal variation pattern and driving factors of CO₂ flux at the water‒air interface in the karst area were evaluated, and the transformation process of water carbon and the source sink changes in water carbon in the open-flow section were explored. The results show that CO₂ flux and pCO₂ in the study area exhibit significant spatial differences, with the outlet of the underground river having the highest values. DO is found to be a key controlling factor affecting changes in CO₂ flux. The metabolism of submerged plants has a regulatory role on the change in CO₂ flux, forming a carbon sink in the river section with submerged plants. Therefore, this study helps to understand the diurnal variation in carbon exchange fluxes at the water‒air interface in the open flow section of karst underground rivers and emphasizes the variation pattern of CO₂ fluxes from source to sink in the open flow section, providing an effective way to promote carbon sequestration and sink enhancement in karst water bodies.

River sediments are vital organic carbon reservoirs, yet the formation mechanisms of mineral-associated organic carbon (MAOC) in karst spring-fed rivers remain poorly understood. Here, we investigated sediments, water chemistry, submerged macrophytes, and sediment-extracted dissolved organic matter (DOM) from five typical karst spring-fed rivers in the Guilin and Jianshui regions of Southwest China. Results showed that the total organic carbon (TOC) in sediments averaged 32.21 ± 24.07 g/kg, and MAOC averaged 5.45 ± 2.68 g/kg. While TOC and MAOC concentrations were similar between Guilin and Jianshui, the dominant MAOC fractions showed clear differences between the two study areas. Guilin sediments were characterized by operationally defined higher iron-bound organic carbon (Fe-OC), whereas operationally defined calcium-bound organic carbon (Ca-OC) concentrations tended to be higher in Jianshui. Random forest (RF) analysis identified stable carbon isotope composition of dissolved inorganic carbon in water (δ¹³C_DIC), fluorescent dissolved organic matter biological index (BIX), and specific conductivity of water (SpC_w) as the most important predictors of MAOC variation. Operationally defined Fe-OC was primarily associated with SpC_w, dissolved oxygen (DO_w), spectral slope ratio (S_R), and specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), whereas operationally defined Ca-OC was mainly related to δ¹³C_DIC, dissolved organic carbon (DOC), and SUVA₂₅₄. These findings indicate that similar MAOC concentrations in karst spring-fed river sediments were accompanied by contrasting contributions of operationally defined Fe- and Ca-associated carbon allocation patterns. Our study highlights the need to move beyond bulk carbon concentration assessment and consider carrier-specific MAOC composition when evaluating the composition and potential persistence of karst sedimentary carbon pools.