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.

The elevational divergence of plant CSR (Competitor–Stress-Tolerator–Ruderal) strategies represents a core theme in global change ecology, yet adaptive patterns across subalpine transition zones remain insufficiently understood. This study focused on 375 plant individuals sampled along an 1143–4361 m elevational gradient on Gongga Mountain. We divided elevations into three zones (low, mid, high) using tertile partitioning, measured leaf functional traits including leaf area (LA), specific leaf area (SLA), and leaf dry matter content (LDMC), and quantified CSR strategy scores with the StrateFy approach. We further explored how hydrothermal factors drive community- and intraspecific-level strategy variation. Results showed that CSR strategies differed significantly along the elevational gradient. The relative abundance of individuals with the C strategy was 53.4% at low elevations, peaked at 72.4% at mid elevations, and decreased to 43.6% at high elevations. By contrast, the S strategy was most abundant at high elevations (32.5%), while the R strategy consistently accounted for less than 5% across all zones. Life form-specific strategies varied strongly: herbs maintained a dominant C strategy (66.7%) at high elevations; trees shifted to S strategy dominance (60%) at high elevations; and shrubs displayed a balanced C–S strategy. Intraspecific plasticity in CSR strategies was pronounced: with increasing elevation, the dominant strategy shifted from C (33.3%) to S (66.7%), with a greater magnitude of variation than at the community level. Temperature was positively correlated with the C strategy (p < 0.001, R² = 0.53) and negatively correlated with the R strategy (p < 0.001, R² = 0.40). Precipitation was negatively correlated with the C strategy (p = 0.001, R² = 0.54) and positively correlated with the R strategy (p = 0.005, R² = 0.42), whereas both factors exerted weak effects on the S strategy. Our findings highlight that shifts in hydrothermal conditions are the primary drivers shaping CSR strategy distributions, and coordinated leaf trait variation serves as a key adaptation mechanism. These results improve mechanistic understanding of mountain plant adaptation and provide a scientific basis for alpine vegetation conservation and management.

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.

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.