Dissolved organic sulfur (DOS) is a crucial yet poorly constrained component linking carbon and sulfur cycles in coastal oceans. To understand how hydrographic dynamics influence its fate, we investigated the chemical evolution of DOS along two distinct plume pathways from the Yangtze River Estuary to the East China Sea: the northward-extending offshore branch (Yangtze River Estuary Northward, YEN) and the southward-confined coastal branch (Yangtze River Estuary Southward, YES). We combined concentration measurements with ultrahigh-resolution mass spectrometry to characterize solid-phase extracted DOS. Results revealed that DOS concentrations decreased seaward but were consistently higher in YEN than in YES. At the molecular level, DOS in the high-salinity zone of YEN exhibited significantly lower aromaticity and higher oxygen-to-carbon ratios compared to YES, indicating divergent photochemical and microbial processing between the pathways. Crucially, in the subsurface hypoxic zone of YEN, we observed a marked increase in both the molecular diversity of dissolved organic matter (DOM) and the relative abundance of sulfonated molecular formulas, providing direct molecular evidence that hypoxia actively facilitates DOM sulfurization. Our results establish that plume trajectory and oxygen availability are key regulators of DOS molecular composition and transformation pathways, offering a mechanistic framework for sulfur-carbon coupling in large river estuaries and marginal seas.

Understanding the multi-scale coupling of carbon and nitrogen (C-N) in agro-pastoral ecotones is complex due to the dual pressures of global change and human activities. This study aimed to investigate C-N dynamics in the Yang River basin (Northern China) using isotopic signatures (delta 13CTOC, delta 15NTN), C/N ratios, and molecular characterization of dissolved organic matter across different spatial scales (1000, 3000, and 5000 m) and land uses (cropland, forest, grassland, and urban). Our findings indicated that terrestrial organic inputs are predominant, with agricultural and urban activities significantly accelerating the transformation between total organic carbon and dissolved organic carbon (DOC) by 88.9%. Land use within 3000 m riparian buffers significantly regulated organic C sources, while N dynamics are predominantly controlled by circular buffers of the same spatial extent. The diversity of nitrogenous heteroatoms in farmland, grassland, and urban areas ranged from 62.7% to 74.7%, which is higher than the 58.7% observed in forested areas. This suggests an increase in the extent of N transformation. Overall, human activities in the Yang River basin drive strong C-N coupling, with dissolved oxygen (DO) availability playing a crucial role in DOC-dissolved inorganic nitrogen dynamics (DO peaked at 16.0 mg/L). This research provides a new multi-scale perspective on C-N interactions in semi-arid ecotones, offering insights that can support targeted strategies for ecological restoration and sustainable resource management.

Water is the most active factor in the karst critical zone and the core driving force of multi-sphere interactions.The unique surface-subsurface dual hydrogeological structure of karst areas,combined with shallow soil layers and rapid connectivity within the fissure-channel system,endows the karst water system with extreme environmental sensitivity and vulnerability.With the aggravation of global climate change and the continuous expansion of human activities,karst regions face multiple challenges,including uneven spatiotemporal distribution of water resources,water environmental pollution,and ecosystem degradation.This study systematically reviews recent research progress in karst water resources,watershed weathering and carbon sink effects,migration and transformation processes of key substances in surface water and groundwater,and pollution prevention and control technologies.It proposes key research directions for karst water resources and the aquatic environment:(1)Promote the transformation of monitoring and simulation technologies towards three-dimensional and intelligent approaches to achieve transparency and visualization of the whole process;(2)Deepen understanding of the nonlinear coupling mechanisms among"water-rock-soil-air-life-human"multi-sphere systems to address the challenge of extrapolating from microscopic mechanisms to watershed-scale effect;(3)Conduct precise accounting and stability assessments of karst carbon sinks in the context of global change;(4)Strengthen research on colloid transport effects,to elucidate the migration mechanisms and ecological and environmental risks of emerging pollutants in karst water systems;(5)Enhance integrated surface and groundwater monitoring,develop coupled surface-groundwater numerical models,and provide scientific guidance for the coordinated prevention and control of surface-groundwater pollution as well as ecological restoration.

Methane (CH4) is both a major energy resource and a key greenhouse gas in the Earth's carbon cycle. The clumped isotope geochemistry of methane (Δ13CH3D and Δ12CH2D2) offers a unique and effective tool for understanding methane sources and sinks. In this work, we comprehensively compiled a global methane clumped isotopes dataset (n = 1061). Combined with our own measured data (n = 26) and machine learning predictions, we discuss the efficacy of clumped isotope analyses of CH4 to infer methane origin mechanisms and to constrain post-generation processes. Insights gleaned through field observations, laboratory-controlled experiments and geochemical modelling allow an isotopologue-scale reconstruction of the biogeochemical methane cycle and its evolution through transformation (or gas migration) fractionation. The most up-to-date evidence suggests that equilibrated and disequilibrium clumped isotope compositions record reservoir geothermal events and subsequent microbial alteration, as well as kinetically-driven methane metabolism, in the low-temperature surface Earth environment. Modern estimates of the atmospheric methane budget, incorporating source and sink contributions through both forward and reverse constraints, are essential for leveraging clumped isotopes to gain a deeper understanding of Earth system dynamics. Future research that expands the observation of methane clumped isotope data on a broader scale and integrates these findings into methane cycle modelling could provide crucial insights into the potential impacts of anthropogenic influences on the global carbon cycle and climate change.

China is the largest emitter of cropland gaseous reactive nitrogen (Ngr, including NH3, N2O, and NOx), which greatly affects regional air quality, climate change, and human health. Despite the substantial spatial and temporal variations in cropland Ngr emissions, their driving mechanisms and net climate change forcings (either cooling or warming) remain unclear. Using revised emission estimates and bottom-up mass flow-based approaches, we evaluated the spatiotemporal changes in and key drivers of China's cropland Ngr emissions from 1978 to 2023. We assessed the relative contribution of China's cropland Ngr emissions to climate change via multiple metrics (global warming and temperature potentials). China's cropland Ngr emissions increased by approximately 2.3-fold from 1978 to 2023. The pace of this increase decelerated over time, from 0.17 Tg N·y-2 before 1996 to 0.15 Tg N·y-2 during 1997-2005 and -0.05 Tg N·y-2 after 2005. Population growth and N-intensive animal-sourced food production accounted for 31.7% and 23.2% of this historical emission change, respectively. The net climate change forcing of Ngr varies with scale. Currently, China's cropland Ngr emissions in 2017 at the county level impart a net cooling effect ascribed to short-lived NH3 and NOx emissions, but this forcing shifts in sign and magnitude to a warming effect over a one-hundred-year time horizon, primarily through long-lived N2O emissions. Our findings suggest that a combination of metrics can comprehensively and adequately capture short-, medium-, and long-term climate impacts and that achieving climate targets will conspire against Ngr mitigation efforts for environmental protection goals.

How the intensity of biological carbon pump (BCP) regulates the accumulation of autochthonous organic carbon (AOC) and the stabilization of recalcitrant organic carbon (ROC) in reservoir sediments remains poorly understood. Here, we integrated stable carbon and nitrogen isotopes (δ13C and δ15N), organic carbon (OC) classification, and molecular fingerprinting to elucidate the sources and stabilization mechanisms of sedimentary OC in the Songbaishan Reservoir. Thermal stratification enhanced the BCP, resulting in a drawdown of dissolved inorganic carbon by 9.1 mg L−1 and concurrent enrichment of δ13CDIC by + 2.7‰ relative to inflowing rivers. This shift, driven by distinct nutrient (carbon-phosphorus) limitations, promoted substantial AOC production. The AOC accounted for 65.8 ± 12.1

Although hydropower is a renewable energy source, reservoirs still emit a certain amount of CO2. However, as natural rivers also release CO2, the net CO2 emissions from reservoirs remain largely unknown. To address this knowledge gap, this study explored the influence of reservoir impoundment on CO2 emissions from the Wujiangdu Reservoir, a typical reservoir in the karst area of Southwest China. Hydrochemical and water environment data in 1972–1974 and 2017 were used to compare CO2 diffusive fluxes () before and after reservoir impoundment. The results indicated that before reservoir impoundment, ranged from 13.42 mmol/(m2⸱d) to 49.73 mmol/(m2⸱d), with an annual average of 26.68 mmol/(m2⸱d). After impoundment, varied between −8.11 mmol/(m2⸱d) and 66.68 mmol/(m2⸱d), with an annual average of 23.54 mmol/(m2⸱d). Variations in after reservoir impoundment were strongly associated with pH. After excluding the effects of changes in air temperature, catchment population, and nutrient inputs, mean showed a slight decline compared to pre-impoundment levels. However, as seasonal fluctuations intensified and the impoundment expanded the water surface area by approximately three times, the net effect of reservoir impoundment resulted in additional CO2 emissions of 7 426 t per year. These findings highlight the net effect of reservoir impoundment on CO2 emissions and underscore the need for effective measures to mitigate CO2 emissions from reservoirs.

Research on carbon cycling in Qinghai-Tibet Plateau rivers has attracted considerable attention. However, the role of carbonate buffering in regulating carbon dioxide (CO2) dynamics within cascade reservoirs remains poorly understood, particularly in plateau regions characterized by carbonate-rich geology and high suspended sediment concentration (SSC). In carbonate-enriched sediments, carbonate buffering prolongs the equilibration timescales of both partial pressure of CO2 (pCO(2)) and stable isotopic composition of dissolved inorganic carbon (delta C-13(DIC)) with the atmosphere by similar to 10-fold and similar to 100-fold, respectively, compared to non-buffered scenarios. Over extended timescales, water-carbonate interactions effectively limit the evasion of metabolically produced CO2 in cascade reservoirs of the Lancang River, reducing CO2 emissions flux (F-CO2) by 24.4%. This buffering effect is particularly pronounced in the hypolimnion, where increased carbonate dissolution occurs, and remains sustained in the outflows. Globally, carbonate-dominated reservoirs with high SSC exhibit 82% lower F-CO2 than those with low SSC, demonstrating that suspended sediments critically amplify carbonate buffering's suppression of CO2 emissions. These findings highlight the necessity of incorporating coupled geological and sedimentary processes into global carbon budgets and climate assessments, particularly in high-altitude regions.

Cascade reservoir development alters hydrological processes and ecosystem functions, particularly in environmentally sensitive karst watersheds. This study evaluated the ecosystem service values (ESVs) of six long-operating cascade reservoirs in the Maotiao River, Southwest China, using an integrated ecosystem-service valuation framework informed by selected Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) modules and supplementary ecological, social, and economic indicators. Provisioning, regulating, supporting, and cultural services were assessed using 23 indicators. Estimated total ESVs ranged from USD 501.82 to 1442.98 million yr⁻1 across reservoirs. Water conservation, industrial and domestic water supply, hydropower generation, and tourism represented the largest positive contributions, whereas erosion control costs, resettlement impacts, and cultural heritage losses constituted the major negative components. Comparative benefit–loss ratios ranged from 2.87 to 10.57, indicating substantial variation in the balance between monetized benefits and losses among reservoirs. Results suggest that karst characteristics, including strong groundwater–surface water connectivity, carbonate weathering, and groundwater dependence, may contribute to differences in ecosystem-service composition and valuation patterns compared with non-karst reservoir systems. Because the assessment integrates heterogeneous valuation domains, the aggregated ESVs and benefit–loss ratios should be interpreted as comparative indicators rather than precise measures of ecosystem value or management efficiency. The results provide a regional-scale perspective for evaluating ecosystem-service trade-offs associated with cascade reservoir development and may inform ecosystem management and planning in karst watersheds.

The association between iron minerals and dissolved organic matter (DOM) plays a crucial role in enhancing sedimentary organic carbon (OC) preservation by promoting Fe-DOM co-precipitation. This study systematically investigated the Fe-DOM co-precipitation behaviors across diverse environments, including cropland, wetlands, estuaries and deep sea environments, using ultraviolet-visible, fluorescence spectroscopy, Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and stable isotope analysis. Our results show that Fe-DOM co-precipitation substantially reduces dissolved organic carbon (DOC) concentrations under a range of hydrological and biogeochemical conditions, with higher removal efficiencies observed in cropland and estuarine environments compared to deep-sea and Yellow River samples. While the condensed aromatics and highly unsaturated compounds were preferentially removed in all environments, Fe-DOM interactions also selectively removed nitrogen-rich peptide-like compounds in deep-sea. Furthermore, carbon isotope analysis indicates that Fe-bound OC is 13C-enriched, reflecting a preferential removal of 13C-rich components due to fractionation among specific DOM compound classes. Overall, by selectively partitioning DOM based on its characteristics, Fe-DOM co-precipitation shapes the quality and quantity of the organic matter available for microbial metabolism and the water-carbon processes.

As a critical component in aquatic systems, dissolved organic matter (DOM) offers valuable insights into water quality and carbon cycling. Karst groundwater ecosystem is extensive across southwest China, and largescale ecological restoration programs, such as the Grain for Green Project, have profoundly altered its vegetation cover and soil organic matter inputs. However, the unique molecular composition of DOM and driving factors under this constantly changing land use context remain largely unknown in such an important Karst critical zone. Herein, we investigated DOM properties in a typical Karst groundwater system, the Houzhai River Basin, to identify the key environmental controls and underlying mechanisms. Our result revealed that land use type exerts a dominant control on DOM, with the 200 m buffer zone emerging as the critical spatial scale at which land use most strongly influences DOM signatures. The forest to cropland transition was associated with altered microbial community assembly, with a notable enrichment on Actinobacteriota and Armatimonadota. Additionally, the effect of microbial communities was further intensified in summer, while the coupling effect of hydrochemistry and microbes was strengthened in winter. Collectively, a multitiered regulatory framework emerges: the coupling effect of land use and microbes acts as the dominant interactive control on DOM signatures, with hydrochemical conditions serving as a secondary modulator. This study provides mechanistic insights into DOM dynamics in the Karst critical zone under large‐scale land use change, highlighting the importance of hydrological-biogeochemical coupling processes in regulating carbon cycling.

Flooded arable fields are considered to emit large amounts of N2O and CH4, but their regional-scale measurements are missing. Here, we measured N2O and CH4 concentrations and calculated their emissions from the flooded maize fields (FMFs) across North China Plain, one of the most important arable regions in China, after the heaviest rainfall in 2023 since 1960s. Our results revealed that both the concentrations and fluxes of N2O and CH4 were high (50.83 +/- 45.41 nmol l(-1) and 1.91 +/- 2.38 mu mol m(-2) h(-1) for N2O, 14.49 +/- 19.71 mu mol l(-1) and 0.69 +/- 0.92 mmol m(-2) h(-1) for CH4). The mean emission factor (EF) of N2O (0.0043) was slightly higher than that of the IPCC default value for paddy fields (0.004), while that of CH4 was about eight-folds larger. Cumulatively, the N2O and CH4 emissions from the completely flooded maize fields were estimated to be 23.3 +/- 18.6 Gg CO2-eq, with CH4 contributing over 90%, and accounted for similar to 0.91% of the regional budget, although the flooding areas were only 0.2% of the entire fields. This emission estimate should be significantly larger if the partly flooded maize fields were included. These findings highlight that flooded arable fields are significant sources of N2O and CH4 at regional scales, and should be included in future estimations.

In the Anthropocene, intensified river regulation via damming fundamentally alters global carbon cycling. While the burial of recalcitrant terrestrial organic carbon (OCter) is well recognized, the mechanisms transforming labile autochthonous organic carbon (AOC) into stable sedimentary pools via the biological carbon pump (BCP) remain poorly constrained, particularly in high dissolved inorganic carbon (DIC) karst reservoirs. Here, we integrated hydrochemistry, stable isotopic signatures (δ13C), and organic carbon (ΟC) fractionation to better understand these stabilization pathways. We find that, in karst reservoirs, elevated DIC provides a strong “fertilization effect” for BCP driven DIC to AOC conversion, whereas prolonged hydraulic retention time may favor the retention, settling, and preservation of the produced AOC. Metabolic mass balance (fOC ≈ −0.2) confirms that primary production dominates epilimnetic cycling, vastly exceeding DIC replenishment via OCter degradation. However, continuous OCter degradation introduces 13C-depleted DIC, exerting a measurable modulation on the isotopic pool as indicated by the deviation of observed δ13CPOC (−29.5‰) from the theoretical Rayleigh prediction (−27.6‰). During vertical transport, settling AOC undergoes substantial degradation despite mineral ballasting, while the remaining AOC derived material may contribute to the recalcitrant organic carbon (ROC) pool together with refractory terrestrial inputs. Consequently, augmented by refractory terrestrial inputs, the ROC pool constitutes the primary vector for long-term preservation, exceeding labile fractions by an order of magnitude (17.9–24.5 g kg−1 dw; 54.1–78.4% of total OC) and sustaining burial fluxes of 213–9273 t yr−1. Our global synthesis of 172 reservoirs reveals that multi-year regulated karst systems sustain burial rates approximately 5.4 times higher than non-karst counterparts (p < 0.05). These findings establish a quantitative framework for evaluating BCP-mediated carbon accumulation, underscoring the disproportionate role of karst reservoirs in the global carbon budget.

Blue carbon ecosystems (BCEs) store a significant amount of organic carbon (OC) in their soils, including OC sequestered by other ecosystems and transported into these ecosystems, referred to as allochthonous OC. This study reviews the state-of-the-art knowledge on the abundance and sources (including still unexplored ones) of allochthonous particulate organic carbon (POC) and reviews the ways allochthonous POC is managed under the current blue carbon policy frameworks for carbon benefits accounting in blue carbon restoration and conservation projects. Based on a review of 102 studies, we find that 56 ± 25% of the soil organic carbon (SOC) deposits in BCEs are allochthonous POC, most commonly identified as originating from terrestrial ecosystems, seston and macroalgae. Whether allochthonous POC should be included in blue carbon accounting is a matter of debate among the scientific community, due to the risks of overestimating carbon benefits and double-counting. Consequently, it is often excluded from voluntary carbon markets, but its inclusion can be justified given the role of BCEs in stabilising OC that may otherwise be remineralised. Based on existing knowledge, a deeper understanding of the stability and behaviour of different types of allochthonous POC under varying environmental conditions is needed to assess whether and to what extent these fractions can be counted toward carbon benefits.

Abstract Inland waters are significant sources of atmospheric nitrous oxide (N2O), and their emissions are expected to rise rapidly due to human activities. However, current estimates remain highly uncertain, partly because sparse field measurements and existing models fail to capture fine‐scale variations in N2O emissions, especially due to the interactions between fluid dynamics and biogeochemical drivers. This study developed an N2O module in the Environmental Fluid Dynamics Code (EFDC), a comprehensive numerical model capable of simulating hydrodynamics and water quality in different inland aquatic systems, by incorporating key nitrogen (N) processes that influence N2O production and emissions. The developed model was applied to a typical mountainous reservoir in China, and reliably reproduced the temporal and spatial variations in N2O concentrations and fluxes. Seasonal patterns of N2O fluxes were primarily driven by temperature, stratification, and nutrient availability, with autumn exhibiting the peak values. In contrast, spatial variations were mainly regulated by fluid dynamics and nutrient availability, with the highest N2O fluxes observed in the main part of the reservoir. Furthermore, the whole‐reservoir N2O budget indicated that sediments were the primary source of N2O, which is mainly produced via denitrification under low dissolved oxygen conditions, underscoring the importance of understanding the sedimentary N cycle. Overall, our model enables the quantification of significant spatiotemporal variations in N2O emissions from inland waters and unravels the underlying mechanisms, thereby enhancing our mechanistic understanding of N2O dynamics and improving the quantification accuracy of N2O emissions at broader scales.

Inland waters constitute vital components in the global carbon (C) cycle. Nevertheless, the regional patterns of aquatic carbon transfer across China remain poorly quantified, and their impacts on the terrestrial carbon budget remain unclear. Here, we quantified the three primary aquatic carbon output fluxes and evaluated their implications for China's terrestrial carbon budget. We showed that the total aquatic carbon output flux was 165 +/- 20 Tg C yr-1, counteracting 29% of China's terrestrial net ecosystem productivity (NEP). Carbon dioxide (CO2) emissions from river networks (86 +/- 19 Tg C yr-1) dominated the aquatic carbon transfer, accounting for 52% +/- 13% of China's aquatic carbon output flux. Carbon burial in reservoirs (20.2 +/- 3.4 Tg C yr-1) represented a significant anthropogenic redistribution of terrestrial carbon equivalent to 46% +/- 8% of the downstream carbon export (43.6 +/- 1.8 Tg C yr-1) across China. Furthermore, the positive correlations between aquatic carbon yield and precipitation and terrestrial NEP indicated a direct climatic control on aquatic carbon dynamics, whereas human activities have amplified these effects. Considerable spatial variations were observed in the net landscape carbon balance (NLCB) across China. Specifically, the NLCB in low-productivity regions was sensitive to aquatic carbon transfer, whereas terrestrial ecosystem productivity exerted the dominant control in high-productivity ecosystems. Our findings elucidate the spatial patterns and regulatory mechanisms governing aquatic carbon transfer across China, establishing a robust framework to refine regional carbon balance assessments and inform targeted climate management strategies.

Watersheds are natural integrators of water, materials, energy, and information flows, and they have long served as the operational units for hydrology and environmental management. Yet under the accelerating pressures of the Anthropocene-climate non-stationarity, land-use intensification, engineered regulation, and socio-economic coupling-the reductionist tradition of decomposing a basin into isolated sub-systems has reached its limits. Persistent bottlenecks include scale mismatch between local processes and global drivers, fragmentation across spheres (water-air-rock-biosphere-human systems), and weak translation of heterogeneous data and model outputs into policy. This review therefore traces the evolution from classical Watershed Science (WS) to Watershed System Science (WSS) and further to Watershed Earth System Science (WESS), arguing for a paradigm that unifies mechanism discovery, prediction, and governance within a single evidence chain. We define WSS as a system-oriented turn that elevates multi-sphere coupling ("water-rock-air-biosphere-intelligence"), emergent behavior (nonlinearity, thresholds, lags), and human-nature coevolution at the basin scale. WSS reframes the basin as a complex adaptive system and shifts inquiry from single-process explanations to mechanism-rich representations that can diagnose how flow paths, residence time, connectivity, biogeochemistry, and human decision-making co-produce macroscopic patterns. WESS extends this logic into the Earth-system domain: under planetary boundary constraints, it seeks two-way coupling and consistent upscaling/downscaling between watershed, regional, and global representations, so that local disturbances (e.g., land conversion, reservoir operation) and global responses (e.g., climate feedbacks, carbon-water budgets) are expressed within a unified modeling and assessment architecture. Methodologically, the paper synthesizes an end-to-end workflow-Observation -> Data Assimilation -> Coupled Modeling -> Scenario Evaluation -> Governance Translation-that closes the loop between evidence and decision. Observation integrates air-space-ground platforms (satellites, UAVs, in-situ sensor webs) with tracers, isotopes, and eDNA to reveal water-biogeochemical coupling at high resolution. Ensemble Kalman/variational schemes then assimilate multi-source data to jointly constrain state variables (soil moisture, LAI, nutrient concentrations) and sensitive parameters (conductivity, reaction rate constants). On this foundation, modular coupling links basin hydrology and river-network water quality with land/vegetation and carbon modules in Earth-system models, while maintaining parsimony through standardized process libraries and interfaces. Scenario analysis leverages ensemble experiments to detect thresholds and early-warning signals under extremes and management options; outputs are mapped to decision-ready metrics-ecological flow, greenhouse-gas fluxes, nutrient loads, and resilience. The review highlights how technological advances (e.g., eDNA, digital twin basins) and global integration (embedding watershed datasets into Earth-system models) are pushing the field from explaining the past to designing futures, including proactive flood/drought risk governance, cross-border water cooperation, and SDG-aligned multi-objective trade-offs. A concise typology of model families (land/vegetation, basin hydrology and water quality, soil/crop GHG, riverine transport, and global couplers) is provided with notes on scale fitness and limitations, underscoring hydrology's role as the coupling "spine" because flow paths, residence time, and connectivity regulate redox environments and reaction opportunities that set biogeochemical rates. Finally, we outline forward paths: (i) deepen cross-scale coupling and parameter transfer with consistent boundary and state spaces; (ii) prioritize key interfaces (riparian zones, wetlands, reservoirs, estuaries) where water-biogeochemistry-human interactions concentrate; (iii) institutionalize open benchmark datasets and joint assimilation for reproducibility; and (iv) co-design governance-facing indicators and platforms that translate science into implementable policy (from ecological flows to carbon-nutrient budgets and resilience diagnostics). Together, WSS and WESS reposition watersheds as "nervous nodes" of the Earth system and provide verifiable, operational decision support to meet water security, climate adaptation, and ecosystem restoration goals under planetary constraints.

River damming globally disrupts hydrological connectivity, transforming reservoirs into critical regulators of the global carbon cycling. However, the molecular characteristics and the specific transformation processes of dissolved organic matter (DOM) in cascade reservoirs remain unclear. To address this knowledge gap, we integrated water chemistry, stable carbon isotopes, and Fourier transform ion cyclotron resonance mass spectrometry to elucidate the molecular fate of DOM across cascade river-reservoir systems on the Eastern Tibetan Plateau. Results demonstrated that cascade damming enhanced autochthonous DOM production, as estimated by a simmr-based stable-isotope mixing model, increasing the phytoplankton-derived fraction of DOM from 27 ± 10% in rivers to 31 ± 12% in cascade reservoirs (posterior mean ± SD). Simultaneously, the favorable thermodynamic condition accelerated the turnover of allochthonous DOM by enhancing both photochemical and microbial decomposition processes within the warming lentic areas. These processes amplified DOM homogenization, evidenced by a decrease in the Jaccard dissimilarity coefficient along the flow path, resulting from consuming labile fractions. Furthermore, the reduced hydrological connectivity due to dam construction facilitated the downstream transport of autochthonous DOM. It is estimated that the global export flux of autochthonous dissolved organic carbon to the oceans amounts to 141 ± 2 Tg C yr-1. Our results highlight that the global proliferation of reservoirs fundamentally alters the biogeochemical fate of DOM, with implications for regional and global carbon cycling.

Unbalanced urban–rural development remains a critical global challenge, particularly in developing countries where widening gaps hinder economic growth and threaten social stability. Traditional Lewis dual-economy theory explains the origins of income disparity but overlooks infrastructure mobility, modern agricultural technology, and the tertiary sector's expanding influence. To address these limitations, this study investigates the Chengdu-Chongqing Economic Circle—an emblematic region of western China with pronounced urban–rural divides—using nighttime light (NTL) data to reveal spatial economic dynamics. A refined spatial division of urban and rural areas was established, and a spatial entropy–based Urban-Rural Development Equilibrium Index (UREDI) was constructed to assess spatial heterogeneity. Pearson correlation and partial correlation analyses, coupled with the XGBoost-SHAP attribution model, were employed to identify key drivers of urban-rural balance. Results show that: (1) high NTL intensity and economic richness are concentrated in Chengdu and Chongqing, with diffusion to surrounding areas after 2010; however, internal disparities have widened. (2) The number of secondary and tertiary industry employees, enterprises above designated size, mechanized harvest area, urbanization rate, and road density are significantly and positively correlated with UREDI. (3) After controlling for other factors, only urbanization rate remains significant (R = 0.394, p < 0.001), and its and its mean absolute SHAP value (0.307) far exceeds others, highlighting it as the core driver of urban–rural equilibrium through factor mobility, industrial integration, and technology diffusion. This study refines the Lewis model framework and provides insights for addressing urban–rural imbalance in developing regions globally.