Abstract Permafrost thaw exposes ancient organic matter to microbial degradation, which is predicted to release globally significant quantities of greenhouse gases into the atmosphere. Though microorganisms drive these processes, the relative importance of biotic (taxonomic and functional community composition) versus environmental (e.g., soil physicochemistry) drivers and their interactions are unknown. Using a novel in situ thaw experiment conducted at the Cold Regions Research and Engineering Laboratory’s Permafrost Tunnel near Fairbanks, Alaska, we experimentally separated the effects of soil physicochemistry and microbial communities under “real-world” thaw conditions. To simulate thaw, active layer soil, Holocene permafrost (2 kya), and Pleistocene permafrost (40 kya) were sterilized, inoculated with microbial communities from the different soils, enclosed in 0.22 µm membrane bags to prevent immigration, and buried in the active layer. We retrieved the bags after two weeks and two months of thaw and characterized microbial community structure (16S rRNA and ITS2 amplicon sequencing), functional potential (metagenome sequencing), and soil organic matter (OM) composition at the molecular level (FT-ICR MS). Soil had a stronger effect on bacterial community and gene assemblages than inoculum, and the effects of inoculum were stronger and longer-lasting on community structure than functional potential. Pleistocene permafrost initially contained approximately eleven times more dissolved organic carbon than the other soils, and was enriched in OM derived from microbial necromass and low molecular weight organic acids. This carbon was rapidly depleted during thaw and OM compositional characteristics became increasingly similar to active layer and Holocene permafrost, paralleling shifts in Pleistocene permafrost functional gene profiles and bacterial community structure towards those of other soils. Overall, this work provides new insights into the susceptibility of OM to microbial degradation in compositionally distinct permafrost soils, and ways in which Pleistocene Yedoma permafrost carbon is likely to be particularly vulnerable to permafrost thaw.

Sunlight penetrates the bare-ice surface of glaciers and ice sheets, giving rise to the presence of a three-dimensional porous matrix of partially melted ice crystals known as the weathering crust. Surface meltwater slowly percolates through this weathering crust, which hosts active and diverse bacterial communities, until it reaches a supraglacial stream. Despite the potential implications of weathering crust dynamics for glacial melting and the export of carbon and nutrients to downstream ecosystems, its role in biogeochemical cycling remains unknown. Here, we use Fourier transform cyclotron resonance mass spectrometry to characterize dissolved organic matter (DOM) along a meltwater flow path in a hydrologically connected micro-catchment on the southern Greenland Ice Sheet. We find a decrease in the relative abundance of aromatic formulae from surface ice (24.9 ± 2.8 %) to weathering crust meltwater (3.5 ± 0.3 %) to supraglacial stream water (2.2 ± 0.2 %), pointing towards photodegradation of aromatic DOM during supraglacial meltwater transit. The relative abundance of aliphatic and peptide-like formulae in supraglacial stream DOM was lower (38.5 ± 4.0 %) than in weathering crust meltwater DOM (50.3 ± 2.4 %), likely as a result of microbial respiration of labile compounds within the weathering crust. Hence, we conclude that the weathering crust plays a thus far unexplored role in supraglacial biogeochemical cycling. In addition, we characterize water-extractable organic matter isolated from surface ice particulate matter, which was predominantly (61.6 ± 8.1 % relative abundance) comprised of aliphatic and peptide-like formulae, providing the first direct evidence of surface ice particulate matter as a potential source of biolabile DOM. As the spatial extent of bare-ice surfaces and the associated weathering crust photic zone is set to increase under a warming climate, our findings underscore the pressing need to further evaluate the role of the weathering crust in supraglacial biogeochemical processes. An understanding of weathering crust biogeochemical cycling is especially critical as climatic warming is predicted to lead to an increase in Arctic rainfall, consequently increasing the frequency of weathering crust degradation events, with unknown impacts on the export of supraglacial DOM to downstream ecosystems.

Glaciers export ancient bioavailable dissolved organic carbon (DOC). Yet, the sources of organic carbon (OC) underpinning bioavailability are poorly constrained. We assessed the isotopic composition of respired OC from bioincubations of glacier DOC. Relative to bulk DOC, respired OC was younger (+4350 to 8940 years) and 13C enriched (+9.2 ‰ to 12.2 ‰), consistent with utilization of an in situ-produced microbial carbon source. These findings provide direct evidence that a hidden pool of young OC may underpin the high bioavailability of ancient glacier DOC.

Intensification of wastewater treatment residual (i.e., biosolid) applications to watersheds can alter the amount and composition of organic matter (OM) mobilized into waterways. To identify novel tracers of biosolids, characterization of biosolids and their impacts on OM composition in recipient ecosystems is required. Here, water-soluble OM was leached from surface soils from Florida pastures with differing levels of biosolid amendment and an adjacent control site. The biosolid endmember was further constrained by extracting water-soluble OM from biosolids sourced from four Florida wastewater treatment facilities. Nontargeted analysis of organic molecules by negative-ion electrospray ionization 21 T Fourier transform ion cyclotron resonance mass spectrometry examined the molecular composition of soil and biosolid leachates and identified molecular formulas unique to these biosolids and biosolid amended soils. Overall, biosolids leachates were enriched in aliphatic (+16.3% relative abundance) and heteroatomic (+42.5% RA) formulas and depleted in aromatic formulas (-33.5% RA) compared to soil leachates. A subset of 297 molecular formulas were present only in biosolids and amended soil leachates (i.e., not present in control soil leachates), the vast majority of which contained nitrogen (66%) or sulfur (27%). The identification of these molecular formulas is a key step in identifying novel tracers of biosolids movement through impacted watersheds.

The Arctic is experiencing unprecedented rates of climate change, leading to numerous disturbances on the terrestrial landscape, including shrubification, increased frequency of wildfires, and permafrost thaw. These changes may impact the mobilization of terrestrial organic carbon into Arctic rivers and are hypothesized to lead to distinct alterations to the molecular composition and thus the reactivity of riverine dissolved organic matter (DOM). To understand how these three major perturbations may impact DOM dynamics in Arctic fluvial and coastal systems, we examined the concentration and bioavailability of dissolved organic carbon (DOC) together with the molecular-level DOM composition of different source endmember leachates from the Yukon River watershed using biodegradation incubation experiments and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Simulated climate-related landscape perturbations generally led to increased leachate DOC concentrations. Incubations demonstrated that the biodegradability of leachate DOC was lowest for vegetation endmembers, particularly for shrubs (12.3% DOC loss), and highest for thawing Yedoma permafrost (64.9% loss) and organic-rich tundra soil (70.9% loss). FT-ICR MS highlighted that aliphatic and high-H/C molecular formulas were preferentially biodegraded, whereas condensed aromatic and polyphenolic compounds were relatively enriched post-biodegradation in all endmember leachates. Together these findings suggest that with continued climate change and landscape perturbation, larger amounts of less bioavailable DOC will be mobilized into Arctic rivers leading to higher relative amounts of highly aromatic, biologically stable DOM being exported into receiving ecosystems and the Arctic Ocean, potentially altering the rates and mechanisms of carbon turnover in the coastal zone.

Wetland and permafrost soils contain some of Earth's largest reservoirs of organic carbon, and these stores are threatened by rapid warming across the Arctic. Nearly half of northern wetlands are affected by permafrost. As these ecosystems warm, the cycling of dissolved organic matter (DOM) and the opportunities for microbial degradation are changing. This is particularly evident as the relationship between wetland and permafrost DOM dynamics evolves, especially with the introduction of permafrost‐derived DOM into wetland environments. Thus, understanding the interplay of DOM composition and microbial communities from wetlands and permafrost is critical to predicting the impact of released carbon on global carbon cycling. As little is understood about the interactions between wetland active layer and permafrost‐derived sources as they intermingle, we conducted experimental bioincubations of mixtures of DOM and microbial communities from two fen wetland depths (shallow: 0–15 cm, and deep: 15–30 cm) and two ages of permafrost soil (Holocene and Pleistocene). We found that the source of microbial inoculum was not a significant driver of dissolved organic carbon (DOC) degradation across treatments; rather, DOM source and specifically, DOM molecular composition, controlled the rate of DOC loss over 100 days of bioincubations. DOC loss across all treatments was negatively correlated with modified aromaticity index, O/C, and the relative abundance of condensed aromatic and polyphenolic formula, and positively correlated with H/C and the relative abundance of aliphatic and peptide‐like formula. Pleistocene permafrost‐derived DOC exhibited ∼70% loss during the bioincubation driven by its initial molecular‐level composition, highlighting its high bioavailability irrespective of microbial source.

Sedimentary organic matter (SOM), an organic mixture of complex compositions with diverse structures, plays a crucial role in lake carbon cycling. The quantity, sources, compositions and biogeochemical behaviors of SOM typically vary with lake ecosystem evolution and mineralization process. However, the dynamic response and transformation pathway of SOM during these joint processes remain incompletely understood, particularly in time dimension. Geochronology, total organic carbon (TOC), nutrients and SOM sources in a sediment core from Erhai Lake were determined using radionuclides, geochemical analyses and stable carbon and nitrogen isotopes. Fluorescence spectroscopy and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to detect the optical and molecular compositions of SOM. A remarkable increase in TOC, total nitrogen (TN), and total phosphorus (TP) contents was observed after the 2000s, accompanied by a rise in autochthonous sources. These changes were largely attributed to lake eutrophication caused by intensive human activities and climate warming, and increased water residence time. Autochthonous sources shifted SOM compositions toward more protein-like component and aliphatic compound, which are more labile and susceptible to microbial degradation. Long-term mineralization led to a gradual decrease in TOC, TN and TP contents over time. Concurrently, the accumulation of humic-like component and CHO-type highly unsaturated and phenolic (HUPs) compound characterized by higher aromaticity and molecular weight promoted the preservation of refractory SOM in deeper sediments, thereby contributing significantly to lake carbon sinks. Network analysis based on mass difference revealed that oxygen addition, dealkylation and decarboxylation were the dominant reactions under the transformation pattern of molecular reactants to products. The CHO formation was related to deamination and desulfonation within N- or S-containing formulae. This study demonstrates that SOM characteristics are sensitive and responsive to lake evolution and mineralization on geological timescales, and offers some new insights about managements of endogenous pollution in lakes.

Small lakes on the Yunnan-Kweichow Plateau of Southwest China are significant carbon sinks, and highly sensitive recorders of global climate change and human disturbance. However, the lack of robust sediment chronologies makes it challenging to unravel the dynamics of dissolved organic matter (DOM) accumulation in these sedimentary successions, and the driving mechanisms of change over historical timescales. In this paper, we constructed a robust chronology (1850-2019 CE) based on radioactive lead (210Pb) for Xihu Lake, and used elemental analysis, optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and stable carbon isotopic measurements to evaluate long-term variations in the abundance, source and composition of sedimentary DOM. We found that DOM abundance increased after 2000 CE due to intensive socio-economic activities and climatic warming within the watershed. Stable carbon isotopic results are consistent with spectral parameters, suggesting the additional DOM abundance was mainly derived from annual input from autochthonous aquatic plants. These autochthonous sources shifted the DOM pool into saturated and S-containing aliphatic compound and protein-like fluorescence component, which had higher bio-lability and mineralization potential. After successive mineralization for hundreds of years, sedimentary DOM underwent a transition characterized by reduced bio-lability and increased persistence with time gradients. More oxygenated, aromatic and high-molecular-weight DOM fractions were preserved in deeper and older sediments, which became a key component of lake carbon sinks. Furthermore, massive diagenetic transformations resulted in an increasing tendency in diversity and richness of molecular assemblage. This work provides a comprehensive understanding of historical inputs, degradations and preservations of sedimentary DOM, and offers new insights into our understanding of DOM dynamics from a paleolimnological perspective.

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Runoff from rapidly melting mountain glaciers is a dominant source of riverine organic carbon in many high-latitude and high-elevation regions. Glacier dissolved organic carbon is highly bioavailable, and its composition likely reflects internal (e.g., autotrophic production) and external (i.e., atmospheric deposition) sources. However, the balance of these sources across Earth's glaciers is poorly understood, despite implications for the mineralization and assimilation of glacier organic carbon within recipient ecosystems. We assessed the molecular-level composition of dissolved organic matter from 136 mountain glacier outflows from 11 regions covering six continents using ultrahigh resolution 21 T mass spectrometry. We found substantial diversity in organic matter composition with coherent and predictable (80% accuracy) regional patterns. Employing stable and radiocarbon isotopic analyses, we demonstrate that these patterns are inherently linked to atmospheric deposition and in situ production. In remote regions like Greenland and New Zealand, the glacier organic matter pool appears to be dominated by in situ production. However, downwind of industrial centers (e.g., Alaska and Nepal), fossil fuel combustion byproducts likely underpin organic matter composition, resulting in older and more aromatic material being exported downstream. These findings highlight that the glacier carbon cycle is spatially distinct, with ramifications for predicting the dynamics and fate of glacier organic carbon concurrent with continued retreat and anthropogenic perturbation.

AbstractUnderstanding the fate of permafrost‐derived dissolved organic matter (DOM) is critical for unraveling its role in carbon cycling. However, it remains unclear whether the high lability of permafrost‐derived DOM can be attributed to intrinsic chemical properties or elevated carbon concentrations. We investigated the dynamics of permafrost DOM from the Qinghai–Tibetan Plateau using both biodegradation and photodegradation experiments. Biodegradation and photodegradation of permafrost‐derived DOM exhibited distinct qualitative preferences for specific chemical groups (i.e., peptide‐like and aromatics, respectively). Notably, reducing the initial concentration of dissolved organic carbon (DOC) by half and a quarter resulted in shifts in biodegradable DOC content from 11.2% to 11.5% and 8.5%, respectively, accompanied by a corresponding decrease in the biodegradation rate from 0.11 to 0.06 and 0.03. This insight highlights the importance of recognizing the interplay between DOM quality and concentration and bears broader significance for our understanding of the fate of permafrost‐derived DOM in natural ecosystems.

West Siberia contains some of the largest soil carbon stores on Earth owing to vast areas of peatlands and permafrost, with the region warming far faster than the global average. Organic matter transported in fluvial systems is likely to undergo distinct compositional changes as peatlands and permafrost warm. However, the influence of peatlands and permafrost on future dissolved organic matter (DOM) composition is not well characterized. To better understand how these environmental drivers may impact DOM composition in warming Arctic rivers, we used ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry to analyze riverine DOM composition across a latitudinal gradient of West Siberia spanning both permafrost-influenced and permafrost-free watersheds and varying proportions of peatland cover. We find that peatland cover explains much of the variance in DOM composition in permafrost-free watersheds in West Siberia, but this effect is suppressed in permafrost-influenced watersheds. DOM from warm permafrost-free watersheds was more heterogenous, higher molecular weight, and relatively nitrogen enriched in comparison to DOM from cold permafrost-influenced watersheds, which were relatively enriched in energy-rich peptide-like and aliphatic compounds. Therefore, we predict that as these watersheds warm, West Siberian rivers will export more heterogeneous DOM with higher average molecular weight than at present. Such compositional shifts have been linked to different fates of DOM in downstream ecosystems. For example, a shift toward higher molecular weight, less energy-rich DOM may lead to a change in the fate of this material, making it more susceptible to photochemical degradation processes, particularly in the receiving Arctic Ocean. West Siberia is warming faster than other regions and contains vast areas of peatlands and permafrost, which contain vast stores of carbon. This carbon is transported off the landscape by rivers and the composition of this exported carbon is likely to change with continued warming, but there is no consensus on exactly what changes will occur. To study these potential changes, we used ultrahigh resolution mass spectrometry to analyze molecular-level organic matter composition across a gradient of permafrost influence and peatland cover in West Siberian watersheds. Warm permafrost-free watersheds had organic matter that was more diverse, of higher molecular weight, and had unique molecular composition compared to cold permafrost-influenced watersheds. We also found that while peatland cover explained much of the compositional diversity between rivers, permafrost ultimately controlled the influence of peatland cover on dissolved organic matter composition, effectively acting as a switch on the compositional signal from peatlands. We predict that as West Siberia warms, the fate of organic matter transported by rivers in the region will thus change and the role of photochemical degradation processes may become more important. West Siberian watersheds exhibit distinct dissolved organic matter (DOM) composition related to permafrost influence and peatland cover Permafrost acts as a switch controlling the influence of peatland cover on the molecular composition of watershed DOM Warming West Siberian watersheds may export more heterogeneous DOM with ramifications for its fate in the Arctic Ocean

Abstract. During the ablation season, active microbial communities colonise large areas of the Greenland Ice Sheet surface and produce dissolved organic matter (DOM) that may be exported downstream by surface melt. Meltwater flow through the bare ice interfluvial area, characterized by a porous weathering crust, is slow (~ 10-2 m d-1), meaning that it presents a potential site for photochemical and/or microbial alteration of supraglacial DOM. Transformations of supraglacial DOM during transport through the supraglacial drainage system remain unexplored, limiting our understanding of supraglacial DOM inputs to downstream subglacial and coastal ecosystems. Here, we employ negative-ion electrospray ionization 21 tesla Fourier transform ion cyclotron resonance mass spectrometry to catalogue the molecular composition of DOM in supraglacial dark ice, weathering crust meltwater, and supraglacial stream water sampled in a hydrologically connected supraglacial micro-catchment to address this knowledge gap. Dark ice DOM contained significantly more aromatic (25 ± 3 %) and less biolabile (13 ± 4 %) DOM than weathering crust meltwater (3 ± 0 and 50 ± 0 %, respectively), pointing to retention of DOM on the ice surface and microbial, as well as photochemical alteration of DOM during transit through the supraglacial drainage system. These findings have implications for our understanding of supraglacial biogeochemical cycling, highlighting the importance of including the weathering crust photic zone when assessing supraglacial inputs to subglacial and downstream ecosystems.

Arctic amplification is leading to increased terrestrial organic carbon (terrOC) mobilization with downstream impacts on riverine and marine biogeochemistry. To improve quantification and characterization of terrOC discharged to the Arctic Ocean, Yukon River delta samples were collected during three stages of the annual hydrograph (ascending limb/peak freshet, descending limb, late summer) and across a land-to-ocean salinity gradient (0.08-29.06 ppt). All samples were analyzed for dissolved organic carbon (DOC) concentration and lignin phenols to determine seasonal variability in riverine terrOC and salinity-induced transformation of highly aromatic terrestrial compounds. Additionally, the relationship between lignin and absorbance at 350 and 412 nm was assessed to determine the feasibility of using optical proxies for accurate quantification, both seasonally and across expansive salinity gradients. Lignin phenols were highest during the ascending limb/peak freshet (0.58-0.97 mg/100 mg OC) when riverine DOC was dominated by young vascular plant sources, whereas lignin phenols were lower (0.15-0.89 mg/100 mg OC) and riverine DOC more variable in terrestrial source and diagenetic state during the descending limb and late summer. Across the sampled salinity gradient, there was disproportionate depletion of lignin (up to 73%) compared to DOC (up to 22%). Finally, while optical proxies can be used to quantify lignin within seasonal or spatial contexts, increased uncertainty is likely when expanding linear correlations across Arctic land-ocean continuums. Overall, results indicate seasonal, spatial, interannual, and climatic controls that are amplified during high-flow conditions and important to constrain when investigating Arctic terrOC cycling and land-ocean DOC flux. The Arctic is experiencing an amplified warming phenomenon that is driving a variety of landscape changes. Through these landscape transformations, large amounts of organic carbon can be transported to nearby rivers. However, the fate and transport of organic carbon along Arctic land-ocean continuums is largely unknown, primarily due to simultaneous complex processes that occur in deltas and coastal zones. To improve the quantification of landscape-derived dissolved organic carbon (DOC) transported from the land to the Arctic Ocean, surface water samples were collected throughout the Yukon River delta (Alaska, USA) during three distinct seasons and across a land-ocean salinity gradient extending from freshwater to high-salinity water. To measure riverine organic carbon coming from the landscape, we analyzed all samples for DOC and an environmental biomarker that is specific to terrestrial sources (lignin phenols). Lignin concentrations and compositions were used to determine seasonal differences in the magnitude and type of terrestrial inputs, as well as coastal processing of organic carbon. Results show spatial and seasonal differences in DOC concentration and composition within the Yukon River delta and coastal zone that highlight the complexity of carbon cycling in Arctic regions. Riverine organic carbon is compositionally distinct in source and signature during the freshet compared to other stages of the hydrograph During the freshet, there is preferential loss of lignin compared to bulk dissolved organic carbon across a land-to-ocean salinity gradient Lignin-chromophoric dissolved organic matter relationships vary across Yukon interfaces (river to delta, plume water to high salinity)

Dissolved organic carbon (DOC) flux from rivers in the pan‐Arctic watershed represents an important connection between major terrestrial carbon stocks and the Arctic Ocean. Previous estimates of Arctic carbon flux and dissolved organic matter (DOM) seasonal dynamics have relied predominantly on measurements from the six major Arctic rivers, yet these may not be representative of northern high‐latitude constrained smaller watersheds. Here, we evaluate DOC concentration and DOM composition in the Onega River, a small Arctic watershed, using optical measurements and ultrahigh resolution mass spectrometry. Compared to the six largest Arctic rivers, DOC, absorbance at a 350 , and indicators of terrestrial DOM (e.g., specific UV absorbance at 254 nm, modified aromaticity index, relative abundance of condensed aromatics and polyphenolics) were elevated in the Onega throughout the year. Seasonality was also generally muted in comparison to the major Arctic rivers with relatively elevated DOC and terrestrial markers in both spring and fall seasons. The Onega exhibits a strong relationship between a 350 and DOC, and its organic‐rich nature is apparent in its high DOC yield (4.85 g m 2 yr −1 ), and higher chromophoric DOM per unit DOC than the six largest Arctic rivers. As DOC yield from the Onega may be more representative of smaller northern high‐latitude rivers, we derived a new pan‐Arctic DOC flux scaling estimate which is over 50% higher than previous estimates scaled solely from the six major Arctic rivers. These observations suggest that smaller northern high‐latitude rivers may be underrepresented in Arctic carbon flux models and highlights uncertainty around constraining the export of DOC to the Arctic Ocean.

The biogeochemistry of rapidly retreating Andean glaciers is poorly understood, and Ecuadorian glacier dissolved organic matter (DOM) composition is unknown. This study examined molecular composition and carbon isotopes of DOM from supraglacial and outflow streams ( n = 5 and 14, respectively) across five ice capped volcanoes in Ecuador. Compositional metrics were paired with streamwater isotope analyses (δ 18 O) to assess if outflow DOM composition was associated with regional precipitation gradients and thus an atmospheric origin of glacier DOM. Ecuadorian glacier outflows exported ancient, biolabile dissolved organic carbon (DOC), and DOM contained a high relative abundance (RA) of aliphatic and peptide‐like compounds (≥27%RA). Outflows were consistently more depleted in Δ 14 C‐DOC (i.e., older) compared to supraglacial streams (mean −195.2 and −61.3‰ respectively), perhaps due to integration of spatially heterogenous and variably aged DOM pools across the supraglacial environment, or incorporation of aged subglacial OM as runoff was routed to the outflow. Across Ecuador, Δ 14 C‐DOC enrichment was associated with decreased aromaticity of DOM, due to increased contributions of organic matter (OM) from microbial processes or atmospheric deposition of recently fixed and subsequently degraded OM (e.g., biomass burning byproducts). There was a regional gradient between glacier outflow DOM composition and streamwater δ 18 O, suggesting covariation between regional precipitation gradients and the DOM exported from glacier outflows. Ultimately, this highlights that atmospheric deposition may exert a control on glacier outflow DOM composition, suggesting regional air circulation patterns and precipitation sources in part determine the origins and quality of OM exported from glacier environments.

Dissolved inorganic carbon (DIC) represents a major global carbon pool and the flux from rivers to oceans has been observed to be increasing. The effect of weathering with respect to increasing DIC has been widely studied in recent decades; however, the influence of dissolved organic matter (DOM) on increasing DIC in large rivers remains unclear. This study employed stable carbon isotopes and Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) to investigate the effect of the molecular composition of DOM on the DIC in the Yangtze River. The results showed that organic matter is an important source of DIC in the Yangtze River, accounting for 40.0 +/- 12.1 % and 32.0 +/- 7.2 % of DIC in wet and dry seasons, respectively, and increased along the river by approximately three times. Nitrogen (N)-containing DOM, an important composition in DOM with a percentage of -40 %, showed superior oxidation state than non N-containing DOM, suggesting that the presence of N could improve the degradable potential of DOM. Positive relationship between organic sourced DIC (DICOC) and Ncontaining DOM formulae indicated that N-containing DOM is crucial to facilitate the mineralization of DOM to DICOC. N-containg molecular formular with low H/C and O/C ratio were positively correlated with DICOC further verified these energy-rich and biolabile compounds are preferentially decomposed by bacteria to produce DIC. Ncontaining components significantly accelerated the degradation of DOM to DICOC, which is important for understanding the CO2 emission and carbon cycling in large rivers.