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• Connected areas shows dynamic extension and contraction during the rainfall. • Aromaticity and unsaturated degree of DOM reach maximums at peak discharge. • Hydrological connectivity accelerates terrestrial aromatics export. • DOC flux dramatically increases at connectivity strength above 0.14. The quantity and quality of dissolved organic matter (DOM) exported from source areas are closely related to hydrological linkage between source areas and streams, that is hydrological connectivity. However, understanding of how hydrological connectivity regulates the export of catchment DOM components remains inadequate. In this study, high-frequency monitoring of groundwater and runoff from subtropical humid catchment was conducted for 20 months, and hydrological connectivity was quantitatively characterized by considering both surface and subsurface hydrological processes. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was utilized to investigate the DOM molecular composition. Results showed that over half of the areas in the catchment could not persistently establish hydrological connectivity with the stream during the rainfall. The average proportion of lignin was the highest in DOM components, followed by tannin and proteins. Additionally, both modified aromaticity index and double bond equivalence reached maximums at peak discharge, reflecting terrestrial materials could increase DOM aromaticity and unsaturated degree. Partial least square-structural equation modeling revealed significantly direct effects of rainfall, antecedent conditions, and hydrological connectivity on dissolved organic carbon (DOC) export. Furthermore, nonlinear relationships were observed between hydrological connectivity and DOC, tannin, and condensed aromatics. Specifically, the instantaneous DOC flux increased dramatically when the hydrological connectivity strength exceeded 0.14; tannin and condensed aromatics exhibited a rapid increase with rising connectivity strength, but remained stable at connectivity strength above 0.25. However, hydrological connectivity showed no significant correlation with unstable components (such as lipids, protein, amino sugars, and carbohydrates). These results provide new insights into hydrological controls on the quantity and quality of DOM export and contribute to developing appropriate catchment management strategies for carbon storage. [Display omitted] [ABSTRACT FROM AUTHOR]

Dissolved black carbon (DBC) involves in many biogeochemical processes in both terrestrial and aquatic environments. About 26.5 Tg of charcoal- or black carbon-derived DBC was released into aquatic environments annually, accounting to ∼10% of the global riverine flux of dissolved organic carbon (DOC). Yet the sorption behaviors of DBC and their effects on water quality in soil-water environment are poorly understood. Here we examined the molecular composition variations of DOC induced by the sorption of two biochar-derived DBCs (pyrolyzed at 300 °C and 500 °C) on three contrasting soils. The DBCs were adsorbed mainly through competitive displacement of soil surface functional groups and co-sorption with soil indigenous DOC, which varied with soil properties and the aromaticity of the DBCs. Ultrahigh resolution mass spectrometry analysis indicated that compounds with rich oxygen content or unsaturated structures such as tannins and unsaturated aromatics from both DBC and soil DOC, were preferentially adsorbed on the soils in the presence of DBC. In contrast, compounds with high aromatic structures including condensed aromatics and lignins were concentrated in the aquatic phase. Molecular fractionation also occurred to the heteroatomic compounds during the sorption, and the heteroatomic dissolved organic sulphur in the DBCs was easier to be adsorbed relative to dissolved organic nitrogen. Our results suggest that DBC sorption in soil-water environment could have important implications for water quality by altering DOC molecular composition and decreasing DOC molecular diversity at the soil-water interface. This study provides essential information for understanding the behavior of DBC in the environments. Image 1 • DBC was adsorbed mainly via co-sorption with soil DOC and ligand displacement on soil. • Molecular fractionation occurred to DBC during sorption at soil-water environment. • DBC sorption induced the decrease of molecular diversity of DOC at soil-water interface. • Biochar-derived dissolved organic N was more prone to be leached than dissolved organic S. [ABSTRACT FROM AUTHOR]

• MPs-DOM release and transformation vary depending on the plastic composition. • Molecular diversity and evolution of MP-DOM from PET- and PE-MPs were examined. • UV irradiation induces greater DOM leaching from PET- than PE-MPs. • PET-DOM became more recalcitrant under UV irradiation compared to PE-DOM. Despite extensive research into the presence and behavior of microplastics (MPs) in the environment, limited attention has been given to the investigation of the characteristics of dissolved organic matter (DOM) that leaches from MPs (MPs-DOM). Herein, two frequently encountered plastic particles in aquatic environments, specifically polyethylene terephthalate (PET)- and polyethylene (PE)-MPs, were subjected to leaching in the aquatic settings for seven days, both in the absence of light and under UV irradiation. Measurements of dissolved organic carbon (DOC) indicated that UV exposure enhanced the liberation of DOM from PET-MPs, while PE-MPs did not exhibit such leaching. After UV treatment for seven days, the DOM released from PET-MPs increased by 25 times, while that from PE-MPs remained almost unchanged. Then, the molecular diversity and the evolving formation of DOM originating from different MPs were comprehensively analyzed with fluorescence excitation-emission matrix (EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Specifically, both PET- and PE-DOM exhibited three fluorescence signatures, with the predominant C1 (tryptophan-like) component showing a decline in PET-DOM and a rise in PE-DOM during aging. The FT-ICR-MS analysis unveiled that PET-DOM grew more recalcitrant under UV exposure, while PE-DOM became increasingly labile. In brief, UV irradiation influences MPs-DOM release and transformation differently, depending on the plastic composition. This highlights the significance of exploring MPs-DOM transformation in securing environmental safety. [Display omitted] [ABSTRACT FROM AUTHOR]

• A significant correlation between δ15N and δ13C of SPE-DOM was observed in the YRE. • CHON with more N atoms was more aromatic and less saturated in the YRE. • CHON with fewer N atoms was more bioavailable than those with more N atoms. • CHON with fewer N atoms was transferred into those with more N atoms in the YRE. Estuaries are hotspots where terrestrially originated dissolved organic matter (DOM) is modified in molecular composition before entering marine environments. However, very few research has considered nitrogen (N) modifications of DOM molecules in estuaries, limiting our understanding of dissolved organic nitrogen (DON) cycling and the associated carbon cycling in estuaries. This study integrated optical, stable isotopes (δ15N and δ13C) and molecular composition (FT-ICR MS) to characterize the transformation of DOM in the Yangtze River Estuary. Both concentration of dissolved organic carbon (DOC) and DON decreased with increasing salinity, while their δ13C and δ15N increased with the increasing salinity. A significant positive correlation was found between δ15N and δ13C during the transportation of DOM to marginal seas, indicating that the behavior of both DOC and DON are primarily controlled by the mixing of freshwater and the seawater in the YRE. During the mixing process, the DON addition was observed using the conservative mixing curves. In the view of molecular composition, DOM molecules became more aromatic as the number of N atoms increased. Spearman correlations reveal that DOM molecules with fewer N atoms exhibited a higher enrichment in protein-like components, while those with more N atoms were more enriched in humic-like components. In addition, the δ15N and δ13C tended to increase as the N content of DOM decreased. Therefore, DON molecules with fewer N atoms were likely to be transformed into those with more N atoms based on the isotopic fractionation theory. This study establishes a linkage between the molecular composition and the δ15N of DOM, and discovers the N transformation pattern within DOM molecules during the transportation to marginal seas. [Display omitted] [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) sustains a substantial part of the organic matter transported seaward in large estuaries, where photochemical reactions significantly influence its transformation and fate. Irradiation experiments can provide valuable information on the photochemical reactivity (photo-labile, photo-resistant, and photo-product) of molecules. However, previous research paid less attention to exploring the controls of the initial DOM chemistry to irradiation experiments and examining the applicability of their further integration with field research. Here, we conducted irradiation experiments for samples from the freshwater and seawater endmember of the Yangtze River Estuary (YRE), which receives organic matter transport from the largest river in China, the Yangtze River. Molecules that occurred before and after irradiation experiments were characterized by the Fourier transform ion cyclotron resonance mass spectrometry. Results show that both post-irradiation samples have the lower aromaticity degree and reduced oxidation state, while the freshwater endmember sample exhibits more dramatic changes, indicating the controls of parent molecules to the effect of irradiation experiments. Integrating with the "molecular matching" approach, we compared the molecules occurring in field samples with the classified molecules (photo-resistant, photo-labile, and photo-product) acquired from performed irradiation experiments and correlated the relative intensity of photochemical reactivity types with salinity. When applying results from different experiments to conduct "molecular matching", the photo-resistant and photo-labile relative intensity possess consistently positive and negative trends with increasing salinity, respectively. This suggests their reliability for molecular matching applications, while the inconsistent trends for the photo-product relative intensity with salinity suggest its uncertainty in assessing the photo-induced effects. Moreover, the molecular composition within the photochemical reactivity types in field samples also evolved along the salinity gradient and showed similar trends with the DOM changes after experimental irradiation. Despite various factors influencing estimations, it is revealed that a fraction of aromatic molecules and majority of carboxyl-rich alicyclic molecules considered with biologically persistent nature in the YRE freshwater zone are simultaneously not susceptible to photochemical transformation to potentially constitute a long-term marine carbon sink. This study emphasizes the importance and limitations of the combination of field research and laboratory-controlled experiments to provide a better understanding of the crucial role of photochemical reactions in affecting carbon cycling in large estuaries. [Display omitted] [ABSTRACT FROM AUTHOR]

• DON in the BS was more recalcitrant based on its molecular and optical properties. • In-situ DON degradation was observed in the YSCW with an ε of 2.5‰. • During DON degradation, more CRAM and HUC molecules were produced. Marginal seas play a crucial role in the cycling of dissolved organic nitrogen (DON) between the terrestrial and marine environments. However, very few studies have considered the molecular transformation of DON in marginal seas, leaving the DON molecular modifications in its cycling largely unknown. Therefore, this study examined DON cycling in the Bohai Sea and Yellow Sea, two semi-closed marginal seas in northern China, using stable isotopes (δ15N and δ13C), optical characteristics, and molecular compositions. Compared to the Yellow Sea, the Bohai Sea had a weaker exchange with the open ocean, resulting in higher concentrations, lower δ15N, and more recalcitrant properties in DON. The DON cycling showed significant differences inside and outside the Yellow Sea Cold Water (YSCW). Degradation was the major sink of DON in the YSCW, during which more highly unsaturated compounds and carboxyl-rich alicyclic molecules were produced. Nitrogen atoms were found to be removed from the molecules with more N atoms to those with fewer ones during the DON degradation. This study discovered the molecular modifications in DON cycling and highlighted the intrinsic mechanisms in the cycling of DON in marginal seas. [Display omitted] [ABSTRACT FROM AUTHOR]

• Peptides-like and protein-like compounds in mass spectrum increased after 28 days. • AOM degraded after 28 days still inhibited the MeHg uptake by phytoplankton. • Inhibition after 28 days on MeHg uptake resulted from bacterial metabolites. Methylmercury (MeHg) uptake by phytoplankton represents a key step in determining the exposure risks of aquatic organisms and human beings to this potent neurotoxin. Phytoplankton uptake is believed to be negatively related to dissolved organic matter (DOM) concentration in water. However, microorganisms can rapidly change DOM concentration and composition and subsequent impact on MeHg uptake by phytoplankton has rarely been tested. Here, we explored the influences of microbial degradation on the concentrations and molecular compositions of DOM derived from three common algal sources and tested their subsequent impacts on MeHg uptake by the widespread phytoplankton species Microcystis elabens. Our results indicated that dissolved organic carbon was degraded by 64.3‒74.1% within 28 days of incubating water with microbial consortia from a natural meso‑eutrophic river. Protein-like components in DOM were more readily degraded, while the numbers of molecular formula for peptides-like compounds had increased after 28 days' incubation, probably due to the production and release of bacterial metabolites. Microbial degradation made DOM more humic-like which was consistent with the positive correlations between changes in proportions of Peaks A and C and bacterial abundance in bacterial community structures as illustrated by 16S r RNA gene sequencing. Despite rapid losses of the bulk DOM during the incubation, we found that DOM degraded after 28 days still reduced the MeHg uptake by Microcystis elabens by 32.7‒52.7% relative to a control without microbial decomposers. Our findings emphasize that microbial degradation of DOM would not necessarily enhance the MeHg uptakes by phytoplankton and may become more powerful in inhibiting MeHg uptakes by phytoplankton. The potential roles of microbes in degrading DOM and changing the uptakes of MeHg at the base of food webs should now be incorporated into future risk assessments of aquatic Hg cycling. [Display omitted] [ABSTRACT FROM AUTHOR]

• Allochthonous organics are the major sources for DOM pool in the Yangtze River. • Natural conditions and anthropogenic forces modify riverine DOM chemo-diversities. • Aromatic, unsaturated and humic-like DOM fractions dominate in up-midstream reaches. • Seasonal phytoplankton growth produces labile aliphatic and protein-like compounds. The Yangtze River, the largest river in Asia, plays a crucial role in linking continental and oceanic ecosystems. However, the impact of natural and anthropogenic disturbances on composition and transformation of dissolved organic matter (DOM) during long-distance transport and seasonal cycle is not fully understood. By using a combination of elemental, isotopic and optical techniques, as well as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated DOM abundance and composition along the whole mainstream at highly spatial resolution in the dry and early wet seasons. Our findings showed that the concentration and flux of dissolved organic carbon (DOC) in the Yangtze River was much lower compared with other worldwide larger rivers. The distribution of δ 13C DOC and higher abundance of humic-like fluorescent component and highly unsaturated and phenolics (HUPs) compound reflected a prominent contribution of allochthonous DOM. Further optical and molecular analysis revealed humic-like fluorescent components were coupled with CHO molecules and HUPs compound with higher aromatic, unsaturated, molecular weight and stable characteristics between upstream and midstream reaches. With increasing agricultural and urban land downstream, there were more heteroatomic formulae and labile aliphatic and protein-like compounds which were derived from human activities and in situ primary production. Meanwhile, DOM gradually accumulates with slow water flow and additional autochthonous organics. Weaker solar radiation and water dilution during the dry/cold season favours highly aromatic, unsaturated and oxygenated DOM compositions. Conversely, higher discharge during the wet/warm season diluted the terrestrial DOM, but warm temperatures could promote phytoplankton growth that releases labile aliphatic and protein-like DOM. Besides, chemical sulfurization, hydrogenation and oxygenation were found during molecular cycling processes. Our research emphasizes the active response of riverine DOM to natural and anthropogenic controls, and provides a valuable preliminary background to better understand the biogeochemical cycling of DOM in a larger river. Highlighted characteristics and compositions of riverine DOM pool of three reaches and two seasons. [Display omitted] [ABSTRACT FROM AUTHOR]

• Storm event induced chemodiversity bloom of DOM in non-TGR area. • Flood control of reservoir reconstrained DOM to more similar chemistry. • Flood control of reservoir enhanced recalcitrant DOC transportation to downstream. With the frequent occurrence of extreme floods under global climate change-induced storm events, reservoir operation has been highlighted for river flood control, complicating the transport and transformation of riverine dissolved organic matter (DOM), one of the largest reactive carbon pools on earth. In particular, the response of riverine DOM chemistry to reservoir flood control during extreme storm events is still unclear. To fill this knowledge gap, the mechanism of DOM variation in Yangtze River with the world's largest Three Gorges Reservoir (TGR) operation during an extreme storm event was explored. Optical and molecular properties of DOM varied significantly from upstream to downstream in non-TGR area, while no significant variation in DOM chemistry was observed in TGR area. The results uncovered a short time transformation of DOM from non-TGR area to TGR area, demonstrating that although storm event induced chemodiversity bloom of riverine DOM, flood control of TGR "re-constrained" DOM to more similar chemistry mainly under the influence of turbidity involved DOM transformation (e.g., adsorption/desorption and flocculation). Furthermore, combined with the hydrological information, we found that although TGR temporarily blocked dissolved organic carbon (DOC) flow during the flood event, the abundance of biologically recalcitrant DOC increased in TGR, which would contribute to its further transportation to downstream watershed. This study emphasizes the impact of TGR on extreme storm event-induced DOM dynamics, which also hints a better understanding of the crucial role of anthropogenic activity in affecting carbon cycling under extreme climate change. [Display omitted] [ABSTRACT FROM AUTHOR]

• Compared with sunny days, rainstorms resulted in elevated DOC and BDOC. • Rainstorms increased the export of DOM with high aromaticity and bio-lability. • Rainstorms resulted in elevated DOC and BDOC fluxes from both rivers. • Future increased rainstorm will increase the input of aromatic DOM. Lakes are hotspots for global carbon cycling, yet few studies have explored how rainstorms alter the flux, composition, and bio-lability of dissolved organic matter (DOM) in inflowing rivers using high-frequency monitoring. We conducted extensive campaigns in the watershed of Lake Taihu and made daily observations for three years in its two largest inflowing tributaries, River Dapu and River Yincun. We found higher DOC, bio-labile DOC (BDOC), and specific UV absorbance (SUVA 254) levels in the northwestern inflowing regions compared with the remaining lake regions. DOC and BDOC increased during rainstorms in River Dapu, and DOC declined due to local dilution and BDOC increased during rainstorms in River Yincun. We found that rainstorms resulted in increased DOM absorbance a 350 , SUVA 254 , and humification index (HIX) and enhanced percentages of humic-like fluorescent components, %polycyclic condensed aromatic and %polyphenolic compounds as revealed from ultrahigh-resolution mass spectrometry (FT-ICR MS), while spectral slope (S 275-295) and the percentages of protein-like C1 and C3 declined during rainstorms compared with other seasons. This can be explained by a combined flushing of catchment soil organic matter and household effluents. The annual inflows of DOC and BDOC to Lake Taihu were 1.15 ± 0.18 × 104 t C yr−1 and 0.23 ± 0.06 × 104 t C yr−1 from River Dapu and 2.92 ± 0.42 × 103 t C yr−1 and 0.53 ± 0.07 × 103 t C yr−1 from River Yincun, respectively, and the fluxes of DOC and BDOC from both rivers increased during rainstorms. We found an elevated frequency of heavy rainfall and rainstorms in the lake watershed during the past six decades. We conclude that an elevated input of terrestrial organic-rich DOM with concurrent high aromaticity and high bio-lability from inflowing rivers is likely to occur in a future wetter climate. [Display omitted] [ABSTRACT FROM AUTHOR]

• Drying-wetting cycles help to retain high aromatic and molecular weight DOM. • The proportion of condensed aromatics increased by 7% after drying-wetting cycles. • Availability of Cd had higher sensibility to dry-wet cycles than that of others. • Available Cd decreasing was related to an increase in aromatic and hydrophobic DOM. Investigating the influence mechanism of drying-wetting cycles on the availability and mobility of heavy metals in sediment from the perspective of the molecular composition of dissolved organic matter (DOM) may gain a new understanding, but little current information exists. Here, we used spectral technologies, high-resolution mass spectrometry, and elemental stoichiometry method to trace the change rules of the molecular composition of DOM in the riparian sediment of the river. Results showed that the drying-wetting cycles could benefit the degradation of labile fractions (e.g., proteins, aliphatics, and lipids) of DOM and retain the fractions with high aromaticity and molecular size (e.g., lignin). The decrease in the availability of Cd after drying-wetting alternation processes was highly related to these changes in DOM composition. However, the availability of Zn and Cu remained almost unchanged, which probably resulted from the release and depletion of N and S in sediment-derived DOM under drying-wetting alternation conditions. As for Cr, its exchangeable fraction was unchanged during the drying-wetting alternation process, likely due to its high stability in the sediment. These results have implications on the environmental geochemical cycling of heavy metals in the riparian sediment with frequent drying-wetting alternation. [Display omitted] [ABSTRACT FROM AUTHOR]

• High DOM chemodiversity is ubiquitous in all headwater streams and a large river. • S-containing and aliphatic compounds originate from rural domestic effluent. • Ecosystem respiration can be controlled by anthropogenic sources of DOM. • H/C of 1.07 and O/C of 0.37 were typical precursors of DBPs. • DOM with 151 to 635 Da was the main part that was hard to remove through degradation. Dissolved organic matter (DOM) is a complicated assembly of organic molecules, including thousands of molecules with various structures and properties. However, how the stream DOM sources respond to carbon compositions and the transformation processes remains unclear. In this study, the chemical characteristics and spectral and mass spectrometry (FT-ICR MS) of DOM were analyzed. Six sampling points of headwater stream (HWSs) were sampled, and an effluent polluted stream (WSR) and a main stream of the Changjiang River (DT) were also sampled for comparison. In situ degradation experiments and FT-ICR MS analysis were also performed to observe the dynamic processes of DOM in HWS. The results showed that the anthropogenic markers of sewage (i.e. sulfur (S) compounds and marker from antibiotics and estrogen) in HWS were higher than those in DT. The molecular weight decreased while the degradation products (S-containing compounds and unsaturated compounds (HU)) increased after in situ degradation due to the influence of both the photodegradation and biodegradation process. In addition, the KMD plots showed that the DOM homologue intensities in range 400–600 Da changed significantly after demethylation by biodegradation. The components of highly refractory substances and the degradation degree of DOM in DT was higher than that in HWS. We extracted the refractory DOM pool in HWS, which was mainly small molecular with molecular weights < 600 Da. These molecular will be difficult to remove in traditional drinking water treatment processes and easily produced disinfection byproducts (DBPs). This study emphasized the necessity of identifying the sources and transformation processes of DOM in HWS and clarified the types and characteristics of DOM that should be considered in future drinking water treatment. [Display omitted] [ABSTRACT FROM AUTHOR]

• Composition of size-fractionated river NOM was characterized by Orbitrap-MS. • The correlations between HBQs and source water NOM components were established. • Molecular tagging revealed the contribution of functional groups on HBQs formation. • HBQs yield was strongly impacted by NOM molecular composition and functional groups. • 50 kDa source water NOM was more conducive to form HBQs during chlorination. Halobenzoquinones (HBQs) are emerging disinfection byproducts generated during the reaction of chlorine disinfectant with natural organic matter (NOM) in source water. In this study, the correlations between molecular weight and HBQs generation of river NOM was evaluated. The compositional and functional characteristics of primary HBQs precursors were revealed by using Orbitrap mass spectrometry combined with molecular tagging. The NOM fraction larger than 50 kDa resulted in approximately 9 times more HBQs (50.9 ± 2.7 ng/mgC) than low molecular weight fractions. Significant correlations were found between the yields of HBQs and lignin-like and highly oxygen compounds in NOM, suggesting their critical roles in HBQs formation. Derivatizating the aldehydes, ketones, hydroxyl and carboxyl groups in NOM could reduce HBQs yields by 90.7%–100%. Unraveling the molecular characteristics of HBQs precursors in NOM would greatly benefit the prediction of HBQs yields of different source water, and develop more efficient disinfection byproduct control strategies. [Display omitted] [ABSTRACT FROM AUTHOR]

• DOM chemistry was investigated in the Three Gorges Reservoir • Higher terrestrial input, molecular and isomeric complexity of DOM were detected in storage period • Hydrological management of reservoirs shifts the DOM chemistry • Hydrological management would affect DOM mineralization and CO 2 emission in young reservoirs Reservoirs are well known as a far-reaching human modification on the functions of natural river networks. However, changes in the chemistry and reactivity of dissolved organic matter (DOM) responding to hydrological management for water retention structures, and its influence on the river carbon cycle, remain poorly understood. Here we show that hydrological management does shape the molecular composition of DOM in the world's largest Three Gorges Reservoir, as revealed by optical spectroscopy and ultrahigh-resolution mass spectrometry. Relatively higher terrestrial input, molecular complexity, isomeric complexity, and environmental stability of DOM were observed during the storage period, whereas the inverse occurred during the drainage period. The results demonstrate that the hydrodynamic processes, which are mainly controlled by water intrusion from mainstream to tributaries, are likely the underlying mechanism controlling DOM chemistry. Integrated with observations from worldwide river reservoirs, the DOM degradation experiments suggest that reservoir hydrological management would enhance DOM mineralization, thereby increase CO 2 emission and change the river carbon cycle. Image, graphical abstract [ABSTRACT FROM AUTHOR]