DOM Associates with Greenhouse Gas Emissions in Chinese Rivers under Diverse Land Uses

Unravelling riverine dissolved organic matter sources using molecular fingerprints and FEAST model in a multi-tributary mountain river basin

Revealing the sources, composition and fate of riverine dissolved organic matter (DOM) is fundamental to understanding the biogeochemical cycles of aquatic ecosystems. This study aimed to reveal the impact of land uses and wastewater treatment plants (WWTPs) on riverine DOM. Spatiotemporal variations in molecular characteristics of riverine DOM in the river network containing 15 tributaries in the mainstream of upper Hanjiang River were studied. Differences in molecular characteristics of DOM in soil leachates of various land uses and the effluent of WWTPs were analyzed and their contributions to riverine DOM in both dry and wet seasons were calculated using FEAST model. DOM in soil leachates was primarily composed of lignin, protein and lipid-like compounds but was dominated by lignin and tannin-like compounds in the effluent of WWTPs. Contribution rates of the soil leachate of each land use calculated by FEAST model showed a significant positive linear correlation with the area-based proportion of each land use in the basins of tributaries. Contributions of area-based proportion of each land use to riverine DOM followed the order of grassland > forest > cropland for both seasons. DOM in the upstream of tributaries contributed more than 50 % to the molecular composition of DOM in the downstream of tributaries but the contribution of the effluent of WWTPs to riverine DOM did not exceed 3 %. These results demonstrated that FEAST model could be used for source identification of riverine DOM based on molecular fingerprint data. Accordingly, this study provides new insights into the carbon cycling and ecological health within the watershed.

Effluent organic matter facilitates anaerobic methane oxidation coupled with nitrous oxide reduction in river sediments

Effluent organic matter (EfOM) from wastewater treatment plants (WWTPs) contains humic-like substances that function as electron shuttles, thereby facilitating microbially-mediated redox reactions. However, the mechanisms governing the coupled processes of anaerobic oxidation of methane (CH4) (AOM) and nitrous oxide (N2O) reduction in river sediments, which receive WWTPs effluents, remain poorly understood. In this study, an incubation experiment with anoxic river sediments was conducted to assess the impacts of EfOM on AOM and nitrous oxide reduction using different effluent dilution ratios. The results showed that EfOM significantly enhanced both processes. Specifically, the AOM rate increased from 8.1 to 14.3 μg gdw-1 d-1, while the N2O reduction rate increased from 29.2 to 56.5 μg gdw-1 d-1. The results of batch tests demonstrated that AOM process enhanced N2O reduction in the presence of EfOM, highlighting the critical role of EfOM in linking these processes. Nitrate-dependent anaerobic methane oxidation (n-DAMO) archaea and denitrifying bacteria dominated the sediment incubated with EfOM. Metagenomic and metatranscriptomic analyses revealed that the denitrifying bacteria exclusively reduce N2O, confirming the role of EfOM in facilitating electron transfer between n-DAMO archaea and N2O reducers. This indicates that effluent discharge could be a potential factor driving the concurrent sinks of methane and nitrous oxide, offering a perspective for investigating the impacts of WWTPs effluent on greenhouse gas sinks in freshwater ecosystems.

Coupling of Mercury Contamination and Carbon Emissions in Rice Paddies: Methylmercury Dynamics versus CO2 and CH4 Emissions

Methylmercury (MeHg) accumulation in rice grains and greenhouse gas emissions are significant environmental concerns in rice paddy ecosystems. Dynamic of MeHg in paddy soils are likely interacted with the emissions of methane (CH4) and carbon dioxide (CO2), given the involvement of methanogenesis and organic matter mineralization in mercury (Hg) methylation, but poorly defined at present. Here, rice-growing pot experiments were performed with varying levels of paddy soil Hg to examine the interactions among CO2 and CH4 emissions and MeHg dynamics under variable Hg amendment scenarios. Mercury addition (20 mg kg-1) significantly enhanced the cumulative emissions of CO2 and CH4 from a paddy system, and shifts in methanogen community explained the increased CH4 emissions. In contrast, such enhancements were not observed at lower Hg addition levels. Under identical total Hg treatments, ecosystem-dependent negative correlations were observed between MeHg concentrations and C emissions during the rice growing period. The divergent kinetics associated with the production of MeHg, CO2, and CH4 likely explained the reason. In addition, methanogens mediated MeHg degradation as well as CH4 production from oxidative demethylation may also contribute to the negative correlations. Overall, this study enhances our understanding of the complex interplay between C emissions and MeHg dynamics in rice paddy ecosystems.

Biological and terrestrial influences on dissolved organic matter in Antarctic surface waters: Insights from mass spectrometry and metagenomic analysis

Global warming increases the surface waters and biodiversity in polar regions. However, the intrinsic biological sources of dissolved organic matter (DOM) in Antarctic surface waters remain poorly understood. This work evaluated the sources and driving mechanisms of DOM in Antarctic lakes systematically, based on fluorescence excitation-emission matrices, ultrahigh-resolution mass spectra, biological detection, and metagenomic analyses. The most abundant DOM in the water was peptides (37.02%), which differed from those in soil (lignins: 26.33%) and penguin guano (lipids: 50.71%). The relative abundance of CHON and CHOP compounds in water was significantly correlated with the distance from the penguin colony (p < 0.05). Both the fluorescence and mass spectrum fingerprints of water and soil/faeces showed low similarities using end-member source tracking methods. This could be attributed to the facilitation of guano-derived nutrients to phytoplankton proliferation, whereas the concentrations of NH4+-N, NO3--N, total phosphorus, and total organic carbon were significantly higher in the penguin-intensive area than in the other areas. Algae had significant positive effects on carbohydrates and amino sugars and positive effects on lignins, compared to zooplankton and bacteria. Zooplankton had significantly more positive effects on peptides than phytoplankton. Secondary bacterial metabolic activity can be positively linked with CHO compounds. Carbohydrates and amino sugars co-occurred with carbohydrate-active enzyme genes and nitrogen cycling genes in one module of the co-occurrence network, whereas the other module was characterised by the co-occurrence patterns of condensed aromatic structures with carbohydrate-active enzyme genes and nitrogen cycling genes. These results emphasise the roles of secondary metabolites from algae and bacteria in species-specific sources of DOM, shedding light on the driving mechanisms of the biogeochemical cycling of DOM in the Antarctic water environment.

The components and aromaticity of dissolved organic matter derived from aquatic plants determine the CO2 and CH4 emission potential

Lakes are integral to the carbon cycle through the processing of dissolved organic matter (DOM). However, the specific contributions of various aquatic plants to carbon emissions during their decomposition remain inadequately understood. In this study, decomposition experiments were performed on three aquatic plants-algae, Phragmites australis (PA), and Potamogeton crispus L. (PC)-using advanced techniques, including FT-ICR-MS and metagenomics, to investigate the mechanisms of carbon dioxide (CO2) and methane (CH4) emissions. The results indicate that algae exhibit a substantial potential for CO2 emissions, with emissions reaching up to 2193 μmol·g-1. Conversely, PA contributes the highest CH4 emissions, reaching up to 2397 μmol·g-1. Factors such as the protein-like content and aromaticity of DOM molecules significantly influence emission levels. DOM with lower aromaticity undergoes easier decomposition in the first 6 days, leading to increased CO2 production. Elevated C/N and C/P ratios in plants enhance the abundance of methanogenic bacteria and genes. Surplus carbon will be mineralized under anaerobic conditions, giving rise to mineralization of organics to CH₄. These findings elucidate the mechanisms underlying CO2 and CH4 emissions during the decomposition of different aquatic plants and provide valuable insights for lake water environment management.

High Resolution Water Quality Dataset of Chinese Lakes and Reservoirs from 2000 to 2023

Water quality parameters (pH, dissolved oxygen (DO), total nitrogen (TN, includes both organic nitrogen and inorganic nitrogen), total phosphorus (TP), permanganate index (CODMn), turbidity (Tur), electrical conductivity (EC), and dissolved organic carbon (DOC)) are important to evaluate the ecological health of lakes and reservoirs. In this research, we developed a monthly dataset of these key water quality parameters from 2000 to 2023 for nearly 180,000 lakes and reservoirs across China, using the random forest (RF) models. These RF models took into account the impacts of climate, soil properties, and anthropogenic activities within basins of studied lakes and reservoirs, and effectively captured the spatial and temporal variations of their water quality parameters with correlation coefficients (R2) ranging from 0.65 to 0.76. Interestingly, an increase in Tur and EC was observed during this period, while pH, DO, and other parameters showed minimal fluctuations. This dataset is of significant value for further evaluating the ecological, environmental, and climatic functions of aquatic ecosystems.

Unveiling the global dynamics of dissolved organic carbon in aquatic ecosystems: Climatic and anthropogenic impact, and future predictions

Dissolved organic carbon (DOC) and its biodegradability (BDOC%) in aquatic ecosystems significantly impact the global carbon cycle, varying greatly across rivers, lakes, and estuaries due to environmental and anthropogenic factors. However, a thorough understanding of these variations is still lacking. This study investigated the interactions between climate, hydrology, physiography, soil, land cover, and human activity on DOC dynamics in rivers, lakes, and estuaries. Utilizing a robust dataset comprising 744 global data points for DOC concentrations (0.18-29.33 mg/L) and 341 samples for BDOC% (0.44 %-81.12 %), spanning a wide range of geographic and climatic gradients across six continents, machine learning techniques were employed to elucidate the relationships between DOC and BDOC% and environmental and anthropogenic factors and to develop predictive models for global DOC and BDOC storage. Results showed that climate primarily affected DOC and BDOC% levels, with other factors varying by ecosystem type. In rivers, soil and human activity had positive influences, while in lakes, hydrology had a positive effect and human activity had a negative one. In estuaries, soil positively impacted the levels of DOC and BDOC%, whereas human activity had a negative effect. Furthermore, we created separate random forest models for DOC and BDOC% based on different factors in each aquatic ecosystem (R2 = 0.50-0.89), and applied to data of environmental and anthropogenic factors worldwide, predicting DOC and BDOC storage for 181 countries. Notably, large countries like Canada, Russia, the United States (U.S.), Brazil, and China accounted for 76.07 % and 51.56 % of the total global DOC and BDOC storage, respectively. Storage prediction models under future climate scenarios indicated significant impacts in Europe under the high fossil fuel use scenario. Thus, prioritizing high-storage, climate-vulnerable areas is essential for effective climate change strategies, aiding in the protection of aquatic ecosystems, maintaining the global carbon balance, and promoting sustainable development.

Terrestrial dissolved organic matter inputs affect the nitrous oxide emission revealed by FT-ICR MS

Nitrous oxide (N2O) emission from lake systems could be affected via intrusion of terrestrial organic matter, causing impairment in biogeochemical cycling. The sources and mechanisms by which DOM (Dissolved organic matter) alters emissions of N2O are poorly understood. Here, we simulate different terrestrial DOM (anthropogenic sources, natural sources, and surface runoff) to assess the mechanisms affecting N2O emissions with variations of DOM. We used a combination of absorption spectroscopy, excitation-emission matrix fluorescence, and Fourier transform ion cyclotron resonance mass spectrometry to characterize DOM comprehensively. For the characterization of DOM, a combination of absorption spectroscopy, excitation-emission matrix fluorescence, and Fourier transform ion cyclotron resonance mass spectrometry was used. Microbial analysis was conducted to identify the potential microbial mechanisms. Different terrestrial DOM inputs primarily impact N2O emissions through the denitrification process (14.52 %, p < 0.05), with significant effects on the abundance of narG (12.97 %, p < 0.05) and nirK+S (10.13 %, p < 0.05). The biodegradable components in sediments directly promote N2O emissions, while in aquatic systems, the labile components (proteins, sugars, and lipids-like) were preferentially metabolized, producing reluctant derivatives. The biodegradable components (i.e., protein-like) from anthropogenic sources rapidly facilitate N2O production. Natural and surface runoff sources were the significant drivers for the continuous release and metabolism of DOM. N2O Loss emissions are negatively influenced by the regulation of carbon and nitrogen metabolism by nitrifiers and denitrifies in the sediment (p < 0.001). Metabolism of carbon and nitrogen regulated by nitrifier and denitrifies in the sediments negatively influences N2O flux (p < 0.001). N2O emissions were mainly influenced by bioavailability of inputs: DOM and varying terrestrial conditions. The results provide a theoretical base for the management of greenhouse gas emissions from lakes.

Effects of land use and dissolved organic matter on pCO2 are dependent on stream orders and hydrological seasonality in a low-order karst river

Human activities and stream accumulation influence carbon loadings, altering the distributions and characteristics of dissolved inorganic and organic carbon in rivers. It is widely recognized that such alterations affect dissolved organic matter (DOM) components, water environment and river carbon dioxide (CO2) degassing, however, the control factors by which land use/land cover (LULC) and DOM components regulate the partial pressure of CO2 (pCO2) are unclear. Here, in the Daning karst river system, an extensive investigation was presented to investigate the role of LULC and DOM components in influencing the spatial and temporal variability of pCO2, as well as to investigate the regulating effect of stream order and hydrological rhythm on this influence. DOM quality and pCO2 levels exhibited significant spatial and temporal variations. In the 3rd - 4th order streams, pCO2 was correlated with protein and lignin compounds in the wet period and with DOM molecular weight in the dry period. Relatively high protein-like components (54.83 % ∼ 71.84 %, on average) and biological index (0.86-0.90, on average) indicated notable autochthonous processes. Significant relationships between pCO2 and water quality parameters were observed in the 3rd - 4th order streams in the wet period, demonstrating the role of runoff and upstream accumulation. Farmland increased pCO2 levels in the 3rd - 4th order streams, whereas forests could potentially mitigate river CO2 saturation. River pCO2 was well predicted by LULC under extended circular buffers (1000 and 2000 m in diameter). This study demonstrated that DOM and LULC directly or indirectly affect pCO2 and that the influences are largely regulated by hydrological seasonality and stream orders, which is better for understanding aquatic CO2 drivers.

Unravelling nitrate transformation mechanisms in karst catchments through the coupling of high-frequency sensor data and machine learning

Nitrate dynamics within a catchment are critical to the earth's system process, yet the intricate details of its transport and transformation at high resolutions remain elusive. Hydrological effects on nitrate dynamics in particular have not been thoroughly assessed previously and this knowledge gap hampers our understanding and effective management of nitrogen cycling in watersheds. Here, machine learning (ML) models were employed to reconstruct the annual variation trend in nitrate dynamics and isotopes within a typical karst catchment. Random forest model demonstrates promising potential in predicting nitrate concentration and its isotopes, surpassing other ML models (including Long Short-term Memory, Convolutional Neural Network, and Support Vector Machine) in performance. The ML-modeled NO3--N concentrations, δ15N-NO3-, and δ18O-NO3- values were in close agreement with field data (NSE values of 0.95, 0.80, and 0.53, respectively), which are notably challenging to achieve for process models. During the transition from dry to wet period, approximately 23.0 % of the annual precipitation (∼269.1 mm) was identified as the threshold for triggering a rapid response in the wet period. The modeled nitrate isotope values were significantly supported by the field data, suggesting seasonal variations of nitrogen sources, with precipitation as the primary driving force for fertilizer sources. Mixing of multiple sources appeared to be the main control of the transport and transformation of nitrate during the rising limb in the wet period, whereas process control (denitrification) took precedence during the falling limb, and the fate of nitrate was controlled by biogeochemical processes during the dry period.

Planting Enhances Soil Resistance to Microplastics: Evidence from Carbon Emissions and Dissolved Organic Matter Stability

Microplastics (MPs) have become a global hotspot due to their widespread distribution in recent years. MPs frequently interact with dissolved organic matter (DOM) and microbes, thereby influencing the carbon fate of soils. However, the role of plant presence in regulating MPs-mediated changes in the DOM and microbial structure remains unclear. Here, we compared the mechanisms of soil response to 3 common nonbiodegradable MPs in the absence or presence of radish (Raphanus sativus L. var. radculus Pers) plants. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis revealed that MPs reduced the chemodiversity and biodiversity of dissolved organic matter (DOM). MPs enhanced the degradation of lignin-like compounds and reduced the DOM stability. Comparative analysis showed that MPs caused less disturbance to the microbial composition and metabolism in planted soil than in unplanted soil. In unplanted soil, MPs stimulated fermentation while upregulating photoautotrophic activity in planted soil, thereby enhancing system stability. The rhizosphere effect mitigated MPs-induced CO2 emissions. Overall, our study highlights the crucial role of rhizosphere effects in maintaining ecosystem stability under soil microbe-DOM-pollutant interactions, which provides a theoretical basis for predicting the resistance, resilience, and transitions of the ecosystem upon exposure to the anthropogenic carbon source.

Seasonal Dynamics of Methane Fluxes from Groundwater to Lakes:Hydrological and Biogeochemical Controls

Methane (CH4) inputs to lakes through lacustrine groundwater discharge (LGD-derived CH4) represent a potentially important but often overlooked source of lake methane emissions. Although great efforts have been made to quantify LGD-derived CH4 fluxes and their spatial variablity, the underlying mechanisms controlling seasonal LGD-derived CH4 fluxes and their influence on lake CH4 emissions remain poorly understood, particularly in humid inland areas. To address this gap, we applied the 222Rn mass balance model, as well as hydrological, isotopic and microbial methods to assess seasonal LGD-derived CH4 fluxes and their influence on the seasonal variability of lake methane emissions in a typical oxbow lake, central Yangtze River. The results revealed wide seasonal differences in LGD-derived CH4 fluxes, which were controlled by hydrological and biogeochemical processes. During the dry season, although more intense methane oxidation and weaker methanogenesis occurred in groundwater, the much higher LGD rate (51.71 mm/d) produced a higher LGD-derived CH4 flux (16.41 mmol/m2/d). During the wet season, methanogenesis was more active and methane oxidation was weaker, but a lower LGD rate (12.16 mm/d) led to a lower LGD-derived CH4 flux (5.33 mmol/m2/d). Furthermore, higher LGD-derived CH4 flux in the dry season resulted in higher CH4 emissions from the lake and diminished the extent of methane oxidation in the lake. In comparison to other regions, the differences in LGD-derived CH4 fluxes and their seasonal variations were found to be controlled by climatic conditions and lake types in different global regions. Higher LGD-derived CH4 fluxes and more pronounced seasonal variations could be associated with higher temperature, larger water depth and more intense water level fluctuations. This study provides a novel perspective and broader implications for the comprehension and evaluation of seasonal methane emissions and understanding the carbon cycle in global lake ecosystems in humid areas with intense water level fluctuations.

A comprehensive conceptual framework for signaling in-lake CO2 through dissolved organic matter

Organic carbon (C) and CO2 pools are closely interactive in aquatic environments. While there are strong indications linking freshwater CO2 to dissolved organic matter (DOM), the specific mechanisms underlying their common pathways remain unclear. Here, we present an extensive investigation from 20 subtropical lakes in China, establishing a comprehensive conceptual framework for identifying CO2 drivers and retrieving CO2 magnitude through co-trajectories of DOM evolution. Based on this framework, we show that lake CO2 during wet period is constrained by a combination of biogeochemical processes, while photo-mineralization of activated aromatic compounds fuels CO2 during dry period. We clearly determine that biological degradation of DOM governs temporal variations in CO2 rather than terrestrial C inputs within the subtropical lakes. Specifically, our results identify a shared route for the uptake of atmospheric polycyclic aromatic compounds and CO2 by lakes. Using machine learning, in-lake CO2 levels are well modelled through DOM signaling regardless of varying CO2 mechanisms. This study unravels the mechanistic underpinnings of causal links between lake CO2 and DOM, with important implications for understanding obscure aquatic CO2 drivers amidst the ongoing impacts of global climate change.

Calcium regulates the interactions between dissolved organic matter and planktonic bacteria in Erhai Lake, Yunnan Province, China

In recent years, the global carbon cycle has garnered significant research attention. However, details of the intricate relationship between planktonic bacteria, hydrochemistry, and dissolved organic matter (DOM) in inland waters remain unclear, especially their effects on lake carbon sequestration. In this study, we analyzed 16S rRNA, chromophoric dissolved organic matter (CDOM), and inorganic nutrients in Erhai Lake, Yunnan Province, China. The results revealed that allochthonous DOM (C3) significantly regulated the microbial community, and that autochthonous DOM, generated via microbial mineralization (C2), was not preferred as a food source by lake bacteria, and neither was allochthonous DOM after microbial mineralization (C4). Specifically, the correlation between the fluorescence index and functional genes (FAPRPTAX) showed that the degree of utilization of DOM was a critical factor in regulating planktonic bacteria associated with the carbon cycle. Further examination of the correlation between environmental factors and planktonic bacteria revealed that Ca2+ had a regulatory influence on the community structure of planktonic bacteria, particularly those linked to the carbon cycle. Consequently, the utilization strategy of DOM by planktonic bacteria was also determined by elevated Ca2+ levels. This in turn influenced the development of specific recalcitrant autochthonous DOM within the high Ca2+ environment of Erhai Lake. These findings are significant for the exploration of the stability of DOM within karst aquatic ecosystems, offering a new perspective for the investigation of terrestrial carbon sinks.

Photo-methanification of aquatic dissolved organic matters with different origins under aerobic conditions: Non-negligible role of hydroxyl radicals

Lingering inconsistencies in the global methane (CH4) budget and ambiguity in CH4 sources and sinks triggered efforts to identify new CH4 formation pathways in natural ecosystems. Herein, we reported a novel mechanism of light-induced generation of hydroxyl radicals (•OH) that drove the production of CH4 from aquatic dissolved organic matters (DOMs) under ambient conditions. A total of five DOM samples with different origins were applied to examine their potential in photo-methanification production under aerobic conditions, presenting a wide range of CH4 production rates from 3.57 × 10-3 to 5.90 × 10-2 nmol CH4 mg-C-1 h-1. Experiments of •OH generator and scavenger indicated that the contribution of •OH to photo-methanificaiton among different DOM samples reached about 4∼42 %. In addition, Fourier transform infrared spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry showed that the carbohydrate- and lipid-like substances containing nitrogen-bonded methyl groups, methyl ester, acetyl groups, and ketones, were the potential precursors for light-induced CH4 production. Based on the experimental results and simulated calculations, the contribution of photo-methanification of aquatic DOMs to the diffusive CH4 flux across the water-air interface in a typical eutrophic shallow lake (e.g., Lake Chaohu) ranged from 0.1 % to 18.3 %. This study provides a new perspective on the pathways of CH4 formation in aquatic ecosystems and a deeper understanding on the sources and sinks of global CH4.

Activated Carbon Application Simultaneously Alleviates Paddy Soil Arsenic Mobilization and Carbon Emission by Decreasing Porewater Dissolved Organic Matter

Flooding of paddy fields during the rice growing season enhances arsenic (As) mobilization and greenhouse gas (e.g., methane) emissions. In this study, an adsorbent for dissolved organic matter (DOM), namely, activated carbon (AC), was applied to an arsenic-contaminated paddy soil. The capacity for simultaneously alleviating soil carbon emissions and As accumulation in rice grains was explored. Soil microcosm incubations and 2-year pot experimental results indicated that AC amendment significantly decreased porewater DOM, Fe(III) reduction/Fe2+ release, and As release. More importantly, soil carbon dioxide and methane emissions were mitigated in anoxic microcosm incubations. Porewater DOM of pot experiments mainly consisted of humic-like fluorophores with a molecular structure of lignins and tannins, which could mediate microbial reduction of Fe(III) (oxyhydr)oxides. Soil microcosm incubation experiments cospiking with a carbon source and AC further consolidated that DOM electron shuttling and microbial carbon source functions were crucial for soil Fe(III) reduction, thus driving paddy soil As release and carbon emission. Additionally, the application of AC alleviated rice grain dimethylarsenate accumulation over 2 years. Our results highlight the importance of microbial extracellular electron transfer in driving paddy soil anaerobic respiration and decreasing porewater DOM in simultaneously remediating As contamination and mitigating methane emission in paddy fields.