Linkages between greenhouse gases (CO2, CH4, and N2O) and dissolved organic matter composition in a shallow estuary

Spatiotemporal variations of dissolved nitrous oxide concentrations in small water bodies in a typical urban landscape in eastern China

Waters in urban landscape are susceptible to nitrogen (N) pollution, potentially leading to supersaturation of dissolved nitrous oxide (N2O) and making them a source of atmospheric N2O. However, the spatiotemporal variations and driving factors of dissolved N2O concentrations in these waters remain unclear. This study examined the spatiotemporal variations of dissolved N2O in an interconnected river-lake system in a subtropical urban landscape. The annual mean dissolved N2O concentration was 0.81 μg N L-1, with a mean N2O saturation of 304 %, indicating the system could act as a N2O emitter. Under the influence of daily fluctuations in water temperature and DO, the diurnal variation of N2O was most pronounced in autumn, with the smallest amplitude observed in winter. Seasonal mean concentrations of dissolved N2O, N, phosphorus (P), chlorophyll-a (Chl-a) and dissolved oxygen (DO) peaked in winter and were lowest in summer. The central lake had significantly lower dissolved N2O, N and P concentrations, but possessed higher water temperature, pH, concentrations of Chl-a and DO than its upstream and downstream rivers. Dissolved N2O concentrations were positively correlated with concentrations of N and P, but negatively with pH, Chl-a, and DO. There was intensifying competition for N between the N2O producers and algae, particularly in summer or in the central lake area. Therefore, the spatiotemporal variations of dissolved N2O concentrations were primarily driven by the integrated effects of seasonal conditions, nutrient fluctuations and algal growth. This research can provide essential scientific guidance for developing the effective strategies to control N pollution and mitigate the risk of N2O emissions from water bodies in urban landscape.

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.

Potential of submerged macrophytes restoration for reducing CH4 and CO2 emissions in a typical urban lake

As one of the areas most affected by human activities, urban lakes play a crucial role in global carbon cycle. Currently, submerged macrophyte restoration is a common ecological practice in urban lakes, primarily aimed at improving water quality and enhancing the aesthetic value of lake environments. Despite its widespread application, its contribution to carbon emission reduction has not received sufficient attention. In this study, we quantified the fluxes, concentrations and isotope signatures of CH4 and CO2 in the restoration zone (RL), the unrestored zone (UR) and the inflow rivers (IR) of Lake Xuanwu, along with potential environmental and dissolved organic matter (DOM) factors over the course of a year. The results indicated that the restoration of submerged macrophytes significantly diminished the emission of CH4 and CO2 from the lake. Compared to the UR and IR zone, the CH4 flux in the RL zone was reduced by 82.27 % and 92.18 %, while the CO2 flux decreased by 464.95 % and 133.12 %, respectively. Further investigation revealed distinct eutrophication levels between the RL zone with submerged macrophytes compared to the UR zone, and higher eutrophication levels were associated with reduced carbon sequestration stability. Nitrogen and phosphorus played critical roles in the emission of CH4 and CO2, respectively. Submerged macrophytes directly reduce carbon emissions through photosynthesis and significantly influence the long-term carbon sequestration capacity of lakes by secreting oxygen, modifying the ecological characteristics of the aquatic environment, and altering the production and mineralization processes of CH4 and CO2 in sediment porewater. These results underscore the potential of submerged macrophytes restoration as a viable strategy for reducing local emissions of CH4 and CO2 in urban lakes.

Enhancing humification in high-temperature composting: Insights from endogenous and exogenous heating strategies

Livestock manure is difficult to manage for high moisture and nutrients. High-temperature composting (> 75 °C) reduces moisture. However, the humification process, crucial for nutrient recycling, remains poorly understood. This study investigated the humification mechanisms of endogenous-heating hyperthermophilic composting (HC) and exogenous-heating continuous thermophilic composting (CTC). Both methods significantly increased humus by 258.70 % in HC and 193.93 % in CTC. Spectral analysis showed stronger humification in the early stage for HC and in the later stage for CTC. Microbial activity primarily drove humification in HC, while environmental factors had more influence in CTC due to its simpler microbial structure. These results suggested that the polyphenol pathway, involving quinones and protein condensation, dominates humification in HC, while the Maillard reaction, with sugars and protein aggregation, dominates in CTC. Humification can be enhanced by increasing protein in HC and sugar in CTC. These findings provided strategies to improve humification efficiency and promote sustainable agriculture.

Tidal-driven N2O emission is a stronger resister than CH4 to offset annual carbon sequestration in mangrove ecosystems

The mangrove ecosystems store a significant amount of "blue carbon" to mitigate global climate change, but also serve as hotspots for greenhouse gases (GHGs: CO2, CH4 and N2O) production. The CH4 and N2O emissions offset mangrove carbon benefits, however, the extent of this effect remains inadequately quantified. By applying the 36 h time-series observations and mapping cruises, here we investigated the spatial and temporal distribution of GHGs and their fluxes in Dongzhaigang (DZG) bay, the largest mangrove ecosystem in China, at tidal and monthly scales. The spatiotemporal variation of GHGs were mainly controlled by tidal-forced water mixing, outgassing and multiple biogeochemical processes. Tidal-driven porewater outwelling and sediment resuspension largely explained the excess addition of dissolved GHGs in tidal creeks. These lateral export combined with river input contribute significantly to surrounding water emission. Salinity controls CH4 emission in river-tidal creek-bay continuum, with concentration decreased by ∼100-fold from freshwater to seawater sites. N2O concentration was controlled by substrate supply for both nitrification and denitrification. Overall, the GHGs emissions in tidal creeks equal to 669-39,000 (7521 ± 6401) g CO2-eq/m2/yr (CO2 contributed ∼88-91 %) to atmosphere. In particular, CH4 and N2O contribute 8-366 (124 ± 78) g CO2-eq/m2/yr and 59-2260 (712 ± 525) g CO2-eq/m2/yr, together offsetting 8.7 ± 2.1 % of annual mangrove carbon sequestration, with a larger contribution from N2O (7.4 %). Our findings highlight the importance of simultaneous quantification of non-CO2 GHGs to accurately assess blue carbon capacity.

Network-Based Methods for Deciphering the Oxidizability of Complex Leachate DOM with •OH/O3 via Molecular Signatures

In landfill leachates containing complex dissolved organic matter (DOM), the link between individual DOM constituents and their inherent oxidizability is unclear. Here, we resolved the molecular signatures of DOM oxidized by •OH/O3 using FT-ICR MS, thereby elucidating their oxidizability and resistance in concentrated leachates. The comprehensive gradual fragmentation of complex leachate DOM was then revealed through a modified machine-learning framework based on 43 key pathways during ozonation. Specifically, humic substances like humic acid (HA) and fulvic acid (FA) were measured to be the dominant DOM fractions in concentrated leachates, accounting for 35.9-51.7% of the total organic carbon, which was consistent with the observation by three-dimensional fluorescence spectroscopy. According to FT-ICR MS, carboxyl-rich alicyclic molecules (CRAMs) or lignin-like substances were the most abundant components, comprising 40.2-54.5% of all substances. The machine learning modeling showed that molecular weight was the most important structural factor for DOM resistance to •OH and O3 degradation (SHAP value 0.84), followed by (DBE-O)/C (0.32), S/C (0.31), and H/C (0.08). During •OH and O3 attacking, unsaturated and reduced compounds were the dominant precursors. For the molecular transformation of CRAMs-DOM, oxygen addition reactions were found to be the predominant O3-attacking process, along with the dealkyl and carboxylic acid reactions during •OH oxidation that often resulted in more complete degradation of DOM. This study proposed a new framework integrating molecular signatures and machine learning for unraveling DOM's inherent reactivity in complexity, which informs strategies for managing concentrated leachates.

Keystone taxa drive the synchronous production of methane and refractory dissolved organic matter in inland waters

The production of both methane (CH4) and refractory dissolved organic matter (RDOM) depends on microbial consortia in inland waters, and it is unclear yet the link of these two processes and the underlying microbial regulation mechanisms. Therefore, a large-scale survey was conducted in China's inland waters, with the measurement of CH4 concentrations, DOM chemical composition, microbial community composition, and relative environmental parameters mainly by chromatographic, optical, mass spectrometric, and high-throughput sequencing analyses, to clarify the abovementioned questions. Here, we found a synchronous production of CH4 and RDOM linked by microbial consortia in inland waters. The increasing microbial cooperation driven by the keystone taxa (mainly Fluviicola and Polynucleobacter) could promote the transformation of labile DOM into RDOM and meanwhile benefit methanogenic microbial communities to produce CH4. As such, CH4 and RDOM showed consistent spatial differences, which were mainly influenced by total nitrogen and dissolved oxygen concentrations. This finding deepened the understanding of microbial-driven carbon transformation and will help to more accurately evaluate the carbon source-sink relationship in inland waters.

Effect of marine anoxia on the conversion of macroalgal biomass to refractory dissolved organic carbon

The input of macroalgal biomass into the deep sea is a crucial process for macroalgal carbon sequestration, but this process may be affected by anoxia. We compared the breakdown of kelp biomass in both normoxic (>4 mg/L O2) and anoxic (<2 mg/L O2) environments. Following 240 days of decomposition experiment, complete degradation of the kelp biomass occurred in normoxic conditions, whereas under anoxic conditions, relatively 13.58% residual biomass remained. Our results suggest that microorganisms facilitated the conversion of dissolved organic carbon (DOC) derived from kelp degradation into refractory dissolved organic carbon (RDOC), a process observed under both normoxic and anoxic conditions. However, different dissolved oxygen levels lead to different bacterial community successions, which affected the conversion process from labile dissolved organic carbon (LDOC) to RDOC differently. Bacteroidia, which possess sulfur metabolic capabilities, play a significant role in RDOC generation under both normoxic and anoxic conditions. In normoxic conditions, the relative abundance of CHO molecules was 2.57% less than that under anoxic conditions, whereas the proportions of CHON was 3.83% higher. Additionally, DBEwa and Almodwa values were 11.04% and 15.63% higher than those observed under anoxic conditions. At the end of the experiment, the relative content of RDOC under normoxic and anoxic conditions was 9.18% and 5.45%, respectively. Despite the reduced production of RDOC, anoxic conditions promote the preservation of a larger amount of macroalgae biomass. However, uncertainty exists regarding the extent to which stored POC reaches deep-sea sequestration. Consequently, it is challenging to assert that anoxia positively influences carbon sequestration in macroalgae.

Cascade reservoirs affected chemical compositions of dissolved organic matter and greenhouse gas dynamics in the Lancang River

Dissolved organic matter (DOM) is an important component in aquatic systems. There has been much debate about the potential effects of cascade reservoirs on the transport and transformation of DOM. Here, through a survey of source to leave-boundary section of Lancang River (LCR) in June and November of 2017-2018, our results revealed that weak spatiotemporal variations were observed for DOC content, whereas DOM parameters were significantly different between natural and reservoir reaches. And DOM showed higher humification degree from allochthonous sources with increasing autochthonous matter in reservoir reach, may due to high particulate organic matter and releasing autochthonous DOM from phytoplankton blooms in the LCR, which can be evidenced by depleted DIC, enriched δ13CDIC and higher BIX. A unique fluorescent fraction (C5) appeared in the reservoir reach and increased along water flow, which was strongly associated with dissolved CO2 and N2O. Meanwhile, BIX value decreased with increasing dam height, hydraulic residence time (HRT), and reservoir capacity, which may promote CH4 production, highlighting variation of DOM compositions in understanding the effect of greenhouse gas (GHG) dynamics in the LCR. The findings were essential for comprehending the influences of cascade reservoirs on carbon cycle, and informed policy development for the sustainable management of transboundary water resources like the LCR.

Microplastics and benthic animals reshape the geochemical characteristics of dissolved organic matter by inducing changes in keystone microbes in riparian sediments

Dissolved organic matter (DOM) in riparian sediments plays a vital role in regulating element cycling and pollutant behavior of river ecosystems. Microplastics (MPs) and benthic animals (BAs) have been frequently detected in riparian sediments, influencing the substance transformation in river ecosystems. However, there is still a lack of systematic investigation on the effects of MPs and BAs on sediment DOM. This study investigated the impact of MPs and BAs on the geochemical characteristics of DOM in riparian sediments and their microbial mechanisms. The results showed that MPs and BAs increased sediment DOC concentration by 34.24%∼232.97% and promoted the conversion of macromolecular components to small molecular components, thereby reducing the humification degree of DOM. Mathematical model verified that the changes of keystone microbes composition in sediments were direct factors affecting the characteristics of DOM in riparian sediment. Especially, MPs tolerant microbes, including Planctomicrobium, Rhodobacter, Hirschia and Lautropia, significantly increased DOC concentration and decreased humification degree (P < 0.05). In addition, MPs and BAs could also influence keystone microbes in sediments by altering the structure of microbial network, thereby indirectly affecting DOM characteristics. The study demonstrates the pollution behavior of MPs in river ecosystems and provides a basis for protecting the ecological function of riparian sediments from MPs pollution.

Fulvic acid impact on constructed wetland-microbial electrolysis cell system performance: Metagenomic insights

This study explores the roles of fulvic acid (FA) in both a conventionally constructed wetland (CCW) and a newly constructed wetland-microbial electrolysis cell (ECW). The results showed that FA increased the average removal efficiency of chemical oxygen demand, total phosphorus, total nitrogen, and ammonia nitrogen in ECW by 8.6, 46.2, 33.0, and 27.9 %, respectively, compared to CCW, and reduced the global warming potential by > 60 %. FA promoted the proliferation of electroactive bacteria (e.g., Chlorobaculum and Candidatus Tenderia) and FA-degrading bacteria (e.g., Anaerolineaceae and Gammaproteobacteria) and reduced methanogens (e.g., Methanothrix) via type-changing. The study's findings suggest that FA influences pollutant removal and microbiome dynamics by altering dissolved oxygen levels and redox potential. In summary, FA and ECW enhanced the efficiency of constructed wetlands by facilitating electron transfer and consumption, and supporting microbial growth and metabolism.

Assessment of carbon flux gradients and dominant processes in a subtropical highly urbanized coastal ecosystem

Highly urbanized coastal ecosystems are vital in the global carbon budget. However, there are limited researches on carbon flux gradients in these nearshore areas, considering both natural and anthropogenic influences. Through on-site measurements and field samplings during wet-to-dry season in 2023, this study investigated spatial variations and factors affecting carbon fluxes, focusing on the impacts of salinity and eutrophic status in five geographically connected coastal waters of the Guangdong-Hong Kong-Macau Greater Bay Area (GBA). By estimating carbon exchange at land-sea-air interface, dominant processes in carbon dynamics were identified as well. Results showed that partial pressure of CO2 (pCO2) varied from 391 to 2290 μatm, and sea-air CO2 exchange fluxes (FCO2) ranged from -3.07 to 70.07 mmol m-2 d-1, indicating significant geographical distinctions among five coastal waters of the GBA. The total carbon transport from rivers to these nearshore waters was approximated at 6.44 Tg C yr-1, with the Pearl River (PR) contributing 99.7%, primarily in dissolved forms. Atmospheric CO2 release was calculated at 0.29 Tg C yr-1 for studied five coastal waters, primarily as carbon sources, except for Dapeng Bay (DPB) as a sink. CO2 emissions inversely correlated with salinity, yet positively with eutrophication status, particularly in river-dominated estuaries. Moreover, CO2 flux decreased 23 times as eco-status shift from eutrophic to non-eutrophic. River plumes, terrestrial pollutant inputs, and economic structure were underlying drivers, influencing carbon species concentrations and fluxes. Elevated CO2 concentrations in eutrophic coastal waters were mainly attributed to terrestrial carbon and nutrients inputs, supporting active biological respiration and microbial decomposition. Conversely, carbon dynamics potentially depend on the balance of respiration and photosynthesis in non-eutrophic coastal waters. This study offers high geographic precision and specificity of carbon species, and provides land-sea integration insight to understand carbon dynamic mechanisms, promoting advancements in water quality management and climate mitigation.

Effects of biochar and wood vinegar co-application on composting ammonia and nitrous oxide losses and fertility

Composting faces challenges with nitrogen (N) losses through ammonia (NH3) and nitrous oxide (N2O) emissions. In this study, wood vinegar (WV) and biochar (BC) were applied individually or combined into wheat straw and chicken manure composting. Results showed that BC and WV reduced NH3 volatilizations by 22-23 % individually, but their combined application achieved a 59 % reduction. However, this combination increased N2O emissions by 174 %. The BC + WV treatment improved compost quality, evidenced by increased total N content by 22 % and enhanced the biological index, promoting additional dissolved organic matter production. Overall, BC and WV applications improved compost quality, reduced gaseous N losses, and supported the re-utilization of agricultural residues. The combined use of BC and WV significantly enhances compost quality and reduces NH3 emissions, offering a promising solution for sustainable agricultural residue management.

Coupled redox cycling of arsenic and sulfur regulates thioarsenate enrichment in groundwater

High‑arsenic groundwater is influenced by a combination of processes: reductive dissolution of iron minerals and formation of secondary minerals, metal complexation and redox reactions of organic matter (OM), and formation of more migratory thioarsenate, which together can lead to significant increases in arsenic concentration in groundwater. This study was conducted in a typical sulfur- and arsenic-rich groundwater site within the Datong Basin to explore the conditions of thioarsenate formation and its influence on arsenic enrichment in groundwater using HPLC-ICPMS, hydrogeochemical modeling, and fluorescence spectroscopy. The shallow aquifer exhibited a highly reducing environment, marked by elevated sulfide levels, low concentrations of Fe(II), and the highest proportion of thioarsenate. In the middle aquifer, an optimal ∑S/∑As led to the presence of significant quantities of thioarsenate. In contrast, the deep aquifer exhibited low sulfide and high Fe(II) concentration, with arsenic primarily originating from dissolved iron minerals. Redox fluctuations in the sediment driven by sulfur‑iron minerals generated reduced sulfur, thereby facilitating thioarsenate formation. OM played a crucial role as an electron donor for microbial activities, promoting iron and sulfate reduction processes and creating conditions conducive to thioarsenate formation in reduced and high‑sulfur environments. Understanding the process of thioarsenate formation and the influencing factors is of paramount importance for comprehending the migration and redistribution of arsenic in groundwater systems.

Using the Potential Transformation of Dissolved Organic Matter to Understand Carbon Emissions from Inland Rivers

Understanding the transformation of river dissolved organic matter (DOM) is important for assessing the emissions of greenhouse gases (GHGs) in inland waters. However, the relationships between the variations in DOM components and GHGs remain largely unknown. Here, parallel factor analysis (PARAFAC) was applied to investigate the DOM components in 46 inland rivers in China. We found that the GHG emissions in peri-urban rivers were 1.10-2.15 times greater than those in urban rivers. Microbial and environmental factors (e.g., living cell numbers, microbial activity and pH) explained more than 70% of the total variance in GHG emissions in rivers. DOM variations relationships between different components ware revealed based on compositional data principal component analysis (CoDA-PCA). Microbial-mediated DOM production and degradation were quantified, and the degradation levels in peri-urban rivers were 11.8-25.2% greater than those in urban rivers. Differences in carbon emission potential between urban and peri-urban rivers were related to DOM variances and transformations and were affected by water chemistry (e.g., NH4-N and As). This study clarifies the regulatory effects of DOM composition variations and transformations on GHG emissions, and enhances the understanding of the DOM biogeochemical cycle.

Persistence of dissolved organic matter in sediments influenced by environmental factors：Implication for nutrition and carbon cycle

The persistence of dissolved organic matter (DOM) plays a crucial role in the cycling and distribution of carbon and nutrients. Nonetheless, our understanding of how environmental alterations affect the persistence of sedimentary DOM remains incomplete. Excitation Emission Fluorescence Matrix-Parallel Factor Analysis (EEM-PARAFAC) was used to examine the fluorescence and compositional characteristics of hydrophilic and hydrophobic DOM (separated using XAD-8 resin) within sediments from twelve lakes and reservoirs. Fluorescence analysis indicated that DOM persistence is dependent on the proportions of the three components derived from PARAFAC. The Mantel test showed that climatic factors had the most significant impact on DOM persistence (Mantel's r = 0.46-0.54, Mantel's p = 0.001-0.007), while anthropogenic (Mantel's r = 0.24-0.32, Mantel's p = 0.03-0.05) and hydrological factors (Mantel's r = 0.03-0.22, Mantel's p = 0.06-0.40) had a somewhat lesser influence. Environmental changes resulted in a consistent decline in DOM persistence from Northeast to Southwest China, accompanied by an increase in gross primary productivity (GPP). Reduced DOM persistence due to climate, hydrological, and anthropogenic factors may lead to elevated concentrations of total phosphorus (TP), contributing to deteriorating water quality and events such as algal blooms. The decline in water quality due to reduced DOM persistence in lakes with high GPP can exacerbate the transition from carbon sinks to carbon sources. Consequently, the persistence of sedimentary DOM significantly influences nutrient and carbon cycling in lakes. Investigating DOM persistence in lakes across diverse geographic locations offers a new perspective on lake eutrophication and carbon emissions. Furthermore, it is crucial to develop targeted recommendations for lake restoration and management.

Role of BP-ANN in simulating greenhouse gas emissions from global aquatic ecosystems via carbon component-environmental factor coupling

Inland waters (IW), estuarine areas (EA), and offshore areas (OA) function as aquatic systems in which the transport of carbon components results in the release of greenhouse gases (GHGs). Interconnected subsystems exhibit a greater greenhouse effect than individual systems. Despite this, there is a lack of research on how carbon loading and its components impact GHG emissions in various aquatic systems. In this study, we analyzed 430 aquatic sites to explore trade-off mechanisms among dissolved organic carbon (DOC), particulate organic carbon, dissolved inorganic carbon (DIC), and GHGs. The results revealed that IW emerged as the most significant GHG source, possessing a comprehensive global warming potential (GWP) of 0.78 ± 0.08 (10-2 Pg CO2-ep ha-1 year-1) for combined carbon dioxide, methane, and nitrous oxide. This surpassed the cumulative potentials of EA and OA (0.35 ± 0.05 (10-2 Pg CO2-ep ha-1 year-1)). Additionally, structural equation modeling indicated that GHG emissions resulted from a combination of carbon component loading and environmental factors. DOC exhibited a positive correlation with GWPs when influenced by biodegradable DOC. Total alkalinity and pH influenced DIC, leading to elevated pCO2 in aquatic systems, thereby enhancing GWPs. Predictive modeling using backpropagation artificial neural networks (BP-ANN) for GWPs, incorporating carbon components and environmental factors, demonstrated a good fit (R2 = 0.6078, RMSEaverage = 0.069, p > 0.05) between observed and predicted values. Enhancing the estimation of aquatic region feedback to GHG changes was achieved by incorporating corresponding water quality parameters. In summary, this study underscores the pivotal role of carbon components and environmental factors in aquatic regions for GHG emissions. The application of BP-ANN to estimate greenhouse effects from aquatic regions is highlighted, providing theoretical and experimental support for future advancements in monitoring and developing policies concerning the influence of water quality on GHG emissions.

Mollisol Erosion-Driven Efflux of Energetic Organic Carbon and Microflora Increases Greenhouse Gas Emissions from Cold-Region Rivers

Massive soil erosion occurs in the world's Mollisol regions due to land use change and climate warming. The migration of Mollisol organic matter to river systems and subsequent changes in carbon biogeochemical flow and greenhouse gas fluxes are of global importance but little understood. By employing comparative mesocosm experiments simulating varying erosion intensity in Mollisol regions of northeastern China, this research highlights that erosion-driven export and biomineralization of terrestrial organic matter facilitates CO2 and CH4 emission from receiving rivers. Stronger Mollisol erosion, as represented by a higher soil-to-water ratio in suspensions, increased CO2 efflux, particularly for the paddy Mollisols. This is mechanistically attributable to increased bioavailability of soluble organic carbon in river water that is sourced back to destabilized organic matter, especially from the cultivated Mollisols. Concurrent changes in microbial community structure have enhanced both aerobic and anaerobic processes as reflected by the coemission of CO2 and CH4. Higher greenhouse gas effluxes from paddy Mollisol suspensions suggest that agricultural land use by supplying more nitrogen-containing, higher-free-energy organic components may have enhanced microbial respiration. These new findings highlight that Mollisol erosion is a hidden significant contributor to greenhouse gas emissions from river water, given that the world's four major Mollisol belts are all experiencing intensive cultivation.

Biochar and hydrochar application influence soil ammonia volatilization and the dissolved organic matter in salt-affected soils

Biochar, which including pyrochar (PBC) and hydrochar (HBC), has been tested as a soil enhancer to improve saline soils. However, the effects of PBC and HBC application on ammonia (NH3) volatilization and dissolved organic matter (DOM) in saline paddy soils are poorly understood. In this research, marsh moss-derived PBC and HBC biochar types were applied to paddy saline soils at 0.5 % (w/w) and 1.5 % (w/w) rates to assess their impact on soil NH3 volatilization and DOM using a soil column experiment. The results revealed that soil NH3 volatilization significantly increased by 56.1 % in the treatment with 1.5 % (w/w) HBC compared to the control without PBC or HBC. Conversely, PBC and the lower application rate of HBC led to decrease in NH3 volatilization ranging from 2.4 % to 12.1 %. Floodwater EC is a dominant factor in NH3 emission. Furthermore, the fluorescence intensities of the four fractions (all humic substances) were found to be significantly higher in the 1.5 % (w/w) HBC treatment applied compared to the other treatments, as indicated by parallel factor analysis modeling. This study highlights the potential for soil NH3 losses and DOM leaching in saline paddy soils due to the high application rate of HBC. These findings offer valuable insights into the effects of PBC and HBC on rice paddy saline soil ecosystems.

Nitrogen migration and transformation during re-suspension and photo-induction in landscape water replenished by reclaimed water

Sediment re-suspension plays a crucial role in releasing endogenous nitrogen and greenhouse gases in shallow urban waters. However, the impacts of repeated re-suspension and photo-induced processes on migration and transformation from endogenous nitrogen, as well as the emission of greenhouse gases, remain unclear. This study simulated three conditions: re-suspension (Rs), re-suspension combined with ultravioletirradiation (Rs + UV), and ultraviolet irradiation (UV). The findings revealed that both repeated sediment re-suspension and exposure to UV light altered the characteristics of surface sediments. Decrease of convertible nitrogen in sediments, leading to the release of ion-exchangeable nitrogen (IEF-N) into NH4+-N and NO3--N, influenced greenhouse gas production differently under various conditions. The study observed the highest concentration of dissolved N2O in under UV irradiation, positively correlated with NO2--N and NO3--N. Re-suspension increased the turbidity of the overlying water and accelerated nitrification, resulting in the highest NO3--N concentration and the lowest dissolved N2O concentration. Additionally, in the Rs + UV dissolved N2O maintained the higher concentrations than in Rs, with greatest amount of N conversion in surface sediments, and a 59.45% reduction in IEF-N. The production of N2O during re-suspension was mainly positively correlated with NH4+-N in the overlying water. Therefore, this study suggest that repeated re-suspension and light exposure significantly influence nitrogen migration and transformation processes in sediment, providing a theoretical explanation for the eutrophication of water and greenhouse gas emissions.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L-1, and 20.3 ± 6.6 nmol L-1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004-2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.

Spatial gradients and molecular transformations of DOM, DON and DOS in human-impacted estuarine sediments

Dissolved organic matter (DOM) constitutes the most active fraction in global carbon pools, with estuarine sediments serving as significant repositories, where DOM is susceptible to dynamic transformations. Anthropogenic nitrogen (N) and sulfur (S) inputs further complicate DOM by creating N-bearing DOM (DON) and S-bearing DOM (DOS). This study delves into the spatial gradients and transformation mechanisms of DOM, DON, and DOS in Pearl River Estuary (PRE) sediments, China, using combined techniques of UV-visible spectroscopy, Excitation-emission matrix (EEM) fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial high-throughput sequencing. Results uncovered a distinct spatial gradient in DOM concentration, aromaticity (SUVA254), hydrophobicity (SUVA260), the content of substituent groups including carboxyl, carbonyl, hydroxyl and ester groups (A253/A203) of chromophoric DOM (CDOM), and the abundances of tyrosine/tryptophan-like protein and humic-like substances in fluorophoric DOM (FDOM). These all decreased from upper to lower PRE, accompanied by a decrease in O3S and O5S components, indicating seaward reduction in the contribution of terrestrial OM, especially anthropogenic inputs. Additionally, sediments exhibited a reduction in molecular diversity (number of formulas) of DOM, DON, and DOS from upper to lower PRE, with molecules tending towards a lower nominal oxidation state of carbon (NOSC) and higher bio-reactivity (MLBL), molecular weight (m/z) and saturation (H/C). While molecular composition of DOM remained similar in PRE sediments, the relative abundance of lignin-like substances decreased, with a concurrent increase in protein-like and lipid-like substances in DON and DOS from upper to lower PRE. Mechanistic analysis identified the joint influence of terrestrial OM, anthropogenic N/S inputs, and microbial processes in shaping the spatial gradients of DOM, DON, and DOS in PRE estuarine sediments. This study contributes valuable insights into the intricate spatial gradients and transformations of DOM, DON, and DOS within human-impacted estuarine sediments.

Terrestrial dissolved organic matter inputs accompanied by dissolved oxygen depletion and declining pH exacerbate CO2 emissions from a major Chinese reservoir

Terrestrial inputs and subsequent degradation of dissolved organic matter (DOM) in lake ecosystems can result in rapid depletion of dissolved oxygen (DO). Inputs of terrestrial DOM including organic acids can also lead to decreases in pH. However, to date, few studies have investigated the linkages between terrestrial DOM inputs, DO and pH levels in the water column, and carbon dioxide (CO2) emissions from lake ecosystems. Based on monthly field sampling campaigns across 100 sites in Lake Qiandao, a major man-made drinking water reservoir in China, from May 2020 to April 2021, we estimated an annual CO2 efflux (FCO2) of 37.2 ± 29.0 gC m-2 yr-1, corresponding to 0.02 ± 0.02 TgC yr-1 from this lake. FCO2 increased significantly with decreasing DO, chlorophyll-a (Chl-a) and δ2H-H2O, while FCO2 increased with increasing specific UV absorbance (SUVA254) and a terrestrial humic-like component (C2). We found that DO concentration and pH declined with increasing terrestrial DOM inputs, i.e. increased SUVA254 and terrestrial humic-like C2 levels. Vertical profile sampling revealed that the partial pressure of CO2 (pCO2) increased with increasing terrestrial DOM fluorescence (FDOM), while DO, pH, and δ13C-CO2 declined with increasing terrestrial FDOM. These results highlight the importance of terrestrial DOM inputs in altering physico-chemical environments and fueling CO2 emissions from this lake and potentially other aquatic ecosystems.

Urbanization influences dissolved organic matter characteristics but microbes affect greenhouse gas concentrations in lakes

Recognition and prediction of dissolved organic matter (DOM) properties and greenhouse gas (GHG) emissions is critical to understanding climate change and the fate of carbon in aquatic ecosystems, but related data is challenging to interpret due to covariance in multiple natural and anthropogenic variables with high spatial and temporal heterogeneity. Here, machine learning modeling combined with environmental analysis reveals that urbanization (e.g., population density and artificial surfaces) rather than geography determines DOM composition and properties in lakes. The structure of the bacterial community is the dominant factor determining GHG emissions from lakes. Urbanization increases DOM bioavailability and decreases the DOM degradation index (Ideg), increasing the potential for DOM conversion into inorganic carbon in lakes. The traditional fossil fuel-based path (SSP5) scenario increases carbon emission potential. Land conversion from water bodies into artificial surfaces causes organic carbon burial. It is predicted that increased urbanization will accelerate the carbon cycle in lake ecosystems in the future, which deserves attention in climate models and in the management of global warming.

The concentration of CH4, N2O and CO2 in the Pearl River estuary increased significantly due to the sediment particle resuspension and the interaction of hypoxia

Hypoxia and sediment particle resuspension (SPR) alter the biogeochemical cycle of estuarine and coastal seas, which in turn affects the production and emission of methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) greenhouse gases (GHGs) in estuaries. Despite the importance of CH4, N2O and CO2 in estuarine ecosystems, little is known about their magnitude and spatiotemporal variation under the combined influence of hypoxia and SPR. This study utilized continuous mooring observations to investigate the temporal and spatial variations of GHGs before and after hypoxia in the Pearl River Estuary (PRE). The results showed that the concentration of GHGs in the water column increased significantly following hypoxia as compared to its absence. The synergistic effect of SPR and hypoxia significantly enhances GHGs production and accumulation in bottom water. Anaerobic mineralization of organic matter (OM) in an environment with severely low dissolved oxygen (DO) is the primary determinant for increased CH4 concentration, while OM and CH4 oxidation are the main drivers for maintaining high CO2 concentration in subsurface water. Hypoxic development enhanced denitrification N2O production in the water column. The presence of SPR enhanced oxygen-consuming coupled hypoxia significantly stimulated the increase of CH4, N2O and CO2 concentrations in the water column. Hypoxic development results in an increased water-air GHGs flux, but this effect may be masked by runoff plumes with high GHGs concentrations in the regions near the river outlets. This study highlights that hypoxia leads to significant increases in anaerobic GHGs production and subsequent emissions from estuarine water columns.

Dynamics of CO2, CH4, and N2O in Ria Formosa coastal lagoon (southwestern Iberia) and export to the Gulf of Cadiz

A first characterization of greenhouse gases had been carried out to study their role and impact in a productive transitional coastal system of the southern Portugal - Ria Formosa lagoon. To this purpose, the partial pressure of CO2 (pCO2) and the concentration of dissolved CH4 and N2O have been measured. Two surveys were carried out during 2020, at low tide under typical conditions of Spring (March) and end of Summer (October). The samplings sites were distributed along the costal lagoon covering: i) inner areas with strong human impact (influence of different flows of treated wastewater discharges); and ii) main channels in connection with the main inlets to study the exchanges with the ocean. In general, the highest values of the three greenhouse gases were found at the inner studied areas, especially affected by the disposal of treated effluents from wastewater treatment plans, in October. The mean water - atmosphere fluxes of the CO2, CH4 and N2O are positive, showing that the study area acts as a source of these gases to the atmosphere. On the other hand, it was calculated a rough estimation of the three gases globally exported from Ria Formosa to the ocean, through the main six inlets to evaluate the magnitude of the supply of these gases from Ria Formosa to the adjacent ocean. The mean CO2, CH4 and N2O horizontal water fluxes exported from all the inlets of Ria Formosa to the Gulf of Cadiz for both seasons, during low water, are 8.7 ± 3.9 mmol m-2 s-1, 8.0 ± 3.5 μmol m-2 s-1 and 3.2 ± 1.5 μmol m-2 s-1, which corresponds to a mass transport through the inlets section of 0.7 ± 0.7 kg s-1, 0.2 ± 0.2 g s-1 and 0.2 ± 0.3 g s-1 respectively. From these estimates, as expected, the higher mass transport was found at the larger and deeper inlets (Faro-Olhão and Armona).

Coupling between chromophoric dissolved organic matter and dissolved inorganic carbon in Indian estuaries

This study investigates the coupling between Chromophoric Dissolved Organic Matter (CDOM) and Dissolved Inorganic Carbon (DIC) in eighteen Indian estuaries across salinity gradient of the east and west coasts during the monsoon season, characterized by significant river discharge. The hypothesis that humic acids (HA) and fulvic acids (FA), prominent in estuarine CDOM, closely correspond to the 'organic alkalinity' (Aorg) component of total alkalinity is examined. In most estuaries, specifically those along the northeast coast (NE) and southwest coast (SW), a significant linear relationship exists between DIC, CDOM abundance, and pH level. Notably, minor estuaries along the southeast coast (SE) and northwest coast (NW) exhibit elevated DIC levels beyond what this relationship predicts. These estuaries also reveal heightened ammonium levels, increased δ15N values, and decreased δ13C values, indicative of anthropogenic influence. CDOM properties, such as spectral slope (S300-500) and spectral slope ratio (SR, S275-295:S350-400), align with these findings, with SE and NW estuaries displaying higher values. On average, CDOM contributes 110.5 μM (6.8 %) to DIC in NE, 390.7 μM (11 %) in SE, 24.4 μM (4.8 %) in SW, and 122.2 μM (4 %) in NW estuaries. The relationship between total alkalinity minus DIC (TA-DIC) and pH25 suggests that CDOM, mediated by HA/FA, buffers the inorganic carbon system in estuaries. This buffering capacity weakens at elevated DIC levels, and this condition is marked by anomalous SR values compared to the baseline salinity-SR linear regression. This Study suggests that estuarine CDOM could largely represent "organic alkalinity" and could help monitor acidification in estuaries.

Dissolved organic matter distribution in the water column and sediment pore water in a highly anthropized coastal lagoon (Mar Menor, Spain): Characteristics, sources, and benthic fluxes

Coastal lagoons are among the most productive and biodiverse systems in the world and are important sentinels of climate change. The Mar Menor is one of the largest coastal lagoons in the Mediterranean, providing a variety of ecosystem services and resources to the community. However, in recent decades this lagoon has suffered drastic changes and degradation caused by human activities. We analyzed the concentration of dissolved organic carbon (DOC) and the optical properties of dissolved organic matter (DOM) in the water column and sediment pore water during the summer and winter of 2018 and during eighteen months from 2016 to 2018. Overall, we found that the composition of DOM is mainly related to and enhanced by anthropogenic activities and microbial metabolism. DOM enters the lagoon via urban and agricultural runoff, drainage systems, and wastewater treatment plants. Additionally, strong microbial metabolism in sediments leads to differences in DOM composition between water and sediments. In the water column, humic-like components accounted for 71 % of the total DOM, while protein-like compounds were most abundant in sediment pore water. We observed a strong seasonal variability associated with precipitation and the system collapse in 2016 (phytoplankton bloom), which resulted in the death of 80 % of macrophytes. The sediments act as a source of DOM to the overlying water, likely due to relatively high organic matter content and intense microbial activity, primarily through anaerobic pathways. Benthic fluxes of DOC ranged from 5.24 to 33.30 mmol m-2 d-1, being higher in winter than summer 2018 and decreasing from north to south, likely related to lower residence time in the northern basin, groundwater discharge and accumulation of organic matter from the dead meadows. We estimate a net flux of DOC from the Mar Menor toward the Mediterranean Sea of 1.57 × 107 mol yr-1.

Seasonal changes of dissolved organic matter chemistry and its linkage with greenhouse gas emissions in saltmarsh surface water and porewater interactions

Dissolved organic matter (DOM) is one of the largest reactive reservoirs of carbon on earth. Saltmarshes play an essential role in shaping the fate of DOM and greenhouse gas (GHG) production in surface water and groundwater interactions in coastal areas. However, the coupling mechanism between DOM and GHG production is poorly understood. In this study, DOM in both surface water and porewater were analyzed by 3D excitation-emission-matrix spectroscopy under different seasonal and tidal conditions in a saltmarsh. Protein-like DOM was likely to produce CH4, while humic-like DOM tended to produce CO2. CH4 concentration was highly enriched in porewater because increasing fresh groundwater flow introduced small-sized protein-like DOM. Based on the mass balance model, >98.5% of CH4 was oxidated to CO2 in sediment-water interface. The degradation of sediment-derived DOC (especially humic-like DOM) contributes ∼80% of the total amount of CO2 in surface water. Both hydrodynamics and chemical reactions are suggested to influence greenhouse gas (GHG) emissions. Hydrodynamics (e.g., tidal pumping) are controlling factors in short timescales (hourly/weekly) while chemical reactions become crucial in influencing DOM chemistry and related degradation rate on seasonal scales. These findings emphasize the importance of the coupling mechanism at different time scales between DOM characteristics and GHG emissions in saltmarshes.

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

Growing evidence indicates that rivers are hotspots of greenhouse gas (GHG) emissions and play multiple roles in the global carbon budget. However, the roles of terrestrial carbon from land use in river GHG emissions remain largely unknown. We studied the microbial composition, dissolved organic matter (DOM) properties, and GHG emission responses to different landcovers in rivers (n = 100). The bacterial community was mainly constrained by land-use intensity, whereas the fungal community was mainly controlled by DOM chemical composition (e.g., terrestrial DOM with high photoreactivity). Anthropogenic stressors (e.g., land-use intensity, gross regional domestic product, and total population) were the main factors affecting chromophoric DOM (CDOM). DOM biodegradability exhibited a positive correlation with CDOM and contributed to microbial activity for DOM transformation. Variations in CO2 and CH4 emissions were governed by the biodegradation or photomineralization of dissolved organic carbon derived from autotrophic DOM and were indirectly affected by land use via changes in DOM properties and water chemistry. Because the GHG emissions of rivers offset some of the climatic benefits of terrestrial carbon (or ocean) sinks, intensified urban land use inevitably alters carbon cycling and changes the regional microclimate.

Interaction between dissolved organic carbon and fungal network governs carbon mineralization in paddy soil under co-incorporation of green manure and biochar

Legume crops in rice cultivation are typically rotated and incorporated into the soil as green manure to improve soil fertility. Biochar has recently been co-incorporated with green manure to simultaneously stimulate soil organic carbon (SOC) mineralization and increase carbon (C) sequestration. However, few studies examine the effects of the co-incorporation of biochar and green manure on C cycling and the underlying microbial mechanisms in paddy fields. In this study, the effects of the co-incorporation of green manure and biochar on C mineralization, dissolved organic carbon (DOC) characteristics, and microbial community structures were investigated. A pot study was conducted with three treatments: inorganic NPK (NPK), inorganic NPK + green manure (GM), and inorganic NPK + green manure + biochar (GMC). Organic amendments significantly increased cumulative C mineralization, with amounts in the order GMC (3,434 mg·kg-1) > GM (2,934 mg·kg-1) > NPK (2,592 mg·kg-1). Fertilizer treatments had similar effects on DOC concentrations, with amounts in the order GMC (279 mg·kg-1) > GM (255 mg·kg-1) > NPK (193 mg·kg-1). According to fluorescence spectra, the highest microbial humic acid-like fraction and biological index were also in GMC. Co-incorporation of green manure and biochar shifted the composition of bacterial and fungal communities but more importantly, increased fungal network complexity and decreased bacterial network complexity. The increase in fungal network complexity with the increase in DOC concentrations and microbially derived components was the dominant factor in promoting C mineralization. Overall, this study reveals the underlying biochemical mechanism, the interaction between DOC and fungal network of C cycling in paddy soil under the co-incorporation of green manure and biochar management, and provides fundamental knowledge for exploring effective approaches to improve soil fertility and health in the future.

The effects of climate warming on microbe-mediated mechanisms of sediment carbon emission

Due to significant differences in biotic and abiotic properties of soils compared to those of sediments, the predicted underlying microbe-mediated mechanisms of soil carbon emissions in response to warming may not be applicable for estimating similar emissions from inland water sediments. We addressed this issue by incubating different types of sediments, (including lake, small river, and pond sediments) collected from 36 sites across the Yangtze River basin, under short-term experimental warming to explore the effects of climate warming on sediment carbon emission and the underlying microbe-mediated mechanisms. Our results indicated that under climate warming CO₂ emissions were affected more than CH₄ emissions, and that pond sediments may yield a greater relative contribution of CO₂ to total carbon emissions than lake and river sediments. Warming-induced CO₂ and CH₄ increases involve different microbe-mediated mechanisms; Warming-induced sediment CO₂ emissions were predicted to be directly positively driven by microbial community network modularity, which was significantly negatively affected by the quality and quantity of organic carbon and warming-induced variations in dissolved oxygen, Conversely, warming-induced sediment CH₄ emissions were predicted to be directly positively driven by microbial community network complexity, which was significantly negatively affected by warming-induced variations in pH. Our findings suggest that biotic and abiotic drivers for sediment CO₂ and CH₄ emissions in response to climate warming should be considered separately when predicting sediment organic carbon decomposition dynamics resulting from climate change.

Exploring the optical properties and molecular characteristics of dissolved organic matter in a large river-connected lake (Poyang Lake, China) using optical spectroscopy and FT-ICR MS analysis

River-connected lakes are complicated and dynamic ecosystems due to their distinctive hydrological pattern, which could significantly impact the generation, degradation, and transformation processes of dissolved organic matter (DOM) and further regulate DOM chemistry in lakes. However, the molecular compositions and characteristics of DOM in river-connected lakes are still poorly understood. Thus, here the spatial variations of optical properties and molecular characteristics of DOM in a large river-connected lake (Poyang Lake) were explored via spectroscopic techniques and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed high degree of spatial heterogeneity of DOM chemistry (variations in DOC concentrations, optical parameters, and molecular compounds) in Poyang Lake, and the diversity at the molecular level was primarily caused by the heteroatom compounds (N- and S- containing). Compared with classic lakes and rivers, DOM compositions of the river-connected lake had distinctive characteristics (differences in the AI_mod and DBE values, and CHOS proportions). And the composition characteristics of DOM between the southern and northern parts of Poyang Lake were different (such as the lability and molecular compounds), suggesting the changes of hydrologic conditions may affect the DOM chemistry. In addition, various sources of DOM (autochthonous, allochthonous, and anthropogenic inputs) were identified agreeably based on optical properties and molecular compounds. Overall, this study first characterizes the DOM chemistry and reveals its spatial variations in Poyang Lake at the molecular level, which could improve our understanding of DOM in large river-connected lake systems. Further studies are encouraged to investigate the seasonal variations of DOM chemistry under different hydrologic conditions in Poyang Lake to enrich the knowledge of carbon cycling in river-connected lake systems.

Chemodiversity of Dissolved Organic Matter Is Governed by Microbial Biogeography in Inland Waters

Dissolved organic matter (DOM) is crucial for the carbon biogeochemical cycle and has a close link with microbiome in aquatic ecosystems; however, the causal relationship between DOM and microbial diversity in inland waters is not very clear so far. Therefore, a national survey of China's inland waters was conducted, and the DOM chemical composition and microbial community composition were determined by Fourier transform ion cyclotron resonance mass spectrometry and high-throughput sequencing to clarify the abovementioned question. Here, we found that DOM chemodiversity was governed by microbial community assembly in inland waters, not vice versa. Under the control of microbial biogeography, DOM chemodiversity showed a clear geographical distribution difference. Water DOM chemodiversity was mainly constrained by bacterial and archaeal community composition, whereas sediment DOM chemodiversity was mainly controlled by eukaryotic and fungal community composition. In addition, the sediment DOM chemical composition was also affected by the interaction of different microbial groups between waters and sediments. The study is the first to clarify the causal relationship and proposes a microbial regulatory mechanism on the geographical distribution pattern of DOM chemodiversity, thus further deepening the understanding of the DOM biogeochemical cycle.

Effects of tidal cycle on greenhouse gases emissions from a tropical estuary

The potential effects of tidal and diel cycles on fluxes and concentrations of carbon dioxide (pCO₂), methane (CH₄), and nitrous oxide (N₂O) along with associated biogeochemical processes remain poorly understood in tropical estuaries. The present study, based on six-hourly sampling for nine consecutive days at three locations along the salinity gradient in the Mahanadi estuary of India, revealed that the tidal forcing affected pCO₂ and CH₄ in the mixing zone with elevated concentrations during low tide with maximum concentrations up to 21,606 μatm and 285 μM, respectively. pCO₂ increased with decrease in tidal height within low and high tide duration as well, possibly due to higher relative contribution of freshwater with high CO₂. N₂O, on the other hand, showed no significant variability with tidal cycle or water level fluctuation during high and low tide. Barring the offshore region, the study area was source of greenhouse gases to the atmosphere.

Greenhouse gas emission during swine manure aerobic composting: Insight from the dissolved organic matter associated microbial community succession

Greenhouse gas emissions during aerobic composting is unavoidable, but good practices can minimize emission. Therefore, to explore the key factors influencing the release of greenhouse gas emissions during composting, the inaction of organic matter conversion, greenhouse gas emissions and bacterial community structure during co-composting with different ratio (pig manure and corn straw) over a 6-week period was studied. The excitation-emission matrix fluorescence spectroscopy with the parallel factor was used to identify that dissolved organic matter associated microbial community succession mainly influenced greenhouse gas emissions. Protein-like fractions of dissolved organic matter were more likely to decompose and promote CH₄ and CO₂ emissions, while the humic-like fractions of dissolved organic matter positively affected N₂O emissions. The largest of greenhouse gas emissions was appeared in MR2 with 12.7 kg CO₂-eq, and the MR3 and MR4 reduced greenhouse gas emissions by 26.8 % and 11.4 %, respectively.

Source and quality of dissolved organic matter in streams are reflective to land use/land cover, climate seasonality and pCO2

Sources and quality of dissolved organic matter (DOM) in streams may be largely controlled by the landscape and season. In this study, we attempted to answer three critical questions: 1) Do land use/land cover (LULC) types affect DOM characteristics? 2) Is there a seasonal fluctuation in DOM components? 3) How do DOM quality and LULC types influence aqueous carbon dioxide partial pressure (pCO₂). To achieve this, we investigated the fluorescence characteristics of DOM and its implication for pCO₂ in three streams draining land with different urban intensities under distinctive dry and wet seasons. Four fluorescence components were identified, including two terrestrial humic-like components, one protein-like component and one microbial humic-like component. We found a significant positive relationship of the maximum fluorescence intensity (F_max) of the four components and fluorescence index (FI₃₇₀) with urbanization intensity in both the dry and wet seasons. The mean F_max, biological index (BIX) and FI₃₇₀ all exhibited an increasing trend from upstream to downstream in the stream with highest proportions of urban and cropland. The fluorescence characteristics were negatively related to proportion of forested land in the both seasons. The terrestrial humic-like DOM was dominating in the studied streams. Moreover, the seasonality altered the DOM composition, with protein-like component emerging only in stream waters during the dry season, while microbial humic-like component exclusively occurred during the wet season. pCO₂ values were positively related to terrestrial humic-like and biological protein-like components, and urban land. The dry season had much higher pCO₂ than the wet season. Results from the Partial Least Squares Path (PLS-PM) models further indicated that LULC types were important in mediating fluorescence DOM whilst pCO₂ was more sensitive to the direct effect from FDOM dynamics. We conclude that DOM source and quality in streams are reflective to LULC and climate seasonality, and are good indicators of pCO₂ via source tracer and quality of fluorescence components.

Spatiotemporal heterogeneous effects of microplastics input on soil dissolved organic matter (DOM) under field conditions

Microplastics (MPs) as emerging persistent pollutants are ubiquitous in terrestrial environments. The effects of MPs input on soil dissolved organic matter (DOM) yet remain largely unclear, which limits our ability to predict how soil carbon dynamics will respond to the intensifying terrestrial plastic contamination, especially under the context of climate change. Here, a long-term field experiment with MPs addition treatment in soils spanning cold temperate zone to the tropics in China was conducted to evaluate the effects of MPs on DOM composition and to explore the spatial and temporal distribution patterns and relevant mechanistic controls of DOM responses after MPs input. We report that the DOM composition in soils with MPs addition was significantly changed relative to the ambient, in which tryptophan-like substances were decreased and humic-like substances were increased. Moreover, we find more apparent transformations of DOM composition in soils for longer treatment time, suggesting a long-lasting effect of MPs on DOM. The overall impact of MPs on DOM is more pronounced in lower latitudes, and nutrient availability and latitude-related climatic variables are associated with the influence degree of MPs input on soil DOM.

Spatial distribution and influencing mechanism of CO2, N2O and CH4 in the Pearl River Estuary in summer

Estuaries, considered as the important carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) sources to the atmosphere, are increasingly affected by near-bottom hypoxia. However, the impact of estuarine hypoxic zone development on GHGs production and discharge remains poorly understood due to the seasonal and spatially distributed heterogeneity of estuarine hypoxia occurrence and the lack of simultaneous monitoring of the distribution of bottom hypoxic waters and the vertical distribution of GHGs. Here, we conducted high spatial resolution vertical stratification sampling and analysis of water column GHGs in the Pearl River Estuary (PRE), a large estuary with frequent hypoxia in recent years. Our results showed that Pearl River runoff is the main source of GHGs in the PRE. Strong nitrification is an important N₂O production mechanism in the PRE. In situ generation of water and resuspension of surface sediments were the main sources of CH₄ in bottom water, while massive organic matter (OM) mineralization is the main driver of CO₂ in bottom water. The development of a hypoxic zone in the PRE significantly increased the concentration of N₂O and CH₄ in the bottom water and thus increased air-water fluxes. The air-water fluxes of N₂O, CH₄ and CO₂ of PRE in summer were 31.9 ± 7.5 μmol m^-2 d^-1, 192.5 ± 229.4 μmol m^-2 d^-1 and 51.9 ± 14.1 mmol m^-2 d^-1, respectively. This study reveals that GHGs fluxes from estuarine waters to the atmosphere will increase significantly with increasing eutrophication caused by human activities and the expansion of hypoxic zones in estuarine waters.

Effects of Photodegradation on the Optical Indices of Chromophoric Dissolved Organic Matter from Typical Sources

Chromophoric dissolved organic matter (CDOM) plays important roles in aquatic environments, and its optical properties provide a series of indices for evaluating the source and composition of dissolved organic matter (DOM). However, little is known about the varying photodegradation of CDOM from different sources and the effects on the optical indices of DOM composition. This was studied for typical natural and anthropogenic sources (plant and leaf litter leachates, the influent and effluent of a wastewater treatment plant, and a river). The CDOM absorption (a₂₈₀) showed a lower degradability for the plant leachate than other sources, mainly due to its low molecular weight and aromaticity. Four fluorescent components were identified with excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC), namely benzoic acid/monolignol-like C1, humic-like C2 and C3, and tryptophan-like C4. The plant leachate contained mainly C1, which was photodegraded moderately, while other sources had more C2 and C3 with higher photodegradability. C4 was photodegraded in most sources but was photoproduced in the leaf litter leachate. The absorption slope (S_275-295) and slope ratio (S_R) increased while the humification index (HIX) decreased, suggesting a decreasing molecular weight and humic content by photodegradation. This was consistent with the decreasing %C2 and %C3 but increasing %C4, which indicated preferential removal of humic-like components. The %C1, %C2, biological index (BIX), and fluorescence index (FI) were less affected by photodegradation than other indices for most sources. These results have implications for a better understanding of the photochemistry of CDOM and the applications of optical indices.

Concentrations of dissolved organic matter and methane in lakes in Southwest China: Different roles of external factors and in-lake biota

Many factors have been reported to affect material cycling in lakes, but the combined and cascading impacts of external environmental factors and in-lake biota on lake carbon cycling are poorly understood. We elucidated the influencing pathways of geoclimatic factors, lake morphometry, land-use type, chemical and physical factors, and biological taxa (phytoplankton and macroinvertebrates) on the concentrations of two important components of carbon cycling, i.e., dissolved organic matter (DOM) and methane (CH₄) based on datasets from 64 plateau lakes in Southwest China. Partial least squares path modelling (PLS-PM) indicated that (1) geoclimatic factors influenced DOM and CH₄ by affecting land use and lake physical factors (e.g., water temperature), (2) lake morphometry (water depth and lake area) had a direct and great negative effect on the CH₄ concentration related to the production and oxidation of CH₄ and affected phytoplankton and macroinvertebrates by influencing chemical and physical factors, (3) land-use type affected DOM and CH₄ concentrations in both direct and indirect ways, (4) terrestrial humic-like DOM was mainly discharged from forestland and also affected by macroinvertebrates, while the impacts of agricultural and construction land on autochthonous DOM and CH₄ concentrations mainly occurred by changing nutrients and then the aquatic biota. Moreover, changes in aquatic biota, primarily affected by water quality, influenced DOM spectral properties, and the two biotas affected DOM and CH₄ concentrations differently. Phytoplankton, especially cyanobacteria contributed to (protein-like and humic-like) DOM in both direct and indirect ways related to eutrophication, whereas macroinvertebrates influenced DOM possibly by utilization, bioturbation, and microbial decomposition of feces according to their different relationships with DOM spectral indices. Additionally, CH₄ production can be enhanced by DOM accumulation, and the significant positive correlations of CH₄ concentrations with protein-like DOM and biological index indicate that autochthonous DOM may play an important role for the CH₄ production. Our findings contribute to the understanding of lake carbon cycling under natural conditions and anthropogenic disturbances.

Tidal variation and litter decomposition co-affect carbon emissions in estuarine wetlands

Estuarine wetlands play important roles in the regional and global carbon cycle as well as greenhouse gas emissions; however, the driving factors and potential carbon emissions mechanisms are unclear. Here, the carbon emission fluxes were investigated in situ from different vegetated areas in the Chongming wetlands. The results showed that the highest methane (CH₄) and carbon dioxide (CO₂) emissions of 178.1 and 21,482.5 mg∙m^-2∙min^-1 were in Scirpus mariqueter and Phragmites australis dominated areas, respectively. A series of microcosms was strategically designed to simulate the influence of tidal variation on carbon emissions and the litter decomposition on daily- and monthly-timescales in estuarine wetlands. All added litter promoted CH₄ and CO₂ emissions from the wetland soils. The CH₄ and CO₂ emission fluxes of the S. mariqueter treatment were higher (367.7 vs. 108.4; 1607.9 vs. 1324.3 mg∙m^-2∙min^-1) than those of the P. australis treatment without tidal variation on a monthly timescale, due to the higher total organic carbon (TOC) content of S. mariqueter. The decomposition of litter also released a large amount of nutrients, which enhanced the abundance of methane-producing archaea (MPA) and methane-oxidizing bacteria (MOB). However, the tidal water level was negatively correlated with CH₄ and CO₂ emission fluxes. The CH₄ and CO₂ emission fluxes in the S. mariqueter treatment at the lowest tide were 556.02 and 604.99 mg∙m^-2∙min^-1, respectively. However, the CH₄ and CO₂ emission fluxes did not change significantly on the daily timescale in the S. mariqueter treatment without tidal variations. Therefore, the prolonged timescales revealed increases in litter decomposition but a decrease in the contribution of tidal variations to carbon emissions in estuarine wetlands. These findings provide a theoretical basis for evaluating the carbon cycle in estuarine wetlands.

The Impacts of Nitrogen Pollution and Urbanization on the Carbon Dioxide Emission from Sewage-Draining River Networks

Carbon dioxide (CO₂) emissions from river water have sparked worldwide concerns due to supersaturate CO₂ levels in the majority of global rivers, while the knowledge on the associations among nitrogen pollution, urbanization, and CO₂ emissions is still limited. In this study, the CO₂ partial pressure (pCO₂), carbon and nitrogen species, and water parameters in sewage-draining river networks were investigated. Extremely high pCO₂ levels were observed in sewage and drainage river waters, such as Longfeng River, Beijing-drainage River, and Beitang-drainage River, which were approximately 4 times higher than the averaged pCO₂ in worldwide rivers. Correlations of carbon/nitrogen species and pCO₂ indicated that carbon dioxide in rural rivers and sewage waters primarily originated from soil aeration zones and biological processes of organic carbon/nitrogen input from drainage waters, while that in urban rivers and lakes was mainly dominated by organic matter degradation and biological respiration. Enhanced internal primary productivity played critical roles in absorbing CO₂ by photosynthesis in some unsaturated pCO₂ sampling sites. Additionally, higher pCO₂ levels have been observed with higher NH₄⁺-N and lower DO. CO₂ fluxes in sewage waters exhibited extremely high levels compared with those of natural rivers. The results could provide implications for assessing CO₂ emissions in diverse waters and fulfilling water management polices when considering water contamination under intense anthropogenic activities.

Warming and humidification mediated changes of DOM composition in an Alfisol

Dissolved organic matter (DOM) represents the most mobile and reactive pool of soil organic matter (SOM). Climate changes, such as global warming and altered precipitation exert considerable influences on the quality and quantity of soil DOM. However, rare reports have focused on the interactive effects of soil warming and increased precipitation. In the present study, we conducted a 90-day incubation experiment to investigate how the concentration, source and chemical composition of DOM from an Alfisol respond to the variations of temperatures (15, 30 and 45 °C) and moistures (40%, 60%, and 80% of saturated soil water content). Four DOM components were identified through fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC). Increased temperature alone aggravated the decomposition of plant-derived aromatic components (C2 and C4) but promoted the accumulation of microbial-derived aliphatic carbon (C1) and tryptophan-like component (C3). Increased fungi/bacteria ratio with warming was responsible for the decomposition of plant-derived components. Warming-induced disassociation of Ca-bearing mineral to colloidal Ca facilitated the accrual of microbial-derived aliphatic DOM. Humidification alone and humidification + warming significantly increased the concentration of DOM and the percentage of plant-derived aromatic carbon (C2, C4), which was attributed to the release of Fe-bearing mineral-OC. Based on the above findings along with the results of two-way ANOVA and Variation partition analysis, we infer that moisture will play a dominant role in regulating the chemical composition of DOM in Alfisols under both warming and humidification which in turn impact global C cycling and the ultimate climate.