Dissolved Organic Matter Composition Research Articles

Online LC-Orbitrap MS method for the rapid molecular characterization of dissolved organic matter.

High-resolution mass spectrometry (HRMS) combined with electrospray ionization (ESI) has been the most useful technique for molecular characterization of dissolved organic matter (DOM) derived from diverse sources. However, the comprehensive detection of DOM composition was hindered by ionization suppression observed in ESI sources. HRMS coupled with liquid chromatography (LC) is a potential tool for comprehensive molecular characterization of DOM. The Suwannee River fulvic acids (SRFAs) and two DOM samples from seawater and refinery wastewater extracted by solid phase extraction (SPE) were analyzed by LC-Orbitrap MS coupled with ESI. The mobile phases, solvent composition, and gradient in the LC-Orbitrap MS analysis were optimized. The number of detected molecular formulae of SRFAs by online LC-Orbitrap MS was significantly increased by approximately 40% compared to direct injection. These additional detected compounds are mainly protein and lignin-like compounds, with a low O/C ratio and high H/C ratio. For the SRFAs, the relative standard deviations (%) of reproducibility are 5.51, 2.33, 7.97, and 1.80 for average O/C, H/C, double bond equivalent, and modified aromaticity index, respectively. This study proposed a simple and rapid method based on LC-Orbitrap MS for an in-depth analysis of the molecular composition of DOM, achieving a remarkable analysis time of only 5 min per sample. The rapid method provides a dependable and efficient approach for the molecular characterization of DOM, thereby advancing our comprehension and investigation of DOM across diverse ecosystems.

Coral high molecular weight carbohydrates support opportunistic microbes in bacterioplankton from an algae-dominated reef.

Dissolved organic matter (DOM) released by benthic primary producers fuels coral reef food webs. Anthropogenic stressors cause shifts from coral to algae dominance on many reefs, and resulting alterations in the DOM pool can promote opportunistic microbes and potential coral pathogens in reef water. To better understand these DOM-induced effects on bacterioplankton communities, we compared the carbohydrate composition of coral- and macroalgae-DOM and analyzed the response of bacterioplankton from an algae-dominated reef to these DOM types. In line with the proposed microbialization of reefs, coral-DOM was efficiently utilized, promoting energy transfer to higher trophic levels, whereas macroalgae-DOM likely stimulated microbial respiration over biomass production. Contrary to earlier findings, coral- and not algal-DOM selected for opportunistic microbial taxa, indicating that a change in the prevalent DOM composition, and not DOM type, may promote the rise of opportunistic microbes. Presented results may also apply to other coastal marine ecosystems undergoing benthic community shifts.

Eutrophication and Dissolved Organic Matter Exacerbate the Diel Discrepancy of CO2 Emissions in China's Largest Urban Lake.

The large variability in the emissions of carbon dioxide (CO2) from urban lakes remains a challenge for partitioning these sources at meaningful spatial and temporal scales. Dissolved organic matter (DOM) governs the spatial and temporal variations in CO2, yet relationships of the CO2 concentration (cCO2) and emission flux (FCO2) with DOM in urban lakes have rarely been reported. In this study, we monitored levels of cCO2, FCO2, and the composition of DOM over a 24 h period at three sites during the dry and wet seasons in China's largest urban lake, Tangxun Lake. Our study found the ratio of day/night FCO2 (millimoles per square meter per day) decreased from the dry season (0.79; 7.68/9.68) to the wet season (0.25; 6.05/24.16), averaging 0.42 (6.77/15.97), implying that accounting for nighttime CO2 emissions can increase regional estimates by 70%. This study revealed that eutrophication affected diurnal CO2 emissions with greater algal growth enhancing daytime CO2 uptake and subsequently increasing nighttime CO2 emissions via DOM degradation (larger protein-like DOM fraction). We anticipate that the relative magnitude of FCO2 between day and night from lakes is likely to increase due to urbanization and climate change, underscoring the importance of treating urban lakes as a distinct group and integrating DOM dynamics into carbon cycling in future research.

The role of nitrogen and iron biogeochemical cycles in the production and export of dissolved organic matter in agricultural headwater catchments

Abstract. To better understand the seasonal variations in environmental conditions regulating dissolved organic matter (DOM) export in agricultural headwater catchments, we combined the monitoring of nitrate, iron, soluble phosphorus, and DOM concentration (as dissolved organic carbon; DOC) and composition (3D fluorescence) in soil and stream waters at regular intervals during 1 hydrological year. We installed 17 zero-tension lysimeters in organic-rich top soil horizons (15 cm below the surface) in the riparian area of a well-monitored agricultural catchment in French Brittany and collected them at a fortnightly frequency from October 2022 to June 2023. We observed a large increase in DOC concentrations in soil waters during the high-flow period linked to the establishment of Fe-reducing conditions and the subsequent release of DOM. We also noted that the timing and the spatial variability in Fe(II) biodissolution in soils was regulated by nitrate from agricultural origin and the heterogeneity of water flow paths at the hillslope scale. Contrary to our current understanding of DOM export in headwater catchments, these results lead us to consider the winter high-flow period as an active phase of both DOM production and export.

Snowmelt seepage fluxes of dissolved organic matter in forest and grassland – a molecular-level case study from the Hainich Critical Zone Exploratory, Germany

Snowfall/cover and snowmelt are essential determinants of winter soil processes/events that may force the biogeochemical dynamics of soils in temperate regions. Increasing variability of the European hydroclimate is expected to lead to more frequent intermittent warm periods, which cause snowmelt during winter and rapidly mobilize large amounts of dissolved organic matter (DOM). This study, conducted at the Hainich Critical Zone Exploratory in Germany, seeks to address a significant gap in understanding the molecular impacts of snowmelt-induced DOM flows and their ability to alter soil ecosystems rapidly. During two snowmelt events between January and March 2021, we observed that DOM concentration and composition varied more in forest soil seepage than in grassland soil seepage. Forest seepage showed a pronounced DOM flux peak and synchronous increases in the relative abundances of aromatic DOM components, indicating surface-derived transport of plant litter carbon. In the grassland, however, peak DOM fluxes were characterized by a marked increase in nitrogen-containing (N-containing) DOM components, indicating a predominance of microbial carbon. Notably, the unique DOM components specific to each ecosystem increased during peak fluxes in the forest but decreased in the grassland. We suggest that an overall higher molecular richness and the broader functional metabolic potentials in grassland may account for its relatively greater DOM stability compared to the forest during peak snowmelt events.

Two-Dimensional Liquid Chromatography Tandem Mass Spectrometry Untangles the Deep Metabolome of Marine Dissolved Organic Matter.

Dissolved organic matter (DOM) is an ultracomplex mixture that plays a central role in global biogeochemical cycles. Despite its importance, DOM remains poorly understood at the molecular level. Over the last decades, significant efforts have been made to decipher the chemical composition of DOM by high-resolution mass spectrometry (HR-MS) and liquid chromatography (LC) coupled with tandem mass spectrometry (MS/MS). Yet, the complexity and high degree of nonresolved isomers still hamper the full structural analysis of DOM. To address this challenge, we developed an offline two-dimensional (2D) LC approach using two reversed-phase dimensions with orthogonal pH levels, followed by MS/MS data acquisition and molecular networking. 2D-LC-MS/MS reduced the complexity of DOM, enhancing the quality of MS/MS spectra and increasing spectral annotation rates. Applying our approach to analyze coastal-surface DOM from Southern California (USA) and open-ocean DOM from the central North Pacific (Hawaii), we annotated in total more than 600 structures via MS/MS spectrum matching, which was up to 90% more than that in iterative 1D LC-MS/MS analysis with the same total run time. Our data offer unprecedented insights into the molecular composition of marine DOM and highlight the potential of 2D-LC-MS/MS approaches to decipher the chemical composition of ultracomplex samples.

Characterization of natural and anthropogenic dissolved organic matter in the yangtze river basin using FT-ICR MS

Dissolved organic matter (DOM) is a complex mixture that plays a crucial role in global carbon cycling and climate dynamics. Understanding the chemical composition of DOM is crucial for studying its biogeochemical behavior. However, identifying individual DOM molecules is challenging. Here, using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry and an in-house database, we developed a framework to investigate DOM characteristics in natural water. Through the developed approach, we successfully identified thousands of individual DOM molecules in the water bodies of the Yangtze River Basin. For the first time, the proportions of natural and anthropogenic organics within DOM were revealed. In total, 9006 unambiguous molecular formulas were assigned to DOM in the Yangtze River Basin. The proportions of CHNO and CHOS compounds increased from upstream to downstream regions. Moreover, 1099 DOM compounds were tentatively identified, with 85 % being endogenous organics and 15 % being exogenous organics. Notably, lipids and pharmaceuticals and personal care products were the most frequently detected endogenous and exogenous compounds. The spatial variation of the identified DOM indicated anthropogenic discharges considerably increased both the number and abundance of DOM in the downstream Yangtze River Basin. This study highlighted the importance of anthropogenic impacts on DOM in water.

Molecular properties of dissolved organic matter leached from microplastics during photoaging process

The occurrence of dissolved organic matter (DOM) derived from microplastics (MPs) and its effect on aquatic systems has attracted great interest recently. However, the photoaging effect on the molecular structure of MP-derived DOM (MP-DOM) remains unclear. This paper presents the characteristics of DOM leached from three commercial MPs, i.e., polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) under UV irradiation. With prolonged aging periods, the surface roughness and oxygen-containing groups on the surface of MPs increase as more DOM leachate is generated. Moreover, the dissolved organic carbon (DOC) content of the leached DOM from PET MPs varies from 0.52 mg/L to 2.25 mg/L, which is higher than PE and PP MPs, due to the larger increased surface reaction area and the cleavage of the benzene ring. According to the excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), the plastic-derived protein/phenolic-like components (C1 and C3) in MP-DOM were changed into photo-induced humic-like components (C2), which were closely related to the intermediates during photo-oxidation. High-performance liquid chromatography-mass spectrometry (HPLC−MS) analysis further identified that the highest proportion of antioxidants (24.8 %∼34.6 %) was contained in MP-DOM. Plasticizers, intermediate additives, and antimicrobial agents were also detected in DOM leachate. Correlation analysis identified that the composition of leached DOM was positively correlated with the surface roughness, the carbonyl index (CI), and the chemical groups of MPs. Moreover, a partial least square structural equation model (PLS−SEM) analysis further verified that the change of morphology and the chemical structure of MPs could affect the DOM structures and fractions directly. This study provides an in-depth understanding of the composition of MP-derived DOM during the aging process, as well as a comprehensive environmental impact assessment of MPs.

Higher Stability of Soil Organic Matter near the Permafrost Table in a Peatland of Northeast China

Understanding the stability of soil organic matter (SOM) is essential for making accurate predictions regarding carbon release rates. However, there is limited information on the role of chemical composition of dissolved organic matter (DOM) in SOM stability. To address this gap, the peatland soil profile in the discontinuous frozen soil region of Northeast China was selected as the focus of this research, and a comprehensive analysis was conducted on the differences between the molecular composition of DOM and the stability of SOM. The results indicate a significant carbon accumulation phenomenon near the permafrost table. Through analyses using TG-50, δ13C, and δ15N, it was determined that SOM near the permafrost table exhibits high stability, whereas SOM within the permafrost layer demonstrates poor stability. Investigations utilizing UV-vis, 3D-EEM, FT-IR, and 1H-NMR technologies revealed that DOM near the permafrost table is of high quality and highly aromatic. Furthermore, compared to near the permafrost table, humic acid materials in the permafrost layer decreased by 17%, while protein materials increased by 17%. These findings offer a novel perspective on the understanding of SOM stability in peatland soil profiles within discontinuous permafrost regions.

Combined Quantification and Characterization of Dissolved Organic Matter by Liquid Chromatography-Mass Spectrometry Using Charged Aerosol Detection.

Dissolved organic matter (DOM) is a complex mixture of thousands of molecular formulas comprised of an unknown number of chemical compounds, the concentration and composition of which are critical to ecosystem function and biogeochemical cycling. Despite its importance, our understanding of the DOM composition is lacking. This is principally due to its molecular complexity, which means that no single method is capable of describing DOM in its entirety. Quantification is typically done by proxy (e.g., relative to carbon content) and does not necessarily match well to compositional data, due to incomplete analytical windows and selectivity of different analytical methods. We present an integrated liquid chromatography (LC)-diode array detector (DAD)-charged aerosol detector (CAD)-mass spectrometry (MS) pipeline designed to both characterize and quantify solid-phase extractable DOM (SPE-DOM) in a single analysis. We applied this method to a set of eight Swedish water bodies sampled in the summer and winter. Chromophoric SPE-DOM was proportionally higher in samples with higher SPE-DOM concentrations but remained relatively consistent between sampling occasions. Ionizable SPE-DOM was relatively consistent across sites but was proportionally higher in summer. Overall, the carbon content of DOM was very consistently ∼40% across sites in both summer and winter. These findings suggest that SPE-DOM concentration at these sites is driven by (presumably allochthonous) chromophoric inputs, with an increased relative contribution in summer of material that is more ionizable and less chromophoric and may be either autochthonous or selectively enriched from allochthonous sources. Thus, with minimal additional effort, this method provided further compositional insights not attained by any single analysis in isolation.

Cold seep nitrogen fixation and its potential relationship with sulfur cycling.

N2 fixation is an important source of biologically available in carbon-dominated cold seep systems as little nitrogen is released by hydrocarbon seepage, thereby promoting biological productivity and the degradation of non-nitrogenous organic matter. Cold seeps are rich in diverse sources of dissolved organic matter (DOM) derived from the sinking of photosynthetic products in euphotic layer and the release of chemosynthesis products on the seafloor. However, it remains unclear whether N2 fixation is coupled to the metabolic processes of DOM, as determined by e.g., carbon, nitrogen, phosphorus, and sulfur content, for energy acquisition in sulfur-rich cold seeps. In this study, diazotroph community structure and its response to DOM compositions were revealed. Moreover, the metagenomics analysis suggested that Dechloromonas genus plays a dominant role in potential coupling N2 fixation and sulfur oxidation. Our study highlighted that sulfur oxidation in deep-sea cold seeps may serve as an energy source to drive N2 fixation.

Characterisation of dissolved organic matter in two contrasting arsenic-prone sites in Kandal Province, Cambodia

Aquifers throughout Asia are impacted by the release of geogenic arsenic (As) into groundwater by microbial reduction of As-bearing Fe(III) (oxy)hydroxide minerals, severely impacting water quality. Groundwater dissolved organic matter (DOM) is likely key to As release, mainly as electron donor or electron shuttles. This study used optical analyses and ultra-high resolution mass spectrometry to examine the sources and composition of groundwater DOM in the As-prone aquifers of Kandal Province, Cambodia, at boreholes with differing host lithology (clay- and sand-dominated). Groundwater and surface water DOM composition were related to As concentrations, to infer the potential role of DOM in promoting As release. Optical and molecular-level analyses indicated an overall dominance of terrestrial-derived DOM in the groundwater samples, with higher freshness index and relative abundance (RA) of aliphatic compounds in clay compared to sand-dominated lithology. Compared to surface water, groundwater DOM had relatively lower O/C ratios and nominal oxidation state of carbon (−0.19 to −0.13 compared to 0.04 for ground and surface water, respectively), with a lower %RA of aliphatic compounds and higher %RA of carboxyl-rich alicyclic molecules, suggesting microbial processing of DOM since percolation into the aquifer. Concentrations of As across both sites were negatively correlated with DOM tryptophan:fulvic-like fluorescence and the %RA of aliphatics, potentially indicating microbial degradation of biolabile DOM in connection with As release, which is consistent with its role as an electron donor source. Together these data support DOM composition as an important control on microbial mediated As release.

In-depth analysis of natural organic matter fractions in drinking water treatment performance: Fate and role of humic substances in trihalomethanes formation potential

In this study we investigate the compositional changes in dissolved organic matter (DOM) fractions across diverse water sources and treatment processes in three Drinking Water Treatment Plants (DWTPs). High-Performance Size Exclusion Chromatography coupled with Diode Array Detection and Organic Carbon Detection (HPSEC-DAD-OCD) was employed to characterize DOM fractions, offering insights into treatment optimization. We examine bulk water parameters, DOM distributions, and the efficiency of treatment trains in reducing DOM fractions. Results reveal distinct DOM composition profiles in river-sourced versus reservoir-sourced waters, with implications for treatment processes. Coagulation, Granular Activated Carbon (GAC) adsorption, Electrodialysis Reversal (EDR), and Ion Exchange (IEX) were evaluated for their efficacy in removing DOM fractions. The analysis highlights the effectiveness of coagulation in reducing high molecular weight (MW) fractions, while GAC filtration targets lower MW fractions. EDR shows significant removal of anions and aromatics, while IEX demonstrates high removal efficiencies for removing humic substances (HS) fractions. Spectroscopic analysis further elucidates changes HS sub-fractions and their role in disinfection by-products (DBP) formation. To quantitatively assess the relationship between HS sub-fractions and trihalomethane formation potentials (THMFP), Pearson correlation analysis were conducted, unveiling robust associations between HS sub-fractions and THM-FP that can be predicted by surrogate parameters such as A254.

Changes in plant resource inputs lead to rapid alterations in soil dissolved organic matter composition in an old-growth tropical forest

Alterations in plant resource inputs to soil affect soil organic matter (OM) dynamics. However, it remains unclear how to alter soil dissolved OM (DOM) composition. Here, we used UV/fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry to analyze soil DOM’s optical and molecular characteristics after eight months of detritus input and removal in an old-growth tropical forest. Changes in plant inputs significantly altered soil DOM’s optical properties, and the most pronounced changes were observed in the humification index and fluorescent components. In litterfall removal and no-input plots, molecular characteristic values increased greatly, such as O/C, double-bond equivalent, aromaticity index, and proportion of carboxyl-rich alicyclic molecules, while biolabile compounds decreased. The abundance of lignin-like and tannin-like compounds was more than 20 % higher in litter removal plots than in no-input plots. Our findings indicate that changes in plant resource inputs can lead to rapid alterations in soil DOM composition.

Applying fluorescence and 2D-correlation spectroscopy to elucidate changes in the chemical composition of DOM from soils with different vegetation types

ABSTRACT To understand the decomposition of the dissolved organic matter (DOM), four different soils (from grassland, spruce forest, oak forest, and agricultural land) were incubated at 15 °C, and the DOM changes were monitored using UV-Vis, synchronized fluorescence and 2D-correlation spectroscopy. The DOM fraction of the agricultural and grassland soils showed a significant increase in both SUVA values in the middle of the incubation ranging from an initial 3–5 L mg−1 m−1 to 40–50 L mg−1 m−1. Synchronous fluorescence spectroscopy revealed that at the beginning of incubation, on day 3, there was an increase in the proportion of low molecular-weight, readily biodegradable compounds (PLF, protein-like fluorescence), while from day 8 onwards, microbial degradation generated a large number of medium-molecular-weight molecules (MHLF, microbial humus-like fluorescence). At further sampling dates, the proportion of compounds of microbial origin decreased systematically, due to a decrease in microbial activity caused by the lack of substrate. 2D-correlation measurements revealed further details of the changes in the quality of DOM, revealing two distinct patterns in the DOM dynamics of the soils, the sequence MHLF→PLF being observed for grassland and arable land and PLF→HLF for forest soils, probably due to the different N supplies of the soils.

Spatial distribution of dissolved free amino acids in three Iberian Atlantic estuaries

Rivers and estuaries are the main link between land and ocean, transferring significant amounts of dissolved organic carbon. These ecosystems receive large amount of dissolved organic matter (DOM) from diverse sources, both allochthonous and autochthonous. Within this pool, dissolved free amino acids (DFAA) represent the most labile fraction, offering valuable insights into DOM composition and diagenetic processes. Our study focused on three Iberian Atlantic estuaries—Guadalquivir, Guadiana, and Tinto-Odiel— that differ in hydrology, land use and DOM sources. We studied the longitudinal distribution of DFAA and their response to tidal cycles across these estuaries. Despite similar DFAA concentrations between estuaries (176.6 nM to 1770 nM) were found, variations in specific amino acids like glutamic acid, taurine, and aspartic acid pointed to a substantial influence of terrestrial inputs in Guadalquivir and Guadiana estuaries and an anthropogenic influence in Tinto-Odiel. Predominant amino acids—serine, glycine, ornithine, and asparagine —comprised more than 50 mol% of the estuarine DFAA pool. The dominance of serine, glycine, and ornithine indicated substantial DOM degradation, possibly associated with the loss of labile DOM during estuarine transport. Concurrently, asparagine prevalence was linked to allochthonous DOM input particularly associated with terrestrial runoff, lateral input, and anthropogenic activities at estuarine margins. Our results underscore the impact of tidal cycles on DFAA distribution and emphasize the potential of DFAA in unraveling estuarine DOM dynamics and their role as indicators of reactivity and composition in estuarine biogeochemistry.

Elucidating the Composition and Transformation of Dissolved Organic Matter at the Sediment-Water Interface Using High-Resolution Mass Spectrometry.

The exchange and transformation of dissolved organic matter (DOM) at the sediment-water interface are crucial factors in regulating watershed biogeochemistry, with the molecular composition of DOM serving as a pivotal determinant in elucidating this process. High-resolution mass spectrometry (HRMS) is an effective tool for resolving the composition of DOM. By analyzing the compositional characteristics of DOM at the sediment-water interface under three different salinities at the same latitude region in northern China, the findings indicate certain variations in component characteristics of DOM between low-salinity inland waters and high-salinity seawaters, with the former exhibiting greater molecular diversity and higher molecular weights, whereas the latter displayed a higher saturation and bioavailability. Notably, the presence of more CHOS substances in the low-salinity inland waters underscores the transformation of the DOM influenced by terrestrial inputs and anthropogenic activities. Conversely, the presence of more CHO and CHNO substances in high-salinity seawater underscores the microbial effects. The chemical transformation process from overlying water to pore water to sediments was characterized by methylation, hydrogenation, decarboxylation, and reduction, as determined by calculating the relations between the H/C and O/C ratios of different compound types. These findings indicate that HRMS can yield more refined results in revealing the process of DOM at the sediment-water interface under different environments, which provides a more reliable basis for a deeper understanding of the source-sink mechanism of sediment organic matter.

Dissolved organic matter (DOM) rather than warming and eutrophication directly affects partial pressure of CO2 (pCO2) in mesocosm systems

Environmental warming and eutrophication pose significant challenges to shallow lake systems, where dissolved organic matter (DOM) serves as a diverse and intricate mixture of organic macromolecules, playing a pivotal role in aquatic ecosystems. Despite its complexity, comprehending the interplay between environmental changes and DOM composition alterations and their subsequent impacts on aqueous CO2 partial pressure (pCO2) is essential for a better understanding of carbon cycling. Yet, our current understanding in this realm remains limited. To address this gap, mesocosm systems were established to investigate how elevated water temperature and eutrophication, alongside changes in DOM composition, influence pCO2 dynamics. Results indicate that while temperature and nutrient levels do not directly influence pCO2 fluctuations, they indirectly affect aqueous pCO2 through their modulation of DOM composition. Elevated temperature and nutrient concentrations notably enhance both the production and degradation of indigenous protein-like organic matter and increase the accumulation of humic-like organic compounds, with phosphorus released from sediment playing a particularly significant role. Furthermore, the degradation rate of protein-like organic matter significantly exceeds its accumulation rate. On the other hand, the impact of water eutrophication on DOM composition surpasses that of temporal temperature variations, with a 2∼4 °C temperature rise showing minimal effects on DOM composition. Notably, the degradation of protein-like organic matter markedly increases aqueous pCO2, while the rise in humic-like organic matter in water exerts minimal influence on pCO2 concentrations. A comprehensive understanding of carbon cycling processes under environmental changes will facilitate effective management of lake ecosystems and the advancement of carbon mitigation technologies.

Plant Root Secretion Alleviates Carbamate-Induced Molecular Alterations of Dissolved Organic Matter.

Studying the interaction between pesticide contamination in the plant system and the dissolved organic matter (DOM) composition is important to understand the impact of pesticides and plants on the ecological function of DOM. The present study investigated the effects of DOM on the bioaccumulation and biotransformation of carbamates in plants, carbamate exposure on DOM composition, and plant root secretion on the interaction between DOM and carbamates. The concentrations of carbamates and their metabolites in living cabbage plants were continuously tracked through an in vivo analytical method. The presence of DOM was found to reduce the highest bioconcentrations and shorten the time it took to reach the highest bioaccumulated amounts of isoprocarb and carbofuran in plants, while it showed no significant effect on the uptake behavior of carbaryl. DOM profiling results indicated that carbamate exposure substantially decreased the number and molecular diversity of DOM. Notably, plant root secretion alleviated carbamate-induced DOM molecular alterations by inducing a higher turnover rate of DOM compared to that in the uncontaminated group, highlighting the role of plants in mitigating the effects of exogenous pesticide exposure on DOM composition and maintaining DOM molecular homeostasis.

Characteristics of Dissolved Organic Matter as Affected by Decomposition of Different Organic Materials in Alpine Wetland

Dissolved organic matter (DOM) plays a significant role in the nutrient supply, energy flow, and pollutant transportation in the wetland ecosystem. However, little is known about the effect of the decomposition of different organic materials in alpine wetland water on the DOM characteristics. By conducting a 90-day decomposition experiment with the addition of different organic materials (peat soil, yak manure, and plant litter) alone or their combinations into alpine wetland water, we characterized the water DOM using three-dimension excitation-emission matrix spectroscopy. The results showed that the decomposition of organic materials significantly affected the chemical properties, sources, humification degree, and composition of the water DOM. The decomposition increased dissolved organic carbon and dissolved total nitrogen in the water. For most of the water samples, a fluorescence index ranging from 1.4 to 1.7 and a biological index of less than 0.8 may indicate that both autochthonous and allochthonous sources contributed to the water DOM, which may primarily rely on allochthonous sources. UVA (37.55–46.81% of total fluorescent components) and UVC fulvic-like substances (29.91–35.53% of total fluorescent components) dominated the water DOM compositions. Among the treatments, additions of peat soil and yak manure led to the highest and the lowest humification degree of the water DOM, respectively. For the treatment of the combination decomposition of all three organic materials, the yak manure may stimulate microbial activity and facilitate the decomposition of plant litter and peat soil and, therefore, boost the humic-like substances in the water DOM. These findings may help the development of wetland biomass management with the objective of maintaining alpine wetland ecosystem services.