Protein-like Dissolved Organic Matter Research Articles

Anthropogenic dissolved organic matter accumulation fuels greenhouse gas diffusive emissions in urban lakes along trophic state levels

Lakes are an important reservoir of dissolved organic matter (DOM) and also emit large amounts of greenhouse gases (GHGs), such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Yet, the linkages between quantity and quality of DOM and GHGs emissions are still not well understood. Hence, we investigated how DOM optical properties mediated dissolved GHGs levels and diffusive fluxes along a trophic state level in subtropical urban lakes, China. Our study showed that anthropogenic protein-like DOM (tryptophan- and tyrosine-like components) dominated the DOM pools, comprising 62% of total fluorescence DOM (FDOM). Partial least squares path modeling (PLS-PM) showed that the direct contribution of relative abundance of DOM to CO2 and CH4 diffusion in the highly-eutrophic state was higher than in the mesotrophic and lightly/moderately-eutrophic states. In particular, protein-like DOM primarily accounted for CO2 variations (44%-49%), while humic- and protein-like DOM contributed comparably to CH4 variations (38%-40% vs. 36%-40%). Furthermore, in the lightly/moderately- and highly-eutrophic states, nutrients contributed to DOM, CO2 and CH4 in both direct and indirect ways, while phytoplankton (chlorophyll a, Chl-a) negatively and directly affected carbon emissions. However, only nitrogen content directly contributed to N2O production and diffusion in the mesotrophic and lightly/moderately-eutrophic states. We concluded that protein-like DOM enrichment has the potential to trigger CO2 and CH4 diffusion and this process is largely mediated by nutrient levels. N2O diffusion is found to be mainly influenced by nitrogen loadings and forms. Our findings highlight the urgency for effective strategies to reduce anthropogenic DOM inputs and control nutrient levels to reduce GHGs diffusion in highly human-impacted aquatic systems.

Production and prediction of hydroxyl radicals in distinct redox-fluctuation zones of the Yellow River Estuary

Hydroxyl radicals (·OH) produced in subsurface sediments play an important role in biogeochemical cycles. One of the major sources of·OH in sediments is associated with reduced compounds (e.g., iron and organic matter) oxygenation. Moreover, the properties of iron forms and dissolved organic matter (DOM) components varied significantly across redox-fluctuation zones of estuaries. However, the influence of these variations on mechanisms of·OH production in estuaries remains unexplored. Herein, sediments from riparian zones, wetlands, and rice fields in the Yellow River Estuary were collected to systematically explore the diverse mechanisms of·OH generation. Rhythmic continuous·OH production (82–730 μmol/kg) occurred throughout the estuary, demonstrating notable spatial heterogeneity. The amorphous iron form and humic-like DOM components were the key contributors to·OH accumulation in estuary wetlands and freshwater restoration wetlands, respectively. The crystalline iron form and protein-like DOM components influenced the capabilities of iron reduction and continuous·OH production. Moreover, the orthogonal partial least squares models outperformed various multivariate models in screening crucial factors and predicting the spatiotemporal production of·OH. This study provides novel insights into varied mechanisms of·OH generation within distinct redox-fluctuation zones in estuaries and further elucidates elemental behavior and contaminant fate in estuarine environments. Environmental implicationGiven that estuaries serve as sinks for anthropogenic pollutants, various organic pollutants (e.g., emerging contaminants such as antibiotics) have been widely detected in estuarine environments. The production of·OH in sediments has been proven to affect the fate of contaminants. Therefore, the varied mechanisms of·OH in estuarine environments, dominated by diverse iron forms and DOM components, were explored in this study. MLR and OPLS models exhibited good performance in screening crucial factors and predicting·OH production. Our work highlights that in estuarine subsurface environments, the presence of·OH potentially leads to a natural degradation of pollutants.

Interaction of reed litter and biochar presences on performances of constructed wetlands

Constructed wetlands (CWs) are frequently used for effective biological treatment of nitrogen-rich wastewater with external carbon source addition; however, these approaches often neglect the interaction between plant litter and biochar in biochar-amended CW environments. To address this, we conducted a comprehensive study to assess the impacts of single or combined addition of common reed litter and reed biochar (pyrolyzed at 300 and 500 °C) on nitrogen removal, greenhouse gas emission, dissolved organic matter (DOM) dynamics, and microbial activity. The results showed that combined addition of reed litter and biochar to CWs significantly improved nitrate and total nitrogen removal compared with biochar addition alone. Compared to those without reed litter addition, CWs with reed litter addition had more low-molecular-weight and less aromatic DOM and more protein-like fluorescent DOM, which favored the enrichment of bacteria associated with denitrification. The improved nitrogen removal could be attributed to increases in denitrifying microbes and the relative abundance of functional denitrification genes with litter addition. Moreover, the combined addition of reed litter and 300 °C-heated biochar significantly decreased nitrous oxide (30.7 %) and methane (43.9 %) compared to reed litter addition alone, while the combined addition of reed litter and 500 °C-heated biochar did not. This study demonstrated that the presences of reed litter and biochar in CWs could achieve both high microbial nitrogen removal and relatively low 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.

MIEX resin and enhanced coagulation treatment of high-bromide natural water: Chlorine reactivity and DBP precursors removal

Both organic and inorganic constituents play a major role in the formation and speciation of disinfection by-products (DBPs) under chlorination. In this study, the removal of organic and inorganic precursors to DBPs by ion exchange and coagulation was examined, and the relationships between characteristics of dissolved organic matter (DOM), chlorine reactivity, and the formation of DBPs in high-bromide natural water were investigated. Formation potential tests were conducted for trihalomethanes (THMs) and haloacetronitriles (HANs) before and after the different treatments (coagulation, magnetic ion exchange (MIEX) resin, and combined MIEX/coagulation). At a low resin dose, MIEX was more effective than coagulation for DOM removal. The advantages of the MIEX resin were more prominent at a high resin dose because substantial amounts of DOM and bromide were simultaneously removed. The maximum bromide uptake on the MIEX resin was 0.02 mg-Br/mL. A 4-component PARAFAC model was developed using a dataset of 148 fluorescence excitation-emission matrices (EEMs) generated from pre-chlorinated and post-chlorinated samples. The maximum fluorescence intensities of humic-like components were found to be strongly correlated to THMs formation (R2 = 0.79–0.89). The MIEX treatment substantially lowered the formation of THMs (19–51%) and HANs (70–95%) due to its ability to remove humic-like DOM and bromide. The fluorescence loss for humic-like DOM was found to increase over a chlorination time of 3 days, suggesting that these compounds had stronger reactivity to chlorine compared to protein-like DOM. The chlorine reactivity of protein-like DOM increased after treatments as the residual chlorine was able to react with the less reactive fractions of DOM after chlorine-reactive compounds were removed.

Seasonal variation and dissolved organic matter influence on the distribution, transformation, and environmental risk of pharmaceuticals and personal care products in coastal zone: a case study of Tianjin, China

Pharmaceuticals and personal care products (PPCPs) are emerging contaminants that have raised urgent environmental issues. The dissolved organic matter (DOM) plays a pivotal role on PPCPs’ migration and transformation. To obtain a comprehensive understanding of the occurrence and distribution of PPCPs, a seasonal sampling focused on the riverine system in coastal zone, Tianjin, Bohai Rim was conducted. The distribution and transformation of thirty-three PPCPs and their interaction with DOM were investigated, and their sources and ecological risks were further evaluated. The total concentration of PPCPs ranges from 0.01 to 197.20 μg/L, and such value is affected by regional temperature, DOM and land use types. PPCPs migration at the soil-water interface is controlled by temperature, sunlight, water flow and DOM. PPCPs have a high affinity to the protein-like DOM, while the humus-like DOM plays a negative influence and facilitates the PPCPs’ degradation. It is also found that protein-like DOM can represent point source pollution, while humus-like substances indicate non-point source (NPS) emission. Specific PPCPs can be used as markers to trace the source of domestic discharge. Additionally, daily use PPCPs such as ketoprofen, caffeine and iopromide are estimated to be the main risk substances, and the PPCPs ecological risk varies on space, season and river hydraulic condition.

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.

Accelerated Hg loss and increased methylmercury covary with soil organic matter mineralization and dissolved organic matter humification under warming conditions in permafrost marsh, Northeast China

Currently, little is available on how mercury (Hg) methylation couples with soil organic matter decomposition in degraded permafrost in high northern latitudes, where the climate is becoming warmer rapidly. Here, we revealed the complex interactions between soil organic matter (SOM) mineralization, dissolved organic matter (DOM) and methylmercury (MeHg) production based on an 87-day anoxic warming incubation experiment. Results supported remarkably promotion effects of warming on MeHg production, by 130%–205% on average. Total mercury (THg) loss under warming treatment depended on marsh types but showed an increasing trend on the whole. Warming yielded higher proportions of MeHg to THg (%MeHg), increased by 123%–569%. As expected, greenhouse gas emission was significantly enhanced by warming. Warming also strengthened fluorescence intensities of fulvic-like and protein-like DOM, with contributions to total fluorescence intensities of 49%–92% and 8%–51%, respectively. DOM and its spectral features explained 60% variation of MeHg, and the explanation increased to 82% in conjunction with greenhouse gas emissions. The structural equation model implied that warming, greenhouse gas emission, and humification of DOM had positive effects on Hg methylation potential, while microbial-derived DOM showed negative effects on MeHg. These results showed that accelerated Hg loss and increased methylation covaried with greenhouse gas emission and DOM formation under warming conditions in permafrost marsh.

How does anthropogenic activity influence the spatial distribution of dissolved organic matter in rivers of a typical basin located in the Loess Plateau, China

River ecosystems interact strongly with adjacent terrestrial environments and receive dissolved organic matter (DOM) from a variety of sources, all of which are vulnerable to human activities and natural processes. However, it is unclear how and to what extent human and natural factors drive DOM quantity and quality changes in river ecosystems. Here, three fluorescence components were identified via optical techniques, including two humic-like substances and one protein-like component. The protein-like DOM was mainly accumulated in anthropogenically impacted regions, while humic-like components exhibit the opposite trend. Furthermore, the driving mechanisms of both natural and anthropogenic factors on the variations in DOM composition were investigated using partial least squares structural equation modelling (PLS-SEM). Human activities, especially agriculture, positively influence the protein-like DOM directly by enhancing anthropogenic discharge with protein signals and also indirectly by affecting water quality. Water quality directly influences the DOM composition by stimulating in-situ production through a high nutrient load from anthropogenic discharge and inhibiting the microbial humification processes of DOM due to higher salinity levels. The microbial humification processes can also be restricted directly by a shorter water residence time during the DOM transport processes. Furthermore, protein-like DOM was more sensitive to direct anthropogenic discharge than indirect in-situ production (0.34 vs. 0.25), especially from non-point source input (39.1%), implying that agricultural industry optimization may be an efficient way to improve water quality and reduce protein-like DOM accumulation.

Spatiotemporal response of dissolved organic matter diversity to natural and anthropogenic forces along the whole mainstream of the Yangtze River

The Yangtze River, the largest river in Asia, plays a crucial role in linking continental and oceanic ecosystems. However, the impact of natural and anthropogenic disturbances on composition and transformation of dissolved organic matter (DOM) during long-distance transport and seasonal cycle is not fully understood. By using a combination of elemental, isotopic and optical techniques, as well as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated DOM abundance and composition along the whole mainstream at highly spatial resolution in the dry and early wet seasons. Our findings showed that the concentration and flux of dissolved organic carbon (DOC) in the Yangtze River was much lower compared with other worldwide larger rivers. The distribution of δ13CDOC and higher abundance of humic-like fluorescent component and highly unsaturated and phenolics (HUPs) compound reflected a prominent contribution of allochthonous DOM. Further optical and molecular analysis revealed humic-like fluorescent components were coupled with CHO molecules and HUPs compound with higher aromatic, unsaturated, molecular weight and stable characteristics between upstream and midstream reaches. With increasing agricultural and urban land downstream, there were more heteroatomic formulae and labile aliphatic and protein-like compounds which were derived from human activities and in situ primary production. Meanwhile, DOM gradually accumulates with slow water flow and additional autochthonous organics. Weaker solar radiation and water dilution during the dry/cold season favours highly aromatic, unsaturated and oxygenated DOM compositions. Conversely, higher discharge during the wet/warm season diluted the terrestrial DOM, but warm temperatures could promote phytoplankton growth that releases labile aliphatic and protein-like DOM. Besides, chemical sulfurization, hydrogenation and oxygenation were found during molecular cycling processes. Our research emphasizes the active response of riverine DOM to natural and anthropogenic controls, and provides a valuable preliminary background to better understand the biogeochemical cycling of DOM in a larger river.

Interactions between particulate organic matter and dissolved organic matter in a weak dynamic bay revealed by stable isotopes and optical properties

Few studies have incorporated the tools of stable isotopes and optical properties to study the biogeochemical process of organic matter (OM), including particulate organic matter (POM) and dissolved organic matter (DOM), which prevents our comprehension of the interactions between POM and DOM in the marine environment. In this study, the origin, distribution, and fate of POM and DOM in Tieshangang Bay, a weak dynamic bay were investigated by measuring δ13C and δ15N of POM, dissolved organic carbon (DOC), and absorption and fluorescence of DOM (CDOM and FDOM). In the upper bay, POM source was mainly originated from terrestrial discharge, whereas the high fluorescence index (FI) indicated that external aquatic organism activity dominated DOM sources. In contrast, in the outer bay, the slightly increased δ13C and enriched δ15N of POM and enhanced fluorescence intensities of protein-like DOM components indicated the considerable contributions of the in situ biological activities to OM. A net addition of DOC and optical components of DOM occurred in both the upper and outer bays. The decomposition of terrestrial POM was responsible for the addition of DOM due to the weak dynamics in the upper bay, whereas the enhanced primary production and the strong decomposition of freshly produced POM jointly contributed to the addition of DOM in the outer bay. Our study suggests that hydrodynamics regulate the biogeochemistry and interactions of POM and DOM in the weak dynamic bay.

Molecular evidence for the production of labile, sulfur-bearing dissolved organic matter in the seep sediments of the South China Sea

Cold seeps with methane-rich fluids leaking out of the seafloor usually support massive biomass of chemosynthetic organisms and associated fauna. A substantial amount of methane is converted to dissolved inorganic carbon by microbial metabolism, and this process also releases dissolved organic matter (DOM) into pore water. Here, pore water samples from “Haima cold seeps” sediments and the non-seep reference sediments in the northern South China Sea were analyzed for optical properties and molecular compositions of pore water DOM. Our results showed that the relative abundance of protein-like DOM, H/Cwa and molecular lability boundary percentage (MLBL%) in the seep sediments were significantly higher than those in the reference sediments, indicating that more labile DOM related to unsaturated aliphatic compounds is produced in the seep sediments. Spearman's correlation of the fluoresce and molecular data suggested that the humic-like components (C1 and C2) mainly constituted the refractory compounds (CRAM, highly unsaturated and aromatics compounds). In contrast, the protein-like component (C3) had high H/C ratios featuring high degree of DOM lability. The amount of S-containing formulas (CHOS and CHONS) was greatly elevated in the seep sediments, likely caused by abiotic and biotic sulfurization of DOM in the sulfidic environment. Although the abiotic sulfurization was proposed to have a stabilizing effect on organic matter, our results implied that the biotic sulfurization in the cold seep sediments would increase DOM lability. Overall, the labile DOM accumulated in the seep sediments is closely linked to methane oxidation, which not only support heterotrophic communities and but also likely have an impact on carbon and sulfur cycling in the sediments and the ocean.

Role of lake dissolved organic matter in cyanobacteria removal by cationic polyacrylamide flocculation and screen filtration

Prompt removal of densely accumulated cyanobacteria is a straightforward method to alleviate the harm of cyanobacterial blooms to lakes. In this study, a cyanobacteria harvesting ship based on flocculation and filtration was used for the removal of cyanobacteria in a bay of Dianchi Lake (Yunnan Province, China), in which the fate and influence of lake dissolved organic matter (DOM) were investigated. After flocculation with 1–3 mg/L cationic polyacrylamide (CPAM) and filtration with the 100-µm-pore-sized screens of a rotary drum filter (RDF), >90 % of cyanobacteria was removed and the DOM was slightly reduced from 7.83 mg/L to 4.61 ∼ 6.49 mg/L. Tryptophan protein-like and high molecular weight (MW) DOM was selectively removed, increasing the humification index and decreasing the biological index in the treated water. Lake DOM, especially the high MW fractions of lake DOM, played an important role in CPAM flocculation. Cyanobacteria could not be effectively flocculated in Milli-Q water while the removal efficiency increased to over 90 % when 1–3 mg/L high MW DOM of Dianchi Lake was added. Three lakes in Yunnan Province were also plagued by cyanobacterial blooms, including Erhai Lake, Qilu Lake, and Xingyun Lake. The high MW DOM (1.7–2.7 mg/L) in the lakes could also deduce the effective flocculation of cyanobacteria with CPAM. The flocculation mechanism could function by the interactions of quaternary ammonium groups of CPAM and carboxylate groups of high MW DOM that form net-like polyelectrolyte complexes (PECs), and coexisting cyanobacteria cells were encapsulated into the PECs, leading to the formation of large flocs under stirring conditions. In the lakes with high densities of cyanobacteria, there might be enough high-MW DOM to promote the flocculation of CPAM, and after flocculation-filtration treatment, both cyanobacteria and cyanobacteria-deprived DOM were effectively removed.

Seasonal Variations of Dissolved Organic Matter in Urban Rivers of Northern China

Dissolved organic matter (DOM) is ubiquitously present in aquatic environments, playing an important role in the global carbon cycle and water quality. It is necessary to reveal the potential sources and explore spatiotemporal variation of DOM in rivers, especially in urban zones impacted by human activities. It was designed to aim to explore spatiotemporal variations of DOM in urban rivers and ascertain the influencing factors. In this study, dissolved organic carbon (DOC) concentrations, UV-Vis absorption spectroscopy, and 3D fluorescence spectroscopy combined with parallel factor analysis were utilized to characterize DOM composition in urban rivers (the Jiyun, Chaobai, and Yongding rivers) in Tianjin city, northern China. The results showed that DOC (1.28 to 25.85 mg·L−1), generally, was at its highest level in spring, followed by summer, and lowest in autumn and winter, and that the absorption parameters E250:365 (condensation degree/molecular weight, 7.88), SUVA254 (aromaticity, 3.88 L mg C−1 m−1), a355 (content of chromophores, 4.34 m−1), a260 (hydrophobicity, 22.02 m−1), and SR (molecular weight, 1.08) of CDOM (chromophoric DOM) suggested that DOM is mainly composed of low-molecular-weight fulvic acid and protein-like moieties, and had the capability of participating in pollutant migrations and transformations. The results demonstrated significant seasonal differences. Generally, high DOC content was detected in rivers in urban suburbs, due to anthropogenic inputs. Three fluorescence components were identified, and the fluorescence intensity of the protein class reached the highest value, 294.47 QSU, in summer. Different types of land use have different effects on the compositions of riverine DOM; more protein-like DOM was found in sections of urban rivers. The correlation between DOC concentration and the CDOM absorption coefficient was found to be unstable due to deleterious input from industrial and agricultural wastewater and from domestic sewage from human activities. HIX and BIX elucidated that the source of CDOM in three river watersheds was influenced by both terrestrial and autochthonous sources, and the latter prevailed over the former. Geospatial data analysis indicated that CDOM in autumn was sourced from plant detritus degradation from forest land or from the urban green belt; construction land had a great influence on DOC and CDOM in riparian buffer areas. It was revealed that DOM in the watershed is highly impacted by nature and human activities through land use, soil erosion, and surface runoff/underground percolation transport; domestic sewage discharge constituted the primary source and was the greatest determiner among the impacts.

Longitudinal patterns of dissolved organic matter quality and bacterial community degradation function along the mainstream of Three Gorges Reservoir during impoundment period

Longitudinal patterns of dissolved organic matter (DOM) influence the biogeochemical cycle in river ecosystems. Yet, linkages between DOM quality and bacterial degradation of emerging contaminants over a river continuum are unclear. In combination with DOM characterization, bioassay, and microbial analysis, this study unraveled the spatial variations of DOM composition and biodegradability as well as bacterial community structures, which determined the estrone biodegradation potential (EBP) along the mainstream in Three Gorges Reservoir during impoundment period. While urban and agricultural utilization increased the protein-like and biodegradable DOM components (BDOC%) in upstream, the in-situ biotransformation preferentially utilized tryptophan-like substances followed by tyrosine-like substances, resulting in the accumulated recalcitrant DOM with higher aromaticity, molecular weight, and humic-like components in downstream. Due to the promotion of biodegradable DOM on bacterial abundances, a significant and positive correlation was constructed between BDOC% and EBP (P < 0.001, R2 = 0.56), both of which were controlled by the ratio of fresh- to humic-like fluorescence. Nevertheless, a higher biomass-normalized EBP was found under recalcitrant conditions in downstream, demonstrating that the degradation function was also associated with the biogeographical distribution of bacterial communities. Results of microbial ecology revealed that the in-situ transformation of aromatic and humic-like DOM exhibited more correlations with bacterial taxa than protein-like DOM, leading to increased diversity and decreased niche breadths along the mainstream. The triggered deterministic selection of community assemblages in downstream favored potential estrone-degraders and up-regulated their specific metabolic capacity. These findings will provide critical insights toward understanding the crucial role of DOM in biogeochemical cycles and the self-purification of emerging contaminants along the river continuum.

Organic matter cycling in a model restored wetland receiving complex effluent

Wetlands have been used to treat anthropogenic effluents for decades due to their intense biogeochemical processes that transform and uptake nutrients, organic matter, and toxins. Despite these known functions, we lack generalizable knowledge of effluent-derived dissolved organic matter (DOM) cycling in wetlands. Here, we quantify the cycling of DOM in one of Canada’s more economically important wetland complexes (Frank Lake, Alberta), restored to hydrologic permanence in the 1980s using urban and agro-industrial effluents. Optical analyses and PARAFAC (parallel factor analysis) modelling showed a clear compositional change from more bioavailable and protein-like DOM at effluent input sites to more aromatic and humic-like at the wetland outflow, likely due to DOM processing and inputs from marsh plants and wetland soils. Microbial incubations showed that effluent DOM was rapidly consumed, with the half-life of DOM increasing from as low as 35 days for effluent, to 462 days at the outflow, as a function of compositional shifts toward aromatic, humic-like material. Long-term averaged dissolved organic carbon (DOC) export was low compared to many wetlands (10.3 ± 2.0 g C m−2 yr−1). Consistent with predictions based on water residence time, our mass balance showed Frank Lake was a net source of DOM across all measured years, but shifted from a source to sink among wet and drought years that respectively shortened or lengthened the water residence and DOM processing times. Overall, Frank Lake processes and transforms effluent DOM, despite being a longer-term net source of DOM to downstream environments.

Optical properties of dissolved organic matter in throughfall and stemflow vary across tree species and season in a temperate headwater forest

Tree-derived dissolved organic matter (DOM) comprises a significant carbon flux within forested watersheds. Few studies have assessed the optical properties of tree-derived DOM. To increase understanding of the factors controlling tree-derived DOM quality, we measured DOM optical properties, dissolved organic carbon (DOC) and calcium concentrations in throughfall and stemflow for 17 individual rain events during summer and fall in a temperate deciduous forest in Vermont, United States. DOC and calcium fluxes in throughfall and stemflow were enriched on average 4 to 70 times incident fluxes in rain. A multiway model was developed using absorbance and fluorescence spectroscopy to further characterize DOM optical properties. Throughfall contained a higher percentage of protein-like DOM fluorescence than stemflow while stemflow was characterized by a higher percentage of humic-like DOM fluorescence. DOM absorbance spectral slopes in yellow birch (Betula alleghaniensis) stemflow were significantly higher than in sugar maple (Acer saccharum) stemflow. DOM optical metrics were not influenced by rainfall volume, but percent protein-like fluorescence increased in throughfall during autumn when leaves senesced. Given the potential influence of tree-derived DOM fluxes on receiving soils and downstream ecosystems, future modeling of DOM transport and soil biogeochemistry should represent the influence of differing DOM quality in throughfall and stemflow across tree species and seasons.

Oxygen availability regulates the quality of soil dissolved organic matter by mediating microbial metabolism and iron oxidation

Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and in determining the responses of soil organic matter (SOM) to global change. Although the quantity of soil DOM has been inventoried across diverse spatio-temporal scales, the underlying mechanisms accounting for variability in DOM dynamics remain unclear especially in upland ecosystems. Here, a gradient of SOM storage across 12 croplands in northeast China was used to understand links between DOM dynamics, microbial metabolism, and abiotic conditions. We assessed the composition, biodegradability, and key biodegradable components of DOM. In addition, SOM and mineral-associated organic matter (MAOM) composition, soil enzyme activities, oxygen availability, soil texture, and iron (Fe), Fe-bound organic matter, and nutrient concentrations were quantified to clarify the drivers of DOM quality (composition and biodegradability). The proportion of biodegradable DOM increased exponentially with decreasing initial DOM concentration due to larger fractions of depolymerized DOM that was rich in small-molecular phenols and proteinaceous components. Unexpectedly, the composition of DOM was decoupled from that of SOM or MAOM, but significantly related to enzymatic properties. These results indicate that microbial metabolism exhibited a dominant role in DOM generation. As DOM concentration declined, increased soil oxygen availability regulated DOM composition and enhanced its biodegradability mainly through mediating microbial metabolism and Fe oxidation. The oxygen-induced oxidation of Fe(II) to Fe(III) removed complex DOM compounds with large molecular weight. Moreover, increased oxygen availability stimulated oxidase-catalyzed depolymerization of aromatic substances, and promoted production of protein-like DOM components due to lower enzymatic C/N acquisition ratio. As global changes in temperature and moisture will have large impacts on soil oxygen availability, the role of oxygen in regulating DOM dynamics highlights the importance of integrating soil oxygen supply with microbial metabolism and Fe redox status to improve model predictions of soil carbon under climate change.

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.

Influence of land cover types and phytoplankton community on the distribution and fate of dissolved organic matter in a typical river located in the semi-arid regions of China

Land use can impact dissolved organic matter (DOM) dynamics directly by changing terrestrial loading in anthropogenic areas, and indirectly by promoting in-situ phytoplankton production due to the high load of nutrients. However, understanding how land use and phytoplankton community interact to affect DOM quantity and quality is deficient. To tease apart the synergistic effects of land use and phytoplankton on DOM dynamics, fluorescence spectroscopy with parallel factor analysis (PARAFAC) and mediation analysis was performed in this study. PARAFAC produced three components that had two humic-like (C1 and C2) and a protein-like (C3) fluorescent components, significantly related to anthropogenic land-use (cropland + urban area) and phytoplankton concentration (p < 0.05). Through the mediation analysis, the altered C1 and C3 at sites with a higher proportion of anthropogenic land-use appeared to originate partly as a result of the phytoplankton production fueled by a high load of nutrients from anthropogenic run-off (26.6% and 12.5%). These results suggested phytoplankton significantly mediated the effects of land use on microbial humic-like and protein-like DOM. Additionally, Redundancy analysis (RDA) results further showed that Phormidium (Cyanophyta) had a notable influence on the production of the protein-like component via phytoplankton. Conversely, the decreases in terrestrial humic-like C2 were mainly attributed to the reduced terrestrial loading in anthropogenic areas. These findings provide insights into the roles of land use and phytoplankton communities in DOM dynamics and advance our understanding of the mechanisms that drive DOM composition patterns.