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Dissolved organic matter (DOM) is a key soil quality property, indicative of the organic matter stored in the soil, which may also be a function of temporal variation. This study examines whether DOM is a robust property of the soil, controlling fertility, or if it may change with time. Altogether eight sets of soil samples were collected in 2018 and 2019 from the cultivated topsoil (0–10 cm) of cropland and from a nearby grassland near Martonvásár, Hungary. The study sites were characterized by Chernozem soil and were part of a long-term experimental project comparing the effects of manure application and fertilization to the control under maize and wheat monocultures. DOM was extracted from the samples with distilled water. The dissolved organic carbon (DOC), total dissolved nitrogen (DN), biological index (BIX), fluorescence index (FI), humification index (HIX), carbon nitrogen (C/N) ratio and specific ultraviolet absorbance at 254 nm (SUVA254) index were studied in the arable soils, and the results showed that all the DOM samples were humified, suggesting relevant microbiological contributions to the decomposition of OM and its conversion into more complex molecules (FI = 1.2–1.5, BIX = ~0.5, and HIX = ~0.9). Temporal variations were detected only for the permanent grassland where higher DOM concentration was found in spring. This increased DOM content mainly originated from humified, solid phase associated, recalcitrant OM. In contrast, there were no differences among fertilization treatments and sampling dates under cropfield conditions. Moreover, climatic conditions were not proven as a general ruler of DOM properties. Therefore, momentary DOM alone is not necessarily the direct property of soil organic matter under cropfield conditions. The application of this measure needs further details of sampling conditions to achieve adequate comparability. [ABSTRACT FROM AUTHOR]

The nitrogen contamination in rivers has become significant concern in arid and semiarid areas due to water resource shortage and extensive anthropogenic activities in relation to land-use changes in China. As a major nitrogen species, identifying driving factors, transformation and sources of nitrate is crucial for managing nitrogen pollution in rivers. In this study, nitrate sources and transformations were deciphered using physicochemical variables, molecular signature of dissolved organic matter and coupled isotopes of nitrate under different land use types in the Yang River, a typical farming-pastoral ecotone in the semi-arid area of North China. The results of river water showed a significant positive correlation between NO3− concentrations, δ15N-NO3− values and percentage of urban land and cropland, which confirmed the critical role of land use in the variations of riverine nitrate. The correlation between dissolved organic matter composition (aliphatic and lignin-like compounds) and NO3−/Cl− ratios as well as Cl− concentrations verified the effect of agricultural activities on nitrate source and transport. The variation in water chemical variables and dual isotopes of nitrate in river and soil extracts (δ15N-NO3− and δ18O-NO3−) was indicative of the concurrence of in-soil nitrification process and assimilation, whereas denitrification was inhibited under aerobic conditions in the semiarid area. The Bayesian model revealed that about 60% of nitrate was derived from non-point sources (manure, soil organic nitrogen and chemical fertilizer) and 36% from sewage. Although urban is not the major land-use type in the farming-pastoral ecotone, sewage contributed to about 36% of nitrate. The source identification of nitrate stresses the importance of the management of non-point pollution and demand for sewage treatment facilities in the farming-pastoral ecotone. This multiple-tracer approach will help gain deeper insights into nitrogen management in semi-arid areas with extensive human disturbance.

Extreme events such as hurricanes and tropical storms often result in large fluxes of dissolved organic carbon (DOC) to estuaries. Precipitation associated with tropical storms may be increasing in the southeastern U.S., which can potentially impact dissolved organic matter (DOM) dynamics and cycling in coastal systems. Here, DOM composition at the Altamaha River and Estuary (Georgia, U.S.A.) was investigated over multiple years capturing seasonal variations in river discharge, high precipitation events, and the passage of two hurricanes which resulted in substantial storm surges. Optical measurements of DOM indicate that the terrigenous signature in the estuary is linearly related to freshwater content and is similar after extreme events with or without a storm surge and during peak river flow. Molecular level analysis revealed significant differences, however, with a large increase of highly aromatic compounds after extreme events exceeding what would be expected by freshwater content alone. Although extreme events are often followed by increased DOC biodegradation, the terrigenous material added during those events does not appear to be more labile than the remainder of the DOM pool that was captured by ultrahigh-resolution mass spectrometry analysis. This suggests that the added terrigenous organic matter may be exported to the coastal ocean, while a fraction of the organic matter that co-varied with the terrigenous DOM may contribute to the increased biomineralization in the estuary, with implications to carbon processing in coastal areas.

Uncovering which biogeochemical processes have a critical role controlling dissolved organic matter (DOM) compositional changes in complex estuarine environments remains a challenge. In this context, the aim of this study is to characterize the dominant patterns of variability modifying the DOM composition in an estuary off the Southeastern U.S. We collected water samples during three seasons (July and October 2014 and April 2015) at both high and low tides and conducted short- (1 day) and long-term (60 days) dark incubations. Samples were analyzed for bulk DOC concentration, and optical (CDOM) and molecular (FT-ICR MS) compositions and bacterial cells were collected for metatranscriptomics. Results show that the dominant pattern of variability in DOM composition occurs at seasonal scales, likely associated with the seasonality of river discharge. After seasonal variations, long-term biodegradation was found to be comparatively more important in the fall, while tidal variability was the second most important factor correlated to DOM composition in spring, when the freshwater content in the estuary was high. Over shorter time scales, however, the influence of microbial processing was small. Microbial data revealed a similar pattern, with variability in gene expression occurring primarily at the seasonal scale and tidal influence being of secondary importance. Our analyses suggest that future changes in the seasonal delivery of freshwater to this system have the potential to significantly impact DOM composition. Changes in residence time may also be important, helping control the relative contribution of tides and long-term biodegradation to DOM compositional changes in the estuary.

In aquatic ecosystems, dissolved organic matter (DOM) composition is driven by land use, microbial activity, and seasonal variation in hydrology and water temperature, and, in turn, its microbial bioavailability is expected to vary due to differences in its composition. It is commonly assumed that DOM of terrestrial origin is resistant to microbial activity because it is composed of more complex aromatic compounds. However, the effect of DOM sources on the microbial reworking and degradation of the DOM pool remains debated. We performed laboratory incubation experiments to examine how temporal changes in DOM composition influence its microbial biodegradability in two contrasting streams (agricultural and forested) in southern Ontario, Canada. Despite a more allochthonous-like DOM signature in the forest stream and a more autochthonous-like DOM signature in the agriculture stream, we found that biodegradation and production of DOC were the same in both streams and synchronous throughout the sampling period. However, the initial DOM composition impacted how the DOM pool changed upon degradation. During the incubations, both autochthonous-like and allochthonous-like fractions of the DOM pool increased. We also found that a greater change in DOM composition during the incubations induced higher degradation of carbon. Finally, temporal variation in DOC biodegradation and production over time or across streams was not related to DOM composition, although there was a significant relationship between BDOC and nutrient concentrations in the agriculture stream. This observation potentially challenges the notion that DOM origin predicts its bioavailability and suggests that broad environmental factors shape DOC consumption and production in aquatic ecosystems. More research is needed to better understand the drivers of microbial biodegradability in streams, as this ultimately determines the fate of DOM in aquatic ecosystems.

In aquatic ecosystems, dissolved organic matter (DOM) composition is driven by land use, microbial activity, and seasonal variation in hydrology and water temperature, and, in turn, its microbial bioavailability is expected to vary due to differences in its composition. It is commonly assumed that DOM of terrestrial origin is resistant to microbial activity because it is composed of more complex aromatic compounds. However, the effect of DOM sources on the microbial reworking and degradation of the DOM pool remains debated. We performed laboratory incubation experiments to examine how temporal changes in DOM composition influence its microbial biodegradability in two contrasting streams (agricultural and forested) in southern Ontario, Canada. Despite a more allochthonous-like DOM signature in the forest stream and a more autochthonous-like DOM signature in the agriculture stream, we found that biodegradation and production of DOC were the same in both streams and synchronous throughout the sampling period. However, the initial DOM composition impacted how the DOM pool changed upon degradation. During the incubations, both autochthonous-like and allochthonous-like fractions of the DOM pool increased. We also found that a greater change in DOM composition during the incubations induced higher degradation of carbon. Finally, temporal variation in DOC biodegradation and production over time or across streams was not related to DOM composition, although there was a significant relationship between BDOC and nutrient concentrations in the agriculture stream. This observation potentially challenges the notion that DOM origin predicts its bioavailability and suggests that broad environmental factors shape DOC consumption and production in aquatic ecosystems. More research is needed to better understand the drivers of microbial biodegradability in streams, as this ultimately determines the fate of DOM in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is a large and complex mixture of compounds with source inputs that differ with location, season, and environmental conditions. Here, we investigated drivers of DOM composition changes in a marsh‐dominated estuary off the southeastern United States. Monthly water samples were collected at a riverine and estuarine site from September 2015 to September 2016, and bulk, optical, and molecular analyses were conducted on samples before and after dark incubations. Results showed that river discharge was the primary driver changing the DOM composition at the mouth of the Altamaha River. For discharge higher than ~150 m3/s, dissolved organic carbon (DOC) concentrations and the terrigenous character of the DOM increased approximately linearly with river flow. For low discharge conditions, a clear signature of salt marsh‐derived compounds was observed in the river. At the head of Sapelo Sound, changes in DOM composition were primarily driven by river discharge and possibly by summer algae blooms. Microbial consumption of DOC was larger during periods of high discharge at both sites, potentially due to the higher mobilization and influx of fresh material to the system. The Georgia coast was hit by Hurricane Matthew in October 2016, which resulted in a large input of carbon to the estuary. The DOC concentration was ~2 times higher and DOM composition was more aromatic with a stronger terrigenous signature compared to the seasonal maximum observed earlier in the year during peak river discharge conditions. This suggests that extreme events notably impact DOM quantity and quality in estuarine regions. Plain Language Summary: Dissolved organic matter (DOM) is a crucial component of aquatic ecosystems and characterizing how its composition and concentration change is important to better understand the carbon cycle. Composition and quantity of DOM can vary spatially and temporally due to a variety of factors, including biological activity, precipitation patterns, and proximity to source inputs such as rivers, salt marshes, and the open ocean. To track these changes, monthly water samples were collected and analyzed over the course of a year in a marsh‐dominated estuary off the Georgia coast, USA. River flow was shown to be an important factor controlling the amount and type of DOM present at both riverine (Altamaha River) and estuarine (Sapelo Sound) locations. In months with high river flow, organic matter contents were higher and had more terrestrially derived compounds compared to months with low river flow. Additionally, Hurricane Matthew was shown to significantly alter the organic matter at Sapelo Sound, suggesting that extreme events greatly impact DOM quantity and quality in estuarine regions. Key Points: Changes in DOM composition over a year at the Altamaha River and Sapelo Sound are strongly correlated with river dischargeHigh river discharge conditions are characterized by DOM with stronger terrigenous signature and higher microbial utilization ratesPassage of Hurricane Matthew resulted in large input of terrigenous DOM to Sapelo Sound, greatly increasing DOC concentrations [ABSTRACT FROM AUTHOR]

Antimony (Sb) is a toxic substance that poses a serious ecological threat when released into the environment. The species and redox state of Sb determine its environmental toxicity and fate. Understanding the redox transformations and biogeochemical cycling of Sb is crucial for analyzing and predicting its environmental behavior. Dissolved organic matter (DOM) in the environment greatly affects the fate of Sb. Microbially produced DOM is a vital component of environmental DOM; however, its specific role in Sb(III) oxidation has not been experimentally confirmed. In this work, the oxidation capacity of several Shewanella strains and their derived DOM to Sb(III) was confirmed. The oxidation rate of Sb(III) shows a positive correlation with DOM concentration, with higher rates observed under neutral and weak alkaline conditions, regardless of the presence of light. Incubation experiments indicated that extracellular enzymes and common reactive oxygen species were not involved in the oxidation of Sb(III). Characteristics of DOM suggests that microbial humic acid-like and fulvic acid-like substances are the potential contributors to Sb(III) oxidation. These findings not only experimentally validate the role of bacterial-derived DOM in Sb(III) oxidation but also reveal the significance of Shewanella and biogenic DOM in the biogeochemical cycling of Sb. [Display omitted] • Shewanella strains, typical DMRB, could oxidize Sb(III) to less toxic Sb(V). • Shewanella -derived DOM also possessed the capability of Sb(III) oxidation. • The oxidation rate of Sb(III) was positively influenced by increasing concentrations of DOM and reaction pH, while being insensitive to light. • Shewanella -derived DOM was rich in humic acid-like and fulvic acid-like components. [ABSTRACT FROM AUTHOR]

Headwater streams influence the biogeochemical characteristics of large rivers and play important roles in regional and global carbon budgets. The combined effects of seasonality and land use change on the biogeochemistry of headwater streams, however, are not well understood. In this study we assessed the influence of catchment land use and seasonality on the composition of dissolved organic matter (DOM) and ecosystem metabolism in headwater streams of a Kenyan river. Fifty sites in 34 streams draining a gradient of catchment land use from 100% natural forest to 100% agriculture were sampled to determine temporal and spatial variation in DOM composition. Gross primary production (GPP) and ecosystem respiration (ER) were determined in 10 streams draining primarily forest or agricultural catchments. Absorbance and fluorescence spectrophotometry of DOM reflected notable shifts in composition along the land use gradient and with season. During the dry season, forest streams contained higher molecular weight and terrestrially derived DOM, whereas agricultural streams were dominated by autochthonous production and low molecular weight DOM. During the rainy season, aromaticity and high molecular weight DOM increased in agricultural streams, coinciding with seasonal erosion of soils and inputs of organic matter from farmlands. Most of the streams were heterotrophic. However, GPP and ER were generally greater in agricultural streams, driven by higher dissolved nutrient (mainly TDN) concentrations, light availability (open canopy) and temperature compared with forest streams. There were correlations between freshly and autochthonously produced DOM, GPP and ER during both the dry and wet seasons. This is one of the few studies to link land-use with organic carbon dynamics and DOM composition. Measures of ecosystem metabolism in these streams help to affirm the role of tropical streams and rivers as important components of the global carbon cycle and demonstrate that even semi-intensive, smallholder agriculture can have measurable effects on riverine ecosystem functioning. [ABSTRACT FROM AUTHOR]

Aim The need to go beyond taxonomy to understand patterns in microbial function has led to an increased use of trait-based approaches, yet we know little about how microbial functional traits vary across large-scale environmental gradients in natural ecosystems. Here, we apply a trait-based approach to explore the large-scale variability in the trait structure underlying the processing of dissolved organic matter (DOM) by boreal bacterioplankton communities, as well as its regulation and links to taxonomic composition. Location Samples were collected from 296 rivers and lakes across five regions in northern Quebec (Canada), which span large gradients in environmental, climatic and geographical properties typical of the boreal zone. Methods We used the metabolic profiles obtained with Biolog EcoPlates® as an imprint of the trait structure underlying bacterial processing of DOM, and Illumina sequencing of the 16SrRNA gene to characterize the taxonomic composition of these bacterial assemblages. The resulting spatial patterns were compared with an array of climatic, landscape and limnological properties varying at the landscape scale. Results Despite a clear regional segregation of the sampled sites based on environmental variables, the trait structure of boreal bacteria did not show any regional or ecosystem-specific patterns, but rather was linked to a gradient of quality of DOM. Community trait configurations diverged progressively with decreasing terrestrial influence, probably due to local processes that transform and diversify the available pool of DOM. This DOM quality gradient did not explain the taxonomic biogeography of these communities, which was controlled by a different set of environmental factors. Main conclusions The functional biogeography of boreal bacterioplankton is driven by the nature of the DOM pool, and particularly by the influence of terrestrial DOM. The lack of coherence between functional and taxonomic biogeographies implies that the environmental controls of freshwater bacterial performance cannot be directly inferred from spatial patterns in taxonomic composition. [ABSTRACT FROM AUTHOR]

Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air-sea CO2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River-ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50-76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large-scale circulation and potentially contribute to the long-term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean. [ABSTRACT FROM AUTHOR]

• DOM composition and NGT removal were measured in seventeen WWTPs during two seasons. • Lab studies proved that DOM composition affected the NGT biodegradation by activated sludge. • Dynamic variations of DOM molecular composition and bacterial communities were explored. • Reveals the intrinsic relationship that the DOM molecules composition affected NGT degradation via regulation of microbes. • Nine major TPs of NGT were identified with varied abundance in different groups. Dissolved organic matter (DOM) mediates the microbial transformation of micropollutants, including norgestrel (NGT) in natural waters. However, little is known of the effect of complex and variable wastewater-derived DOM composition on NGT degradation during wastewater treatment. In this study, the relationship between the compositions of initial DOM and NGT removal efficiencies of 17 wastewater treatment plants (WWTPs) in spring and summer were analyzed. The different molecular composition of DOM was selected in the lab to further explore its effect on NGT degradation by activated sludge. Results indicated that the DOM composition was a substantial driver of NGT removal in WWTPs. The discrepancies in the initial DOM composition contributed to the differences in the kinetics of NGT degradation by activated sludge. The larger rapid decay phase rates of NGT are usually accompanied by a large proportion of labile substances in DOM. High-throughput sequencing and ultrahigh-resolution mass spectrometry were used to further analyze the evolution of bacterial communities and DOM molecular composition were combined with network analysis to reveal the intrinsic relationship that how DOM composition affected NGT degradation by regulating core microbes. Eighty-nine core OTUs were significantly associated with NGT degradation, and 73 occurred in the rapid decay phase, implying that NGT degradation was mainly regulated by the initial composition of DOM. Nine major transformation products were identified in different groups with widely varying concentrations or relative abundances of these transformation products. This work provides valuable insights into the effects of wastewater-derived DOM composition on NGT degradation by activated sludge and innovatively explores the influence mechanisms from the bacterial community and molecular characterization perspectives. [Display omitted]. [ABSTRACT FROM AUTHOR]

We explored catchment processes that control stream nutrient concentrations at an upland forest in northeastern Vermont, USA, where inputs of nitrogen via atmospheric deposition are among the highest in the nation and affect ecosystem functioning. We traced sources of water, nitrate, and dissolved organic matter (DOM) using stream water samples collected at high frequency during spring snowmelt. Hydrochemistry, isotopic tracers, and end-member mixing analyses suggested the timing, sources, and source areas from which water and nutrients entered the stream. Although stream-dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) both originated from leaching of soluble organic matter, flushing responses between these two DOM components varied because of dynamic shifts of hydrological flow paths and sources that supply the highest concentrations of DOC and DON. High concentrations of stream water nitrate originated from atmospheric sources as well as nitrified sources from catchment soils. We detected nitrification in surficial soils during late snowmelt which affected the nitrate supply that was available to be transported to streams. However, isotopic tracers showed that the majority of nitrate in upslope surficial soil waters after the onset of snowmelt originated from atmospheric sources. A fraction of the atmospheric nitrogen was directly delivered to the stream, and this finding highlights the importance of quick flow pathways during snowmelt events. These findings indicate that interactions among sources, transformations, and hydrologic transport processes must be deciphered to understand why concentrations vary over time and over space as well as to elucidate the direct effects of human activities on nutrient dynamics in upland forest streams. [ABSTRACT FROM AUTHOR]

Abstract: In sandy soils the behavior of heavy metals is largely controlled by soil organic matter (solid and dissolved organic matter; SOC and DOC). Therefore, knowledge of the partitioning of organic matter between the solid phase and soil solution is essential for adequate predictions of the total dissolved metal concentration in the soil solution. At present, only a few studies have incorporated solid/liquid partitioning of organic matter in metal mobility predictions. In order to gain more insight in the behavior of DOC, we have studied the effect of the extraction solution, solid to liquid ratio (SLR), pH and storage time on the extractability of DOC and related metal concentration in the soil solution of a sandy soil. Furthermore, the composition of the DOC was measured and free metal concentrations were analyzed in the soil solution with the Donnan Membrane Technique. Extraction solution, SLR and pH affected the extracted amount of both DOC and metals. The DOC concentrations were highest in pore water and decreased further from water extracts to CaCl2 extracts. In general, with increasing SLR the metal/DOC ratio decreased, which indicated that the increased DOC at higher SLR had a lower average metal binding capacity than DOC released at the lowest SLR. Storage time of the samples and changes in the field greatly affected the extracted amount of DOC; the DOC concentration of samples taken 20 years ago and stored for a long period of time are 10 times higher than for samples from the same area and stored for less than one year. The extracted DOC is comprised mainly of fulvic acids (FA) and hydrophilic acids. Humic acids (HA) played a minor role in the DOC, which is in contrast to the often used assumptions on the DOC compositions in modeling studies. The composition of DOC is also affected by the soil pH; at low pH the contribution of FA to the total DOC concentration is lower because more FA is adsorbed to Fe-oxides in the soil. Speciation calculations assuming that DOC comprised of HA, FA and citric acid (as a model substance for hydrophilic acids) showed that for Cu the citrate-bound Cu could be up to 20% of the total Cu in solution. This study shows that more research is needed to gain more insight in the composition, concentration and behavior of DOC under field conditions in order to improve the prediction of metal leaching in the field. [Copyright &y& Elsevier]

Sponges are critical components of marine reefs due to their high filtering capacity, wide abundance, and alteration of biogeochemical cycling. Here, we characterized dissolved organic matter (DOM) composition in the sponge holobiont exhalent seawater of a loggerhead sponge (Spheciospongia vesparium) and in ambient seawater in Florida Bay (USA), as well as the microbial responses to each DOM pool through dark incubations. The sponge holobiont removed 6% of the seawater dissolved organic carbon (DOC), utilizing compounds that were low in carbon and oxygen, yet high in nitrogen content relative to the ambient seawater. The microbial community accessed 7% of DOC from the ambient seawater during a 5-day incubation but only 1% of DOC from the sponge exhalent seawater, suggesting a decrease in lability possibly due to holobiont removal of nitrogen-rich compounds. If this holds true for other sponges, it may have important implications for DOM lability and cycling in coastal environments. • S. vesparium holobiont altered the DOM composition of seawater in Florida Bay. • Sponge-microbial holobiont uptook ~6% of DOC from ambient seawater. • Compounds with low carbon, low oxygen, and high nitrogen content were depleted. • Sponge exhalent water was enriched in large, more aromatic compounds. • Microbial consumption of DOC was higher in seawater than in sponge exhalent water. [ABSTRACT FROM AUTHOR]