Dissolved Organic Carbon Characteristics Research Articles

Photochemical evolution of the molecular composition of dissolved organic carbon and dissolved brown carbon from wood smoldering

Recently, extreme wildfires occur frequently around the world and emit substantial brown carbon (BrC) into the atmosphere, whereas the molecular compositions and photochemical evolution of BrC remain poorly understood. In this work, primary smoke aerosols were generated from wood smoldering, and secondary smoke aerosols were formed by the OH radical photooxidation in an oxidation flow reactor, where both primary and secondary smoke samples were collected on filters. After solvent extraction of filter samples, the molecular composition of dissolved organic carbon (DOC) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The molecular composition of dissolved BrC was obtained based on the constraints of DOC formulae. The proportion of dissolved BrC fractions accounted for approximately 1/3–1/2 molecular formulae of DOC. The molecular characteristics of dissolved BrC showed higher levels of carbon oxidation state, double bond equivalents, and modified aromaticity index than those of DOC, indicating that dissolved BrC fractions were a class of organic structures with relatively higher oxidation state, unsaturated and aromatic degree in DOC fractions. The comparative analysis suggested that aliphatic and olefinic structures dominated DOC fractions (contributing to 70.1%-76.9%), while olefinic, aromatic, and condensed aromatic structures dominated dissolved BrC fractions (contributing to 97.5%-99.9%). It is worth noting that dissolved BrC fractions only contained carboxylic-rich alicyclic molecules (CRAMs)-like structures, unsaturated hydrocarbons, aromatic structures, and highly oxygenated compounds. CRAMs-like structures were the most abundant species in both DOC and dissolved BrC fractions. Nevertheless, the specific molecular characteristics for DOC and dissolved BrC fractions varied with subgroups after aging. The results highlight the similarities and differences in the molecular compositions and characteristics of DOC and dissolved BrC fractions with aging. This work will provide insights into understanding the molecular composition of DOC and dissolved BrC in smoke.

Comparison of Fe‐based and Al‐based coagulants in the removal of organic and disinfection by‐product precursors

AbstractSurface water is renowned for its natural organic matter, constituting approximately 45% of total dissolved organic carbon (DOC) which can be removed in water treatment plants. However, residual DOC in water can react with chlorine to form several carcinogenic disinfectant by‐products (DBPs). This study aimed to examine the molecular weight of organic fractions dissolved in three different water sources that act as precursors to the formation of DBPs species. The coagulants used were Al‐ and Fe‐based, frequently used in water treatment plants to remove organic fractions. Characterization of DOC in source water served as the first step in determining the performance of both coagulants in terms of organic properties. The results showed that the selected surface waters had similar DOC characteristics, including biopolymers, humic substances, building blocks, and a low molecular weight. These fractions contributed to the formation of trihalomethanes (THMs) and haloacetic acids (HAAs). The Fe‐based coagulant was more effective than the Al‐based coagulant in removing all organic fractions and reducing THMs compared to HAAs. Furthermore, one‐way ANOVA analysis showed a significant difference in the average removal of organic fractions and DBP species between the Fe‐based and Al‐based coagulants. The Fe‐based coagulant showed higher efficiency in removing biopolymers, dibromochloromethane, and chlorodibromoacetic acid than the Al‐based coagulant. In contrast, the Al‐based coagulant had better performance in reducing dibromo HAA and tribromo HAA. Both coagulants had no significant difference in extracting other organic fractions or DBPs species.

Dissolved organic carbon fractionation in wet deposition and its potential impact on radiative forcing in the central Tibetan Plateau

As an important component of carbonaceous matters, dissolved organic carbon (DOC) can absorb and scatter the solar radiation at ultraviolet and blue wavelengths. The wet deposition process has great impact on the concentration and light absorption ability of precipitation DOC, affecting the climatic effect caused by DOC in the atmosphere. In this study, light absorption and fluorescence characteristics of precipitation DOC was investigated in the central Tibetan Plateau (TP). The results showed that the mean DOC concentration and mass absorption cross-section measured at 365 nm (MAC365) in Tanggula (TGL) station were 0.59 ± 0.42 mg/L and 0.37 ± 0.19 m2/g, respectively, while both values showed much higher volatilities than those of aerosols. DOC concentrations had significant negative correlation with the precipitation amount, while MAC365 values increase with the precipitation amount in TGL station. Therefore, DOC with high light-absorbing ability was preferred to be retained in the atmosphere during wet deposition. In this study, precipitation DOC contained three fluorescent components (one humic-like component and two tyrosine-like components) mainly from local biomass burning sources. DOC concentration showed a negative relationship with MAC365 value in TGL station. The wet deposition of DOC with low light-absorbing ability can reduce the strong negative radiative forcing caused by secondary organic aerosol due to high proportion of DOC in secondary organic carbon. Similar phenomenon was also found in Nam Co, Lulang and Everest stations of previous study, which may have a potential impact on radiative forcing in the atmosphere of TP.

Responses of dissolved organic carbon to freeze–thaw cycles associated with the changes in microbial activity and soil structure

Abstract. Arctic warming accelerates snowmelt, exposing soil surfaces with shallow or no snow cover to freeze–thaw cycles (FTCs) more frequently in early spring and late autumn. FTCs influence Arctic soil C dynamics by increasing or decreasing the amount of dissolved organic carbon (DOC); however, mechanism-based explanations of DOC changes that consider other soil biogeochemical properties are limited. To understand the effects of FTCs on Arctic soil responses, we designed microcosms with surface organic soils from Alaska and investigated several soil biogeochemical changes for seven successive temperature fluctuations of freezing at −9.0 ± 0.3 ∘C and thawing at 6.2 ± 0.3 ∘C for 12 h each. FTCs significantly changed the following soil variables: soil CO2 production (CO2), DOC and total dissolved nitrogen (TDN) contents, two DOC quality indices (SUVA254 and A365 / A254), microaggregate (53–250 µm) distribution, and small-sized mesopore (0.2–10 µm) proportion. Multivariate statistical analyses indicated that the FTCs improved soil structure at the scale of microaggregates and small-sized mesopores, facilitating DOC decomposition by soil microbes and changes in DOC quantity and quality by FTCs. This study showed that FTCs increased soil CO2 production, indicating that FTCs affected DOC characteristics without negatively impacting microbial activity. Soil microaggregation enhanced by FTCs and the subsequent increase in microbial activity and small-sized pore proportion could promote DOC decomposition, decreasing the DOC quantity. This study provides a mechanism-based interpretation of how FTCs alter DOC characteristics of the organic soil in the active layer by incorporating structural changes and microbial responses, improving our understanding of Arctic soil C dynamics.

Dissolved Organic Carbon Dynamics Variability from Ponds Draining Different Landscapes in a Typical Agricultural Watershed

Dissolved organic carbon (DOC) in inland waters (rivers, reservoirs, lakes, and small ponds) plays a significant role in the global carbon cycle and affects global climate change. In addition, DOC is also a vital indicator of the water environment due to its multiple physical, chemical, and ecological roles. Lakes and ponds of small sizes are abundant on a global and regional scale, and a large increase in ponds is expected with global agricultural land expansion. However, the DOC characteristics of ponds in agricultural watersheds are still unclear, posing a challenge to better understanding the carbon cycle of inland waters. In this study, we explored the DOC variability and their influencing factors in ponds draining different landscapes in a typical agricultural watershed to address the issue. The field measurements over a year showed the DOC concentration varied among ponds draining different landscapes. Specifically, the mean DOC concentrations in the natural pond, sewage pond, aquaculture pond, and irrigation pond were (6.17 ± 1.49) mg/L, (12.08 ± 2.92) mg/L, (9.36 ± 2.92) mg/L, and (8.91 ± 2.71) mg/L, respectively. Meanwhile, monthly measurements found the DOC varied across sampling dates. The DOC variability was positively correlated with nutrients, primary production, and precipitation, suggesting anthropogenic loadings, an internal production rate, and hydrological regime that regulated the substantial variability of DOC in these ponds at the watershed scale. Further, large pollutant discharge and high primary production led to peak DOC occurring in the sewage pond. Our results implied that more attention should be paid to ponds in agricultural watersheds to better understand the roles of inland waters in the global carbon cycle.

Effects of regional climate, hydrology and river impoundment on long-term patterns and characteristics of dissolved organic matter in semi-arid northern plains rivers

Diverse environmental and anthropogenic factors, such as the ongoing reservoir constructions may influence riverine dissolved organic matter (DOM) properties. This has important implications for river water quality, particularly when reservoirs are a source of drinking water. Simultaneous studies of multidecadal trends in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are scarce. We studied the patterns in DOC and DON concentration in two major rivers of the South Saskatchewan River (SSR) basin over a 42-year period (1978–2019). We also examined the impact of a large reservoir on riverine DOC properties. Contrary to many studies, we did not find a long-term increase in DOC and DON concentration, and DOC and DON patterns were not always synchronous. In an agriculture dominated watershed like the SSR basin, agricultural land use (e.g., nitrogen-fertilizer application) could influence DOC and DON concentration differently, potentially resulting in asynchronous patterns over time. River discharge was an important driver of DOM patterns. Regional precipitation in the lower SSR basin may also influence DOM patterns in locations where runoff contribution is greater. These regional factors explained greater variability in DOM compared to global scale indices (e.g., Pacific decadal oscillation) due to their direct control on DOM. A travel time corrected approach to account for the lengthy reservoir turnover time showed that a large reservoir caused a reduction in allochthonous DOC characteristics through photodegradation and perhaps, an increase in autochthonous characteristics. Our results illustrate: 1) the increase in DOM concentrations seen in the northern hemisphere is not present in semi-arid prairie rivers, 2) Controls on different DOM components could be different, and 3) large reservoirs may modify riverine DOC composition due to longer water residence time.

Soil dissolved organic carbon leaching characteristics of the coastal saline seepweed wetlands in the Wuleidaowan national wetland

As the most active component of soil TOC (Total Organic Carbon), DOC (Dissolved Organic Carbon) is either adsorbed by the soil and used by plants or transported by soil water into the water column, where it participates in the broader carbon cycle. As a substantial "mobile carbon pool" in soils, its leaching process is an essential pathway for the loss of soil TOC. This paper analysed the investigated the leaching characteristics of soil DOC by simulating leaching in an indoor soil column based on the spatial distribution of soil TOC and DOC contents in order to reveal the leaching and migration characteristics of soil DOC at different depths and its main influencing factors. The DOC content of soil leachate varies somewhat under conditions such as PH and soil phosphorous content. However, the overall trend is that the DOC content of soil leachate generally increases with the depth of the soil layer. The DOC content in the soil leachate gradually decreased with increasing leaching time. In the absence of exogenous carbon recharge, the loss of TOC from the upper layers of the soil was significantly more remarkable due to the prolonged leaching. In addition, soil TOC loss declined after leaching and decreased significantly with increasing soil depth. Because the soil DOC is adsorbed and trapped in other layers during leaching, the DOC in the upper layer was always lost during the leaching test. The experiments used highly concentrated DOC solutions as drench solutions. According to the analysis of the intensity of the fluorescence centres of each soil layer in the three-dimensional fluorescence pattern of the drench solution of each soil layer, the ability of different soil layers to adsorb and retain organic matter of different fractions differs. In addition, the selected soil layers planted with Saline seepweed show a general decreasing trend in the ability to adsorb and retain organic matter as the depth increases.

Effects of management on the content and spectral characteristics of soil dissolved organic carbon in a subtropical forest

The differences of artificial measures, such as logging residue management, between assisted natural regeneration and afforestation may change the content and structure of soil dissolved organic carbon (DOC) and affect forest carbon cycle. In this study, we investigated the effects of managements on the content and spectral characteristics of DOC in a subtropical forest, which contained the forest of assisted natural regeneration (Ⅱ), and the plantation (Ⅲ), both were converted from mature secondary forests (Ⅰ). Results showed that DOC content in the 0-10 cm soil layer was significantly decreased by 21% and 50% in Ⅱ and Ⅲ, respectively, compared with that in Ⅰ. The DOC/SOC (soil organic carbon) ratios of 0-10 cm and 10-20 cm soil layers were significantly decreased by 27% and 43% after the conversion, respectively. In the 0-10 cm soil layer, the aromatic index and humification index of DOC in Ⅱ were significantly higher than that in Ⅲ. The infrared absorption ratio of soil DOC in the range of 3700-3000 cm-1, 1650-1620 cm-1, 1160-1000 cm-1, and 690-530 cm-1 in Ⅱ was higher than that in Ⅲ, indicating that the DOC in Ⅱ had higher carboxylic acids and aromatic substances than Ⅲ. The fluorescence index of DOC in Ⅱ and Ⅲ ranged from 1.4 to 1.9, and the biological index of Ⅱ was significantly higher than that of Ⅲ, indicating that Ⅲ had higher protein components in DOC and being more bioavailable. Thus, the differences of the content and structure of DOC between Ⅱ and Ⅲ might cause higher soil carbon pool of Ⅱ than that of Ⅲ.

Analysis of UV–Vis spectral characteristics and content estimation of soil DOM under mulching practices

Dissolved organic matter (DOM), which is generally expressed in terms of dissolved organic carbon (DOC), is a key indicator used to monitor the short-term dynamics of soil quality. However, its content and molecular composition are strongly influenced by agricultural practices, and the determination of DOC content is cumbersome and expensive. A field experiment with three treatments, including (i) no mulching (CK), (ii) gravel mulching (GM) and (iii) film mulching (FM), was selected to study the content and composition characteristics of DOC under different agricultural practices using ultraviolet visible (UV–Vis) technology. The results showed that the GM and FM treatments significantly decreased the DOC contents in topsoil (0–20 cm) by 6.15% and 12.18%, respectively, in comparison with the control. The FM treatment significantly decreased the values of specific UV absorbance at 254 nm (SUVA254), 260 nm (SUVA260) and Chromophoric DOM (CDOM) in each soil layer, indicating the aromaticity, hydrophobicity and CDOM content in the FM treatment were significantly decreased in each soil layer. To establish an accurate model for rapid determination of soil DOC, Spearman correlation analysis was performed on normally and non-normally distributed datasets, and linear regression analysis was utilized for modeling with normalized data. The results of linear regression analysis showed that the regression models established among CDOM/DOC, CDOM, SUVA254, SUVA260, and specific UV absorbance at 280 nm (SUVA280) had a high degree of fit, with coefficients of determination (R2) all exceeding 0.95. The model validation results showed that the regression model established between CDOM/DOC and CDOM had the highest accuracy with the lowest root mean squared error (RMSE) (3.58 mg kg−1) and the lowest relative error (RE) (2.74%), which was DOC = 58.065 + 184.355 × A(355) mg kg−1. The results showed that the estimation of soil DOC can be achieved with good prediction using UV–Vis technology. This study highlights the potential of UV–Vis spectroscopic technology to rapidly estimate soil DOM in agriculture.

Integrating remote sensing of hydrological processes and dissolved organic carbon fluxes in long-term Lake Studies

Dissolved organic carbon (DOC) is an important fraction of organic carbon (OC) that together with colored dissolved organic matter (CDOM) plays a key role in the carbon cycling of lakes. Lake Võrtsjärv is a large and shallow eutrophic lake where water quality measurements have been carried out for a long time. The monthly characteristics of DOC and CDOM in Võrtsjärv and its input/output rivers were traced in 5-years observations. We attempted to link the hydro-morphological indices as lake area (A), water level (WL) and volume variations (Qc) with lake DOC fluxes and CDOM-DOC relationships to improve understanding of carbon fluxes in a very shallow, large and highly eutrophic lake, as well as of the possibility to estimate DOC fluxes by CDOM via remote sensing. The research questions were: how the properties and quantity of DOC, its fluxes and CDOM-DOC relationship in a very shallow, large and highly eutrophic lake are responding to 1) variable proportions of allochthonous and autochthonous processes and 2) changing hydrological conditions. The increased lake volume variation in spring caused the higher level of CDOM, signifying the terrestrial (allochthonous) DOC (DOCAlloch) domination in Lake Võrtsjärv. The proportion of DOCAlloch increased together with increasing WL, A and CDOM concentration. Although the generally high proportion of DOCAlloch in Võrtsjärv, the autochthonous DOC determines the water color (CDOM/DOC) in this highly productive ecosystem with short residence time (280–365 days). Our study on DOC flux revealed that the internal carbon sources, driven by hydrological factors, contribute significantly to the role of large and shallow lakes in global carbon cycling. The further development of water color remote sensing could provide novel directions for understanding the feedbacks of lake DOC flux and changing hydrology.

Characteristics of Dissolved Organic Carbon in Boreal Lakes: High Spatial and Inter‐Annual Variability Controlled by Landscape Attributes and Wet‐Dry Periods

AbstractConcentration and chemical composition of dissolved organic carbon (DOC) influence several lake functions; greenhouse gas exchange, nutrient cycling, food webs, and water treatability. To assess spatial and inter‐annual controls on DOC characteristics, 34 lakes were sampled annually for 8 years on the Boreal Plains, Western Canada—a region with heterogeneous surficial geology, and a sub‐humid climate with pronounced inter‐annual wet‐dry periods. Large spatial variability in long‐term average DOC concentration (10–49 mg C L−1) and aromaticity (SUVA254: 1.2–3.9 L mg−1 C m−1) among lakes was found. Higher DOC concentrations and aromaticity were associated with lakes in watersheds with fine‐textured surficial geology and with relatively large contributions through shallow, organic‐rich flow paths. Lake DOC aromaticity was also higher in lakes with lower evaporative enrichment, regardless of surficial geology, indicating shorter lake water residence times and less within‐lake degradation of allochthonous DOC. High inter‐annual variability for both DOC and aromaticity was observed, with coefficients of variation at 10.9 ± 4.6% and 11.1 ± 2.5% among lakes, respectively. Inter‐annual variability in DOC concentrations had low synchronicity among lakes, with patterns of variability linked to surficial geology and primarily responsive to short‐term cumulative precipitation. Conversely, inter‐annual variability in aromaticity had high synchronicity among lakes, driven by longer‐term cumulative precipitation and shifts in lake water residence times. Our study shows it is necessary to consider lake hydrogeomorphic setting and climate to understand spatial and inter‐annual variability in lake DOC characteristics and associated lake functions, and that Boreal Plains lakes have high climate sensitivity.

The physiology of fish in acidic waters rich in dissolved organic carbon, with specific reference to the Amazon basin: Ionoregulation, acid-base regulation, ammonia excretion, and metal toxicity.

Although blackwaters, named for their rich content of dissolved organic carbon (DOC), are often very poor in ions and very acidic, they support great fish biodiversity. Indeed, about 8% of all freshwater fish species live in the blackwaters of the Rio Negro watershed in the Amazon basin. We review how native fish survive these harsh conditions that would kill most freshwater fish, with a particular focus on the role of DOC, a water quality parameter that has been relatively understudied. DOC, which is functionally defined by its ability to pass through a 0.45-µm filter, comprises a diverse range of compounds formed by the breakdown of organic matter and is quantified by its carbon component that is approximately 50% by mass. Adaptations of fish to acidic blackwaters include minimal acid-base disturbances associated with a unique, largely unknown, high-affinity Na+ uptake system that is resistant to inhibition by low pH in members of the Characiformes, and very tight regulation of Na+ efflux at low pH in the Cichliformes. Allochthonous (terrigenous) DOC, which predominates in blackwaters, consists of larger, more highly colored, reactive molecules than autochthonous DOC. The dissociation of protons from allochthonous components such as humic and fulvic acids is largely responsible for the acidity of these blackwaters, yet at the same time, these components may help protect organisms against the damaging effects of low water pH. DOC lowers the transepithelial potential (TEP), mitigates the inhibition of Na+ uptake and ammonia excretion, and protects against the elevation of diffusive Na+ loss in fish exposed to acidic waters. It also reduces the gill binding and toxicity of metals. At least in part, these actions reflect direct biological effects of DOC on the gills that are beneficial to ionoregulation. After chronic exposure to DOC, some of these protective effects persist even in the absence of DOC. Two characteristics of allochthonous DOC, the specific absorbance coefficient at 340 nm (determined optically) and the PBI (determined by titration), are indicative of both the biological effectiveness of DOC and the ability to protect against metal toxicity. Future research needs are highlighted, including a greater mechanistic understanding of the actions of DOCs on gill ionoregulatory function, morphology, TEP, and metal toxicity. These should be investigated in a wider range of native fish Orders that inhabit one of the world's greatest biodiversity hotspots for freshwater fishes.

Natural and anthropogenic impacts on the DOC characteristics in the Yellow River continuum

The Yellow River is the second largest river in China. Carbon transport by the Yellow River has significant influence on riverine carbon cycles in Asia. During the wet season, the riverine carbon was mainly found in dissolved form, i.e., dissolved organic carbon (DOC), along the entire course of the river. The distinct spatial variations of DOC concentration were observed at different reaches of the mainstream (p < 0.01), while the highest mean DOC concentration was generally observed at midstream (4.13 ± 0.91 mg/L). Carbon stable isotope analysis δ13C and C: N ratio of DOC, evidenced the sources of DOC in headwater and upstream were primarily the terrestrial plants (94% and 61%), but it was changed to soil organic matter (SOM) in mid- and downstream (36% and 37%), and the contribution of sewage to DOC were also increased to 17% and 18%. In the whole mainstream of the Yellow River, water temperature (WT) had a significant impact on DOC concentration, and it could explain 67% of the DOC variance. However, in a large catchment, the driving mechanisms on the DOC variations in headwaters will not necessarily be those controlling DOC trends in downstream. The study firstly quantified, in headwater and upstream, the natural factors explained as much as 65% and 73% of the DOC variations, respectively. In mid- and downstream areas, DOC was significantly influenced by the amount of wastewater discharged by the industry and the use of chemical fertilizers (p < 0.05). These findings may facilitate a better assessment of global riverine carbon cycling and may help to reveal the importance of the balance between development and environmental sustainability with the changing DOC transport features in the Yellow River due to human disturbances.

Characteristics of dissolved organic carbon and nitrogen in precipitation in the northern Tibetan Plateau

Dissolved organic carbon (DOC) and nitrogen (N) play essential roles in global C and N cycles. To address the possible role of DOC and N in precipitation and enrich the related global database, the characteristics of DOC and N in precipitation were investigated in a typical remote permafrost region (upper Heihe River Basin) of the northern Tibetan Plateau (TP) from February 2019 to March 2020. The results demonstrated that the average DOC and total dissolved N (TDN) concentrations in the precipitation were 1.41 ± 1.09 μg mL−1 and 0.84 ± 0.48 μg mL−1, respectively, with relatively lower concentrations in the summer. The annual DOC and TDN fluxes were estimated to be 6.42 kg ha−1 yr−1 and 3.39 kg ha−1 yr−1, respectively, indicating that precipitation was a significant factor in C and N deposition. The light-absorbing properties of precipitation DOC from the SUVA254 and spectral slope revealed that precipitation DOC containing more aromatic components and lower molecular weights mostly was present during the summer; the mass cross-section (at the wavelength of 365 nm) ranged 0.26–1.84 m2 g−1, suggesting the potential impact of DOC on climatic forcing in the area. The principal component analysis combined with air mass backward trajectories indicated that the air masses from west Siberia, Central Asia, and northwestern China most significantly influenced the precipitation C and N in the study area. The WRF-Chem simulations and aerosol vertical distributions further illustrated the air mass transport pathways, demonstrating that dust and anthropogenic emissions could be transported over the studied area by westerlies and monsoonal winds. In the study basin, the precipitation deposition of DOC and N contributed largely to the riverine DOC and N exportation during the summer and had potential ecological effects. These results highlight the importance of DOC and N deposition from precipitation in the northern TP.

DOC export is exceeded by C fixation in May Creek: A late-successional watershed of the Copper River Basin, Alaska.

Understanding the entirety of basin-scale C cycling (DOC fluxes and CO2 exchanges) are central to a holistic perspective of boreal forest biogeochemistry today. Shifts in the timing and magnitude of dissolved organic carbon (DOC) delivery in streams and eventually into oceans can be expected, while simultaneously CO2 emission may exceed CO2 fixation, leading to forests becoming stronger CO2 sources than sinks amplifying rising trace gases in the atmosphere. At May Creek, a representative late-successional boreal forest watershed at the headwaters of the Copper River Basin, Alaska, we quantified the seasonality of DOC flux and landscape-scale CO2 exchange (eddy covariance) over two seasonal cycles. We deployed in situ fDOM and conductivity sensors, performed campaign sampling for water quality (DOC and water isotopes), and used fluorescence spectroscopy to ascertain DOC character. Simultaneously, we quantified net CO2 exchange using a 100 ft eddy covariance tower. Results indicate DOC exports were pulse-driven and mediated by precipitation events. Both frequency and magnitude of pulse-driven DOC events diminished as the seasonal thaw depth deepened, with inputs from terrestrial sources becoming major contributors to the DOC pool with decreasing snowmelt contribution to the hydrograph. A three-component parallel factorial analysis (PARAFAC) model indicated DOC liberated in late-season may be bioavailable (tyrosine-like). Combining Net Ecosystem Exchange (NEE) measurements indicate that the May Creek watershed fixes 142-220 g C m-2 yr-1 and only 0.40-0.57 g C m-2 yr-1 is leached out as DOC. Thus, the May Creek watershed and similar mature spruce forest dominated watersheds in the Copper River Basin are currently large ecosystem C sinks and exceeding C conservative. An understanding of DOC fluxes from Gulf of Alaska watersheds is important for characterizing future climate change-induced seasonal shifts.

An evaluation of measurement techniques for algal-derived organic nitrogen

Algal-derived organic matter (AOM) from algal blooms in water supply systems contains dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) among other constituents. The DON and DOC are disinfection by-product (DBP) precursor compounds, and must be well characterised to facilitate effective removal, thus minimising DBP formation during disinfection. While DOC character has been studied extensively, DON analysis suffers from inaccuracies due to sample pre-treatment and instrument sensitivities. A liquid chromatography method that combines size exclusion chromatography with highly sensitive organic carbon and nitrogen detectors (LC-OCND) has been widely adopted for DOC analysis; however, its potential for application for DON charactersation has been suggested as a viable alternative to existing DON characterisation techniquesnot been assessed despite its potential. Hence, the aim was to compare the effectiveness of conventional total dissolved N–dissolved inorganic N (TN–DIN), and LC-OCND methods for analysing DON in AOM. A suite of N-containing model compounds representative of DON and AOM extracted from Chlorella vulgaris CS-42/7 and Microcystis aeruginosa CS-555/1 were used to evaluate the techniques. The DON of both model compounds and AOM was first analysed using the conventional method and, then, via LC-OCND. It was observed that LC-OCND had a better precision for DON when TN contained more DIN. LC-OCND provided direct quantitative measurements for bulk and fractionated DON and DIN, with little interference caused by DIN. Additionally, LC-OCND provided information on MW distribution and protein content of the AOM. For example, LC-OCND results showed that M. aeruginosa AOM contained more HMW material than C. vulgaris AOM. However, as LC-OCND uses UV oxidation, it could not completely oxidise complex aromatic structures, and thus had a lower recovery for HMW model compounds and algal DON in comparison to the conventional method that used high temperature catalytic oxidation. Overall, it is advised that a combination of LC-OCND and TN analysis be used to provide a more detailed characterisation of N-containing AOM and other similar HMW aquatic NOM samples.

Association between extracted copper and dissolved organic matter in dairy-manure amended soils

Dairy manure often has elevated concentrations of copper (Cu) that when applied to soil may create toxicity risks to seedlings and soil microbes. Manure application also increases dissolved organic matter (DOM) in soil solution. We hypothesize that high rates of dairy manure amendment over several years will cause increased DOM in the soil that complexes Cu, increasing its mobility. To test this hypothesis, this study investigated water soluble Cu concentrations and dissolved organic carbon (DOC) in soil samples from 3 years of manure-amended soils. Samples were collected at two depths over the first 3 years of a long-term manure-amendment field trial. DOC, Cu, Fe, and P concentrations were measured in water extracts from the samples. Ultraviolet/visible (UV/Vis) spectra were used to assess the DOC characteristics. After 3 years of manure application, extractable Cu concentration was approximately four times greater in the surface and two times greater in subsurface samples of manure-amended soils as compared to non-amended control soils and traditional mineral fertilizer-amended soils. The extractable Cu concentration was greatest in plots that had the highest manure amendment rates (35 t ha−1 and 52 t ha−1, dry weight). The UV/Vis parameters SUVA254 and E2/E3 correlated with Cu concentration in the extracts (p < 0.05), suggesting that DOC characteristics are important in Cu-binding. The molecular characteristics of the DOC in the subsurface after 3 years of manure amendment were distinct from the DOC in the control plot, suggesting that manure amendment creates mobile DOC that may facilitate Cu mobilization through soil. The 10-fold increase in extractable Cu concentration after only 3 years of manure application indicates that repeated applications of the dairy manure sources used in this study at rates of 35 t/ha or greater may create risks for Cu toxicity and leaching of Cu into ground and surface waters.

Concentration Variations and Flux Estimation of Dissolved Carbon in Karst Spring of a Typical Karst Area

Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) are two important indices for studying karstification, carbon sinks, and the carbon cycle. In order to further understand the migration characteristics of DIC and DOC in karst springs in small watersheds and improve the estimation accuracy of carbon flux under conditions of discrete and limited water quality monitoring data, the concentration variations of DIC and DOC were studied in karst outlet springs of Chenqi small watershed in Puding County, Guizhou Province, China. The flux estimation regression equations of DIC and DOC were established by the LOADEST model, and the carbon sink intensity in Chenqi karst spring basin was estimated. The results showed that the concentrations of DIC and DOC were 16.47-42.31 mg·L-1 and 0.87-6.89 mg·L-1, which displayed exponential decrease and increase with increased instantaneous runoff, respectively. Based on the regression equations constructed by the LOADEST model, the daily flux load of DIC was mainly affected by runoff, whereas that of DOC was affected by both time and runoff. The estimated total fluxes of DIC and DOC were 9490.01 kg·a-1 (95% confidence interval of 11293.58-7972.33 kg·a-1) and 1704.87 kg·a-1 (95% confidence interval of 1895.24-1553.24 kg·a-1), respectively. The carbon sink intensity of the Chenqi karst spring basin was 3.40 g·(m2·a)-1[95% confidence interval of 2.85-4.05 g·(m2·a)-1]. The LOADEST model fully utilized discrete and limited water quality data to improve flux estimation accuracy from the monthly average to the daily average. Therefore, it is an effective tool to estimate the fluxes of DIC and DOC in karst springs under low frequency water quality monitoring conditions.

Fluorescent DOC characteristics are related to streamflow and pasture cover in streams of a mixed landscape

A growing body of research suggests that different land use activities may alter both the quantity and quality of dissolved organic carbon (DOC) exported from terrestrial landscapes. However, little is known about DOC from mixed-use landscapes where hydrology varies seasonally. This study examined how DOC and chemical properties of stream water were related to land use, drainage area, and streamflow in mixed-use landscape in the Willamette River Basin, Oregon. Stream water samples were collected at roughly monthly intervals over three water years from 21 sites whose drainage areas ranged from 1 to > 11,000 km2 and included pasture, forest, and developed land uses. DOC properties were characterized using PARAFAC (parallel factor) analyses of 3-D excitation and emission matrices (EEMs). We used the Cory-McKnight (CM) model, and we also developed a model unique to our samples. Thirteen components were identified using the CM model, and these were highly correlated with, and similar with respect to excitation and emission to the three components identified in our site-specific model. Fluorescent components of DOC were related to discharge and land cover, but not to drainage area. In our model, one component (C1) appeared to be associated with terrestrial detritus and was greater in streamflow from sites with forest/developed cover than from sites with high pasture cover. A second component (C2) was greater in streamflow from high-pasture sites than from high-forest sites. C2 was strongly correlated with a protein component identified in the CM model, and therefore we attributed this second component to more microbially-processed DOC. C1 increased significantly with discharge for both land covers, suggesting that periods of high flow produce less microbially-processed DOC from more surficial flow. C2 decreased significantly with discharge for both land covers, suggesting that deeper hydrologic flow paths produce more soil-associated, microbially processed DOC. SUVA254, often used as an index of chemical quality and aromaticity of DOC, was related only to streamflow but not land use, suggesting that while the chemistry of DOC differed among land use types as detected by EEMs analysis, the aromaticity of the DOC did not differ. The CM model and our site-specific model provided consistent results: those components that were highest in high pasture sites were also highest at low flow and appeared to be more microbially processed; components that were highest in low pasture sites were also greatest at high flow and appeared to reflect less microbially processed DOC. Taken together, these findings are consistent with a conceptual model of how differences in DOC chemistry among landscape types reflect differences in plant litter input chemistry, hydrologic connectivity, and degree of microbial processing. Such changes in DOC quality with land-use change can affect foodweb dynamics in receiving waters and change the balance between carbon storage and carbon flux to the atmosphere, and they imply that even non-intensive agricultural land use can have significant effects on terrestrial-aquatic carbon processes.

Differences in the distribution and optical properties of DOM between fresh and saline lakes in a semi-arid area of Northern China

In limnological environments, most organic carbon is present in the dissolved form. Dissolved organic carbon (DOC) is the main source of energy for microbial metabolism and biosynthesis, and also affects photosynthetic radiation level and the attenuation of ultraviolet irradiation to protect aquatic organisms. There are large differences in DOC concentration, source, and characteristics due to regional variations in water quality and basin characteristics. Reliable estimates of DOC and analysis of optical characteristics are crucial to understand the true role of lakes in the global carbon cycle. In this article, the distribution of DOC across 30 lakes in semi-arid areas of Northern China is reported. The data shows that saline lakes exhibited higher DOC concentrations than freshwater lakes, and the positive relationship between salinity and DOC was established (R2 = 0.42, p < 0.01, n = 196). The mean DOC concentration in eutrophic lakes was lower than in mesotrophic and oligotrophic lakes. Analysis of optical characteristics of CDOM indicated that saline lakes in this semi-arid regions contained abundant fulvic acid, and greater levels of autochthonous dissolved organic matter (DOM) with a lower molecular mass than fresh waters. The total suspended matter (TSM) is the main factor influencing on SUVA254 in both freshwater and saline lakes with a negative correlation. SUVA254 was negatively correlated with the salinity only in freshwater lakes, and with pH only in saline lakes. The result suggests that it was doubtful whether CDOM or SUVA254 alone can be a predictor of DOC concentration and other water quality parameters, especially in different types of lakes with different optical and physicochemical characteristics.