Composition Of Dissolved Organic Nitrogen Research Articles

Dominant heterocyclic composition of dissolved organic nitrogen in the ocean: A new paradigm for cycling and persistence.

Marine dissolved organic nitrogen (DON) is one of the planet's largest reservoirs of fixed N, which persists even in the N-limited oligotrophic surface ocean. The vast majority of the ocean's total DON reservoir is refractory (RDON), primarily composed of low molecular weight (LMW) compounds in the subsurface and deep sea. However, the composition of this major N pool, as well as the reasons for its accumulation and persistence, are not understood. Past characterization of the analytically more tractable, but quantitatively minor, high molecular weight (HMW) DON fraction revealed a functionally simple amide-dominated composition. While extensive work in the past two decades has revealed enormous complexity and structural diversity in LMW dissolved organic carbon, no efforts have specifically targeted LMW nitrogenous molecules. Here, we report the first coupled isotopic and solid-state NMR structural analysis of LMW DON isolated throughout the water column in two ocean basins. Together these results provide a first view into the composition, potential sources, and cycling of this dominant portion of marine DON. Our data indicate that RDON is dominated by 15N-depleted heterocyclic-N structures, entirely distinct from previously characterized HMW material. This fundamentally new view of marine DON composition suggests an important structural control for RDON accumulation and persistence in the ocean. The mechanisms of production, cycling, and removal of these heterocyclic-N-containing compounds now represents a central challenge in our understanding of the ocean's DON reservoir.

Characterization, Transformation, and Bioavailability of Dissolved Organic Nitrogen in Biofilters Treating Domestic Onsite Wastewater

This study investigated the transformation, bioavailability, and seasonal changes of dissolved organic nitrogen (DON) in nitrogen removing biofilters (NRBs) treating domestic onsite wastewater. The results demonstrated that NRBs were able to remove 92.6–94.3% DON from septic tank effluent with low effluent DON concentrations (0–5.8 mg N L–1), comparable to the levels detected in the tertiary municipal wastewater treatment plant. Both hydrophilic and hydrophobic DON were efficiently removed by the sand filters, while additional hydrophilic DON (0.8–1.8 mg N L–1) was produced in the lignocellulose/sand layer due to soluble microbial product formation. Seasonal change (i.e., temperature variation) did not impact the effluent DON composition or its removal efficiency by NRBs, while higher concentrations of hydrophilic DON (1.7–2.5 mg N L–1) were detected in the final effluent during the summer months (16.4–22.9 °C). DON in the NRB effluent had a higher potential to stimulate algal growth than other nitrogen forms (NH4+ and NOx–), confirmed by higher chlorophyll-a concentrations observed in the bioassay tests. The results suggest the DON levels in the final effluent of an onsite wastewater treatment system shall also be considered in onsite wastewater quality management and the effluent discharge regulations.

Synthesis and characterization of a novel magnetic resin (m-MAR resin) and its removal performance for alkaline amino acids

Alkaline amino acids as dissolved organic nitrogen (DON) have raised much concern in drinking water treatment due to poor removal in conventional treatment process and high potential for nitrogenous disinfection by-products (N-DBPs). This work was intended to devise a new magnetic adsorption resin (noted as m-MAR resin) for the efficient reduction of alkaline amino acids and explore the application potential of combined MIEX and m-MAR resins. The distribution and composition of DON and amino acids was clarified for different water sources in Lake Taihu basin, in which alkaline amino acids accounted for a higher proportion. The removal of different nitrogenous organics by MIEX resin was also examined, where the resin was effective in removing phycocyanin (65.6%) and glutamic acid (74.2%), reducing the generation of disinfection by-products (DBPs). The m-MAR resin was manufactured and characterized to cope with alkaline amino acids, and batch experiments were undertaken to investigate its adsorption behaviors on histidine and arginine under different operating conditions. The maximal adsorption capacities of arginine and histidine onto m-MAR resin were 2.84 mg/g and 1.62 mg/g, respectively, which was better than MIEX resin. The removal mechanism of the two basic amino acids by m-MAR resin was mainly due to the hydrogen bonding and the acid-base reaction. Moreover, the reusability of the m-MAR resin was elucidated after six successive adsorption-desorption cycles. Finally, the effectiveness of combined MIEX and m-MAR resin in treating DON derived from Microcystis aeruginosa reached 35.2% and the DON concentration in Lake Taihu could be reduced from 0.56 to 0.16 mg/L, which simultaneously decreased the generation potential of N-DBPs. The enhancement of coagulation by the combined process of m-MIER and m-MAR as pretreatment was estimated.

A protocol for nitrogen isotopic measurement of dissolved organic nitrogen with a combination of oxidation-denitrification and gas phase diffusion methods

ABSTRACT Isotopic tracing technique is one of the most effective methods to identify nitrogen source and fate in aquatic environments. Although dissolved organic nitrogen (DON) is a key component in nitrogen cycles, information on nitrogen stable isotope ratios in DON (δ 15N-DON) is limitedly available for its low recovery through a direct measurement. Indirect measurement is based on mass balance calculations and easy to use with high recovery of DON. However, in theory, the result from mass balance calculation is sensitive to the level of DON content, and its applicability remains to be examined for waters containing a variety of DON content in total dissolved nitrogen (TDN). In this study, we established a protocol for indirect measurement of δ 15N-DON values based on the combination of multiple analytical methods. Precision and accuracy in the measurement were assessed by varying the composition of DON and dissolved inorganic nitrogen, and quantitation thresholds were presented at different acceptable levels. The results illustrated an advantage of the developed protocol possibly applicable to water samples particularly with low DON content that is commonly detected in freshwater. This method is expected to expand the use of isotope tracing techniques for understanding the nitrogen cycle in water environments.

Are nitrous oxide emissions indirectly fueled by input of terrestrial dissolved organic nitrogen in a large eutrophic Lake Taihu, China?

Lakes actively transform nitrogen (N) and emit disproportionately large amounts of N2O relative to their surface area. Studies have investigated the relative importance of denitrification or nitrification on N2O emissions; however, the linkage between N2O efflux and dissolved organic nitrogen (DON) and carbon (DOC) remains largely unknown. Long-term (2012–2017) seasonal field observations and a series of degradation experiments were used to unravel how DON composition impacts N2O emissions from Lake Taihu, China. In the northwestern part of the lake, large riverine inflow and high N2O emissions occur in all seasons (24.6 ± 25.2 μmol m−2 d−1), coincident with high levels of terrestrial DON and DOC here. The degradation of labile DON and DOC likely enhanced ammonification as supported by the correlations between NH4+-N and DON, DOC, a(350), and terrestrial humic-like C3. The area with large riverine inputs in the northwestern part of the lake was characterized by low DO which may enhance incomplete aerobic nitrification and incomplete denitrification, both leading to N2O production. Twenty days laboratory experiments indicated greater N2O production in the northwest inflow samples (N2O on day 20: 120.9 nmol L−1 and 17.3 nmol L−1 for bio- and photo-degradation samples, respectively) compared with the central lake samples (N2O on day 20: 20.3 nmol L−1 and 12.3 nmol L−1 for bio- and photo-degradation samples, respectively), despite both having low Chl-a. Our DON and DOC degradation experiments confirmed the occurrence of ammonification along with consumption of NH4+-N and thereafter NO3−-N. Our results collectively suggest that terrestrial DON fueled ammonification, enhanced nitrification and incomplete denitrification, and thereby became an important contributor to the N2O efflux from Lake Taihu.

Longitudinal trends in concentration and composition of dissolved organic nitrogen (DON) in a largely unregulated river system

Dissolved organic nitrogen (DON) can comprise up to 80% of the dissolved N pool in riverine ecosystems, but concentration and compositional responses to catchment conditions has received limited attention. We examined the suite of nitrogenous nutrients along the length of the Ovens River, Victoria, Australia, a river with identifiable regions of native vegetation, agricultural activity and floodplain forest connection, carrying out longitudinal surveys in winter during a period of high flow and in summer during a period of stable base flow. We examined: the concentrations of DON, the proportion of DON that occurs as dissolved combined amino acids (DCAAs), whether concentration and DCAA composition varied between flow and whether land-use and tributaries have an impact upon nutrient concentration and DON composition. DON concentrations were greater than dissolved inorganic nitrogen under both base flow and high flow conditions. Under base flow DON exhibited a continuous increase in concentration downstream (ranging from 50 to 300 μg/L), compared to a much larger increase under high flow (150–600 μg/L) coupled with a major discrete increase of ~ 350 μg/L at a tributary input (King River). Concentrations of NOx (oxides of nitrogen) species were much higher under high flow conditions (range 50–250 μg/L) compared to 0–50 µg/L at base flow, and showed a significant increase in concentration with distance downstream. A discrete change in NOx concentrations was also observed at the King River confluence under high flow, although in this case causing a decrease in concentration of ~100 µg/L. DCAA concentrations varied little along the length of the river at base flow but increased with distance downstream at high flow. The DCAA concentrations were of the same order of magnitude as ammonium at both base and high flows and nitrate concentrations at base flow. The proportion of DON that was in the form of DCAA was reasonably uniform during high flow (3–6%), but highly variable at base flow (5–44%). The amino acid (AA) composition of the DCAA varied along the river and differed between flow regimes (except below the confluence with the King River where AA composition under the two flow conditions converged) suggesting a strong influence of land use. We show that DON is potentially a large component (4–81%) of the total N budget and given that 5–23% is in the form of peptide/protein, represents an important source of N. DON and more specifically DCAAs should therefore be considered both when constructing N budgets and monitoring levels of in-stream nitrogen.

Sources of reactive nitrogen in marine aerosol over the Northwest Pacific Ocean in spring

Abstract. Atmospheric deposition of long-range transport of anthropogenic reactive nitrogen (Nr, mainly comprised of NHx, NOy and water-soluble organic nitrogen, WSON) from continents may have profound impact on marine biogeochemistry. In addition, surface ocean dissolved organic nitrogen (DON) may also contribute to aerosol WSON in the overlying atmosphere. Despite the importance of off-continent dispersion and Nr interactions at the atmosphere–ocean boundary, our knowledge of the sources of various nitrogen species in the atmosphere over the open ocean remains limited due to insufficient observations. We conducted two cruises in the spring of 2014 and 2015 from the coast of China through the East China seas (ECSs, i.e. the Yellow Sea and East China Sea) to the open ocean (i.e. the Northwest Pacific Ocean, NWPO). Concentrations of water-soluble total nitrogen (WSTN), NO3- and NH4+, as well as the δ15N of WSTN and NO3- in marine aerosol, were measured during both cruises. In the spring of 2015, we also analysed the concentrations and δ15N of NO3- and the DON of surface seawater (SSW; at a depth of 5 m) along the cruise track. Aerosol NO3-, NH4+ and WSON decreased logarithmically (1–2 orders of magnitude) with distance from the shore, reflecting strong anthropogenic emission sources of NO3-, NH4+ and WSON in China. Average aerosol NO3- and NH4+ concentrations were significantly higher in 2014 (even in the remote NWOP) than in 2015 due to the stronger wind field in 2014, underscoring the role of the Asian winter monsoon in the seaward transport of anthropogenic NO3- and NH4+. However, the background aerosol WSON over the NWPO in 2015 (13.3 ± 8.5 nmol m−3) was similar to that in 2014 (12.2 ± 6.3 nmol m−3), suggesting an additional non-anthropogenic WSON source in the open ocean. Obviously, marine DON emissions should be considered in model and field assessments of net atmospheric WSON deposition in the open ocean. This study contributes information on parallel isotopic marine DON composition and aerosol Nr datasets, but more research is required to explore complex Nr sources and deposition processes in order to advance our understanding of anthropogenic influences on the marine nitrogen cycle and nitrogen exchange at land–ocean and atmosphere–ocean interfaces.

Spatial-temporal variations of dissolved organic nitrogen molecular composition in agricultural runoff water

Leaching of dissolved organic nitrogen (DON) has been reported as a pathway of N loss from agriculture, but the molecular composition of DON in agricultural water is poorly understood. Runoff water samples were collected from citrus grove furrows (CGF), ditches (CGD) and pasture ditches (PD) in four seasons. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was used to investigate molecular composition of DON. Chemodiversity index of DON had spatiotemporal variations, while the molecular composition of total DON showed minimal variations, except for PD in November. Lignin derivatives constituted 61% of the total DON compounds. Relative abundance of aliphatic compounds, char and condensed aromatics of unique DON compounds varied spatiotemporally and had a significant correlation with DON concentration. Aromaticity index decreased from CGF to connected CGD, implying that photodegradation is possibly the dominant process that alters molecular composition of aquatic DON during the transport. Significant differences in unique DON composition between CGD and PD indicates that fertilization and land use affect DON composition. The information on molecular characterization of DON should be useful for tracking DON source and developing technologies to remove DON in the agricultural runoff water.

Comparative study of activated sludge with different individual nitrogen sources at a low temperature: Effluent dissolved organic nitrogen compositions, metagenomic and microbial community

The objective of this study was to explore nitrogen removal, especially effluent dissolved organic nitrogen (DON) composition, relative genes and microbial community structures with four individual nitrogen sources at 5°C. Results show that effluent DON did not have dependent relationship with the TN removal rate (urea>ammonia chloride>L-Alanine>D-Alanine). With the same influent TN, the highest effluent DON was formed with urea; the lowest DON was fed with ammonia chloride. The main DON composition was the product of cell metabolism excluding urea, rather than the original substrate. Glutamic acid synthesizing process was of great importance to DON accumulation at 5°C. The nitrogen source type was important to the diversity and heterogeneity of the nitrogen removal genes. Bacterial population structure using redundancy analysis (RDA) showed Simplicispira occupied a higher abundance remarkably in the reactors feeding with urea, and Dyadobacter occupied higher feeding with l-Alanine.

Molecular composition and bioavailability of dissolved organic nitrogen in a lake flow-influenced river in south Florida, USA

Dissolved organic nitrogen (DON) represents a large percentage of the total nitrogen in rivers and estuaries, and can contribute to coastal eutrophication and hypoxia. This study reports on the composition and bioavailability of DON along the Caloosahatchee River (Florida), a heavily managed system receiving inputs from Lake Okeechobee as well as agricultural and urban runoff from the surrounding watershed. Water samples were collected bimonthly for 1 year beginning December 2014 at three stations along the river. Treatments included 28-day dark incubations with and without prior photo-irradiation. Concentrations of DON, ammonium, nitrate–nitrite, total hydrolyzable amino acids (THAA), and urea, as well as bacterial numbers, leucine aminopeptidase activity, and fluorescent optical properties were measured. Ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize the molecular composition of DON before and after incubation for selective samples. The total dissolved N pool was dominated by DON (61–99%), with low inorganic N (1–39%), and small amounts of THAA-N (0.1–23%) and urea-N (0.6–3.2%). The mean percentage of biologically available DON (BDON) for the study was 15% (−12–61% range) with highest values occurring when water inputs from Lake Okeechobee were the most dominant freshwater source. FT-ICR MS analysis revealed the presence of a wide range of N-containing formulas and the generation of aliphatic and ‘peptide-like’ structures likely due to microbial alteration of the carbon skeleton of DON compounds. Effects of light exposure prior to incubation did not have a measurable effect on %BDON but did affect bacterial biomass and DON composition. These findings may help predict nutrient loading effects to the Caloosahatchee River estuary and may aid in understanding wetland potential as a treatment technology for removing N in this and other freshwater systems sensitive to N loading.

Characterization of dissolved organic nitrogen in wet deposition from Lake Erhai basin by using ultrahigh resolution FT-ICR mass spectrometry

Dissolved Organic Nitrogen (DON) of wet deposition in Erhai basin (EWD) was characterized at the molecular level by using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The structure and composition of DON were investigated by the combined ESI FT-ICR MS, UV–Vis absorbance and fluorescence techniques. The FT-ICR MS measurements indicate that a large (∼790) number of organic species present in the wet deposition, in which DON account for 18.3%, with most of DON containing a single nitrogen atom. The typical relative molecular mass of the DON species was found to be in the range of 200–400 Da. Approximately 57.2% of DON species are highly unsaturated (DBE (Double Bond Equivalent) > 5) with the nitrogen- and sulfur-containing species, which are probably represented mainly by active nitrooxy organosulfates, accounting for ∼ 19.3% of the total DON. The low average SUVA254 and A253/A203 values (0.02 and 0.06, respectively), indicates that the aromaticity of the EWD samples is particularly weak. The average values of E2/E3 and E4/E6 in the EWD samples were 6.84 and 1.84, respectively. This is a clear indication of the low degree of humification of EWD samples, in agreement with ESI FT-ICR MS measurements. Our study demonstrates that multiple experimental techniques combined with FT-ICR MS, UV–Vis absorbance and fluorescence can be efficiently used for in-depth studying the DON at the molecular level. Thus it allows us to achieve a deep and insightful understanding of the DON structure and composition.

Dissolved organic nitrogen in urban streams: Biodegradability and molecular composition studies

A portion of the dissolved organic nitrogen (DON) is biodegradable in water bodies, yet our knowledge of the molecular composition and controls on biological reactivity of DON is limited. Our objective was to investigate the biodegradability and molecular composition of DON in streams that drain a gradient of 19–83% urban land use. Weekly sampling over 21 weeks suggested no significant relationship between urban land use and DON concentration. We then selected two streams that drain 28% and 83% urban land use to determine the biodegradability and molecular composition of the DON by coupling 5-day bioassay experiments with high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Both urban streams contained a wide range of N-bearing biomolecular formulas and had >80% DON in lignin-like compounds, with only 5–7% labile DON. The labile DON consisted mostly of lipid-and protein-like structures with high H/C and low O/C values. Comparison of reactive formulas and formed counterparts during the bioassay experiments indicated a shift toward more oxygenated and less saturated N-bearing DON formulas due to the microbial degradation. Although there was a little net removal (5–7%) of organic-bound N over the 5-day bioassay, there was some change to the carbon skeleton of DON compounds. These results suggest that DON in urban streams contains a complex mixture of compounds such as lipids, proteins, and lignins of variable chemical structures and biodegradability.

Biodegradability and Molecular Composition of Dissolved Organic Nitrogen in Urban Stormwater Runoff and Outflow Water from a Stormwater Retention Pond.

Dissolved organic nitrogen (DON) can be a significant part of the reactive N in aquatic ecosystems and can accelerate eutrophication and harmful algal blooms. A bioassay method was coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to determine the biodegradability and molecular composition of DON in the urban stormwater runoff and outflow water from an urban stormwater retention pond. The biodegradability of DON increased from 10% in the stormwater runoff to 40% in the pond outflow water and DON was less aromatic and had lower overall molecular weight in the pond outflow water than in the stormwater runoff. More than 1227 N-bearing organic formulas were identified with FT-ICR-MS in the stormwater runoff and pond outflow water, which were only 13% different in runoff and outflow water. These molecular formulas represented a wide range of biomolecules such as lipids, proteins, amino sugars, lignins, and tannins in DON from runoff and pond outflow water. This work implies that the urban infrastructure (i.e., stormwater retention ponds) has the potential to influence biogeochemical processes in downstream water bodies because retention ponds are often a junction between the natural and the built environment.

Dissolved organic nitrogen in wet deposition in a coastal city (Keelung) of the southern East China Sea: Origin, molecular composition and flux

In this study, we collected and analyzed rainwater samples from Keelung, Taiwan, which is a coastal city located south of the East China Sea (ECS). From January 2012 until June 2013, 78 rainwater samples were collected over an 18-month period and were analyzed to examine the total dissolved nitrogen (TDN) and major ions in the rainwater. TDN can be divided into dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON). This study, which focused on the composition of DON, is the first study to employ ultrafiltration to separate DON in wet deposition into low molecular weight-DON (LMW-DON) and high molecular weight-DON (HMW-DON). The concentrations of dissolved nitrogen species observed in the research area between November 2012 and April 2013 were relatively high, whereas those observed between May 2013 and October 2012 were relatively low. The patterns of changes over time were similar to those of non-sea-salt (nss) ions. The concentration of nss-ions was high during months in which the total dissolved nitrogen concentration was also high, which occur frequently during the spring and winter. In addition, the concentration of nss-ions was low during months in which the TDN concentration was low, which primarily occurs during the summer. The amounts of DIN and DON accounted for 63 ± 5% and 37 ± 5% of the TDN, respectively, and the percentage of the DIN was higher during the spring and winter. The concentrations of LMW-DON and HMW-DON, which accounted for 84 ± 3% and 16 ± 3% of the DON, respectively, were both high in the winter and low in the summer. The percentage of LMW-DON increased in the summer, possibly because of the numerous oceanic air masses and typhoons. Furthermore, the percentage of HMW-DON increased in the spring, potentially due to biomass burning during agricultural activities. Regarding the wet deposition fluxes, the DIN (197 ± 10.27 mmol m−2 yr−1) and DON (129 ± 6.82 mmol m−2 yr−1) accounted for approximately 64% and 36% of the TDN, respectively, and the LMW-DON (108 ± 5.97 mmol m−2 yr−1) and HMW-DON (19 ± 1.02 mmol m−2 yr−1) accounted for 85% and 15% of the DON, respectively. The wet deposition flux of the nitrogen species observed in the research area was 332 ± 16.0 mmol m−2 yr−1, and the total flux (dry and wet deposition) was approximately 393.4 ± 25.2 mmol m−2 yr−1 (5508 ± 353 mg N m−2 yr−1).

Molecular‐level characterization of reactive and refractory dissolved natural organic nitrogen compounds by atmospheric pressure photoionization coupled to Fourier transform ion cyclotron resonance mass spectrometry

Dissolved organic nitrogen (DON) represents a significant fraction of the total dissolved nitrogen pool in most surface waters and serves as an important nitrogen source for phytoplankton and bacteria. As with other natural organic matter mixtures, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) is the only technique currently able to provide molecular composition information on DON. Although electrospray ionization (ESI) is the most commonly used ionization method, it is not very efficient at ionizing most DON components. Positive- and negative-mode atmospheric pressure photoionization (APPI) coupled with ultrahigh resolution FTICRMS at 9.4 T were compared for determining the composition of DON before and after bioassays. Toluene was added as the APPI dopant to the solid-phase DON extracts, producing a final sample that was 90% methanol and 10% toluene by volume. Positive-mode (+) APPI proved significantly more efficient at ionizing DON; 62% of the formulas that could be assigned in the positive-ion spectrum contained at least one nitrogen atom vs. 31% in the negative-ion spectrum. FTICR mass spectral data indicated that most of the refractory DON compounds (i.e. nonreactive during the 5 days of the incubation) had molecular compositions representative of lignin-like molecules, while lipid-like and protein-like molecules comprised most of the small reactive component of the DON pool. From these data we conclude that (+) APPI FTICRMS is a promising technique for describing the molecular composition of DON mixtures. The technique is particularly valuable in assessing the bioavailability of individual components of DON when combined with bioassays.

Effect of fertilization on composition and spatial distribution of dissolved organic nitrogen in paddy soil microbial systems

Dissolved organic nitrogen (DON) has high bioavailability and is an important source of soil nutrients. In order to determine the concentrations and the composition of DON in different depths (0–30 cm), and provide a theoretical basis for further deep research into the paddy soil nitrogen supply, experiments based on the lysimeter method were carried out to investigate the effect of fertilization on composition and spatial distribution of DON in paddy soil. Paddy soil was treated under chemical fertilization treatment (CT) and mixed fertilization treatment (MT). With methods of PCR-DGGE and HPLC-MS, the results of the experiments suggested that the samples in 10–20 cm and 20–30 cm under MT had significantly higher bacterial diversity than those under CT except in 0–10 cm. DON had a high percentage (63.1–79.9%) in N<sub>tot</sub> of soil solution. The results of the correlation analysis revealed that DON had a significant positive correlation with organic matter content, Shannon-Wiener diversity index (H') and a significant negative correlation with pH. The possible results of HPLC-MS identification of DON from paddy soil solution were that (a) 3-(4-thiazolyl)-l-alanine; (b) 2-phenylbenzimidazole-5-sulfonic acid; (c) 4-(2, 4-difluorophenyl)-3-nitrobenzene carbaldehyde; (d) fendizoic acid.    

Measurement of dissolved organic nitrogen in a drinking water treatment plant: Size fraction, fate, and relation to water quality parameters

This paper investigates the characteristics of dissolved organic nitrogen (DON) in raw water from the Huangpu River and also in water undergoing treatment in the full-scale Yangshupu drinking water treatment plant (YDWTP) in Shanghai, China. The average DON concentration of the raw water was 0.34 mg/L, which comprised a relatively small portion (~ 5%) of the mass of total dissolved nitrogen (TDN). The molecular weight (MW) distribution of dissolved organic matter (DOM) was divided into five groups: > 30, 10–30, 3–10, 1–3 and < 1 kDa using a series of ultrafiltration membranes. Dissolved organic carbon (DOC), UV absorbance at wavelength of 254 nm (UV254) and DON of each MW fraction were analyzed. DON showed a similar fraction distribution as DOC and UV254. The < 1 kDa fraction dominated the composition of DON, DOC and UV254 as well as the major N-nitrosodimethylamine formation potential (NDMAFP) in the raw water. However, this DON fraction cannot be effectively removed in the treatment line at the YDWTP including pre-ozonation, clarification and sand filtration processes. The results from linear regression analysis showed that DON is moderately correlated to DOC, UV254 and trihalomethane formation potential (FP), and strongly correlated to haloacetic acids FP and NDMAFP. Therefore, DON could serve as a surrogate parameter to evaluate the reactivity of DOM and disinfection by-products FP.

Utilization of marine sedimentary dissolved organic nitrogen by native anaerobic bacteria

Bioavailability and chemical composition of dissolved organic nitrogen (DON) in marine pore water were followed and related to bacterial activity in a 14–d anoxic decomposition experiment. For the experiment 0.2–µm filtered pore water from a coastal marine sediment was inoculated with native sediment bacteria. The initial composition of DON was characterized by a high contribution of dissolved free amino acids (DFAA), dissolved combined amino acids (DCAA), and urea, which together accounted for 82% of the DON pool. During the experiment, 54% of the DON was used by bacteria, indicating that DON was readily available to bacterial degradation. The consumption of DFAA and DCAA accounted for more than half of the DON consumed. The unidentified DON pool accounted for 33% of the net DON consumption, and the unidentified DON pool was fully depleted by the end of the experiment. Systematic changes in the amino acid composition occurred with time of incubation, as demonstrated by use of a principal component analysis based on the mole percent contribution of amino acids. The results indicated that amino acids of both the DFAA and the DCAA pools were useful indicators of the diagenetic state of DON. The present study presents the first direct evidence of short‐term (within 14 d) changes in DFAA and DCAA of the DON pool. These changes were similar to decompositional changes in the molecular composition of amino acids observed in traditional studies of bulk sediment diagenesis.

Free amino acids in marine rains: evidence for oxidation and potential role in nitrogen cycling

Previous studies of dissolved organic nitrogen (DON) in precipitation have addressed various aspects of nutrient transport and global nitrogen cycling1. In most of these studies however, the detailed chemical composition of DON was not determined. Analyses of specific organic nitrogen compounds within precipitation can yield new information about sources and transformations of DON as well as about heterogeneous oxidative processes in the atmosphere. Dissolved free amino acids (DFAA) are a class of compounds for which surprisingly few analyses in precipitation have been published1. We report here the first detailed analyses of DFAA and primary amines in marine rains. Unexpectedly high concentrations of total DFAA were measured, averaging about 6.5 μM (16 marine rain samples) and ranging from 1.1 to 15.2 μM (0.015 to 0.21 p.p.m. N); these values are similar to those obtained for inorganic nitrogen species2. Amino acids are predominantly found in rain as their L optical isomers and therefore are most likely biological in origin; however, the exact sources and modes of enrichment in rain over the open ocean are not known. There is evidence that some of the amino acids, in particular methionine, are oxidized in the atmosphere possibly by a heterogeneous photochemical pathway. The finding of high DFAA and primary amine concentrations in marine rain may have important implications in global nitrogen cycling and also may contribute locally to available nitrogen at the sea surface.