Dissolved Organic Nitrogen Concentrations Research Articles

Effects of pipe material on nitrogen transformation, microbial communities and functional genes in raw water transportation

Raw water transportation pipelines are vital in an urban water supply system for transporting raw water to drinking water treatment plants. This study investigated the effects of pipe material on nitrogen transformation, microbial communities and characteristics of related function genes in paint-lined steel pipe (PLSP) and cement-lined steel pipe (CLSP) raw water model systems. We established quantitative relationships between specific functional genes and change rates of nitrogen pollutants, which were verified by field investigation on nitrogen pollutant transformations in real raw water transportation systems. The results showed that the CLSP produced higher ammonia nitrogen (NH4+-N) transformation rates and higher effluent concentrations of nitrate nitrogen (NO3−-N) and dissolved organic nitrogen (DON) than the PLSP. Both pipes achieved high and stable nitrite nitrogen (NO2−-N) and low total nitrogen (TN) removal efficiency. Nitrification was found to be the dominant process in both model systems, especially in the CLSP. Characteristics of microbial communities and nitrogen functional genes, which were analysed by high-throughput pyrosequencing and quantitative polymerase chain reaction (qPCR), respectively, varied between the two pipe systems. Nitrogen transformation pathways, identified by path analysis, were also different between the PLSP and CLSP due to different microbial community characteristics and synergistic effects of nitrogen functional genes. In the CLSP, (NH4+-N→NO2−-N) with part denitrification, was the primary transformation pathway of ammonia nitrogen (NH4+-N), while only ammonia oxidization contributed to NH4+-N transformation in the PLSP. (NO2−-N→NO3−-N) was the main pathway involved in NO2−-N transformation and NO3−-N accumulation. The TN removal showed complex relationships with nitrification, denitrification and nitrogen fixation processes. These findings provided molecular-level insights into nitrogen pollutant transformations during the transportation of raw water through different types of pipes and technical support for the selection of raw water pipe materials. In our study area, the Taihu basin, China, PLSP was better than CLSP for distributing raw water in a short transportation distance, due to the lower effluent concentrations of DON and NO3−-N and less abundance of microorganisms.

Effect of hydraulic retention time on nitrogen removal and functional gene quantity/transcription in biochar packed reactors at 5 °C: A control-strategy study

This study investigated the effect of hydraulic retention time (HRT = 8, 12, 16, and 24 h) on effluent dissolved organic nitrogen (DON), dissolved total nitrogen (DTN), and functional gene quantity/transcription in biochar packed reactors over a 125-day operation at 5 °C. The lowest effluent DON concentration (0.21 ± 0.14 mg/L) and DTN concentration (10.74 ± 0.41 mg/L) were in R12h and R24h, respectively. Adequate HRT (>12 h) was a necessary parameter for poly-β-hydroxybutyrate (PHB) accumulation (PHB/MLSS: 0.1181–0.1522), but higher HRT was detrimental to adenosine triphosphate (ATP) accumulation from 62.77–66.31 µg/g SS (R8h) to 48.21–48.39 µg/g SS (R24h). Effluent DON had a negative correlation relation with ATP and the relative abundance of amoA (p = 0.02), and effluent DTN had a negative correlation relation with PHB (p < 0.01), the relative abundance of napA (p = 0.01), and the transcriptional quantity of nirS (p = 0.14) and nxrA (p = 0.03).

Dissolved organic nitrogen distribution in differently fertilized paddy soil profiles: Implications for its potential loss

Dissolved organic nitrogen (DON) is recognized as an important nitrogen (N) pool in soil N cycling, but its role in the N cycling of paddy soils, which are intensively fertilized, is not fully predicted. In this study, we investigated DON in flooded layer and soil solution along soil profiles with suction cups in fertilized paddy fields. The DON concentration showed a relative decrease in the deeper layer of paddy soil, while free amino acid N (FAA-N) exhibited a drastic increase along with nutrient profiles of soil. In the upper layer (0–20 cm), DON accounted for 54–64% of total dissolved N (TDN), but this value increased up to 63–97% in the deeper layer (40–60 cm). Low concentrations (9.6–15.0 μg L−1) of FAA-N and low percentage of FAA-N/DON (0.1–0.2%) were observed in the upper layer, but higher concentrations (111–307 μg L−1) and increased percentage (8–36%) were examined in the deeper layer. The high percentage of DON/TDN indicated that DON was the predominant N pool in the deeper layer. Concentrations of DON were significantly and positively correlated with organic matter, total N, and electrical conductivity (EC), while negatively related to soil pH. Additionally, capillary porosity, air porosity, bulk density and particle density were also found to be significantly associated with DON. We suggest the DON and FAA in the paddy field could be an important source for N leaching, which is most strongly related with soil nutrient profiles and physical properties. It is estimated that a total loss of 4.0 kg N ha−1 yr−1 is potentially linked to DON in the paddy field, which implied that ca. 3.35% of the applied N fertilizers could be lost via DON.

Using In‐Situ Optical Sensors to Understand the Biogeochemistry of Dissolved Organic Matter Across a Stream Network

AbstractThe advent of high‐frequency in situ optical sensors provides new opportunities to study the biogeochemistry of dissolved organic matter (DOM) in aquatic ecosystems. We used fDOM (fluorescent dissolved organic matter) to examine the spatial and temporal variability in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) across a heterogeneous stream network that varies in concentration. Across the ten study streams fDOM explained twice the variability in the concentration of DOC (r2 = 0.82) compared to DON (r2 = 0.39), which suggests that the N‐rich fraction of DOM is either more variable in its sources or more bioreactive than the more stable C‐rich fraction. Among sites, DON molar fluorescence was approximately 3x more variable than DOC molar fluorescence and was correlated with changes in inorganic N, indicating that DON is both more variable in composition as well as highly responsive to changes in inorganic N. Laboratory results also indicate that the fDOM sensors we used perform as well as the excitation‐emission wavelength pair generally referred to as the “tryptophan‐like” peak when measured under laboratory conditions. However, since neither the field sensor not the laboratory measurements explained a large percentage of variation in DON concentrations, challenges still remain for monitoring the ambient pool of dissolved organic nitrogen. Sensor networks provide new insights into the potential reactivity of DOM and the variability in DOC and DON biogeochemistry across sites. These insights are needed to build spatially explicit models describing organic matter dynamics and water quality.

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.

Effect of pre-ozonation-enhanced coagulation on dissolved organic nitrogen in municipal wastewater treatment plant effluent

ABSTRACTMunicipal wastewater treatment plant (WWTP) effluent is increasingly used as reclaimed water and as water sources for downstream areas. Effluent dissolved organic nitrogen (DON) is considered to be the precursor to nitrogen disinfection by-products (N-DBPs). Here, we investigated DON removal by pre-ozonation-enhanced coagulation and found that (1) pre-ozonation significantly enhanced the removal of DON, dissolved organic carbon (DOC), and UV254 by coagulation. (2) pH had an important impact on the pre-ozonation-enhanced coagulation. At lower pH, the removal of DON, DOC, and UV254 is higher. (3) As ozone dosage increased, zeta potential gradually increased, while dissolved calcium concentrations declined and there was a strong correlation between zeta potential and dissolved calcium concentrations. (4) Small (<6 kDa) molecular weight DON was dominant in the effluent. Hydrophilic and hydrophobic DON accounted for 58.5% and 30.8% of the total, respectively. (5) Coagulation reduced overall DBP formation potentials to less than 20%. In contrast, overall DBP formation potentials increased after ozonation. However, pre-ozonation-enhanced coagulation not only removed 71% DON, but also reduced overall DBP formation potentials by more than 40%. (6) 3-DEEM spectra indicated that DBP formation potentials and DON concentrations were related to three main peaks, which corresponded to tryptophan-based proteins, aromatic proteins, and fulvic acids.

Long‐term variability of nutrient and dissolved organic matter concentrations in Tokyo Bay between 1989 and 2015

AbstractTokyo Bay is surrounded by a growing urban metropolis and has experienced varying levels of water quality over time. Vertical profiles of nutrients and chlorophyll a (Chl a) were obtained monthly from 1989 to 2015 at two sampling stations located in the center of Tokyo Bay. From 2006 to 2015, profiles of dissolved organic nitrogen (DON) and phosphorus were also obtained. Mann–Kendall trend analysis was used to establish the significance of the long‐term trends for all parameters. Water‐column integrated nutrient concentrations showed significant decreases at both stations, except for integrated phosphate concentration at one station. Water‐column integrated Chl a concentration also decreased, indicating that active nutrient consumption due to an increase in primary production was not responsible for the declining nutrient concentrations. Rather, the decreases reflect the reduced loading of nutrients into the bay due to the implementation of advanced wastewater treatment techniques. Phosphate concentrations in the surface waters of Tokyo Bay were frequently below detection limits during spring and summer after 2000. During the 10‐yr period following 2006, dissolved organic phosphorus concentrations decreased significantly at the surface at both stations, whereas DON concentrations did not change significantly. The ratios of total dissolved nitrogen to total dissolved phosphorus in the surface waters were much higher than the Redfield ratio for both stations, and these ratios increased significantly during the most recent decade. Continued declines in total dissolved phosphorus concentration could strengthen phosphorus limitation of primary production in Tokyo Bay.

Concentrations and Compositions of Different Forms of Nitrogen and Phosphorus in Atmospheric Aerosols in the Qingdao Coastal Region and over the Yellow and Bohai Sea

The total suspended particulate (TSP) samples were collected in the Qingdao coastal region and over the Yellow and Bohai Sea from June to July in 2016. The diurnal and nightly TSP samples were also continuously collected in the Qingdao coastal region from August 6 to 15. The concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved total nitrogen (DTN), dissolved total phosphorus (DTP), total nitrogen (TN), and total phosphorus (TP) in the TSP samples were analyzed. Results showed that the concentrations of different forms of nitrogen and phosphorus in Qingdao were higher than those over the Yellow and Bohai Sea during the same sampling period. The contribution of dissolved N was similar to that of insoluble N to TN in Qingdao, with the ratio of DTN to TN of 56%. However the DTN was the dominant contributor of TN over the Yellow and Bohai Sea, accounting for 72% of TN on average. The inorganic nitrogen was the dominant species of DTN in Qingdao and over the Yellow and Bohai Sea (YBS), accounting for 67% and 75% of DTN, respectively. The contribution of dissolved P to TP was similar to that of insoluble P to TP in Qingdao and over the Yellow and Bohai Sea, and the DTP accounted for 49% and 58% of TP in Qingdao and over YBS, respectively. The ratio of IP to DTP was slightly higher than that of OP, with values of 56% and 59% in Qingdao and over the YBS, respectively. The origin of the air mass affected the concentrations and compositions of nitrogen and phosphorus in the aerosols. The concentrations of DIN, dissolved organic nitrogen (DON), TN, DIP, and dissolved organic phosphorus (DOP) in the aerosols from southern air mass were higher than those from northern and marine air masses. The concentration of DON in the diurnal aerosol samples was similar to that in the nightly samples; however, the concentrations of DIN and TN were higher in the diurnal aerosols than those in the nightly aerosols. The DTN was the dominant species of TN in the diurnal and nightly aerosol samples, accounting for 79% of TN on average. Inorganic nitrogen was the dominant species of DTN in the diurnal and nightly aerosols. The ratio of DIN to DTN decreased from 70% in diurnal samples to 61% in nightly samples. The concentrations of DIP were close to that of DOP in the diurnal and nightly aerosols; however, the concentrations of TP were higher in the diurnal aerosols than in the nightly ones. The insoluble P was the dominant form of TP in the aerosols, accounting for 83% and 62% of TP during the day and night, respectively. The contribution of DTP to TP in the nightly aerosols samples was much higher than that in the diurnal aerosols. For both day and night samples, inorganic phosphorus was the dominant species of DTP in aerosols, with a ratio of 71%-77%.

Removal Characteristics of Dissolved Organic Nitrogen and Its Bioavailable Portion in a Postdenitrifying Biofilter: Effect of the C/N Ratio.

Addition of external carbon sources to postdenitrification biofilters (DNFs) is frequently used in municipal wastewater treatment plants to enhance dissolved inorganic nitrogen removal. However, little is known about its influence on the removal of dissolved organic nitrogen (DON). This study investigated the effect of the carbon-to-nitrogen (C/N) ratio (3, 4, 5, and 6) on the removal characteristics of DON and bioavailable DON (ABDON) in the pilot-scale DNFs treating real secondary effluent. Results showed that DNFs effluent DON accounted for 31.2-39.8% of the effluent total nitrogen. The maximum effluent DON and ABDON concentrations both occurred in DNF operated at a C/N ratio of 3. There was no significant difference in effluent DON concentrations in DNFs at C/N ratios of 4, 5, and 6; however, effluent ABDON and DON bioavailability significantly decreased with C/N ratios (p < 0.05, t-test). According to the chemical composition analysis, effluent DON at high C/N ratios tends to contain less % molecular weight < 1 kDa nitrogenous organic compounds and proteins/amino sugars-like nitrogenous organic formulas, which is likely responsible for its low bioavailability. Overall, this study indicates the benefit of a high C/N ratio during the DNF process in terms of controlling the DON forms that readily stimulate algal growth.

Assessing net-uptake of nitrate and natural dissolved organic matter fractions in a revitalized lowland stream reach

Lowland streams are often revitalized by hydrologically reconnecting their surrounding terrestrial environment, which likely alters central ecosystem functions such as autotrophic and heterotrophic carbon and nutrient metabolism. However, such central ecosystem functions of stream reaches following revitalization have rarely been investigated. Here, we measured net-uptake of nitrate and of various dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) fractions (using size-exclusion chromatography, SEC), as well as fluorescence components (established by parallel factor analysis, PARAFAC) repeatedly along a revitalized 1.4km reach of a Danish lowland stream during base flow conditions. Samples were taken at six stations every three hours for one day. We measured discharge at five of the six stations to calculate whole-stream net-uptake. Moreover, we continuously measured oxygen at the start and at the end of the reach to assess stream metabolism, and took piezometer samples from ten shallow groundwater sites to assess potential determinants of organic matter and nitrate metabolism. We found high metabolic activity within the stream reach with a gross-primary production of 4.8 and 3.6g O2 m−2day−1 and a production to respiration ratio of 0.8 and 0.9 at the start and end of the reach, respectively. Nitrate exhibited relatively constant high net-uptake rates of 0.41–0.52gNm−2 d−1, which varied little and were not related to the time of the day. Therefore, autotrophic nitrate uptake for was likely of minor importance, despite the apparently high primary production. In contrast, SEC DON and DOC fractions, as well as PARAFAC components did not suggest net-uptake or release. Instead, DOC and DON concentration were highly variable among the six stations and sampling times, a pattern that was not explainable by measurement errors but was likely related to the high variability of DOC and DON concentrations in the hyporheic zone and adjacent groundwater bodies. This pointed to a potentially high interaction with the hyporheic and riparian zone, underlining the strong linkage of DOM-related processes across the terrestrial-aquatic boundary. The high nitrate uptake points to the high retention potential of revitalized stream reaches, which, however, would need to be corroborated by further studies with reference reaches.

Distribution Characteristics of Chromophoric Dissolved Organic Matter and Nutrients from the Yellow Sea and Bohai Sea in Autumn

Water samples were collected from the Yellow and Bohai seas during November 2013 and the chromophoric dissolved organic matter (CDOM) and nutrients concentrations were investigated, including their composition, source and distribution characteristics. CDOM was analyzed by excitation-emission matrix spectroscopy (EEMS) in combination with a parallel factor analysis (PARAFAC). Three terrestrial humic-like substances (C1, C2, and C3) and one protein-like (C4) substances were identified. The three terrestrial humic-like components had a similar horizontal distribution, decreasing from inshore to offshore. The protein-like component, showed higher values for both inshore and offshore areas in the Yellow Sea at the surface layer and the middle layer of the Bohai Sea, while in other layers it had a similar distribution pattern as the terrestrial humic-like components. In the Bohai Sea, the dissolved inorganic nitrogen (DIN) concentration showed a decreasing trend from inshore to offshore areas and the concentration of dissolved inorganic phosphorus (DIP) gradually decreased from the Caofeidian coastal and central areas to other adjacent areas. The dissolved organic nitrogen (DON) showed an increasing from inshore to the central area of the Bohai Sea. DIN and DON were higher in surface layer than in the bottom layer and for DIP this was the opposite in the Bohai Sea. In the Yellow Sea, DON concentrations showed a decreasing trend from inshore to offshore areas. DIN and DIP had higher concentrations at inshore areas in the surface and middle layers, while in the bottom layer they had higher concentrations in offshore areas. The vertical distribution of the DIN and DIP showed higher values in the bottom layer than surface layers and for DON this was the opposite. These results also showed that the concentrations of DIN, DON, and DIP in the Bohai sea were overall higher than the Yellow Sea. A discriminant analysis was performed through redundancy analysis (RAD) of these water quality parameters, including the four fluorescent components, an absorption coefficient (a355), chlorophyll a concentration, conductivity, dissolved oxygen (DO), dissolved organic carbon (DOC), DIN, DON, and DIP. The RDA indicated that the four fluorescent components are mainly affected by terrestrial inputs. DOC was affected by both terrestrial and marine sources but terrestrial inputs were the major contributor. It was also indicated that the DIN concentration was affected by terrestrial inputs in the Bohai Sea area and by both terrestrial and marine sources in the Yellow Sea. DON concentrations were mainly affected by marine sources in the Bohai Sea and terrestrial inputs in the Yellow Sea. DIP concentrations were affected by both terrestrial and marine sources in the studied areas.

Characterizing Limits of Precision for Dissolved Organic Nitrogen Calculations

The usefulness of dissolved organic nitrogen (DON) data to environmental science depends on the precision of each calculated concentration. The precision of a calculated DON concentration is based on propagation of error from the measured nitrogen fractions used to calculate the DON concentration. We present an improved approach for calculating DON precision that strengthens conventional error propagation by developing empirical relationships between precision and concentration for each of the measured nitrogen fractions used to calculate DON concentration. Because the concentration and relative importance of each measured nitrogen fraction differ among samples, DON precision and the corresponding detection limit are likely to be different for each sample. Case studies from two different research efforts, a synoptic study of surface water DON and an experimental assessment of DON leached from heat-treated soils, demonstrate how the proposed approach can be tailored to the analytical methods and concentrat...

Nitrogen transformations along a shallow subterranean estuary

Abstract. The transformations of chemical constituents in subterranean estuaries (STEs) control the delivery of nutrient loads from coastal aquifers to the ocean. It is important to determine the processes and sources that alter nutrient concentrations at a local scale in order to estimate accurate regional and global nutrient fluxes via submarine groundwater discharge (SGD), particularly in boreal environments, where data are still very scarce. Here, the biogeochemical transformations of nitrogen (N) species were examined within the STE of a boreal microtidal sandy beach located in the Magdalen Islands (Quebec, Canada). This study revealed the vertical and horizontal distribution of nitrate (NO3−), nitrite (NO2−), ammonia (NH4+), dissolved organic nitrogen (DON) and total dissolved nitrogen (TDN) measured in beach groundwater during four spring seasons (June 2011, 2012, 2013 and 2015) when aquifer recharge was maximal after snowmelt. Inland groundwater supplied high concentrations of NOx and DON to the STE, whereas inputs from seawater infiltration were very limited. Non-conservative behaviour was observed along the groundwater flow path, leading to low NOx and high NH4+ concentrations in the discharge zone. The long transit time of groundwater within the beach (∼ 166 days), coupled with oxygen-depleted conditions and high carbon concentrations, created a favourable environment for N transformations such as heterotrophic and autotrophic denitrification and ammonium production. Biogeochemical pathways led to a shift in nitrogen species along the flow path from NOx-rich to NOx-poor groundwater. An estimate of SGD fluxes of N was determined to account for biogeochemical transformations within the STE based on a N-species inventory and Darcy's flow. Fresh inland groundwater delivered 37 mol NOx yr−1 per metre of shoreline and 63 mol DON m−1 yr−1 to the STE, and NH4+ input was negligible. Near the discharge zone, the potential export of N species was estimated around 140, 1.5 and 33 mol yr−1 per metre of shoreline for NH4+, NOx and DON respectively. In contrast to the fresh inland groundwater, the N load of beach groundwater near the discharge zone was dominated by NH4+ and DON. Our study shows the importance of tidal sands in the biogeochemical transformation of the terrestrial N pool. This local export of bioavailable N probably supports benthic production and higher trophic levels leading to its rapid transformation in surface sediments and coastal waters.

Seasonality of dissolved nitrogen from spring melt to fall freezeup in Alaskan Arctic tundra and mountain streams

Predicting the response of dissolved nitrogen export from Arctic watersheds to climate change requires an improved understanding of seasonal nitrogen dynamics. Recent studies of Arctic rivers emphasize the importance of spring thaw as a time when large fluxes of nitrogen are exported from Arctic watersheds, but studies capturing the entire hydrologic year are rare. We examined the temporal variability of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) concentrations in six streams/rivers in Arctic Alaska from spring melt to fall freeze-up (May through October) in 2009 and 2010. DON concentrations were generally high during snowmelt and declined as runoff decreased. DIN concentrations were low through the spring and summer and increased markedly during the late summer and fall, primarily due to an increase in nitrate. The high DIN concentrations were observed to occur when seasonal soil thaw depths were near maximum extents. Concurrent increases in DIN and DIN-to-chloride ratios suggest that net increases from nitrogen sources contributed to these elevated DIN concentrations. Our stream chemistry data, combined with soil thermistor data, suggest downward penetration of water into seasonally thawed mineral soils, and reduction in biological nitrogen assimilation relative to remineralization, may increase DIN export from Arctic watersheds during the late summer and fall. While this is part of a natural cycle, improved understanding of seasonal nitrogen dynamics is particularly important now because warmer temperatures in the Arctic are causing earlier spring snowmelt and later fall freeze-up in many regions.

Land use affects total dissolved nitrogen and nitrate concentrations in tropical montane streams in Kenya

African tropical montane forests are facing fast and dynamic changes in land use. However, the impacts of these changes on stream water quality are understudied. This paper aims at assessing the effect of land use and physical catchment characteristics on stream water concentrations of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), nitrate (NO3-N) and dissolved organic nitrogen (DON) in the Mau Forest, the largest tropical montane forest in Kenya. We conducted five synoptic stream water sampling campaigns at the outlets of 13–16 catchments dominated by either natural forest, smallholder agriculture or commercial tea and tree plantations. Our data show a strong effect of land use on TDN and NO3-N, with highest concentrations in stream water of catchments dominated by tea plantations (1.80±0.50 and 1.62±0.60mgNl−1, respectively), and lowest values in forested catchments (0.55±0.15 and 0.30±0.08mgNl−1, respectively). NO3-N concentration increased with stream temperature and specific discharge, but decreased with increasing catchment area. DOC concentrations increased with catchment area and precipitation and decreased with specific discharge, drainage density and topographic wetness index. Precipitation and specific discharge were also strong predictors for DON concentrations, with an additional small positive effect of tree cover. In summary, land use affects TDN and NO3-N concentrations in stream water in the Mau Forest region in Kenya, while DOC and DON were more related to hydrologic regimes and catchment properties. The importance of land use for NO3-N and TDN concentrations emphasizes the risk of increased nitrogen export along hydrological pathways caused by intensified land use and conversion of land to agricultural uses, which might result in deterioration of drinking water quality and eutrophication in surface water in tropical Africa.

Nitrate formation during ozonation as a surrogate parameter for abatement of micropollutants and the N-nitrosodimethylamine (NDMA) formation potential

In this study, nitrate formation from ammonium and/or dissolved organic nitrogen (DON) was investigated as a novel surrogate parameter to evaluate the abatement of micropollutants during ozonation of synthetic waters containing natural organic matter (NOM) isolates, a natural water and secondary wastewater effluents. Nitrate formation during ozonation was compared to the changes in UV absorbance at 254 nm (UVA254) including the effect of pH. For low specific ozone doses UVA254 was abated more efficiently than nitrate was formed. This is due to a relatively slow rate-limiting step for nitrate formation from the reaction between ozone and a proposed nitrogen-containing intermediate. This reaction cannot compete with the fast reactions between ozone and UV-absorbing moieties (e.g., activated aromatic compounds). To further test the kinetics of nitrate formation, two possible intermediates formed during ozonation of DON were tested. At pH 7, nitrate was formed during ozonation of acetone oxime and methyl nitroacetate with second-order rate constants of 256.7 ± 4.7 M−1 s−1 and 149.5 ± 5.8 M−1 s−1, respectively. The abatement of the selected micropollutants (i.e., 17α-ethinylestradiol (EE2), carbamazepine (CBZ), bezafibrate (BZF), ibuprofen (IBU), and p-chlorobenzoic acid (pCBA)) was investigated for specific ozone doses ≤1.53 mgO3/mgDOC and its efficiency depended strongly on the reactivity of the selected compounds with ozone. The relative abatement of micropollutants (i.e., EE2 and CBZ) with high ozone reactivity showed linear relationships with nitrate formation. The abatement of micropollutants with intermediate-low ozone reactivity (BZF, IBU, and pCBA) followed one- and two-phase behaviors relative to nitrate formation during ozonation of water samples containing high and low concentrations of nitrate-forming DON, respectively. During ozonation of a wastewater sample, the N-nitrosodimethylamine formation potential (NDMA-FP) during chloramination decreased with increasing specific ozone doses. A good correlation was obtained between NDMA-FP abatement and nitrate formation. Therefore, nitrate formation after pre-ozonation may be a useful parameter to estimate the reduction of the NDMA-FP during post-chloramination. Overall, the results of this study suggest that nitrate formation (possibly in combination with UVA254 abatement) during ozonation of DON-containing waters may be a good surrogate for assessing the abatement of micropollutants and the NDMA-FP.

Peatland restoration and the dynamics of dissolved nitrogen in upland freshwaters

This study assesses the impact of peatland restoration on fluvial N dynamics of south Pennine headwaters (UK) using a space-for-time approach. We monitored dissolved nitrogen in catchment drainage waters at intact, bare, and early stage restoration peatland sites over a two year period (Jan 2013–Dec 2014). Our study demonstrates that peatland restoration is effective in reducing dissolved inorganic nitrogen (DIN) leaching to levels lower than, or comparable to, the intact peatland site despite the adoption of a restoration approach involving fertilizer application in the revegetation process. In comparison with the bare site, DIN leaching was ∼92% (10.2kgNha−1yr−1) lower at the restored site − where vegetation cover has been recently reintroduced. Whilst restoration increased the proportional significance of dissolved organic nitrogen (DON) when compared to the bare site, it was not to a level significantly different from what existed at the intact site. The results also reveal a strong positive relationship (P<0.001) between DON and dissolved organic carbon (DOC) at all the sites, suggesting similarity of source material. Nitrate decreased with increasing DOC concentrations across the sites, suggesting the influence of organic C supply on NO3− immobilisation. In all but the bare site, average DON concentration was low in winter, but high in summer, and DIN concentration exhibited a winter-high and summer-low pattern; although year-on-year variations in this seasonal pattern was observed mainly at the bare site. Overall, our study has shown that restoration/revegetation is effective in advancing ecosystem recovery of degraded peatlands. Understanding nitrogen behaviour and trajectories as peatland restoration moves beyond early phases will require long-term catchment-scale monitoring.

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.

Subtropical urban turfs: Carbon and nitrogen pools and the role of enzyme activity

Urban grasslands not only provide a recreational venue for urban residents, but also sequester organic carbon in vegetation and soils through photosynthesis, and release carbon dioxide through respiration, which largely contribute to carbon storage and fluxes at regional and global scales. We investigated organic carbon and nitrogen pools in subtropical turfs and found that dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) were regulated by several factors including microbial activity which is indicated by soil enzymatic activity. We observed a vertical variation and different temporal patterns in both soil DOC, DON and enzyme activities, which decreased significantly with increasing soil depths. We further found that concentration of soil DON was linked with turf age. There were correlations between grass biomass and soil properties, and soil enzyme activities. In particular, soil bulk density was significantly correlated with soil moisture and soil organic carbon (SOC). In addition, DOC correlated significantly with DON. Significant negative correlations were also observed between soil total dissolved nitrogen (TDN) and grass biomass of Axonopus compressus and Zoysia matrella. Specifically, grass biomass was significantly correlated with the soil activity of urease and β-glucosidase. Soil NO3-N concentration also showed negative correlations with the activity of both β-glucosidase and protease but there were no significant correlations between cellulase and soil properties or grass biomass. Our study demonstrated a relationship between soil C and N dynamics and soil enzymes that could be modulated to enhance SOC pools through management and maintenance practices.

Evaluation of leachate dissolved organic nitrogen discharge effect on wastewater effluent quality

Nitrogen is limited more and more frequently in wastewater treatment plant (WWTP) effluents because of the concern of causing eutrophication in discharge waters. Twelve leachates from eight landfills in Florida and California were characterized for total nitrogen (TN) and dissolved organic nitrogen (DON). The average concentration of TN and DON in leachate was approximately 1146mg/L and 40mg/L, respectively. Solid-phase extraction was used to fractionate the DON based on hydrophobic (recalcitrant fraction) and hydrophilic (bioavailable fraction) chemical properties. The average leachate concentrations of bioavailable (bDON) and recalcitrant (rDON) DON were 16.5mg/L and 18.4mg/L, respectively. The rDON fraction was positively correlated, but with a low R2, with total leachate apparent color dissolved UV254, chemical oxygen demand (COD), and humic acid (R2 equals 0.38, 0.49, and 0.40, respectively). The hydrophobic fraction of DON (rDON) was highly colored. This fraction was also associated with over 60% of the total leachate COD. Multiple leachate and wastewater co-treatment simulations were carried out to assess the effects of leachate on total nitrogen wastewater effluent quality using removals for four WWTPs under different scenarios. The calculated pass through of DON suggests that leachate could contribute to significant amounts of nitrogen discharged to aquatic systems.