Dissolved Inorganic Nitrogen Species Research Articles

Spatiotemporal variation, composition, and implications for transport flux of nitrogen in Leizhou Peninsula coastal water, China

Nitrogen, as the main bioactive element, plays an important role in biological productivity, ecosystem function, and biogeochemical processes in marine environment. In this study, seawater samples collected from China’s Leizhou Peninsula coastal water (LZPCW) during dry, normal, and wet seasons in 2018 were explored to reveal the spatiotemporal variation, composition, and transport flux of dissolved inorganic nitrogen (DIN) linked to hydrographic condition. DIN concentration and speciation showed significantly seasonal variation (P < 0.01), and the concentration of DIN species was significantly higher in dry season than other seasons. The annual mean DIN concentration was 8.01 ± 5.79 μmol L−1. In addition, the N–NO2−, N–NO3− and N–NH4+ were significantly different in the DIN bulk in different seasons. The largest fraction of DIN was N–NO3−, followed by N–NH4+, the lowest was N–NO2−, comprising up to 67.92 ± 23.20%, 23.90 ± 23.19% and 8.18 ± 8.19%, respectively. Besides, the Beibu Gulf was subjected to 7.28 × 1010 mol DIN annual flux through the Qiongzhou Strait. The annual transport fluxes of N–NO2−, N–NO3−, and N–NH4+ accounted for 6.43%, 77.20% and 16.36%, respectively. The DIN concentration and coastal water flow led to the largest DIN flux transport in dry season. This study revealed that the coastal ocean currents, river plumes and human activities jointly drove the dynamic variations of N species in LZPCW. It provides a baseline data for studying the spatiotemporal effects of hydrographic condition on nitrogen distribution and transport flux in the LZPCW, which is implications for understanding nutrients dynamics and coastal water quality management in future.

Spatial-Temporal Distribution, Morphological Transformation, and Potential Risk of Dissolved Inorganic Nitrogen in the Contaminated Unconfined Aquifer from a Retired Nitrogenous Fertilizer Plant.

The accumulation of nitrogen in groundwater in the industrial plots, especially the high ammonium, can result in a serious threat to the groundwater system in the urban area. This study monitored the dissolved inorganic nitrogen (DIN) of the polluted groundwater four times in one year in a retired nitrogenous fertilizer plant site with a production history of nearly 40 years, to analyze the spatial-temporal characteristics of DIN species (NH4+-N, NO3−-N, and NO2−-N) and the effects of groundwater environment on their transfer and transformation. The results showed that NH4+-N (<0.025 to 1310 mg/L) was the main DIN species (61.38–76.80%) with low mobility, whereas the concentration of NO3−-N and NO2−-N was 0.15–146 mg/L and <0.001–12.4 mg/L, accounting for 22.34–36.07% and 0.53–2.83% of total DIN, respectively. The concentration and proportion of NO3−-N and NO2−-N showed an upward trend with time, posing a threat to the safety of surrounding groundwater, and their high spatial-temporal variation was related to the morphological transformation and the transport. In the wet season, the pH and redox condition benefited the nitrification, and NO3−-N easily migrated from the deep soil solution to groundwater, hence the NO3−-N can be accumulated. Therefore, the analysis of species and behaviors of DIN in shallow groundwater is indispensable for environmental risk assessment.

The dynamics of dissolved inorganic nitrogen species mediated by fresh submarine groundwater discharge and their impact on phytoplankton community structure

Submarine groundwater discharge (SGD)-driven nutrient inputs have long been speculated to sustain the high frequency of red tide occurrence in Tolo Harbour, Hong Kong, for its larger flux and higher nutrient loadings than river discharge. Based on analysis of high resolution time series biogeochemical and climatological data from 2000 to 2015, fresh SGD-derived dissolved inorganic nitrogen (DIN) is found to be a significant regulator of the annual cycle of phytoplankton community structure in the harbour. In the wet season, fresh SGD supplies nutrients with NH4+:NO3– ratio < 1 to the seawater, meanwhile creates an intensive vertical stratification environment. As a result, diatom which is a NO3– specialist, is prone to be the major group in the harbour. Fresh SGD delivers a same orders of magnitude of DIN as river and precipitation, but it is more important to phytoplankton community structure dynamics because fresh groundwater has smaller NH4+:NO3– ratio that significantly changes the ratio in the harbour. In the dry season, with the decline of fresh SGD and the ease of stratification, vertical mixing uplifts the nutrient (NH4+:NO3– ratio > 1) released from the bottom sediment leading to a NH4+ dominant environment in water column. Dinoflagellate and other groups then become dominant species of phytoplankton in the harbour. Fresh SGD has a major influence on the NH4+:NO3– ratio in the seawater compared to tide-driven SGD, even though the latter contributes a larger proportion SGD. Tide-driven SGD also produces NH4+ and NO3–, but NH4+:NO3– ratio are mainly subject to the beach environment (bare/mangrove beach), which does not change much seasonally, thus dominant DIN species do not change significantly throughout a year. In a conclusion, fresh SGD plays the most important role among all the endmembers in regulating the DIN composition in Tolo Harbour and its fluctuation mediates the phytoplankton community structure.

Dissolved inorganic nitrogen in a tropical estuary in Malaysia: transport and transformation

Abstract. Dissolved inorganic nitrogen (DIN), including nitrate, nitrite and ammonium, frequently acts as the limitation for primary productivity. Our study focused on the transport and transformation of DIN in a tropical estuary, i.e., the Rajang River estuary, in Borneo, Malaysia. Three cruises were conducted in August 2016 and February–March and September 2017, covering both dry and wet seasons. Before entering the coastal delta, decomposition of the terrestrial organic matter and the subsequent soil leaching was assumed to be the main source of DIN in the river water. In the estuary, decomposition of dissolved organic nitrogen was an additional DIN source, which markedly increased DIN concentrations in August 2016 (dry season). In the wet season (February 2017), ammonium concentrations showed a relatively conservative distribution during the mixing, and the nitrate addition was weak. La Niña events induced high precipitations and discharge rates, decreased reaction intensities of ammonification and nitrification. Hence similar distribution patterns of DIN species in the estuary were found in September 2017 (end of the dry season). The magnitude of riverine DIN flux varied between 77.2 and 101.5 t N d−1, which might be an important support for the coastal primary productivity.

Tracing the cycling and fate of the munition, Hexahydro-1,3,5-trinitro-1,3,5-triazine in a simulated sandy coastal marine habitat with a stable isotopic tracer, 15N-[RDX

Coastal marine habitats become contaminated with the munitions constituent, Hexahydro-1,3,5-trinitro-1,3,5-trazine (RDX), via military training, weapon testing and leakage of unexploded ordnance. This study used 15N labeled RDX in simulated aquarium-scale coastal marine habitat containing seawater, sediment, and biota to track removal pathways from surface water including sorption onto particulates, degradation to nitroso-triazines and mineralization to dissolved inorganic nitrogen (DIN). The two aquaria received continuous RDX inputs to maintain a steady state concentration (0.4 mg L−1) over 21 days. Time series RDX and nitroso-triazine concentrations in dissolved (surface and porewater) and sorbed phases (sediment and suspended particulates) were analyzed. Distributions of DIN species (ammonium, nitrate + nitrite and dissolved N2) in sediments and overlying water were also measured along with geochemical variables in the aquaria. Partitioning of RDX and RDX-derived breakdown products onto surface sediment represented 13% of the total added 15N as RDX (15N-[RDX]) equivalents after 21 days. Measured nitroso-triazines in the aquaria accounted for 6–13% of total added 15N-[RDX]. 15N-labeled DIN was found both in the oxic surface water and hypoxic porewaters, showing that RDX mineralization accounted for 34% of the 15N-[RDX] added to the aquaria over 21 days. Labeled ammonium (15NH4+, found in sediment and overlying water) and nitrate + nitrite (15NOX, found in overlying water only) together represented 10% of the total added 15N-[RDX]. The production of 15N labeled N2 (15N2), accounted for the largest individual sink during the transformation of the total added 15N-[RDX] (25%). Hypoxic sediment was the most favorable zone for production of N2, most of which diffused through porous sediments into the water column and escaped to the atmosphere.

Effects of pH and H2O2 on ammonia, nitrite, and nitrate transformations during UV254nm irradiation: Implications to nitrogen removal and analysis

In order to achieve better removal and analyses of three dissolved inorganic nitrogen (DIN) species via ultraviolet-activated hydrogen peroxide (UV/H2O2) process, this study systematically investigated the rates of photo-oxidations of ammonia/ammonium (NH3/NH4+) and nitrite (NO2−) as well as the photo-reduction of nitrate (NO3−) at varying pH and H2O2 conditions. The results showed that the mass balances of nitrogen were maintained along irradiation despite of interconversions of DIN species, suggesting that no nitrogen gas (N2) or other nitrogen-containing compound was formed. NH3 was more reactive than NH4+ with hydroxyl radical (OH), and by a stepwise H2O2 addition method NH3/NH4+ can be completely converted to NOx−; NO2− underwent rapid oxidation to form NO3− when H2O2 was present, suggesting that it is an intermediate compound linking NH3/NH4+ and NO3−; but once H2O2 was depleted, NO3− can be gradually photo-reduced back to NO2− at high pH conditions. Other than H2O2, the transformation kinetics of DINs were all dependent on pH, but to varying aspects and extents: the NH3 photo-oxidation favored a pH of 10.3, which fell within the pKa values of NH4+ (9.24) and H2O2 (11.6); the NO3− photo-reduction increased with increasing pH provided that it exceeds the pKa of peroxynitrous acid (6.8); while the NO2− photo-oxidation remained stable unless the pH neared the pKa of H2O2 (11.6). The study thereby demonstrates a picture of the evolutions of DIN species together during UV/H2O2 irradiation process, and for the first time presents a method to achieve complete conversion of NH4+ to NO3− with UV/H2O2 process.

Contribution of urban runoff in Taipei metropolitan area to dissolved inorganic nitrogen export in the Danshui River, Taiwan.

A previous study has demonstrated that Danshui River has almost the highest dissolved inorganic nitrogen (DIN) yield in the world and exports most of the DIN in the form of ammonium unlike the world's large rivers. However, the DIN sources are poorly constrained. In this study, the contributions of major sources in the Taipei metropolitan area to the DIN export in the Danshui River were investigated. It is observed that ammonium is the major DIN species in the downstream reaches, resulting from the ammonium-dominated inputs of the effluents of wastewater treatment plants (WWTP) and rain water pumping stations (RWPS). DIN concentrations in the downstream (urban) reaches are substantially elevated. The upstream tributaries annually discharge ∼2709t DIN to the downstream reaches. However, the DIN discharge off the downstream reaches rises to ∼17,918t, resulting from the contribution of RWPS-collected water, i.e., ∼14,632t, and the effluents of two WWTP, i.e., ∼577t. RWPS-collected water inherently contains the contribution of atmospheric deposition, ∼2937t DIN. This finding implies that ∼11,695t (∼66% of the downstream output) DIN flux off the Danshui River is from urban runoff and can be attributed to human activities in the Taipei metropolitan area. To improve the water quality in the Danshui River, water quality controls in urban runoff are important.

Biofilm growth and nitrogen uptake responses to increases in nitrate and ammonium availability

Nitrate (NO3 −) and ammonium (NH4 +) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO3 −:NH4 + ratio. However, few studies have examined biofilm responses to enrichment of both DIN species. We examined biofilm responses to variation in ambient concentrations and enrichments in either NO3 − or NH4 +. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO3 − and +NH4 +) in five streams. Biomass-specific uptake rates (U spec ) of NO3 − and NH4 + were then measured using in situ additions of 15N-labeled NO3 − and NH4 +. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U spec -NO3 − of biofilms in DIN-free treatments varied among the streams in which the NDS had been incubated. Higher ambient DIN concentrations were only correlated with enhanced biofilm growth rates. U spec -NO3 − was one order of magnitude greater and more variable than U spec -NH4 +, however similar relative preference index (RPI) suggested that biofilms did not show a clear preference for either DIN species. Biofilm growth and DIN uptake in DIN-amended NDS (i.e., +NO3 − and +NH4 +) were consistently lower than in DIN-free NDS (i.e., control). Lower values in controls with respect to amended NDS were consistently more pronounced for algal accrual rates and U spec -NO3 − and for the +NH4 + than for the +NO3 − treatments. In particular, enrichment with NH4 + reduced biofilm U spec -NO3 − uptake, which has important implications for N cycling in high NH4 + streams.

Improved analysis of dissolved organic nitrogen in water via electrodialysis pretreatment.

This study evaluated electrodialysis (ED) for direct, accurate, and precise dissolved organic nitrogen (DON) analysis in water. Unlike conventional methods that calculate DON as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN), we designed a compact ED reactor as a pretreatment tool that completely separates DIN from DON in water and then measures DON by equating DON to TDN. The experiments confirmed that the ED pretreatment process can achieve 99% removal of all three major DIN species (i.e., ammonia, nitrite, and nitrate) and an average recovery rate of 88% for an array of model DON compounds of varying characteristics (e.g., urea, amino acids, tripeptide, protein, and humic substances). Variations in nitrogen removal and recovery might be explained by a combined effect of molecular weight, acid dissociation ability (pK(a)), aromaticity, and ED reactor configurations. For model solutions with DIN/DON ratios varying from 1 to 10 mg-N/mg-N, the relative standard deviations in DON concentrations were considerably lower with ED pretreatment (<10%) than without pretreatment (47%). A survey of seven field samples, including lake water, tap water, and treated wastewater, also demonstrated the benefits of using ED pretreatment as compared with a conventional DON analysis method. Overall, this study provides evidence and mechanistic insight for a new DON detection method that uses ED pretreatment. The ED unit is robust for separating DIN and DON, and thus it may facilitate more frequent detection of DON and ultimately enhances understanding of DON issues in the environmental studies.

Speciation and dynamics of dissolved inorganic nitrogen export in the Danshui River, Taiwan

Abstract. Human-induced excess nitrogen outflowing from land through rivers to oceans has resulted in serious impacts on terrestrial and coastal ecosystems. Oceania, which occupies &lt; 2.5% of the global land surface, delivers 12% of the freshwater and dissolved materials to the ocean on a global scale. However, there are few empirical data sets on riverine dissolved inorganic nitrogen (DIN) fluxes in the region, and their dynamics are poorly understood. In this study, a river monitoring network covering different types of land uses and population densities was implemented to investigate the mechanism of DIN export. The results show that DIN concentration/yield varied from ∼20 μM/∼300 kg-N km−2 yr−1 to ∼378 μM/∼10 000 kg-N km−2 yr−1 from the relatively pristine headwaters to the populous estuary. Agriculture and population density control DIN export in less densely populated regions and urban areas, respectively, and runoff controls DIN at the watershed scale. Compared to documented estimates from global models, the observed DIN export from the Danshui River is 2.3 times larger, which results from the region-specific response of DIN yield to dense population and abundant runoff. The dominating DIN species change gradually from NO3− in the headwaters (∼97%) to NH4+ in the estuary (∼60%) following the urbanization gradient. The prominent existence of NH4+ is probably the result of the anaerobic water body and short residence time, unlike in large river basins. Given the analogous watershed characteristics of the Danshui River to the rivers in Oceania, our study could serve as a first example to examine riverine DIN fluxes in Oceania.

Nitrogen Stable Isotopes in Primary Uptake Compartments Across Streams Differing in Nutrient Availability

High variability in the natural abundance of nitrogen stable isotopes (δ(15)N) has been reported for primary uptake compartments (PUCs; e.g., epilithon, filamentous algae, bryophytes, macrophytes) in human-impacted aquatic ecosystems, but the origin of this variability is not yet well understood. We examined how δ(15)N of different PUC types relate to δ(15)N of dissolved inorganic nitrogen (DIN) species (nitrate and ammonium) and to the stream nutrient concentrations in which they grow. We selected 25 reaches located across the fluvial network of La Tordera catchment (NE Spain, 868.5 km(2)), encompassing a gradient of human pressures from headwaters to the river valley. δ(15)N-PUC variability was mostly explained by location within the fluvial network and was strongly related to the δ(15)N of DIN species, especially of ammonium. Models were stronger for PUCs growing within the stream channel and thus using streamwater as their main source of nutrients. Regression models including nutrient concentrations improved the prediction power for δ(15)N-PUCs, suggesting that nutrient concentrations and stoichiometry cannot be ignored in explaining the natural abundance of nitrogen isotopes in PUCs. These results provide insights into what controls variability in δ(15)N of PUCs within a stream network, with implications for the application of stables isotopes as an ecological tool.

Measurements of Dissolved Organic Nitrogen in Wastewater Samples with Nanofiltration Pretreatment

Conventional dissolved organic nitrogen (DON) measurements for water samples with high dissolved inorganic nitrogen (DIN) to total dissolved nitrogen (TDN) ratio are incorrect due to the cumulative analytical errors of independently measured nitrogen species (DIN and TDN). In this study, we present a nanofiltration (NF) pretreatment to increase the accuracy and precision of DON measurements by selectively concentrating DON while passing through DIN species in water samples to reduce the DIN/TDN ratio. Compared to the available dialysis pretreatment method, the NF pretreatment method shows a similar improved performance for DON measurement for aqueous samples and can save at least 20 h of operating time and a large volume of deionized water, which is beneficial for laboratories involved in DON analysis. To prove the feasibility of the pretreatment, we collected and compared effluent samples from two sewage water treatment plants. The result shows that with the pretreatment, the DIN to TDN ratio has significantly decreased while the dissolved organic carbon (DOC) recovery has been increased. So this NF pretreatment is available.

A semianalytical three‐dimensional process‐based model for hyporheic nitrogen dynamics in gravel bed rivers

We present a three‐dimensional semianalytical process‐based model of dissolved oxygen and dissolved inorganic nitrogen (DIN) transformation within the hyporheic zone of gravel bed rivers. Oxygen and multispecies solute transport is solved within a Lagrangian framework with transformation of DIN species modeled by linearized Monod's kinetics, with temperature‐dependent reaction rate coefficients derived from field experiments. Our solutions, which are obtained under the assumptions of sediments with uniform hydraulic properties and negligible local dispersion, highlight the importance of morphological characteristics of the streambed on DIN transformations within the hyporheic zone. By means of this model we explore the effects of streambed topography and relative abundance of ammonium and nitrate in stream waters on the reactive nitrogen cycle in the hyporheic zone of gravel bed rivers with a pool and riffle morphology. Our model shows complex concentration dynamics within the hyporheic zone that may act as a source or a sink of nitrogen depending on the residence time distribution, which can be parameterized in terms of streambed morphology, and the ratio between the in‐stream concentrations of ammonium and nitrate. Application of the model to seven natural streams shows good agreement between predicted and measured nitrous oxide emissions from their hyporheic zone.

Dissolved Organic Nitrogen Measurement Using Dialysis Pretreatment

Dissolved organic nitrogen (DON) is important for ecological and engineering researches. Quantification of low DON concentrations in waters with elevated dissolved inorganic nitrogen (DIN) using existing methods is inaccurate. In this study, a dialysis-based pretreatment technique was optimized and adopted to reduce the interference from DIN to the quantification of DON in natural water. A cellulose ester dialysis tube (nominal molecular weight cutoff = 100 Da) was used in batch and continuous-flow dialysis steps with model compounds, natural organic matter isolates, and bulk waters to develop a dialysis pretreatment approach that selectively reduces DIN from solutions containing DON. By reducing DIN concentrations, propagation of analytical variance in total dissolved nitrogen (TDN) and DIN species concentrations allows more accurate determination of DON (DON = TDN - NO3 - NO2- - NH3/NH4+). Dialysis for 24 h against continuously flowing distilled water reduced DIN species by 70%. With dialysis pretreatment, DON recoveries of more than 95% were obtained for surface water and finished drinking water, but wastewater experienced a slight loss (approximately 10%) of DON possibly due to the adsorption of organics onto the dialysis membrane, permeation of low molecular weight fractions, or biodegradation. Dialysis experiments using surface water spiked with different DIN/TDN ratios concluded that dialysis pretreatment leads to more accurate DON determination than no dialysis when DIN/TDN ratios exceed 0.6 mg of N/mg of N.