Stable Isotope Ratios Of Nitrate Research Articles

Evaluation of stable isotope ratios (δ15N and δ18O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types

Stable isotope ratios of nitrate are a powerful tool to evaluate aquatic environment stress from treated and untreated sewage. However, there is generally a lack of knowledge on the change in stable isotope ratios within wastewater treatment plants. We investigated nitrogen and oxygen stable isotope ratios (δ15N and δ18O) of nitrate in four types of advanced treatment processes operated in parallel; (A) extended aeration activated sludge, (B) anaerobic-anoxic-aerobic (A2O), (C) recycled nitrification-denitrification, and (D) modified Bardenpho. The results exhibited spatial variation of δ15N and δ18O for nitrate within the treatment steps. The changes in δ15N and δ18O may result from the reactor conditions (aerobic, anoxic, and anaerobic) and the order of these processes. As decreasing nitrate concentration in the anoxic stages, the δ15N/δ18O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ15N and δ18O signatures were attributed to process performance in regard to nitrogen removal. In particular, the modified Bardenpho process has higher nitrogen removal efficiency over other processes, producing effluent with lower nitrate concentration and higher stable isotopes (δ15N: 23.6 to 25.5‰, δ18O: 2.8 to 4.5‰). We concluded that the stable isotope signatures mirrored the treatment efficiency and effluent characteristics.

Temporal changes in δ18O and δ15N of nitrate nitrogen and H2O in shallow groundwater: Transit time and nitrate-source implications for an irrigated tract in southern Idaho

Intensive irrigated agriculture in semiarid southern Idaho contributes to nitrate loads in shallow groundwater. To determine the temporal character and source of leached nitrate in the Twin Falls Irrigation Tract, and investigate the soil N cycling process involved, we measured stable isotope ratios of nitrate (δ15N-NO3, δ18O-NO3) and water (δ2H-H2O, δ18O-H2O) in 1) tunnel drain and irrigation waters during 2003-07, and 2) leachate from incubated urea- and manure-amended soil endmembers. The tunnel water δ2H-H2O vs. δ18O-H2O plots showed clear overlap with waters of Milner reservoir, which stores the tract’s irrigation water prior to distribution. The δ18O-H2O time series and a correlation analysis with climate/hydrologic parameters indicated a 20 to 23 month, transit time between the surface and shallow groundwater. The tunnel waters had a mean δ15N-NO3 of +6.1 ± 0.7‰ (±Std. Dev.) and mean δ18O-NO3 of −6.1 ± 0.7‰. Decreasing tunnel-water δ15N-NO3, δ18O-NO3, and nitrate concentrations in tunnel drains located closer to the Snake R indicate increasing contributions of regional groundwater to shallow groundwater and dilution of the latter’s NO3–N content as proximity to the river increases. Relative to all-other tunnels, water from the two tunnels closest to Snake River were depleted in δ15N-NO3 (+5.4‰ vs. + 6.3‰) and included a greater contribution of regional groundwater (71% vs. 47%). Nitrate δ15N-NO3 and δ18O-NO3 ratios for tunnel waters plotted between those of urea (fixed-N) amended soil (4.6 ± 0.5‰ and −4.9 ± 1.4‰), manure-amended soil (13.4 ± 1.3‰ and −4.4 ± 1.2‰), and regional groundwater endmembers. A dual-isotopic element, three-source, simple linear mixing model indicated that, on average, 1.5X more N is sourced from fertilizer and fixed N than animal waste. The dominant N-cycling process in the system at the scale observed here is the nitrification of NH4-N derived from applied fertilizer and manure, whereas denitrification has a minor influence.

Nitrogen dynamics in a highly urbanized coastal area of western Japan: impact of sewage-derived loads

In this study, we examined the nitrogen dynamics of a highly urbanized coastal area, focusing on the impacts of sewage-derived nitrogen. High levels of dissolved inorganic nitrogen were detected in seawater near treated sewage effluent (TSE) discharge points before decreasing in the offshore direction, suggesting that the impact zone of sewage effluent is about 1–2 km from the discharge point. The stable isotope ratios of nitrate and particulate organic nitrogen suggest nitrogen uptake by phytoplankton as well as dilution by offshore seawater, which contributed to a decrease in sewage-derived nitrogen levels. However, the extent of the impact zone was controlled by tidal variations and differences in temperature between the TSE and seawater. Our results also identify nitrogen transport processes, through exchange between seawater and sediment pore water, as an additional important source of nitrogen in the study area.

Modifications to the azide method for nitrate isotope analysis.

The azide method for measuring the stable isotope ratios of nitrate (NO3- ) is easy to set up. However, the method requires spongy cadmium (Cd) or activated Cd powder which are not easy to prepare, and a toxic azide buffer is used. We aimed to use Cd powder directly to simplify preparation and to substantially reduce the azide dose. The reaction conditions were optimized in order to maximize the NO3- reduction yield. The original azide buffer was diluted by 10- to 10000-fold with or without addition of sodium acetate to reduce O-exchange between nitrite (NO2- ) and H2 O. The isotope ratios of the produced nitrous oxide (N2 O), used to examine the overall reaction performance, were measured using a purge and cryogenic trap system coupled to an isotope ratio mass spectrometer. It was found that Cd powder could be directly used to reduce NO3- to NO2- . A 100-fold diluted azide buffer could be used to reduce NO2- to N2 O when only the δ15 N value was measured, and the diluted azide buffer with sodium acetate when both δ15 N and δ18 O values were measured. Using the modified method, the standard deviations of the δ15 N and δ18 O measurements of international NO3- standards were 0.1 to 1.0‰ and often better than 0.3‰ (3 replicates). Compared with the original azide method, the techniques described here can reduce preparation time by using Cd powder without activation in the first reaction step and substantially (by >60-fold) reduce the dose of extremely toxic reagents containing azide by incorporating sodium acetate in the second reaction step. Our modified method is suitable for samples with small volume (5 mL), being different from previous methods in which 50 or 70 mL samples were used. Copyright © 2016 John Wiley & Sons, Ltd.

Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica

Abstract. Stable isotope ratios of nitrate preserved in deep ice cores are expected to provide unique and valuable information regarding paleoatmospheric processes. However, due to the post-depositional loss of nitrate in snow, this information may be erased or significantly modified by physical or photochemical processes before preservation in ice. We investigated the role of solar UV photolysis in the post-depositional modification of nitrate mass and stable isotope ratios at Dome C, Antarctica, during the austral summer of 2011/2012. Two 30 cm snow pits were filled with homogenized drifted snow from the vicinity of the base. One of these pits was covered with a plexiglass plate that transmits solar UV radiation, while the other was covered with a different plexiglass plate having a low UV transmittance. Samples were then collected from each pit at a 2–5 cm depth resolution and a 10-day frequency. At the end of the season, a comparable nitrate mass loss was observed in both pits for the top-level samples (0–7 cm) attributed to mixing with the surrounding snow. After excluding samples impacted by the mixing process, we derived an average apparent nitrogen isotopic fractionation (15ϵapp) of −67.8 ± 12 ‰ for the snow nitrate exposed to solar UV using the nitrate stable isotope ratios and concentration measurements. For the control samples in which solar UV was blocked, an apparent average 15ϵapp value of −12.0 ± 1.7 ‰ was derived. This difference strongly suggests that solar UV photolysis plays a dominant role in driving the isotopic fractionation of nitrate in snow. We have estimated a purely photolytic nitrogen isotopic fractionation (15ϵphoto) of −55.8 ± 12.0 ‰ from the difference in the derived apparent isotopic fractionations of the two experimental fields, as both pits were exposed to similar physical processes except exposure to solar UV. This value is in close agreement with the 15ϵphoto value of −47.9 ± 6.8 ‰ derived in a laboratory experiment simulated for Dome C conditions (Berhanu et al., 2014). We have also observed an insensitivity of 15ϵ with depth in the snowpack under the given experimental setup. This is due to the uniform attenuation of incoming solar UV by snow, as 15ϵ is strongly dependent on the spectral distribution of the incoming light flux. Together with earlier work, the results presented here represent a strong body of evidence that solar UV photolysis is the most relevant post-depositional process modifying the stable isotope ratios of snow nitrate at low-accumulation sites, where many deep ice cores are drilled. Nevertheless, modeling the loss of nitrate in snow is still required before a robust interpretation of ice core records can be provided.

Tracking sources of groundwater nitrate contamination using nitrogen and oxygen stable isotopes at Beijing area, China

The identification of sources and behavior of contaminants is important to control and manage groundwater quality of aquifer systems in urban areas. In this study, hydrogeochemistry of major constituents and stable isotope ratios of nitrate in groundwater were determined to identify contamination sources and transformation processes occurring in soils and deeper groundwater of Beijing with intense human activities. The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from groundwater samples indicate at least three potential sources of nitrate in groundwaters at Beijing. Stable isotope analyses from this study site, which has atmospheric, chemical fertilizer and human waste nitrate sources, provide a tool to distinguish nitrate sources in a confined aquifer where concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Beijing are wastewater and denitrification process occurred specially in the Central area. NO3–N and cation and anion concentrations (Ca2+, Mg2+ Cl− and SO 4 2 ) showed strong correlations indicating that they originated from the same sources. This study demonstrates that a thorough evaluation of hydrodynamic and hydrochemical parameters with dual isotopes of NO3 − constitutes an effective approach for identifying sources and transformation processes of NO3 − in deeper groundwater systems.

窒素飽和過程における渓流水硝酸イオン濃度および硝酸イオン中酸素の安定同位体比

We measured major inorganic ions and stable isotope ratios of nitrogen and oxygen of nitrate in the stream water in the Chichibu region during the course of nitrogen saturation. Nitrate concentration showed a high west-east gradient in the study area. Stable isotope ratios of oxygen in nitrate showed clear relationships with nitrate concentration. A direct relationship was apparent in the lower range of nitrate concentrations in that the stable isotope ratio of oxygen increases as the nitrate concentration increases. On the other hand, oxygen stable isotope ratios of nitrate hardly increase in the middle to higher concentrations of nitrate. This phenomenon may suggest that the oxygen stable isotope ratios of nitrate reflect a phase of the nitrogen saturation in the forest ecosystems.

Tracing the Origin and Fate of NO x in the Arctic Atmosphere Using Stable Isotopes in Nitrate

Atmospheric nitrogen oxides (NOx =NO+ NO2) play a pivotal role in the cycling of reactive nitrogen (ultimately deposited as nitrate) and the oxidative capacity of the atmosphere. Combined measurements of nitrogen and oxygen stable isotope ratios of nitrate collected in the Arctic atmosphere were used to infer the origin and fate of NOx and nitrate on a seasonal basis. In spring, photochemically driven emissions of reactive nitrogen from the snowpack into the atmosphere make local oxidation of NOx by bromine oxide the major contributor to the nitrate budget. The comprehensive isotopic composition of nitrate provides strong constraints on the relative importance of the key atmospheric oxidants in the present atmosphere, with the potential for extension into the past using ice cores.