Nitrogen-15 fractionation in the thermal decomposition of nitrous oxide of natural isotopic composition

The 15N fractionation in the thermal decomposition of nitrous oxide (N2O) of natural isotopic composition has been investigated in quartz reaction vessel in the temperature interval 888-1073 K. The formulas relating the observed experimentally 15N fractionations with the primary 15N kinetic isotope effect, (k14/k15)p for 14N15N16O, and secondary 15N kinetic isotope effect, (k14/k15)s for 15N14N16O, have been derived. The experimentally estimated 15N kinetic isotope effects have been compared with the primary and secondary 15N kinetic isotope effects calculated with the absolute rate theory formulations applied to linear three atom molecules. A good agreement was found for the primary 15N kinetic isotope effect, (k14/k15)p, in the temperature interval 888-1007 K. But at 1073 K the decompositions of N2O, accompanied by NO (nitric oxide) formation proceed with a twice times smaller primary kinetic isotope effect, (k14/k15)p of 1.0251 +/- 0.0009, only, suggesting the nonlinear transition state structures with participation of the fourth external atom at high temperature decompositions of nitrous oxide. The nitrogen isotope effects determined in this study correlate well with nitrogen isotope fractionations observed in the natural biological, earth and atmospheric processes.     

The Position Dependent 15N Enrichment of Nitrous Oxide in the Stratosphere

Abstract

The position dependent ¹⁵N fractionation of nitrous oxide (N₂O), which cannot be obtained from mass spectrometric analysis on molecular N₂O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO⁺ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent ¹⁵N fractionations occur in the photolysis of N₂O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N₂O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for ¹⁵N¹⁴NO and ¹⁴N¹⁵NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N₂O with O(¹D).     

Sensitive detection of methane and nitrous oxide isotopomers using a continuous wave quantum cascade laser

A continuous wave quantum cascade laser (QCL), operating near 8.1 μm, was used for wavelength modulation spectroscopy of methane (CH₄) and nitrous oxide (N₂O) stable isotopes. Several rotational transitions of ¹⁴N₂ ¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N₂ ¹⁸O, ¹⁴N₂ ¹⁷O, ¹³CH₄ and ¹²CH₄ fundamental bands were detected. The noise-equivalent absorbance was measured to be less than 10^-5 in a 1-Hz bandwidth. A characterization of the laser source was also performed. The use of a QCL spectrometer for high-precision isotope ratio measurements is discussed. Received 14 March 2002     

15N-Traceruntersuchungen zum Mechanismus der N2O-Bildung in Böden

Nitrous oxide is a potential environmental hazard responsible for the green house effect and the destruction of the ozone layer in the lower stratosphere. Biological denitrification under anaerobic conditions in soils results in the formation of both N₂O and N₂, whereby highly nitrogen-fertilized agricultural soils contribute to a considerable extent of the N₂O emission. Latest results in the literature indicate that nitrous oxide can also be formed as a byproduct of the microbial nitrification. This is of importance for soils in central Germany because of the non-existence of typical denitrification conditions in a semiaride climate.

This study was conducted to measure the path of N₂O formation in Haplic Phaeozen: using [¹⁵N] ammonium and [¹⁵N] nitrat and a GC-MS aided incubation system. The kinetic isotope method was used to evaluate the experimental data. The results are:

- Under anaerobic conditions (～ 90% of the water holding capacity = WHC) N₂O originates mainly from the nitrate pool by denitrification.

- As expected, the N₂O formation is low under aerobic conditions (～ 80% WHC) but the gas originates directly from the ammonium and not from the nitrate pool, probably as a byproduct of the nitrification process.     

Measurements of the 3ν 3 band of 14N15N16O and 15N14N16O using continuous-wave cavity ring-down spectroscopy

The absorption spectra of the 3ν₃ band of nitrous oxide isotopologues, ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O, have been measured using diode laser cavity ring-down spectroscopy in 6400–6463 and 6465–6532 cm^-1, respectively. Spectroscopic parameters and the rotational line intensities of the bands have been determined. We have applied this spectroscopic technique to the measurements of the absolute isotope ratio of those isotopologues using the absolute line intensities. PACS 32.10.Bi; 33.20.Vq; 33.70.-w; 42.55.Px; 42.62.Fi     

Tree species of the central amazon and soil moisture alter stable isotope composition of nitrogen and oxygen in nitrous oxide evolved from soil

The use of stable isotopes of N and O in N₂O has been proposed as a way to better constrain the global budget of atmospheric N₂O and to better understand the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N₂O formation in soil. This study compared the isotopic composition of N₂O emitted from soils under different tree species in the Brazilian Amazon. We also compared the effect of tree species with that of soil moisture, as we expected the latter to be the main factor regulating the proportion of nitrifier- and denitrifier-derived N₂O and, consequently, isotopic signatures of N₂O. Tree species significantly affected δ ¹⁵N in nitrous oxide. However, there was no evidence that the observed variation in δ ¹⁵N in N₂O was determined by varying proportions of nitrifier- vs. denitrifier-derived N₂O. We submit that the large variation in δ ¹⁵N-N₂O is the result of competition between denitrifying and immobilizing microorganisms for NO 3 m . In addition to altering δ ¹⁵N-N₂O, tree species affected net rates of N₂O emission from soil in laboratory incubations. These results suggest that tree species contribute to the large isotopic variation in N₂O observed in a range tropical forest soils. We found that soil water affects both ¹⁵N and ¹⁸O in N₂O, with wetter soils leading to more depleted N₂O in both ¹⁵N and ¹⁸O. This is likely caused by a shift in biological processes for ¹⁵N and possible direct exchange of ¹⁸O between H₂O and N₂O.     

Theoretical vibrational terms and rotational constants for the 15N substituted isotopologues of N2O calculated using normal hyperspherical coordinates

Variational calculations of the vibrational terms G_v and rotational constants B_v of the ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁵N¹⁵N¹⁶O isotopologues of nitrous oxide are carried out using normal hyperspherical coordinates. The Morse-cosine potential energy surface for N₂O previously determined by the authors by fitting to a set of experimental vibrational frequencies is employed. The G_v and B_v spectroscopic constants calculated for the ¹⁵N substituted isotopologues show an satisfactory agreement with those experimentally observed for a large number of vibrational bands of these isotopologues recently measured. Predicted calculated values of these spectroscopic constants for unobserved vibrational bands of the ¹⁵N substituted isotopologues are given in order to be of help in the identification and characterization of such bands, as a complement to the use of global effective Hamiltonians.     

The position dependent 15N enrichment of nitrous oxide in the stratosphere.

The position dependent 15N fractionation of nitrous oxide (N2O), which cannot be obtained from mass spectrometric analysis on molecular N2O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO+ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent 15N fractionations occur in the photolysis of N2O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N2O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for 15N14NO and 14N15NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N2O with O(1D).     

N2O concentration and isotope signature along profiles provide deeper insight into the fate of N2O in soils†

Nitrous oxide is an important greenhouse gas and its origin and fate are thus of broad interest. Most studies on emissions of nitrous oxide from soils focused on fluxes between soil and atmosphere and hence represent an integration of physical and biological processes at different depths of a soil profile. Analysis of N₂O concentration and isotope signature along soil profiles was suggested to improve the localisation of sources and sinks in soils as well as underlying processes and could therefore extend our knowledge on processes affecting surface N₂O fluxes. Such a mechanistic understanding would be desirable to improve N₂O mitigation strategies and global N₂O budgets. To investigate N₂O dynamics within soil profiles of two contrasting (semi)natural ecosystem types (a temperate acidic fen and a Norway spruce forest), soil gas samplers were constructed to meet the different requirements of a water-saturated and an unsaturated soil, respectively. The samplers were installed in three replicates and allowed soil gas sampling from six different soil depths. We analysed soil air for N₂O concentration and isotope composition and calculated N₂O net turnover using a mass balance approach and considering diffusive fluxes. At the fen site, N₂O was mainly produced in 30–50 cm soil depth. Diffusion to adjacent layers above and below indicated N₂O consumption. Values of δ¹⁵N and δ¹⁸O of N₂O in the fen soil were always linearly correlated and their qualitative changes within the profile corresponded with the calculated turnover processes, suggesting further reduction of N₂O. In the spruce forest, highest N₂O production occurred in the topsoil, but there was also notable production occurring in the subsoil at a depth of 70 cm. Changes in N₂O isotope composition as to be expected from local production and consumption processes within the soil profile did hardly occur, though. This was presumably caused by high diffusive fluxes and comparatively low net turnover, as isotope signatures approached values measured for ambient N₂O towards the topsoil. Our results demonstrate a highly variable influence of diffusive versus production/consumption processes on N₂O concentration and isotope composition, depending on the type of ecosystem. This finding indicates the necessity of further N₂O concentration and isotope profile investigations in different types of natural and anthropogenic ecosystems in order to generalise our mechanistic understanding of N₂O exchange between soil and atmosphere.     

Release of nitrous oxide and dinitrogen from a transition bog under drained and rewetted conditions due to denitrification: results from a [15N]nitrate–bromide double-tracer study

Denitrification is well known being the most important nitrate-consuming process in water-logged peat soils, whereby the intermediate compound nitrous oxide (N₂O) and the end product dinitrogen (N₂) are ultimately released. The present study was aimed at evaluating the release of these gases (due to denitrification) from a nutrient-poor transition bog ecosystem under drained and three differently rewetted conditions at the field scale using a ¹⁵N-tracer approach ([¹⁵N]nitrate application, 30?kg N ha^?1) and a common closed-chamber technique. The drained site is characterized by a constant water table (WT) of –30?cm (here referred to as D30), while rewetted sites represent a constant WT of –15?cm, a constant WT of 0?cm (i.e. waterlogged), and an initial WT of 0?cm (which decreased slightly during the experiment), respectively, (here referred to as R15, R0, and R0_d, respectively). The highest N₂O emissions were observed at D30 (291?µg N₂O–N m^?2 h^?1) as well as at R0_d (665?µg N₂O–N m^?2 h^?1). At the rewetted peat sites with a constant WT (i.e. R15 and R0), considerably lower N₂O emissions were observed (maximal 37?µg N₂O–N m^?2 h^?1). Concerning N₂ only at the initially water-logged peat site R0_d considerable release rates (up to 3110?µg N₂–N m^?2 h^?1) were observed, while under drained conditions (D30) no N₂ emission and under rewetted conditions with a constant WT (R15 and R0) significantly lower N₂ release rates (maximal 668?µg N₂–N m^–2 h^?1) could be detected. In addition, it has been found that natural WT fluctuations at rewetted peat sites, in particular a rapid drop down of the WT, can induce high emission rates for both N₂O and N₂.     

PreCon: A Fully Automated Interface for the Pre-Gc Concentration of Trace Gases on Air for Isotopic Analysis

Abstract

For the high precision isotope analysis of atmospheric trace gases a computer controlled concentration interface has been developed. From small air samples it collects either N₂O or CO₂ derived from CH₄ at their respective concentrations (0.3 ppm for N₂O, 1.7 ppm for CH₄) into a small diameter cold trap (?196°C) and interfaces via GC and open split to an isotope ratio mass spectrometer (Finnigan MAT 252) for on-line isotope evaluation. External reproducibilities for repeated measurements of 100 ml air samples from the same source of < 0.2° (δ-notation) have been achieved for ¹³C/¹²C from CH₄ and for ¹⁵N/¹⁴N and ¹⁸O/¹⁶O from N₂O. The precision is adequate to monitor the isotopic changes in these gases during a day's course.     

Sextic force field of nitrous oxide

The sextic force field in the curvilinear internal coordinates has been studied for the nitrous oxide molecule from the spectroscopic data of ¹⁴N₂¹⁶O, ¹⁴N¹⁵N¹⁶O, and ¹⁵N¹⁴N¹⁶O. The bands below 6600 cm⁻¹ have been used. The force constants in the internal coordinates are converted to those in dimensionless normal coordinates by two successive transformations. The vibration Hamiltonian matrix for each symmetry species of a given isotopic species has been constructed from the harmonic oscillator basis functions, and it is then diagonalized numerically to give the vibrational energy levels and the wavefunctions. The latter have been used for the evaluation of ratational constants. The least-squares refinement has been very successful in the present study, and it is shown that the general quartic force field supplemented by the quintic and sextic stretching diagonal force constants estimated from the Morse function, provided that the terms up to sextic are kept in the dimensionless normal coordinate space, well reproduces the spectroscopic constants such as the vibrational levels, rotational constants, l-type doubling constants, and centrifugal distortion constants. The spectroscopic constants of the isotopic molecules which are excluded from the refinement process are also in good agreement with the computed ones. The bond dissociation energies of the NN and NO bonds estimated from the present results have been critically examined.     

Vibration-rotation bands of nitrous oxide: 4.1 micron region

The infrared spectrum of nitrous oxide has been measured and analyzed from 2265 cm^?1 to 2615 cm^?1. Newly refined effective rotational constants for twenty-one vibrational states of ¹⁴N₂O, three vibrational states each of ¹⁴N₂¹⁸O and ¹⁵N¹⁴N¹⁶O, two states of ¹⁴N¹⁵N¹⁶O and one state of ¹⁴N₂¹⁷O have been calculated.The most interesting features observed are two Δ-Σ “forbidden” bands, 04^2c0-00⁰0 and 12^2c0-00⁰0. These bands occur because of Coriolis interaction between unperturbed vibrational states having l = 0 and l = 2.     

Vibration-rotation bands of 15N216O14N218O

The infrared spectrum of 64 bands of the isotopic species ¹⁵N₂¹⁶O of nitrous oxide and of 37 bands of ¹⁴N₂¹⁸O have been analyzed. The studied spectral range extends from 1750 to 6000 cm^?1 for ¹⁵N₂¹⁶O and from 1750 to 3100 cm^?1 for ¹⁴N₂¹⁸O. The effective rotational constants are given for 44 levels of ¹⁵N₂¹⁶O comprising 21Σ, 12Π, 7Δ, 4Φ levels and also for 29 levels of ¹⁴N₂¹⁸O comprising 13Σ, 7Π, 6Δ, 3Φ levels. Thirty-one levels (20Σ, 11Π) of the following isotopic species have also been studied: ¹⁵N₂¹⁷O, ¹⁵N₂¹⁸O, ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁴N¹⁸O, ¹⁴N₂¹⁷O. In ¹⁵N₂¹⁶O a local Coriolis resonance affects the 10⁰1 level. The “forbidden” Δ-Σ transition 12^2c0-00⁰0 is observed in the spectrum of ¹⁵N₂¹⁶O. The equilibrium values for the internuclear distances have been calculated.     

Development of a field-deployable method for simultaneous,real-time measurements of the four most abundant N2O isotopocules*

Understanding and quantifying the biogeochemical cycle of N₂O is essential to develop effective N₂O emission mitigation strategies. This study presents a novel, fully automated measurement technique that allows simultaneous, high-precision quantification of the four main N₂O isotopocules (¹⁴N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁸O) in ambient air. The instrumentation consists of a trace gas extractor (TREX) coupled to a quantum cascade laser absorption spectrometer, designed for autonomous operation at remote measurement sites. The main advantages this system has over its predecessors are a compact spectrometer design with improved temperature control and a more compact and powerful TREX device. The adopted TREX device enhances the flexibility of the preconcentration technique for higher adsorption volumes to target rare isotope species and lower adsorption temperatures for highly volatile substances. All system components have been integrated into a standardized instrument rack to improve portability and accessibility for maintenance. With an average sampling frequency of approximately 1 h^–1, this instrumentation achieves a repeatability of 0.09, 0.13, 0.17 and 0.12?‰ for δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O and site preference of N₂O, respectively, for pressurized ambient air. The repeatability for N₂O mole fraction measurements is better than 1 ppb (parts per billion, 10^–9 moles per mole of dry air).     

Analysis of the coexisting pathways for NO and N2O formation in Chernozem using the 15N-tracer SimKIM-Advanced model

The nitrogen (N) cycle consists of a variety of microbial processes. These processes often occur simultaneously in soils, but respond differently to local environmental conditions due to process-specific biochemical restrictions (e.g. oxygen levels). Hence, soil nitrogen cycling (e.g. soil N gas production through nitrification and denitrification) is individually affected through these processes, resulting in the complex and highly dynamic behaviour of total soil N turnover. The development and application of methods that facilitate the quantification of individual contributions of coexisting processes is a fundamental prerequisite for (i) understanding the dynamics of soil N turnover and (ii) implementing these processes in ecosystem models. To explain the unexpected results of the triplet tracer experiment (TTE) of Russow et al. (Role of nitrite and nitric oxide in the processes of nitrification and denitrification in soil: results from ¹⁵N tracer experiments. Soil Biol Biochem. 2009;41:785–795) the existing SimKIM model was extended to the SimKIM-Advanced model through the addition of three separate nitrite subpools associated with ammonia oxidation, oxidation of organic nitrogen (N_org), and denitrification, respectively. For the TTE, individual treatments with ¹⁵N ammonium, ¹⁵N nitrate, and ¹⁵N nitrite were conducted under oxic, hypoxic, and anoxic conditions, respectively, to clarify the role of nitric oxide as a denitrification intermediate during N₂O formation. Using a split nitrite pool, this analysis model explains the observed differences in the ¹⁵N enrichments in nitric oxide (NO) and nitrous oxide (N₂O) which occurred in dependence on different oxygen concentrations. The change from oxic over hypoxic to anoxic conditions only marginally increased the NO and N₂O release rates (1.3-fold). The analysis using the model revealed that, under oxic and hypoxic conditions, N_org-based N₂O production was the dominant pathway, contributing to 90 and 50 % of the total soil N₂O release. Under anoxic conditions, denitrification was the dominant process for soil N₂O release. The relative contribution of N_org to the total soil NO release was small. Ammonia oxidation served as the major pathway of soil NO release under oxic and hypoxic conditions, while denitrification was dominant under anoxic conditions. The model parameters for soil with moderate soil organic matter (SOM) content were not scalable to an additional data set for soil with higher SOM content, indicating a strong influence of SOM content on microbial N turnover. Thus, parameter estimation had to be re-calculated for these conditions, highlighting the necessity of individual soil-dependent parameter estimations.     

A frequency-modulated quantum-cascade laser for spectroscopy of CH4 and N2O isotopomers

We report the development of a novel laser spectrometer for high-sensitivity detection of methane and nitrous oxide. The system relies on a quantum-cascade laser source emitting wavelength of around 8.06 μm, where strong fundamental absorption bands occur for the considered species and their isotopomers. The detection technique is based on audio-frequency and radio-frequency modulation of laser radiation. First experimental tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of ¹³C/¹²C, ¹⁸O/¹⁶O, ¹⁷O/¹⁶O and ¹⁵N/¹⁴N isotope ratios.     

The 14N2/15N2 and 14N2/14N15N liquid-vapour isotope separation factor

A theoretical calculation of the ¹⁴N₂/¹⁵N₂ and ¹⁴N₂/¹⁴N¹⁵N liquid-vapour isotope separation factors using a diatomic, or atom-atom potential is undertaken. The results show that this potential is capable of representing the isotope separation factors with some permissible variation in the parameter set.     

Simultaneous and sequential adsorption of deuterium and nitrous oxide by rhenium

The addition of nitrous oxide to a stream of deuterium passing over a rhenium filament reduced the initial sticking probability of the latter gas from 0.24 to 0.09 when the proportion of N₂O exceeded 40%. For the addition of deuterium to nitrous oxide the equivalent figures were 0.45 and 0.30 when deuterium exceeded 30% of the gas phase. These results are attributed to a competition between the two gases for places in the precursor state on the surface. The replacement of adsorbed deuterium from a saturated layer by the oxygen atom of nitrous oxide proceeded initially with a high probability, 0.27, at room temperature and with each oxygen atom replacing one deuterium atom. However, the reaction was incomplete, about 2 × 10¹⁴ atoms cm^?2 of deuterium remaining on the surface. It is suggested that kinetic rather than thermodynamic factors are responsible for the incomplete reaction, possibly as the result of a high activation energy for the migration of deuterium atoms over an oxygenated rhenium surface.     

Rapid isotopic analysis of trace gases at atmospheric levels

A method is reported allowing simultaneous isotopic analysis of N₂ and N₂O at atmospheric concentrations. A gas chromatograph fitted with a thermal conductivity detector and micro desorption trap was interfaced to an isotope ratio mass spectrometer (Tracer Mass, Europa Scientific, Crewe, UK). CO₂ and H₂O were chemically removed prior to the complete sorption of N₂O onto a zeolite bed. N₂ was chromatographically purified prior to isotopic analysis. The N₂O was thermally desorbed into a slow carrier stream (300 μL min^?1) prior to analysis at m/z 44, 45 and 46. Precision of measurement was 0.3663 ± 0.000013 and 0.365 ± 0.0025 Atom % ¹⁵N as N₂ and N₂O respectively. The method was then evaluated for a soil denitrification experiment.

Es wird über eine Methode zur simultanen Isotopenanalyse am N₂ und N₂O bei atmosphärischer Konzentration berichtet. Ein Gaschromatograph mit Wärmeleitfähigkeitsdetektor und Mikrodesorptionsfalle wurde an ein IRM-Massenspektrometer (Tracer Mass, Europa Scientific, Crewe, GB) angekoppelt. CO₂ und H₂O wurden vor der vollständigen Adsorption des N₂O auf einem Zeolit-Bett chemisch entfernt. Das N₂ wurde vor der Isotopenanalyse chromatographisch gereinigt. Vor der Analyse bei M/Z 44, 45 und 46 wurde das N₂O thermisch desorbiert und in einen langsamen Trägergasstrom (300 μl min^?1) gebracht. Die Meßgenauigkeit betrug 0,3663 ± 0,000013 At-% ¹⁵N für N₂ und 0,365 ± 0,0025 At.-% für N₂O. Die Methode wurde dann für ein Boden-Denitrifizierungsexperiment erprobt.