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.     

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.     

A new 15N tracer method to determine N turnover and denitrification of Pseudomonas stutzeri

Although denitrification is one of the key processes of ecosystem N turnover, the understanding of the regulation of the denitrification pathway is still limited due to the lack of feasible methods for the quantification of N₂ formation. Based on the previously developed isotope pairing method, we present a new in vitro ¹⁵N tracer method for the quantification of N₂ released from denitrification by bacterial cultures. The application of the new method was enabled by replacing the background air in the sample flasks with a gas mixture of He and O₂ with an approximately 50-fold reduced N₂ background (1.7% v/v), allowing for a direct and sensitive quantification of N₂ formation with isotope-ratio mass spectrometry after ¹⁵N-labelling on the one hand, but leaving the method relatively insensitive to intrusion of ambient N₂ on the other hand. The method was tested on bacterial cultures of Pseudomonas stutzeri grown at different oxygen levels. Additionally, NO and N₂O formation were determined with a chemoluminescence analyser and a gas chromatograph, respectively. Following labelling with ¹⁵N-ammonium and ¹⁵N-nitrate, it could be shown that P. stutzeri used ammonium preferably for biomass build-up, and nitrate preferably as electron acceptor. Between 84–107% of the total available N could be recovered. Due to the high sensitivity of the new method only low levels of ¹⁵N tracer were necessary, minimising substrate-induced effects and making this method also an appropriate tool for the use on soil cores. By that it offers a new method for studying denitrification in terrestrial ecosystems.     

N2O emissions and source processes in snow-covered soils in the Swiss Alps

Nitrous oxide (N₂O) emissions from snow-covered soils represent a significant fraction of the annual flux from alpine, subalpine or cold-temperate regions. In winter 2010–2011, we investigated the temporal variability of N₂O emissions and source processes from a subalpine valley in the Swiss Alps. The study included regular measurements of N₂O snow profiles at a fixed location and an intensive sampling campaign along a transversal cut through the valley with grassland at the bottom and coniferous forest at the slopes. During the intensive campaign, recently developed laser spectroscopy was employed for high-precision N₂O isotopomer analysis. Maximum N₂O fluxes (0.77±0.64 nmol m^?2 h^?1) were found for periods with elevated air temperature and, in contrast to our expectations, were higher from forest than from grassland in mid-February. At maximum snow height (63 cm) the main N₂O source processes were heterotrophic denitrification and nitrifier denitrification. The reduction of N₂O by heterotrophic denitrifiers was much more pronounced for the grassland compared with the forest soil, as indicated by the ¹⁵N site preferences of 16.4±11.5 ‰ (grassland) and?1.6±2.1 ‰ (forest). This illustrates the potential of laser spectroscopic N₂O isotopomer analysis for the identification of source processes even at low emission rates in nutrient poor ecosystems.     

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.     

Bestimmung der 15N Häufigkeit bei nichtstatistischer 15N-verteilung in N2 sowie bei N2O in Bodenluftproben mittels GC-R-IRMS-Kopplung in einem Probenlauf

Abstract

An analysis technique based on GC-R-IRMS coupling (Gas-Chromatography-Reduction-Isotope Ratio Mass Spectrometry) is demonstrated. The ¹⁵N abundance of N² and N²O in atmospheric air or soil atmosphere from nitrification or denitrification processes with nonrandom distribution is determined in one run. The 12 ml sample is separated from CO² and transported by a helium gas stream through a cooling trap. The N²O is trapped in the cooling trap while the N² passes through it and enters the GC. After GC separation and O² removal in a reduction column, part of the N² enters an isotope mass spectrometer to determine the masses m/z 28,29 and 30. The interferences on mass 30 by the formation of NO in the ion source of the mass spectrometer are eliminated by a calibration and a correction procedure. Upon removing the cooling trap, the N²O is injected into the GC, where it is separated and then quantitatively reduced to N² in a reduction column. The measurement of one sample takes 16 minutes. The detection limit of the ³⁰R_t in alteration N₂ is Δ³⁰R_t = 5 · 10^?7. The detection limit of the N₂O is 3.6 nl.     

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.     

Equilibrating of 15N-Gases by Electrodeless Discharge: A Method of Indirect Mass Spectrometric Analysis of 30N2 for Denitrification Studies in Soils

Abstract

The estimation of denitrification in soil by the ¹⁵N tracer technique includes isotope analysis of gas samples with a nonrandom distribution of the N₂ mole masses of 28, 29 and 30. In that case the emission of total ¹⁵N is underestimated by calculating ¹⁵N atom fractions from the ²⁹N₂/²⁸N₂ ratio if ³⁰N₂ is not considered. ³⁰N₂ can be measured indirectly in N₂ enriched with ¹⁵N with nonrandom distribution of mole masses by mass spectrometric analysis. The nitrogen fraction of gas samples was transferred to discharge tubes. Microwaves (60 sec) generated an electrodeless discharge of the gas which caused a temporary split-up of N₂ molecules and thus established an equilibrium distribution of the mole masses. The ²⁹N₂/²⁸N₂ ratio was measured in equilibrated and in untreated samples to calculate the real emission of ¹⁵N. The measurements of ¹⁵N standard gases by this method satisfactorily coincided with calculated values for ¹⁵N atom fraction above a concentration of 50 δ‰.     

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).     

15N tracing model SimKIM to analyse the NO and N2O production during autotrophic, heterotrophic nitrification, and denitrification in soils

An adjusted model was developed to analyse measured data of nitrous oxide and nitric oxide fluxes from an arable black earth soil. The existing models for kinetic isotope studies ignore N-trace gas fluxes. The novel model includes both N-gas production by heterotrophic and autotrophic nitrification and N-gas production and consumption by denitrification. Nitrous oxide and nitric oxide production through nitrification was simulated following the 'hole-in-the-pipe' model (4: M.K. Firestone et al. Microbiological basis of NO and N2O production and consumption in soil), N-gas production by denitrification was described with first-order kinetics.The model has been evaluated in a triplicate laboratory experiment, which involved three treatments (glycine, , or -pool labeled) to distinguish the different sources of N2O and NO. Heterotrophic nitrification was negligible, whereas autotrophic nitrification and denitrification occur simultaneously in soils. Nitrification was the main source of NO and N2O in the black earth soil by field capacity (water content: 0.22 g H2O g(-1) soil). The NO release was higher than the N2O release, the N2O/NO ratio was 0.05 in this soil.     

Determination of N and O – Atom and N2(A) Metastable Molecule Densities in the Afterglows of N2 and N2‐O2 Microwave Discharges

Early afterglows of N₂ and N₂‐O₂ flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atom and N₂(A) metastable molecule densities are determined by optical emission spectroscopy after calibration by NO titration for N‐atoms and measurements of NO and N₂ band intensities for O‐atoms and N₂(A) metastable molecules. By using N₂ tanks with 50 and 10 ppm impurity, it is determined in the afterglow an O‐ atom impurity of 150‐200 ppm. Variations of the N and O‐atom and N₂(A) metastable molecule densities are obtained in the early afterglow of N₂–(9·10^–5–3·10^–3)O₂ gas mixtures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The role of methylene in prompt NO formation

We address the plausibility of singlet methylene (¹CH₂) in the prompt NO formation mechanism via examination of experimental species profiles and kinetic flame modeling of several low-pressure methane-oxygen-nitrogen flames. Existing kinetic models assuming CH as the only prompt NO precursor greatly underpredict NO formation under very fuel-lean conditions. We have constructed a kinetic pathway initiated by the recombination of singlet CH₂ with molecular nitrogen to form diazomethane, CH₂NN, early in the flame. Although the majority of the diazomethane is predicted to react with flame radicals to regenerate N₂, a small percentage (approximately 10%) is predicted to react via cleavage of the NN bond leading to NO formation. This leads to accurate prediction of the experimental measurements of NO formation in lean, low-pressure flames. Assuming reasonable kinetic parameters for the reactions of CH₂, the large underprediction of NO under lean conditions can be rectified by the inclusion of the ¹CH₂ prompt NO pathway in the kinetic mechanism.     

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.     

Adsorption of NO and N2O on Fe-BEA and H-BEA zeolites

FT-IR (Fourier-transform infrared) spectroscopy and density function theory (DFT) methods have been applied to the investigation of the interaction of NO and N₂O with Fe³⁺ species in a beta zeolite (BEA). The geometries for H-BEA and Fe-BEA represented as 10T cluster, and NO and N₂O adsorption on them in η¹-O and η¹-N modes have been completely optimized. The results show that NOx could be adsorbed on Fe³⁺ species and Brønsted acid sites in two modes, but NOx is mainly bonded by N to H or Fe atom and the iron site is preferred. NOx adsorbed on Fe³⁺ species is more stable than on Brønsted acid sites. Adsorption energies for N₂O and NO follow the order of NO > N₂O, predicating that the affinity of NO molecule on BEA zeolite is much stronger than N₂O molecule on BEA zeolite.     

Nitrogen-15 Fractionation in the Thermal Decomposition of Nitrous Oxide of Natural Isotopic Composition

Abstract

The ¹⁵N fractionation in the thermal decompostion of nitrous oxide (N₂O) of natural isotopic composition has been investigated in quartz reaction vessel in the temperature interval 888–1073K. The formulas relating the observed experimentally ¹⁵N fractionations with the primary ¹⁵N kinetic isotope effect, (k ¹⁴/k ¹⁵)_p for ¹⁴N¹⁵N¹⁶O, and secondary ¹⁵N kinetic isotope effect, (k ¹⁴/k ¹⁵)_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, (k ¹⁴/k ¹⁵)_p, in the temperature interval 888–1007K. But at 1073K the decompositions of N₂O, accompanied by NO (nitric oxide) formation proceed with a twice times smaller primary kinetic isotope effect, (k ¹⁴/k ¹⁵)_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 chemisorption and decomposition of NO on the (110) surface of iridium

The chemisorption of NO on Ir(110) has been studied with thermal desorption mass spectrometry (including isotopic exchange experiments), X-ray and UV-photoelectron spectroscopies, Auger electron spectroscopy,LEED and CPD measurements. Chemisorption of NO proceeds by precursor kinetics with the initial probability of adsorption equal to unity independent of surface temperature. Saturation coverage of molecular NO corresponds to 9.6 × 10¹⁴ cm^?2 below 300 K. Approximately 35% of the saturated layer desorbs as NO in two well separated features of equal integrated intensity in the thermal desorption spectra. The balance of the NO desorbs as N₂ and O₂ with desorption of N₂ beginning after the low-temperature peak of NO has desorbed almost completely. Molecular NO desorbs with activation energies of 23.4–28.9 and 32.5–40.1 kcal mole^?1, assuming the preexponential factor for both processes is between 10¹³–10¹⁶ s^?1. At low coverages of NO, N₂ desorbs with an activation energy of 36–45 kcal mole^?1, assuming the preexponential factor is between 10^?2 and 10 cm²s^?1. Levels at 13.5, 10.4 and 8.5 eV below the Fermi level are observed with HeI UPS, associated with the 4σ, 5σ and 1π orbitals of NO, respectively. Core levels of NO appear at 531.5 eV [O(1s)] and 400.2 eV [N(1s)], and do not shift in the presence of oxygen. Oxygen overlayers tend to stabilize chemisorbed NO as reflected in thermal desorption spectra and a downshift in the 1π level to 9.5 eV.     

Accuracy and Precision for Measurements of the Mass Ratio 30/28 in Dinitrogen from Air Samples and its Application to the Investigation of N Losses from Soil by Denitrification

Abstract

In the 1950s Hauck introduced a special version of the ¹⁵N dilution technique (¹⁵N flux method) for the determination of N losses from the soil by denitrification. Although this method is very useful and reliable its application has been rather infrequent up to now. This is mainly due to the need to measure the m/z 30 in addition to the usually measured m/z 28 and 29 for dinitrogen, because the ¹⁵N in the enriched air sample taken from an enclosure (cover box) at the soil surface is nonrandom. The signal from the m/z 30 is very low and difficult to measure with sufficient precision because other species (e.g. NO) also having the m/z 30 often interfere with its measurement. In this study the accuracy and precision of an easy to use CF-IRMS with sample batch operation to measure the ratio 30/28 was investigated. The relative standard deviation (RSD = precision) from natural abundance up to 2 at.% was always <1%. After correction of the mass ratio 30/28 (R30), by means of a formula obtained by linear regression of theoretical R30 against measured R30, the accuracy of the abundance calculated from this corrected R30 was very high. From the achieved precision and assuming a cover box height of 10 cm (headspace volume of 7 1), and a collection time of 2 h, a limit of detection for N₂ losses by denitrification equivalent to 16 g N/ha*d or 6 kg N/ha*a can be estimated. The performance of the ¹⁵N dilution method using the equipment and procedure described is demonstrated by means of results from an incubation experiment with [¹⁵N]nitrate-amended soils.     

N2O molecular tagging velocimetry

A new seeded velocity measurement technique, N₂O molecular tagging velocimetry (MTV), is developed to measure velocity in wind tunnels by photochemically creating an NO tag line. Nitrous oxide “laughing gas” is seeded into the air flow. A 193 nm ArF excimer laser dissociates the N₂O to O(¹D) that subsequently reacts with N₂O to form NO. O₂ fluorescence induced by the ArF laser “writes” the original position of the NO line. After a time delay, the shifted NO line is “read” by a 226-nm laser sheet and the velocity is determined by time-of-flight. At standard atmospheric conditions with 4% N₂O in air, ∼1000 ppm of NO is photochemically created in an air jet based on experiment and simulation. Chemical kinetic simulations predict 800–1200 ppm of NO for 190–750 K at 1 atm and 850–1000 ppm of NO for 0.25–1 atm at 190 K. Decreasing the gas pressure (or increasing the temperature) increases the NO ppm level. The presence of humid air has no significant effect on NO formation. The very short NO formation time (<10 ns) makes the N₂O MTV method amenable to low- and high-speed air flow measurements. The N₂O MTV technique is demonstrated in air jet to measure its velocity profile. The N₂O MTV method should work in other gas flows as well (e.g., helium) since the NO tag line is created by chemical reaction of N₂O with O(¹D) from N₂O photodissociation and thus does not depend on the bulk gas composition.