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Background: The sustainability of bioenergy is strongly affected by direct field‐derived greenhouse gas (GHG) emissions and indirect emissions form land‐use change. Marginal land in low mountain ranges is suitable for feedstock production due to small impact on indirect land‐use change. However, these sites are vulnerable to high N2O emissions because of their fine soil texture and hydrology. Aims: The perennial cup plant (Silphium perfoliatum L.) might outperform silage maize (Zea mays L.) on cold, wet low mountain ranges sites regarding yield and ecosystem services. The aim of this study was to assess whether the cultivation of cup plant also provides GHG mitigation potential compared to the cultivation of maize. Methods: A t‐year field experiment was conducted in a low mountain range region in western Germany to compare area and yield‐scaled GHG emissions from cup plant and maize fields. GHG emissions were quantified using the closed chamber method. Results: Cup plant fields emitted an average of 3.6 ± 4.3 kg N2O‐N ha–1 year–1 (–85%) less than maize fields. This corresponded to 74.0 ± 94.1 g CO2‐eq kWh–1 (–78%) less emissions per produced electrical power. However, cup plant had a significantly lower productivity per hectare (–34%) and per unit of applied nitrogen (–32%) than maize. Conclusion: Cup plant as a feedstock reduces direct field‐derived GHG emissions compared to maize but, due to lower yields cup plant, likely increases emissions associated with land‐use changes. Therefore, the increased sustainability of bioenergy from biogas by replacing maize with cup plant is heavily dependent on the performance of maize at these sites and on the ecosystem services of cup plant in addition to GHG savings. [ABSTRACT FROM AUTHOR]

Sustainability of biogas production is strongly dependent on soil‐borne greenhouse gas (GHG) emissions during feedstock cultivation. Maize (Zea mays) is the most common feedstock for biogas production in Europe. Since it is an annual crop requiring high fertilizer input, maize cropping can cause high GHG emissions on sites that, due to their hydrology, have high N2O emission potential. On such sites, cultivation of cup plant (Silphium perfoliatum) as a perennial crop could be a more environmentally friendly alternative offering versatile ecosystem services. To evaluate the possible benefits of perennial cup plant cropping on GHG emissions and nitrogen losses, an incubation study was conducted with intact soil cores from a maize field and a cup plant field. The 15N gas flux method was used to quantify N source‐specific N2 and N2O fluxes. Cumulated N2O emissions and N2+N2O emissions did not differ significantly between maize and cup plant soils, but tended to be higher in maize soil. Soils from both systems exhibited relatively high and similar N2O/(N2+N2O) ratios (N2Oi). N2O emissions originating from sources other than the 15N‐labelled NO3 pool were low, but were the only fluxes exhibiting a significant difference between the maize and cup plant soils. Missing differences in fluxes derived from the 15N pool indicate that under the experimental conditions with high moisture and NO3‐ level, and without plants, the cropping system had little effect on N fluxes related to denitrification. Lower soil pH and higher bulk density in the cup plant soil are likely to have reduced the mitigation potential of perennial biomass cropping. [ABSTRACT FROM AUTHOR]

Rationale: Existing methods for the measurement of the 15N/14N isotopic composition of ammonium and nitrate are either only suitable for labelled samples or require considerable sample preparation efforts (or both). Our goal was to modify an existing analytical approach to allow for natural abundance precision levels. Methods: Published reaction protocols were used to convert ammonium into N2 by NaOBr and nitrate into N2O by TiCl3. A membrane inlet system was developed and coupled to an isotope ratio mass spectrometer to allow precise determination of the analytes. Results: Concentrations of ≥35 μmol/L N for both ammonium or nitrate could be analysed for δ15N values with precisions of better than 0.9 mUr. While ammonium analyses exhibited a small concentration dependency and an offset of 2.7 mUr at high ammonium concentrations irrespective of the standard isotopic composition, nitrate analysis showed no offset but a blank contribution visible at very low concentrations. Conclusions: The presented method is capable of fast measurement of δ15N values in ammonium and nitrate from aqueous samples with reasonable accuracy at natural abundance levels. It will thus facilitate the application of isotopic methods to studies of nitrogen cycling in ecosystems. [ABSTRACT FROM AUTHOR]

Rationale: N2O isotopomers are a useful tool to study soil N cycling processes. The reliability of such measurements requires a consistent set of international N2O isotope reference materials to improve inter-laboratory and inter-instrument comparability and avoid reporting inaccurate results. All these are the more important given the role of N2O in anthropogenic climate change and the pressing need to develop our understanding of soil N cycling and N2O emission to mitigate such emissions. Cavity ring-down spectroscopy (CRDS) could potentially overcome resource requirements and technical challenges, making N2O isotopomer measurements more feasible and less expensive than previous approaches (e.g., gas chromatography [GC] and isotope ratio mass spectrometry [IRMS]). Methods: A combined laser spectrometer and small sample isotope module (CRDS & SSIM) method enabled N2O concentration, δ15Nbulk, δ15Nα, δ15Nβ and site preference (SP) measurements of sample volumes <20 mL, such as static chamber samples. Sample dilution and isotopic mixing as well as N2O concentration dependence were corrected numerically. A two-point calibration procedure normalised d values to the international isotope-ratio scales. The CRDS & SSIM repeatability was determined using a reference gas (Ref Gas). CRDS & SSIM concentration measurements were compared with those obtained by GC, and the isotope ratio measurements from two different mass spectrometers were compared. Results: The repeatability (mean ± 1s; n = 10) of the CRDS & SSIM measurements of the Ref Gas was 710.64 ppb (± 8.64), 2.82? (± 0.91), 5.41? (± 2.00), 0.23? (± 0.22) and 5.18? (± 2.18) for N2O concentration, δ15Nbulk, δ15Nα, δ15Nβ and SP, respectively. The CRDS & SSIM concentration measurements were strongly correlated with GC (r = 0.99), and they were more precise than those obtained using GC except when the N2O concentrations exceeded the specified operating range. Normalising CRDS & SSIM d values to the international isotope-ratio scales using isotopic N2O standards (AK1 and Mix1) produced accurate results when the samples were bracketed within the range of the d values of the standards. The CRDS & SSIM δ15Nbulk and SP precision was approximately one order of magnitude less than the typical IRMS precision. Conclusions: CRDS & SSIM is a promising approach that enables N2O concentrations and isotope ratios to be measured by CRDS for samples <20 mL. The CRDS & SSIM repeatability makes this approach suitable for N2O "isotopomer mapping" to distinguish dominant source pathways, such as nitrification and denitrification, and requires less extensive lab resources than the traditionally used GC/IRMS. Current study limitations highlighted potential improvements for future users of this approach to consider, such as automation and physical removal of interfering trace gases before sample analysis. [ABSTRACT FROM AUTHOR]

Crop residue amendment in conjunction with synthetic nitrogen (N) fertilization is a common agricultural practice that increases soil fertility and crop yield. However, such a practice may also change soil denitrification process. Here, we conducted an incubation experiment with a robotized continuous flow N2 free incubation system [using helium (He) and oxygen (O2)] and measured fluxes of N2O and N2 at a high resolution in an intensively managed soil to examine the interaction effect of straw amendment and N fertilization on soil denitrification. Four treatments were set consisting of (a) a nonamended treatment (Control); (b) a Straw treatment (2 g straw kg−1 dry soil); (c) a KNO3 treatment (KNO3, 15 mM KNO3); and (d) a Straw + KNO3 treatment (2 g straw kg−1 dry soil and 15 mM KNO3). During the oxic phase (80% He plus 20% O2) of the experiment (initial 2 days), flux rates of N2O were 0.54 and 0.38 kg N/ha day−1 in the Control and KNO3 treatments, respectively. Meanwhile, straw amendment triggered N2O fluxes immediately after the onset of treatments, which was more evident in the Straw + KNO3 treatment. During the anoxic phase (100% He), both N2O and N2 emissions increased in all treatments, with the effect being more pronounced in the Straw and Straw + KNO3 treatments. In line with the observed differences in gas fluxes, the abundances of nirK, nirS, and nosZ genes increased clearly in these treatments. Overall, the mean N2O/(N2O + N2) ratio (0.69 ± 0.03) in the Control treatment was significantly lower compared to the KNO3 treatment (18.8%) and higher than the straw‐amended soils (31.9% and 17.4% compared to the Straw and Straw + KNO3 treatments, respectively). Taken together, our results suggest straw amendment significantly altered N2O and N2 fluxes by decreasing the N2O/(N2O + N2) ratio; however, the effects of straw amendment depended on soil nitrate content. [ABSTRACT FROM AUTHOR]

Rationale: Field measurement of denitrification in agricultural ecosystems using the 15N gas flux method has been limited by poor sensitivity because current isotope ratio mass spectrometry is not precise enough to detect low 15N2 fluxes in the presence of a high atmospheric N2 background. For laboratory studies, detection limits are improved by incubating soils in closed systems and under N2‐depleted atmospheres. Methods: We developed a new procedure to conduct the 15N gas flux method suitable for field application using an artificially N2‐depleted atmosphere to improve the detection limit at the given precision of mass spectrometry. Laboratory experiments with and without 15N‐labelling and using different flushing strategies were conducted to develop a suitable field method. Subsequently, this method was tested in the field and results were compared with those obtained from the conventional 15N gas flux method. Results: Results of the two methods were in close agreement showing that the denitrification rates determined were not biased by the flushing procedure. Best sensitivity for N2 + N2O fluxes was 10 ppb, which was 80‐fold better than that of the reference method. Further improvement can be achieved by lowering the N2 background concentration below the values established in the present study. Conclusions: In view of this progress in sensitivity, the new method will be suitable to measure denitrification dynamics in the field beyond peak events. [ABSTRACT FROM AUTHOR]

Rationale: Isotopic signatures of N2O can help distinguish between two sources (fertiliser N or endogenous soil N) of N2O emissions. The contribution of each source to N2O emissions after N‐application is difficult to determine. Here, isotopologue signatures of emitted N2O are used in an improved isotopic model based on Rayleigh‐type equations. Methods: The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N2O, N2 and CO2 over a 12‐day incubation period. To determine the source of N2O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2O and those from the 15N‐tracing technique. Results: The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The 15N‐enrichment analysis shows that initially ~70% of the emitted N2O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N‐pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh‐type model applied to N2O isotopocules data (δ15Nbulk‐N2O values) shows poor agreement with the measurements for the original one‐pool model for treatment 1c; the two‐pool models gives better results when using a third‐order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches. Conclusions: The importance of N2O emissions from different N‐pools in soil for the interpretation of N2O isotopocules data was demonstrated using a Rayleigh‐type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N‐pool sizes. [ABSTRACT FROM AUTHOR]

Summary: The global budget for nitrous oxide (N2O), an important greenhouse gas and probably dominant ozone‐depleting substance emitted in the 21st century, is far from being fully understood. Cycling of N2O in terrestrial ecosystems has traditionally exclusively focused on gas exchange between the soil surface (nitrification–denitrification processes) and the atmosphere. Terrestrial vegetation has not been considered in the global budget so far, even though plants are known to release N2O.Here, we report the N2O emission rates of 32 plant species from 22 different families measured under controlled laboratory conditions. Furthermore, the first isotopocule values (δ15N, δ18O and δ15Nsp) of N2O emitted from plants were determined.A robust relationship established between N2O emission and CO2 respiration rates, which did not alter significantly over a broad range of changing environmental conditions, was used to quantify plant‐derived emissions on an ecosystem scale. Stable isotope measurements (δ15N, δ18O and δ15Nsp) of N2O emitted by plants clearly show that the dual isotopocule fingerprint of plant‐derived N2O differs from that of currently known microbial or chemical processes.Our work suggests that vegetation is a natural source of N2O in the environment with a large fraction released by a hitherto unrecognized process. [ABSTRACT FROM AUTHOR]

Rationale: Despite a long history and growing interest in isotopic analyses of N2O, there is a lack of isotopically characterized N2O isotopic reference materials (standards) to enable normalization and reporting of isotope‐delta values. Here we report the isotopic characterization of two pure N2O gas reference materials, USGS51 and USGS52, which are now available for laboratory calibration ( https://isotopes.usgs.gov/lab/referencematerials.html). Methods: A total of 400 sealed borosilicate glass tubes of each N2O reference gas were prepared from a single gas filling of a high vacuum line. We demonstrated isotopic homogeneity via dual‐inlet isotope‐ratio mass spectrometry. Isotopic analyses of these reference materials were obtained from eight laboratories to evaluate interlaboratory variation and provide preliminary isotopic characterization of their δ15N, δ18O, δ15Nα, δ15Nβ and site preference (SP) values. Results: The isotopic homogeneity of both USGS51 and USGS52 was demonstrated by one‐sigma standard deviations associated with the determinations of their δ15N, δ18O, δ15Nα, δ15Nβ and SP values of 0.12 mUr or better. The one‐sigma standard deviations of SP measurements of USGS51 and USGS52 reported by eight laboratories participating in the interlaboratory comparison were 1.27 and 1.78 mUr, respectively. Conclusions: The agreement of isotope‐delta values obtained in the interlaboratory comparison was not sufficient to provide reliable accurate isotope measurement values for USGS51 and USGS52. We propose that provisional values for the isotopic composition of USGS51 and USGS52 determined at the Tokyo Institute of Technology can be adopted for normalizing and reporting sample data until further refinements are achieved through additional calibration efforts. [ABSTRACT FROM AUTHOR]

Rationale: Enhanced nitrous oxide (N2O) emissions can occur following grassland break‐up for renewal or conversion to maize cropping, but knowledge about N2O production pathways and N2O reduction to N2 is very limited. A promising tool to address this is the combination of mass spectrometric analysis of N2O isotopocules and an enhanced approach for data interpretation. Methods: The isotopocule mapping approach was applied to field data using a δ15NspN2O and δ18ON2O map to simultaneously determine N2O production pathways contribution and N2O reduction for the first time. Based on the isotopic composition of N2O produced and literature values for specific N2O pathways, it was possible to distinguish: (i) heterotrophic bacterial denitrification and/or nitrifier denitrification and (ii) nitrification and/or fungal denitrification and the contribution of N2O reduction. Results: The isotopic composition of soil‐emitted N2O largely resembled the known end‐member values for bacterial denitrification. The isotopocule mapping approach indicated different effects of N2O reduction on the isotopic composition of soil‐emitted N2O for the two soils under study. Differing N2O production pathways in different seasons were not observed, but management events and soil conditions had a significant impact on pathway contribution and N2O reduction. N2O reduction data were compared with a parallel 15N‐labelling experiment. Conclusions: The field application of the isotopocule mapping approach opens up new prospects for studying N2O production and consumption of N2O in soil simultaneously based on mass spectrometric analysis of natural abundance N2O. However, further studies are needed in order to properly validate the isotopocule mapping approach. [ABSTRACT FROM AUTHOR]

Abstract: To evaluate the risk of nitrate (NO3−‐N) in groundwater, it is necessary to know the denitrification capacity. In this study, observations were carried out for 2 years to investigate the groundwater denitrification capacity below three arable land systems in eastern China. Denitrification capacity was assessed by measuring the concentration and distribution patterns of nitrous oxide (N2O) and dinitrogen (N2) in groundwater. The results revealed that groundwater denitrification activity was high and consumed 76%, 83%, and 65% of the NO3−‐N in the vineyard (VY), vegetable field (VF), and paddy field (PF), respectively. The high denitrification activity might be attributed to the strong reducing conditions, with high dissolved oxygen concentrations in groundwater, which promotes denitrification. During the sampling period, we observed high dinitrogen (excess N2) accumulation in groundwater, which exceeded the total reactive nitrogen (N) in the deep layer. The large amount of excess N2 observed in VY and VF indicated that considerable N was stored as gaseous N2 in groundwater and should not be ignored when balancing N budgets in aquifers where denitrification is high. [ABSTRACT FROM AUTHOR]

Abstract: Nitrous oxide (N2O) emissions from agricultural land are often estimated by measuring changes in N2O concentrations over a given period in the headspace of a gas‐sampling chamber covering a specific soil area. This technique is particularly challenging in tall growing row crops such as maize (Zea mays L.), to which farmers regularly apply fertilizer banded below the seeds to ensure good crop development. Placing chambers in the inter‐row space leads to bias in flux measurements, due to exclusion of fertilized and rhizosphere soil. Chambers for N2O flux measurements should therefore be placed centered over the row. A new split chamber for gas sampling was developed in this study from a closed, rectangular chamber (original chamber: 78 cm × 78 cm, 51 cm height). The new chamber is applicable for use for the complete maize growing cycle until harvest. For each flux measurement, the two parts of the chambers are placed in a gas‐tight seal on a collar previously inserted into soil covering a representative area of land. In a later growth stage, when plant height exceeds chamber height, stalks of developed maize plants can be fixed between the two chamber parts through a rubber‐tightening opening on the top of the chamber. Air tightness of the split chamber was tested in the laboratory and the split chamber was compared with the original chamber in a field experiment with slurry injection under maize seeds. The laboratory test demonstrated similar air tightness of both chamber types. The field test yielded almost identical N2O fluxes for the original chamber (244 µg N2O‐N m−1 h−1) and the split‐chamber (254 µg N2O‐N m−1 h−1). It can be concluded that the split chamber is an adequate gas‐sampling unit, with particular advantages when flux measurements are conducted in tall growing row crops. [ABSTRACT FROM AUTHOR]

Rationale Fungal denitrifiers can contribute substantially to N2O emissions from arable soil and show a distinct site preference for N2O (SP(N2O)). This study sought to identify another process-specific isotopic tool to improve precise identification of N2O of fungal origin by mass spectrometric analysis of the N2O produced. Methods Three pure bacterial and three fungal species were incubated under denitrifying conditions in treatments with natural abundance and stable isotope labelling to analyse the N2O produced. Combining different applications of isotope ratio mass spectrometry enabled us to estimate the oxygen (O) exchange accelerated by denitrifying enzymes and the ongoing microbial pathway in parallel. This experimental set-up allowed the determination of δ18O(N2O) values and isotopic fractionation of O, as well as SP(N2O) values, as a perspective to differentiate between microbial denitrifiers. Results Oxygen exchange during N2O production was lower for bacteria than for fungi, differed between species, and depended also on incubation time. Apparent O isotopic fractionation during denitrification was in a similar range for bacteria and fungi, but application of the fractionation model indicated that different enzymes in bacteria and fungi were responsible for O exchange. This difference was associated with different isotopic fractionation for bacteria and fungi. Conclusions δ18O(N2O) values depend on isotopic fractionation and isotopic fractionation may differ between processes and organism groups. By comparing SP(N2O) values, O exchange and the isotopic signature of precursors, we propose here a novel tool for differentiating between different sources of N2O. [ABSTRACT FROM AUTHOR]

RATIONALE: Triple oxygen and nitrogen isotope ratios in nitrate are powerful tools for assessing atmospheric nitrate formation pathways and their contribution to ecosystems. N2O decomposition using microwave-induced plasma (MIP) has been used only for measurements of oxygen isotopes to date, but it is also possible to measure nitrogen isotopes during the same analytical run. METHODS: The main improvements to a previous system are (i) an automated distribution system of nitrate to the bacterial medium, (ii) N2O separation by gas chromatography before N2O decomposition using the MIP, (iii) use of a corundumtube for microwave discharge, and (iv) development of an automated system for isotopic measurements. Three nitrate standards with sample sizes of 60, 80, 100, and 120 nmol were measured to investigate the sample size dependence of the isotope measurements. RESULTS: The δ17O, δ18O, and δ17O values increased with increasing sample size, although the δ15N value showed no significant size dependency. Different calibration slopes and intercepts were obtained with different sample amounts. The slopes and intercepts for the regression lines in different sample amounts were dependent on sample size, indicating that the extent of oxygen exchange is also dependent on sample size. The sample-size-dependent slopes and intercepts were fitted using natural log (ln) regression curves, and the slopes and intercepts can be estimated to apply to any sample size corrections. When using 100 nmol samples, the standard deviations of residuals from the regression lines for this system were 0.5‰, 0.3‰, and 0.1‰, respectively, for the δ18O, δ17O, and δ15N values, results that are not inferior to those from other systems using gold tube or gold wire. CONCLUSIONS: An automated system was developed to measure triple oxygen and nitrogen isotopes in nitrate using N2O decomposition by MIP. This system enables us to measure both triple oxygen and nitrogen isotopes in nitrate with comparable precision and sample throughput (23 min per sample on average), and minimal manual treatment. [ABSTRACT FROM AUTHOR]

RATIONALE: The gaseous N losses mediated by soil denitrifiers are generally inferred by measuring N2O fluxes, but should include associated N2 emissions, which may be affected by abiotic soil characteristics and biotic interactions. Soil fauna, particularly anecic earthworms and euedaphic collembola, alter the activity of denitrifiers, creating hotspots for denitrification. These soil fauna are abundant in perennial agroecosystems intended to contribute to more sustainable production of bioenergy. METHODS: Two microcosm experiments were designed to evaluate gaseous N emissions from a silty loam and a sandy soil, both provided with litter from the bioenergy crop Silphium perfoliatum (cup-plant) and inoculated with an anecic earthworm (Lumbricus terrestris), which was added alone or together with an euedaphic collembola (Folsomia candida). In experiment 1, litter-derived N flux was determined by adding 15N-labelled litter, followed by mass spectrometric analysis of N2 and N2O isotopologues. In experiment 2, the δ18O values and 15N site preference of N2O were determined by isotope ratio mass spectrometry to reveal underlying N2O formation pathways. RESULTS: Lumbricus terrestris significantly increased litter-derived N2 emissions in the loamy soil, from 174.5 to 1019.3 μg N2-N kg-1 soil, but not in the sandy soil (non-significant change from 944.7 to 1054.7 μg N2-N kg-1 soil). Earthworm feeding on plant litter resulted in elevated N2O emissions in both soils, derived mainly from turnover of the soil mineral N pool during denitrification. Folsomia candida did not affect N losses but showed a tendency to redirect N2O formation pathways from fungal to bacterial denitrification. The N2O/(N2+N2O) product ratio was predominantly affected by abiotic soil characteristics (loamy soil: 0.14, sandy soil: 0.26). CONCLUSIONS: When feeding on S. perfoliatum litter, the anecic L. terrestris, but not the euedaphic F. candida, has the potential to cause substantial N losses. Biotic interactions between the species are not influential, but abiotic soil characteristics have an effect. The coarse-textured sandy soil had lower gaseous N losses attributable to anecic earthworms. [ABSTRACT FROM AUTHOR]

Subsoils play an important role within the global C cycle, since they have high soil organic carbon (SOC) storage capacity due to generally low SOC concentrations. However, measures for enhancing SOC storage commonly focus on topsoils. This study assessed the long-term storage and stability of SOC in topsoils buried in arable subsoils by deep ploughing, a globally applied method for breaking up hard pans and improving soil structure to optimize crop growing conditions. One effect of deep ploughing is translocation of SOC formed near the surface into the subsoil, with concomitant mixing of SOC-poor subsoil material into the 'new' topsoil. Deep-ploughed croplands represent unique long-term in situ incubations of SOC-rich material in subsoils. In this study, we sampled five loamy and five sandy soils that were ploughed to 55-90 cm depth 35-50 years ago. Adjacent, similarly managed but conventionally ploughed subplots were sampled as reference. The deep-ploughed soils contained on average 42 ± 13% more SOC than the reference subplots. On average, 45 years after deep ploughing, the 'new' topsoil still contained 15% less SOC than the reference topsoil, indicating long-term SOC accumulation potential in the topsoil. In vitro incubation experiments on the buried sandy soils revealed 63 ± 6% lower potential SOC mineralisation rates and also 67 ± 2% lower SOC mineralisation per unit SOC in the buried topsoils than in the reference topsoils. Wider C/N ratio in the buried sandy topsoils than in the reference topsoils indicates that deep ploughing preserved SOC. The SOC mineralisation per unit SOC in the buried loamy topsoils was not significantly different from that in the reference topsoils. However, 56 ± 4% of the initial SOC was preserved in the buried topsoils. It can be concluded that deep ploughing contributes to SOC sequestration by enlarging the storage space for SOC-rich material. [ABSTRACT FROM AUTHOR]

Rationale The oxygen isotope anomaly, Δ17O, of N2O and nitrate is useful to elucidate nitrogen oxide dynamics. A comparison of different methods for Δ17O measurement was performed. Methods For Δ17O measurements, N2O was converted into O2 and N2 using microwave-induced plasma in a quartz or corundum tube reactor, respectively, or conversion was carried out in a gold wire oven. In each case, isotope ratios were measured by isotope ratio mass spectrometry. Results All the tested methods showed acceptable precision (coefficient of variation <2.4 % at 160 nmol N2O) with high sample size but the sample size dependence was lowest when using microwave-induced plasma in a corundum tube reactor. Conclusions The use of microwave-induced plasma in a corundum tube yields best results for Δ17O measurement on N2O gas samples. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We applied a 15N dilution technique called "Integrated Total Nitrogen Input" (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7 mg N pot -1) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2 mg N pot -1. Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55 kg N ha -1 yr -1. Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5-10 kg N ha -1 yr -1. [ABSTRACT FROM AUTHOR]

RATIONALE This study aimed (i) to determine the isotopic fractionation factors associated with N2O production and reduction during soil denitrification and (ii) to help specify the factors controlling the magnitude of the isotope effects. For the first time the isotope effects of denitrification were determined in an experiment under oxic atmosphere and using a novel approach where N2O production and reduction occurred simultaneously. METHODS Soil incubations were performed under a He/O2 atmosphere and the denitrification product ratio [N2O/(N2 + N2O)] was determined by direct measurement of N2 and N2O fluxes. N2O isotopocules were analyzed by mass spectrometry to determine δ18O, δ15N and 15 N site preference within the linear N2O molecule (SP). An isotopic model was applied for the simultaneous determination of net isotope effects (η) of both N2O production and reduction, taking into account emissions from two distinct soil pools. RESULTS A clear relationship was observed between 15 N and 18O isotope effects during N2O production and denitrification rates. For N2O reduction, diverse isotope effects were observed for the two distinct soil pools characterized by different product ratios. For moderate product ratios (from 0.1 to 1.0) the range of isotope effects given by previous studies was confirmed and refined, whereas for very low product ratios (below 0.1) the net isotope effects were much smaller. CONCLUSIONS The fractionation factors associated with denitrification, determined under oxic incubation, are similar to the factors previously determined under anoxic conditions, hence potentially applicable for field studies. However, it was shown that the η18O/η15N ratios, previously accepted as typical for N2O reduction processes (i.e., higher than 2), are not valid for all conditions. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE In recent years, research and applications of the N2O site-specific nitrogen isotope composition have advanced, reflecting awareness of the contribution of N2O to the anthropogenic greenhouse effect, and leading to significant progress in instrument development. Further dissemination of N2O isotopomer analysis, however, is hampered by a lack of internationally agreed gaseous N2O reference materials and an uncertain compatibility of different laboratories and analytical techniques. METHODS In a first comparison approach, eleven laboratories were each provided with N2O at tropospheric mole fractions (target gas T) and two reference gases (REF1 and REF2). The laboratories analysed all gases, applying their specific analytical routines. Compatibility of laboratories was assessed based on N2O isotopocule data for T, REF1 and REF2. Results for T were then standardised using REF1 and REF2 to evaluate the potential of N2O reference materials for improving compatibility between laboratories. RESULTS Compatibility between laboratories depended on the analytical technique: isotope ratio mass spectrometry (IRMS) results showed better compatibility for δ15N values, while the performance of laser spectroscopy was superior with respect to N2O site preference. This comparison, however, is restricted by the small number of participating laboratories applying laser spectroscopy. Offset and two-point calibration correction of the N2O isotopomer data significantly improved the consistency of position-dependent nitrogen isotope data while the effect on δ15N values was only minor. CONCLUSIONS The study reveals that for future research on N2O isotopocules, standardisation against N2O reference material is essential to improve interlaboratory compatibility. For atmospheric monitoring activities, we suggest N2O in whole air as a unifying scale anchor. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE The contribution of fungal denitrification to the emission of the greenhouse gas nitrous oxide (N2O) from soil has not yet been sufficiently investigated. The intramolecular 15N site preference (SP) of N2O could provide a tool to distinguish between N2O produced by bacteria or fungi, since in previous studies fungi exhibited much higher SP values than bacteria. METHODS To further constrain isotopic evidence of fungal denitrification, we incubated six soil fungal strains under denitrifying conditions, with either NO3− or NO2− as the electron acceptor, and measured the isotopic signature (δ18O, δ15Nbulk and SP values) of the N2O produced. The nitrogen isotopic fractionation was calculated and the oxygen isotope exchange associated with particular fungal enzymes was estimated. RESULTS Five fungi of the order Hypocreales produced N2O with a SP of 35.1 ± 1.7 ‰ after 7 days of anaerobic incubation independent of the electron acceptor, whereas one Sordariales species produced N2O from NO2− only, with a SP value of 21.9 ± 1.4 ‰. Smaller isotope effects of 15Nbulk were associated with larger N2O production. The δ18O values were influenced by oxygen exchange between water and denitrification intermediates, which occurred primarily at the nitrite reduction step. CONCLUSIONS Our results confirm that SP of N2O is a promising tool to differentiate between fungal and bacterial N2O from denitrification. Modelling of oxygen isotope fractionation processes indicated that the contribution of the NO2− and NO reduction steps to the total oxygen exchange differed among the various fungal species studied. However, more information is needed about different biological orders of fungi as they may differ in denitrification enzymes and consequently in the SP and δ18O values of the N2O produced. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE Fungi can contribute greatly to N2O production from denitrification. Therefore, it is important to quantify the isotopic signature of fungal N2O. The isotopic composition of N2O can be used to identify and analyze the processes of N2O production and N2O reduction. In contrast to bacteria, information about the oxygen exchange between denitrification intermediates and water during fungal denitrification is lacking, impeding the explanatory power of stable isotope methods. METHODS Six fungal species were anaerobically incubated with the electron acceptors nitrate or nitrite and 18O-labeled water to determine the oxygen exchange between denitrification intermediates and water. After seven days of incubation, gas samples were analyzed for N2O isotopologues by isotope ratio mass spectrometry. RESULTS All the fungal species produced N2O. N2O production was greater when nitrite was the sole electron acceptor (129 to 6558 nmol N2O g dw-1 h-1) than when nitrate was the electron acceptor (6 to 47 nmol N2O g dw-1 h-1). Oxygen exchange was complete with nitrate as electron acceptor in one of five fungi and with nitrite in two of six fungi. Oxygen exchange of the other fungi varied (41 to 89 % with nitrite and 11 to 61 % with nitrate). CONCLUSIONS This is the first report on oxygen exchange with water during fungal denitrification. The exchange appears to be within the range previously reported for bacterial denitrification. This adds to the difficulty of differentiating N2O producing processes based on the origin of N2O-O. However, the large oxygen exchange repeatedly observed for bacteria and now also fungi could lead to less variability in the δ18O values of N2O from soils, which could facilitate the assessment of the extent of N2O reduction. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE N2O isotopomer ratios may provide a useful tool for studying N2O source processes in soils and may also help estimating N2O reduction to N2. However, remaining uncertainties about different processes and their characteristic isotope effects still hamper its application. We conducted two laboratory incubation experiments (i) to compare the denitrification potential and N2O/(N2O+N2) product ratio of denitrification of various soil types from Northern Germany, and (ii) to investigate the effect of N2O reduction on the intramolecular 15N distribution of emitted N2O. METHODS Three contrasting soils (clay, loamy, and sandy soil) were amended with nitrate solution and incubated under N2-free He atmosphere in a fully automated incubation system over 9 or 28 days in two experiments. N2O, N2, and CO2 release was quantified by online gas chromatography. In addition, the N2O isotopomer ratios were determined by isotope-ratio mass spectrometry (IRMS) and the net enrichment factors of the 15N site preference (SP) of the N2O-to-N2 reduction step (ηSP) were estimated using a Rayleigh model. RESULTS The total denitrification rate was highest in clay soil and lowest in sandy soil. Surprisingly, the N2O/(N2O+N2) product ratio in clay and loam soil was identical; however, it was significantly lower in sandy soil. The IRMS measurements revealed highest N2O SP values in clay soil and lowest SP values in sandy soil. The ηSP values of N2O reduction were between -8.2 and -6.1 ‰, and a significant relationship between δ18O and SP values was found. CONCLUSIONS Both experiments showed that the N2O/(N2O+N2) product ratio of denitrification is not solely controlled by the available carbon content of the soil or by the denitrification rate. Differences in N2O SP values could not be explained by variations in N2O reduction between soils, but rather originate from other processes involved in denitrification. The linear δ18O vs SP relationship may be indicative for N2O reduction; however, it deviates significantly from the findings of previous studies. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE The oxygen isotope anomaly, Δ17O, of N2O and nitrate is useful to elucidate nitrogen oxide dynamics. The previously developed method for Δ17O measurement presents difficulty in maintaining optimal conditions of the gold tube for thermal decomposition of N2O to O2 and the Δ17O value is also sample size dependent. METHODS Trace amounts (5-40 nmol) of N2O were decomposed quantitatively to O2 in a quartz tube by microwave discharge. The O2 was purified using gas chromatography. Triple oxygen isotopes were measured using isotope ratio mass spectrometry. Each step was connected online and was applied to the analysis of nitrate in precipitation samples collected in Yokohama, Japan. RESULTS Precision (1σ) of Δ17O analysis was better than 0.26‰ when more than 20 nmol of N2O with a small Δ17O value (approx. 1‰) was measured. It was better than 0.76‰ when more than 60 nmol of nitrate was converted into N2O using the denitrifier method and then measured on the developed system. The obtained Δ17O values in precipitation samples (14.5-26.4‰) agreed with findings from previous studies. CONCLUSIONS A novel on-line analytical method was developed to measure the triple oxygen isotopes of N2O using microwave discharge to decompose N2O. This easy-to-use method is free from conditioning of reaction devices, and is applicable to molecules other than N2O such as NO and NO2. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE An enhanced analytical approach for analyzing gaseous products from 15N-enriched pools has been developed. This technique can be used to quantify nitrous oxide (N2O) and dinitrogen (N2) fluxes from denitrification. It can also help in distinguishing different N2- and N2O-forming processes, such as denitrification, nitrification, anaerobic ammonium oxidation or co-denitrification. METHODS The measurement instrumentation was based on a commercially available automatic preparation system allowing collection and separation of gaseous samples. The sample transfer paths, valves, liquid nitrogen traps, gas chromatography column and open split of the original system were modified. A reduction oven (Cu) was added in order to eliminate oxygen and measure N2O-N as N2. Gases leaving the separation system entered an isotope ratio mass spectrometer where masses 28N2, 29N2 and 30N2 were measured. RESULTS The enhanced technique enabled rapid simultaneous measurement of stable isotope ratios 29N2/28N2 and 30N2/28N2 originating from dinitrogen alone (N2) and from the sum of the denitrification products (N2+N2O) as well as the determination of 15N enrichment in N2O. The 15N fraction in the N pool undergoing N2 and N2O production (15 XN) and the contribution of N2 and N2O originating from this pool ( d) were determined with satisfactory accuracy of better than 3.3% and 2.9%, respectively. CONCLUSIONS The precision and accuracy of this method were comparable with or better than previously reported for similar measurements. The proposed method allows for the analysis of all quantities within one run, thus reducing the measurement and sample preparation time as well as increasing the reliability of the results. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE Nitrous oxide (N2O), a highly climate-relevant trace gas, is mainly derived from microbial denitrification and nitrification processes in soils. Apportioning N2O to these source processes is a challenging task, but better understanding of the processes is required to improve mitigation strategies. The N2O site-specific 15 N signatures from denitrification and nitrification have been shown to be clearly different, making this signature a potential tool for N2O source identification. We have applied for the first time quantum cascade laser absorption spectroscopy (QCLAS) for the continuous analysis of the intramolecular 15 N distribution of soil-derived N2O and compared this with state-of-the-art isotope ratio mass spectrometry (IRMS). METHODS Soil was amended with nitrate and sucrose and incubated in a laboratory setup. The N2O release was quantified by FTIR spectroscopy, while the N2O intramolecular 15 N distribution was continuously analyzed by online QCLAS at 1 Hz resolution. The QCLAS results on time-integrating flask samples were compared with those from the IRMS analysis. RESULTS The analytical precision (2σ) of QCLAS was around 0.3 ‰ for the δ15Nbulk and the 15 N site preference (SP) for 1-min average values. Comparing the two techniques on flask samples, excellent agreement (R2 = 0.99; offset of 1.2 ‰) was observed for the δ15Nbulk values while for the SP values the correlation was less good (R2 = 0.76; offset of 0.9 ‰), presumably due to the lower precision of the IRMS SP measurements. CONCLUSIONS These findings validate QCLAS as a viable alternative technique with even higher precision than state-of-the-art IRMS. Thus, laser spectroscopy has the potential to contribute significantly to a better understanding of N turnover in soils, which is crucial for advancing strategies to mitigate emissions of this efficient greenhouse gas. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article presents a study which investigates the indirect nitrous oxide emissions from a shallow aquifer in Germany over a one-year period. The study investigated the vertical nitrous oxide emissions by measuring nitrous oxide concentrations and calculating fluxes from the surface ground water to the unsaturated zone and at the soil surface. Lateral nitrous oxide was also examined through measuring ground water nitrous oxide and nitrogen trioxide concentrations at five multilevel wells and at a waterworks well. The study found that the Intergovernmental Panel on Climate Change (IPCC) emission factor of 0.0025 kilograms Nitrous oxide-nitrogen per kilograms of nitrogen might be very high for reducing the Fuhrberger Feld Aquifer (FFA).

Examines the use of in situ measurement of subsoil denitrification. Principle of nitrogen and nitrate gas production from denitrification of added nitrate; Identification of soil physical parameters; Impact of model parameters on steady-state concentrations.

Examines a combination probe constructed to detect denitrification in the vadose zone. Problems found in investigations of subsoil denitrification; Reason for the design of the probe; Collection of nitrogen gas diffusing.

A letter to the editor is presented in response to the article "A test of a field-based 15N-nitrous oxide pool dilution technique to measure gross N2O production in soil" by Yang et al.

Background Aims Methods Results Conclusion Incorporation of cover crop (cc) shoot and root biomass can have different effects on nitrogen (N) dynamics and the transformation of soil‐derived N and cc N.The objective was to determine the effects of different ccs, cc compartments (roots and shoots), and pretreatment of cc biomass (fresh vs. dried) on mineralization processes and on the transformation of soil and cc N following incorporation into a silty loam soil.Soil columns with incorporated 15N‐labeled root and shoot biomass of two cc species (winter rye and oil radish) and different pretreatments (dried and fresh) were incubated for 70 days at a constant temperature and soil moisture (8°C, 40% water‐filled pore space). Carbon and N transformation dynamics were determined repeatedly, distinguishing between N originating from cc biomass and from soil.Net CO2 emission was related to the amount of soluble cell components added with ccs. Net N mineralization was negatively related to the C:N ratio of cc biomass. The incorporation of dried cc biomass caused higher initial soil respiration and N immobilization than fresh biomass. All treatments with cc incorporation showed increased N2O emission. Emitted N2O‐N consisted mainly of cc N (55%–57%) in treatments with fresh shoot biomass, whereas soil N was the main source of N2O (75%) in the treatment with fresh oil radish roots. Recovery of cc 15N was affected by crop compartment and pretreatment. At the end of the incubation, it was 17.5%–42.3% in soil NO3−, 0.1%–8.1% in microbial biomass N, and less than 0.23% of cc N was found in cumulative N2O emission.The incorporation of cc roots and shoots had different effects on N mobilization and immobilization processes and on the partitioning of cc N. These processes can be influenced significantly by pretreatment of the added plant biomass (dried vs. fresh). [ABSTRACT FROM AUTHOR]