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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]

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]

Known as biogeochemical hotspots in landscapes, riparian buffer zones exhibit considerable potential concerning mitigation of groundwater contaminants such as nitrate, but may in return enhance the risk for indirect N2O emission. Here we aim to assess and to compare two riparian gray alder forests in terms of gaseous N2O and N2 fluxes and dissolved N2O, N2, and NO3- in the near-surface groundwater. We further determine for the first time isotopologue ratios of N2O dissolved in the riparian groundwater in order to support our assumption that it mainly originated from denitrification. The study sites, both situated in Estonia, northeastern Europe, receive contrasting N loads from adjacent uphill arable land. Whereas N2O emissions were rather small at both sites, average gaseous N2-to-N2O ratios inferred from closed-chamber measurements and He-O laboratory incubations were almost four times smaller for the heavily loaded site. In contrast, groundwater parameters were less variable among sites and between landscape positions. Campaign-based average 15N site preferences of N2O (SP) in riparian groundwater ranged between 11 and 44 o. Besides the strong prevalence of N2 emission over N2O fluxes and the correlation pattern between isotopologue and water quality data, this comparatively large range highlights the importance of denitrification and N2O reduction in both riparian gray alder stands. [ABSTRACT FROM AUTHOR]

Abstract: Quantifying denitrification in arable soils is crucial in predicting nitrogen fertiliser losses and N2O emissions. Stable isotopologue analyses of emitted N2O (δ15N, δ18O and SP= 15N site preference within the linear N2O molecule) may help to distinguish production pathways and to quantify N2O reduction to N2. However, such interpretations are often ambiguous due to insufficient knowledge on isotopic fractionation mechanisms. Here we present a complex experimental approach to determine the net fractionation factors (η) associated with denitrification. This determination is based on three laboratory experiments differing in their experimental set-up and soil properties. Static and dynamic incubation techniques were compared. All available methods for independent determination of N2O reduction contribution were used, namely, N2-free atmosphere incubation, acetylene inhibition technique and 15N gas-flux method. For N2O production: (i) the determined difference in δ18O between soil water and produced N2O vary from +18‰ to +42‰ and show very strict negative correlation with soil water saturation; (ii) the determined η15N of N2O production vary from −55‰ to −38‰ and the fractionation decreases with decreasing substrate availability; (iii) the determined SP of produced N2O vary from −3‰ to +9‰. For N2O reduction: (i) the determined η18O and η15N of N2O reduction vary in very wide ranges from −18‰ to +4‰ and from −11‰ to +12‰, respectively, and depend largely on the differences in experimental setups; whereas (ii) the determined ηSP of N2O reduction shows a very consistent value with all previous studies and varies in a rather narrow range from −2‰ to −8‰. It can be concluded that η values of N2O production determined during laboratory incubations yield only roughly estimates for respective values expectable under field study conditions. η18O and η15N associated with N2O reduction may vary largely, probably depending on spatial and temporal coincidence of N2O production and reduction, and are hence not yet predictable for natural conditions. However, the ηSP of N2O reduction appeared to be relatively robust and a most probable value of about −5‰ can be used to constrain N2O reduction based on SP of soil emitted N2O. [Copyright &y& Elsevier]

N2O gas is involved in global warming and ozone depletion. The major sources of N2O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N2O produced by soil AOA and associated N2O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N2O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ15Nbulk and δ18O -N2O of soil AOA strains were 13-30%, −13 to −35% and 22-36%, respectively, and strains MY1-3 and other soil AOA strains had distinct isotopic signatures. A 15N-NH4+-labeling experiment indicated that N2O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N2O from AOA may be attributable to the relative contributions of these two processes. The highest N2O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N2O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N2O emissions from soil nitrification may be attributable to AOA. [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 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]

Abstract: Denitrifying aquifers are sources of the greenhouse gas N2O. Isotopic signatures reflect processes of production and reduction of N2O, but it is not clear to which extent these can be used to quantify those processes. We investigated the spatial distribution of isotopologue values of N2O (δ18O, average δ15N, and 15N site preference, SP) in two denitrifying sandy aquifers to study N2O production and reduction and associated isotope effects in groundwater. For the first time, we combined this approach with direct estimation of N2O reduction from excess-N2 analysis. Groundwater samples were collected from 15 monitoring wells and four multilevel sampling wells and analysed for , dissolved N2O, dissolved O2, excess N2 from denitrification and isotopic signatures of and N2O. Both aquifers exhibited high concentrations with average concentrations of 22 and 15mgNL−1, respectively. Evidence of intense denitrification with associated N2O formation was obtained from mean excess-N2 of 3.5 and 4.3mgNL−1, respectively. Isotopic signatures of N2O were highly variable with ranges of 17.6–113.2‰ (δ18O), −55.4 to 89.4‰ (δ15Nbulk) and 1.8–97.9‰ (SP). δ15N and δ18O of ranged from −2.1‰ to 65.5‰ and from −5‰ to 33.5‰, respectively. The relationships between δ15N of , δ15Nbulk and SP were not in good agreement with the distribution predicted by a Rayleigh-model of isotope fractionation. The large ranges of δ18O and SP of N2O as well as the close correlation between these values could be explained by the fact that N2O reduction to N2 was strongly progressed but variable. We confirm and explain that a large range in SP and δ18O is typical for N2O from denitrifying aquifers, showing that this source signature can be distinguished from the isotopic fingerprint of N2O emitted from soils without water-logging. We conclude that isotopologue values of N2O in our sites were not suitable to quantify production or reduction of N2O or the contribution of different processes to the total N2O flux, apparently because these values were not only governed by individual pathways but eventually also by the spatial distribution of substrates and activity within the aquifers. These observations could be explained by the dynamics of N2O production, reduction and transport in water-saturated systems with heterogenic distribution of microbial activity and by a combination of diffusive and enzymatic isotope effects. [Copyright &y& Elsevier]

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).

Abstract: Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N2O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to determine variability and control of the isotopic fingerprint of N2O fluxes following NH4 +-fertilization and dominated by nitrification. We conducted a microcosm study with three arable soils fertilized with 0–140mgNH4 +–Nkg−1. Fractions of N2O derived from nitrification and denitrification were determined in parallel experiments using the 15N tracer and acetylene inhibition techniques or by comparison with unfertilized treatments. Soils were incubated for 3–10 days at low moisture (30–55% water-filled pore space) in order to establish conditions favoring nitrification. Dual isotope and isotopomer ratios of emitted N2O were determined by mass spectrometric analysis of δ 18O, average δ 15N (δ 15Nbulk) and 15N site preference (SP=difference in δ 15N between the central and peripheral N positions of the asymmetric N2O molecule). N2O originated mainly from nitrification (>80%) in all treatments and the proportion of NH4 + nitrified that was lost as N2O ranged between 0.07 and 0.45%. δ 18O and SP of N2O fluxes ranged from 15 to 28.4‰ and from 13.9 to 29.8‰, respectively. These ranges overlapped with isotopic signatures of N2O from denitrification reported previously. There was a negative correlation between SP and δ 18O which is opposite to reported trends in N2O from denitrification. Variation of average 15N signatures of N2O (δ 15Nbulk) did not supply process information, apparently because a strong shift in precursor signatures masked process-specific effects on δ 15Nbulk. Maximum SP of total N2O fluxes and of nitrification fluxes was close to reported SP of N2O from NH4 + or NH2OH conversion by autotrophic nitrifiers, suggesting that SP close to 30‰ is typical for autotrophic nitrification in soils following NH4 +-fertilization. The results suggest that the δ 18O/SP fingerprint of N2O might be used as a new indicator of the dominant source process of N2O fluxes in soils. [Copyright &y& Elsevier]

Abstract: Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N2O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to evaluate isotopomer analysis of N2O (intramolecular distribution of 15N) as well as conventional nitrogen and oxygen isotope ratios (i) as a tool to identify N2O production processes in soils and (ii) to constrain the isotopic fingerprint of soil-derived N2O. We conducted a microcosm study with arable loess soil fertilized with 20mgNkg−1 of 15NO3 −-labeled or non-labeled ammonium nitrate. Soils were incubated for 16d at varying moisture (55%, 75% and 85% water-filled pore space (WFPS)) in order to establish different levels of nitrification and denitrification. Dual isotope and isotopomer ratios of emitted N2O were determined by mass spectrometric analysis of δ 18O, average δ 15N (δ 15Nbulk) and 15N site preference (SP=difference in δ 15N between the central and peripheral N-positions of the asymmetric N2O molecule). Total rates and N2O emission of denitrification and nitrification were determined by 15N analysis of headspace gases and soil extracts of the 15NO3 − treatment. N2O emission and denitrification increased with moisture whereas gross nitrification was almost constant. In the 55% WFPS treatment, more than half of the N2O flux was derived from nitrification, whereas denitrification was the dominant N2O source in the 75% WFPS and 85% WFPS treatments. Moisture conditions were reflected by the isotopic signatures since highly significant differences were observed for average δ 15Nbulk, SP and δ 18O. Experiment means of the 75% WFPS and 85% WFPS treatments gave negative δ 15Nbulk (−18.0‰ and −34.8‰, respectively) and positive SP (8.6‰ and 15.3‰, respectively), which we explained by the fractionation during N2O production and partial reduction to N2. In the 55% WFPS treatment, mean SP was relatively low (1.9‰), which suggests that nitrification produced N2O with low or negative SP. The observed influence of process condition on isotopomer signatures suggests that the isotopomer approach might be suitable for identifying N2O source processes. However, more research is needed to determine the impact from process rates and microbial community structure. Isotopomer signatures were within the range reported from previous soil studies which supports the assumption that SP of soil-derived N2O is lower than SP of tropospheric N2O. [Copyright &y& Elsevier]

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]

Soil N2O fluxes originate from a multiple of mostly microbial processes where the currently known include production by nitrification (including hydroxylamine oxidation and nitrifier denitrification), fungal and bacterial denitrification, co-denitrification, DNRA as well as N2O reduction to N2 by bacterial denitrification. Better knowledge on their significance and control is needed to better predict gaseous N fluxes from soil. In recent years, stable isotope signatures of N2O such as δ18O, average δ15N (δ15Nbulk) and 15N site preference (SP = difference in δ15N between the central and peripheral N positions of the asymmetric N2O molecule) have been used to characterize N2O turnover processes including N2O production and reduction by microbial denitrification. While it is generally accepted that different microbial processes of N2O production are associated with specific isotope effects leading to characteristic "endmember" values of N2O produced, there is also consensus that a clear distinction and identification of processes contributing to N2O fluxes is hampered by several factors including the impact of N2O reduction and its (variable) isotope effect, variability of endmember values as well as isotopic values of N2O precursors and their spatial variability. This leads to substantial uncertainty in identification and quantification of different N2O processes, unless some of these factors can be estimated or excluded, which we will illustrate by Monte-Carlo modelling. Therefore, in order to obtain useful information from N2O isotopocules, it is necessary to constrain as many unknowns as possible. We will show examples how this can be done. Moreover, we will illustrate why δ15Nbulk is currently a poor indicator for source processes due to the difficulty to determine δ15N of the precursors of N2O. Finally, we show a comparison of N2O reduction in the field determined by the isotopocule approach and by the 15N gas flux method as independent reference method. We conclude that the isotopocule approach is principally a powerful tool to identify N2O processes that are difficult or impossible to determine otherwise, but to obtain meaningful results its prerequisites and limitations must be taken into account. [ABSTRACT FROM AUTHOR]

The absence of N2 flux measurements in liquid manure-amended soils has resulted in a poor understanding of the effect of manure application on gaseous N losses. The aim of this study was to quantify N2, N2O, CO2, N2O reduction to N2, depth distribution of moisture, water-extractable organic C, NO3−, NH4+, pH, and diffusivity in a laboratory incubation experiment with an arable silt-loam soil. To quantify N processes and gaseous fluxes, 15N tracing was applied. An artificial livestock slurry-mixture was added to the soil in various treatments (control, surface or injected application; slurry-application rate: 42.9 kg N ha− 1; soil water content of either 40% or 60% water-filled pore space (WFPS)). The soil was incubated for 10 days. The depth distribution of the control parameters was measured twice during the experiment on days 5 and 10. The average increase in N2 and N2O fluxes from denitrification was about 900% in slurry-amended soils. The highest N2 and N2O fluxes from denitrification were measured in the slurry injection, 60% WFPS treatment (7.83 ± 3.50 and 11.22 ± 7.60 mg N m− 2 d− 1, respectively). The hypothesis that injected slurry at a higher water content enhances denitrification was confirmed. This study provides important insights into the formation, spatial and temporal variation of the manure-soil hotspot and its impact on the denitrification process. The results will form part of a dataset to develop, improve and test manure application submodules of biogeochemical models and will help to understand in detail the effect of hotspots on N-cycling in manure-treated soils. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a potent greenhouse gas that is produced during microbial nitrogen transformation processes such as nitrification and denitrification. Soils represent the largest sources of N2O emissions with nitrogen fertilizer application being the main driver of rising atmospheric N2O concentrations. Soil biochar amendment has been proposed as a promising tool to mitigate N2O emissions from soils. However, the underlying processes that cause N2O emission suppression in biochar-amended soils are still poorly understood. We set up microcosm experiments with fertilized, wet soil in which we used 15N tracing techniques and quantitative polymerase chain reaction (qPCR) to investigate the impact of biochar on mineral and gaseous nitrogen dynamics and denitrification-specific functional marker gene abundance and expression. In accordance with previous studies our results showed that biochar addition can lead to a significant decrease in N2O emissions. Furthermore, we determined significantly higher quantities of soil-entrapped N2O and N2 in biochar microcosms and a biochar-induced increase in typical and atypical nosZ transcript copy numbers. Our findings suggest that biochar-induced N2O emission mitigation is based on the entrapment of N2O in water-saturated pores of the soil matrix and concurrent stimulation of microbial N2O reduction resulting in an overall decrease of the N2O/(N2O + N2) ratio. [ABSTRACT FROM AUTHOR]

Abstract Straw application in combination with synthetic N fertilizer could increase crop yield and improve soil fertility, however, contradictory observations have been reported on the effects of straw addition on soil N 2 O emission. Straw application can affect both denitrification rate and its product stoichiometry (N 2 O/(N 2 O + N 2) ratio), whereas the latter remains rather unclear since the ratio is strongly regulated by other soil parameters, e.g. nitrate and oxygen concentrations at denitrifying micro-sites. In this context, we conducted an incubation experiment with a robotized continuous flow incubation system using a He/O 2 atmosphere and measured N 2 O and direct N 2 fluxes over 22 days. Soil amended with and without rice straw (2.5 g kg−1 soil) in conjunction with nitrate fertilizer (10 mM KNO 3) and non-amended control soil were incubated under 85% water-filled pore space. To simulate a short soil anoxic period, three different O 2 partial pressures phases were set (20%, 5% and 10%). Additionally, N 2 O site preference signatures of soil-emitted N 2 O were analyzed to identify the processes contributing to N 2 O fluxes. Addition of nitrate increased cumulative N 2 O fluxes and decreased cumulative N 2 fluxes compared with non-fertilized control, while rice straw amendment increased both N 2 O and N 2 emissions drastically compared with the nitrate only treatment. The N 2 O SP values ranged from 0.4 to 2.7‰ among all treatments, indicating denitrification/nitrifier denitrification was the dominating source. The results suggest that straw amendment can trigger high denitrification rate, whereas the effect of straw amendment on the amount of emitted N 2 O and the N 2 O/(N 2 O + N 2) product ratio strongly depends on soil NO 3 − concentration. As a conclusion, the present study suggests that straw amendment in conjunction with nitrate-N can increase soil N 2 O emissions under conditions favoring denitrification, even though it may decrease the overall N 2 O/(N 2 O + N 2) product ratio. Highlights • Combined effect of straw addition and soil NO 3 − was shown on direct N 2 fluxes. • Decrease in soil NO 3 − caused a quick shift from N 2 O production to reduction. • Effect of straw addition on N 2 O and N 2 fluxes depends on soil NO 3 − content. [ABSTRACT FROM AUTHOR]

European policy recommends that biomass production occur on marginal land, such as poorly draining Stagnosols. Compared to annual cropping, perennial crops may better mitigate N2O emissions at such sites, through more complete denitrification. To test that hypothesis, we compared N2 and N2O fluxes from the soils of a perennial crop (cup plant, Silphium perfoliatum L.) and an annual crop (silage maize, Zea mays L.). Intact soil columns (35 cm height, 14.4 cm diameter) were incubated for 37 days. The soils were fertilized with 60 or 120 kg N ha−1 and exposed to successive phases of waterlogging: free drainage, waterlogging of 1/3-, and waterlogging of 2/3- of the column. Source-specific N2O and N2 fluxes were measured using the 15 N gas flux method. Denitrification was higher in cup plant than maize soil and total N losses from denitrification were dominated by emissions from the third phase. Cup plant soil emitted 33.6 ± 78.1 mg N m−2 and 95.8 ± 64.4 mg N m−2 more N2O than maize soil in the low and high N treatments, respectively. The product ratio of denitrification (N2Oi = N2O/(N2 + N2O)) increased with waterlogging in maize soil, while remaining stable in cup plant soil. Emissions from the top 10 cm dominated the N2Oi rather than N2 fluxes from the saturated soil. This study did not show N2O mitigation in cup plant soil, instead highlighting the complexity of plant-soil effects on denitrification. We clearly showed that the application of a general N2Oi for agricultural soils across annual and perennial cropping is not recommended. [ABSTRACT FROM AUTHOR]

[Display omitted] • Soil gaseous N emissions were altered by land use conversion. • Land use conversion increased N 2 O emissions from bacterial denitrification. • Land use conversion altered the abundance and composition of soil denitrifiers. • Increased moisture in upland soils induced more gaseous N losses dominated by N 2 O. In this study, soil-borne N 2 , NO, and N 2 O emissions induced by land use conversion and water management were investigated in intact soil cores under a helium/oxygen atmosphere by a robotized incubation system in combination with the N 2 O 15N site preference signature and molecular-based microbial analysis. The experiment consisted of five treatments: i) paddy-flooded (PF); ii) orchard-wet (OW, 70% WFPS); iii) orchard-dry (OD, 43% WFPS); iv) vegetable-wet (VW, 70% WFPS); and v) vegetable-dry (VD, 43% WFPS). The vessels of each treatment received 200 mg urea-nitrogen (N) (equivalent to 210 kg of urea-N ha−1), and soil moisture in the OW and VW treatments was adjusted to a higher constant moisture to simulate a scenario after irrigation or rainfall. The results showed that total gaseous N losses during the incubation period were 25.33 ± 0.33 kg N ha−1 in the PF treatment, whereas smaller losses were recorded in the OD and VD treatments (4.28 ± 2.04 and 9.75 ± 3.75 kg N ha−1, respectively). The potential contribution of bacterial denitrification to N 2 O emissions in the OD and VD treatments was 11.1% and 15.4% higher, respectively, than that in the PF treatment (58.8% ± 0.5%). Furthermore, the corresponding N 2 O/(N 2 O + N 2) ratio in the OD and VD treatments decreased by 50% and 73.8%, respectively, relative to the ratio in the PF treatment (0.42 ± 0.01). Such changes indicated the crucial role of altered soil properties caused by land use conversion in regulating the production and consumption of N 2 O. Relative to the normal moisture (dry) condition, enhanced soil moisture increased total gaseous N losses in the orchard and vegetable soils by 386.9% and 67.4%, accompanied by a higher N 2 O/(N 2 O + N 2) product ratio, but decreased the share of bacterial N 2 O by 11.1% and 15.4%, respectively. The changes in the abundance and community composition of soil denitrifiers caused by land use conversion from rice paddies to orchards and vegetable fields could partly explain the differences in gaseous N loss therein. These findings highlight the influence of land use conversion on soil gaseous N emissions and demonstrate that increased moisture in upland soils reduced the dominance of bacterial denitrification in N 2 O production. [ABSTRACT FROM AUTHOR]

Numerous interrelated factors (e.g., the labile C, soil NO3− concentration, and soil moisture content) are involved in controlling the microbial sources of N2O and the product stoichiometry of denitrification; however, the interactions among different factors are still poorly understood. Here, a fully robotized continuous flow soil incubation system (allowing simultaneous measurements of N2 and N2O fluxes) was employed to investigate the interactive effects of a 51-day duration of moist spell, straw amendment, and the NO3− level on the rate and product stoichiometry (N2O/(N2O + N2) ratio) of denitrification in heavily N loaded arable soils (i.e., paddy, vegetable, and orchard soils). The rewetting-induced N2O emissions mainly originated from bacterial denitrification in all soil types, with a clear shift to fungal denitrification (plus contingent nitrification) over time. The vegetable and orchard soils showed a higher share of bacterial N2O (62–70%) than that in the paddy soils (50–54%), which may be attributed to more labile-C driven bacterial activity induced by the greater manure and crop residue input therein. Interestingly, the inhibitory effect of high soil NO3− on N2O reduction in these soils was offset by a 51-day-long moist spell, regardless of the amendment of straw. To our knowledge, our study is the first to show that the inhibitory effect of high residual NO3− on N2O reduction is suppressed by a moist spell with a certain duration in heavily N loaded arable soils, suggesting that the water regime history should be considered when optimizing the N fertilizer application timing to mitigate soil N2O emissions. [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]

Since it is known that nitrous oxide (N2O) production and consumption pathways are affected by soil pH, optimising the pH of agricultural soils can be an important approach to reduce N2O emissions. Because liming effects on N2O reduction had not been studied under ambient atmosphere and typical bulk density of arable soils, we conducted mesoscale incubation experiments with soils from two liming trials to investigate the impact of long-term pH management and fresh liming on N transformations and N2O production. Soils differed in texture and covered a range of pH levels (3.8–6.7), consisting of non-limed controls, long-term field-limed calcite and dolomite treatments, and freshly limed soils. Both soils were amended with 15N-labelled potassium nitrate (KNO3) and incubated with and without incorporated maize litter. Packed soil mesocosms were cycled through four phases of alternating temperatures and soil moistures for at least 40 days. Emissions of N2O and dinitrogen (N2) as well as the product ratio of denitrification N2O/(N2O + N2), referred to as N2Oi were measured with the 15N gas flux method in N2-reduced atmosphere. Emissions of N2O increased in response to typical denitrifying conditions (high moisture and presence of litter). Increased temperature and soil moisture stimulated microbial activity and triggered denitrification as judged from 15NO3− pool derived N2O + N2 emissions. Fresh liming increased denitrification in the sandy soil up to 3-fold but reduced denitrification in the loamy soil by 80%. N2Oi decreased throughout the incubation in response to fresh liming from 0.5–0.8 to 0.3–0.4, while field-limed soils had smaller N2Oi (0.1–0.3) than unlimed controls (0.9) irrespective of incubation conditions. Our study shows that the denitrification response (i.e., N2O + N2 production) to liming is soil dependent, whereas liming effects on N2Oi are consistent for both long- and short-term pH management. This extends previous results from anoxic slurry incubation studies by showing that soil pH management by liming has a good mitigation potential for agricultural N2O emissions from denitrification under wet conditions outside of cropping season. [ABSTRACT FROM AUTHOR]

Denitrification is an important component of the nitrogen cycle in soil, returning reactive nitrogen to the atmosphere. Denitrification activity is often concentrated spatially in anoxic microsites and temporally in ephemeral events, which presents a challenge for modelling. The anaerobic fraction of soil volume can be a useful predictor of denitrification in soils. Here, we provide a review of this soil characteristic, its controlling factors, its estimation from basic soil properties and its implementation in current denitrification models. The concept of the anaerobic soil volume and its relationship to denitrification activity has undergone several paradigm shifts that came along with the advent of new oxygen and microstructure mapping techniques. The current understanding is that hotspots of denitrification activity are partially decoupled from air distances in the wet soil matrix and are mainly associated with particulate organic matter (POM) in the form of fresh plant residues or manure. POM fragments harbor large amounts of labile carbon that promote local oxygen consumption and, as a result, these microsites differ in their aeration status from the surrounding soil matrix. Current denitrification models relate the anaerobic soil volume fraction to bulk oxygen concentration in various ways but make little use of microstructure information, such as the distance between POM and air-filled pores. Based on meta-analyses, we derive new empirical relationships to estimate the conditions for the formation of anoxia at the microscale from basic soil properties and we outline how these empirical relationships could be used in the future to improve prediction accuracy of denitrification models at the soil profile scale. [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]

Nitrite (NO2−) is a crucial compound in the N soil cycle. As an intermediate of nearly all N transformations, its isotopic signature may provide precious information on the active pathways and processes. NO2− analyses have already been applied in 15N tracing studies, increasing their interpretation perspectives. Natural abundance NO2− isotope studies in soils were so far not applied and this study aims at testing if such analyses are useful in tracing the soil N cycle. We conducted laboratory soil incubations with parallel natural abundance and 15N treatments, accompanied by isotopic analyses of soil N compounds (NO3−, NO2−, NH4+). The double 15N tracing method was used as a reference method for estimations of N transformation processes based on natural abundance nitrite dynamics. We obtained a very good agreement between the results from nitrite isotope model proposed here and the 15N tracing approach. Natural abundance nitrite isotope studies are a promising tool to our understanding of soil N cycling. [ABSTRACT FROM AUTHOR]

The greenhouse gas nitrous oxide (N 2 O) is produced in soil as a consequence of complex co-occurring processes conducted by diverse microbial species, including fungi. The fungal p450nor gene encodes a nitric oxide reductase associated with fungal denitrification. We thus hypothesized that p450nor gene expression is a marker for ongoing fungal denitrification. Specific PCR primers and quantitative PCR (qPCR) assays were developed targeting p450nor genes and transcripts. The novel PCR primers successfully amplified p450nor from pure cultures, and were used in an mRNA targeted qPCR to quantify p450nor gene transcription (i.e., gene expression) during denitrification activity in cultures of the fungal model denitrifier Fusarium oxysporum. Gene expression was induced by high (5 mM) and low (0.25 mM) nitrite concentrations. Nitrite stimulated N 2 O production rates by F. oxysporum , which correlated well with an up to 70-fold increase in p450nor gene expression during the first 12–24 h of anoxic incubation. The relative p450nor gene peak expression and peak N 2 O production rates declined 20- and 2-fold on average, respectively, towards the later phase of incubation (48–120 h). The 15N site preference of N 2 O (SP(N 2 O)) was high for F. oxysporum and independent of reaction progress, confirming the fungal origin of N 2 O produced. In conclusion, the developed fungal p450nor gene expression assay together with the analysis of SP(N 2 O) values provide a basis to improve current tools for the identification of fungal denitrification and/or N 2 O production in natural systems like soils. • New quantitative PCR assay targeting fungal p450nor. • Fusarium oxysporum p450nor expression correlates with nitrous oxide production. • F. oxysporum produced nitrous oxide with high 15N site preference. • F. oxysporum tolerates high concentrations of nitrite (>5 mM). [ABSTRACT FROM AUTHOR]

• First study comparing mechanical and chemical grassland conversion for more than 1 year. • Mineralization dynamics differ between chemical and mechanical conversion. • Similar cumulative N 2 O emission and net-N mineralization for both conversion types. • Accelerated N mineralization after conversion lasts for more than one year. Grassland conversion to cropland bears a risk of increased nitrate (NO 3 −) leaching and nitrous oxide (N 2 O) emission due to enhanced nitrogen (N) mineralization. This study investigates the dynamics of mineral N and N 2 O emissions following chemical and mechanical conversion from permanent grassland to cropland (maize) at two sites with different texture (clayey loam and sandy loam) and fertilization regime (with and without mineral N-fertilization) over a two-year period. Soil mineral N levels increased shortly after conversion and remained elevated in converted plots compared to permanent grassland or long-term cropland in the second year of investigation. Fluxes of N 2 O were higher from converted plots than permanent grassland or cropland. However, soil mineral N contents and cumulative N 2 O emissions did not differ between conversion types. Only the distribution of N 2 O losses over the two years differed: while losses were of similar magnitude in both years in mechanically converted plots, the major part of N 2 O loss in chemically converted plots occurred in the first year after conversion while emissions approximated grassland level in the second year. N 2 O fluxes were mainly controlled by water-filled pore space and soil NO 3 − levels. Despite differences in N levels at the two sites, these key findings are similar on both study sites. They indicate strongly accelerated mineralization after conversion, an effect that still lasted in the converted plots at the end of the two-year investigation irrespective of the conversion type used. [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]

Combined application of organic amendment with synthetic fertilizer is an emerging management technique for maximized agronomic benefits without drastic soil health effects. However, little attention has been paid to the environmental costs of such management as the primary focus has remained on agronomic outcomes. In present study, we investigated the impacts of three organic amendments including dairy cattle manure (20 t/ha), poultry manure (20 t/ha), and a biochar (30 t/ha) applied along with urea (55 kg-N/ha) on soil N2O fluxes and explored the mechanisms behind N2O emissions. Plots with urea-only application were used as a control. For source partitioning of soil-emitted N2O, we used isotopocule mapping approach. The results showed significantly higher N2O emission rates from manure-treated plots compared with biochar or control treatments plots. The cumulative N2O emissions rates for 125 days from control, biochar, dairy cattle, and poultry treatments were 0.76, 0.70, 0.96, and 1.05 kg N2O-N/ha, respectively. Isotopocule mapping approach revealed that bacterial denitrification was the dominant pathway for temporal high N2O emission events as the fractions of bacterial denitrification ranged between 0.5 and 0.9 among treatments. Soil DNA-based Q-PCR assays showed significant increase in abundance of NO2− reducing denitrifiers in dairy cattle and poultry treated plots suggesting acceleration in N2O emissions was due to shift in the molecular ratios of (nirS + nirK)/nosZ denitrifying bacterial community. In contrast to manure treatments, the combined application of biochar along fertilizer significantly improved soil C contents with a slight decrease in N2O emission rates. Therefore, biochar appeared to be the best option to minimize soil quality loss without additional environmental cost. [ABSTRACT FROM AUTHOR]

The 15N gas flux (15NGF) method allows for direct in situ quantification of dinitrogen (N2) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N2 emissions of a wheat rotation using the 15NGF method, to compare these N2 emissions with those obtained from a lysimeter-based 15N fertilizer mass balance approach, and to contextualize N2 emissions with 15N enrichment of N2 in soil air. For four sampling periods, fertilizer-derived N2 losses (15NGF method) were similar to unaccounted fertilizer N fates as obtained from the 15N mass balance approach. Total N2 emissions (15NGF method) amounted to 21 ± 3 kg N ha− 1, with 13 ± 2 kg N ha− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the 15N mass balance approach overall indicated fertilizer-derived N2 emissions of 11%, equivalent to 18 ± 13 kg N ha− 1. Nitrous oxide (N2O) emissions were small (0.15 ± 0.01 kg N ha− 1 or 0.1% of fertilizer N), resulting in a large mean N2:(N2O + N2) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N2 emissions was well explained by the δ15N2 in soil air down to 50 cm depth. We conclude the 15NGF method provides realistic estimates of field N2 emissions and should be more widely used to better understand soil N2 losses. Moreover, combining soil air δ15N2 measurements with diffusion modeling might be an alternative approach for constraining soil N2 emissions. [ABSTRACT FROM AUTHOR]