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From the Ground Up: Global Nitrous Oxide Sources are Constrained by Stable Isotope Values

  • David M. Snider ,

    dave.snider@ec.gc.ca (DMS); jvenkiteswaran@wlu.ca (JJV)

    Affiliation National Water Research Institute, Canada Centre for Inland Waters, Environment Canada, Burlington, ON, L7R 4A6, Canada

  • Jason J. Venkiteswaran ,

    dave.snider@ec.gc.ca (DMS); jvenkiteswaran@wlu.ca (JJV)

    Affiliations Department of Geography and Environmental Studies, Wilfrid Laurier University, Waterloo, ON, N2L 3C5, Canada, Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

  • Sherry L. Schiff,

    Affiliation Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

  • John Spoelstra

    Affiliations National Water Research Institute, Canada Centre for Inland Waters, Environment Canada, Burlington, ON, L7R 4A6, Canada, Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

From the Ground Up: Global Nitrous Oxide Sources are Constrained by Stable Isotope Values

  • David M. Snider, 
  • Jason J. Venkiteswaran, 
  • Sherry L. Schiff, 
  • John Spoelstra
PLOS
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Correction

9 Feb 2016: Snider DM, Venkiteswaran JJ, Schiff SL, Spoelstra J (2016) Correction: From the Ground Up: Global Nitrous Oxide Sources are Constrained by Stable Isotope Values. PLOS ONE 11(2): e0149290. https://doi.org/10.1371/journal.pone.0149290 View correction

Abstract

Rising concentrations of nitrous oxide (N2O) in the atmosphere are causing widespread concern because this trace gas plays a key role in the destruction of stratospheric ozone and it is a strong greenhouse gas. The successful mitigation of N2O emissions requires a solid understanding of the relative importance of all N2O sources and sinks. Stable isotope ratio measurements (δ15N-N2O and δ18O-N2O), including the intramolecular distribution of 15N (site preference), are one way to track different sources if they are isotopically distinct. ‘Top-down’ isotope mass-balance studies have had limited success balancing the global N2O budget thus far because the isotopic signatures of soil, freshwater, and marine sources are poorly constrained and a comprehensive analysis of global N2O stable isotope measurements has not been done. Here we used a robust analysis of all available in situ measurements to define key global N2O sources. We showed that the marine source is isotopically distinct from soil and freshwater N2O (the continental source). Further, the global average source (sum of all natural and anthropogenic sources) is largely controlled by soils and freshwaters. These findings substantiate past modelling studies that relied on several assumptions about the global N2O cycle. Finally, a two-box-model and a Bayesian isotope mixing model revealed marine and continental N2O sources have relative contributions of 24–26% and 74–76% to the total, respectively. Further, the Bayesian modeling exercise indicated the N2O flux from freshwaters may be much larger than currently thought.

Introduction

Since the advent of the Haber-Bosch process one century ago, humans have vastly perturbed the global nitrogen (N) cycle. Current anthropogenic activities contribute 51% of the total N fixed worldwide (210 of 413 Tg N yr−1) [1]. One negative consequence of this is an increase in atmospheric nitrous oxide (N2O) [2], a long-lived trace gas that contributes to climate warming and the destruction of stratospheric ozone [3]. The current concentration of N2O in the troposphere is 325 parts per billion (ppb) [4]. Future concentrations of atmospheric N2O are difficult to predict, yet this information is an essential input parameter for global climate change models. Further, both the prediction and mitigation of N2O concentrations depend on an accurate understanding of the emissions from key N2O sources.

Most emissions of N2O (natural and anthropogenic) occur from terrestrial, freshwater, and marine environments, where N compounds are processed by nitrifying and denitrifying microorganisms. These processes account for ∼89% of the total annual N2O emissions, or almost 16 Teragrams (Tg = 1012 g) N/year [5]. However, scientists’ best estimates of the N2O budget are still highly uncertain. The most recent Intergovernmental Panel on Climate Change Assessment Report (IPCC-AR5) reveals wide ranges in the relative uncertainty of many individual N2O sources. In addition, the uncertainty on the annual cumulative emissions of N2O for 2006 from natural soils, oceans, rivers, estuaries, coastal zones, and agriculture combined ranged between 6.9–26.1 Tg N [5].

The clear separation and accounting of individual N2O sources remains challenging, but is essential if we are to make meaningful reductions in emissions. Measurements of stable isotope ratios (δ15N-N2O and δ18O-N2O) and the intramolecular site preference (SP) of 15N are one way to track sources if they are isotopically distinct. Several accounts of the global N2O budget have used ‘top-down’ isotope mass-balance models to estimate the strength and isotopic composition of anthropogenic and natural N2O sources [2,611]. In this approach, changes in atmospheric N2O over time are modelled by comparing our modern-day atmosphere (a mixture of post-industrial, anthropogenic N2O and natural N2O) to relic air trapped in glacial firn and ice. All these studies have assumed that soils are the main source of post-industrial N2O because its calculated isotopic composition was most similar to a limited body of published soil N2O measurements. Yet we do not have a clear synthesis of the isotopic character of individual N2O sources. For example, freshwaters and estuaries may contribute up to 25% of anthropogenic N2O emissions [5], but prior to 2009 there was only one publication reporting freshwater δ15N-N2O and δ18O-N2O values [12] (S1 Dataset). In reality, there is extreme variation in the measured values of δ15N-N2O and δ18O-N2O (Fig. 1), and no systematic examination of individual sources has occurred.

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Fig 1. Global N2O isotope measurements from atmospheric, marine, and terrestrial samples.

All the data compiled in this study that fit within the axis ranges shown are plotted here. Each point represents one measurement, or in a few cases a reported average value, and is two-thirds transparent to allow the density of data to be displayed. Standard ellipses encompass ∼40% of the data (Fig. 2) and are shown for the six non-atmospheric categories. Most data fall to the left of the current tropospheric value. Stratospheric data falls along a line (δ18O = 0.89 × δ15N + 38.4) (R2 = 0.999) that originates from the tropospheric value, and is caused by isotopic fractionation during N2O destruction [13].

https://doi.org/10.1371/journal.pone.0118954.g001

In this paper, we use a ‘bottom-up’ approach to define key N2O sources and demonstrate that their global average δ15N and δ18O values are isotopically unique. Further we use these in situ N2O isotope data to substantiate what ‘top-down’ global atmospheric models have predicted; soils, and not marine or freshwater ecosystems, are the main source of rising atmospheric N2O levels.

Methods

We mined 1920 data points from 52 studies that measured in situ δ15N-N2O and δ18O-N2O in atmospheric, terrestrial and marine systems from 1987 to present [2,1060]. If the published data was not tabulated, we used the software ‘g3data’ (http://www.frantz.fi/software/g3data.php) to extract data from figures [61]. The accuracy of our method was tested by plotting a subset of data from Well et al. [51], re-extracting it, and then comparing it to the original values. The mean (min/max) difference (‰) was 0.06 (0.00/0.13) for δ15N and 0.02 (0.00/0.07) for δ18O. This represents a worst-case accuracy of our ability to extract data from figures because the test data had an unusually wide range (−80 to +120‰ for δ15N, and 0 to +120‰ for δ18O; n = 53) and all other published graphs had much smaller scales. Values of δ18O-N2O reported vs. atmospheric O2 were converted to δ18O-N2O vs. Vienna Standard Mean Ocean Water (VSMOW) according to Kim and Craig [19].

Twenty-seven studies also measured the intramolecular distribution of 15N in the linear NNO molecule (780 data points) and these data are provided in the supplementary datasets (S1 Dataset and S2 Dataset). This difference between the central (δ15Nα) and terminal (δ15Nβ) 15N enrichment is often expressed as the site preference (SP). This parameter is thought to be a unique indicator of the microbial pathway that produces N2O and not to be affected by variations in the isotopic ratios of substrates. Recently, this idea has been called into question by Yang et al. [62], who showed that SP can vary depending on the growth conditions of microbial cultures. Regardless, if the SP of different global sources is unique it can be used in conjunction with traditional measures of δ15N-N2O and δ18O-N2O values to separate sources in three-dimensional isotope space.

To this compendium of published data we added 1367 new in situ δ15N-N2O and δ18O-N2O data from 16 sites across Ontario and New Brunswick, Canada (S1 Dataset). Soil pore gas and static flux chambers were sampled at four Ontario sites. Urban wastewater treatment plants, streams, rivers, and agricultural drainage tile outlets were sampled across four watersheds in Ontario and New Brunswick. Groundwaters were sampled from numerous domestic and monitoring wells (some multi-level) distributed across nine research sites in Ontario and New Brunswick.

Liquid samples were stripped of N2O using an off-line purge-and-trap system described in Baulch et al. [35]. With the exception of samples from one location (ERS) that were analyzed at UC Davis-SIF, all analyses occurred at the University of Waterloo on an IsoPrime isotope ratio mass spectrometer (IRMS) with a TraceGas pre-concentrator with an analytical precision of 0.2‰ (δ15N-N2O) and 0.4‰ (δ18O-N2O). All samples were analyzed alongside an internal N2O isotope standard that was previously calibrated at the University of Waterloo against local tropospheric air (assumed to be equal to 6.72‰ for δ15N and 44.62‰ for δ18O [17]). This internal standard gas was also submitted to UC Davis-SIF for isotopic analysis, and the standard deviation of 8 replicates (at varying concentrations including ambient) was 0.34‰ (for δ15N) and 0.77‰ (for δ18O). The absolute difference in the assigned value of this internal standard gas (blind inter-lab comparison) was 0.29‰ (for δ15N) and 0.81‰ (for δ18O). Given there are no internationally-recognized standardization methods or materials for N2O isotope analysis, these inter-laboratory results are in good agreement with one another. All values are reported here in units of per mill (‰) relative to air-N2 and VSMOW for δ15N and δ18O, respectively.

All data were categorized as either Antarctic, freshwater, groundwater, marine, soil, stratosphere, troposphere, or urban wastewater, and a bivariate ellipse-based metric [63] was used to analyze and describe individual N2O reservoirs (Table 1). This circular statistical analysis is an improvement over other techniques that qualitatively summarize isotope data with a polygon or a freeform shape e.g., [6,15,46,47]. There is often a high degree of covariance between δ15N-N2O and δ18O-N2O and this statistical technique provides an accurate description of the central tendency of the data. By definition, the standard ellipse contains ∼40% of the data, is centered on the mean and has standard deviations of the bivariate data as semi-axes (Fig. 2) [63,64]. The data and an R file that contains the code to perform the statistical analyses and create the figures shown here are found at https://github.com/jjvenky/Global-N2O-Ellipses.

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Fig 2. The standard ellipse of a bivariate sample.

The urban wastewater N2O isotope data from Townsend-Small et al. [41], Toyoda et al. [65], and this study (n = 83) are summarized here with a standard ellipse. The centre of the ellipse is located at the sample mean (, ), where the semi-major (a) and semi-minor (b) axes intersect. The major axis is inclined versus the positive x axis by the angle θ. The tangent lines parallel to the x and y axes are related to the standard deviations (σx, σy) and the correlation coefficient (r). The two regression lines shown intersect the ellipse at the points of tangency [64].

https://doi.org/10.1371/journal.pone.0118954.g002

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Table 1. Summary statistics of global δ15N-N2O and δ18O-N2O values as standard ellipses*.

https://doi.org/10.1371/journal.pone.0118954.t001

Results and Discussion

The data are highly non-uniform within and between categories (Figs. 1 and 3), and even within individual field sites (S1 Dataset). Historically, this has made it challenging to define an ‘isotopic signature’ for a given environment. Multiple factors cause this variability: (1) N2O is produced by nitrification and denitrification, and the isotopic composition of N and oxygen (O) endmembers can vary widely [26,31,32,54]; (2) the apparent fractionation of 15N/14N and 18O/16O during N transformations is not constant, nor is it easily predicted; and (3) oxygen exchange between N2O precursors and water imparts a large control on δ18O-N2O values during N2O formation. While the exact mechanisms are not fully understood, it appears that greater amounts of exchange occur in unsaturated environments than in saturated ones [6668]. Additionally, the reduction of N2O to N2 in anaerobic environments causes enrichment of 15N and 18O isotopes in the remaining N2O pool, which displaces δ15N-N2O and δ18O-N2O values away from their original source values [26,51]. An initial analysis of all the data compiled in this study shows there is no clear separation of sources because each is described by an ellipse that overlaps at least one other source category (Figs. 1 and 3).

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Fig 3. Global N2O isotope measurements from atmospheric samples and key environmental sources: freshwater, marine and soil (n = 2117).

Data from municipal wastewater treatment plants is also included (n = 92). Each point represents one measurement, or in a few cases a reported average value. The colour of each point is two-thirds transparent to allow the density of data to be displayed. Although the ellipses are the same as in Fig. 1, the scales of the axes are narrowed to better show the data relative to current atmospheric values.

https://doi.org/10.1371/journal.pone.0118954.g003

A similar comparison of all the published SP data (excluding Antarctic and groundwater categories) shows poor isotopic separation of sources (Fig. 4). It is difficult to determine how much of this variability is real, and how much is due to standardization issues and differences in measurement techniques. A recent inter-laboratory assessment of the methods used to determine nitrogen isotopomers revealed poor SP reproducibility [69]. Eleven laboratories employing either IRMS or laser spectroscopy techniques analyzed a single N2O target gas and the resulting standard deviation for SP was 4.24‰. Further, the inter-lab variation in the mean SP value was high, spanning a range of 11.62‰ [69]. This may help explain why there are two distinct groupings of SP data in each of the troposphere [2,17] and the stratosphere [29,45,58] (Fig. 4). Reaching an international consensus on standardization methods for the measurement and reporting of nitrogen isotopomers should vastly improve the utility of this data in source-apportionment studies at all scales. Finally, we note the reproducibility of δ15N-N2O and δ18O-N2O measurements in this recent round-robin test was much better than for SP, which gives us confidence in our ability to use these data here to make useful comparisons. The standard deviation (and range) of the N2O target gas in the inter-lab comparison was 1.37‰ (1.89‰) for δ15N-N2O and 1.00‰ (3.47‰) for δ18O-N2O [69].

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Fig 4. A comparison of 15N site preference values (y-axes) and (a) δ15N-N2O (left panel) and (b) δ18O-N2O (right panel) measurements from freshwaters, oceans, soils, atmosphere, and urban wastewater (n = 651).

Measurements of 15N site preference from the Antarctic (n = 18) and groundwaters (n = 111) are not shown here, but are tabulated in S1 Dataset.

https://doi.org/10.1371/journal.pone.0118954.g004

Much of the low concentration data from soils and surface waters are highly influenced by mixing with tropospheric N2O. This is evident by the high density of soil, freshwater and marine data that lies near the tropospheric N2O value (Fig. 3). In contrast, groundwater N2O, which does not mix with the atmosphere following recharge, is unaffected by this mixing process. Other processes such as substrate enrichment and N2O consumption control the isotopic composition of groundwater N2O, which displays extreme variability even within the same location (Fig. 1) [50,51]. Only 15 studies reported flux-weighted average δ15N-N2O, SP, and/or δ18O-N2O values, or provided enough information for us to calculate these values (2 freshwater studies, 2 marine studies, 10 soil studies, and 1 urban wastewater study; Fig. 5; S2 Dataset). The available flux-weighted data from soil and freshwater environments shows much overlap among this combined continental source, but the flux-weighted marine source appears to be unique. Importantly, there are very few flux-weighted data from all sources so robust conclusions cannot be made at this time. Additionally, these data were not weighted equally across studies so conclusions drawn from this analysis can be misleading. Only some of the values are time-weighted, and the sample size used to calculate the flux-weighted average varies from 3 to ∼50 (S2 Dataset). Emissions of N2O from soils (and potentially freshwaters and oceans) are inherently episodic, so future estimates of the flux-weighted average should attempt to include multiple measurements made over long timescales (months to years and encompassing seasonal differences) whenever possible.

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Fig 5. Emission-weighted average δ15N-N2O and δ18O-N2O from continental and marine environments.

A small number of studies reported flux-weighted or flux and time-weighted average values. A few other studies provided information that allowed us to calculate these values. Equal weighting criteria were not applied in each case because not all values are time-weighted. Additional factors such as the sample size (n), antecedent conditions of N2O substrate(s), and time of year also different among studies (see S2 Dataset).

https://doi.org/10.1371/journal.pone.0118954.g005

The results of our analyses shown in Figs. 1 and 3 are not flux-weighted, nor are all the categories important atmospheric sources. For example, the most recent IPCC assessment reports that human excreta (all forms of treated/untreated sewage) contributes between 0.1–0.3 Tg N yr−1 as N2O, or only ∼1.1% of all natural and anthropogenic sources [5]. Of this, N2O emissions from urban wastewater treatment plants constitutes a very small fraction. To address this, we analyzed subsets of the data from important atmospheric sources (freshwaters, oceans, and soils) that were not strongly influenced by mixing with tropospheric N2O, and thereby make an important contribution to the flux-weighted average source value (Table 2; Fig. 6). To do this we filtered the data to include: (i) all reports of emitted N2O, regardless of the strength of the flux (two freshwater studies [13,46], two marine studies [16,43] and several soil studies) (see S2 Dataset); (ii) isotope data in the soil profile that had concentrations of N2O >650 ppb v/v (or 200% ambient); and (iii) isotope data in freshwater and near-surface marine environments (depths >100 m) with dissolved N2O concentrations >200% saturation with respect to atmospheric N2O [70].

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Fig 6. Nitrous oxide isotope measurements of key sources in the global isotope budget.

Soil, freshwater and marine data were filtered to exclude samples that were highly influenced by mixing with tropospheric N2O: (a) δ15N-N2O vs. δ18O-N2O (top panel, n = 1383); (b) SP vs. δ15N-N2O (middle panel; n = 235); and (c) SP vs. δ18O-N2O (bottom panel; n = 235). The filtering criteria are described in the text.

https://doi.org/10.1371/journal.pone.0118954.g006

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Table 2. Summary statistics of filtered δ15N-N2O and δ18O-N2O data. These values show a subset of data that were not strongly influenced by mixing with tropospheric N2O, and thereby make an important contribution to the flux-weighted average source value (see Fig. 6).

https://doi.org/10.1371/journal.pone.0118954.t002

Most of the freshwater and soil data were retained (71% and 90%, respectively), and the ellipses of these data subsets are similar to the ellipses for all data in these categories (Table 1). Although the median δ15N and δ18O values of freshwater and soil N2O are significantly different (p < 0.001, Mann-Whitney test), their ellipses intersect one another at the 1σ level (Fig. 6–top panel), and we conclude that these sources are not isotopically distinct at the global scale. In order to further delineate freshwater and soil N2O more stable isotope measurements from freshwaters are needed; especially from non-temperate environments because the current data coverage from these systems is lacking. Measurements of SP may prove to be a useful means of separating these sources because the ellipses that describe SP vs. δ15Nbulk for freshwater and soils do not overlap (Fig. 6–middle panel).

Of the 495 published marine values compiled here, only 62 originated from the top 100 m of the ocean and were >200% saturation. Relative to continental N2O sources, the δ15N and δ18O values of near-surface oceanic N2O sources are poorly constrained. We found no reports of N2O isotope values from estuaries, which could represent an important fraction of the marine source but should be similar to marine or freshwater values. Tropical systems including reservoirs are also poorly studied. Future campaigns to more fully characterize δ15N and δ18O values of N2O emissions from aquatic environments, especially those impacted by anthropogenic N sources, are needed.

Although N2O generated from fossil fuel and biomass combustion may contribute ∼8% of the total source (or ∼20% of the anthropogenic source [5]), its isotopic composition is largely unknown. A lab-scale investigation of coal combustion revealed δ15N-N2O and δ18O-N2O values that were both enriched (unstaged combustion) and slightly depleted (air-staged combustion) relative to tropospheric N2O [71], indicating coal-derived N2O isotope values might be similar to marine sources but are dependent upon combustion conditions. A controlled study of gasoline-powered automobile exhaust concluded the average δ15N-N2O and δ18O-N2O values are similar to freshwater sources (−4.9 ±8.2‰ and +43.5 ± 13.9‰, respectively) [72]. Finally, N2O derived from biomass burning appears to closely resemble the δ15N and δ18O values of its endmembers; biomass-N and atmospheric O2, respectively [73]. We recognize the need to further investigate these potentially important sources and evaluate how they might affect the global N2O isotope budget.

After analyzing the filtered δ15N-N2O and δ18O-N2O data, the ‘bottom-up’ global N2O sources defined here were compared to estimates derived from ‘top-down’ atmospheric models (Fig. 7). Modelled estimates of the average anthropogenic and natural source fall within (or very close to) the soil ellipse and along a mixing line between soil and tropospheric N2O. All but two of the modelled estimates fall outside the freshwater ellipse, indicating the bulk of the combined anthropogenic and natural sources are not from freshwaters. If freshwaters were a major source of atmospheric N2O, a mixing line between freshwater and tropospheric N2O would be much closer to the anthropogenic and natural source values. It is not, and therefore we confirm what 'top-down' approaches have previously inferred: soil is the main source of N2O to the atmosphere.

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Fig 7. A comparison of bottom-up measurements to top-down estimates of N2O sources.

Previous top-down studies have used a variety of modelling approaches to apportion the global N2O budget into different sources, identified here by colour. Ishijima et al. [10] measured N2O in firn air and calculated the isotopic composition of the anthropogenic source for two time periods: 1952‒1970 and 1970‒2001 that differed markedly in δ15N. Park et al. [2] constrained the pre-industrial, natural N2O source from δ15N-N2O and δ18O-N2O measurements in firn air and then calculated the current anthropogenic N2O source using recent archived air samples. Rahn and Wahlen [6] evaluated a depleted ocean scenario [DOS, originally proposed by Kim and Craig [20]], and an enriched ocean scenario [EOS, originally proposed by Kim and Craig [19]] to calculate corresponding terrestrial N2O sources. Röckmann et al. [9] measured N2O in firn air and modelled the pre-industrial (natural) source, the modern global average source (pink circle with black outline), and the anthropogenic source under the IPCC3 (higher value) and IPCC2 (lower value) scenarios. Sowers et al. [11] measured firn air and gas trapped in an ice core to calculate a range of values for the isotopic composition of the average anthropogenic N2O source. Toyoda et al. [7] estimated the δ15N and δ18O value of the oceanic N2O source using ‘Keeling Plots’ of detailed water column data. Toyoda et al. [8] monitored the isotopic ratio of tropospheric N2O in the northern hemisphere on a monthly basis from 2000–2011, and then used a box-model to estimate the current anthropogenic source. For reference, we show the Continental N2O source, which is used along with the Marine source in our box-model calculations, and is operationally defined as Soil + Freshwater.

https://doi.org/10.1371/journal.pone.0118954.g007

We used the newly constrained δ15N-N2O and δ18O-N2O values for freshwater and soil (the combined continental source) and the ocean source to model their relative contributions to the total annual flux of N2O. We began with a box-model approach similar to that presented in [8,9,11,29]. We adapted the following isotope mass-balance equation for atmospheric N2O from Park et al. [29]: (1) where, Burden = Present-day burden of N2O in the troposphere [1553 (± 21.742) Tg N] as reported by Stocker et al. [5]. This estimate is for year 2011, and is updated from data provided in Prather et al. [74], who report an uncertainty of 1.4% for year 2010.

= Deseasonalized, linear trend in archived samples of tropospheric N2O measured by Park et al. [2]. The linear trends for δ15N-N2O and δ18O-N2O are −0.035‰ yr−1 (± 0.002) and −0.022‰ yr−1 (± 0.004), respectively.

Source = Annual N2O emissions from all sources (17.9 Tg N yr−1) [5]. For our calculations we applied an uncertainty of 25% to this parameter.

δSources = Flux-weighted δ15N-N2O or δ18O-N2O value (‰) of the average modern source (all natural and anthropogenic sources).

δTrop = δ15N-N2O or δ18O-N2O value (‰) of the modern troposphere (provided in Table 1).

ε = Apparent enrichment factor (‰) for N2O destruction processes in the stratosphere. These values are taken from Table 3 in Park et al. [29], and are −14.9‰ (± 0.5) for 15N and −13.5‰ (± 0.5) for 18O. Note, the ratio of enrichment factors provided by Park et al. [29] (18O: 15N = 0.906) is very close to the slope of the regression line of the stratospheric N2O data shown in Fig. 118O-N2O:δ15N-N2O = 0.886).

L = Photochemical loss rate of N2O in the stratosphere (14.3 Tg N yr−1) [5]. Following [29], we applied an uncertainty of 25% in our calculations.

The term (− ε × L) is a very close approximation of the 'Net Isotope Flux' (‰ Tg N yr−1) as defined in [29], and is the net annual flux of N2O isotopologues from the stratosphere to the troposphere.

Equation 1 can be rearranged to solve for δSources (Eq. 2), and all the known quantities provided above can be substituted into Eq. 2 to derive a flux-weighted, average modern source value (δSources) for δ15N-N2O and δ18O-N2O (‰).

(2)

Accordingly, we derive an average modern source value (± propagated standard deviation) for δ15N-N2O and δ18O-N2O of −8.4‰ (± 4.0) and +31.7‰ (± 13.9), respectively.

If we were to assume that all N2O fluxes (F) originate only from marine and continental sources, then: (3) and the flux-weighted modern source value is approximated by: (4) where the δ value of the continental and ocean sources are given in Table 2.

Combining Eq. 2 with Eq. 4 yields:

(5)

Given the assumption that FCont ≈ ∑ SourcesFOcean (Eq. 3), we can approximate Focean by:

(6)

Accordingly, using N isotope ratios we derive a value for Focean of ∼4.6 (± 12.6) Tg N yr−1, which is ∼26% of all sources (17.9 Tg N yr−1) [5]. The FCont is found by difference, and is approximately equal to 13.3 (± 13.4) Tg N yr−1, or 74% of all natural and anthropogenic N2O sources. The largest source of uncertainty in the N isotope mass-balance lies in the δ15N value of the continental source (1σ = 11.5‰), followed by ∑ Sources and L, which have a relative uncertainty of 25% in our model.

The most recent N2O budget estimates the combined soil, freshwater, and ocean flux to be ∼15.7 Tg N yr−1, or 87.7% of the total source [5]. Our approach assumes the ∑ Sources = 17.9 Tg N yr−1 (as reported in IPCC-AR5) because other terms in the mass-balance (e.g., N2O burden and loss rate) are based on a budget that includes all known sources. As such, we ignore the contributions from smaller sources such as human sewage, fossil fuels, industry, biomass combustion, and chemical production processes in the atmosphere, which have a combined annual flux of ∼2.2 Tg N yr−1 [5]. Despite this, our result for Focean is similar to the estimate provided in IPCC-AR5, which shows oceans contribute 21% of the annual N2O budget [5].

The O isotope mass-balance fails to derive a positive ocean flux (Focean = −4.5 ± 19.9 Tg N yr−1). This is because the δ18O separation between the troposphere and the continental source is smaller than it is for δ15N, and the term ∑ Sources (δTropδCont) is too small to make the numerator in Eq. 6 a net positive number. However, decreasing the loss rate (L) by 4 Tg N yr−1 and increasing ∑ Sources by the same amount yields a positive ocean flux = 4.6 Tg N yr−1. Therefore, if the uncertainty of these parameters is reduced in the future we may find that the O isotope budget balances.

Finally, we used a stable isotope Bayesian mixing model (MixSIAR) [75] to determine the proportions of soil, freshwater, and marine N2O that best predicted the average modern source (anthropogenic plus natural) (Eq. 7). (7) where, FSoil + FFreshwater + FOcean = 1.

MixSIAR, which is a front-end interface of the model SIAR (Stable Isotope Analysis in R) [76], is an ecological mixing model traditionally used to describe food web and predator-prey relationships. Values of δ15Nbulk-N2O, δ15Nα-N2O, δ15Nβ-N2O, and δ18O-N2O for the average modern source (the mixture) were taken from Röckmann et al. [9]. These estimates are almost identical to the ones calculated in our 2-box-model (above), and Röckmann et al. [9] provided 15N isotopomers, which allowed us to use 3 variables in our model runs (δ15Nbulk, δ18O, and SP). A series of Markov Chain Monte Carlo simulations, using values for soil, freshwater, and marine N2O (filtered, raw data compiled in this study), were done to find mixing solutions that best fit the average modern source (Table 3). Gelman-Rubin and Geweke diagnostic tests indicated a chain length of 300,000, burn in of 200,000, thinning of 50 (2-isotope) or 100 (3-isotope), and 3 chains were appropriate.

Model runs using only the δ15N-N2O and δ18O-N2O data (2-isotope mixing model, n = 1383 data pairs) produced results very similar to the model runs that also included SP data (3-isotope mixing model, n = 235 data triads). Overall, this Bayesian modeling exercise predicted the soil, freshwater, and ocean contributions (± 1σ) to the average modern N2O source were 0.43 (0.20), 0.34 (0.22), and 0.24 (0.16), respectively (Table 3). Unlike the box-model, this approach does not place a priori bounds on the data, and is not constrained by terms such as the stratospheric N2O loss rate (L), which have large uncertainty. However, this method also ignores the contributions of several small sources, which have a combined contribution of ∼12.3% to the total budget presented in IPCC-AR5 [5].

Both SIAR and the box-model predict the ocean flux to be 24% and 26% of the total, respectively, which closely confirms the scientific community's best estimate of the ocean flux as presented in IPCC-AR5 (21% of the total source). Further, the SIAR model output shows that freshwaters may contribute much more N2O than previously thought. The current N2O budget estimates the combined flux from rivers, estuaries, and coastal zones is 0.6 Tg N yr−1, or just 3% of the total source. While we acknowledge that there is δ15N-δ18O overlap in the soil and freshwater source (Fig. 6–top and bottom panels), these sources appear to be unique in δ15N-SP space (Fig. 6–middle panel). Therefore, we suggest there is a great need to quantify N2O fluxes from freshwaters, estuaries, and coastal zones, which have received considerably less attention than soil and off-shore marine environments.

Supporting Information

S1 Dataset. A comma-delimited text file with all the data collected and analyzed in this study.

In addition to δ15N-N2O, SP and δ18O-N2O values, we provide a reference citation, the category, a brief site description, and the criteria used to filter the data subsets (S1_Dataset.csv).

https://doi.org/10.1371/journal.pone.0118954.s001

(CSV)

S2 Dataset. A comma-delimited text file with weighted average δ15N-N2O, SP and δ18O-N2O values from select freshwater, soil and urban wastewater studies (S2_Dataset.csv).

https://doi.org/10.1371/journal.pone.0118954.s002

(CSV)

Acknowledgments

We thank the following people for their contributions in obtaining the new data presented in this study: R. Aravena, R. Elgood, J. Harbin, M. Levesque, W. Mark, M. Rempel, W. Robertson, M. Rosamond, A. Rossi, N. Senger, S. Thuss, A. Vandenhoff, and C. Wagner-Riddle. We are indebted to Hadley Wickham for the creation of ggplot2; an implementation of the Grammar of Graphics in R. We are especially grateful to all the anonymous reviewers who provided us with helpful suggestions as this manuscript evolved.

Author Contributions

Conceived and designed the experiments: DMS JJV SLS JS. Performed the experiments: DMS JJV. Analyzed the data: DMS JJV. Wrote the paper: DMS JJV SLS JS.

References

  1. 1. Fowler D, Coyle M, Skiba U, Sutton MA, Cape JN, Reis S, et al. The global nitrogen cycle in the twenty-first century. Philos Trans R Soc B Biol Sci. 2013;368: 20130164–20130164. pmid:23713126
  2. 2. Park S, Croteau P, Boering KA, Etheridge DM, Ferretti D, Fraser PJ, et al. Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940. Nat Geosci. 2012;5: 261–265.
  3. 3. Ravishankara AR, Daniel JS, Portmann RW. Nitrous Oxide (N2O): The dominant ozone-depleting substance emitted in the 21st century. Science. 2009;326: 123–125. pmid:19713491
  4. 4. Prinn RG, Weiss RF, Fraser PJ, Simmonds PG, Cunnold DM, Alyea FN, et al. A history of chemically and radiatively important gases in air deduced from ALE/GAGE/AGAGE. J Geophys Res Atmospheres. 2000;105: 17751–17792.
  5. 5. Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, et al., editors. Climate Change 2013: The Physical Science Basis. Contributions of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press; 2013. 1535 p.
  6. 6. Rahn T, Wahlen M. A reassessment of the global isotopic budget of atmospheric nitrous oxide. Glob Biogeochem Cycles. 2000;14: 537–543.
  7. 7. Toyoda S, Yoshida N, Miwa T, Matsui Y, Yamagishi H, Tsunogai U. Production mechanism and global budget of N2O inferred from its isotopomers in the western North Pacific. Geophys Res Lett. 2002;29: 7-1–7-4.
  8. 8. Toyoda S, Kuroki N, Yoshida N, Ishijima K, Tohjima Y, Machida T. Decadal time series of tropospheric abundance of N2O isotopomers and isotopologues in the Northern Hemisphere obtained by the long-term observation at Hateruma Island, Japan. J Geophys Res Atmospheres. 2013;118: 3369–3381.
  9. 9. Röckmann T, Kaiser J, Brenninkmeijer CAM. The isotopic fingerprint of the pre-industrial and the anthropogenic N2O source. Atmospheric Chem Phys. 2003;3: 315–323.
  10. 10. Ishijima K, Sugawara S, Kawamura K, Hashida G, Morimoto S, Murayama S, et al. Temporal variations of the atmospheric nitrous oxide concentration and its delta N-15 and delta O-18 for the latter half of the 20th century reconstructed from firn air analyses. J Geophys Res-Atmospheres. 2007;112.
  11. 11. Sowers T, Rodebaugh A, Yoshida N, Toyoda S. Extending records of the isotopic composition of atmospheric N2O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core. Glob Biogeochem Cycles. 2002;16: 76-1–76-10.
  12. 12. Boontanon N, Ueda S, Kanatharana P, Wada E. Intramolecular stable isotope ratios of N2O in the tropical swamp forest in Thailand. Naturwissenschaften. 2000;87: 188–192. pmid:10840807
  13. 13. Rahn T, Wahlen M. Stable isotope enrichment in stratospheric nitrous oxide. Science. 1997;278: 1776–1778. pmid:9388175
  14. 14. Croteau P, Atlas EL, Schauffler SM, Blake DR, Diskin GS, Boering KA. Effect of local and regional sources on the isotopic composition of nitrous oxide in the tropical free troposphere and tropopause layer. J Geophys Res. 2010;115. pmid:20463844
  15. 15. Dore JE, Popp BN, Karl DM, Sansone FJ. A large source of atmospheric nitrous oxide from subtropical North Pacific surface waters. Nature. 1998;396: 63–66.
  16. 16. Frame CH, Deal E, Nevison CD, Casciotti KL. N2O production in the eastern South Atlantic: Analysis of N2O stable isotopic and concentration data. Glob Biogeochem Cycles. 2014;28.
  17. 17. Kaiser J, Röckmann T, Brenninkmeijer CAM. Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide. J Geophys Res. 2003;108. pmid:14686320
  18. 18. Kaiser J, Engel A, Borchers R, Röckmann T. Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N2O isotope distribution. Atmospheric Chem Phys. 2006;6: 3535–3556.
  19. 19. Kim K, Craig H. Two-isotope characterization of N2O in the Pacific Ocean and constraints on its origin in deep water. Nature. 1990;347: 58–61.
  20. 20. Kim K, Craig H. Nitrogen-15 and Oxygen-18 characteristics of nitrous oxide: a global perspective. Science. 1993;262: 1855–1857. pmid:17829632
  21. 21. Koba K, Osaka K, Tobari Y, Toyoda S, Ohte N, Katsuyama M, et al. Biogeochemistry of nitrous oxide in groundwater in a forested ecosystem elucidated by nitrous oxide isotopomer measurements. Geochim Cosmochim Acta. 2009;73: 3115–3133.
  22. 22. Koehler B, Corre MD, Steger K, Well R, Zehe E, Sueta JP, et al. An in-depth look into a tropical lowland forest soil: nitrogen-addition effects on the contents of N2O, CO2 and CH4 and N2O isotopic signatures down to 2-m depth. Biogeochemistry. 2012;111: 695–713.
  23. 23. Li L, Spoelstra J, Robertson WD, Schiff SL, Elgood RJ. Nitrous oxide as an indicator of nitrogen transformation in a septic system plume. J Hydrol. 2014;519: 1882–1894.
  24. 24. Maeda K, Toyoda S, Shimojima R, Osada T, Hanajima D, Morioka R, et al. Source of nitrous oxide emissions during the cow manure composting process as revealed by isotopomer analysis of and amoa abundance in betaproteobacterial ammonia-oxidizing bacteria. Appl Environ Microbiol. 2010;76: 1555–1562. pmid:20048060
  25. 25. Mander Ü, Well R, Weymann D, Soosaar K, Maddison M, Kanal A, et al. Isotopologue ratios of N2O and N2 measurements underpin the importance of denitrification in differently N-loaded riparian alder forests. Environ Sci Technol. 2014;48: 11910–11918. pmid:25264900
  26. 26. Mandernack KW, Rahn T, Kinney C, Wahlen M. The biogeochemical controls of the δ15N and δ18O of N2O produced in landfill cover soils. J Geophys Res-Atmospheres. 2000;105: 17709–17720.
  27. 27. Naqvi SWA, Yoshinari T, Jayakumar DA, Altabet MA, Narvekar PV, Devol AH, et al. Budgetary and biogeochemical implications of N2O isotope signatures in the Arabian Sea. Nature. 1998;394: 462–464.
  28. 28. Ostrom NE, Sutka R, Ostrom PH, Grandy AS, Huizinga KM, Gandhi H, et al. Isotopologue data reveal bacterial denitrification as the primary source of N2O during a high flux event following cultivation of a native temperate grassland. Soil Biol Biochem. 2010;42: 499–506.
  29. 29. Park S, Atlas EL, Boering KA. Measurements of N2O isotopologues in the stratosphere: Influence of transport on the apparent enrichment factors and the isotopologue fluxes to the troposphere. J Geophys Res. 2004;109.
  30. 30. Park S, Pérez T, Boering KA, Trumbore SE, Gil J, Marquina S, et al. Can N2O stable isotopes and isotopomers be useful tools to characterize sources and microbial pathways of N2O production and consumption in tropical soils? Glob Biogeochem Cycles. 2011;25, GB1001.
  31. 31. Pérez T, Trumbore SE, Tyler SC, Matson PA, Ortiz-Monasterio I, Rahn T, et al. Identifying the agricultural imprint on the global N2O budget using stable isotopes. J Geophys Res-Atmospheres. 2001;106: 9869–9878.
  32. 32. Pérez T, Trumbore SE, Tyler SC, Davidson EA, Keller M, de Camargo PB. Isotopic variability of N2O emissions from tropical forest soils. Glob Biogeochem Cycles. 2000;14: 525–535.
  33. 33. Peters B, Casciotti KL, Samarkin VA, Madigan MT, Schutte CA, Joye SB. Stable isotope analyses of NO2−, NO3−, and N2O in the hypersaline ponds and soils of the McMurdo Dry Valleys, Antarctica. Geochim Cosmochim Acta. 2014;135: 87–101.
  34. 34. Priscu JC, Christner BC, Dore JE, Westley MB, Popp BN, Casciotti KL, et al. Supersaturated N2O in a perennially ice-covered Antarctic lake: Molecular and stable isotopic evidence for a biogeochemical relict. Limnol Oceanogr. 2008;53: 2439–2450.
  35. 35. Baulch HM, Schiff SL, Thuss SJ, Dillon PJ. Isotopic character of nitrous oxide emitted from streams. Environ Sci Technol. 2011;45: 4682–4688. pmid:21534582
  36. 36. Rock L, Ellert BH, Mayer B, Norman AL. Isotopic composition of tropospheric and soil N2O from successive depths of agricultural plots with contrasting crops and nitrogen amendments. J Geophys Res. 2007;112.
  37. 37. Samarkin VA, Madigan MT, Bowles MW, Casciotti KL, Priscu JC, McKay CP, et al. Abiotic nitrous oxide emission from the hypersaline Don Juan Pond in Antarctica. Nat Geosci. 2010;3: 341–344.
  38. 38. Santoro AE, Casciotti KL, Francis CA. Activity, abundance and diversity of nitrifying archaea and bacteria in the central California Current: Nitrification in the central California Current. Environ Microbiol. 2010;12: 1989–2006. pmid:20345944
  39. 39. Sasaki Y, Koba K, Yamamoto M, Makabe A, Ueno Y, Nakagawa M, et al. Biogeochemistry of nitrous oxide in Lake Kizaki, Japan, elucidated by nitrous oxide isotopomer analysis. J Geophys Res. 2011;116. pmid:24307747
  40. 40. Smemo KA, Ostrom NE, Opdyke MR, Ostrom PH, Bohm S, Robertson GP. Improving process-based estimates of N2O emissions from soil using temporally extensive chamber techniques and stable isotopes. Nutr Cycl Agroecosystems. 2011;91: 145–154.
  41. 41. Townsend-Small A, Pataki DE, Tseng LY, Tsai C-Y, Rosso D. nitrous oxide emissions from wastewater treatment and water reclamation plants in Southern California. J Environ Qual. 2011;40: 1542. pmid:21869516
  42. 42. Townsend-Small A, Pataki DE, Czimczik CI, Tyler SC. Nitrous oxide emissions and isotopic composition in urban and agricultural systems in Southern California. J Geophys Res. 2011;116. pmid:24307747
  43. 43. Townsend-Small A, Prokopenko MG, Berelson WM. Nitrous oxide cycling in the water column and sediments of the oxygen minimum zone, eastern subtropical North Pacific, Southern California, and Northern Mexico (23°N-34°N). J Geophys Res Oceans. 2014;119: 3158–3170.
  44. 44. Toyoda S, Yoshida N, Urabe T, Aoki S, Nakazawa T, Sugawara S, et al. Fractionation of N2O isotopomers in the stratosphere. J Geophys Res-Atmospheres. 2001;106: 7515–7522.
  45. 45. Toyoda S, Yoshida N, Urabe T, Nakayama Y, Suzuki T, Tsuji K, et al. Temporal and latitudinal distributions of stratospheric N2O isotopomers. J Geophys Res. 2004;109.
  46. 46. Toyoda S, Iwai H, Koba K, Yoshida N. Isotopomeric analysis of N2O dissolved in a river in the Tokyo metropolitan area. Rapid Commun Mass Spectrom. 2009;23: 809–821. pmid:19222057
  47. 47. Toyoda S, Yano M, Nishimura S, Akiyama H, Hayakawa A, Koba K, et al. Characterization and production and consumption processes of N2O emitted from temperate agricultural soils determined via isotopomer ratio analysis. Glob Biogeochem Cycles. 2011;25, GB2008.
  48. 48. Tumendelger A, Toyoda S, Yoshida N. Isotopic analysis of N2O produced in a conventional wastewater treatment system operated under different aeration conditions. Rapid Commun Mass Spectrom. 2014;28: 1883–1892. pmid:25088132
  49. 49. Van Groenigen JW, Zwart KB, Harris D, van Kessel C. Vertical gradients of δ15N and δ18O in soil atmospheric N2O—temporal dynamics in a sandy soil. Rapid Commun Mass Spectrom. 2005;19: 1289–1295. pmid:15838846
  50. 50. Well R, Flessa H, Jaradat F, Toyoda S, Yoshida N. Measurement of isotopomer signatures of N2O in groundwater. J Geophys Res. 2005;110.
  51. 51. Well R, Eschenbach W, Flessa H, von der Heide C, Weymann D. Are dual isotope and isotopomer ratios of N2O useful indicators for N2O turnover during denitrification in nitrate-contaminated aquifers? Geochim Cosmochim Acta. 2012;90: 265–282.
  52. 52. Westley MB, Yamagishi H, Popp BN, Yoshida N. Nitrous oxide cycling in the Black Sea inferred from stable isotope and isotopomer distributions. Deep Sea Res Part II. 2006;53: 1802–1816.
  53. 53. Xiong ZQ, Khalil MAK, Xing G, Shearer MJ, Butenhoff C. Isotopic signatures and concentration profiles of nitrous oxide in a rice-based ecosystem during the drained crop-growing season. J Geophys Res. 2009;114.
  54. 54. Yamagishi H, Westley MB, Popp BN, Toyoda S, Yoshida N, Watanabe S, et al. Role of nitrification and denitrification on the nitrous oxide cycle in the eastern tropical North Pacific and Gulf of California. J Geophys Res. 2007;112.
  55. 55. Yamulki S, Toyoda S, Yoshida N, Veldkamp E, Grant B, Bol R. Diurnal fluxes and the isotopomer ratios of N2O in a temperate grassland following urine amendment. Rapid Commun Mass Spectrom. 2001;15: 1263–1269. pmid:11466781
  56. 56. Yano M, Toyoda S, Tokida T, Hayashi K, Hasegawa T, Makabe A, et al. Isotopomer analysis of production, consumption and soil-to-atmosphere emission processes of N2O at the beginning of paddy field irrigation. Soil Biol Biochem. 2014;70: 66–78.
  57. 57. Yoshinari T, Altabet MA, Naqvi SWA, Codispoti L, Jayakumar A, Kuhland M, et al. Nitrogen and oxygen isotopic composition of N2O from suboxic waters of the eastern tropical North Pacific and the Arabian Sea-Measurement by continuous-flow isotope-ratio monitoring. Mar Chem. 1997;56: 253–264.
  58. 58. Yoshida N, Toyoda S. Constraining the atmospheric N2O budget from intramolecular site preference in N2O isotopomers. Nature. 2000;405: 330–334. pmid:10830958
  59. 59. Zhu R, Liu Y, Li X, Sun J, Xu H, Sun L. Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions. Rapid Commun Mass Spectrom. 2008; 22: 3570–3578. pmid:18932270
  60. 60. Zou Y, Hirono Y, Yanai Y, Hattori S, Toyoda S, Yoshida N. Isotopomer analysis of nitrous oxide accumulated in soil cultivated with tea (Camellia sinensis) in Shizuoka, central Japan. Soil Biol Biochem. 2014;77: 276–291.
  61. 61. Bauer B, Reynolds M. Recovering data from scanned graphs: Performance of Frantz’s g3data software. Behav Res Methods. 2008;40: 858–868. pmid:18697681
  62. 62. Yang H, Gandhi H, Ostrom NE, Hegg EL. Isotopic fractionation by a fungal P450 nitric oxide reductase during the production of N2O. Environ Sci Technol. 2014;48: 10707–10715. pmid:25121461
  63. 63. Jackson AL, Inger R, Parnell AC, Bearhop S. Comparing isotopic niche widths among and within communities: SIBER—Stable Isotope Bayesian Ellipses in R: Bayesian isotopic niche metrics. J Anim Ecol. 2011;80: 595–602. pmid:21401589
  64. 64. Batschelet E. Circular Statistics in Biology. London: Academic Press; 1981.
  65. 65. Toyoda S, Suzuki Y, Hattori S, Yamada K, Fujii A, Yoshida N, et al. Isotopomer analysis of production and consumption mechanisms of N2O and CH4 in an advanced wastewater treatment system. Environ Sci Technol. 2011;45: 917–922. pmid:21171662
  66. 66. Kool DM, Wrage N, Oenema O, Harris D, Van Groenigen JW. The 18O signature of biogenic nitrous oxide is determined by O exchange with water. Rapid Commun Mass Spectrom. 2009;23: 104–108. pmid:19061209
  67. 67. Snider DM, Venkiteswaran JJ, Schiff SL, Spoelstra J. Deciphering the oxygen isotope composition of nitrous oxide produced by nitrification. Glob Change Biol. 2012;18: 356–370.
  68. 68. Snider DM, Venkiteswaran JJ, Schiff SL, Spoelstra J. A new mechanistic model of δ18O- N2O formation by denitrification. Geochim Cosmochim Acta. 2013;112: 102–115.
  69. 69. Mohn J, Wolf B, Toyoda S, Lin C-T, Liang M-C, Brüggemann N, et al. Interlaboratory assessment of nitrous oxide isotopomer analysis by isotope ratio mass spectrometry and laser spectroscopy: current status and perspectives. Rapid Commun Mass Spectrom. 2014;28: 1995–2007. pmid:25132300
  70. 70. Thuss SJ, Venkiteswaran JJ, Schiff SL. Proper interpretation of dissolved nitrous oxide isotopes, production pathways, and emissions requires a modelling approach. PLoS ONE. 2014;9: e90641. pmid:24608915
  71. 71. Ogawa M, Yoshida N. Intramolecular distribution of stable nitrogen and oxygen isotopes of nitrous oxide emitted during coal combustion. Chemosphere. 2005;61: 877–887. pmid:15993467
  72. 72. Toyoda S, Yamamoto S, Arai S, Nara H, Yoshida N, Kashiwakura K, et al. Isotopomeric characterization of N2O produced, consumed, and emitted by automobiles. Rapid Commun Mass Spectrom. 2008;22: 603–612. pmid:18247408
  73. 73. Ogawa M, Yoshida N. Nitrous oxide emission from the burning of agricultural residue. Atmos Environ. 2005;39: 3421–3429. pmid:15952345
  74. 74. Prather MJ, Holmes CD, Hsu J. Reactive greenhouse gas scenarios: Systematic exploration of uncertainties and the role of atmospheric chemistry. Geophys Res Lett. 2012;39, L09803.
  75. 75. Stock BC, Semmens BX. MixSIAR GUI User Manual, version 2.1.2. 2013. Available: https://github.com/brianstock/MixSIAR.
  76. 76. Parnell AC, Inger R, Bearhop S, Jackson AL. Source partitioning using stable isotopes: coping with too much variation. PLoS ONE. 2010;5: e9672. pmid:20300637