B41G-01 INVITED
Isotope fractionation in the stratospheric removal of CH4 and N2O - effects of transport and chemistry and relevance for the global isotope budgets
The stratospheric removal reactions of the long-lived trace gases methane and nitrous oxide are associated with strong isotope fractionation. Since photochemically processed stratospheric air re-enters the troposphere as part of the global Brewer-Dobson circulation, the isotope fractionation in this stratospheric sink is an important component of the global isotope budgets for those gases, in particular for N2O, where the stratospheric represents is the only sink process. At the same time, the isotope signatures in the stratosphere itself reveal information about the individual chemical removal processes and about atmospheric transport. A comprehensive set of stratospheric balloon and aircraft samples from between 1987 and 2005 was analyzed for isotopic composition of nitrous oxide and methane. Cryogenic balloon samples were obtained at polar (Kiruna/Sweden, 68° N), mid-latitude (southern France, 44° N) and tropical sites (Hyderabad/India, 18° N). Aircraft samples were collected on board of the high-altitude aircraft M55 Geophysica during the EUPLEX 2003, TROCCINOX and SCOUT-O3 campaigns. In the lower stratosphere, relative isotope enrichments (delta values) and mixing ratios display a compact relationship, which is nearly independent of latitude and season and which can be explained equally well by Rayleigh fractionation or mixing. This well-established correlation allows quantifying the effect of the stratospheric sinks on the global isotope budgets. In the middle stratosphere this compact relationship gives way to meridional, seasonal and interannual variations. In general, the magnitude of the apparent fractionation constants (i.e., apparent isotope effects) increases continuously with altitude. Strong deviations from Rayleigh fractionation behavior occur where polar vortex air mixes with upper stratospheric/mesospheric air (e.g., during the boreal winters of 2003 and possibly 1992).
B41G-02
Constraining the N2O budget using isotopologues and isotopomers
The continued increase of N2O in the atmosphere is a serious environmental concern. As a consequence, N2O is one of the gases targeted for regulation by the Kyoto Protocol. Therefore, to stabilize concentrations at the present level, an immediate reduction of the additional flux of N2O that has occurred since the Industrial Revolution would be necessary. However, the N2O budget is at present not well quantified, making it difficult to determine the sources and the cause of its increase precisely (IPCC 2007). In order to find the best constraints to the N2O budget, we carried out a time dependent box model study similar to that of Rahn and Wahlen (2000) for N2O and its isotopologues and isotopomers from 1700 A.D. to the present. An adjoint code was constructed from the box model via the Tangent linear and Adjoint Model Compiler (TAMC) to calculate the gradient of a cost function, in this case the least squared sum of residuals, with respect to the initial conditions. These gradient calculations were then fed to optimization algorithms to minimize the value of the cost function, thereby acquiring the optimal parameters and the best fit to the observed data. Preliminary results are as follows: (1) The IPCC 2007 N2O budget appears to be consistent with the isotopic data, which help to refine some of the fluxes; (2) The mean nitrification/denitrification ratio for N2O sources is obtained from this analysis in order to explain the time evolution of both 15-N and 18-O data; (3) Our model confirms and extends the "depleted ocean" model of Rahn and Wahlen (2000) for the N2O oceanic source; and (4) Site preference data for 15-N provides the most sensitive discrimination between natural and anthropogenic sources of N2O.
B41G-03 INVITED
Isotopomeric Signature of Nitrous Oxide Discharged from Lake Biwa in Japan and a Polluted River in Mongolia
Nitrous oxide (N2O) is one of the greenhouse gases and considered to be substantially emitted from aquatic ecosystems eutrophicated by anthropogenic nitrogen. N2O is produced by nitrification (hydroxylamine oxidation) and denitrification (nitrite reduction) which also consumes N2O. Recent studies have demonstrated that isotopomeric signatures of N2O (δ15N, δ18O and site preference; SP) are useful to gain insight into production and consumption processes of N2O. Sutka et al. (2004, 2006) clearly revealed that SP can be used to identify production process of N2O, and Ostrom et al. (2007) expanded the usefulness of SP for tracing consumption process. Isotope ratios of substrates such as ammonia, nitrate, dissolved oxygen, and water could give much information to support interpretations from SP data. In this study, firstly, multiple stable isotope measurements have been conducted in Lake Biwa, a monomictic lake in Japan whose oxygen concentration is moderate. Stable isotope ratios of ammonium and nitrate in both lake water and sediment suggested several possible processes for N2O production, such as nitrification in water column and sediment, and denitrification in sediment. However, a lack of significant correlations in δ15N and δ18O of N2O with substrates, and δ15N and SP for sedimental N2O source estimated from Keeling plot suggested that N2O accumulated in Lake Biwa would be produced by nitrification in the sediment and diffuse into water column. Secondly, diurnal variation of N2O has been investigated in a river of Mongolia where a large amount of ammonium was directly flowed from sewage plant. Stable isotope ratios of ammonium, nitrate, and nitrite indicated that nitrification would occur in daytime and denitrification would occur in night time. Site Preference of N2O were always close to that produced by nitrite reduction though dissolved oxygen concentration increased in daytime. Thus, SP is particularly useful tool to diagnose processes of N2O emission.
B41G-04
Quantification of nitrous oxide emissions from nitrification and denitrification based on intramolecular site preference measurements
The intramolecular site preference of N2O was measured during a 2 week field campaign at a dairy farm in south eastern Australia. Field chambers were treated with 15N labelled ammonium or nitrate, either at application rates of 100 or 50 kg-N.ha-1, followed by irrigation. N2O emissions, 15N composition of N2O, soil mineral nitrogen concentrations and 15N composition, as well as soil moisture were monitored. From the interpretation of the collected data, site preference signatures were assigned to nitrification and denitrification. The calculated site preference signatures for nitrification and denitrification were -28 (±8) and 7 (±3) ‰ respectively. The assigned site preference signatures were used to determine the contribution of nitrification and denitrification to the emitted N2O. For all treatments denitrification sourced N2O emissions far outweighed (>90%) nitrification. N2O emission factors (amount of N emitted as N2O per amount of N applied) were also calculated for nitrification and denitrification from each treatment. This study illustrates the capacity of site preference measurements to quantify the relative contributions of nitrification and denitrification in field studies.
B41G-05
Denitrification is the Primary Source of Soil-Derived Nitrous Oxide in a Terrestrial Ecosystem: Evidence From Isotopologues
Isotopologues were applied to address the origins of N2O released from soils at the W.K. Kellogg Biological Station, a temperate agricultural field station in SW Michigan. Soils sampled included: unfertilized, tilled fields in soybean (AGsoy); tilled fields in corn receiving fertilizer (AGcorn), and an early-successional (SUCC) field abandoned in1989. Soil-derived N2O fluxes and isotopologues were measured in May, July, Sept. and Nov. in 2006. Fluxes were greatest in AGcorn, with a maximum average in May of 46.9 g N2O-N ha-1 d-1. The δ15N and δ15O values for N2O ranged between -16.1 and 9.7 ‰, and 22.2 and 57.3 ‰, respectively. Site Preference (SP), the difference between the δ15N of the central and outer N atoms in N2O, values weighted by flux were 3.0, 10.6 and 14.9 ‰ at AGcorn, SUCC and AGsoy, respectively. Based on SP values of 0 ‰ for denitrification and 33 ‰ for nitrification [Sutka et al., 2006], between 55 and 91 % of soil-derived N2O was from denitrification in all three fields. We further find that N2O reduction alters SP to a small extent and results in an overestimate of the importance of nitrification. Thus, our overall finding that 85% of the N2O flux is from denitrification is an underestimate.
B41G-06 INVITED
Diurnal Studies of Methane in the London Region: the use of Mixing Ratio and Automated Stable Isotope Measurements From a Fixed Site to Evaluate Sources
Over the last decade methane mixing ratios in the London area have decreased, with a reduction in mean annual methane mixing ratio of 4% between 1996 (2056 ppb) and 2006 (1974 ppb). Stable isotope studies can be used to consider which sources of the gas are changing. A continuous-flow gas chromatography isotope ratio mass spectrometry system has been developed to carry out high precision isotopic analysis of methane at ambient atmospheric mixing ratios. The repeatability (1σ) obtainable with this system is ± 0.05‰ for δ13C of CH4. An automated inlet system, connected to an outside air intake has been set up for diurnal studies of methane isotopes at Royal Holloway, 32 km WSW of the centre of London. Studies of known local sources of methane have also been carried out to identify source signatures of emissions. The mean methane δ13C source signature in air from the London sector for diurnal studies carried out at Royal Holloway in 2005 was 50.1 ± 0.7‰. This indicated a larger proportion of methane from biogenic sources, such as landfill sites, than was recognised in the London emissions inventories. The isotope studies carried out using the new system can be compared with large volume samples collected at the same site in the mid to late 1990s for which methane δ13C was analysed using a conventional off-line extraction line and dual inlet mass spectrometer. The methane source signature has decreased by 1.4 ± 1.0‰ over the last decade. Stable isotopic analysis provides a top-down approach of partitioning methane between its sources and is independent of statistical emissions inventories. The modified instrumentation has been shown to be suitable for measuring diurnal variations at urban sites and has great potential for measuring seasonal variations at background stations. The small sample volume, rapid analysis time and high precision enables a significant increase in the number of methane isotope measurements in air samples to be made, providing additional data for developing global models of isotopic change, an important step to understanding changing emissions.
B41G-07
A coupled molecular and field-based approach to study microbial controls on methane flux in upland soils
Predicting the responses of ecosystems to global change depends, in part, on understanding how soil microbial communities respond to external controls. To address this question, we are studying a relatively simple biogeochemical process: methane consumption in upland (i.e., well-drained, oxic) soils. In this process, methane molecules diffuse from the atmosphere into the soil, where they are consumed by methanotrophic bacteria. Because of the simplicity of this process, we have been able to develop a reaction-diffusion model that allows us to directly quantify methanotroph activity in situ from chamber-based measures of flux and diffusivity. Moreover, because the bacteria that oxidize methane come from a phylogenetically cohesive group, we can use molecular tools to quantify the size of methanotroph community and determine its species composition. Our application of these approaches on the Shortgrass Steppe Long-Term Ecological Research (SGS LTER) site in northeastern Colorado has revealed strong temporal and spatial patterns in methane uptake rates that are driven primarily by methanotroph activity, and very little by soil diffusivity. The temporal patterns in methanotroph activity follow seasonal changes in soil temperature and water content, with sharp reductions in activity associated with hot, dry conditions. Spatial patterns in activity follow differences in soil texture, with sandier soils expressing a greater range of methanotroph activity than clay soils. Although methanotroph abundances did not vary across soil types, the phylogenetic structure of the methanotroph communities differed significantly between clay and sand soil types. In addition, we found that the majority of methanotrophs were not the usual Type I or Type II, but instead were of the JR2 and JR3 types previously found only in a dry California grassland by Horz et al. AEM (2005). Together, these observations suggest that the species composition of methanotroph communities reflects changes in the physical environment. Given the potential for specialization among methanotrophs (e.g., for desiccation tolerance, or for methane affinity), we anticipate that the specific phylogenetic and physiological characteristics of methane oxidizer communities will modulate the responses of upland methane fluxes to climate change.
B41G-08
Revising Estimates of the Methane Production Pathway in Peatland Porewater Using Intramolecular Isotopic Analyses of Acetate
Stable isotopic measurements of methane and carbon dioxide are routinely applied to environmental samples to assess the relative importance of methane production by either aceticlastic or hydrogenotrophic methanogenesis. Such estimates rely upon assumptions about isotopic fractionation during methane production and oxidation. Rigorous isotope-based pathway estimates require knowledge of the carbon isotopic composition of both carbon dioxide and acetate. In practice, technical barriers have limited measurements of the isotopic composition of whole acetate in natural samples. Yet, the estimate of whole acetate isotopic values, even when available, may not represent accurately the composition of the methyl carbon, which is, in fact, the precursor to methane. It is exceedingly rare to find carbon isotopic measurements of acetate-methyl in the literature, and, to our knowledge, the d13C of the acetate-methyl precursor to methane has never before been reported from peatland porewater samples. Extremely 13C-depleted methane, -70 permil VPDB, and 13C-enriched carbon dioxide from acidic northern peat bogs are typically interpreted as signatures of hydrogenotrophic methanogenesis. The hypothesized dominance of methane production from hydrogen in acidic bogs contrasts with the vast majority of freshwater wetlands in which aceticlastic methanogenesis dominates. Using a new technique for the online analysis of the intramolecular carbon isotopic composition of acetate in natural samples, we find the acetate-methyl in peat porewaters can be significantly depleted relative to bulk organic matter. In porewater profiles from both winter and summer, acetate is as much as 15 permil depleted relative to bulk carbon. We hypothesize that acetate- methyl isotopic depletion results from conditions that favor autotrophic acetogenesis and subsequent acetate consumption by aceticlastic methanogens. Porewater depth profiles during winter and summer illustrate depth- dependent increases in the fraction of methane derived from carbon dioxide, with deeper peat dominated by hydrogenotrophic methanogenesis, but shallow peat dominated by aceticlastic methanogens. Significant aceticlastic methane production from autotrophically produced acetate challenges the ability of hydrogen isotopic measurements of methane to represent the pathway of methanogenesis. Supplementing our field observations, intramolecular acetate measurements of incubation experiments confirm that an aceticlastic methanogen can facilitate significant acetate-carboxyl exchange with DIC. This novel technique confirms two caveats associated with whole acetate carbon isotopic data: 1, the carboxyl carbon isotopic composition may not accurately reflect the composition of the parent molecule, and 2, the acetate methyl may be derived from inorganic carbon or the fractionation effect of fermentation in acidic porewaters may be significant.