B21A-0847 0800h
The Isotopomer Signal of N$_{2}$O Produced From Biological Sources
Atmospheric concentrations of N$_{2}$O have increased significantly since the industrial revolution; this is predominantly the result of the increasing nitrogen inputs to the biosphere. Although this is accepted as the major source of increasing atmospheric N$_{2}$O, the uncertainty associated with the quantification of these emissions is quite large. This uncertainty is partly because of both the poor understanding of the biological mechanisms producing N$_{2}$O, and their relative contributions to N$_{2}$O emissions. Isotopic fractionation provides insight into the processes responsible for N$_{2}$O emissions. Here we present measurements of the isotopic composition of N$_{2}$O emitted from different sources. To provide an improved signal for the minor isotopomers (specifically $^{15}$N$^{14}$N$^{16}$O and $^{14}$N$^{15}$N$^{16}$O), some of the analysis sites were fertilised with substrate that was 10% $^{15}$N enriched, providing a 20 fold increase in the signal for the minor isotopes. Collection of samples involved the cryogenic trapping of N$_{2}$O collected from the headspace of chambers over soil plots or estuarine sediments. The samples were analysed in the laboratory using high resolution FTIR spectrometry. The isotopomer measurements were augmented with soil parameters such as, nitrate and ammonium concentration, water filled pore space (WFPS) and pH. The combined measurements provide insight into the likely N$_{2}$O production mechanisms.
B21A-0848 0800h
Response of Marine Denitrifying Bacterial Populations to Nitrous Oxide: a Natural Sink?
The influence of nitrous oxide upon denitrifying populations was investigated in moderately saline shallow Texas coastal bays. Homogenized sediment samples were incubated under a N2O atmosphere for one week, after which {\it nosZ} DNA marker for nitrous oxide reductase enzyme was extracted and analyzed by quantitative polymerase chain reaction (qPCR) and semi-quantitative polymerase chain reaction/denaturing gradient gel electrophoresis (PCR/DGGE). qPCR analysis indicated a significant (over an order of magnitude) increase in {\it nosZ} copy number in response to N2O addition, with PCR/DGGE indicating a significant population shift towards a small number of select organisms. Intriguingly, {\it nirK} and {\it nirS} markers for nitrite reductase enzymes in denitrifying bacteria did not show a corresponding increase, suggesting that observed {\it nosZ} peak was not associated with typical denitrifying populations. It is possible that N2O consumption in these sediments might not be driven by normally abundant denitrifiers, but rather by a niche-specific guild of bacteria converting N2O to N2 and naturally present in sediments at low numbers. These organisms are capable of responding rapidly to increased N2O supply. Our results suggest that net biological emission of N2O from the sediments is regulated by bacteria at both production and consumption stage, and disturbance of either could result in enhanced N2O emission.
B21A-0849 0800h
Exploring Nitrogen and Oxygen Isotopic Relationships Among Nitrification Products
The process of nitrification plays a central role in the production of nitrite, nitrate, nitric oxide and nitrous oxide in the marine environment. The mass fluxes and isotopic signatures of these compounds are interrelated through various metabolic pathways and intermediates of this process, including interactions between multiple groups of bacteria. In this presentation, simple metabolic network models will be combined with measured isotope effects to explore the nitrogen and oxygen isotopic relationships between nitrification product pools. This work focuses on potential role of ammonia transport and oxygen isotopic exchange in setting the isotopic signatures in nitrite, nitrate, and nitrous oxide produced by marine nitrifiers. Current uncertainties and priorities for future research will be highlighted.
B21A-0850 0800h
Site-specific N$_{2}$O isotopic compositions from Brazilian Amazon soils and their implications for the global N$_{2}$O isotope budget
The site-specific $^{15}$N signature of N$_{2}$O (i.e., the $^{15}$N isotopic composition at the central or terminal position, expressed as $\delta$$^{15}$N$^{\alpha}$ or $\delta$$^{15}$N$^{\beta}$) from natural sources (e.g., tropical rain forest soils) remains poorly explored, despite indications from the few measurements available that the signature can be used for distinguishing microbiological N$_{2}$O production mechanisms in soils and for reducing uncertainties in the global isotope budget. We present measurements of site-specific $\delta$$^{15}$N$^{\alpha}$, $\delta$$^{15}$N$^{bulk}$ (i.e., the average of the two N atom positions), and $\delta$$^{18}$O of N$_{2}$O of soil gas samples collected in March 2002 during the rainy season in the Tapajos National Forest (TNF), Para, Brazil and on N$_{2}$O emissions from soil incubation experiments using the TNF soils and soils from Nova Vida Farm, Rondonia, Brazil. The microbiological and physical processes that could play roles in the observed distribution and variation of N$_{2}$O isotopologues from the tropical forest soils will be discussed and compared with results for N$_{2}$O produced in laboratory cultures of denitrifying bacteria and with stratospheric N$_{2}$O observations. We then use the combination of data from tropical soils, bacterial cultures and the stratosphere to constrain the contribution of the tropical forest soils to the atmospheric N$_{2}$O isotope budget.
B21A-0851 0800h
Ammonia Oxidizing, Nitrite Reducing Bacteria and the Cycling of Nitrous Oxide in the Oxygen Minimum Zone (OMZ) of the Eastern South Pacific
The distribution of nitrous oxide, oxygen, nitrite and nitrate, and 16S rDNA and functional genes ({\it amoA},{\it nirS}) richness of ammonia oxidizing (AOB) and nitrite reducing bacteria (NRB) were studied in the water column of one of the shallowest ($<$50 m depth) and most severe (oxygen $<$4.5 $\mu$M) OMZ of the world's ocean. Results show two distinct environments in which nitrous oxide gets cycled. The first one is at the oxyclines, where high concentrations of up to 0.4 $\mu$M (3630%) of nitrous oxide are present. The second one is at the core of the OMZ, where low nitrous oxide concentration ($<$0.01 $\mu$M) are observed. A secondary nitrite maximum ($>$8 $\mu$M) is also present at the OMZ core. The relationship among apparent oxygen utilization (AOU), apparent nitrous oxide production, and nitrate distribution allowed the differentiation among nitrification, denitrification, and the coupling between both, at AOU values of $<$200, $>$230 and 200-230 $\mu$mol kg$^{-1}$, respectively. The richness of the AOB ribotypes (DGGE) and the NRB {it\nirS} TRFs increased towards the OMZ core, as well as the presence of the same kind of ribotypes and TRFs in the upper boundary and OMZ core. Our results suggest the existence of three nitrous oxide cycling layers in the upper 400 m (according to AOU analyses), where nitrification and denitrification contributes differently. In addition, the molecular evidence supports the coexistence of AOB and NRB in the upper boundary and core of the OMZ, in contrast with the conventional view of their vertical separation due to their different oxygen requirements.
B21A-0852 0800h
Characterization of Isotopomer Factionation During Consumption of Nitrous Oxide in Pure Culture and Soils
Previous work in our laboratory demonstrates that the intermolecular distribution of $^{15}$N in the N$_{2}$O molecule can be used distinguish N$_{2}$O derived from nitrification and denitrification in pure culture. However, if consumption of N$_{2}$O during denitrification also imparts an isotopomeric effect, it needs to be accounted for. We determined fractionation factors associated with N$_{2}$O consumption for bulk $^{15}$N, $^{18}$O and site preference (difference in $\delta$$^{15}$N between the central and outer N atoms) in a pure culture of {\it Pseudomonas denitrificans} and soils from the Kellogg Biological Station's Long Term Ecosystem Research Site. The soils derive from a deciduous forest and agricultural fields with different management histories. Soils were collected by auger, homogenized and stored dry prior to initiation of mesocosm experiments. Denitrification in soil mesocosms was promoted by a headspace of pure N$_{2}$ and addition of water to near saturation levels. The isotopic enrichment factor, $\epsilon$ (defined as ($\alpha$-1)*1000)) for $^{15}$N, $^{18}$O and site preference during N$_{2}$O consumption by {\it Pseudomonas denitrificans} was found to be 10.9, 24.8, and 20.3 $\permil$, respectively. Isotopic enrichment factors in soils ranged from 1.0 to 9.2 $\permil$ for $^{15}$N, 2.7 to 24.5 $\permil$ for $^{18}$O, and 6 to 11.9 $\permil$ for the site preference. The enrichment factors in soils were generally less than those observed in pure culture. This likely reflects suppression of fractionation during diffusion in wet soils. Isotopic enrichment factors for nitrogen and oxygen were strongly correlated such that fractionation for $^{18}$O exceeded that associated with $^{15}$N by a factor of 2.5. This relationship may provide a basis to identify N$_{2}$O consumption and account for it in models of source apportionment.
B21A-0853 0800h
Hydrologic controls on nitrous oxide production and consumption in a forested headwater catchment in central Japan
We investigated producing and consuming processes of N$_{2}$O by measuring the N$_{2}$O concentration in the soil gas and of dissolved phase in groundwater in a forested headwater catchment in central Japan. According to the previous study in the same site showing that N$_{2}$O has mainly produced by denitrification in the groundwater, we focused N$_{2}$O dynamics in the groundwater. The concentrations of N$_{2}$O, NO $_{3}$$^{-}$, DOC, DO, and \delta$^{15}$N- N$_{2}$O in the groundwater were measured with monitoring the groundwater revels. Concentrations of dissolved N$_{2}$O in the groundwater ranged from 0.12 to 32.5mgN/l. It is considered that N$_{2}$O produced by denitrification was controlled by DO, NO $_{3}$$^{-}$ and DOC. For the groundwaters taken from the edge part of groundwater body where the highest dissolved N$_{2}$O was observed, negative correlation (r$^{2}$=0.81, p$<$0.005) was found between DO concentration and dissolved N$_{2}$O concentration, while there were no correlations with NO $_{3}$$^{-}$ and DOC concentrations. This indicates that the N$_{2}$O production at the edge part of the groundwater body was regulated only by DO concentration. The edge part of groundwater body could receive both DOC and NO $_{3}$$^{-}$ rich water generated by decomposition of litters in surface soils. This was supplied by saturated through flow from the upstream hillslope, that had relatively short residence time according to the geochemical solute concentration such as Na$^{+}$. \delta$^{15}$N- N$_{2}$O of this point was the smallest (from +1.5 to -19.6 permil, -9.6 permil as the average) and had a negative correlation (r$^{2}$=0.61, p$<$0.05) with N$_{2}$O concentration, confirming that that dissolved N$_{2}$O was produced mainly by denitrification. These observed results suggest that hydrological controls are critical for N$_{2}$O dynamics as a transporting conditions of DOC and NO $_{3}$$^{-}$ from the surface soil to the groundwater body.
B21A-0854 0800h
Isotopomer Signatures of Nitrous Oxide Emissions From Soils During Nitrification, Denitrification and Chemodenitrification
New Zealand's greenhouse gas budget is atypical amongst developed nations in that 49.2% of total emissions in 2002 were produced by the agriculture sector. Agricultural soils are the source of most of the N$_{2}$O emissions in New Zealand and have increased by 27.6% since 1990. Emissions of N$_{2}$O arise directly from agriculture soils as a result of fertilizer, predominantly urea, and animal urine deposition onto grazed pastures. Identification of N$_{2}$O source mechanisms in soils would enable better targeting, timing and development of appropriate mitigation strategies. N$_{2}$O isotopomers have shown promise as a means for distinguishing the mechanisms that generate N$_{2}$O emissions due to the differential enrichment in the central N position ($\alpha$) relative to the terminal N position ($\beta$). P\'{e}rez et al. (2001) reported the first isotopomer measurements from agricultural soils with nitrification preferentially enriching the $\alpha$ position. Sutka et al. (2003) reported the first measured N$_{2}$O isotopomer values for isolated microbial processes finding significant differences in site preference following hydroxylamine oxidation by Nitrosomonas and Methylococcus species The results of Yamulki et al. (2001) suggested a possible change from nitrification to denitrification processes following cow urine applications to soil. However, in the urine patch there are three possible mechanisms for N2O formation, nitrification, chemodenitrification, and denitrification. These mechanisms may possibly occur separately or simultaneously. The objective of our work was to measure isotopomer signatures of N$_{2}$O, from the three N$_{2}$O production mechanisms mentioned above, from soils in the laboratory under carefully delineated conditions. Interim results to date are reported on.
B21A-0855 0800h
Apportionment of Nitrous Pxide Flux From a Successional Midwest Grassland to Denitrification and Nitrification Based on Isotopomers.
Nitrous oxide (N$_{2}$O) is an important greenhouse gas that in agricultural environments may contribute more to global warming than CO$_{2}$. We investigated nitrogen cycling and N$_{2}$O flux following the initial tillage of a previously uncultivated successional Midwest grassland. This grassland has now been tilled annually for a total of 3 years. Tillage resulted in a 5-10-fold increase in NO$_{3}$-N; a 5-fold increase in nitrifier enzyme activity (0.61 vs. 3.04 mg N kg$^{-1}$ soil d$^{-1}$); and increased N$_{2}$O flux during the first two years. Following each tillage event, NH$_{4}$+ concentration increases, followed successively by increases in NO$_{3}$- concentration and N$_{2}$O flux. Soil trace gas flux chambers were placed on a tilled plot and a control plot that had not been tilled approximately 1.5 weeks after the third annual tillage. Samples for isotopic analysis of N$_{2}$O were taken over the course of one-hour chamber closings during four different times of day from early morning to evening. Fluxes of 14.6 to 22.8 g N$_{2}$O-N ha$^{-1}$ day$^{-1}$ were measured in the tilled treatment plot as compared to 0.6 to 1.9 g N$_{2}$O-N ha$^{-1}$ day$^{-1}$ in the non-tilled control treatment. Fluxes increased in the control and tilled plots and were highest at the afternoon (4 PM) and evening (8:30 PM) sampling points. The isotopomer composition of N$_{2}$O was evaluated to determine the relative importance of nitrification and denitrification to soil-derived N$_{2}$O. The bulk $\delta$$^{15}$N-N$_{2}$O from the tilled field decreased during each closing and ranged from - 10.2 to 1.3 $\permil$. The nitrogen isotopic composition in the $\alpha$, or central, N atom similarly decreased during each closing and ranged from - 4.7 to 11.2 $\permil$. Values for the isotopomer site preference ($\delta$$^{15}$N$\alpha$ - $\delta$$^{15}$N$\beta$) varied between 9.8 to 19.7 $\permil$ and decreases with time were also evident. Variation in isotopomer values during closings reflected mixing of atmospheric and soil-derived N$_{2}$O. Based on the measured site preference and prior knowledge of values for site preference for tropospheric N$_{2}$O (18.7 $\permil$) and for N$_{2}$O derived from denitrification (0 $\permil$) and nitrification (33 $\permil$) isotope-mixing models were used to determine soil-derived isotopomer values (Keeling plot) and to apportion soil-derived N$_{2}$O to nitrification and denitrification. Within the first closing in the tilled plot N$_{2}$O originated completely from denitrification whereas a mixture of 70$%$ and 30$%$ from denitrification and nitrification, respectively, was determined for the fourth closing.
B21A-0856 0800h
N$_{2}$O Production and Consumption in Semi-Arid Soils
We investigated how summer monsoon precipitation affects surface and subsurface N$_{2}$O production and consumption in semi-arid rangeland soils, southern Arizona, USA. Surface fluxes and profiles up to 2 m deep were sampled for N$_{2}$O concentration, $\delta$$^{15}$N and $\delta$$^{18}$O. Soil samples for incubations and C and N stable isotope composition were taken at the surface through 50 cm depth. Pre-monsoon soils were dry with small N$_{2}$O fluxes (1.6$\pm$0.6 $\mu$g/m$^{2}$/hr). Initial precipitation events wetted the surface 10 cm and triggered much larger N$_{2}$O fluxes (11.7$\pm$6.7 $\mu$g/m$^{2}$/hr). $\delta$$^{15}$N values of N$_{2}$O surface fluxes averaged -9.7$\pm$1.9$\permil$ after precipitation and increased as the soil surface dried (-6.5$\pm$2.1$\permil$). Soil profile $\delta$$^{15}$N values after rainfall ranged from -3.8 to -1.5$\permil$, with one outlier at 8.6$\permil$. $\delta$$^{18}$O values of N$_{2}$O fluxes following rainfall were highly variable (29$\pm$32$\permil$), but were more consistent (43.4$\pm$0.1$\permil$) when the soil surface dried. Soil profile $\delta$$^{18}$O values following rainfall averaged 44.1$\pm$1.0$\permil$, with one outlier 55.2$\permil$. Analysis of N$_{2}$O trapped from aerobic laboratory incubations of surface soil had $\delta$$^{15}$N (-10.8$\permil$) and $\delta$$^{18}$O (22.0$\permil$) values indicative of nitrification processes. Assuming that incubation isotope values represent production, we calculated that 40% of N$_{2}$O was consumed before leaving the soil. Our results suggest that after precipitation, surface produced N$_{2}$O preserved isotopic signal of production but as the surface dried, flux and soil profile N and O isotope values reflected considerable N$_{2}$O consumption. Isotopic variability of fluxes following rainfall indicates that several distinct surface production mechanisms.
B21A-0857 0800h
The Effect of Temperature and Hydrogen Limited Growth on the Fractionation of Sulfur Isotopes by {\it Thermodesulfatator indicus}, a Deep-sea Hydrothermal Vent Sulfate-Reducing Bacterium
Sulfate-reducing bacteria fractionate sulfur isotopes during dissimilatory sulfate reduction, producing sulfide depleted in $^{34}$S. Although isotope fractionation during sulfate reduction of pure cultures has been extensively studied, most of the research to date has focused on mesophilic sulfate reducers, particularly for the species {\it Desulfovibrio desulfuricans}. Results from these studies show that: 1) fractionations range from 3-46$\permil$ with an average around 18$\permil$, 2) when organic electron donors are utilized, the extent of fractionation is dependent on the rate of sulfate reduction, with decreasing fractionations observed with higher specific rates, 3) fractionations are suppressed with low sulfate concentrations, and when hydrogen is used as the electron donor. High specific sulfate-reduction rates are encountered when sulfate-reducing bacteria metabolize at their optimal temperature and under non-limiting substrate conditions. Changes in both temperature and substrate availability could shift fractionations from those expressed under optimal growth conditions. Sulfate reducers may frequently experience substrate limitation and sub-optimal growth temperatures in the environment. Therefore it is important to understand how sulfate-reducing bacteria fractionate sulfur isotopes under conditions that more closely resemble the restrictions imposed by the environment. In this study the fractionation of sulfur isotopes by {\it Thermodesulfatator indicus} was explored during sulfate reduction under a wide range of temperatures and with both hydrogen-saturating and hydrogen-limited conditions. {\it T. indicus} is a thermophilic (temperature optimum = $70\deg$ C) chemolithotrophic sulfate-reducing bacterium, which was recently isolated from a deep-sea hydrothermal vent on the Central Indian Ridge. This bacterium represents the type species of a new genus and to date is the most deeply branching sulfate-reducing bacterium known. {\it T. indicus} was grown in carbonate-buffered salt-water medium with H$_{2}$ as the sole electron donor, and CO$_{2}$ as primary carbon source. The fractionation of sulfur isotopes was measured in batch cultures and in a thermal gradient block over the full temperature range of growth (40-$80\deg$ C). For experiments in the gradient block, cell-specific rates of sulfate reduction increased with increasing temperatures to $70\deg$ C after which sulfate-reduction rates rapidly decreased. The range of fractionations (1.5-10$\permil$) was typical for growth with hydrogen as the electron donor. Fractionations decreased with increasing temperature from 40--$60\deg$ C, and increased with increasing temperatures from 60-$80\deg$ C. Growth under H$_{2}$-limited conditions in a fed-batch culture revealed high fractionations of 24-37$\permil$. This is the first report of sulfur isotope fractionation under H$_{2}$ limited growth and indicates that large fractionations are produced when H$_{2}$ is supplied as a limiting substrate. Our results suggest that fractionation is controlled by the competition of forward and reverse enzymatic reaction rates during sulfate reduction and by sulfate transport into the cell.
B21A-0858 0800h
Aquifer Microbial Diversity Represented by Whole Cell and Dissolved DNA
Microbial diversity measurements of aquifers are faced with two major sources of bias. These are a reflection of molecular (caused by amplification, labeling and extraction dissimilarities) and sampling aspects. Sampling of aquifers represents a considerable and often insurmountable bias since almost all evaluations rely on water samples where free living bacteria will predominate. This highly problematic when one desires to know the metagenomic potential of zone of well influence. We present a method designed to avoid cellular collection biases imposed by the difficulty in collecting attached biomass. Dissolved DNA (d-DNA) and whole bacterial cells were concentrated from Snake River Plain aquifer water. 96% of dDNA was recovered from DNA spiked surrogates using an anion-exchange membrane method. Approximately 2,000 ng DNA was recovered per liter of water. Total DNA turnover rate was measured at 49.3 ng/ml/day, at DNA concentrations above 1 mg/l, indicating that aquifer dDNA represents a dynamic steady state balanced between dDNA release and native DNA degradation activities. 16s, dual labeled T-RFLP analysis was used analyze the genomic complexity of whole bacterial cells collected via filtration versus d-DNA sources of bacterial DNA from 16 l of water. Highly similar T-RFLP profiles (with two restriction endonucleases) were obtained for both dDNA (200 nm filtrate) and the whole cells harvested from the same filter. This demonstrates that dDNA was an accurate reflection of the total bacterial community found within the well volume. We present evaluations of the anion-exchange system with respect to DNA recovery parameters, and demonstrate the suitability of this DNA template for PCR amplification reactions.
B21A-0859 0800h
Constraining Flows of Nitrogen by Assimilating Data into a Multi-Element Marine Ecosystem Model
We have developed a new, multi-element ecosystem model to simulate a set of batch incubation experiments of natural phytoplankton assemblages. The model divides organisms into classes based on differences in size and function and simulates the flexible-composition of phytoplankton. Nutrient concentrations and plankton community differed among the incubations, allowing us to examine the functioning of the ecosystem by comparison. Data included concentrations of nutrients, organic matter (particulate and dissolved) and plankton (biomass by species). We used a Monte Carlo Markov Chain method to assimilate these data into our model and examined the distributions of simulated values from the ensemble of simulations. The model simulated well the changes in C:N ratio of bulk particulate organic matter (POM), and its difference between experiments. We examined the simulated gross flows of carbon and nitrogen (which cannot be directly measured), dividing the ecosystem between the Microbial Food Web (MFW) and Grazing Food Web (GFW) based on the size of organisms. The MFW dominated the flow of nitrogen in all incubations. The bulk POC:PON ratio varied inversely with the gross amount of nitrogen remineralized in a given incubation. The flexible composition of phytoplankton is a key link between remineralization and the dynamic stoichiometry of POM.
B21A-0860 0800h
S and O Isotope Studies of Microbial S Cycling in the Deep Biosphere of Marine Sediments: Eastern Equatorial Pacific Ocean
We have determined the oxygen ($^{18}$O/$^{16}$O) and sulfur ($^{34}$S/$^{32}$S) isotope ratios of porewater sulfate to depths of over 400 mbsf in sediments from open-ocean and upwelling sites in the Eastern Equatorial Pacific ocean. Sulfate $\delta$$^{18}$O ranges from near-normal seawater values (9.5 permil) at organic-poor open-ocean sites, to approximately 30 permil at sites with higher organic matter content and higher associated microbial activity. Depth-correlative trends of $\delta$$^{18}$O, $\delta$$^{34}$S, alkalinity, methane, ammonium and the presence of sulfide, indicate significant oxidation of sedimentary organic matter by sulfate-reducing microbial populations as well as anaerobic oxidation of methane. $\delta$$^{18}$O-SO$_{4}$ values at low-activity sites reveal the presence of significant microbial sulfur-cycling activity despite relatively flat sulfate concentration and $\delta$$^{34}$S profiles. This activity may include contributions from several processes including: enzyme-catalyzed equilibration between oxygen in sulfate and water superimposed upon microbial sulfate reduction, sulfide oxidation, and bacterial disproportionation of sulfur intermediates. Large isotope enrichment factors observed at low-activity sites (40-80 permil) likely reflect concurrent processes of: kinetic isotope fractionation, equilibrium fractionation between sulfate and water, and sulfide oxidation at low rates of sulfate reduction. Results of this study indicate that coupled measurements of S and O isotope ratios of porewater sulfate are a powerful tool for tracing microbial activity and sulfur cycling in marine sediments.
B21A-0861 0800h
Using Pure Cultures to Define the Site Preference of Nitrous Oxide Produced by Microbial Nitrification and Denitrification
Defining the site preference of nitrous oxide (N$_{2}$O) produced in pure culture studies is crucial to interpreting field data. We have previously demonstrated that the intramolecular distribution of nitrogen isotopes (isotopomers) can be used to differentiate N$_{2}$O produced by nitrifier denitrification and nitrification in cultures of Nitrosomonas europaea. Here, we have expanded on our initial results and evaluated the isotopomeric composition of N$_{2}$O produced during nitrification and nitrifier denitrification with cultures of Nitrosospira multiformis. In addition, we have analyzed N$_{2}$O produced during methanotrophic nitrification, denitrification, and fungal denitrification. To evaluate N$_{2}$O production during nitrification and nitrifier denitrification, we compared the site preference of N$_{2}$O formed as a result of nitrite reduction and hydroxylamine oxidation with {\it Nitrosomonas europaea} and {\it Nitrosospira multiformis}. The average site preference of N$_{2}$O produced by hydroxylamine oxidation was similar for {\it Nitrosomonas europaea} (33.0 $\pm$ 3.5 $\permil$) and {\it Nitrosospira multiformis} (33.1 $\pm$ 4.2 $\permil$). Nitrous oxide produced by nitrifier-denitrification by {\it Nitrosomonas europaea} and {\it Nitrosospira multiformis} had a similar site preference of - 1.4 $\pm$ 4.4 $\permil$ and - 1.1 $\pm$ 2.6 $\permil$ respectively. The results indicate that it is possible to differentiate between N$_{2}$O produced by nitrite reduction and hydroxylamine oxidation by ammonia oxidizing bacteria. Methanotrophic nitrification was evaluated by analyzing the N$_{2}$O produced during hydroxylamine oxidation in concentrated cell suspensions of two methane oxidizing bacteria. The site preference of N$_{2}$O produced by the two methane oxidizers, {\it Methylococcus capsulatus} Bath and {\it Methylosinus trichosporium} was 31.8 $\pm$ 4.7 $\permil$ and 33.0 $\pm$ 4.5 $\permil$ respectively. The results indicate that a site preference of 33 $\permil$ is applicable for nitrification regardless of whether a methane oxidizer or ammonia oxidizer is involved in the reaction. To determine the site preference of N$_{2}$O produced during denitrification we used concentrated cell suspensions of two organisms ({\it Pseudomonas chlororaphis} and {\it Pseudomonas aureofaciens}) that lack N$_{2}$O reductase. The site preference of N$_{2}$O produced during nitrite reduction was similar for {\it P. chlororaphis} (0.3 $\pm$ 2.7 $\permil$) and P. aureofaciens (- 0.3 $\pm$ 1.7 $\permil$ ). The results indicate that the site preference of N$_{2}$O produced during nitrite reduction is 0 $\permil$ regardless of whether the organism is a denitrifier or nitrifier. Fungal denitrification was investigated using pure cultures of {\it Fusarium oxysporum} and {\it Cylindrocarpon tonkinense}. The site preference of N$_{2}$O produced during nitrite reduction was similar for the cultures with an average site preference of 34.7 $\pm$ 2.2 $\permil$ for {\it Fusarium oxysporum} and 29.7 $\pm$ 1.7 $\permil$ for {\it Cylindrocarpon tonkinense}. The data indicate that fungal denitrification and bacterial denitrification can be distinguished based on site preference. The results from all of the pure culture studies indicate that isotopomers can be used to apportion bacterial nitrification and denitrification and in field studies.
B21A-0862 0800h
Self-organizing patterns of peat decomposition in mires and implications for greenhouse gas emissions
In many peatlands occurring in both permafrost and temperate terrain, vegetation develops in the form of spatially heterogeneous patterns or pools. Because GHG fluxes and their responses to allogenic forcing from different elements of mire patterns (eg., pools and raised peat strings) can vary, quantifying the role of peatlands in climate change may require using models for the dynamics of patterns. To this end, we investigate patterns of consistently-spaced $\approx$ 2 m diameter pools that occur in permafrost mires across a 4500 y chronosequence at Espenberg, NW Alaska. Pools first occur in small groups ($<5$) on 1200 y old mires where peat depth is $\approx$ 1 m, and pool patterns increase in size thereafter. Dissolved oxygen in pool water and peat stratigraphy in pool walls and floors are consistent with formation by peat decomposition amplified by thaw-derived subsidence of ice-rich organic soil. Natural patterns are reproduced by a model in which new pools principally initiate in $\approx$ 3 m wide annuli around existing pools, as evaluated using Kolmogorov-Smirnov tests of pool spacing distributions. Models encapsulating spatially random mechanisms for pool formation cannot reproduce natural patterns. Pool replication occurs by suppression of peat accumulation adjacent to 0.2--1.0 m high frost-heaved peat rings by drifting snow that persists into the growing season. Because pool initiation depends on distribution of earlier pools, rates and patterns of peat decomposition in the mire may nonlinearly depend upon, and have intrinsic time-scales exceeding, external influences on decomposition such as climate. Variations in peat accumulation or decomposition, and hence carbon accumulation or release, are localized in pools rather than occurring as a bulk change across a peatland surface. The influence of emergent pattern dynamics in this environment, and others, such as the patterned peatlands of Cape Breton, Nova Scotia, will be discussed.