15N2 Gas Research Articles

First evidence of nitrogen fixation associated with bryophytes from coastal Wabanaki–Acadian forests

Associations between bryophytes and dinitrogen (N2)-fixing bacteria are a significant source of exogenous N in unmanaged boreal and possibly temperate ecosystems. However, the extent to which biological N2-fixation (BNF) applies to the boreal–temperate ecotone remains elusive. The current focus on common species limits our understanding of BNF at the community level. Our objective was to characterize the presence of cyanobacteria and BNF activity associated with bryophytes in the coastal forests of Fundy National Park (New Brunswick, Canada). In 2021, we harvested three liverwort and 11 moss species from two sites (71 samples) and measured environmental covariates (e.g., canopy composition and soil pH). We used stable isotope incubations with 15N2 gas in growth chambers to quantify potential BNF activity and used phycocyanin extractions as a cyanobacteria abundance proxy. Many species presented detectable BNF rates, which were similar to or higher than those of well-studied feather mosses. These included species that have rarely been found to contribute to BNF. While cyanobacteria were present on most samples, we found no positive association between abundance and BNF. Our findings are among the first records for bryophyte-associated BNF in the boreal–temperate ecotone of eastern Canada and offer insights into the potential role of this process in N cycling in coastal conifer-dominated forests.

From Dinitrogen to N‐Containing Organic Compounds: Using Li2CN2 as a Synthon

AbstractThrough the synergies of a heterogeneous synthetic approach and a homogeneous synthetic methodology, N‐containing organic compounds can be synthesized via activated N‐containing species prepared from N2 gas and suitable carbon sources. From N2, carbon, and LiH, we have previously succeeded in the high‐yield preparation of Li2CN2 as the activated N‐containing species. In this work, we applied Li2CN2 as a novel synthetic synthon for constructing N‐containing organic compounds. A series of reaction models, including a substitution reaction, cycloaddition reaction, and transition metal‐catalyzed coupling reaction, were successfully performed using Li2CN2 under mild conditions. Various valuable cyanamides, carbodiimides, N‐aryl cyanamides and 1,2,4‐triazole derivatives were readily synthesized in moderate to excellent yields. With this method, the 15N‐labeled products, including oxazolidine derivatives with anti‐cancer activity, could also be facilely prepared from 15N2 gas.

Limnospira fusiformis harbors dinitrogenase reductase (nifH)-like genes, but does not show N2 fixation activity

East African soda lakes (EASLs), some of them world-renowned for their large flocks of flamingos, range amongst the most productive aquatic ecosystems worldwide. The non-heterocytous filamentous cyanobacterium Limnospira fusiformis (formerly Arthrospira fusiformis or Spirulina platensis), forming almost unialgal blooms, is supposed to be a key driver in those ecosystems and is gaining increasing attention because of its nutritional value. Compared to phosphorus and carbon availability, these lakes show reduced nitrogen supply. We studied the possibility of molecular nitrogen (N2) fixation in Limnospira, as contradictory statements have been published, and some closely related taxa were confirmed as N2 fixers (diazotrophs). We cultivated nine isolates originating from various EASLs under nitrate-rich and nitrate-depleted conditions. We detected dinitrogenase reductase (nifH)-like genes in all strains; however, the genes grouped within nifH cluster IV that mostly contains nitrogenases not functioning in N2 fixation. Accordingly, incubations with 15N2 gas did not support N2 fixation activity of the investigated strains. Under laboratory conditions, all strains faded during nitrate-depleted growth after approximately three weeks. Both phycocyanin and chlorophyll-a dropped to a threshold, and chlorophyll fluorescence indicated a severe problem with nitrogen supply. In summary, our data indicate that the investigated Limnospira fusiformis strains are not capable of N2 fixation.

Compound-specific amino acid 15N-stable isotope probing for the quantification of biological nitrogen fixation in soils

Biological nitrogen fixation (BNF) performed by diazotrophs is vital to our understanding of ecosystem functions, as plant nitrogen (N) is commonly a limiting nutrient for primary productivity. However, significant limitations have remained in our knowledge of the controls and rates of this process, due to technical difficulties in directly quantifying nitrogen (N2) fixation rates. To address this, we developed a novel compound-specific 15N-stable isotope probing method involving analysis of acid hydrolysable soil amino acids (AAs) by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) for the quantification of BNF in soils. By analysing 15N-enriched AAs (as N-acetyl, O-isopropyl derivatives), this new approach aimed to provide greater specificity compared to existing methods, and to contribute previously unobtainable quantitative information on the capture and flow of N2 fixed in soils. Laboratory incubations using 15N2 gas were carried out on surface peat over 15 days to obtain quantitative measures of N2 fixation by the microbial community. Longer incubations with the addition of a glucose energy source significantly increased the level of 15N enrichment, i.e. N fixed. The enhanced detection limits of 15N-AAs by GC-C-IRMS, compared to bulk soil δ15N value determinations, was key to assessments of N2 fixation. Valuable insights into the assimilation pathway of the applied 15N2-substrate were revealed; for peat soils, 15N incorporation into glutamate dominated over other AAs.

The relationship of C and N stable isotopes to high-latitude moss-associated N2 fixation.

Moss-associated N2 fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N2 fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N2 fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N2 fixation with 15N2 gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that δ15N values would increase toward 0‰ with higher N2 fixation to reflect the increasing contribution of fixed N2 in moss biomass. Second, we hypothesized that δ13C and N2 fixation would be positively related, as enriched δ13C signatures reflect abiotic conditions favorable to N2 fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N2 fixation rates and δ15N in a structural equation model. We found a significant positive relationship between δ13C and N2 fixation only in Hypnales, where the probability of N2 fixation activity reached 95% when δ13C values exceeded -30.4‰. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N2 fixation rates.

Dinitrogen fixation rates in the Bay of Bengal during summer monsoon

Biological dinitrogen (N2) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as ammonium) to photosynthetic microorganisms. N2 fixation is dominant in nutrient poor and warm surface waters. The Bay of Bengal is one such region where no measurements of phototrophic N2 fixation rates exist. The surface water of the Bay of Bengal is generally nitrate-poor and warm due to prevailing stratification and thus, could favour N2 fixation. We commenced the first N2 fixation study in the photic zone of the Bay of Bengal using 15N2 gas tracer incubation experiment during summer monsoon 2018. We collected seawater samples from four depths (covering the mixed layer depth of up to 75 m) at eight stations. N2 fixation rates varied from 4 to 75 μmol N m−2 d−1. The contribution of N2 fixation to primary production was negligible (<1%). However, the upper bound of observed N2 fixation rates is higher than the rates measured in other oceanic regimes, such as the Eastern Tropical South Pacific, the Tropical Northwest Atlantic, and the Equatorial and Southern Indian Ocean.

Quantifying the overestimation of planktonic N2 fixation due to contamination of 15N2 gas stocks

AbstractThe 15N2-tracer assay [Montoya et al. (1996) A simple, high-precision, high-sensitivity tracer assay for N2 fixation. Appl. Environ. Microbiol., 62, 986–993.] is the most used method for measuring biological N2 fixation in terrestrial and aquatic environments. The reliability of this technique depends on the purity of the commercial 15N2 gas stocks used. However, Dabundo et al. [(2014) PLoS One, 9, e110335.] reported the contamination of some of these stocks with labile 15N-labeled compounds (ammonium, nitrate and/or nitrite). The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and 142 ammonium and consequences for nitrogen fixation measurements. Considering that the tracer assay relies on the conversion of isotopically labeled 15N2 into organic nitrogen, this contamination may have led to overestimated N2 fixation rates. We conducted laboratory and field experiments in order to (i) test the susceptibility of 15N contaminants to assimilation by non-diazotroph organisms and (ii) determine the potential overestimation of the N2 fixation rates estimated in the field. Our findings indicate that the contaminant 15N-compounds are assimilated by non-diazotrophs organisms, leading to an overestimation of N2 fixation rates in the field up to 16-fold under hydrographic conditions of winter mixing.

True or False in Electrochemical Nitrogen Reduction

Electrochemical nitrogen reduction to ammonia has recently gathered enormous interest as an attractive carbon-neutral alternative to the Haber-Bosch process. However, this novel process severely suffers from poor reproducibility and reliability. As reported recently in Nature, Chorkendorff and colleagues implemented a systematic benchmarking protocol to eliminate the false and retain the true in nitrogen electroreduction research.

A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements.

The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1-4 to the energy-intensive Haber-Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7-9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designedto identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia.

CH4 oxidation-dependent 15N2 fixation in rice roots in a low-nitrogen paddy field and in Methylosinus sp. strain 3S-1 isolated from the roots

Methane (CH4) oxidation and nitrogen (N2) fixation are simultaneously activated in the roots of rice plants grown in paddy fields with low N input (LN). However, the mechanism of CH4 oxidation-dependent N2 fixation remains largely unknown. In the present study, a15N2-feeding experiment was adopted to evaluate methanotrophic N2 fixation in LN rice roots and in a methanotroph isolated from the rice roots. The presence of CH4 significantly enhanced 15N incorporation from 15N2 gas in LN rice roots, indicating methanotrophic N2 fixation. Methylosinus sp. strain 3S-1 was isolated from LN rice roots, and it grew well in N-free liquid medium under different levels of oxygen stress (2%, 10%, and 20% O2). N2 fixation of strain 3S-1 was directly measured using 15N in biomass; when 3S-1 cells were exposed to a gas mixture consisting of 15N2 (35%), CH4 (5%), and O2 (2% or 10%) in argon (Ar) balance, the 15N concentration in the cells rapidly increased at both O2 concentrations. The addition of difluoromethane, a potent inhibitor of methane monooxygenase, immediately stopped CH4 oxidation and reduced the 15N enrichment rate, indicating that N2 fixation depended on CH4 oxidation in pure culture. These results suggest that N2 fixation was stimulated by CH4 oxidation in LN rice roots and that type II methanotrophs in LN rice roots, including Methylosinus, are responsible for CH4 oxidation-dependent N2 fixation.

New Perspectives on Nitrogen Fixation Measurements Using 15N2 Gas

Recently, the method widely used to determine 15N2 fixation rates in marine and freshwater environments was found to underestimate rates because the dissolution of the added 15N2 gas bubble in seawater takes longer than theoretically calculated. As a solution to the potential underestimate of rate measurements, the usage of the enriched water method was proposed to provide constant 15N2 enrichment. Still, the superiority of enriched water method over the previously used bubble injection remains inconclusive. To clarify this issue, we performed laboratory based experiments and implemented the results into an error analysis of 15N2 fixation rates. Moreover, we conducted a literature search on the comparison of the two methods to calculate a mean effect size using a meta-analysis approach. Our results indicate that the error potentially introduced by an equilibrium phase of the 15N2 gas is -72% at maximum for experiments with very short incubation times of 1 hour. In contrast, the underestimation was negligible for incubations lasting 12 to 24 hours (error is -0.2%). Our meta-analysis indicates that 84 % of the measurements in the two groups will overlap and there is a 61% chance that a sample picked at random from the enriched water group will have a higher value than one picked at random from the bubble group. Overall, the underestimation of N2 fixation rates when using the bubble method relative to the enriched water method is highly dependent on incubation time and other experimental conditions and cannot be generalized.

Effect of light on N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation and net nitrogen release of &amp;lt;i&amp;gt;Trichodesmium&amp;lt;/i&amp;gt; in a field study

Abstract. Dinitrogen fixation (NF) by marine cyanobacteria is an important pathway to replenish the oceanic bioavailable nitrogen inventory. Light is the key to modulating NF; however, field studies investigating the light response curve (NF-I curve) of NF rate and the effect of light on diazotroph-derived nitrogen (DDN) net release are relatively sparse in the literature, hampering prediction using models. A dissolution method was applied using uncontaminated 15N2 gas to examine how the light changes may influence the NF intensity and DDN net release in the oligotrophic ocean. Experiments were conducted at stations with diazotrophs dominated by filamentous cyanobacterium Trichodesmium spp. in the western Pacific and the South China Sea. The effect of light on carbon fixation (CF) was measured in parallel using the 13C tracer method specifically for a station characterized by Trichodesmium bloom. Both NF-I and CF-I curves showed a Ik (light saturation coefficient) range of 193 to 315 µE m−2 s−1, with light saturation at around 400 µE m−2 s−1. The proportion of DDN net release ranged from ∼ 6 to ∼ 50 %, suggesting an increasing trend as the light intensity decreased. At the Trichodesmium bloom station, we found that the CF ∕ NF ratio was light-dependent and the ratio started to increase as light was lower than the carbon compensation point of 200 µE m−2 s−1. Under low-light stress, Trichodesmium physiologically preferred to allocate more energy for CF to alleviate the intensive carbon consumption by respiration; thus, there is a metabolism tradeoff between CF and NF pathways. Results showed that short-term (&lt; 24 h) light change modulates the physiological state, which subsequently determined the C ∕ N metabolism and DDN net release by Trichodesmium. Reallocation of energy associated with the variation in light intensity would be helpful for prediction of the global biogeochemical cycle of N by models involving Trichodesmium blooms.

Temporal variability of nitrogen fixation and particulate nitrogen export at Station ALOHA

AbstractWe present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) dinitrogen (N2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N2 fixation rates were measured based on adding the 15N2 tracer as a gas bubble. Beginning in August 2012, 15N2 was first dissolved into filtered seawater and the 15N2‐enriched water was subsequently added to N2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N2‐enriched seawater resulting in twofold greater depth‐integrated rates than those derived from adding a 15N2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N2 fixation averaged 230 ± 136 μmol N m−2 d−1 for the full time series (n = 71). Analysis of the 15N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N2 fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N2 fixation rates ranged between 502 and 919 μmol N m−2 d−1, which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m−2 yr−1, consistent with previous independent estimates of NCP at this site.

Self-assembly of (NH4)0.3TiO1.1F2.1 crystal by dinitrogen fixation as a precursor of N-doped TiO2 nanosheets

Semiconductor TiO2 has been always extensively studied for maximizing its environmental- and energy-related applications with tailored facets or doped crystal structure, especially for photocatalysis, solar cell devices, sensors, catalyst supports, and supercapacitor. In this study, in air or argon atmosphere, a novel facile and effective method has been successfully exploited to prepare N-doped TiO2 nanosheets (air-TiO2 and Ar-TiO2) by annealing treatment of non-stoichiometric ammonium oxofluorotitanates crystal ((NH4)0.3TiO1.1F2.1) with layer-by-layer self-assembled morphology, which is obtained based on the dinitrogen fixation of TiCl4 in the presence of hydrogen fluoride under mild condition. In this method, the conversion of dinitrogen to ammonium salt has been demonstrated by means of isotopic labeling experiments with labeled 15N2 gas acting as benign oxidant and isopropanol as suitable reductant. The resultant of N-doped TiO2 nanosheets exhibits an enhancement of photocatalytic activity by degrading RhB under visible-light irradiation. In addition, the as-prepared Ar-TiO2 of N-doped TiO2 nanosheets obtained from Ar atmosphere also shows a higher electrochemical performance compared with reported pure TiO2 in the application of supercapacitor. In this study, in air or argon atmosphere, a novel facile and effective method has been successfully exploited to prepare N-doped TiO2 nanosheets (air-TiO2 and Ar-TiO2) by annealing treatment of non-stoichiometric ammonium oxofluorotitanates crystal ((NH4)0.3TiO1.1F2.1) with layer-by-layer self-assembled morphology, which is obtained based on the dinitrogen fixation of TiCl4 in the presence of hydrogen fluoride under mild condition.

The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and ammonium and consequences for nitrogen fixation measurements.

We report on the contamination of commercial 15-nitrogen (15N) N2 gas stocks with 15N-enriched ammonium, nitrate and/or nitrite, and nitrous oxide. 15N2 gas is used to estimate N2 fixation rates from incubations of environmental samples by monitoring the incorporation of isotopically labeled 15N2 into organic matter. However, the microbial assimilation of bioavailable 15N-labeled N2 gas contaminants, nitrate, nitrite, and ammonium, is liable to lead to the inflation or false detection of N2 fixation rates. 15N2 gas procured from three major suppliers was analyzed for the presence of these 15N-contaminants. Substantial concentrations of 15N-contaminants were detected in four Sigma-Aldrich 15N2 lecture bottles from two discrete batch syntheses. Per mole of 15N2 gas, 34 to 1900 µmoles of 15N-ammonium, 1.8 to 420 µmoles of 15N-nitrate/nitrite, and ≥21 µmoles of 15N-nitrous oxide were detected. One 15N2 lecture bottle from Campro Scientific contained ≥11 µmoles of 15N-nitrous oxide per mole of 15N2 gas, and no detected 15N-nitrate/nitrite at the given experimental 15N2 tracer dilutions. Two Cambridge Isotopes lecture bottles from discrete batch syntheses contained ≥0.81 µmoles 15N-nitrous oxide per mole 15N2, and trace concentrations of 15N-ammonium and 15N-nitrate/nitrite. 15N2 gas equilibrated cultures of the green algae Dunaliella tertiolecta confirmed that the 15N-contaminants are assimilable. A finite-differencing model parameterized using oceanic field conditions typical of N2 fixation assays suggests that the degree of detected 15N-ammonium contamination could yield inferred N2 fixation rates ranging from undetectable, <0.01 nmoles N L−1 d−1, to 530 nmoles N L−1 d−1, contingent on experimental conditions. These rates are comparable to, or greater than, N2 fixation rates commonly detected in field assays. These results indicate that past reports of N2 fixation should be interpreted with caution, and demonstrate that the purity of commercial 15N2 gas must be ensured prior to use in future N2 fixation rate determinations.

The Impact of Relict Organic Materials on the Denitrification Capacity in the Unsaturated-Saturated Zone Continuum of Three Volcanic Profiles

The denitrification capacity of wetlands, riparian zones, and aquifers in glacial outwash areas is well documented, but little or no information exists for volcanic profiles, particularly those containing relict organic matter contained in or on top of paleosols (old soils buried by volcanic deposits) below the groundwater table. Relict carbon contained in these layers could provide the necessary electrons to fuel heterotrophic denitrification. To the best of our knowledge, this is the first study investigating the denitrification capacity in both the unsaturated and saturated zone of volcanic profiles. Samples from three profile types with differing organic matter distribution were amended with N-enriched nitrate (NO-) and incubated in the laboratory under anaerobic conditions. Dinitrogen (N) dominated the N gas fluxes; averaged across all samples, it accounted for 96% of the total N (nitrous oxide [NO] and N) gas fluxes. Dinitrogen fluxes were generally highest in the A horizon samples (4.1-6.2 nmol N g h), but substantial fluxes were also observed in some paleosol layers (up to 0.72 nmol N g h). A significant correlation ( < 0.001) was found between the concentration of extractable dissolved organic carbon and the total N gas flux produced in samples from below the A horizon, suggesting that heterotrophic denitrification was the dominant NO attenuation process in this study. Extrapolation of lab-derived denitrification capacities to field conditions suggests that the denitrification capacity of profiles containing relict soil organic matter in the saturated zone exceeds the estimated N leaching from the root zone.

First direct measurements of N2fixation during aTrichodesmiumbloom in the eastern Arabian Sea

[1] We report the first direct estimates of N2 fixation rates measured during the spring, 2009 using the 15N2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13C tracer technique. The N2 fixation rates vary from ∼0.1 to 34 mmol N m−2d−1 after correcting for the isotopic under-equilibrium with dissolved air in the samples. These higher N2 fixation rates are consistent with higher chlorophyll a and low δ15N of natural particulate organic nitrogen. Our estimates of N2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.

A Nitridoniobium(V) Reagent That Effects Acid Chloride to Organic Nitrile Conversion: Synthesis via Heterodinuclear (Nb/Mo) Dinitrogen Cleavage, Mechanistic Insights, and Recycling

The transformation of acid chlorides (RC(O)Cl) to organic nitriles (RC[triple bond]N) by the terminal niobium nitride anion [N[triple bond]Nb(N[Np]Ar)3]- ([1a-N]-, where Np = neopentyl and Ar = 3,5-Me2C6H3) via isovalent N for O(Cl) metathetical exchange is presented. Nitrido anion [1a-N]- is obtained in a heterodinuclear N2 scission reaction employing the molybdenum trisamide system, Mo(N[R]Ar)3 (R = t-Bu, 2a; R = Np, 2b), as a reaction partner. Reductive scission of the heterodinuclear bridging N2 complexes, (Ar[R]N)3Mo-(mu-N2)Nb(N[Np]Ar)3 (R = t-Bu, 3b; R = Np, 3c) with sodium amalgam provides 1 equiv each of the salt Na[1a-N] and neutral N[triple bond]Mo(N[R]Ar)3 (R = t-Bu, 2a-N; R = Np, 2b-N). Separation of 2-N from Na[1a-N] is readily achieved. Treatment of salt Na[1a-N] with acid chloride substrates in tetrahydrofuran (THF) furnishes the corresponding organic nitriles concomitant with the formation of NaCl and the oxo niobium complex O[triple bond]Nb(N[Np]Ar)3 (1a-O). Utilization of 15N-labeled 15N2 gas in this chemistry affords a series of 15N-labeled organic nitriles establishing the utility of anion [1a-N]- as a reagent for the 15N-labeling of organic molecules. Synthetic and computational studies on model niobium systems provide evidence for the intermediacy of both a linear acylimido and niobacyclobutene species along the pathway to organic nitrile formation. High-yield recycling of oxo 1a-O to a niobium triflate complex appropriate for heterodinuclear N2 scission has been developed. Specifically, addition of triflic anhydride (Tf2O, where Tf = SO2CF3) to an Et2O solution of 1a-O provides the bistriflate complex, Nb(OTf)2(N[Np]Ar)3 (1a-(OTf)2), in near quantitative yield. One-electron reduction of 1a-(OTf)2 with either cobaltocene (Cp2Co) or Mg(THF)3(anthracene) provided the monotriflato complex, Nb(OTf)(N[Np]Ar)3 (1a-(OTf)), which efficiently regenerates complexes 3b and 3c when treated with the molybdenum dinitrogen anions [N2Mo(N[t-Bu]Ar)3]- ([2a-N2]-) or [N2Mo(N[Np]Ar)3]- ([2b-N2]-), respectively.

Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

ABSTRACTIn this study, the response of N2 fixation to elevated CO2 was measured in Scirpus olneyi, a C3 sedge, and Spartina patens, a C4 grass, using acetylene reduction assay and 15N2 gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO2 significantly (P&lt; 0·05) caused a 35% increase in nitrogenase activity and 73% increase in 15N incorporated by Scirpus olneyi. In Spartina patens, elevated CO2 (660 ± 1 μmol mol−1) increased nitrogenase activity and 15N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N2 fixation in Scirpus olneyi under elevated CO2 was 611 ± 75 ng 15N fixed plant−1 h−1 compared with 367 ± 46 ng 15N fixed plant−1 h−1 in ambient CO2 plants. In Spartina patens, however, the rate of N2 fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng 15N fixed plant−1 h−1 for elevated and ambient CO2, respectively. Heterotrophic non‐symbiotic N2 fixation in plant‐free marsh sediment also increased significantly (P&lt; 0·05) with elevated CO2. The proportional increase in 15N2 fixation correlated with the relative stimulation of photosynthesis, in that N2 fixation was high in the C3 plant in which photosynthesis was also high, and lower in the C4 plant in which photosynthesis was relatively less stimulated by growth in elevated CO2. These results are consistent with the hypothesis that carbon fixation in C3 species, stimulated by rising CO2, is likely to provide additional carbon to endophytic and below‐ground microbial processes.

Synthesis and Sintering of AlN using Enriched Nitrogen

A high purity Al powder was directly reacted with isotopically enriched 15N2 gas at 1,200°C in a vacuum furnace to synthesize Al15N. In the initial reaction process, a small amount of AlF3, was mixed with the Al powder to suppress excess temperature rise due to the heat of formation of the nitride. Then, the Al15N thus formed was used as the diluent for the second run, and the same procedure was repeated. A total of 100g of Al15N powder with 15N isotopic content 99.6 at% was formed with average gas efficiency of 75.6%. Then the Al15N powder was pressureless-sintered at 1,830°C in an argon atmosphere by adding a small amount of Y2O3 as the sintering agent. An Al15N ceramic plate with dimensions 50mm × 50mm × 1.6 mm and relative density 97.3% was obtained with the 15N isotopic content 99.2 at%. Some physical properties and stability in air of an Al15N ceramic plate were studied.