Source identification of nitrate by means of stable nitrogen isotopes in the river Daugava and loads of nitrogen to the Gulf of Riga

Nitrogen isotope ratio of nitrate provides a powerful tool to investigate nitrate sources and cycling mechanisms. Although the use of an isotope ratio method for ¹⁵N/¹⁴N allows identifying the nitrate sources in rivers by estimating a seasonal variation of N-NO₃ concentration, however, there are some restrictions. Nitrification, the conversion of NH₄⁺ to NO₃-, can proceed with significant nitrogen isotope fractionation, preferentially accumulating ¹⁴N in the produced NO₃-, and can make it difficult to identify the nitrate source with a high proportion of the isotope δ¹⁵N. However, the uptake and assimilation of NH₄⁺ and NO₃- have the capability of affecting isotopic compositions of riverine nitrogen compounds, and this may hinder the determination of whether the impact of the nitrate source with a high proportion of the isotope δ¹⁵N reduces. In addition, this study demonstrates that nitrate nitrogen concentration may correlate with δ¹⁵N_NO3 values both positively and negatively. Such correlations are the result of isotope effects during nitrogen transformation processes (e.g. nitrification and assimilation) and isotopic variability in the various nitrate sources. A comparison of NO₃- concentration and δ¹⁵N_NO3 can be used to further distinguish mixing from biological processing. However, in order to get a more precise answer regarding the nitrate sources, it would be useful to take both the data of nitrogen isotopes and data of oxygen isotopes present in nitrates.     

Evaluation of the influence of arsenic species on the nitrogen metabolism of a model angiosperm: nasturtium,Tropaeolum majus

How the various organic and inorganic arsenic species affect the nitrogen metabolism of a model plant, Tropaeolum majus, was studied in order to evaluate the toxicological impact of the various chemical forms of arsenic. For this purpose, the effects on the (a) entire nitrogen pool, (b) protein fraction, and (c) non‐protein fraction were distinguished. The arsenic‐dependent effects on the nitrogen cycle were assessed by using ¹⁵N‐labelled KNO₃ as a nutritive substance and optical emission spectroscopy to analyse how ¹⁵N is incorporated into the nitrogen cycle. In addition to the ¹⁵N‐tracer experiments, the uptake and metabolization of the arsenic compounds were examined. The work shows that biochemical indicator systems like ¹⁵N‐tracer studies are able to characterize the degree of the influence of metabolic processes by arsenic species. For example, the incorporated ¹⁵N concentration decreased linearly and independently of the ¹⁵N fraction with increasing dimethylarsinate (DMA) concentrations. This behaviour indicates that DMA has prevented the uptake of ¹⁵N and hence the formation of amino acids and proteins. Arsenite‐treated plants exhibited an elevated concentration of non‐protein ¹⁵N, which could be an indication either for a stimulated uptake of nitrate or for an interrupted amino acid/protein synthesis. Copyright © 2005 John Wiley & Sons, Ltd.     

N2: a potential pitfall for bulk 2H isotope analysis of explosives and other nitrogen‐rich compounds by continuous‐flow isotope‐ratio mass spectrometry

Observations made during the ¹³C isotope analysis of gaseous CO₂ in the simultaneous presence of argon in the ion source of the isotope ratio mass spectrometer prompted us to investigate what influence the simultaneous presence of nitrogen would have on both accuracy and precision of bulk ²H isotope analysis of nitrogen‐rich organic compounds. Initially an international reference material, IAEA‐CH7, was mixed with silver nitrate in various ratios to assess the impact that N₂ evolved from the pyrolysis of nitrogen‐rich organic compounds would have on measured δ²H‐values of IAEA‐CH7. In a subsequent experiment, benzoic acid was mixed with silver nitrate to mimic the N:H ratio of organic‐rich nitrogen compounds such as cellulose nitrate and RDX. The results of both experiments showed a significant deterioration of both accuracy and precision for the expected δ²H values for IAEA‐CH7 and benzoic acid when model mixtures were converted into hydrogen and nitrogen, and subsequently separated by gas chromatography using standard experimental conditions, namely a 60 cm packed column with molecular sieve 5 Å as stationary phase held at a temperature of 85°C. It was found that bulk ²H stable isotope analysis of nitrogen‐rich organic compounds employing published standard conditions can result in a loss of accuracy and precision yielding δ²H values that are 5 to 25‰ too negative, thus suggesting, for example, that tree‐ring ²H isotope data based on cellulose nitrate may have to be revised. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrochemical Oxidation of Nitrogen towards Direct Nitrate Production on Spinel Oxides

Nitrates are widely used as fertilizer and oxidizing agents. Commercial nitrate production from nitrogen involves high‐temperature‐high‐pressure multi‐step processes. Therefore, an alternative nitrate production method under ambient environment is of importance. Herein, an electrochemical nitrogen oxidation reaction (NOR) approach is developed to produce nitrate catalyzed by ZnFe_xCo_2?xO₄ spinel oxides. Theoretical and experimental results show Fe aids the formation of the first N?O bond on the *N site, while high oxidation state Co assists in stabilizing the absorbed OH^? for the generation of the second and third N?O bonds. Owing to the concerted catalysis, the ZnFe_0.4Co_1.6O₄ oxide demonstrates the highest nitrate production rate of 130±12 μmol h^?1 g_MO^?1 at an applied potential of 1.6 V versus the reversible hydrogen electrode (RHE).     

Stable isotope ratio analysis to differentiate temporal diets of a free‐ranging herbivore

Stable isotope ratio analysis (SIRA) of carbon (δ¹³C) and nitrogen (δ¹⁵N) in tissue samples of herbivores can identify photosynthetic pathways (C₃ vs. C₄) of plants consumed. We present results from free‐ranging Rocky Mountain elk (Cervus elaphus) that highlight the ability to differentiate diets using tissue δ¹³C and δ¹⁵N. The signatures of δ¹³C and δ¹⁵N differed in tissues of varying metabolic activity: muscle, a short‐term dietary indicator (i.e., 1–2 months) and hoof, a long‐term dietary indicator (i.e., 3–12 months). We also documented that δ¹³C and δ¹⁵N values along elk hooves (proximal, middle, distal sections) elucidated temporal shifts in dietary selection. The carbon isotopes of the composite hoof were similar to those of the middle section, but the composite hoof differed in δ¹³C from the distal and proximal sections. The δ¹³C and δ¹⁵N signatures also differed among elk populations, indicating temporal dietary shifts of individuals occupying disparate native range and human‐derived agricultural landscapes. Analyses of stable isotopes in various tissues highlighted carbon and nitrogen assimilation through time and differences in the foraging ecology of a rangeland herbivore. Published in 2009 by John Wiley & Sons, Ltd.     

The effect of formaldehyde on the growth ofCandida diddensii 74-10 andCandida tropicalis R-70

The effects of formaldehyde on the growth of two strains of fodder yeastsCandida diddensii 74-10 andCandida tropicalis R-70 were investigated using the method of continuous cultivation under conditions of carbon limitation and at dilution rates of 0.1/h and 0.25/h. The results indicate that formaldehyde induces a decrease in the yield of biomass, but stimulates the synthesis of protein and RNA. The authors studied the activities of the following enzymes: NADPH-linked glutamate dehydrogenase, NADH-linked glutamate dehydrogenase, alanine dehydrogenase, glutamine synthetase, and glutamate synthase, which are utilized in nitrogen metabolism. The data obtained showed an increase in the activity of the glutamate dehydrogenase pathway of ammonium assimilation. It was also established that formaldehyde caused considerable changes in the micro-organisms at the higher dilution rate.     

Belowground fate of 15N injected into sweetgum trees (Liquidambar styraciflua) at the ORNL FACE Experiment

Nitrogen (N) cycling can be an important constraint on forest ecosystem response to elevated atmospheric CO₂. Our objective was to trace the movement of ¹⁵N, injected into tree sap, to labile and stable forms of soil organic matter derived partly from the turnover of tree roots under elevated (545 ppm) and ambient (394 ppm) atmospheric CO₂ concentrations at the Oak Ridge National Laboratory (ORNL) FACE (Free‐Air Carbon Dioxide Enrichment) Experiment. Twenty‐four sweetgum trees, divided equally between CO₂ treatments, were injected with 3.2 g ¹⁵N‐ammonium sulfate (99 atom %), and soil samples were collected beneath the trees over a period of 89 weeks. For 16 cm deep soil samples collected beneath the study trees, there was 28% more fine root (less than or equal to 2 mm diameter) biomass under elevated CO₂ (P = 0.001), but no significant treatment effect on the amounts of necromass, coarse root biomass, or on the N concentrations in tree roots and necromass. Nitrogen‐15 moved quickly into roots from the stem injection site and the ¹⁵N content of roots, necromass, and labile organic matter (i.e. particulate organic matter, POM) increased over time. At 89 weeks post‐injection, approximately 76% of the necromass ¹⁵N originated from fine root turnover. Nitrogen‐15 in POM had a relatively long turnover time (47 weeks) compared with ¹⁵N in roots (16 to 22 weeks). Over the 1.7 year period of the study, ¹⁵N moved from roots into slower cycling POM and the disparity in turnover times between root N and N in POM could impose progressive limitations on soil N availability with stand maturation irrespective of atmospheric CO₂, especially if the release of N through the decomposition of POM is essential to sustain forest net primary production. Published in 2009 by John Wiley & Sons, Ltd.     

Absorption and translocation of nitrogen in rhizomes of Leymus chinensis

Leymus chinensis is a dominant species in the Inner Mongolia steppe, northern China. Plant growth in northern China grassland is often limited by low soil nitrogen availability. The objective of this study is to investigate whether rhizomes of Leymus chinensis are involved in the contribution of N uptake. The N concentration, ¹⁵N concentration and ¹⁵N proportion in roots, rhizomes and shoots after 48 h exposure of roots (L_root) and rhizomes (L_rhizo) separately and roots and rhizomes together (L_r+r) to 0.1 mM ¹⁵NHNO₃ solution were measured using root‐splitting equipment and stable isotope (¹⁵N) techniques, respectively. The N content and dry mass were not affected by the labeling treatment. In contrast, the ¹⁵N concentration in shoots, rhizomes and roots was significantly increased by the labeling in rhizomes, indicating that the inorganic nitrogen was absorbed via rhizomes from the solution and can be transported to other tissues, with preference to shoots rather than roots. Meanwhile, the absolute N absorption and translocation among compartments were also calculated. The N absorption via rhizomes was much smaller than via roots; however, the uptake efficiency per surface unit via rhizomes was greater than via roots. The capacity and high efficiency to absorb N nutrient via rhizomes enable plants to use transient nutrient supplies in the top soil surface. Copyright © 2011 John Wiley & Sons, Ltd.     

Characterization of 2‐aryl‐1,3,4‐oxadiazoles by 15N and 13C NMR spectroscopy

¹⁵N NMR chemical shifts of 2‐aryl‐1,3,4‐oxadiazoles were assigned on the basis of the ¹H–¹⁵N HMBC experiment. Chemical shifts of the nitrogen and carbon atoms in the oxadiazole ring correlate with the Hammett σ‐constants of substituents in the aryl ring (r² ≥ 0.966 for N atoms). ¹⁵N NMR data are a suitable and sensitive means for characterizing long‐range electronic substituent effects. Additionally, ¹³C NMR data for these compounds are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Comparison of methods for simultaneous identification of bacterial species and determination of metabolic activity by protein‐based stable isotope probing (Protein‐SIP) experiments

We developed a concept for analysing carbon and nitrogen fluxes in microbial communities by employing protein‐based stable isotope probing (Protein‐SIP) in metabolic labelling experiments with stable isotope labelled substrates. For identification of microbial species intact protein profiling (IPP) can be used, whereas the assessment of their metabolic activity is achieved by shotgun mass mapping (SMM). Microbial cultures were grown on substrates containing ¹³C or ¹⁵N. For identification of species we tested both the IPP and the SMM approaches. Mass spectra (MALDI‐MS) were taken from mixtures of either intact proteins or peptides from tryptic digestion for generating species‐specific peak patterns. In the case of SMM, the fragmentation of peptides was additionally used to obtain sequence information for species identification. Mass spectra of peptide sequences allow calculation of the amount of ¹³C or ¹⁵N incorporation within peptides for determining metabolic activity of the specific species. The comparison of IPP and SMM revealed a higher robustness of species identification by SMM. In addition, the assessment of incorporation levels of ¹³C and ¹⁵N into peptides by SMM revealed a lower uncertainty (0.5–0.8 atom %) compared to IPP (6.4–8.9 atom %). The determination of metabolic activity and function of individual species by Protein‐SIP can help to analyse carbon and nitrogen fluxes within microbial communities. Copyright © 2009 John Wiley & Sons, Ltd.     

Compositing Amorphous TiO2 with N‐Doped Carbon as High‐Rate Anode Materials for Lithium‐Ion Batteries

Compositing amorphous TiO₂ with nitrogen‐doped carbon through Ti? N bonding to form an amorphous TiO₂/N‐doped carbon hybrid (denoted a‐TiO₂/C? N) has been achieved by a two‐step hydrothermal–calcining method with hydrazine hydrate as an inhibitor and nitrogen source. The resultant a‐TiO₂/C? N hybrid has a surface area as high as 108 m² g^?1 and, when used as an anode material, exhibits a capacity as high as 290.0 mA h g^?1 at a current rate of 1 C and a reversible capacity over 156 mA h g^?1 at a current rate of 10 C after 100 cycles; these results are better than those found in most reports on crystalline TiO₂. This superior electrochemical performance could be ascribed to a combined effect of several factors, including the amorphous nature, porous structure, high surface area, and N‐doped carbon.     

Fe2O3‐N‐doped Honeycomb‐like Porous Carbon Derived from Nature Silk Sericin as Electrocatalysts for Oxygen Evolution Reaction

A facile method is reported to form a honeycomb‐like porous nanomaterial by intercalation of iron nitrate using nature silk sericin (SS) as nitrogen and carbon source. A series of Fe₂O₃ nanoparticles anchored on Fe₂O₃‐N‐doped graphite carbon electrocatalysts (SS‐Fe) were synthesized, exhibits well‐defined pore structure and excellent oxygen evolution reaction (OER) catalytic activities. Among these materials, SS‐Fe‐0.5 shows the best performance, the overpotential of SS‐Fe‐0.5 at 10 mA · cm^–2 is 440 mV (vs. RHE) and the Tafel slope is only 68 mV · dec^–1. The results indicate that it is promising to the preparation of carbon catalyst materials using natural, renewable and abundant resources for electrocatalysis.     

Determination of nitrogen‐15 isotope fractionation in tropine: evaluation of extraction protocols for isotope ratio measurement by isotope ratio mass spectrometry

N‐Demethylation of tropine is an important step in the degradation of this compound and related metabolites. With the purpose of understanding the reaction mechanism(s) involved, it is desirable to measure the ¹⁵N kinetic isotope effects (KIEs), which can be accessed through the ¹⁵N isotope shift (Δδ¹⁵N) during the reaction. To measure the isotope fractionation in ¹⁵N during tropine degradation necessitates the extraction of the residual substrate from dilute aqueous solution without introducing artefactual isotope fractionation. Three protocols have been compared for the extraction and measurement of the ¹⁵N/¹⁴N ratio of tropine from aqueous medium, involving liquid‐liquid phase partitioning or silica‐C18 solid‐phase extraction. Quantification was by gas chromatography (GC) on the recovered organic phase and δ¹⁵N values were obtained by isotope ratio measurement mass spectrometry (irm‐MS). Although all the protocols used can provide satisfactory data and both irm‐EA‐MS and irm‐GC‐MS can be used to obtain the δ¹⁵N values, the most convenient method is liquid‐liquid extraction from a reduced aqueous volume combined with irm‐GC‐MS. The protocols are applied to the measurement of ¹⁵N isotope shifts during growth of a Pseudomonas strain that uses tropane alkaloids as sole source of carbon and nitrogen. The accuracy of the determination of the ¹⁵N/¹⁴N ratio is sufficient to be used for the determination of ¹⁵N‐KIEs. Copyright © 2009 John Wiley & Sons, Ltd.     

Forazoline A: Marine‐Derived Polyketide with Antifungal In Vivo Efficacy

Forazoline A, a novel antifungal polyketide with in vivo efficacy against Candida albicans, was discovered using LCMS‐based metabolomics to investigate marine‐invertebrate‐associated bacteria. Forazoline A had a highly unusual and unprecedented skeleton. Acquisition of ¹³C–¹³C gCOSY and ¹³C–¹⁵N HMQC NMR data provided the direct carbon–carbon and carbon–nitrogen connectivity, respectively. This approach represents the first example of determining direct ¹³C–¹⁵N connectivity for a natural product. Using yeast chemical genomics, we propose that forazoline A operated through a new mechanism of action with a phenotypic outcome of disrupting membrane integrity.     

Surface modification of malonic acid‐catalyzed carbon xerogels and their high performance for adsorption of Cu (II) ions

The preparation of malonic acid‐catalyzed carbon xerogels modified with nitric acid and their high performance for adsorption of Cu²⁺ were investigated. The treated and untreated carbon xerogels (nitrogen‐doped carbon xerogel and carbon xerogel) are mainly microporous with high surface areas (1150.18 and 1201.46 m² g^?1) based on the analysis of N₂ adsorption isotherm. Fourier transform infrared spectroscopy study demonstrates that modification process generates a number of functional groups such as carboxyl, carbonyl, and nitrate groups. X‐ray photoelectron spectra analysis shows an increase in the content of O and N after oxidation. The adsorption performance for Cu²⁺ using different process parameters like initial concentration, contact time, and temperature was investigated. The result indicates that the pseudo‐second order correlates with the experimental data, and the activation energy of Cu²⁺ adsorption onto nitrogen‐doped carbon xerogel and carbon xerogel is calculated as 15.62 kJ mol^?1 and 2.79 kJ mol^?1, respectively, indicating the coexistence of chemisorption and ion‐exchange. Langmuir and Freundlich isotherms were used to describe the adsorption behavior of Cu²⁺. The adsorption of Cu²⁺ by carbon xerogels modified with nitric acid was fast and had noticeable adsorption capacity, with a higher adsorption capacity than the original carbon xerogels (299.41 vs 260.42 mg g^?1). Copyright © 2013 John Wiley & Sons, Ltd.     

Orientational disorder in Λ‐cobalt(III) sepulchrate trinitrate

The crystal structure of Λ‐(1,3,6,8,10,13,16,19‐octaazabicyclo[6.6.6]eicosane)cobalt(III) trinitrate, [Co(C₁₂H₃₀N₈)](NO₃)₃, consists of a sepulchrate moiety that serves as a macrobicyclic nitrogen cage for the Co³⁺ cation, which is six‐coordinated by N atoms, and three nitrate anions. The Co–sepulchrate group lies on a threefold axis (site symmetry 32), as do two symmetry‐related and ordered nitrate groups (site symmetry 3), with which it is connected via N—H...O hydrogen bonds [Co—N = 5.1452 (12) Å]. The third nitrate group is disordered as a result of symmetry requirements around the origin (site symmetry 32), and is further away from the Co–sepulchrate cage [Co—N = 6.3160 (8) Å]. The structure is described by applying orientational disorder over six equivalent orientations for the disordered nitrate group, which is considered as an ideal planar molecule of regular trigonal geometry with its molecular plane rotated out of the ab plane and the molecular centre of gravity slightly shifted away from the origin. This new model for disorder clearly improves a previous crystal structure determination.     

Imines in the Titanium Coordination Sphere – Transformation of Imidoyl Chlorides to Nitrilium Ions

In the reaction of TiCl₄ in benzene as solvent with the imidoyl chloride p‐Tolyl(Cl)C=NPh ( 1 ) the abstraction of the chloride substituent is observed, leading to the nitrilium salt [p‐Tolyl–C≡N–Ph]⁺[Ti₂Cl₉]^– ( 2 ) in quantitative yield. The highly electrophilic salt 2 is characterized by IR‐ and NMR spectroscopy. The observed band for the C≡N stretching mode of 2 clearly indicates the formation of a nitrilium ion. Especially a characteristic line broadening of the ¹³C{¹H}‐NMR signals related to carbon atoms next to the nitrogen is observed. By ¹⁵N,¹H‐HMBC NMR experiments it is shown that the nitrogen signal of 2 is significantly shifted to high‐field in relation to nitriles and imines. The molecular structure of 2 was confirmed by single‐crystal X‐ray diffraction. The C≡N bond length and the linearity of the C–C≡N–C unit in 2 confirm the triple bond character of this bond.     

Rheological Analysis of Biopolymer Produced by Aureobasidium Pullulans in Different Sources of Nitrogen

Summary: In this study, the rheological behaviors of media fermented by two Aureobasidium pullulans strains (IOC 3467 and IOC 3011) were evaluated in different nutritional conditions. The media consisted of crystal sugar (sucrose), as the main carbon source, and different nitrogen sources (ammonium sulfate, sodium nitrate, ammonium nitrate, urea or residual brewery yeast - RBY). Viscosity measurements were performed on cell-free supernatants, from 48-hour fermentation assays, at 25 ^°C. Shear rates ranged between 0.1000 and 500 s⁻¹. All samples showed pseudoplastic behavior. Nevertheless, the viscosimetric profile of each one varied according to the nitrogen source, its concentration and the strain used. The maximum viscosity of 0.06 Pa.s. was achieved at 15.6 s⁻¹ for IOC 3011 strain grown on RBY as nitrogen source.     

Carbon‐Dot‐Sensitized,Nitrogen‐Doped TiO2 in Mesoporous Silica for Water Decontamination through Nonhydrophobic Enrichment–Degradation Mode

Mesoporous silica synthesized from the cocondensation of tetraethoxysilane and silylated carbon dots containing an amide group has been adopted as the carrier for the in situ growth of TiO₂ through an impregnation–hydrothermal crystallization process. Benefitting from initial complexation between the titania precursor and carbon dot, highly dispersed anatase TiO₂ nanoparticles can be formed inside the mesoporous channel. The hybrid material possesses an ordered hexagonal mesostructure with p6mm symmetry, a high specific surface area (446.27 m² g^?1), large pore volume (0.57 cm³ g^?1), uniform pore size (5.11 nm), and a wide absorption band between λ=300 and 550 nm. TiO₂ nanocrystals are anchored to the carbon dot through Ti?O?N and Ti?O?C bonds, as revealed by X‐ray photoelectron spectroscopy. Moreover, the nitrogen doping of TiO₂ is also verified by the formation of the Ti?N bond. This composite shows excellent adsorption capabilities for 2,4‐dichlorophenol and acid orange 7, with an electron‐deficient aromatic ring, through electron donor–acceptor interactions between the carbon dot and organic compounds instead of the hydrophobic effect, as analyzed by the contact angle analysis. The composite can be photocatalytically recycled through visible‐light irradiation after adsorption. The narrowed band gap, as a result of nitrogen doping, and the photosensitization effect of carbon dots are revealed to be coresponsible for the visible‐light activity of TiO₂. The adsorption capacity does not suffer any clear losses after being recycled three times.