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1.
An improved method for emission spectrometric determination of 15N content with a small amount of nitrogen (2 μg, minimum) is described. Ammonium nitrogen and organic nitrogen are converted to N 2 gas by the method of Rittenberg and by the method of Dumas, respectively. The N 2 gas is purified, introduced into a MacLeod vacuum gauge for measuring the total quantity, and then an appropriate amount of the N 2 gas is collected in a discharge tube containing a molecular sieve 5A so as to give a gas pressure of about 5 Torr. Emission is stable and reproducible, and 15N abundance can be determined with an error less than 0.01 and 0.10 atom% at a natural abundance level of 0.366 and at 9 atom%, respectively. Predetermination of N content of samples is not required. 相似文献
2.
13N, generated by proton bombardment of 13C powder, is rapidly and easily converted to 13N-N 2, 0.01 atm pressure, ca. 10 mCi/ml, by automated Dumas combustion. 13N fixed (as 13N-N 2) by algal filaments was localized by an autoradiographic technique which permits track autoradiography with isotopes having short half-lives. Our findings show directly that a minimum of about 25% of the N 2 fixation by intact, aerobically grown filaments of Anabaena cylindrica is carried out by the heterocysts. If all of the N 2 fixation takes place in the heterocysts, then the movement of nitrogen along the filaments can be characterized by a constant τ < ca. 5 s (cell -2). 相似文献
3.
To evaluate the denitrification abilities of many Bradyrhizobium field isolates, we developed a new 15N-labeled N 2 detection methodology, which is free from interference from atmospheric N 2 contamination. 30N 2 ( 15N 15N) and 29N 2 ( 15N 14N) were detected as an apparent peak by a gas chromatograph equipped with a thermal conductivity detector with N 2 gas having natural abundance of 15N (0.366 atom%) as a carrier gas. The detection limit was 0.04% 30N 2, and the linearity extended at least to 40% 30N 2. When Bradyrhizobium japonicum USDA110 was grown in cultures anaerobically with 15NO 3−, denitrification product ( 30N 2) was detected stoichiometrically. A total of 65 isolates of soybean bradyrhizobia from two field sites in Japan were assayed by this method. The denitrification abilities were partly correlated with filed sites, Bradyrhizobium species, and the hup genotype. 相似文献
4.
Summary The 15N/ 14N ratios of plant and soil samples from Northern California ecosystems were determined by mass spectrometry. The 15N abundance of 176 plant foliar samples averaged 0.0008 atom % 15N excess relative to atmospheric N 2 and ranged from-0.0028 to 0.0064 atom % 15N excess relative to atmospheric N 2. Foliage from reported N 2-fixing species had significantly lower mean 15N abundance (relative to atmospheric N 2 and total soil N) and significantly higher N concentration (% N dry wt.) than did presumed non-N 2-fixing plants growing on the same sites. The mean difference between N 2-fixing species and other plants was 0.0007 atom % 15N. N 2-fixing species had lower 15N abundance than the other plants on most sites examined despite large differences between sites in vegetation, soil, and climate. The mean 15N abundance of N 2-fixing plants varied little between sites and was close to that of atmospheric N 2. The 15N abundance of presumed non-N 2-fixing species was highest at coastal sites and may reflect an input of marine spray N having relatively high 15N abundance. The 15N abundance of N 2-fixing species was not related to growth form but was for other plants. Annual herbaceous plants had highest 15N abundance followed in decreasing order by perennial herbs, shrubs, and trees. Several terrestrial ferns (Pteridaceae) had 15N abundances comparable to N 2-fixing legumes suggesting N 2-fixation by these ferns. On sites where the 15N abundance of soil N differs from that of the atmosphere, N 2-fixing plants can be identified by the natural 15N abundance of their foliage. This approach can be useful in detecting and perhaps measuring N 2-fixation on sites where direct recovery of nodules is not possible. 相似文献
5.
Improvement of dinitrogen fixation in beans ( Phaseolus vulgaris L.) will depend on the selection of superior plant genotypes and the presence of efficient rhizobial strains. This study
was conducted to evaluate diverse bean lines for N 2 fixation potential using the 15N-depleted dilution technique under field conditions in Wisconsin, USA. Plants of 21 bean lines and three non-nodulating isolines
of soybean received appliin Wisconsin, USA. Plants of 21 bean lines and three non-nodulating isolines of soybean received
applications of 15N-depleted ammonium sulphate. Shoots harvested at the V 6, R 3 and R 7 stages and dry seeds were analyzed for total N using the Kjeldahl procedure, and the ratio of 15N to 14N was determind on a MAT 250 mass spectrometer. Nodule occupancy of the applied strain of R. leguminosarum biovar phaseoli, CIAT 899, was determined in five of the bean lines. Total shoot N content showed a pattern of accumulation similar to shoot
dry weight and fixed N 2 in the shoot. Based on shoot total N, N 2 fixed in the shoot and shoot dry weight Riz 30 and Preto Cariri were identified as being as good fixers as Puebla 152 and
Cargamanto appear to begin N 2 fixation early. Furthermore, some bean lines that fixed considerable N 2 did not translocate a large amount of N to the grains. Preto Cariri accumulated 21.2 kg N ha −1 in the seeds compared to Puebla 152 which accumulated 43.8 kg N ha −1 of the fixed N 2 into the grains. At the early sampling, Puebla 152 and 22–27 had a considerable higher percentage of their crown nodules
formed by the inoculant strain CIAT 899, than did Rio Tibagi which has been considered a poor N 2 fixer. 相似文献
6.
Simultaneous determination of 15N and total N using an automated nitrogen analyser interfaced to a continuous-flow isotope ratio mass spectrometer (ANA-MS method) was evaluated. The coefficient of variation (CV) of repeated analyses of homogeneous standards and samples at natural abundance was lower than 0.1%. The CV of repeated analyses of 15N-labelled plant material and soil samples varied between 0.3% and 1.1%. The reproductibility of repeated total N analyses using the automated method was comparable to results obtained with a semi-micro Kjeldahl procedure. However, the automated method gave results which were 3% to 5% higher than those obtained with the Kjeldahl procedure. Since only small samples can be analysed, careful sample homogenization and fine grinding are very important. Evaluation of a diffusion method for preparing nitrate and ammonium in solution for automated 15N analysis showed that the recovery of inorganic N in the NH 3 trap was lower when the N was diffused from water than from 2 M KCl. The results also indicated that different proportions of the NO 3
- and the NH 4
+ in aqueous solution were recovered in the trap after combined diffusion. The method is most suited for diffusing either NO 3
- or NH 4
+ alone, but can be used for combined diffusion of the two ions. 相似文献
7.
If the quality and quantity of yields from cowpea ( Vigna unguiculata [L.] Walp.) are to be maximised, a complete understanding of the N nutrition of the plant must be achieved. The N requirement for developing pods of this species may come from mobilization of N in vegetative tissue, biological N fixation and uptake of N from soil. In this study, the fate of a pulse of fixed 15N 2 or of 15NO 3-given to different cowpea plants during pod development was determined. The plants were grown in vermiculite in plastic pots that were able to be sealed with silicone adhesive and equipped with a rubber septum so that 15N 2 gas could be injected into the air space above the vermiculite, and gas losses would be eliminated. Nineteen days after injection of 15N 2 the pods, leaves, nodules and roots contained 65%, 15%, 9%, and 4%, respectively of the quantity of 15N 2 fixed. When 15NO 3- 15N was taken up by other plants during this period, these plant parts contained 40%, 26%, 3% and 19%, respectively, of the total plant 15N. The percentage 15N in roots was greater, and that of 15N in nodules was lower, when 15NO 3- 15N was applied than when 15N 2 was utilised by plants. These results indicate that, while a high percentage of fixed-N or NO 3-N given to cowpea plants moved to the developing pods, other sinks were competing for this newly-aquired N. 相似文献
8.
A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of
(N 2+N 2O)− 15N from 98 atom % 15N-labeled urea broadcast at 29 kg N ha −1 into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N 2+N 2O)− 15N during the 19 days following urea application never exceeded 28 g N ha −1 day −1. The total recovery of (N 2+N 2O)− 15N evolved from the field was only 0.51% of the applied N, whereas total gaseous 15N loss estimated from unrecovered 15N in the 15N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded
0.14 g N m −3 or 0.3% of the applied N. Prior cropping of cowpea [ Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha −1, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N 2+N 2O)− 15N and total 15N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N 2+N 2O)− 15N and total 15N loss from 27 kg applied nitrate-N ha −1 were 20% and 53% during the same 19-day period. The failure of directly-recovered (N 2+N 2O)− 15N to match total 15N loss from added nitrate- 15N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere
through rice plants. The rapid conversion of added nitrate-N to (N 2+N 2O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g −1 in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss
from urea was controlled by supply of nitrate rather than by availability of organic C. 相似文献
9.
The aims of this study were to simulate wet deposition of atmospheric nitrate (NO 3?) onto forest soils and trace its fate via conversion spatially and temporally into gaseous products nitrous oxide (N 2O) and dinitrogen (N 2). The most likely mechanism is microbial denitrification, but an intermediate product nitrite (NO 2?) can fuel N 2O production via a chemical pathway involving reactions with iron and/or organic matter referred to as chemodenitrification. During summer months, we applied tracer amounts of 15N-labeled NO 3? onto forest soils (pH ~ 4) at three sites in the White Mountain Region of New Hampshire, USA. We recovered 15N as N 2O in 210 of 504 measurements (42%) versus 15N as N 2 in 51 of 504 measurements (10%), suggesting partial microbial denitrification and/or chemodenitrification. When recovery occurred, the mean percent recovery of added 15N was just 1.1% as N 2O, although N 2 recovery was 33%. A site with old-growth trees had a larger percentage recovery as N 2 (48%), whereas a site that had burned 100 years ago had a small percentage recovery as N 2O (0.24%). The 15N composition of N 2O in ambient air, collected before addition of the label, was markedly enriched in 15N. Since flux measurements were made 2 h after the addition, the results suggest that denitrification enzymes and conditions for chemodenitrification are present throughout the summer months but account for small amounts of NO 3? conversion into N 2O and N 2. 相似文献
10.
Detached cowpea nodules that contained a nitrous oxide reductase-positive (Nor +) rhizobium strain (8A55) and a nitrous oxide reductase-negative (Nor −) rhizobium strain (32H1) were incubated with 1% 15N 2O (95 atom% 15N) in the following three atmospheres: (i) aerobic with C 2H 2 (10%), (ii) aerobic without C 2H 2, and (iii) anaerobic (argon atmosphere) without C 2H 2. The greatest production of 15N 2 occurred anaerobically with 8A55, yet very little was formed with 32H1. Although acetylene reduction activity was slightly higher with 32H1, about 10 times more 15N 2 was produced aerobically by 8A55 than by 32H1 in the absence of acetylene. The major reductive pathway of N 2O reduction by denitrifying rhizobium strain 8A55 is by nitrous oxide reductase rather than nitrogenase. 相似文献
11.
The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N 2O), the effects of N 2O were evaluated. N 2O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N 2O-anesthetized animals, measured with [U- 14C]glucose and [1- 2H 2,2- 2H 2]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O 2 (partial pressure of oxygen, Pao 2= 120 mm Hg) were compared with rats exposed to 15% O 2 (Pao 2= 57 mm Hg) or 10% O 2 (Pao 2= 42 mm Hg). The synthesis of acetylcholine, measured with [U- 14C]glucose, was decreased by 35 and 54% with 15% O 2 and 10% O 2 respectively; acetylcholine synthesis, measured with [1- 2H 2,2- 2H 2]choline, was decreased by 50 and 68% with 15% O 2 and 10% O 2 respectively. Animals breathing either 15% or 10% O 2 had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O 2. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia. 相似文献
12.
Nitrate, nitrite and nitrous oxide were denitrified to N 2 gas by washed cells of Rhizobium japonicum CC706 as well as by bacteroids prepared from root nodules of Glycine max (L.) Merr. (CV. Clark 63). Radiolabelled N 2 was produced from either K 15NO 3 or Na 15NO 2 by washed cells of Rh. japonicum CC705 grown with either nitrate only (5 mM) or nitrate (5 mM) plus glutamate (10 mM). Nitrogen gas was also produced from N 2O. Similar results were obtained with bacteroids of G. max. The stoichiometry for the utilization of 15NO
3
-
or 15NO
2
-
and the produciton of 15N 2 was 2:1 and for N 2O utilization and N 2 production it was 1:1. Some of the 15N 2 gas produced by denitrification of 15NO
3
-
in bacteroids was recycled via nitrogenase into cell nitrogen. 相似文献
13.
N 2O reductase activity in soybean nodules formed with Bradyrhizobium japonicum was evaluated from N 2O uptake and conversion of 15N-N 2O into 15N-N 2. Free-living cells of USDA110 showed N 2O reductase activity, whereas a nosZ mutant did not. Complementation of the nosZ mutant with two cosmids containing the nosRZDFYLX genes of B. japonicum USDA110 restored the N 2O reductase activity. When detached soybean nodules formed with USDA110 were fed with 15N-N 2O, they rapidly emitted 15N-N 2 outside the nodules at a ratio of 98.5% of 15N-N 2O uptake, but nodules inoculated with the nosZ mutant did not. Surprisingly, N 2O uptake by soybean roots nodulated with USDA110 was observed even in ambient air containing a low concentration of N 2O (0.34 ppm). These results indicate that the conversion of N 2O to N 2 depends exclusively on the respiratory N 2O reductase and that soybean roots nodulated with B. japonicum carrying the nos genes are able to remove very low concentrations of N 2O. 相似文献
14.
In this study, the response of N 2 fixation to elevated CO 2 was measured in Scirpus olneyi, a C 3 sedge, and Spartina patens, a C 4 grass, using acetylene reduction assay and 15N 2 gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO 2 significantly ( P < 0·05) caused a 35% increase in nitrogenase activity and 73% increase in 15N incorporated by Scirpus olneyi. In Spartina patens, elevated CO 2 (660 ± 1 μ mol mol − 1) increased nitrogenase activity and 15N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N 2 fixation in Scirpus olneyi under elevated CO 2 was 611 ± 75 ng 15N fixed plant − 1 h − 1 compared with 367 ± 46 ng 15N fixed plant − 1 h − 1 in ambient CO 2 plants. In Spartina patens, however, the rate of N 2 fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng 15N fixed plant − 1 h − 1 for elevated and ambient CO 2, respectively. Heterotrophic non-symbiotic N 2 fixation in plant-free marsh sediment also increased significantly ( P < 0·05) with elevated CO 2. The proportional increase in 15N 2 fixation correlated with the relative stimulation of photosynthesis, in that N 2 fixation was high in the C 3 plant in which photosynthesis was also high, and lower in the C 4 plant in which photosynthesis was relatively less stimulated by growth in elevated CO 2. These results are consistent with the hypothesis that carbon fixation in C 3 species, stimulated by rising CO 2, is likely to provide additional carbon to endophytic and below-ground microbial processes. 相似文献
15.
Triacontanol (TRIA) increased fresh and dry weight and total reducible nitrogen (total N) of rice ( Oryza sativa L.) seedlings within 40 minutes. Increases in total N in the supernatants from homogenates of corn ( Zea mays L.) and rice leaves treated with TRIA for one minute before grinding occurred within 30 and 80 minutes, respectively. The source for the increase was investigated utilizing atmospheric substitution and enrichment and depletion studies with 15N. The increase in total N in seedlings was shown to be independent of method of N analysis and the presence of nitrate in the plants. Automated Kjeldahl determinations showing apparent increases in N composition due to TRIA were shown to be correlated with hand Kjeldahl, elemental analysis, and chemiluminescent analysis in three independent laboratories. TRIA did not alter the nitrate uptake or endogenous levels of nitrate in corn and rice seedlings. Enrichment experiments revealed that the total N increases in rice seedlings, in vivo, and in supernatants of corn leaf homogenates, in vitro, are not due to atmospheric N 2. TRIA increased the soluble N pools of the plants, specifically the free amino acid and soluble protein fractions. No differences in depletion or enrichment of 15N incorporated into soluble and insoluble N fractions of rice seedlings could be detected on an atom per cent 15N basis. The apparent short-term total N increases cannot be explained by current knowledge of major N assimilation pathways. TRIA may stimulate a change in the chemical composition of the seedlings, resulting in interference with standard methods of N analysis. 相似文献
16.
Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha ?1 year ?1) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (N 2O), carbon dioxide (CO 2) and methane (CH 4), as well as natural abundance isotopic signatures of N 2O, nitrate (NO 3 ?) and ammonium (NH 4 +). In the control plots during dry season, we deduced limited N 2O production by denitrification in the topsoil (0.05–0.40 m) as indicated by: ambient N 2O concentrations and ambient 15N-N 2O signatures, low water-filled pore space (35–60%), and similar 15N signatures of N 2O and NO 3 ?. In the subsoil (0.40–2.00 m), we detected evidence of N 2O reduction to N 2 during upward diffusion, indicating denitrification activity. During wet season, we found that N 2O at 0.05–2.00 m was mainly produced by denitrification with substantial further reduction to N 2, as indicated by: lighter 15N-N 2O than 15N-NO 3 ? throughout the profile, and increasing N 2O concentrations with simultaneously decreasing 15N-N 2O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between 18O-N 2O and 15N-N 2O site preferences. Long-term N addition did not affect dry-season soil N 2O-N contents, doubled wet-season soil N 2O-N contents, did not affect 15N signatures of NO 3 ?, and reduced wet-season 15N signatures of N 2O compared to the control plots. These suggest that the increased NO 3 ? concentrations have stimulated N 2O production and decreased N 2O-to-N 2 reduction. Soil CO 2-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO 2-C contents. The N-addition plots showed reduced dry-season soil CH 4-C contents and threshold CH 4 concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH 4 uptake rates remained similar between treatments possibly because diffusive CH 4 supply from the atmosphere largely limited CH 4 oxidation. 相似文献
17.
Plants take up inorganic nitrogen and store it unchanged or convert it to organic forms. The nitrogen in such organic compounds is stoichiometrically recoverable by the Kjeldahl method. The sum of inorganic nitrogen and Kjeldahl nitrogen has long been known to equal the total nitrogen in plants. However, in our attempt to study the mechanism of nitrogen dioxide (NO 2) metabolism, we unexpectedly discovered that about one-third of the total nitrogen derived from 15N-labeled NO 2 taken up by Arabidopsis thaliana (L.) Heynh. plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen ( UN). The generality of the formation of UN across species, nitrogen sources and cultivation environments for plants has been shown as follows. Firstly, all of the other 11 plant species studied were found to form the UN in response to fumigation with 15NO 2. Secondly, tobacco ( Nicotiana tabacum L.) plants fed with 15N-nitrate appeared to form the UN. And lastly, the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. In addition, the UN appeared to comprise a substantial proportion of total nitrogen in these plant species. Collectively, all of our present findings imply that there is a novel nitrogen mechanism for the formation of UN in plants. Based on the analyses of the exhaust gas and residue fractions of the Kjeldahl digestion of a plant sample containing the UN, probable candidates for compounds that bear the UN were deduced to be those containing the heat-labile nitrogen–oxygen functions and those recalcitrant to Kjeldahl digestion, including organic nitro and nitroso compounds. We propose UN-bearing compounds may provide a chemical basis for the mechanism of the reactive nitrogen species (RNS), and thus that cross-talk may occur between UN and RNS metabolisms in plants. A mechanism for the formation of UN-bearing compounds, in which RNS are involved as intermediates, is proposed. The important broad impact of this novel nitrogen metabolism, not only on the general physiology of plants, but also on plant substances as human and animal food, and on plants as an integral part of the global environment, is discussed.Abbreviations NO Nitric oxide - NO2 Nitrogen dioxide - RNS Reactive nitrogen species - UN Unidentified nitrogen - TNNAT, RNNAT, INNAT and UNNAT Total, Kjeldahl, inorganic and unidentified nitrogen in naturally fed plants, respectively - TNNIT, RNNIT, INNIT and UNNIT Total, Kjeldahl, inorganic and unidentified nitrogen derived from nitrate, respectively - TNNO2, RNNO2, INNO2 and UNNO2 Total, Kjeldahl, inorganic and unidentified nitrogen derived from NO 2, respectively 相似文献
18.
The methodology, characteristics and application of the sensitive C 2H 2-C 2H 4 assay for N 2 fixation by nitrogenase preparations and bacterial cultures in the laboratory and by legumes and free-living bacteria in situ is presented in this comprehensive report. This assay is based on the N 2ase-catalyzed reduction of C 2H 2 to C 2H 4, gas chromatographic isolation of C 2H 2 and C 2H 4, and quantitative measurement with a H 2-flame analyzer. As little as 1 μμmole C 2H 4 can be detected, providing a sensitivity 10 3-fold greater than is possible with 15N analysis. A simple, rapid and effective procedure utilizing syringe-type assay chambers is described for the analysis of C2H2-reducing activity in the field. Applications to field samples included an evaluation of N2 fixation by commercially grown soybeans based on over 2000 analyses made during the course of the growing season. Assay values reflected the degree of nodulation of soybean plants and indicated a calculated seasonal N2 fixation rate of 30 to 33 kg N2 fixed per acre, in good agreement with literature estimates based on Kjeldahl analyses. The assay was successfully applied to measurements of N2 fixation by other symbionts and by free living soil microorganisms, and was also used to assess the effects of light and temperature on the N2 fixing activity of soybeans. The validity of measuring N2 fixation in terms of C2H2 reduction was established through extensive comparisons of these activities using defined systems, including purified N2ase preparations and pure cultures of N2-fixing bacteria. With this assay it now becomes possible and practicable to conduct comprehensive surveys of N2 fixation, to make detailed comparisons among different N2-fixing symbionts, and to rapidly evaluate the effects of cultural practices and environmental factors on N2 fixation. The knowledge obtained through extensive application of this assay should provide the basis for efforts leading to the maximum agricultural exploitation of the N2 fixation reaction. 相似文献
19.
We report on the contamination of commercial 15-nitrogen ( 15N) N 2 gas stocks with 15N-enriched ammonium, nitrate and/or nitrite, and nitrous oxide. 15N 2 gas is used to estimate N 2 fixation rates from incubations of environmental samples by monitoring the incorporation of isotopically labeled 15N 2 into organic matter. However, the microbial assimilation of bioavailable 15N-labeled N 2 gas contaminants, nitrate, nitrite, and ammonium, is liable to lead to the inflation or false detection of N 2 fixation rates. 15N 2 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 15N 2 lecture bottles from two discrete batch syntheses. Per mole of 15N 2 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 15N 2 lecture bottle from Campro Scientific contained ≥11 µmoles of 15N-nitrous oxide per mole of 15N 2 gas, and no detected 15N-nitrate/nitrite at the given experimental 15N 2 tracer dilutions. Two Cambridge Isotopes lecture bottles from discrete batch syntheses contained ≥0.81 µmoles 15N-nitrous oxide per mole 15N 2, and trace concentrations of 15N-ammonium and 15N-nitrate/nitrite. 15N 2 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 N 2 fixation assays suggests that the degree of detected 15N-ammonium contamination could yield inferred N 2 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, N 2 fixation rates commonly detected in field assays. These results indicate that past reports of N 2 fixation should be interpreted with caution, and demonstrate that the purity of commercial 15N 2 gas must be ensured prior to use in future N 2 fixation rate determinations. 相似文献
20.
Rice ( Oryza sativa L.) plants growing in pots of flooded soil were exposed to a 15N 2-enriched atmosphere for 3 to 13 days in a gas-tight chamber. The floodwater and soil surface were shaded with a black cloth to reduce the activity of phototrophic N 2-fixing micro-organisms. The highest 15N enrichments were consistently observed in the roots, although the total quantity of 15N incorporated into the soil was much greater. The rate of 15N incorporation into roots was much higher at the heading than at the tillering stage of growth. Definite enrichments were also found in the basal node and in the lower outer leaf sheath fractions after 3 days of exposure at the heading stage. Thirteen days was the shortest time period in which definite 15N enrichment was observed in the leaves and panicle. When plants were exposed to 15N 2 for 13 days just before heading and then allowed to mature in a normal atmosphere, 11.3% of the total 15N in the system was found in the panicles, 2.3% in the roots, and 80.7% in the subsurface soil. These results provide direct evidence of heterotrophic N 2 fixation associated with rice roots and the flooded soil and demonstrate that part of the newly fixed N is available to the plant. 相似文献
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