The use of stable isotopes in ecosystem research. First results of a field study with 15N

Terrestrial ecosystems, e.g. forest ecosystems, are characterized by a complex and sensitive network of biotic and abiotic factors and their interactions. By using stable isotopes (e.g. labelled nitrogen compounds), very small addition rates of highly enriched compounds can be applied, which do not change or disturb the investigated system, but provide information about single processes, their interactions and especially about their dynamics.

First results of a field study in the Fichtelgebirge, Northeast-Bavaria, Germany, are presented. The distribution of labelled nitrogen (as ¹⁵N-NH₄ ⁺ and ¹⁵N[sbnd]NO₃ ^?) within a spruce ecosystem (Picea abies (L.) Karst. in competition with understory vegetation of Vaccinium myrtillus, Calluna vulgaris and Deschampsia flexuosa) showed maximum ¹⁵N concentrations in tissues of the understory vegetation. During the first six weeks after the ¹⁵N application, the nitrogen uptake of all investigated species was higher after the ¹⁵N[sbnd]NO₃ ^? treatment than after the ¹⁵N[sbnd]NH₄ ⁺ treatment.     

15N Distribution in Wheat and Chemical Fractionation of Root-Borne 15N in the Soil

Abstract

Spring wheat plants were grown in split-root containers and labelled with ¹⁵N by fertilizing one compartment of the container with ¹⁵NH₄ ¹⁵NO₃ (95 at.-% ¹⁵N exc.). After the harvest, approx. 90% of the ¹⁵N incorporated by the plants were found in the shoots and 3% in the roots; approx. 7% had been released into the soil of the unlabelled compartment. The ¹⁵N in the soil of the unlabelled compartment was extracted with KCl and hydrolysed with HCl. Approx. 60% of the ¹⁵N was found in the hydrolysable organic N pool of the soil and 9% in the fraction of the soluble and exchangeable inorganic nitrogen.     

The Influence of Feeding Frequency on Nitrogen Turnover and Whole Body Protein Synthesis in Adult Rats (Estimation using [14C]- and [15N]- Labelled Leucine)

Abstract

Male Wistar rats (17 wks. old, body weight ～400 g), fitted with an intra gastric cannula and with a catheter in the vena jugularis were divided into 3 groups and given a marginal ration of the feeding solution Nutrison Standard (1g protein and 350 kJ ME per day). Group 1 had ad lib. access to the drinking bottle, the groups 2 and 3 were pair fed by gastric infusion, splitted up into 2 greater meals for group 2 respectively into 6 smaller meals for group 3. After adaptation all animals get an i.p. injection of doubly labelled tracer solution (200μl) containing 2.5mg L-[¹⁵N]leucine (72 atom% ¹⁵N) combined with either [1-¹⁴C]- or [U-¹⁴C] leucine (37 kBq).

The course of ¹⁴CO₂ expiration was estimated by breath test over 4h in intervals of 15 min and the course of urinary ¹⁵N excretion over 24h in intervals of 45 resp. 90 min. An infusion of saline (0.9% 5ml/h) into the vena jugularis was used to provoke sustained urine production of the animals during the experiment.

From the parameters of the excretion curves of breath ¹⁴CO₂ resp. urine ¹⁵N (cumulative end value) and from the N balance the portions of leucine-C and leucine-N used for protein synthesis, transamination decarboxylation and total oxidation as well as the kinetic parameters for whole body protein metabolism were computed.

The following conclusions were drawn:

6 x feeding regime produces a small but measurable amino acid economy effect in comparison to 2 x feeding regime.

Protein gain for 2 x feeding group was significant smaller than for 6 x feeding group, though protein synthesis rate was higher, but was overcompensated by a greater increase of protein breakdown rate for the 2 x feeding group. Energy storage in form of fat and glycogen built from decarboxylation was unaffected by feeding frequency. The amount of leucine oxidized for heat production was 4% higher for the 6 x feeding group. Transamination rate for leucine was estimated to 8–15%. Absolute values for protein flux, protein synthesis and protein breakdown may be overestimated or underestimated because the metabolism of [¹⁵N] leucine does not exactly agree with that of total N; but the proportions of them and therefore also the conclusions will be true. Better results for absolute values will be obtained using a mixture of ¹⁵N labelled AA, ¹⁵N labelled protein or hydrolysate of ¹⁵N labelled protein (yeast) as the tracer source.     

Course of Uptake of Weed-Borne Nitrogen by Maize,Tested with 15N

Abstract

The course of uptake of weed-borne nitrogen by maize was tested with ¹⁵N in a pot experiment with silty loam after common growth of maize and Chenopodium album L., and mulching the weed in the 5-leaf stage of maize. Harvests 4,8 and 12 weeks after mulching show that the maize took up 35, 63 and 70% of the weed-borne nitrogen, resp., in consequence of a rapid and almost complete mineralization. The portion of weed-borne nitrogen in total N of the maize was 16% at all harvest dates. The differences in yield between weeded and unweeded maize were not significant neither at 5-leaf stage nor at corn maturity.     

Estimation of Urea Production Rate With [15N2]Urea and [13C]Urea to Measure Catabolic Rates in Diabetes Mellitus

Abstract

For verifying catabolic states in insulin-dependent patients and dogs the method estimating urea production rates with ¹³C and with doubly ¹⁵N labeled urea, respectively, has been established. For a fast steady state of urea tracer dilution, a prime of 600 times the continuous infusion rate had to be injected. Urea was isolated from plasma samples by protein precipitation and cation exchange chromatography with a consecutive derivatization of the dried urea fraction (trimethylsilyl derivatives). The masses of the fragment ions m/z 189 (¹⁴N¹⁴N), 190 (¹⁴N¹⁵N) and 191 (¹⁵N¹⁵N) urea are monitored to estimate the [¹⁵N₂]urea frequency in the overall body urea pool in mol percent excess (MPE). 1 to 15 ng of derivatized urea were measured efficiently. An excellent correlation between expected standard and measured MPE (r = 0.9977) was achieved from solutions containing 1 to 7% [¹⁵N₂]urea. The interassay coefficient of variation amounted to < 10% for a [¹⁵N₂]urea portion of ≥ 3%.

Normoglycemic diabetic patients who were treated with insulin overnight showed significantly higher urea production compared to healthy controls (9.22 ± 2.07 vs. 5.4 ± 0.32 μmol·kg^?1 · min^?1; p < 0.05). Measurements in chronic diabetic dogs proved an increased rate of amino acid catabolism (+ 20% urea production) in systemic versus portal application of insulin in paired studies. This increased nitrogen load in diabetics may accelerate progression of diabetic nephropathy. - Thus, the established stable isotope technique may serve as a sensitive and useful indicator of amino acid catabolism in clinical and experimental research.     

The Effect of Catch Crops on the 15N Nitrogen Percolation and Leaching During the Early Winter Period

Abstract

Lysimeter experiments with application of ¹⁵N in growth chambers were used to investigate to what extent the growth of oil radish can prevent by temporary biological N conservation the nitrogen percolation and leaching during late autumn and early winter periods. It could be shown that the oil radish plants incorporated 47% of the applied ¹⁵N and thus reduced substantially the ¹⁵N percolation to the deeper soil layers (60–100 cm) and the ¹⁵N leaching losses. Before giving final recommendations, the fate of the ¹⁵N contained in the oil radish must be examined in the late winter and early spring periods, after freezing of the plants.     

A Dual 13C and 15N Long Term Labelling Technique to Investigate Uptake and Translocation of C and N in Beech (Fagus sylvatica L.)

Abstract

A continuous dual ¹³CO₂ and ¹⁵NH₄ ¹⁵NO₃ labelling experimental set-up is presented that was used to investigate the C and N uptake and allocation within 3-year old beech (Fagus sylvatica L.) during one growing season. The C and N allocation pattern was determined after six, twelve and eighteen weeks of growth. The carbon uptake was distinctly different in the three phases examined: The first six weeks after budbreak were dedicated to leaf growth with a R/S (root to shoot) ratio of 0.14 for the new carbon. The second growth phase showed a balanced R/S ratio of C allocation and after week 13, the root compartment was the main carbon sink (R/S = 6.97).

Nitrogen allocation was more basipetal as compared to carbon. In the second growth phase, R/S of N_new was 5.57 but fell to 3.54 for the third growth phase probably due to formation of reserves in buds and stem.     

Estimation of protein synthesis rates using the flooding method and [15N] lysine

After injection of a single dose of double labelled lysine (1 MBq L-[u-¹⁴C]lysine and 150 μmol L-[α¹⁵N]lysine (95 atom% ¹⁵N)) to growing rats the fractional protein synthesis rates (FSR) of some organs were estimated by the “large dose-” or “flooding method”. Data obtained with both substances were compared and the following conclusions can be drawn:

—In principle lysine is suited as a flooding substance.

—In flooding experiments [¹⁴C]lysine and [¹⁵N]lysine gave identical FSR-values for the investigated organs.

—By application of [¹⁵N]lysine instead of the ¹⁴C labelled amino acid as flooding substance this method is suited for larger animals (pigs, sheep, ruminants) too, because of the absence of any radioactive burden.

Nach einmaliger Verabfolgung von doppelt markiertem Lysin (1 MBq L-[u-¹⁴C]Lysin und 150 μmol L-[α¹⁵N]Lysin (95 Atom% ¹⁵N)) an wachsende Ratten wurden nach der “Large dose-” oder “Flooding-Methode” die fraktionellen Proteinsyntheseraten (FSR) einiger Organe bestimmt und die für beide Substanzen erhaltenen Werte verglichen. Es ergaben sich folgende Schlußfolgergungen:

—Lysin ist grundsätzlich als Flooding-Substanz geeignet.

—[¹⁴C]Lysin und [¹⁵N]Lysin ergeben im Floodingversuch am gleichen Tier für die FSR der Organe identische Werte.

—Durch den Fortfall der radioaktiven Belastung bei Einsatz von [¹⁵N]Lysin anstelle von [¹⁴C]Lysin als Flooding-Substanz ist die Methode auch bei Croßtieren anwendbar.     

Nitrogen Isotope Ratios in Different Compartments of a Mixed Stand of Spruce,Larch and Beech Trees and of Understorey Vegetation Including Fungi

Abstract

Natural nitrogen isotope ratios were measured in different compartments (needles or leaves and twigs of different age classes and crown positions, roots and soil of different horizons) of spruce (Picea abies), larch (Larix decidua) and beech (Fagus sylvatica) trees in an 11-year-old mixed stand in the Fichtelgebirge, NE Bavaria, Germany. In addition, samples of understorey vegetation (mainly ericaceous shrubs and grass) and of ectomycorrhizal and saprophytic fungi were analyzed. The δ¹⁵N values found for all samples ranged between ?7.5 and + 4.5‰. No significant differences were found for the nitrogen isotope ratios of the three tree species despite of their evergreen versus deciduous foliage and despite of their different rooting depth. Ericaceous shrubs had the most negative and fungi and soil from the mineral horizon the most positive δ¹⁵N values. Positive δ¹⁵N values of the fungi indicate their ability to utilize organic soil nitrogen, but the data do not unequivocally show that plants forming mycorrhizas profit from this organic nitrogen source.     

Uptake of 15NH3 by Picea abies in Closed Chamber Experiments

Abstract

Above-ground deposition of anthropogenic trace gases like NH₃ and NO_x is considered as a main factor for nitrogen (N) loading of Picea abies ecosystems. In order to quantify NH₃ deposition, tracer experiments with ¹⁵N labelled NH₃ were carried out in fumigation chambers (GSF München).

NH₃ uptake is linearly related to the gas concentration in the air, but the relation differs between organs and depends on N-nutrition of the organs. Plants well supplied with N have a lower NH₃ uptake per g dry weight then plants deprived of N. Only a small amount of the offered gas deposits to the external plant surfaces. The NH₃ uptake rates of spruce indicate that NH₃ may be regarded as being just as or even more important as environmental pollutant than NO_x with respect to N loading of spruce ecosystems.     

The Fate of [15N]Ammonium and [15N]Nitrate in the Soil of a 140-Year-Old Spruce Stand (Picea Abies) in the Fichtelgebirge (NE-Bavaria)

Abstract

A ¹⁵N tracer-experiment was carried out in a 140-year-old spruce stand (Picea abies (L.) Karst.) in the Fichtelgebirge (NE-Bavaria, Germany). Highly enriched (98 at%) [¹⁵N]ammonium and [¹⁵N]nitrate were applied as tracers by simulation of a deposition of 41.3 mol N ha^?1 with 11 water m^?2. To examine seasonal variations of uptake by spruce and understorey vegetation, different plots were labelled in spring, summer and autumn 1994.

One aim of the present study was to perfect a method of preparation of soil extracts for isotope ratio mass spectrometry (IRMS) measurements. Ammonium and nitrate from soil extracts were prepared for IRMS measurements by steam distillation and subsequent freeze drying. Additionally, tracer distribution and transformations in the soil nitrogen pools were examined. Ammonium, nitrate and total nitrogen were examined in the organic layer and the upper 10 cm of the mineral soil during 3 months after the first tracer application in spring 1994.

In July 1994, three months after tracer application, 40% of the [¹⁵N]ammonium label and 29% of the [¹⁵N]nitrate label, respectively, were recovered in the total N pool of the investigated soil horizons. In the organic layer the L/Of horizon retained most of the recovered tracers. Nitrification, immobilisation and mineralisation occurred even under the conditions of high soil acidity at the study site.     

Theoretical study on the influence of different NˆN ligands on the electronic structures and optoelectronic properties of heteroleptic Iridium(III) complexes

DFT/TDDFT calculations were carried out to investigate the electronic structures, absorption and phosphorescence properties of a series of heteroleptic Ir(III) complexes consisting of two N-heterocyclic carbene ligands and a conjugated bicyclic N,N′-heteroaromatic (N?N) ligand. On the basis of the results reported herein, we attempt to explain the experimental observations according to which complex (mpmi)₂Ir(pybi) (1) [Hmpmi = 1-(4-tolyl)-3-methyl-imidazole; Hpybi = 2-(pyridin-2-yl)-1H-benzo[d]imidazole] emits green light with an extremely high-quantum phosphorescence efficiency (Φ _PL ) of 79.3%, while a relatively lower Φ _PL (only 11%) was measured for (fpmi)₂Ir(tfpypz) (2) [fpmi = 1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C, C^2′; tfpypz = 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato] emitting blue light by tuning the N?N ligands. Besides, we also designed (fpmi)₂Ir(pyN3) (3) [pyN3H = 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine] and (fpmi)₂Ir(pyN4) (4) [pyN4H = 2-(1H-tetrazol-5-yl)pyridine] to explore the influence of electron-withdrawing substituents on N?N ligands on the electronic and optical properties of these Ir(III) complexes. The results revealed that electron-withdrawing substituents can stabilise both HOMOs and LUMOs and induce HOMO–LUMO energy gap change. Moreover, the emission properties can be significantly tuned by introducing different N?N ligands. While new insights were gained on structural and electronic properties, the extremely high Φ _PL of 1 was found to be not inherent to spin-orbital coupling effects, but determined by its large transition dipole moment (μ_{S 1}) upon S ₀–S ₁ transition compared with that of 2. On the basis of these results, the designed complexes 3 and 4 are considered to be the promising candidates for blue-emitting phosphorescence materials with higher Φ _PL than the complex 2.     

Lysimeter Investigations on the Effect of Winter Catch Crops and Weeded Fallow on the N-Dynamics in a Sandy Treposol Soil of Northeast Germany

Abstract

Lysimeter experiments (soil: sandy treposol, from the region “Havelländisches Luch”, Brandenburg, Germany) with application of ¹⁵N labelled fertilizer (80 kg N per ha as ¹⁵NH₄ ¹⁵NO₃, 10 at.-%¹⁵N exc.; for simulating mineralization in the early autumn period) were carried out to determine to what extent the amount of mineral- N was temporary conserved by winter catch crops, taken up subsequently in the vegetation periods by following crops, taken by subsequently in the vegetation periods by following crops, or percolated in the leaching water, respectively. The results were as follows:

1) Until winter or spring respectively, the catch crop uptake rates of applied mineral-N were 32% for phacelia (Phacelia tanacetifolia BENTH.), 25% for winter rape (Brassica napus L. cv. ‘AKELA’), and 16% for white mustard (Sinapis alba L.).

2) In the year after, following maize incorporated from 2.1 to 4.5% of the fertilizerborne N. The following plant community of fallow took up from 0.2 to 0.5% N originating from the fertilizer-N.

3) In comparison with the catch crops, N-leaching losses under fallow conditions were highest and equivalent to 17% of the applied fertilizer-N amount. In contrast to 3% of white mustard, phacelia and winter rape reduced N-leaching losses to 0.2 and 0.3% of the applied fertilizer-N amount.

4) In spring of the first year after the beginning of investigations, N-leaching losses were highest under fallow conditions and white mustard cultivation. Thus, the amounts of nitrate losses would exceed the EU limit for drinking water.

5) Three years after the investigations had been started, 10% (white mustard) and 20% (fallow) of the applied fertilizer-N was still found in th lysimeter soil.     

Incorporation of 15NO2 Nitrogen into Individual Amino Acids by Sunflowers Using Gc-C-Irms

Abstract

The nitrogen isotopic composition of individual amino acids in sunflower leaves after exposures to ¹⁵NO₂ in the range of ambient NO₂ concentrations (5–37 ppb) was analysed by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS). Amino acids as well as the amides glutamine and asparagine were converted with MTBSTFA (N-methyl-N-(tert.-butyldimethylsilyl)-tri-fluoroacetamid) in pyridine to their corresponding TBDMS derivatives (N, O-tert.-butyldimethylsilyl) in a simple one-step silylation reaction. The derivatized amino acids were separated by gas chromatography, combusted on-line, and the products were sent continuously to an isotope ratio mass spectrometer. Accurate measurements were obtained, when more than 7 nmol N₂ were introduced into the ion source of the mass spectrometer per gas chromatographically separated and combusted compound. No interferences of the silicate and fluor containing derivatization agents on the performance of the system were observed.

In the range of ambient NO₂ concentrations sunflower leaves predominately incorporate the nitrogen derived from atmospheric NO₂ into soluble amino acids. The highest δ¹⁵N values were measured for alanine. The ¹⁵N enrichments of the detectable amino acids increased with increasing ¹⁵NO₂ concentration.     

Gel-Electrophoresis and Subsequent Optical Emission 15N Analysis to Identify 15N-Labelled Protein Fractions

Abstract

A combined procedure to detect of ¹⁵N/¹⁴N isotope ratios by emission spectrometric analysis after starch gel-electrophoresis was developed. ¹⁵N-labelled proteins of human serum were used to optimise this method. Electrophorised gel slices with protein fractions were directly digested for subsequent isotope analysis. This method is proposed for use in routine analysis for clinical application.     

Inkorporation von 15N-markiertem Harnstoff in Organogene Dünge- und Abfallstoffe während der Rotte

Abstract

Eight organic fertilizers and wastes enriched with ¹⁵N labelled urea were stored at 25°C for one year. At the end of the experiment the ¹⁵N recovery rate ranged between 24 and 100%. The distributions of inorganic nitrogen (NH₄- and NO₃-N) and organic nitrogen (fulvic acids, humic acids and non extractable substances) are ascertained. Between the test materials, there are great differences in incorporation parameters.     

Amino Acid 15N/14N Analysis at Natural Abundances: A New Tool for Soil Organic Matter Studies in Agricultural Systems

Abstract

The effects of landuse, fertilizer history and soil type on the quantity and isotopic quality of hydrolysable soil amino acids were examined in 3 grassland and 2 arable soils. Results showed, (i) that overall concentrations of individual amino acids were highest in the grassland soils, (ii) that ‰δ¹⁵N values of the individual amino acids differed considerably between the five soils, and (iii) that the combination of amino acid ‰δ¹⁵N values and concentrations could be used to distinguish between landuse, crop type and fertilizer history. This preliminary study indicates that the pathways of transformation of soil amino acid N are influenced by long term N inputs and that associated biological processes are reflected in differences in concentrations and ‰δ¹⁵N values of individual soil amino acids.     

A New Approach to Determining the Content and 15N Abundance of Total Dissolved Nitrogen in Aqueous Samples: TOC Analyser-QMS Coupling

Abstract

The standard method for determining the ¹⁵N abundance of total dissolved nitrogen (TDN) in aqueous samples (e.g., soil leachate, sewage, urine) is currently Kjeldahl digestion followed by steam distillation or diffusion to isolate the ammonium, and then ¹⁵N measurement using IRMS. However, this technique is both time-consuming and laborious. One way of overcoming these disadvantages could be to couple a TOC analyser to determine the TDN with a sufficient quadrupole MS to determine the ¹⁵N abundance. The highTOC analyser (Elementar Analysensysteme Hanau, Germany), which catalytically oxidises the sample's total nitrogen with a high, constant yield to nitrogen monoxide (NO), appeared particularly suitable. The quadrupole-MS ESD 100 (InProcess Instruments Bremen, Germany) proved to be a suitable mass spectrometer for the ¹⁵N determination of NO. This combination of instruments was found to provide a workable method in numerous measurements of standard and actual samples. The detection limit concerning the N amount required per analysis is 2 μg, corresponding to an N concentration of 0.7mg/l in a maximum sample volume of 3ml. Depending on the N concentration, ¹⁵N abundances starting from 0.5 at.% can be measured with the required precision of better than 3% (simple standard deviation). For example, measuring the abundance of 0.5 at.% requires about 50 μg N, whereas for 1 at.% or more only about 5 μg N is needed per analysis.     

Conference Reports

Abstract

One purpose of new land use concepts for degraded fens (organic soils with high N content) is the reduction of the mineralization process due to very high groundwater levels. However, knowledge of nitrogen mineralization process (net and gross) in degraded fen soils affected by reflooding is very small. Therefore, the objectives of our study were (a) to evaluate the suitability of ¹⁵N pool dilution method for measurements of gross mineralization rates in degraded fen soils and (b) to investigate how the reflooding of a degraded fen affects the net and gross nitrogen mineralization in a short-term incubation experiment. The usability of the ¹⁵N pool dilution method was diminished by the low recovery of the applied ¹⁵NH₄ ^? at time zero. The recovery of the added ¹⁵NH₄ ^? in the extractable soil NH₄ ^? pool was only 13.5% for the drained soil and 59.6% for the reflooded soil. However, the gross mineralization rates were similar for both soils and exceeded always the net rates substantially. The cumulative net mineralization rate was higher for the reflooded soil (1.58 μg N?cm^?3?d^?1) than for the drained soil (-0.67 μg N?cm^?3?d^?1). Differences between the two soils were also found in the nitrification intensity and the loss of ¹⁵N. This was probably one reason for the higher net mineralization rate in the reflooded soil.     

In vivo NMR spectroscopy: in situ 15N pulse labelling NMR spectroscopy with photoautotrophic microorganisms

For studying the nitrogen metabolism in plants ¹⁵N NMR spectroscopy can be used. For in vivo ¹⁵N NMR (natural abundance of ¹⁵N: 0.37%) enrichment of the sample with the isotope ¹⁵N is compulsory. The detection of time courses of ¹⁵N assimilation from cells, which are enriched in culture is restricted in scope. Here, a method, the ¹⁵N pulse labelling NMR spectroscopy, is demonstrated, which permits labelling of different nitrogen compounds in photoautotrophic microorganisms during the NMR spectroscopic measurement. Using an effective illumination system it is possible to maintain photosynthesis in plant samples of high biomass densities in the magnet necessary for ammonia assimilation. The technique thus enables to directly observe ammonia assimilation pathways by application of a ¹⁵NO₃ ^? or ¹⁵NH₄ ^? pulses.

Für das Studium des Stickstoffstoffwechsels der Pflanzen kann die ¹⁵N-NMR-Spektroskopie herangezogen werden. Hierzu ist bei der in-vivo-¹⁵N-NMR (natürliche Häufigkeit von ¹⁵N: 0.37%) eine Anreicherung der Probe mit dem Isotop ¹⁵N unerläßlich. Eine Verfolgung der ¹⁵N-Assimilationskinetik mit Zellen, die in der Kultur angereichert wurden, ist jedoch nur bedingt möglich. In dieser Arbeit wird die ¹⁵N-Pulsmarkierungs-NMR-Spektroskopie als eine Methode vorgestellt, die es erlaubt, eine Markierung von Stickstoffverbindungen in photoautotrophen einzelligen Mikroorganismen während der NMR-Messung im Magneten vorzunehmen. Es wird ein spezielles Beleuchtungssystem verwendet, das eine für die Stickstoffassimilation ausreichende Photosyntheseleistung der Zellen unter NMR-Bedingungen bei hoher Biomassedichte ermöglicht. Diese Technik erlaubt durch die Applikation eines ¹⁵NO₃ ^?-oder ¹⁵NH₄ ⁺-Pulses eine direkte Verfolgung von Ammonium-Assimilationswegen.