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Published in Soil Sci. Soc. Am. J. 67:1862-1868 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-7—FOREST & RANGE SOILS

Dinitrogen and Nitrous Oxide Formation in Beech Forest Floor and Mineral Soils

Inken Wolf and Rainer Brumme*

Univ. of Göttingen, Institute of Soil Science and Forest Nutrition, Büsgenweg 2, D-37077 Göttingen, Germany

* Corresponding author (rbrumme{at}gwdg.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A 15N tracer study was conducted to determine N2 and N2O fluxes and the processes responsible for the formation of N2O in two beech (Fagus silvatica L.) forest soils: an acid mineral soil (AM) (pH = 3.8) and the overlying acid forest floor (AFF) (pH = 3.8) from the Solling and a less acid mineral soil (LAM) (pH = 5.2) from the Göttinger Wald. Ammonium and nitrate of undisturbed soil cores were labeled by injecting 15N. The evolved gases, N2O and N2, and the ammonium and nitrate concentrations in the soils were measured together with the 15N abundances over a period of 8 d. Nitrous Oxide was produced in all soils by denitrification. Nitrate was reduced to N2 at higher soil pH (LAM) and in the AFF. The end product of denitrification at the lower soil pH (AM) was N2O. The N2O and N2 emission calculated on an areal basis decreased from LAM, AFF, to AM. The N2O/(N2O + N2) ratios decreased from AM (1.0), AFF (0.97), to LAM (0.80) during the initial period indicating that the main product of denitrification was N2O. On prolonged incubation the N2O/(N2O + N2) ratios decreased for AFF and LAM to 0.78 and 0.32, respectively, and was attributed to a gradual decrease in nitrate concentration. We estimate the in situ N2 emissions to be 0.71 and 0.51 kg N ha-1 yr-1 for the Göttinger Wald and the Solling, using published annual in situ N2O emissions and our N2/N2O ratios. The in situ N2O emissions of the Göttinger Wald and Solling are 0.17 and 3 kg ha-1 yr-1 and the N2 emissions increased the annual denitrification losses to about 0.88 and 3.51 kg ha-1 yr-1. The approach for estimating in situ N2 emissions has to be improved in future studies.

Abbreviations: AFF, acid forest floor • Am, ammonium • AM, acid mineral soil • KIM, Kinetic Isotope Model • LAM, less acid mineral soil • Nat, nitrate • Nit, nitrite • WFPS, water-filled pore space • WHC, water holding capacity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOILS ARE A SOURCE of N2 and N2O that are released during denitrification; however, N2O can also be formed during nitrification (Bouwman, 1990). Terrestrial N2 emission is the most important pathway of N back into the atmosphere in the global N-cycle (Schlesinger, 1997). Our knowledge about the N2 flux from soils is very limited because there has been no reliable and sensitive in situ method (Payne, 1991). Most available data has been obtained by the acetylene block method, which measures denitrified N2 and N2O losses but not nitrified N2O. Moreover, the acetylene block method has been found to have large uncertainties (Payne, 1991; Bollmann and Conrad, 1997b). As a consequence, the N balances of ecosystems and the global N2 cycle imply large uncertainties expressed in the large range of (N2 + N2O) emissions (13–233 x 1012 g N yr-1) compared with the N2O emissions (7–16 x 1012 g N yr-1) (Bowden, 1986). Nitrous Oxide is a greenhouse gas and much more effective in the absorption of infrared radiation than CO2 (IPCC, 1996) and is also involved in the catalytic decomposition of ozone in the stratosphere (Crutzen, 1981). Therefore we have to obtain more information about the source strength of N2 and N2O and the processes responsible for their formation. Temperate forests are a focus of concern because of their high anthropogenic N load. High N deposition or soil acidification already might have increased the source strength of N2 and N2O or their proportions by changing the contributing nitrification and denitrification processes.

Our knowledge about N2O fluxes from temperate forests has increased in recent decades. Temperate forests have been found to have low or high emissions of N2O. A recent approach makes it possible to separate forests into emission types (Brumme et al., 1999). These authors distinguished forests with low emission during the year (Background Emission Type) and assigned 70% of the investigated forests using whole-year measurements, with an average emission of 0.39 ± 0.27 kg N2O-N ha-1 yr-1 to this type. The highest emissions with 1 to 8 kg N2O-N ha-1 yr-1 were recorded in forests belonging to the Seasonal Emission Type. This type has high emissions in wet/warm periods. Background Emissions occur in needle and broad-leaved forests of the temperate zone (n = 21) whereas high Seasonal Emissions have been observed in tropical (n = 3) and temperate broad-leaved forests (n = 5). For both types additional emissions, for example, during freezing/thawing periods in the temperate zone or after rewetting, are possible (Event Emission Type). A key factor responsible for the differences between Seasonal and Background Emissions is O2 availability within the soils, which suggests that different processes are involved in the formation of N2O. An in situ study with 15N indicated that the N2O of the Seasonal Emission Type at Solling is formed by denitrification (Wolf and Brumme, 2002). The processes responsible for the formation of N2O by the Background Emission Type are not known.

The aim of this work was to determine the formation processes and the rates of N2 and N2O emissions from an AM and the overlaying AFF (Solling, Seasonal Emission Type) and a LAM (Göttinger Wald, Background Emission Type). We used undisturbed soil cores in a laboratory study. In situ N2O measurements at these sites have already been conducted (Brumme et al., 1999). We combined the measured ratio between N2 and N2O and in situ N2O measurements to obtain information about the relevance of N2 emissions under field conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The study was conducted with soil samples from two beech forests. The 154-yr-old beech forest at Solling is located at 504 m above sea level, receives 1090 mm precipitation and has a mean annual temperature of 6.4°C. The beech forest at Göttinger Wald is 136 yr old and receives a lower precipitation (680 mm), has exposed to a higher mean annual temperature (7.8°C), and is situated at 420 m above sea level. In contrast to Solling, the soil at the Göttinger Wald is completely covered by a ground vegetation, primary Allium ursinum L. and Anemone nemorosa L. The podsolic mineral soil at Solling is covered by a moder humus with a thickness of about 5 cm and has a pH of 3.8 (H2O) in both layers (Table 1). The soil is a Dystric Cambisol (FAO) derived from loess over Triassic sandstone. The soil of the Göttinger Wald has a pH of 5.2 (H2O) in the upper mineral soil. A high microbial and earthworm (Lumbricus terrestris) activity prevent an accumulation of litter at the top mineral soil, forming a mull humus, and resulted in a lower bulk density compared with the acid soil at Solling. The soil is a Rendzic Laptosol (FAO) derived from limestone but is free of carbonate in the top 20 cm.


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  		Table 1. Soil properties and N-transformation rates calculated by the Kinetic Isotope Method during 8 d of incubation at 20°C (standard deviation in parentheses) for the less acid mineral soil from Göttinger Wald (LAM), the acid mineral soil at Solling (AM), and the acid forest floor from Solling (AFF).

 
Undisturbed soil samples were randomly selected and taken from the upper AM and the AFF at Solling, and the LAM from the Göttinger Wald using stainless steel cylinders with a diameter of 8 cm and a 5-cm depth. The soil cores were stored at 4°C and at 20°C for 24 h before conducting the experiment.

Two tracer experiments were conducted: one with 15N labeled ammonium and one with 15N labeled nitrate (60 atom% 15N). The N content was adjusted in total to 50 mg of NH+4–N and 50 mg of NO-3–N to provide sufficient level of N. Between 30 and 47 mg 15N labeled NH+4–N and 47 and 49 mg 15N labeled NO-3–N was added per kilogram of dry weight. The 15N tracer was applied by nine injections per soil core using plastic syringes to achieve a homogeneous distribution of the tracer. After 15N application the water content equals about 100% water holding capacity (WHC). The incubation temperature was 20°C. For each tracer experiment, 10 soil cores were used for each soil type. For the determination of the mineral N contents and the 15N abundance, two soil cores were sampled after 1, 2, and 4 d of incubation and the remaining four soil cores were sampled after 8 d of incubation. The design for measuring gas emissions and their 15N abundance was the same as for soil N concentrations, such that fluxes were measured on 10 cores on Day 0.5 and 1 in closed vessels before two cores were taken for soil analysis on Day 1. Flux measurements were made on eight cores on Day 1.5 and 2, on six cores on Day 2.5 and 4, and on four cores on Day 7.5 and 8. The incubation time for gas flux measurements was 16 h. For the soil analysis each soil core was homogenized, an aliquot of the soil was extracted with 0.5 M K2SO4. The extract was analyzed for ammonium and nitrate. The net N mineralization of ammonium and nitrate (Fig. 1) were calculated by the differences in N concentrations between Day 1 and 8.



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  		Fig. 1. Net N-mineralization of ammonium, nitrate, and ammonium + nitrate (NNM) during 8 d of incubation at 20°C of the less acid mineral soil from Göttinger Wald (LAM), the acid forest floor (AFF), and the acid mineral soil from Solling (AM).

 
The 15N abundance of ammonium and nitrate in the soil were determined after extraction with the diffusion method of Jensen (1991). Total 15Nt was measured directly with an element analyzer coupled with a mass spectrometer (IRMS, Finnigan MAT, Bremen, Germany). The 15N Kinetic Isotope Method (KIM), described below, was used to determine the production and consumption of ammonium and nitrate given in Table 1 (Neiman and Gal, 1971; Sich and Russow, 1999; Wolf and Russow, 2000). The mass spectrometric measurement of 15N2 was performed using the microwave equilibration method of Well et al. (1998). Dinitrogen formation was determined by the 15N gas flux method (Hauck and Melsted, 1958; Arah, 1992; Russow et al., 1996). The 15N content of N2O was determined directly with a mass spectrometer coupled with a preconcentration unit (Precon-GC-IRMS, Finnigan MAT; Brand, 1995). The gaseous emissions of N2O were measured with a gas chromatograph (Loftfield et al., 1997).

Kinetic Isotope Method
For a description of the production and consumption of chemical products in chemistry and biochemistry the KIM can be used if the products are labeled (Neiman and Gal, 1971). In the case of short incubation times that reduce the cycling of labeled compounds, the production and consumption correspond to the gross rates. In the case of microbial N transformation processes in soils, the method can be used to quantify the production and consumption of ammonium, nitrite, nitrate, and the gaseous products (NO, N2O, N2) (mg kg-1 h-1) as described below.



The variables are r0, rate of mineralization; r1, rate of nitrification from ammonium (Am) to nitrite (Nit); r2, rate of nitrification from nitrite (Nit) to nitrate (Nat); r3, rate of denitrification from Nat to Nit; and r4, rate of denitrification from Nit to N gases.

As an example, the KIM for the transformations of Am is given. The concentration of ammonium (CAm) changed as a function of time (t) and is described by the difference of Am production (r0) and Am consumption (r1):

[1]

The change of the concentration of 15N-labeled Am (d15CAm, mg kg-1) is described by multiplying the transformation rates with the 15N abundance (a):

[2]

The change of the concentration of 15N-labeled ammonium in excess of the natural abundance is described by multiplying the transformation rates with the excess abundance (a'). The organic matter (OM) is unlabeled. If we use the excess abundance, the term aOM x r0 is zero, and the equation changes to:

[3]

Another equation is needed because Eq. [3] contains two unknown parameters, d15CAm and r1:

[4]

Calculating the first derivative of 15CAm yields:

[5]

Equation [3] and [5] were equated and the solution after the consumption of ammonium (r1) is:

[6]

Using Eq. [1] and [6] the production of Am (r0) is calculated as followed:

[7]

On the conditions that the initial N concentration and other parameters, for example, temperature and water content are similar in the soils compared, the KIM can be used to determine the production and the consumption. We used an initial N concentration of 50 mg N kg-1 and calculated the production and the consumption of ammonium and nitrate between the first and the last measurements.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Nitrogen Transformation
To understand the processes and rates of gaseous N formation in soils, the transformation rates of ammonification and nitrification under the given experimental conditions must be known. The increase in concentration of ammonium during the incubation (Fig. 1) indicated a higher production than consumption in all treatments (Table 1). Ammonification diluted the 15N enriched ammonium pool and resulted in decreased 15N abundance during the incubation in all soils (Fig. 2) . The strongest decrease of 15N abundance of ammonium was found in the AFF of the Solling, indicating a 12 to 17 times higher ammonium production compared with the LAM from the Göttinger Wald and the AM from Solling (Table 1).



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  		Fig. 2. Temporal changes in the 15N abundance of nitrate, ammonium, and N2O in the experiments with 15N labeled nitrate and 15N labeled ammonium during 8 d of incubation at 20°C of the less acid mineral soil from Göttinger Wald (LAM), the acid forest floor (AFF), and the acid mineral soil from Solling (AM).

 
The concentration of nitrate decreased during the experiment (Fig. 1), indicating a lower production than consumption in all treatments (Table 1). A constant 15N abundance of nitrate showed that nitrification is not occurring in AM (Fig. 2). A similar result was obtained for AFF, where nitrate production was delayed as shown by a constant 15N abundance of nitrate during the first 4 d of incubation. The Solling soil is known to have a much lower net nitrification than the Göttinger Wald, but it has also been shown at Solling that nitrification is due to heterotrophic microorganisms (Lang and Beese, 1985). In total, the nitrate production was more than two times higher at Göttinger Wald compared with Solling (Table 1). Although the net accumulation of nitrate was negative, nitrate availability did not limit the initial gaseous N losses. Only at the end of the experiment was a decreased N2O production accompanied by decreased nitrate concentrations.

Identification of N2O Formation Processes
The 15N abundance of N2O followed the pattern of the 15N abundance of nitrate and not of ammonium (Fig. 2). Directly after increasing the 15N abundance of nitrate up to about 30 to 45 atom% 15N by injecting 15N-labeled nitrate, a similar 15N abundance was measured for N2O. With increasing dilution by nitrification the 15N abundance of nitrate decreased during the incubation period to 20 atom% 15N in the LAM and AFF, and the 15N abundance of N2O was similar. The AM did not nitrify, indicated by a constant 15N abundance of nitrate, and was also reflected in the constant 15N abundance of N2O. The same conclusion was drawn for the responsible process after injecting labeled ammonium. The 15N abundance of ammonium increased to 45 to 60 atom% 15N after labeling. But the 15N abundance of N2O was similar to the 15N abundance of nitrate indicating that nitrate was the source of N2O formation. In the case of an exclusive formation by nitrification the 15N abundance of ammonium and N2O would be similar. These results showed clearly that from the beginning of the experiments N2O was formed from nitrate by denitrification.

Dinitrogen oxide in forest soils is formed either solely by nitrification (Robertson and Tiedje, 1987; Castaldi, 2000), denitrification (Ambus, 1998; Bauhus et al., 1996), or as a combination of both processes (Robertson and Tiedje, 1987; Davidson et al., 1986; Martikainen et al., 1993; Kester et al., 1997; Gödde and Conrad, 2000). Although a number of studies investigating the responsible processes are available, an evaluation at an ecosystem level is very difficult. Most studies used acetylene, which is known to create a lot of uncertainties such as blocking nitrification, which in turn would result in an underestimation of N2O emission from nitrification and from denitrification if nitrate is limiting. Also acetylene may serve as a substrate for denitrifying bacteria, which would result in an overestimation if C were limiting to denitrification. Additional diffusion limitations of acetylene in field studies must be considered (Payne, 1991; Myrold, 1990). Bollmann and Conrad (1997b) found that acetylene increased the production of NO2, which was taken up by the soil, and consequently underestimated denitrification. Moreover, very often soils were sieved or mixed which disturbed the natural field conditions and thus could not be used for an evaluation at an ecosystem scale. Nevertheless, there is a general agreement that the highest N2O emissions occur at a field capacity between 45 and 75% water-filled pore space (WFPS), and that they are due to denitrification and nitrification (Granli and Bøckman, 1994; Davidson et al., 2000). Higher WFPS increases the proportion of denitrification N2O, while lower WFPS increases the proportion of nitrification N2O. In our laboratory study, field conditions during rainy weather were simulated (70–84% WFPS, Table 1) and the general finding of a high proportion of denitrification N2O at high soil moisture content was confirmed. This finding was also confirmed by a field study at Solling. The N2O emission at Solling was formed by denitrification indicated by an in situ 15N experiment (Wolf and Brumme, 2002).

N2O and N2 Emission
The highest average N2O emission over the entire period was measured from the AFF (189 µg N kg-1 h-1, SD of 154) and LAM (133 µg N kg-1 h-1, SD of 69), the lowest from the AM (38 µg N kg-1 h-1, SD of 38). On an areal basis the order of N2O emission changed due to differences in bulk densities (Table 1). The emissions decreased from LAM (4.1 mg N m-2 h-1, SD of 2.1) to the AFF (2.3 mg N m-2 h-1, SD of 1.1) and AM (2.0 mg N m-2 h-1, SD of 2.0) (Fig. 3) . Owing to the higher soil temperature, higher soil water content, and higher nitrate concentrations in the laboratory study, the N2O emissions were much higher compared with field studies (Brumme et al., 1999). Even the N2O emission at LAM at the Göttinger Wald was increased, a site, which is known to have low in situ N2O emissions (0.17 kg N ha-1 yr-1) compared with the Solling (3 kg N ha-1 yr-1) (Brumme et al., 1999).



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  		Fig. 3. Average emissions of N2O, N2, and (N2O + N2) at the less acid mineral soil at Göttinger Wald (LAM), the acid forest floor (AFF), and the acid mineral soil at Solling (AM). The emission of N2 at AM is zero.

 
Dinitrogen was not formed under the acid conditions of the AM and therefore N2O must be the end product of denitrification. The N2 emissions were an average 51 (SD of 37) and 27 (SD of 26) µg N kg-1 h-1 at LAM and AFF, respectively. On an areal basis the emissions decreased from LAM (1.5 mg N m-2 h-1, SD of 1.1) to the AFF (0.3 mg N m-2 h-1, SD of 0.4) (Fig. 3). Higher N2 emissions from less acid soils like LAM have often been reported (Granli and Bøckman, 1994) and were explained by a higher nitrate-reducing potential at higher pH levels (Nômmik, 1956; Nägele and Conrad, 1990). However, in most studies examining the effect of soil pH on denitrification, the soil pH was rapidly manipulated by adding acidity or lime, which has often resulted in increased microbial activity or probably enabled growth of unnatural microbial populations (Granli and Bøckman, 1994). Although our study with natural differences in the soil pH confirmed the general findings, the reasons may be different. The LAM at Göttinger Wald has a higher pH and a much higher microbial activity, the cation-exchange capacity is dominated by base cations and not by Al, and the bulk density is very low (Table 1) resulting in a seven times higher gas diffusion (Ball et al., 1997) compared with the AM at Solling. Therefore direct and indirect effect of soil acidity may be responsible for differences between the sites.

Differences in the N2 emission between the AFF and AM with similar pH values at Solling could be explained by other soil characteristics (Table 1), for example, a much higher microbial activity in the forest floor. However, we have to stress that to our knowledge these are the first reliable results showing that under natural conditions a moder humus is a source of N2. The only study performed with soil from an undisturbed forest floor reported that N2 losses were absent in a raw humus of a spruce forest in central Sweden (Pluth and Nômmik, 1981). This difference may have resulted from different leaf structures. Litter of broad-leaved trees has been found to function as a diffusion barrier compared with needle litter (Ball et al., 1997) and has been held responsible for high N2O emissions in temperate forests with Seasonal Emission Pattern by reducing the O availability (Brumme et al., 1999).

The N2O and N2 production from LAM and AFF increased during the initial 4 d, and constant N2O/(N2O + N2) ratios indicate that the increases in production of N2O and N2 were similar. Initial N2O/(N2O + N2) ratios of 0.97 and 0.80 were calculated for AFF and LAM. Afterwards N2O production decreased, whereas N2 production remained more or less constant and reduced the N2O/(N2O + N2) ratios to 0.78 and 0.32 for AFF and LAM at the end of the experiment (Day 7 and 8). During the experiment, a decrease in the nitrate concentration from about 47 to 25 mg N kg-1 was observed which may have influenced the production of N2O. A positive effect of nitrate on N2O production has often been described in the literature (reviewed by Granli and Bøckman, 1994). Therefore nitrate seems to be a more sensitive factor influencing the N2O/(N2O + N2) ratio than soil pH. A high mean N2O/(N2O + N2) ratio of 0.89 was also found in a 15N study with undisturbed soil cores by Nômmik and Larsson (1989), where soil cores from seven sites were adjusted to a soil water content of 100% WHC and incubated at 21°C. On prolonged incubation the proportion of N2O in the evolved gases decreased, averaging 64%, and was attributed to a gradual decrease in nitrate concentration, similar to our study.

Implications for the Ecosystem Level
Since the late eighties, our knowledge about gaseous losses has been increased through the widespread use of chambers for whole-year measurements of N2O emissions from forest ecosystems, as reported in several general reviews (Brumme et al., 1999, Davidson et al., 2000; Groffman et al., 2000). Most information about N2 emission at an ecosystem scale, however, is primarily based on the acetylene method and most measurements of in situ denitrification rates were performed using soil cores (Barton et al., 1999). As discussed above, the reliability of the acetylene method has often been questioned (Payne, 1991; Terry and Duxbury, 1985; Bollmann and Conrad, 1997a, 1997b) and the direct measurement with the 15N technique is limited due to low detection levels under field conditions (Payne, 1991). We therefore combined information obtained from in situ flux measurements of N2O with the N2/N2O ratios obtained in our laboratory study to present a preliminary estimate of in situ N2 emissions. We multiplied the ratios by the field fluxes for scaling-up, assuming that the observed influence of the laboratory conditions on both the N2O and N2 emission were in the same order as in the field.

We used the N2/N2O ratios at the end of the laboratory experiment because of the more realistic nitrate concentrations in the soils at the end of the experiment. The low in situ Background N2O Emissions at the Göttinger Wald of 0.17 kg N ha-1 yr-1 (Brumme et al., 1999) and the high N2/N2O ratio of 4.2 resulted in a relatively low N2 production of 0.71 kg N ha-1 yr-1. At the Solling, high in situ Seasonal N2O Emissions of 3 kg N ha-1 yr-1 were measured, to which the forest floor, where N2 production was detected, contributed to about 53% (Brumme et al., 1999), assuming that N2O production at lower depths is negligible. The calculated N2 production at the Solling of 0.51 kg N ha-1 yr-1 is only slightly higher than at the Göttinger Wald because of the lower N2/N2O ratio of 0.32 at Solling. The annual denitrification losses (N2O + N2) from the Solling and the Göttinger Wald would be about 3.51 and 0.88 kg N ha-1 yr-1 and are in the same range with the values by Barton et al. (1999) (0.3 to 28 kg N ha-1 yr-1) which based on reported whole-year measurements using predominantly acetylene inhibition in soil cores from deciduous forests.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Nitrogen-15 tracer studies are a powerful tool for identification of which processes (nitrification or denitrification) are responsible for N2O production. The only source for N2O production in the beech forest soils was denitrification. Dinitrogen was not produced in the acid mineral soil, indicating that N2O was the end product of denitrification. The production of N2 in the acid forest floor showed that O-horizon could be a significant source of this highly reduced N gas even under acid soil conditions. The use of laboratory derived N2/N2O ratios and annual in situ measurements of N2O emissions for estimating annual in situ N2 emissions have to further improve in future experiments. The necessary addition of 15N-labeled N may have lead to an underestimation of in situ N2 losses. For future experiments we recommend to double the amount of mineral N background in the soil with highly labeled 15N (e.g., 100 atom% 15N) to get 50% enrichment, which is required for an optimal signal for N2 in mass spectrometry analysis. Furthermore, whole-year in situ measurements with intact soil cores taken at various dates during a year are necessary to cover seasonal fluctuations in N2 emission. Whether the methodical uncertainties of the 15N approach are more distinct compared with the acetylene method is still not clear.


   ACKNOWLEDGMENTS
 
This work was funded by the German Science Foundation.

Received for publication February 15, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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