Dierent pathways of formation of N
2
O, N
2
and NO in black
earth soil
I. Wolf
a, b,*, R. Russow
aaDepartment of Soil Science, UFZ Centre for Environmental Research Leipzig-Halle, Theodor-Lieser-Strasse 4, D-06120 Halle, Germany bInstitute of Soil Science and Forest Nutrition, University of GoÈttingen, BuÈsgenweg 2, D-37077 GoÈttingen, Germany
Accepted 17 August 1999
Abstract
The use of15N tracer provides a suitable technique to investigate the processes of N transformation in soils and the origin of
the environmentally relevant gaseous N compounds N2O and NO from nitri®cation and denitri®cation. The results of
incubation experiments with black earth soil under two dierent water contents are presented here. Nitri®cation and denitri®cation proceeded simultaneously, but the importance of these two microbial processes shifted depending on the water content of the soil. Under water-unsaturated conditions the microbial oxidation of NH4
+
to NO3ÿpredominated, but a reduction
of NO3ÿ also occurred. The emission of NO exceeded the emission of N2O by a factor of up to 20 at the beginning of the
experiments. Under water-saturated conditions denitri®cation was the dominant process of N transformation in the soil. However, nitri®cation also occurred to a considerable extent. The emission of N2O was greater than under unsaturated
conditions. The formation of NO could hardly be observed. N loss by molecular nitrogen from denitri®cation could be detected under saturated conditions. The N loss amounted to 60% of NO3ÿ and thereby the cumulative N ratio of N2 to N2O was 3.
Under either unsaturated or saturated conditions NO arose from NO2ÿ or during the microbial oxidation of NH4+ to NO2ÿ.
However, N2O mainly formed from denitri®cation under both conditions. Furthermore, NO could not be observed as a
precursor of N2O and the free NO2ÿ could not be detected as a common N pool for the formation of N2O and NO. High
emissions of NO could be a problem for the black earth soil in the semi-arid climate in central Germany, if there are large amounts of NH4+in the soil after fertilisation.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Denitri®cation; Di-nitrogen; Nitri®cation;15N; Nitric oxide; Nitrous oxide
1. Introduction
Nitrous oxide and nitric oxide are directly or in-directly involved in global warming, the destruction of the stratospheric ozone, the production and consump-tion of atmospheric oxidants, and the photochemical formation of nitric acid (Bouwman, 1990; Williams et al., 1992). Additionally, the emission of N2O, NO and
N2 from agricultural soils is a loss of N fertiliser
(Granli and Bùckman, 1994).
During nitri®cation and denitri®cation N2O and NO
as well as N2 are emitted (Firestone and Davidson,
1989). Nitrous oxide and NO can also be consumed by microorganisms in the soil (Hutchinson and Davidson, 1993). Although the mechanism of N2O and NO
for-mation in soils is generally well-known, it is still not clear how nitri®cation and denitri®cation contribute to the formation of N2O under dierent amounts of
water saturation in the soil. Further uncertainties include the role of NO in these processes. Nitric oxide could be a by-product or an obligate and free precur-sor of N2O in the denitri®cation path. Not much is
known about the ratios between N2 and N2O-N or
N2O-N and NO-N under dierent conditions.
Recent 15N studies with agricultural soil have shown that under unsaturated conditions nitri®cation was the
Soil Biology & Biochemistry 32 (2000) 229±239
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* Corresponding author. Tel.: 393518; fax: +49-551-393310.
dominant process for N2O production (Stevens et al.,
1997). The formation of N2O and NO via nitri®cation
could be signi®cant for the black earth soils in central Germany because of the absence of typical denitri®ca-tion condidenitri®ca-tions (water saturadenitri®ca-tion) under the prevailing semi-arid climate conditions. However, N2O can be
formed in soils by denitri®cation due to anoxic micro-sites within soil aggregates (Renault and Stengel, 1994) or by aerobic denitri®cation (Robertson and Kuenen, 1984). Studies on the formation of NO by these mi-crobial processes using 15N have not been carried out yet.
The eect of soil moisture on the formation of N2O,
NO and N2 was studied by the kinetic 15N isotope
method (Neiman and Gal, 1971) using15NH4+, 15NO3ÿ
and 15NO2ÿ as tracer. Comparisons of the 15N
abun-dance of NH4+, NO3ÿ, NO2ÿ, N2O, NO and N2allowed
us to identify the N processes in the soil and the for-mation processes of the gaseous N compounds.
2. Materials and methods
2.1. Soil
Soil was taken from the surface of a plot at the research ®eld belonging to the Centre for Environmental Research Leipzig-Halle (Bad LauchstaÈdt). The soil type is Haplic Phaeozem and the soil form is a well-textured loess±black earth (21% clay, 68% silt, 11% sand, Nt=0.31%, Ct=3.45%).
The soil was air-dried and then sieved (2 mm).
2.2. Gas analysis
The concentrations of N2O, NO and N2, as well as
their 15N abundance in the incubation atmosphere were recorded by a gas chromatograph linked to a quadrupole mass spectrometer (GC±QMS, Shimadzu
QP 2000) with column switching of a PORAPAK Q packed column and a PLOT column containing mo-lecular sieve 5 AÊ (Russow et al., 1995). The gas was injected via a gas injection and preconcentration device (Sich and Russow, 1998).
2.3. Analysis of NH4+, NO3ÿand NO2ÿ
For the quantitative analysis of NH4+ and NO3ÿ in
the soil samples, steam distillation of the KCl extract was used (Faust et al., 1981; Bremner and Mulvaney, 1982) followed by the determination of 15N abundance using emission spectrometry (Fischer and Meier, 1992). The content of NO2ÿ and its 15N enrichment was
determined by producing nitric oxide followed by an analysis with a continuous ¯ow quadrupole mass spec-trometer (CF±QMS; Russow et al., 1996a).
2.4.15N tracer methods
The experiments were carried out on the basis of the kinetic isotope method explained in detail by Neiman and Gal (1971) using the stable isotope 15N (15NH4+, 15
NO3ÿ and15NO2ÿ). It included the comparison of the 15
N abundance and concentration of the participating N compounds during the incubation. With the analyti-cal equipment used and developed by us, it was pos-sible to record and quantify the concentration and15N abundance of NH4+, NO3ÿ and NO2ÿ as well as N2O,
NO and N2during incubation.
N2 losses due to denitri®cation were determined by
the 15N gas ¯ux method (Hauck and Melsted, 1958; Boast et al., 1988; Arah, 1992; Russow et al., 1996b).
2.5. Conditions for the series of experiments and experimental design
Experiments were conducted under either unsatu-rated (50±55% water-holding capacity (WHC)) or
saturated conditions (about 95% WHC). The uniform application of the N fertiliser at the start of the exper-iments was important (154 mg N kgÿ1 air-dried soil;
about 280 kg N haÿ1). Application was carried out
using NH4+, NO2ÿ and NO3ÿ salts dissolved in distilled
water. To promote nitri®cation under unsaturated con-ditions the N ratio of NH4+ to NO3ÿ was 2, and to
promote denitri®cation under saturated conditions the N ratio of NH4+ to NO3ÿ was 0.5. The initial
concen-tration of NO2ÿ was 5 mg N kgÿ1 air-dried soil in all
experiments. Three separate experiments were done at each moisture content with the ®rst experiment using
15
NH4+, the second experiment using 15NO2ÿ, and the
third experiment using15NO3ÿas tracer. An incubation
system was used which permitted a high frequency sampling of the soil and the N gases evolved (Figs. 1 and 2). After a conditioning phase (air-dried soil incu-bated with 8 mg NH4+-N kgÿ1air-dried soil and 40%
WHC over 3 d) 15 g of the soil were placed in an incu-bation vessel and compressed (soil bulk density 1.3 g
cmÿ3). The application of the fertiliser was carried out
by pipetting the solutions of the salts over the soil. After the treatment the resulting moisture contents of the soil were 50±55% WHC or about 95% WHC. The vessels were kept at 308C in the dark. With three repli-cations a total of 39 incubation vessels were used for one 15N tracer experiment (Fig. 2). For each GC± QMS measurement of the soil atmosphere seven vessels (soil mass 105 g air-dried soil) were placed in one of three containers. The other vessels were incu-bated in parallel outside of the containers. Each con-tainer with the incubation vessels was connected to the analytical system (GC±QMS) via a gas circulation pump leading to the GC's injection and preconcentra-tion device (Fig. 3). After measurement of the emitted N gases the soil of one vessel from each container was extracted with 75 ml of 1 M KCl (Fig. 2). The extract was analysed for concentrations and 15N contents of NH4+, NO3ÿand NO2ÿ. For each vessel removed a new
one from the stock of vessels was placed into the
con-Fig. 2. Procedure for sampling the soil and the N gases evolved for one15N tracer experiment with three replications.
tainer to ensure equal number of vessels in the con-tainer for the next GC±QMS measurement of the soil atmosphere.
3. Results
3.1. Water-unsaturated conditions
The inorganic N compounds exhibited a typical nitri®cation time course. The concentration of NH4+
decreased and the concentration of NO3ÿ increased
during the course of incubation (Fig. 4a), and N2O
and NO were formed (Fig. 4b). The emission of NO was high at the beginning of the experiments but decreased thereafter. The emissions of N2O followed
an opposite temporal course. It increased constantly from 1±2 mg N kgÿ1 air-dried soil hÿ1during the ®rst
4 d to 3±6mg N kgÿ1air-dried soil hÿ1after d 4.
Mineralisation was slow as indicated by the constant
15
N abundance of the NH4+ at the start of the 15NH4+
experiment (Fig. 5a). Due to the decrease of the amounts of NH4+ during the experiments (Fig. 4a),
small amounts of NH4+ from the mineralisation
con-tributed to the remarkable 15N dilution of the labelled
15
NH4+ pool after d 8. Nitri®cation was the dominant
N transformation process, 15N from the NH4+ pool
being transformed via NO2ÿ to NO3ÿ to increase the 15
N enrichment of the NO2ÿand NO3ÿ(Fig. 5a).
From the pro®les of the15N abundance of the NO2ÿ,
which is between the 15N abundance of NH4 +
and NO3ÿ (Fig. 5a,c,e), it is apparent that the NO2ÿ pool Fig. 3. Container connected to the analytical system (injection device, GC±QMS).
Table 1
15N balances for the experiments under unsaturated conditions
Nitrogen label Amount of15N (mg15N kgÿ1soil) (15N recovery (%))
start end
Ntotal NH4 +
NO2ÿ NO3ÿ Norg N2O NO Ntotal
15 NH4
+
45.59 0 0.01 (0.02) 40.37 (88.6) 3.82 (8.4) 0.30 (0.7) 0.20 (0.4) 44.70 (98.1)
15
NO2ÿ 5.63 0 0 4.21 (74.8) 0.91 (16.2) 0.02 (0.4) 0.10 (1.8) 5.24 (93.2)
15
NO3ÿ 16.53a 0 0 16.48 (99.7) 0.89 (5.4) 0.09 (0.5) 0.04 (0.2) 17.50 (105.8)
aImpurity with nitrite 410
was fed by both NH4 +
via nitri®cation and NO3ÿ via
denitri®cation. This is in agreement with the results of Burns et al. (1996). However, there is also the possi-bility that two or more separate pools of NO2ÿexist in
dierent soil aggregates. As we analysed a mixture of NO2ÿ in the soil KCl extract, we were unable to
dis-tinguish two or more pools of the very reactive NO2ÿ
in the soil.
In the15NO3ÿexperiments we have to take into
con-sideration the high 15N abundance of NO2ÿ at the
beginning of the experiments. These high 15N
abun-dance was due to impurities of the used salt (K15NO3)
with15NO2ÿ.
In the15N tracer experiment with15NO3ÿ, the
enrich-ment of the NO3ÿ pool decreased due to an input of
unlabelled N from the NH4+and NO2ÿpools by
nitri®-cation (Fig. 5e). After 8 to 9 d the 15N abundance of the NO3ÿ reached a constant value in all cases (Fig.
5a,c,e). In all experiments NO3ÿwas the ®nal product.
At the end of the experiments between 75 and 100% of the added 15N was found in the NO3ÿ pool (Table
1).
Fig. 4. Changes in concentrations and emission rates of the N components with time under water-unsaturated (a, b) and water-saturated con-ditions (c, d) (mean values for the three experiments, error bars=standard deviation).
Fig. 5. Changes in the15N abundance of the N components with time under water-unsaturated conditions with15NH 4
+(a, b),15NO 2
ÿ(c, d) and
15
Fig. 6. Changes in the15N abundance of the N components with time under water-saturated conditions with15NH4 +
(a, b),15NO2ÿ(c, d) and 15
NO3ÿ(e, f) as tracer (error bars=standard deviation).
From the time-course of the15N abundance of N2O
and NO during the incubation we inferred dierent pathways for their formation. On d 1 of the incubation the15N abundance of N2O and NO was the same, and
were similar to that for NO2ÿ (Fig. 5). Thus, the N2O
and NO may have originated directly from NO2ÿ.
After d 2, the15N abundance of N2O approached the 15
N abundance of NO3ÿ. From d 8 onwards N2O and
NO3ÿ had the same 15N abundance. Therefore, N2O
was formed only from NO3ÿ via denitri®cation. The 15
N abundance of NO followed more or less the value of NO2ÿ after d 2. Therefore, NO must have had a
quite dierent formation path to N2O. It could have
originated during the oxidation of NH4+ to NO2ÿ or
directly from NO2ÿ. Due to the dierent pro®les of the 15
N abundance of N2O and NO after d 2 of incubation
(Fig. 5b,d,f) extracellular NO was not an intermediate product or a precursor for the formation of N2O
under unsaturated conditions.
3.2. Water-saturated conditions
Incubations under saturated conditions produced data which were more variable and more disparate than under unsaturated incubations. The concen-trations of NH4+ and NO3ÿ decreased during the
incu-bation (Fig. 4c). Denitri®cation was the most common microbial process, but nitri®cation also occurred to some extent. In comparison with the experiments under unsaturated conditions large amounts of N2O
were formed at the beginning of the incubation, but little emission of NO was ascertained (Fig. 4d).
The 15N abundance of the 15N labelled NH4+ pool
decreased from the beginning of the experiment (Fig. 6a). Therefore, a rapid N mineralisation process took place during the incubation. It exceeded the N min-eralisation rate under unsaturated conditions. The recovery of 15N in the organic N pool indicated that immobilisation of NH4+ was also faster than under
unsaturated conditions (Tables 1 and 2). Similar to unsaturated conditions the NO3ÿ pool supplied from
the NH4+pool by nitri®cation was enriched during the
incubation (Fig. 6a). A decrease in 15N abundance of the NO3ÿin the experiment with 15NO3ÿ as tracer
con-®rmed the N input to the NO3ÿ pool by nitri®cation
(Fig. 6e).
The 15N abundance of NO2ÿ was between the 15
N abundance of NH4
+
and NO3ÿ (Fig. 6a,e). Therefore
similar to unsaturated conditions the NO2ÿwas formed
from both N pools, by reduction of the NO3ÿ and by
oxidation of the NH4+.
On d 1 of the experiments the 15N abundance of N2O and NO were similar and followed the value of
NO2ÿ (Fig. 6). N2O and NO must have originated
from the same NO2ÿ pool and is similar to the
exper-iment under unsaturated conditions. After d 1 of incu-bation, the 15N abundance of N2O was the same as
the 15N abundance of NO3ÿ. This indicated that the
NO3ÿwas the mean N pool for the formation of N2O.
The15N abundance of NO followed either those of the NO2ÿor was between the 15N abundance of NH4+ and
NO2ÿ. Again, the NO was coming directly from the
NO2ÿ pool or from the oxidation of NH4 +
to NO2ÿ.
Therefore, under these typical denitri®cation con-ditions (water saturation) the role of NO as a free in-termediate of N2O in the denitri®cation path could not
be con®rmed.
From the 15N balance it was obvious that there is a high loss of N gases (Table 2). This could not be explained by the emission of N2O and NO. However,
the emission of N2 from denitri®cation could lead to
these N losses. 15N in N2could be detected only in the
experiment with 15NO3ÿ as tracer due to the high 15N
enrichment in the NO3ÿpool. The N2 rate of
denitri®-cation was calculated according to the 15N gas ¯ux method. About 60% of the soil NO3ÿ was reduced to
N2. At the end of the experiment the cumulative N
ratio of N2to N2O was 3.
4. Discussion
soil under unsaturated and saturated conditions, and the resulting emission of N2O, NO and N2(Fig. 7).
In the soil, nitri®cation and denitri®cation proceeded under both unsaturated (nearly aerobic) and saturated (nearly anaerobic) conditions.
The total gaseous N loss and the portions of N2,
N2O and NO in this N loss during the saturated and
unsaturated incubations were clearly dierent. Under saturated conditions the total gaseous N loss was 90.4 mg N kgÿ1 air-dried soil for the experiment with 15
NO3ÿas tracer, apportioned between N2, N2O-N and
NO-N in the ratio 1:0.3:0.002. However, the cumula-tive emission of N2O and NO under unsaturated
con-ditions averaged only 2.2 mg N kgÿ1 air-dried soil.
The proportion of N2O-N and NO-N in the total N
gases evolved were equal. The N ratios of N2O to NO
were considerably larger under saturated conditions than under unsaturated conditions during incubation (Table 3). This is in contrast to the results provided by VanCleemput and Baert (1984) and BaumgaÈrtner and Conrad (1992), who observed high emissions of NO under saturated conditions.
Under unsaturated as well as saturated conditions N2O was mainly formed by denitri®cation of NO3ÿ
(Fig. 7). Only at the beginning of soil incubation did a large part originate from nitri®cation in the aerobic zones. As incubation under unsaturated conditions progressed, denitri®cation became the dominant pro-cess for N2O formation, due to the development of
an-aerobic microsites (Renault and Stengel, 1994).
Increasing water saturation promoted N2O emission
via denitri®cation caused by an increasing number of anaerobic sites in the soil. It was possible to calculate the contribution of NO3ÿ reduction to N2O formation
by using the isotope dilution equation for the exper-iment with 15NO3ÿ as tracer (Table 4; Sich, 1997).
Under unsaturated conditions the percentage of NO3ÿ
reduction to N2O formation was between 69±98%
from d 3 to 17. This is in contrast to other investi-gations with agricultural soil, where under unsaturated conditions the nitri®cation was responsible for 70% of the N2O ¯ux (Stevens et al., 1997). Under saturated
conditions from d 2 onwards 64±100% of the N2O
was formed from NO3ÿ reduction as expected.
However, N2O was probably not directly formed from
the NO3ÿ pool. Nitrous oxide may have arose from a
stage between NO3ÿ and free NO2ÿ, possibly an
enzyme-bound NO2ÿ([NO2ÿ]e, Fig. 7) (Ye et al., 1994).
The formation of N2O from the free NO2ÿpool in the
soil as suggested by Firestone and Davidson (1989) and Kroneck and Zumft (1990) could not be con-®rmed by our results.
The free NO desorbed into the soil atmosphere was mainly produced by nitri®cation as a by-product of the oxidation of NH4
+
to NO2ÿor directly by NO2ÿ
de-composition under oxygen limitation through nitri®er denitri®cation (Poth and Focht, 1985) (Fig. 7). No evi-dence was found for NO being a free obligatory inter-mediate of N2O in the consecutive elementary
denitri®cation steps NO2ÿ4NO4N2O4N2 as we Fig. 7. Flow chart of the paths of N transformation in black earth soil under water-unsaturated (a) and water-saturated (b) conditions related to N2O, NO and N2formation (thickness of the arrows refer to the importance of processes of N transformation in the soil and of formation of the N gases; SOM=soil organic matter; [NO2ÿ]e=enzyme-bound NO2ÿ).
Table 3
N ratio of N2O to NO during the experiments under unsaturated and saturated conditions
Duration (d) Unsaturated conditions N2O-N to NO-N Saturated conditions N2O-N to NO-N
minimum maximum minimum maximum
0±3 0.05 0.5 60.3 293.0
4±11 1.0 16.0 7.9 53.8
12±17 16.3 38.8 10.9 52.8
assumed from the literature (Firestone and Davidson, 1989; Ye et al., 1994). This ®nding could be caused by diusion limitation of NO in the soil water of the an-aerobic microsites. Nitric oxide arising from NO3ÿ can
not desorb from the liquid phase into the gaseous phase before its denitri®cation continued to N2O.
Further investigations without diusion limitation and
15
N labelled NO are needed to elucidate this problem. The emission of N2 from denitri®cation was only
detected under saturated conditions (Fig. 7b). Thereby the N loss via N2 was about 60% of the NO3ÿ and
exceeded the emission of N2O by a factor of 3.
5. Conclusion
The results obtained and the ¯ow chart developed show that in the black earth soil the N transformation processes nitri®cation and denitri®cation took place simultaneously under aerobic as well as anaerobic con-ditions and were closely coupled. The relationship between nitri®cation and denitri®cation depended on the degree of water saturation and thus the develop-ment of aerobic and anaerobic microsites within the same soil aggregate.
The processes of formation of N2O and NO in the
black earth soil were dierent. The N2O was mainly
formed via denitri®cation and NO via nitri®cation or nitrite decomposition. These formation pathways for N2O and NO were the same under unsaturated as well
as saturated conditions. During the pathway of denitri-®cation NO was not a free precursor of N2O.
Black earth soils in the semi-arid climate of central Germany (unsaturated conditions) are a permanent, low source for N2O. Under saturated conditions the
emission of N2O could increase to 280mg N kgÿ1
air-dried soil hÿ1. However, N
2O mainly originated from
the anaerobic microsites via denitri®cation.
These experiments highlighted the formation of NO from black earth soil. Under water unsaturation and
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