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Soil Biology & Biochemistry 32 (2000) 1251±1259
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Gaseous N emission during simultaneous nitri®cation±
denitri®cation associated with mineral N fertilization to a
grassland soil under ®eld conditions
M.K. Abbasi a,*, W.A. Adams b
a

Department of Soil Science, University College of Agriculture, University of Jammu and Kashmir, Rawalakot, Azad Kashmir, Pakistan
b
Soil Science Unit, Institute of Biological Sciences, University of Wales Aberystwyth, Aberystwyth, Ceredigion SY23 3DE, UK
Received 12 April 1999; received in revised form 5 August 1999; accepted 6 March 2000

Abstract
Gaseous emission of N from soil is essentially related to microbial activity, which includes nitri®cation and denitri®cation. In
grassland soils subjected to high annual rainfall and intensive grazing, aerobic and anaerobic zones can develop in close
proximity in the upper few centimeters of the soil, hence nitri®cation and denitri®cation can occur concurrently and adjacently.
The objective of this study was to demonstrate the occurrence of simultaneous nitri®cation and denitri®cation following the
+
ÿ

ÿ1
addition of NOÿ
3 and NH4 fertilizers to a grassland soil under ®eld conditions. After applying 100 kg NO3 ±N ha , ca. 25±75
kg haÿ1 of the added N disappeared from the mineral N pool in 7 days. Emission of N2O and total denitri®cation was
substantial, and 5±22 kg haÿ1 of the added N was evolved as gaseous N. In the soil where NH+
4 ±N was added, almost 50% of
the N that disappeared from the mineral pool could not be accounted for. A substantial proportion of the applied N (7 kg haÿ1)
+
was evolved as gaseous N. The rate and amount of N loss and ¯uxes of N2O from both NOÿ
3 and NH4 sources were greater in
soils at 84% water-®lled pore space (WFPS) compared with 71% and 63% WFPS. Emission of N2O from soil following NOÿ
3
ÿ
addition can therefore be attributed to denitri®cation. In the soils to which NH+
4 was added, accumulation of NO3 ±N was
greatest at low moisture content (63% WFPS), while the gaseous emissions were greatest at the highest WFPS. The study
demonstrated that nitri®cation and denitri®cation occur simultaneously in compacted silty grassland soils at moisture conditions
close to ®eld capacity. 7 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Denitri®cation; Grassland; Nitri®cation; Nitrous oxide

1. Introduction
Soils rarely supply sucient N for productive grass
cultivars to achieve their potential yield. Application
of mineral N fertilizers has been the key factor in
bringing about the very substantial increase in grassland productivity that has been achieved over the last
four or ®ve decades (Jarvis et al., 1995). Increased N
fertilization, on the other hand, may increase the
release of N2O from soils through nitri®cation and

* Corresponding author. Tel.: +92-58710-42688; fax: +92-5871042628.

denitri®cation and thereby contribute to the global
warming and the destruction of the stratospheric
ozone layer. Skiba et al. (1996) reported that intensively managed grassland soils are the major agricultural source of N2O emission in the UK.
In West Britain, soils may be wetter than ®eld capacity for over half a year due to an annual rainfall in
excess of 1000 mm. High rainfall on silty soils compacted due to intensive grazing, creates stagnogley
conditions conducive to denitri®cation (Davies et al.,
1989; Naeth et al., 1990). Soil compaction restricts
oxygen di€usion within the soil, and may lead to changing N transformations and particularly increased
N2O production rates (Oenema et al., 1997). High

0038-0717/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 4 2 - 0

1252

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

emissions have been reported for agricultural grassland
in moist temperate environments (Clayton et al.,
1997).
Denitri®cation is considered to be the main source
of N2O in soils but N2O is also produced during nitri®cation. Most researchers have reported that the relative ¯uxes of N gases and their possible sources
depend mainly on water-®lled pore space of the soil.
Data from di€erent sites have shown that denitri®cation rates rapidly increase when water-®lled pore space
(WFPS) exceeds 60%, whereas in the range 30±70%
WFPS, nitri®cation is comparable with denitri®cation
as a source of N2O (Davidson, 1991; Granli and Bockman, 1994). Denitri®cation only provides a direct
source of N2O from NO3± applied in fertilizers but
both nitri®cation and denitri®cation can be involved in

the production and emission of N2O from NH+
4 or
NH+
4 producing compounds (Skiba et al., 1993). It is
also possible that soil conditions may allow both nitri®cation and denitri®cation to occur simultaneously
and at adjacent locations so that N2O may be produced without an obvious accumulation of NOÿ
3
(Petersen et al., 1991; Abbasi and Adams, 1998).
Nitrous oxide emission during simultaneous nitri®cation and denitri®cation is possible because applications
of NH+
4 sources increase the potential for nitri®cation
in grassland soils. Accumulated NOÿ
3 ±N in the presence of an excess amount of root-derived organic
matter creates conditions conducive to the production
of N2O by denitri®cation under favourable moisture
and temperature (Stevens and Laughlin, 1997). Heterogeneity of soil physical conditions at shallow depths
increases the possibility of simultaneous nitri®cation
and denitri®cation in adjacent microsites of di€erent
aerobicity (Hutchinson and Davidson, 1993; Jarvis et
al., 1994). The primary aim of the present study was

to demonstrate the occurrence of simultaneous nitri®cation and denitri®cation under ®eld conditions. An
additional objective was to quantify N loss as N2O following application of either NO3 or NH4 sources of
N.

2. Materials and methods
2.1. Experimental site
The experimental site was situated on an area of
almost ¯at pasture land at Blaendolau near Aberystwyth, Wales, UK. The soil was a Dystric Eutrochrept
and classi®ed as Conway series (Grid Ref. SN 597804)
as described in detail in an earlier study (Abbasi and
Adams, 1998). Brie¯y, the land had been in pasture
(pure grass) until 1988. Thereafter, the sward was
maintained by mowing weekly at a height of about 5
cm. The site was located at an altitude of 20 m above

sea level. The stone-free soil had developed from silty
alluvium and had a clay loam surface texture (0±8
cm). The soil (0±7.5 cm) had a pH of 5.1 (1:1 water),
an organic matter content of 81.4 g kgÿ1, a total N
content of 3.43 g kgÿ1 and a bulk density of 1.05 Mg

mÿ3. The pasture was dominated by perennial ryegrass
(Lolium perenne L) and common bent (Agrostis capillaris l).
2.2. Field management
An area of 10  10 m was selected in May 1997
from an actively growing grassland ®eld. The ®eld was
not heavily compacted as it had not been grazed by
cattle since 1988. Therefore, the site was uniformly
compacted before commencing the experiment, using
successive passes in di€erent directions for 20 min
using a roller weighing 2.5 ton. The mean bulk density
of the top 8 cm of soil was 1.17 mg mÿ3after it had
been rolled. Broad-leaved weeds present in the ®eld
were eradicated by spraying Supertox (active ingredients mecaprop and 2, 4D) at the rate of 50 ml lÿ1
water. The main plot was divided into two parts, one
for the nitrate and one for the ammonium treatment.
Three moisture regimes were established: (A) plots
with no extra water, i.e., natural conditions, (B) plots
saturated completely and allowed to drain for 2 days,
i.e., ®eld capacity conditions, and (C) plots saturated
completely then drained for 1 day, saturated again and

drained for 1 day i.e., slightly wetter than ®eld capacity. A randomized complete block design with split
treatments was used for the experiment.
2.3. Experiment 1: N transformations in NOÿ
3 treated
soil
Experiment 1 was conducted on a 3  6 m area with
the three moisture regimes, one applied N and a control treatment with three replicates of each. A total of
18 sub-plots (1  1 m) were established. KNO3 was
applied once supplying 100 kg N haÿ1. An appropriate
amount of fertilizer for a 1 m2 area was dissolved in 5
l of tap water and applied evenly with a watering can.
The WFPS at the start of the experiment were 63%,
71%, and 84%, respectively for the A, B, and C plots.
These moisture contents were maintained throughout
the experiment by applying tap water twice weekly.
Gaseous N emissions were measured each day for a
7-day period using a similar method to that described
by Ryden and Dawson (1982). However, instead of
their enclosure system we used specially adapted plastic buckets having an internal diameter of 18 cm and a
height of 20 cm with a gas sampling port and suba

seal on top. In each sub-plot at di€erent locations
three buckets without lid were inverted and inserted
into the soil to a depth of about 7.5 cm. Soil around

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

the bucket edge was ®rmly compressed to minimize the
chance of gas leakage. Samples of gas were collected
every day at ca. 10 am after sealing overnight for 18 h.
The suba seals were left open during daytime to limit
heat buildup within the buckets. The weather was
overcast for a large part of the experimental period.
The daytime air temperature within the buckets was
checked on several occasions and on no occasion was
it more than 28C greater than the ambient air. At each
sampling time, 50 ml of headspace gas was removed
by a 50 ml syringe, transported to the laboratory
(within 1 km) and immediately analysed for N2O. For
this purpose 1 ml gas from the 50 ml syringe was
removed and the N2O concentration determined with a

Pye±Unicam 104 series gas chromatagraph with an
electron capture detector.
Emission of N2O was also measured in an acetylene
enriched atmosphere under laboratory conditions. Immediately after fertilizer application, intact soil cores
(7 dia  7.5 cm deep) were collected from each plot as
described in an earlier study (Abbasi and Adams,
1998) and taken to the laboratory. Three cores of each
moisture level were wrapped in foil and placed in 1 l
Kilner jars ®tted with gas tight lids and a gas sampling
port. After sealing, 50 ml of the headspace in the jars
were removed and replaced by 50 ml of acetylene (5%
v/v). After treatment, the jars were placed in an incubator for 18 h. The temperature of the incubator was
adjusted at the ®eld soil temperature of the 0±7.5 cm
depth, which was recorded immediately after sampling.
The rate of denitri®cation was determined between day
1 and 7. For this purpose 1 mL of headspace gas was
removed and the N2O concentration was determined
as described earlier. Sampling procedure, timings, and
duration of the experiment were the same as in the
®eld experiment.

2.4. Experiment 2: N transformations in NH+
4 treated
soil
In this experiment, (NH4)2 HPO4 was applied as N
source at a rate of 250 kg N haÿ1. This experiment
was conducted on a 3  6 m plot in the selected area.
Experimental layout, soil conditions, and treatments
were the same as described for Experiment 1. Gaseous
N emissions were measured on days 7, 10, 18, 21, and
28 after fertilizer application either in the presence or
absence of acetylene. Two buckets per sub-plot were
established for gas ¯ux measurements. Concentrations
of acetylene of approximately 2% (v/v) were established in the soil air, calculated on the basis of the top
7.5 cm of soil having 50% air-®lled porespace, by diffusion from ¯exible nylon tubes (four per pot) inserted
into the soil to a depth of ca. 7.5 cm inside the buckets. Acetylene was injected through the sampling port
into the tubes. Immediately after injecting, the buckets

1253

were sealed with a suba seal for 48 h. Acetylene was

used to inhibit the reduction of N2O to N2 during
denitri®cation and thus allowing estimation of total
denitri®cation by measurement of the accumulated
N2O. Two more buckets were also ®xed in each subplot for N2O measurement without acetylene injection.
At each sampling time at 24 and 48 h after sealing, 50
ml of headspace gas was removed via a 50 ml syringe,
taken to the laboratory and 1 ml gas was used to
measure the N2O concentration as described in Experiment 1. After sampling, the location of the buckets
was changed within the sub-plot. Denitri®cation and
N2O emission rates were calculated for each sampling
date and were corrected for N2O dissolved in the
aquatic phase of the soil, by using the Bunsen coecient (Moraghan and Buresh, 1977). Total denitri®cation and N2O losses were estimated for each
experimental period by integrating the daily losses over
time.
2.5. Soil sampling and analysis
At time zero and at the end of Experiment 1 at day
7, soil cores were taken both from the ®eld and the
laboratory, sectioned into 0±2.5, 2.5±5 and 5±7.5 cm
layers. In Experiment 2, soil cores were taken from the
®eld at day 0, 7, 14, 21, and 28 and sectioned in the
same way. The concentration of total mineral N and
NH+
4 ±N in the soil samples was determined by
extracting 40 g sub-samples of fresh soil for 1 h with
200 ml of 2 M KCl followed by steam distillation and
titration (Keeney and Nelson, 1982). NOÿ
3 ±N plus any
NO2±N present was determined by subtracting NH+
4 ±
N from the total mineral N. Total N in the soil was
determined using the Kjeldahl method of Bremner and
Mulvaney (1982). Organic matter was estimated as
weight loss on ignition at 4008C (Ball, 1964). Soil pH
was measured in a water suspension (1:1 v/v). Particle
size distribution was determined by the pipette method
described by Avery and Bascomb (1974). Soil moisture
content was determined gravimetrically by drying subsamples at 1058C for 24 h. Total porespace and WFPS
were calculated from known bulk density and particle
density predicted from organic matter content (Adams,
1973). Soil temperature at 7.5 cm depth was measured
in the ®eld on every sampling day, and varied around
17228C, during both experiments.
2.6. Statistical analysis
All data were statistically analysed by multifactorial
analysis of variance (ANOVA) using the software
package Statgraphics (Statgraphics Manugistics, 1992).
Least signi®cant di€erences are given to show the
variability between means for selected subsets of data.
Con®dence values (P ) are given in the text for the sig-

1254

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

Table 1
ÿ1
Changes in the concentration of NOÿ
soil) in grassland soil at di€erent depths and at di€erent moisture contents following the
3 ±N (mg N kg
addition of KNO3 under (a) laboratory and, (b) ®eld conditions over a 7-day period
Depth (cm)

Day-0

(a) Laboratory incubation
0±2.5
2.5±5.0
5.0±7.5
(b) Field conditions
0±2.5
2.5±5.0
5.0±7.5
a

LSDa(P < 0.05)

Water-®lled pore space (%) Day-7
63

71

84

118
34
24

45.0
17.4
7.6

31.5
17.7
12.9

0
2.9
5.0

11.2
8.5
5.9

118
34
24

29.7
22.8
19.2

15.6
4.4
6.7

3.7
2.4
3.2

11.8
1.2
7.3

LSD represent least signi®cant di€erence (P < 0.05) between values at di€erent moisture levels over three depths.

ni®cance
depths.

between treatment, time intervals, and

3. Results
3.1. Experiment 1: N transformations in NOÿ
3 treated
soil
3.1.1. Changes in NOÿ
3 ±N concentration
Before KNO3 addition the soil NOÿ
3 ±N concentrations were uniform over the 0±7.5 cm depth range
with a mean value of R5 mg kgÿ1. Addition of KNO3
increased the NOÿ
3 ±N concentration in all layers especially in the 0±2.5 cm (Table 1). A signi®cant reduction (P < 0.05) in NOÿ
3 ±N occurred over time and
by day 7, 60±100% of the added NOÿ
3 ±N had disappeared from the 0±2.5 cm layer. The depletion in
NOÿ
3 ±N increased with increase in WFPS and virtually
no NOÿ
3 remained in any layer at day 7 at 84%

WFPS. At 63% and 71% WFPS the extent of NOÿ
3
depletion was greater, the shallower the layer. After
taking into account the NOÿ
3 ±N remaining in the soil
at day 7 (Table 1) and the uptake of N by the herbage
(data not shown), the predicted proportion of the
applied NOÿ
3 ±N lost ranged from 75% at 84% WFPS
to around 25% at 63% WFPS. The overall pattern of
NOÿ
3 ±N loss was similar whether the changes were
monitored in the ®eld or in the laboratory.
3.1.2. Denitri®cation and N2O ¯uxes
Soil to which KNO3 had been added displayed substantial ¯uxes of N2O measured over the 7-day period
(Table 2). N2O gas ¯uxes were detected 1 day after
KNO3 application and in almost all cases, the maximum ¯uxes occurred between day 2 and 4. Emissions
then dropped o€ sharply probably because of the
decrease in NOÿ
3 ±N in the mineral N pool. During
days 1±5 the N2O emissions were greatest from the
soil with 84% WFPS. The regression analysis of the

Table 2
Emission of nitrous oxide (kg N2O±N haÿ1 dÿ1) from the nitrate added grassland soil (100 kg N haÿ1) at di€erent moisture contents under laboratory conditions in the presence of acetylene and ®eld conditions in the absence of acetylene
Treatment C2H2 (% v/v)

(a) Laboratory incubations
5
5
5
LSD
(b) Field conditions
0
0
0
LSD
a

WFPSa (%)

LSDb

Days after fertilizer application
1

2

3

4

5

6

7

63
71
84
(P < 0.05)

0.46
0.92
3.38
0.10

0.77
2.15
8.00
0.33

1.38
6.15
11.54
0.42

3.85
8.62
13.08
0.50

1.69
4.06
4.69
0.17

1.38
2.62
2.00
NS

0.15
0.00
0.00
NS

2.39
5.01
5.76
±

63
71
84
(P < 0.05)

0.10
0.79
1.32
0.30

0.23
1.55
5.28
2.20

0.26
3.00
5.44
2.88

0.09
2.13
3.87
1.39

0.01
0.35
0.26
NS

0.00
0.03
0.00
NS

0.00
0.00
0.03
NS

0.15
1.03
4.40
±

WFPS = water-®lled pore space.
LSD within each row indicate least signi®cant di€erences (P < 0.05) at di€erent time intervals at each moisture level whereas LSD within
each column represent signi®cant di€erences within three moisture levels at selected time intervals.
b

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

1255

Fig. 1. Loss of N from a grassland soil under three di€erent moisture regimes following the addition of nitrate and ammonium N. (a) represents
+
total N loss as N2O from the NOÿ
3 treated soil, (b) indicates total N loss from the NH4 added soil in the presence and absence of acetylene.

data showed that emissions of N2O were positively
correlated with WFPS …r 2 ˆ 0:99).
Total N2O±N losses from soil with added KNO3
under the di€erent moisture regimes are illustrated in
Fig. 1. A maximum of 22 kg N2O±N haÿ1 was
detected from the soil at 84% WFPS. The corresponding loss at 63% and 71% WFPS was in the range 0.4±
13 kg N2O±N haÿ1. Gaseous emissions of N from the
acetylene treated soils were three times greater than
the untreated ones. This suggests that N2 was the main

gaseous product and denitri®cation was the most likely
source of N2O.
3.2. Experiment 2: N transformations in NH+
4 treated
soil
3.2.1. Changes in mineral±N concentration
When NH+
4 ±N was applied to the soil, its concentration decreased progressively from the time of application until the end (Table 3). Mineral N equivalent to

1256

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

Table 3
ÿ1
Changes in the concentration of NH+
soil) from the
4 ±N (mg N kg
grassland soil at di€erent moisture contents after adding
(NH4)2HPO4 (250 kg N haÿ1) during a 28-day study under ®eld conditions

Table 4
Changes in the concentration of NO3±N (mg N kgÿ1 soil) from the
grassland soil at di€erent moisture contents after adding
(NH4)2HPO4 (250 kg N haÿ1) during a 28-day study under ®eld condition

WFPSa (%) Depth (cm) Days after fertilizer application

WFPSa (%) Depth (cm) Days after fertilizer application

63
LSD
71
LSD
84
LSD

0±2.5
2.5±5.0
5.0±7.5
(P < 0.05)
0±2.5
2.5±5.0
5.0±7.5
(P < 0.05)
0±2.5
2.5±5.0
5.0±7.5
(P < 0.05)

0

7

14

21

28

456
96
36
52.6
457
110
35
92.5
467
114
38
69.7

275
22
15
73.7
224
32
20
64.6
185
93
47
95.4

148
36
25
67.6
97
47
43
60.4
81
13
2
43.0

123
51
28
60.9
85
18
9
23.6
42
14
16
18.8

36
21
18
31.8
31
10
8
15.2
20
20
12
15.8

LSDb

69.9
49.7
NS
±
211.5
29.2
NS
±
70.5
52.2
21.6
±

0±2.5
2.5±5.0
5.0±7.5
(P < 0:05)
0±2.5
2.5±5.0
5.0±7.5
(P < 0:05)
0±2.5
2.5±5.0
5.0±7.5
(P < 0:05)

63
LSD
71
LSD
84
LSD

a

0

7

14

21

28

3
3
0
NS
6
0
0
NS
4
2
0
2.4

17
3
0
6.6
9
1
1
3.5
7
6
4
NS

19
1
2
NS
15
1
3
10.7
13
0
0
6.9

22
2
3
13.0
12
2
1
10.2
5
2
2
2.8

27
1
4
18.7
9
1
1
NS
4
1
2
NS

LSDb

20.8
NS
NS
±
NS
NS
1.9
±
6.2
NS
NS
±

a

WFPS = water-®lled pore space.
LSD within each row indicate least signi®cant di€erences (P <
0.05) at di€erent time intervals of each moisture level whereas LSD
within each column represent signi®cant di€erences within three
moisture levels at selected time intervals.

WFPS = water-®lled pore space.
LSD within each row indicate least signi®cant di€erences
…P < 0:05† at di€erent time intervals of each moisture level whereas
LSD within each column represent signi®cant di€erences within
three moisture levels at selected time intervals.

50±95% of the added NH+
4 ±N, disappeared from the
mineral pool over the course of the experiment. The
maximum reduction occurred in the 0±2.5 cm layer.
The apparent recovery of applied N in the herbage
ranged from 40±50% (data not shown). Taking this
recovery into account and the N remaining in the soil
at the end, still 40±50% of the applied N that disappeared from the mineral pool could not be accounted
for. The concentration of NOÿ
3 ±N increased signi®cantly …P < 0:05† in the 0±2.5 cm layers especially in
the plots with 63% WFPS. But in lower layers acÿ1
soil
cumulation of NOÿ
3 ±N never exceeded 6 mg kg
(Table 4). The maximum accumulation of NOÿ
3 ±N

recorded during the study was 27 mg kgÿ1 which was
ca. 6% of the NH+
4 ±N that disappeared.

b

b

3.2.2. Denitri®cation and N2O emission
Fluxes of N2O from soil with NH+
4 added under
®eld conditions at di€erent moisture contents were
determined in the presence or absence of acetylene
(Table 5). The N2O ¯uxes from plots with 63% WFPS
never exceeded 0.24 kg N2OÿN haÿ1 dayÿ1 even when
acetylene was used. In plots with 71% WFPS, emissions of N2O increased substantially to 1 kg N2O±N
haÿ1 dayÿ1 at day 10. Thereafter, the emission
decreased progressively until the end. The pattern of
N2O emission in the plots with 84% WFPS was simi-

Table 5
Emission of N2O (kg N haÿ1 dayÿ1) from the grassland soil at di€erent moisture contents in the presence and absence of acetylene after adding
ammonium N (250 kg N haÿ1) during a 28-day study under ®eld conditions
Treatment C2H2 (%v/v)

2
2
2
LSD
0
0
0
a

WFPSa (%)

63
71
84
(P < 0.05)
63
71
84

LSDb

Days after fertilizer application
7

10

14

18

21

28

0.00
0.11
0.07
0.11
0.00
0.00
0.01

0.01
0.94
3.60
2.85
0.00
0.02
0.08

0.12
0.24
0.89
0.32
0.02
0.01
0.02

0.04
0.04
0.49
NS
0.00
0.07
0.05

0.24
0.02
0.07
NS
0.01
0.01
0.02

0.04
0.01
0.01
NS
0.01
0.00
0.01

0.12
0.52
1.82
±
±
±
±

WFPS = water-®lled pore space.
LSD within each row indicate least signi®cant di€erences (P < 0.05) at di€erent time intervals of each moisture level whereas LSD within
each column represent signi®cant di€erences within three moisture levels at selected time intervals.
b

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

lar to that of 71% WFPS but the rates and concentrations were much higher. A maximum of 4 kg N2O±
N haÿ1 dayÿ1 was evolved at day 10, which decreased
progressively with time. The gradual decrease of N2O
in the later stages of the study was probably because
of the decrease in NH+
4 ±N from the mineral N pool.
The total loss of N as N2O over 28 days amounted to
7 kg N haÿ1from the soil with 84% WFPS. This was
equivalent to 2.8% of applied N which can be compared with 0.15% and 0.40% from soils with 63% and
71% WFPS (Fig. 1). Emissions of N2O in the absence
of acetylene were only just detectable and a maximum
of 0.06% of N2O±N was found at 84% WFPS. No
detectable amount of N2O was observed in the control
plots without NH+
4 ±N either in the presence or
absence of acetylene.

4. Discussion
Between 60% and 100% of the added N disappeared from the soil mineral N pool over a 7-day
period when KNO3 was the N source. When NH+
4 ±N
was added it took 28 days for a similar loss from the
mineral N pool. Fluxes of N2O and total denitri®cation losses measured from plots receiving KNO3 were
much greater than from the NH+
added plots. A
4
maximum of 5±22 kg haÿ1 of applied N was evolved
as gaseous N when NO3 was added compared with
0.4±7 kg haÿ1 from NH+
4 added plots at the same
moisture content. Rates of nitri®cation in grassland
soils are relatively slow compared with the process of
denitri®cation (Abbasi and Adams, 1998). Since NH+
4
must ®rst undergo nitri®cation prior to denitri®cation,
¯uxes and total losses of mineral N may be expected
to be lower.
The greater disappearance of NOÿ
3 ±N from the plots
with the greatest WFPS was expected because conditions were more favourable for denitri®cation. In the
experiment, where NH+
4 ±N was the source of added
mineral N, the greatest accumulation of NOÿ
3 was in
the plots with the lowest WFPS. This was also
expected due to the enhanced oxygen di€usion into the
soil with low moisture, limiting N2O production, and
favouring nitri®cation. However, the greatest loss of
mineral N and the greatest N2O ¯uxes were recorded
from plots with the greatest WFPS (84%) con®rming
that conditions close to ®eld capacity or wetter favour
N2O production. High rates of N2O emissions in soils,
which have matric potentials above ®eld capacity (less
negative), are in agreement with the ®ndings of Bandibas et al. (1994). Soil texture also plays an important
role in this regard. de Klein and van Logtestijn (1996)
stated that in a ®ne-textured soil with small pores, a
small increase in moisture content could saturate
whole soil aggregates, resulting in a sharp increase in

1257

denitri®cation. Sextone et al. (1988) also showed a
stronger e€ect of soil water content on denitri®cation
rates in a clayey loam, compared to a sandy loam. The
soil used in the present experiment had a clayey loamy
texture. It was compacted by repeated passes with a
roller before the start of the experiment and a linear
relationship between WFPS in the soil and N2O production has been found.
Emissions of N2O from the soil to which KNO3 had
been added were 3±8 times greater than from those
where NH+
4 ±N was added. Greater N2O ¯uxes from
the NO3 source than from the NH+
4 source suggest
that denitri®cation was the main mechanism of N2O
production. McTaggart et al. (1997) and Velthof et al.
(1997) drew similar conclusions from N2O measurements from grassland soils. In the NH+
4 treated soils
the accumulation of NOÿ
3 ±N was greatest in plots with
low moisture content (63% WFPS) while the denitri®cation activity and N2O ¯uxes were greatest in plots
with the greatest WFPS. If nitri®cation had been the
main source of N2O, the maximum ¯uxes would be
expected from plots with the greatest NOÿ
3 accumulation because N2O emission due to nitri®cation has
been reported under aerobic conditions. This was not
the case and we conclude that gaseous emission under
greater WFPS was due to simultaneous nitri®cation
and denitri®cation. On the basis of the e€ect of WFPS
on N2O emissions, Whitehead (1995) reported maximum ¯uxes of N2O from a moisture range where both
nitri®cation and denitri®cation were at their maximum.
In the present study, the maximum ¯uxes of N2O from
+
both NOÿ
3 and NH4 sources were recorded in soils at
84% WFPS. Denitri®cation would be expected to
occur in soils with such a small air-®lled porosity,
however, there is evidence that nitri®cation can occur
in wet or virtually waterlogged soils (Adams and Akhtar, 1994; Aulakh and Singh, 1997). In such conditions
nitri®cation may be restricted to shallow depths or
local microsites. The ultimate source of emitted N2O
+
was NOÿ
3 derived from nitri®cation of the added NH4
ÿ
since NO3 was barely detectable in the control soil
without N addition throughout the study. In addition,
+
in the NH+
4 treated soil, as NH4 decreased, the conÿ
centration of NO3 increased to some extent in the surface layer indicating that nitri®cation was occurring.
However, the build-up of NOÿ
3 was much smaller reladisappearance.
The very limited active to the NH+
4
cumulation of NOÿ
in
the
soil
at
any time suggests
3
that the products of nitri®cation di€used to less
aerobic zones where they became the substrate of denitrifying organisms in locations adjacent to their production (Petersen et al., 1991). Measurement of
nitri®cation and denitri®cation potential of the soil
used indicated that the potential of the latter was 4±5
times greater than the former (Abbasi and Adams,
1998). Thus a substantial accumulation of NOÿ
3 ±N

1258

M.K. Abbasi, W.A. Adams / Soil Biology & Biochemistry 32 (2000) 1251±1259

would not be expected where both processes are occurring simultaneously. Velthof et al. (1997) reported a
+
similar pattern of N loss from NOÿ
3 and NH4 added
grassland soils and suggested that denitri®cation rates
for NH+
4 fertilized soils may have been dependent on
the release of NOÿ
3 from nitri®cation of fertilizer
NH+
4 .
Despite being unable to account for the loss of a
large proportion of added NH+
4 nitri®cation and denitri®cation occurring concurrently in¯uence the fate of
NH+
4 ±N in grassland soils under moisture conditions
that apply for a substantial part of the year in West
Britain. The processes occur simultaneously at shallow
depths and result in the gaseous loss of N from the
soil mineral N pool. Thus far it has not been possible
to quantify N losses in the ®eld due to these linked
processes. Further research is needed to identify the
precise location of the simultaneous processes and the
ecology and physiology of the microorganisms directly
involved.

Acknowledgements
The authors gratefully acknowledge the Government
of Pakistan for ®nancial support, the University of
Jammu and Kashmir, Muza€erabad for nominating
the senior author for postgraduate studies. We express
our appreciation to Mr. Tom Lewis of the Soil Science
Unit, Meirion Morgan, Meuring Jones, Huw Evans
and Aled Morgans of the ground sta€ for providing
all necessary facilities and help during this ®eld study.

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