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
ba
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 addition of NO3ÿand NH4+fertilizers to a grassland soil under ®eld conditions. After applying 100 kg NO3ÿ±N haÿ1, 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 haÿ1of the added N was evolved as gaseous N. In the soil where NH4+±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 NO3ÿand 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 NO3ÿ
addition can therefore be attributed to denitri®cation. In the soils to which NH4+ was added, accumulation of NO3ÿ±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.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Denitri®cation; Grassland; Nitri®cation; Nitrous oxide
1. Introduction
Soils rarely supply sucient 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 grass-land 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
denitri®cation and thereby contribute to the global warming and the destruction of the stratospheric ozone layer. Skiba et al. (1996) reported that inten-sively managed grassland soils are the major
agricul-tural source of N2O emission in the UK.
In West Britain, soils may be wetter than ®eld ca-pacity for over half a year due to an annual rainfall in excess of 1000 mm. High rainfall on silty soils com-pacted due to intensive grazing, creates stagnogley conditions conducive to denitri®cation (Davies et al., 1989; Naeth et al., 1990). Soil compaction restricts oxygen diusion within the soil, and may lead to chan-ging N transformations and particularly increased
N2O production rates (Oenema et al., 1997). High
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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 rela-tive ¯uxes of N gases and their possible sources depend mainly on water-®lled pore space of the soil. Data from dierent sites have shown that denitri®ca-tion 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
Bock-man, 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 NH4+ or
NH4+ producing compounds (Skiba et al., 1993). It is
also possible that soil conditions may allow both nitri-®cation and denitrinitri-®cation to occur simultaneously
and at adjacent locations so that N2O may be
pro-duced without an obvious accumulation of NO3
ÿ
(Petersen et al., 1991; Abbasi and Adams, 1998). Nitrous oxide emission during simultaneous nitri®ca-tion and denitri®canitri®ca-tion is possible because applicanitri®ca-tions
of NH4+ sources increase the potential for nitri®cation
in grassland soils. Accumulated NO3ÿ±N in the
pre-sence 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). Hetero-geneity of soil physical conditions at shallow depths increases the possibility of simultaneous nitri®cation and denitri®cation in adjacent microsites of dierent 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®nitri®-cation under ®eld conditions. An
additional objective was to quantify N loss as N2O
fol-lowing 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 Aberyst-wyth, 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 kgÿ1and a bulk density of 1.05 Mg
mÿ3. The pasture was dominated by perennial ryegrass
(Lolium perenne L) and common bent (Agrostis capil-larisl).
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 dierent 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
ingredi-ents 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 ca-pacity. A randomized complete block design with split treatments was used for the experiment.
2.3. Experiment 1: N transformations in NO3ÿtreated soil
Experiment 1 was conducted on a 36 m area with
the three moisture regimes, one applied N and a con-trol 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 m2area 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.
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. Im-mediately after fertilizer application, intact soil cores
(7 dia7.5 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 incu-bator for 18 h. The temperature of the incuincu-bator 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 NH4+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 36 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 estab-lished in the soil air, calculated on the basis of the top 7.5 cm of soil having 50% air-®lled porespace, by dif-fusion from ¯exible nylon tubes (four per pot) inserted into the soil to a depth of ca. 7.5 cm inside the buck-ets. Acetylene was injected through the sampling port into the tubes. Immediately after injecting, the buckets
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
sub-plot 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
Exper-iment 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®ca-tion 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
NH4+±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). NO3ÿ±N plus any
NO2±N present was determined by subtracting NH4+±
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
sub-samples 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 dierences are given to show the variability between means for selected subsets of data.
sig-ni®cance between treatment, time intervals, and depths.
3. Results
3.1. Experiment 1: N transformations in NO3ÿtreated soil
3.1.1. Changes in NO3
ÿ
±N concentration
Before KNO3 addition the soil NO3ÿ±N
concen-trations were uniform over the 0±7.5 cm depth range
with a mean value ofR5 mg kgÿ1. Addition of KNO3
increased the NO3ÿ±N concentration in all layers
es-pecially in the 0±2.5 cm (Table 1). A signi®cant
re-duction (P< 0.05) in NO3ÿ±N occurred over time and
by day 7, 60±100% of the added NO3ÿ±N had
disap-peared from the 0±2.5 cm layer. The depletion in
NO3ÿ±N increased with increase in WFPS and virtually
no NO3ÿ remained in any layer at day 7 at 84%
WFPS. At 63% and 71% WFPS the extent of NO3ÿ
depletion was greater, the shallower the layer. After
taking into account the NO3ÿ±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 NO3ÿ±N lost ranged from 75% at 84% WFPS
to around 25% at 63% WFPS. The overall pattern of
NO3ÿ±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
sub-stantial ¯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
maxi-mum ¯uxes occurred between day 2 and 4. Emissions then dropped o sharply probably because of the
decrease in NO3ÿ±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 1
Changes in the concentration of NO3ÿ±N (mg N kgÿ1soil) in grassland soil at dierent depths and at dierent moisture contents following the addition of KNO3under (a) laboratory and, (b) ®eld conditions over a 7-day period
Depth (cm) Day-0 Water-®lled pore space (%) Day-7 LSDa(P< 0.05)
63 71 84
(a) Laboratory incubation
0±2.5 118 45.0 31.5 0 11.2
2.5±5.0 34 17.4 17.7 2.9 8.5
5.0±7.5 24 7.6 12.9 5.0 5.9
(b) Field conditions
0±2.5 118 29.7 15.6 3.7 11.8
2.5±5.0 34 22.8 4.4 2.4 1.2
5.0±7.5 24 19.2 6.7 3.2 7.3
aLSD represent least signi®cant dierence (P< 0.05) between values at dierent moisture levels over three depths.
Table 2
Emission of nitrous oxide (kg N2O±N ha ÿ1
dÿ1) from the nitrate added grassland soil (100 kg N haÿ1) at dierent moisture contents under lab-oratory conditions in the presence of acetylene and ®eld conditions in the absence of acetylene
Treatment C2H2(% v/v) WFPSa(%) Days after fertilizer application LSDb
1 2 3 4 5 6 7
(a) Laboratory incubations
5 63 0.46 0.77 1.38 3.85 1.69 1.38 0.15 2.39
5 71 0.92 2.15 6.15 8.62 4.06 2.62 0.00 5.01
5 84 3.38 8.00 11.54 13.08 4.69 2.00 0.00 5.76
LSD (P< 0.05) 0.10 0.33 0.42 0.50 0.17 NS NS ±
(b) Field conditions
0 63 0.10 0.23 0.26 0.09 0.01 0.00 0.00 0.15
0 71 0.79 1.55 3.00 2.13 0.35 0.03 0.00 1.03
0 84 1.32 5.28 5.44 3.87 0.26 0.00 0.03 4.40
LSD (P< 0.05) 0.30 2.20 2.88 1.39 NS NS NS ±
a
WFPS = water-®lled pore space. b
data showed that emissions of N2O were positively
correlated with WFPS r20:99).
Total N2O±N losses from soil with added KNO3
under the dierent moisture regimes are illustrated in
Fig. 1. A maximum of 22 kg N2O±N ha
ÿ1
was detected from the soil at 84% WFPS. The correspond-ing 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 N2was the main
gaseous product and denitri®cation was the most likely
source of N2O.
3.2. Experiment 2: N transformations in NH4 +
treated soil
3.2.1. Changes in mineral±N concentration
When NH4+±N was applied to the soil, its
50±95% of the added NH4+±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 disap-peared from the mineral pool could not be accounted
for. The concentration of NO3ÿ±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
ac-cumulation of NO3ÿ±N never exceeded 6 mg kgÿ1soil
(Table 4). The maximum accumulation of NO3ÿ±N
recorded during the study was 27 mg kgÿ1 which was
ca. 6% of the NH4+±N that disappeared.
3.2.2. Denitri®cation and N2O emission
Fluxes of N2O from soil with NH4+ added under
®eld conditions at dierent 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 haÿ1dayÿ1even when
acetylene was used. In plots with 71% WFPS,
emis-sions 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 3
Changes in the concentration of NH4+±N (mg N kgÿ1soil) from the grassland soil at dierent moisture contents after adding (NH4)2HPO4(250 kg N ha
ÿ1
) during a 28-day study under ®eld con-ditions
WFPSa(%) Depth (cm) Days after fertilizer application LSDb
0 7 14 21 28
WFPS = water-®lled pore space. b
LSD within each row indicate least signi®cant dierences (P< 0.05) at dierent time intervals of each moisture level whereasLSD
within each column represent signi®cant dierences within three moisture levels at selected time intervals.
Table 4
Changes in the concentration of NO3±N (mg N kgÿ1soil) from the grassland soil at dierent moisture contents after adding (NH4)2HPO4(250 kg N ha
ÿ1
) during a 28-day study under ®eld con-dition
WFPSa(%) Depth (cm) Days after fertilizer application LSDb
0 7 14 21 28
WFPS = water-®lled pore space. b
LSD within each row indicate least signi®cant dierences P<0:05at dierent time intervals of each moisture level whereas
LSD within each column represent signi®cant dierences within three moisture levels at selected time intervals.
Table 5
Emission of N2O (kg N haÿ1dayÿ1) from the grassland soil at dierent 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) WFPSa(%) Days after fertilizer application LSDb
7 10 14 18 21 28
WFPS = water-®lled pore space. b
lar to that of 71% WFPS but the rates and
concen-trations 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 NH4
+
±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 com-pared 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 NH4+±N either in the presence or
absence of acetylene.
4. Discussion
Between 60% and 100% of the added N disap-peared from the soil mineral N pool over a 7-day
period when KNO3 was the N source. When NH4+±N
was added it took 28 days for a similar loss from the
mineral N pool. Fluxes of N2O and total
denitri®ca-tion losses measured from plots receiving KNO3 were
much greater than from the NH4
+
added plots. A
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 NH4+ 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 NH4+
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 NO3ÿ±N from the plots
with the greatest WFPS was expected because con-ditions were more favourable for denitri®cation. In the
experiment, where NH4+±N was the source of added
mineral N, the greatest accumulation of NO3
ÿ was in the plots with the lowest WFPS. This was also expected due to the enhanced oxygen diusion 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 Bandi-bas 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
denitri®cation. Sextone et al. (1988) also showed a stronger eect 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
pro-duction has been found.
Emissions of N2O from the soil to which KNO3had
been added were 3±8 times greater than from those
where NH4+±N was added. Greater N2O ¯uxes from
the NO3 source than from the NH4+ 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
measure-ments from grassland soils. In the NH4+ treated soils
the accumulation of NO3ÿ±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 NO3ÿ
accumu-lation 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 eect of WFPS
on N2O emissions, Whitehead (1995) reported
maxi-mum ¯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 NO3ÿand 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 Akh-tar, 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 NO3
ÿ
derived from nitri®cation of the added NH4
+
since NO3ÿ was barely detectable in the control soil
without N addition throughout the study. In addition,
in the NH4+ treated soil, as NH4+ decreased, the
con-centration of NO3ÿincreased to some extent in the
sur-face layer indicating that nitri®cation was occurring.
However, the build-up of NO3ÿwas much smaller
rela-tive to the NH4+ disappearance. The very limited
ac-cumulation of NO3ÿ in the soil at any time suggests
that the products of nitri®cation diused to less aerobic zones where they became the substrate of deni-trifying organisms in locations adjacent to their pro-duction (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 NO3
would not be expected where both processes are occur-ring simultaneously. Velthof et al. (1997) reported a
similar pattern of N loss from NO3ÿ and NH4+ added
grassland soils and suggested that denitri®cation rates
for NH4+ fertilized soils may have been dependent on
the release of NO3
ÿ
from nitri®cation of fertilizer
NH4+.
Despite being unable to account for the loss of a
large proportion of added NH4+nitri®cation and
deni-tri®cation occurring concurrently in¯uence the fate of
NH4+±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, Muzaerabad 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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