Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol52.Issue3-4.Oct1999:
Soil & Tillage Research 52 (1999) 191±201
Soil and residue management effects on cropping conditions and
nitrous oxide ¯uxes under controlled traf®c in Scotland
2. Nitrous oxide, soil N status and weather
B.C. Ball*, J.P. Parker, A. Scott
Environmental Division, SAC, West Mains Road, Edinburgh, EH9 3JG, UK
Received 16 February 1999; received in revised form 21 July 1999; accepted 5 August 1999
Abstract
Nitrogen from fertilisers and crop residues can be lost as nitrous oxide (N2O), a greenhouse gas that causes an increase in
global warming and also depletes stratospheric ozone. Nitrous oxide emissions, soil chemical status, temperature and N2O
concentration in the soil atmosphere were measured in a ®eld experiment on soil compaction in loam and sandy loam
(cambisols) soils in south-east Scotland. The overall objective was to discover how the intensity and distribution of soil
compaction by tractor wheels or by roller just before sowing in¯uenced crop performance, soil conditions and production and
emissions of N2O under controlled traf®c conditions. Compaction treatments were zero, light compaction by roller (up to
1 Mg per metre of length) and heavy compaction by loaded tractor (up to 4.2 Mg). In this paper we report the effects on
production and emissions of N2O and relate them to soil and crop conditions. Nitrous oxide ¯uxes were substantial only when
the soil water content was high (>27 g per 100 g). Fertiliser application stimulated emissions in the spring whereas crop
residues stimulated emissions in autumn and winter. Heavy compaction increased N2O emissions after fertiliser application or
residue incorporation more than light or zero compaction. The bulk densities of the heavily and lightly compacted soils were
up to 89% and 82% of the theoretical (Proctor) maxima. Higher soil cone resistances, temperatures and nitrogen availability
and lower gas diffusivities and air-®lled porosities combined to make the heavily compacted soil more anaerobic and likely to
denitrify than the zero or lightly compacted soil. Compaction suf®cient to increase N2O emissions signi®cantly corresponded
with adverse soil conditions for winter barley (Hordeum vulgare L.) growth. Soil tillage, which ensures that soil compaction is
no greater than in our light treatment and is con®ned to near the soil surface, may help to mitigate both surface ¯uxes of N2O
and losses to the subsoil. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Compaction; Residues; Nitrous oxide; Soil
1. Introduction
*
Corresponding author. Tel.: 44-131-535-4392; fax: 44-131667-2601
E-mail address: b.ball@ed.sac.ac.uk (B.C. Ball)
The main production processes of nitrous oxide
(N2O) are microbial. These are denitri®cation, dominant in anaerobic conditions, and nitri®cation, dominant in aerobic conditions (Firestone and Davidson,
0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 8 1 - 1
192
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
1989). Although nitrogen fertiliser applications have a
major in¯uence on N2O emissions from arable soils
(Clayton et al., 1994), soil physical conditions, particularly near the surface, are also important mainly
through their in¯uence on soil aeration (Arah et al.,
1991). Compaction increases N2O emissions by
increasing water-®lled porosity and increasing the
likelihood of anaerobic soil conditions and denitri®cation, which are particular problems in moist, temperate climates (Douglas and Crawford, 1993; Hansen
et al., 1993). Nitrous oxide emissions can be reduced
by avoiding soil compaction, improving soil structure
and controlling drainage and irrigation to avoid excessive soil wetness (Beauchamp, 1997).
Incorporated crop residues can also increase N2O
emissions, particularly after ploughing in the autumn
(Smith et al., 1997). Such residues can enhance metabolic activity and form local anaerobic zones, giving
favourable sites for denitri®cation and contribute to
`hot spots' of emission (Flessa and Beese, 1995).
These, along with variability of soil water content
(Ambus and Christensen, 1994), contribute signi®cantly to high spatial variability of emission. The
contribution of crop residues to N2O emissions and
the complex interactions with other soil properties are
still subject to considerable uncertainty (Beauchamp,
1997).
The production, consumption and transport of N2O
are strongly in¯uenced by the changes in soil structural quality, residue incorporation and water content
associated with tillage and compaction. A complex
interaction between water-®lled pore space and soil
compaction in¯uences microbial activity and nitrogen
losses by denitri®cation (Torbert and Wood, 1992).
Since the in¯uence of residues on denitri®cation under
different tillage systems has been associated with
compaction (Ball and Robertson, 1990), the interactions of residue incorporation, soil compaction and
N2O emission necessitate assessment.
Our objective was to identify the optimum level of
seedbed compaction (including zero) after normal
ploughing of residues, which gives the best cropping
conditions while minimising losses of nitrogen as
N2O. A previous paper (Ball and Ritchie, 1999)
reported the various management options evaluated
and the soil and crop responses. Heavy compaction
was applied by a laden tractor and light compaction by
a roller. This resulted in an increase in soil bulk density
of 75±89% of the theoretical (Proctor) maxima for
heavy compaction whereas the range for light compaction was only 71±82%. Heavy compaction also
limited crop growth due to poor soil physical conditions particularly when the soil was wet. Both zero and
light compaction gave improved crop performance.
This paper reports the N2O emissions and how they
relate to environmental, soil and crop conditions.
2. Methods and materials
2.1. Field experiment
The ®eld experiment was located at two sites. The
®rst site at Boghall, 10 km south of Edinburgh, contained spring and winter barley from spring 1995 to
summer 1996. Daily rainfall was monitored at a
weather station 1.3 km south of the site. The second
site, located 3 km south of North Berwick, contained
winter oil-seed rape (Brassica spp. L.) from late
summer 1996 to harvest 1997. Daily rainfall was
not monitored at this site, though monthly total rainfalls from a weather station 1 km southwards were
given by Ball and Ritchie (1999) who also gave full
details of the site, soil and treatments. Brie¯y, treatments were applied to soil which had been mouldboard ploughed the previous day, incorporating the
chopped residues of the previous cereal crop. Treatments were (1) zero compaction, (2) light compaction
(target depth of 10 cm) using a heavy roller, (3) heavy
compaction (target depth of 25 cm) using a laden
tractor and (4) heavy compaction with the soil subsequently loosened to 10 cm depth with a rotary
cultivator. An additional treatment of no-tillage was
also applied at Boghall.
Treatments were applied on 12 April and 27 September 1995 (Boghall) and on 27 August 1996 (North
Berwick). At Boghall, rainfall in the month after
sowing spring barley was low. Therefore on 15 May
we decided to irrigate an area of about 0.5 m2 at and
around the closed chamber location on plots 1±10.
This provided the equivalent of 77 mm of rainfall. At
Boghall we also re-applied the no-tillage, heavy compaction and zero compaction treatments to half of the
plots and occasionally monitored ¯uxes throughout
the winter. Nitrogen fertiliser was applied to the spring
barley at sowing at 120 kg N haÿ1 on 12 April. For the
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
winter barley, nitrogen was applied to the growing
crop at 70 kg N haÿ1 on 7 March 1996 and at 110
kg N haÿ1 on 17 April 1996. At North Berwick,
nitrogen fertiliser was applied to the growing crop
at 78 kg haÿ1 on 3 March 1997, at 88 kg haÿ1 on 20
March 1997 and at 43 kg haÿ1 on 25 March 1997.
2.2. Measurement of gas fluxes
Gas ¯uxes were measured using closed chamber
systems. The atmosphere within the chamber is
sampled 1 h after closure. For a constant net emission
of N2O, we have found that the increase in concentration within closed chambers is linear over a period of
up to 3 h (Scott et al., 1999). This change in concentration is a result of net emission from the soil and
enables gas ¯ux to be determined. We have developed
gas sampling techniques using both manually and
automatically closed chambers (Scott et al., 1999).
One manually closed chamber was installed on
every plot shortly after sowing and on some plots
after ®nal harvest. These chambers (Clayton et al.,
1994) were 0.2 m tall polypropylene cylinders of
diameter 0.4 m and were pushed into the soil to a
depth of 5 cm to provide a head space of 0.02 m3 on
enclosure with an aluminium lid. Gas samples were
taken in syringes or aluminium sampling tubes and
subsequently analysed in the laboratory by gas chromatography. For ¯ux assessment, the manual chambers were only closed for 1 h duration once or twice
per week. Periodically these chambers were re-sited
within each plot to overcome potential microclimate
artefacts.
Automatic chambers were also used to provide
more regular measurements. The automatic chambers
(0.7 m 0.7 m) are similar to those described by
Scott et al. (1999) and have an actuator-driven, lidclosing system. The actuator is controlled by an
external, battery-operated, timing and sampling unit,
which allows remote collection of gas samples to be
carried out at programmed time intervals. Samples
(1 cm3) are collected by pumping into one of 24
isolated copper loops, attached to two rotary valves.
The entire valve/loop assembly is removed and
replaced by another assembly in order to preserve
continuity of sampling. The ®lled loop assembly is
transported to the laboratory for analysis by gas
chromatograph. In both manual and automated cham-
193
ber systems, ambient air is collected and used as the
reference for calculating gas ¯uxes. The automated
chambers were programmed to close for 1 h starting at
13:00 h and remain open for 3 h, thereby giving 8 ¯ux
assessments per day. Due to the large number of
samples generated by the auto-systems, only one
replicate per treatment was possible.
Flux monitoring using the chambers continued until
the crop was too high for lid closure to be effective.
This corresponded to late spring for the barley crops,
but was as early as the beginning of April for the
winter oil-seed rape due to its rapid growth rate.
In order to determine if the chopped oil-seed rape
residues at the soil surface contributed to the N2O
¯uxes after harvest at North Berwick, we took intact
core samples from 0±5 and 5±10 cm depth from the
®eld. We incubated these samples 11 times for 1 h in
the 30 day period immediately after sampling. The
residues were removed from one set of samples taken
from 0±5 cm. The average fresh weight of residues
was 5.8 g per sample. For comparison we also incubated 25 g of fresh oil seed rape residues only.
2.3. Measurement of N2O
Nitrous oxide was measured using a Pye Unicam
4500 gas chromatograph ®tted with a 63 Ni electron
capture detector at 3608C. Argon (Pureshield grade,
BOC) is used as carrier gas, with a ¯ow rate of
35 ml minÿ1. Gas separation is carried out on a 1 m
column (558C) packed with HayeSep Q, 60±80 mesh
(Haye Separations, Inc., Bandera, Texas).
2.4. Production and consumption of N2O in soil
In order to estimate the likely depths of production
and consumption of N2O in the soil, we occasionally
measured its concentration in the soil atmosphere
using 4 mm i.d. and 6 mm o.d. brass tubes of lengths
between 20 and 40 cm. These were installed to 10, 15,
20, 25 and 30 cm depths and were sealed at the top
with a rubber/te¯on septum. Duplicate 1 cm3 soil air
samples were taken and analysed as previously
described. Soil temperature was recorded at 90 min
intervals at Boghall using thermistor probes inserted at
2.5, 7.5 and 15 cm depths on two plots of each
treatment. Ammonium and nitrate N were measured
by continuous ¯ow colorimetric analysis of 1M KCl
194
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
extracts prepared from ®eld-moist soil using a soil:
solution ratio of 1 : 5. Soluble organic carbon was
considered as a relevant measure of the microbial
energy source and was estimated at the same time
with a Rosemount-Dohrmann DC80 Total Carbon
Analyser, on water extracts prepared from ®eld-moist
soil using a soil : solution ratio of 1 : 2. Soil pH was
measured on suspensions of 10 ml fresh soil in 25 ml
water.
3. Results
3.1. Nitrous oxide fluxes
Cumulative N2O ¯uxes in the ®rst six weeks after
sowing spring barley (Table 1) were low and were not
affected by the compaction. Although the average
effect of the irrigation treatment was to increase the
¯ux from 250 to 500 g N2O±N haÿ1, the effect was not
signi®cant because the ¯uxes generally were small.
Similarly the effects of compaction on post irrigation
¯uxes were not signi®cant, but there was a considerable enhancement of ¯ux in the no-tillage treatment
(Table 1). Under spring barley, N2O emissions showed
marked differences between replicates, but as for the
yields and soil physical properties, neither soil water
nor clay content gave a signi®cant covariate effect in
analysis of variance. Under winter barley, ¯uxes did
not differ signi®cantly between replicates.
Cumulative emissions of N2O were greater after
sowing winter barley (Table 1). Treatment differences
were signi®cant and likely to be associated with the
wet soil conditions and the greater compactive effort
applied in the heavy compaction treatment than under
spring barley.
The lowest emitting treatment was light compaction
and the highest emitting treatments involved heavy
compaction. The difference in emission between the
zero and the heavy compaction treatments is shown
more clearly by the ¯uxes from the automatic chambers (Fig. 1). The high frequency of measurements
revealed an irregular, episodic pattern of N2O ¯ux,
with some of the temporal variation associated with
Fig. 1. Nitrous oxide fluxes, assessed using automatic chambers, and soil surface temperature in the zero (Z) and heavy (H) compaction
treatments under winter barley at Boghall. Nitrogen fertiliser was applied at 70 kg haÿ1 on day 431 (not shown) and at 110 kg haÿ1 on day 472
(marked with an arrow).
Crop
Spring barley (1995)
Spring barley (1995)
Winter barley (1995±1996)
Winter barley (1996)
Winter oil-seed rape (1996)
Winter oil-seed rape (1997)
Winter oil-seed rape (1997)
Period
5 April±15 May (pre-irrigation)
16 May±14 July (post irrigation)
29 September±7 March (pre-fertilisation)
8 March±8 May (post-fertilisation)
1 September±13 November (autumn)
24 January±1 April (spring)
5±10 September (post harvest)
Days in Cumulative N2O flux (g N2O±N haÿ1)
period
Zero
Light
Heavy
compaction
compaction
compaction
Heavy compaction
loosened to 10 cm
No-tillage
LSD
(P < 0.05)
41
60
161
62
74
68
6
201
529
1033
304
170
144
1080
201
1123
638
313
±
±
±
ns
235 (***)
ns
196(*)
ns
ns
±
152
320
661
245
123
161
1137
107
310
622
210
97
151
851
153
401
905
578
136
196
258
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
Table 1
Cumulative nitrous oxide emissions measured using the manual chambers split according to irrigation, fertiliser application and season (the short period of high flux post oil-seed
rape harvest is included)
195
196
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
Fig. 2. Daily rainfall and mean soil temperature at 2.5 cm depth in the no-tilled (NT) treatment at Boghall. The arrows indicate
days of irrigation of spring barley (I), of sowing of winter barley (W) and of fertiliser application to the winter barley (F1 and F2).
soil surface temperature (Fig. 1). Although no data
were collected for the ®rst 19 days after the ®rst
fertiliser application, ¯uxes and the differences
between compaction treatments were greatest in the
month after nitrogen fertiliser applications. During
these periods rainfall and temperature at 2.5 cm depth
increased (Fig. 2). In Fig. 2 temperature is shown for
the no-tilled treatment which is taken as a reference. In
Fig. 3 the differences between this reference and the
zero and heavy compaction treatments are shown. In
the spring periods (April±May) temperature in the
heavily compacted soil (Fig. 3) was generally higher
than under no-tillage or zero compacted soil, particularly in spring 1996 when ¯uxes were high (Fig. 1).
Under winter oil seed rape, N2O emissions (Table 1)
were low and were associated with the dry conditions
at sowing which continued throughout the growing
season. However, N2O emissions were greater than in
any other period in the six days after harvest when the
soil was wet and the chopped residues provided a
source of available carbon. Core incubations (Table 2)
showed that the presence of chopped residues stimulated N2O production and that the isolated residues
provided a signi®cant component of emission. The
Table 2
Average and standard deviation of flux during incubation of intact soil cores and decomposing oil-seed rape residues after harvest, North
Berwick
Sample
type
Soil core,
residues removed
Depth (cm)
N2O (g N2O±N haÿ1 dÿ1)
CO2 (kg CO2±C haÿ1 dÿ1)
0±5
89 56
300 42
5±10
41 34
120 32
Soil core with
residues in place
Loose
residues only
0±5
820 1150
960 150
0
290 158
1980 82
197
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
Fig. 3. Relative difference in soil temperature at 2.5 cm depth between the zero compaction and no-tillage treatments (Z±NT) and between the
heavy compaction and no-tillage treatments (H±NT) at Boghall. The actual temperatures in the no-tilled treatments are shown in Fig. 2. The
times indicated by the arrows are explained in the key of Fig. 2.
high rate of CO2 production from the residues indicated that the microbial biomass was highly active.
At Boghall, ¯uxes after the spring barley harvest
(when under winter barley, 1995±1996, Table 1) were
moderate, particularly from the heavy compaction
treatment loosened to 10 cm. However, after the
winter barley harvest, when conditions were dry,
emissions were consistently low (ranging from 0.8
to 2.0 g N2O±N haÿ1 dÿ1) and were unaffected by
treatment.
3.2. Production and consumption of N2O in soil
Nitrous oxide was present in the soil air (Fig. 4)
between 10 and 30 cm depth and concentrations were
highest under heavy compaction. These concentra-
Table 3
Soil concentrations of NH4±N, NO3±N and soluble carbon, pH and gravimetric moisture content between 0 and 10 cm depth at North
Berwicka
Date of sampling
28 September 1996
29 October 1996
5 March 1997
24 March 1997
NH4±N (mg kgÿ1)
1.11
0.086
14.2
2.8
58.4
5.7
108
8.9
NO3±N (mg kgÿ1)
9.50
0.30
18.8
1.8
67.6
8.0
88.9
7.1
Soluble organic carbon (mg kgÿ1)
26.0
0.39
44.5
3.1
27.2
0.46
36.0
0.93
pH
6.57
0.034
6.78
0.053
6.37
0.048
6.22
0.042
Soil water content (g kgÿ1)
113
2.3
228
3.1
234
2.8
243
2.6
a
Values provided are overall means with standard errors given below in italics.
198
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
were shown for any property in a given treatment
except for pH which tended to decrease after treatment
application, particularly under no-tillage.
4. Discussion
4.1. Nitrous oxide fluxes
Fig. 4. Nitrous oxide concentration in the soil profile to 30 cm
depth in the zero and the heavy compaction treatments in the late
spring under winter barley at Boghall. The concentrations at depth
zero were detected in the manual chambers during routine
measurement of flux.
tions were much greater than those measured in the
chambers for calculation of surface ¯uxes. Although
soil chemical properties were measured regularly to
20 cm depth, no strong variation with depth was found
except at Boghall. Differences were con®ned mainly
to the top 10 cm layer within the zero compaction,
heavy compaction and no-tillage treatments (Fig. 5)
and were only signi®cant in spring 1996 after fertiliser
was applied to the winter barley. The increases in
ammonium and nitrate N re¯ected the levels of fertiliser application and were greatest under heavy
compaction. At North Berwick, treatment differences
were small and not signi®cant and are summarised in
Table 3. At both sites, no consistent trends with time
Increasing soil moisture content by rainfall or irrigation stimulated N2O emissions. However, heavy
compaction increased these emissions from winter
barley. Several factors such as high moisture, nitrate,
ammonium (Fig. 5) and near-soil surface temperature
levels (Fig. 2) probably combined to produce poor soil
aeration. Soil aeration status is particularly important
in the period after fertiliser application when N2O
emissions, as a result of denitri®cation, tend to peak
(Clayton et al., 1994). Under heavy compaction soil
physical conditions in the 0±10 cm layer were
adverse, with cone resistances of 2 MPa and bulk
densities of 1.4 Mg mÿ3 close to critical levels for
restriction of crop growth of 2.5 MPa and 1.41 Mg
mÿ3 (Ball and Ritchie, 1999). Such conditions were
also likely to have contributed to poor aeration. Air®lled porosity was below 10% m3 mÿ3 for much of the
growing season and gas diffusivities were low under
heavy compaction. In a pot experiment, Prade and
Trolldenier (1988) varied air ®lled porosity by applying different levels of compaction. Denitri®cation
increased exponentially with decreasing air-®lled porosity (
Soil and residue management effects on cropping conditions and
nitrous oxide ¯uxes under controlled traf®c in Scotland
2. Nitrous oxide, soil N status and weather
B.C. Ball*, J.P. Parker, A. Scott
Environmental Division, SAC, West Mains Road, Edinburgh, EH9 3JG, UK
Received 16 February 1999; received in revised form 21 July 1999; accepted 5 August 1999
Abstract
Nitrogen from fertilisers and crop residues can be lost as nitrous oxide (N2O), a greenhouse gas that causes an increase in
global warming and also depletes stratospheric ozone. Nitrous oxide emissions, soil chemical status, temperature and N2O
concentration in the soil atmosphere were measured in a ®eld experiment on soil compaction in loam and sandy loam
(cambisols) soils in south-east Scotland. The overall objective was to discover how the intensity and distribution of soil
compaction by tractor wheels or by roller just before sowing in¯uenced crop performance, soil conditions and production and
emissions of N2O under controlled traf®c conditions. Compaction treatments were zero, light compaction by roller (up to
1 Mg per metre of length) and heavy compaction by loaded tractor (up to 4.2 Mg). In this paper we report the effects on
production and emissions of N2O and relate them to soil and crop conditions. Nitrous oxide ¯uxes were substantial only when
the soil water content was high (>27 g per 100 g). Fertiliser application stimulated emissions in the spring whereas crop
residues stimulated emissions in autumn and winter. Heavy compaction increased N2O emissions after fertiliser application or
residue incorporation more than light or zero compaction. The bulk densities of the heavily and lightly compacted soils were
up to 89% and 82% of the theoretical (Proctor) maxima. Higher soil cone resistances, temperatures and nitrogen availability
and lower gas diffusivities and air-®lled porosities combined to make the heavily compacted soil more anaerobic and likely to
denitrify than the zero or lightly compacted soil. Compaction suf®cient to increase N2O emissions signi®cantly corresponded
with adverse soil conditions for winter barley (Hordeum vulgare L.) growth. Soil tillage, which ensures that soil compaction is
no greater than in our light treatment and is con®ned to near the soil surface, may help to mitigate both surface ¯uxes of N2O
and losses to the subsoil. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Compaction; Residues; Nitrous oxide; Soil
1. Introduction
*
Corresponding author. Tel.: 44-131-535-4392; fax: 44-131667-2601
E-mail address: b.ball@ed.sac.ac.uk (B.C. Ball)
The main production processes of nitrous oxide
(N2O) are microbial. These are denitri®cation, dominant in anaerobic conditions, and nitri®cation, dominant in aerobic conditions (Firestone and Davidson,
0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 8 1 - 1
192
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
1989). Although nitrogen fertiliser applications have a
major in¯uence on N2O emissions from arable soils
(Clayton et al., 1994), soil physical conditions, particularly near the surface, are also important mainly
through their in¯uence on soil aeration (Arah et al.,
1991). Compaction increases N2O emissions by
increasing water-®lled porosity and increasing the
likelihood of anaerobic soil conditions and denitri®cation, which are particular problems in moist, temperate climates (Douglas and Crawford, 1993; Hansen
et al., 1993). Nitrous oxide emissions can be reduced
by avoiding soil compaction, improving soil structure
and controlling drainage and irrigation to avoid excessive soil wetness (Beauchamp, 1997).
Incorporated crop residues can also increase N2O
emissions, particularly after ploughing in the autumn
(Smith et al., 1997). Such residues can enhance metabolic activity and form local anaerobic zones, giving
favourable sites for denitri®cation and contribute to
`hot spots' of emission (Flessa and Beese, 1995).
These, along with variability of soil water content
(Ambus and Christensen, 1994), contribute signi®cantly to high spatial variability of emission. The
contribution of crop residues to N2O emissions and
the complex interactions with other soil properties are
still subject to considerable uncertainty (Beauchamp,
1997).
The production, consumption and transport of N2O
are strongly in¯uenced by the changes in soil structural quality, residue incorporation and water content
associated with tillage and compaction. A complex
interaction between water-®lled pore space and soil
compaction in¯uences microbial activity and nitrogen
losses by denitri®cation (Torbert and Wood, 1992).
Since the in¯uence of residues on denitri®cation under
different tillage systems has been associated with
compaction (Ball and Robertson, 1990), the interactions of residue incorporation, soil compaction and
N2O emission necessitate assessment.
Our objective was to identify the optimum level of
seedbed compaction (including zero) after normal
ploughing of residues, which gives the best cropping
conditions while minimising losses of nitrogen as
N2O. A previous paper (Ball and Ritchie, 1999)
reported the various management options evaluated
and the soil and crop responses. Heavy compaction
was applied by a laden tractor and light compaction by
a roller. This resulted in an increase in soil bulk density
of 75±89% of the theoretical (Proctor) maxima for
heavy compaction whereas the range for light compaction was only 71±82%. Heavy compaction also
limited crop growth due to poor soil physical conditions particularly when the soil was wet. Both zero and
light compaction gave improved crop performance.
This paper reports the N2O emissions and how they
relate to environmental, soil and crop conditions.
2. Methods and materials
2.1. Field experiment
The ®eld experiment was located at two sites. The
®rst site at Boghall, 10 km south of Edinburgh, contained spring and winter barley from spring 1995 to
summer 1996. Daily rainfall was monitored at a
weather station 1.3 km south of the site. The second
site, located 3 km south of North Berwick, contained
winter oil-seed rape (Brassica spp. L.) from late
summer 1996 to harvest 1997. Daily rainfall was
not monitored at this site, though monthly total rainfalls from a weather station 1 km southwards were
given by Ball and Ritchie (1999) who also gave full
details of the site, soil and treatments. Brie¯y, treatments were applied to soil which had been mouldboard ploughed the previous day, incorporating the
chopped residues of the previous cereal crop. Treatments were (1) zero compaction, (2) light compaction
(target depth of 10 cm) using a heavy roller, (3) heavy
compaction (target depth of 25 cm) using a laden
tractor and (4) heavy compaction with the soil subsequently loosened to 10 cm depth with a rotary
cultivator. An additional treatment of no-tillage was
also applied at Boghall.
Treatments were applied on 12 April and 27 September 1995 (Boghall) and on 27 August 1996 (North
Berwick). At Boghall, rainfall in the month after
sowing spring barley was low. Therefore on 15 May
we decided to irrigate an area of about 0.5 m2 at and
around the closed chamber location on plots 1±10.
This provided the equivalent of 77 mm of rainfall. At
Boghall we also re-applied the no-tillage, heavy compaction and zero compaction treatments to half of the
plots and occasionally monitored ¯uxes throughout
the winter. Nitrogen fertiliser was applied to the spring
barley at sowing at 120 kg N haÿ1 on 12 April. For the
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
winter barley, nitrogen was applied to the growing
crop at 70 kg N haÿ1 on 7 March 1996 and at 110
kg N haÿ1 on 17 April 1996. At North Berwick,
nitrogen fertiliser was applied to the growing crop
at 78 kg haÿ1 on 3 March 1997, at 88 kg haÿ1 on 20
March 1997 and at 43 kg haÿ1 on 25 March 1997.
2.2. Measurement of gas fluxes
Gas ¯uxes were measured using closed chamber
systems. The atmosphere within the chamber is
sampled 1 h after closure. For a constant net emission
of N2O, we have found that the increase in concentration within closed chambers is linear over a period of
up to 3 h (Scott et al., 1999). This change in concentration is a result of net emission from the soil and
enables gas ¯ux to be determined. We have developed
gas sampling techniques using both manually and
automatically closed chambers (Scott et al., 1999).
One manually closed chamber was installed on
every plot shortly after sowing and on some plots
after ®nal harvest. These chambers (Clayton et al.,
1994) were 0.2 m tall polypropylene cylinders of
diameter 0.4 m and were pushed into the soil to a
depth of 5 cm to provide a head space of 0.02 m3 on
enclosure with an aluminium lid. Gas samples were
taken in syringes or aluminium sampling tubes and
subsequently analysed in the laboratory by gas chromatography. For ¯ux assessment, the manual chambers were only closed for 1 h duration once or twice
per week. Periodically these chambers were re-sited
within each plot to overcome potential microclimate
artefacts.
Automatic chambers were also used to provide
more regular measurements. The automatic chambers
(0.7 m 0.7 m) are similar to those described by
Scott et al. (1999) and have an actuator-driven, lidclosing system. The actuator is controlled by an
external, battery-operated, timing and sampling unit,
which allows remote collection of gas samples to be
carried out at programmed time intervals. Samples
(1 cm3) are collected by pumping into one of 24
isolated copper loops, attached to two rotary valves.
The entire valve/loop assembly is removed and
replaced by another assembly in order to preserve
continuity of sampling. The ®lled loop assembly is
transported to the laboratory for analysis by gas
chromatograph. In both manual and automated cham-
193
ber systems, ambient air is collected and used as the
reference for calculating gas ¯uxes. The automated
chambers were programmed to close for 1 h starting at
13:00 h and remain open for 3 h, thereby giving 8 ¯ux
assessments per day. Due to the large number of
samples generated by the auto-systems, only one
replicate per treatment was possible.
Flux monitoring using the chambers continued until
the crop was too high for lid closure to be effective.
This corresponded to late spring for the barley crops,
but was as early as the beginning of April for the
winter oil-seed rape due to its rapid growth rate.
In order to determine if the chopped oil-seed rape
residues at the soil surface contributed to the N2O
¯uxes after harvest at North Berwick, we took intact
core samples from 0±5 and 5±10 cm depth from the
®eld. We incubated these samples 11 times for 1 h in
the 30 day period immediately after sampling. The
residues were removed from one set of samples taken
from 0±5 cm. The average fresh weight of residues
was 5.8 g per sample. For comparison we also incubated 25 g of fresh oil seed rape residues only.
2.3. Measurement of N2O
Nitrous oxide was measured using a Pye Unicam
4500 gas chromatograph ®tted with a 63 Ni electron
capture detector at 3608C. Argon (Pureshield grade,
BOC) is used as carrier gas, with a ¯ow rate of
35 ml minÿ1. Gas separation is carried out on a 1 m
column (558C) packed with HayeSep Q, 60±80 mesh
(Haye Separations, Inc., Bandera, Texas).
2.4. Production and consumption of N2O in soil
In order to estimate the likely depths of production
and consumption of N2O in the soil, we occasionally
measured its concentration in the soil atmosphere
using 4 mm i.d. and 6 mm o.d. brass tubes of lengths
between 20 and 40 cm. These were installed to 10, 15,
20, 25 and 30 cm depths and were sealed at the top
with a rubber/te¯on septum. Duplicate 1 cm3 soil air
samples were taken and analysed as previously
described. Soil temperature was recorded at 90 min
intervals at Boghall using thermistor probes inserted at
2.5, 7.5 and 15 cm depths on two plots of each
treatment. Ammonium and nitrate N were measured
by continuous ¯ow colorimetric analysis of 1M KCl
194
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
extracts prepared from ®eld-moist soil using a soil:
solution ratio of 1 : 5. Soluble organic carbon was
considered as a relevant measure of the microbial
energy source and was estimated at the same time
with a Rosemount-Dohrmann DC80 Total Carbon
Analyser, on water extracts prepared from ®eld-moist
soil using a soil : solution ratio of 1 : 2. Soil pH was
measured on suspensions of 10 ml fresh soil in 25 ml
water.
3. Results
3.1. Nitrous oxide fluxes
Cumulative N2O ¯uxes in the ®rst six weeks after
sowing spring barley (Table 1) were low and were not
affected by the compaction. Although the average
effect of the irrigation treatment was to increase the
¯ux from 250 to 500 g N2O±N haÿ1, the effect was not
signi®cant because the ¯uxes generally were small.
Similarly the effects of compaction on post irrigation
¯uxes were not signi®cant, but there was a considerable enhancement of ¯ux in the no-tillage treatment
(Table 1). Under spring barley, N2O emissions showed
marked differences between replicates, but as for the
yields and soil physical properties, neither soil water
nor clay content gave a signi®cant covariate effect in
analysis of variance. Under winter barley, ¯uxes did
not differ signi®cantly between replicates.
Cumulative emissions of N2O were greater after
sowing winter barley (Table 1). Treatment differences
were signi®cant and likely to be associated with the
wet soil conditions and the greater compactive effort
applied in the heavy compaction treatment than under
spring barley.
The lowest emitting treatment was light compaction
and the highest emitting treatments involved heavy
compaction. The difference in emission between the
zero and the heavy compaction treatments is shown
more clearly by the ¯uxes from the automatic chambers (Fig. 1). The high frequency of measurements
revealed an irregular, episodic pattern of N2O ¯ux,
with some of the temporal variation associated with
Fig. 1. Nitrous oxide fluxes, assessed using automatic chambers, and soil surface temperature in the zero (Z) and heavy (H) compaction
treatments under winter barley at Boghall. Nitrogen fertiliser was applied at 70 kg haÿ1 on day 431 (not shown) and at 110 kg haÿ1 on day 472
(marked with an arrow).
Crop
Spring barley (1995)
Spring barley (1995)
Winter barley (1995±1996)
Winter barley (1996)
Winter oil-seed rape (1996)
Winter oil-seed rape (1997)
Winter oil-seed rape (1997)
Period
5 April±15 May (pre-irrigation)
16 May±14 July (post irrigation)
29 September±7 March (pre-fertilisation)
8 March±8 May (post-fertilisation)
1 September±13 November (autumn)
24 January±1 April (spring)
5±10 September (post harvest)
Days in Cumulative N2O flux (g N2O±N haÿ1)
period
Zero
Light
Heavy
compaction
compaction
compaction
Heavy compaction
loosened to 10 cm
No-tillage
LSD
(P < 0.05)
41
60
161
62
74
68
6
201
529
1033
304
170
144
1080
201
1123
638
313
±
±
±
ns
235 (***)
ns
196(*)
ns
ns
±
152
320
661
245
123
161
1137
107
310
622
210
97
151
851
153
401
905
578
136
196
258
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
Table 1
Cumulative nitrous oxide emissions measured using the manual chambers split according to irrigation, fertiliser application and season (the short period of high flux post oil-seed
rape harvest is included)
195
196
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
Fig. 2. Daily rainfall and mean soil temperature at 2.5 cm depth in the no-tilled (NT) treatment at Boghall. The arrows indicate
days of irrigation of spring barley (I), of sowing of winter barley (W) and of fertiliser application to the winter barley (F1 and F2).
soil surface temperature (Fig. 1). Although no data
were collected for the ®rst 19 days after the ®rst
fertiliser application, ¯uxes and the differences
between compaction treatments were greatest in the
month after nitrogen fertiliser applications. During
these periods rainfall and temperature at 2.5 cm depth
increased (Fig. 2). In Fig. 2 temperature is shown for
the no-tilled treatment which is taken as a reference. In
Fig. 3 the differences between this reference and the
zero and heavy compaction treatments are shown. In
the spring periods (April±May) temperature in the
heavily compacted soil (Fig. 3) was generally higher
than under no-tillage or zero compacted soil, particularly in spring 1996 when ¯uxes were high (Fig. 1).
Under winter oil seed rape, N2O emissions (Table 1)
were low and were associated with the dry conditions
at sowing which continued throughout the growing
season. However, N2O emissions were greater than in
any other period in the six days after harvest when the
soil was wet and the chopped residues provided a
source of available carbon. Core incubations (Table 2)
showed that the presence of chopped residues stimulated N2O production and that the isolated residues
provided a signi®cant component of emission. The
Table 2
Average and standard deviation of flux during incubation of intact soil cores and decomposing oil-seed rape residues after harvest, North
Berwick
Sample
type
Soil core,
residues removed
Depth (cm)
N2O (g N2O±N haÿ1 dÿ1)
CO2 (kg CO2±C haÿ1 dÿ1)
0±5
89 56
300 42
5±10
41 34
120 32
Soil core with
residues in place
Loose
residues only
0±5
820 1150
960 150
0
290 158
1980 82
197
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
Fig. 3. Relative difference in soil temperature at 2.5 cm depth between the zero compaction and no-tillage treatments (Z±NT) and between the
heavy compaction and no-tillage treatments (H±NT) at Boghall. The actual temperatures in the no-tilled treatments are shown in Fig. 2. The
times indicated by the arrows are explained in the key of Fig. 2.
high rate of CO2 production from the residues indicated that the microbial biomass was highly active.
At Boghall, ¯uxes after the spring barley harvest
(when under winter barley, 1995±1996, Table 1) were
moderate, particularly from the heavy compaction
treatment loosened to 10 cm. However, after the
winter barley harvest, when conditions were dry,
emissions were consistently low (ranging from 0.8
to 2.0 g N2O±N haÿ1 dÿ1) and were unaffected by
treatment.
3.2. Production and consumption of N2O in soil
Nitrous oxide was present in the soil air (Fig. 4)
between 10 and 30 cm depth and concentrations were
highest under heavy compaction. These concentra-
Table 3
Soil concentrations of NH4±N, NO3±N and soluble carbon, pH and gravimetric moisture content between 0 and 10 cm depth at North
Berwicka
Date of sampling
28 September 1996
29 October 1996
5 March 1997
24 March 1997
NH4±N (mg kgÿ1)
1.11
0.086
14.2
2.8
58.4
5.7
108
8.9
NO3±N (mg kgÿ1)
9.50
0.30
18.8
1.8
67.6
8.0
88.9
7.1
Soluble organic carbon (mg kgÿ1)
26.0
0.39
44.5
3.1
27.2
0.46
36.0
0.93
pH
6.57
0.034
6.78
0.053
6.37
0.048
6.22
0.042
Soil water content (g kgÿ1)
113
2.3
228
3.1
234
2.8
243
2.6
a
Values provided are overall means with standard errors given below in italics.
198
B.C. Ball et al. / Soil & Tillage Research 52 (1999) 191±201
were shown for any property in a given treatment
except for pH which tended to decrease after treatment
application, particularly under no-tillage.
4. Discussion
4.1. Nitrous oxide fluxes
Fig. 4. Nitrous oxide concentration in the soil profile to 30 cm
depth in the zero and the heavy compaction treatments in the late
spring under winter barley at Boghall. The concentrations at depth
zero were detected in the manual chambers during routine
measurement of flux.
tions were much greater than those measured in the
chambers for calculation of surface ¯uxes. Although
soil chemical properties were measured regularly to
20 cm depth, no strong variation with depth was found
except at Boghall. Differences were con®ned mainly
to the top 10 cm layer within the zero compaction,
heavy compaction and no-tillage treatments (Fig. 5)
and were only signi®cant in spring 1996 after fertiliser
was applied to the winter barley. The increases in
ammonium and nitrate N re¯ected the levels of fertiliser application and were greatest under heavy
compaction. At North Berwick, treatment differences
were small and not signi®cant and are summarised in
Table 3. At both sites, no consistent trends with time
Increasing soil moisture content by rainfall or irrigation stimulated N2O emissions. However, heavy
compaction increased these emissions from winter
barley. Several factors such as high moisture, nitrate,
ammonium (Fig. 5) and near-soil surface temperature
levels (Fig. 2) probably combined to produce poor soil
aeration. Soil aeration status is particularly important
in the period after fertiliser application when N2O
emissions, as a result of denitri®cation, tend to peak
(Clayton et al., 1994). Under heavy compaction soil
physical conditions in the 0±10 cm layer were
adverse, with cone resistances of 2 MPa and bulk
densities of 1.4 Mg mÿ3 close to critical levels for
restriction of crop growth of 2.5 MPa and 1.41 Mg
mÿ3 (Ball and Ritchie, 1999). Such conditions were
also likely to have contributed to poor aeration. Air®lled porosity was below 10% m3 mÿ3 for much of the
growing season and gas diffusivities were low under
heavy compaction. In a pot experiment, Prade and
Trolldenier (1988) varied air ®lled porosity by applying different levels of compaction. Denitri®cation
increased exponentially with decreasing air-®lled porosity (