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 vulgareL.) 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
The main production processes of nitrous oxide (N2O) are microbial. These are denitri®cation,
nant in anaerobic conditions, and nitri®cation, domi-nant in aerobic conditions (Firestone and Davidson, Soil & Tillage Research 52 (1999) 191±201
*Corresponding author. Tel.:44-131-535-4392; fax:
44-131-667-2601
E-mail address: b.ball@ed.sac.ac.uk (B.C. Ball)
1989). Although nitrogen fertiliser applications have a major in¯uence on N2O emissions from arable soils
(Clayton et al., 1994), soil physical conditions, parti-cularly 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, tem-perate 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 exces-sive 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 meta-bolic 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 struc-tural 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 interac-tions 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 com-paction was only 71±82%. Heavy comcom-paction also limited crop growth due to poor soil physical condi-tions 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, con-tained 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 rain-falls 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, treat-ments were applied to soil which had been mould-board ploughed the previous day, incorporating the chopped residues of the previous cereal crop. Treat-ments 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 sub-sequently loosened to 10 cm depth with a rotary cultivator. An additional treatment of no-tillage was also applied at Boghall.
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
concentra-tion within closed chambers is linear over a period of up to 3 h (Scott et al., 1999). This change in concen-tration 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 m3on enclosure with an aluminium lid. Gas samples were taken in syringes or aluminium sampling tubes and subsequently analysed in the laboratory by gas chro-matography. For ¯ux assessment, the manual cham-bers 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 m0.7 m) are similar to those described by Scott et al. (1999) and have an actuator-driven, lid-closing 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-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 incu-bated 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 63Ni 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
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 ha
ÿ1
, 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 consider-able 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 cham-bers (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 haÿ1on day 431 (not shown) and at 110 kg haÿ1on day 472
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)
Crop Period Days in
period
Cumulative N2O flux (g N2O±N ha
ÿ1)
Zero compaction
Light compaction
Heavy compaction
Heavy compaction loosened to 10 cm
No-tillage LSD (P< 0.05)
Spring barley (1995) 5 April±15 May (pre-irrigation) 41 152 107 153 201 201 ns
Spring barley (1995) 16 May±14 July (post irrigation) 60 320 310 401 529 1123 235 (***) Winter barley (1995±1996) 29 September±7 March (pre-fertilisation) 161 661 622 905 1033 638 ns Winter barley (1996) 8 March±8 May (post-fertilisation) 62 245 210 578 304 313 196(*) Winter oil-seed rape (1996) 1 September±13 November (autumn) 74 123 97 136 170 ± ns
Winter oil-seed rape (1997) 24 January±1 April (spring) 68 161 151 196 144 ± ns
Winter oil-seed rape (1997) 5±10 September (post harvest) 6 1137 851 258 1080 ± ±
B.C.
Ball
et
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/
Soil
&
T
illage
Resear
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52
(1999)
191±201
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, particu-larly 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 stimu-lated N2O production and that the isolated residues
provided a signi®cant component of emission. The 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).
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
Soil core with residues in place
Loose residues only
Depth (cm) 0±5 5±10 0±5 0
N2O (g N2O±N ha
ÿ1dÿ1) 8956 4134 8201150 290158
CO2(kg CO2±C ha
high rate of CO2production from the residues
indi-cated 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 ha
ÿ1
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-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.
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 kg
ÿ1) 1.11 14.2 58.4 108
0.086 2.8 5.7 8.9
NO3±N (mg kg
ÿ1) 9.50 18.8 67.6 88.9
0.30 1.8 8.0 7.1
Soluble organic carbon (mg kgÿ1) 26.0 44.5 27.2 36.0
0.39 3.1 0.46 0.93
pH 6.57 6.78 6.37 6.22
0.034 0.053 0.048 0.042
Soil water content (g kgÿ1) 113 228 234 243
2.3 3.1 2.8 2.6
aValues provided are overall means with standard errors given below in italics.
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 fer-tiliser 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
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
Increasing soil moisture content by rainfall or irri-gation 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% m3mÿ3
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 apply-ing different levels of compaction. Denitri®cation increased exponentially with decreasing air-®lled por-osity (<10±12%), particularly in the presence of a respiring root system which depleted oxygen in the rhizosphere.
Stimulation of N2O emission, by compaction
asso-ciated with adverse soil physical conditions, also corresponded with adverse conditions for winter bar-ley growth (Ball and Ritchie, 1999). Similarly, in Norway, Bakken et al. (1987) found that compaction of wet soil by tractor traf®c increased N loss by denitri®cation 3±4 fold and decreased wheat yield by 25%. The observed restricted growth increased the likelihood of emissions by reducing uptake of available nitrogen and of plant available water. The reduced crop cover may have reduced insulation from the sun thereby increasing soil temperatures near the soil surface. Hansen et al. (1993) also found that N2O
¯uxes from soil compacted by a tractor (similar to our Fig. 4. Nitrous oxide concentration in the soil profile to 30 cm
heavy compaction treatment) were greater than in soil subject to normal traf®c, and attributed the ¯uxes to denitri®cation. We observed symptoms of nitrogen
de®ciency in our late autumn winter barley in heavily compacted soil. Maidl and Fischbeck (1988) attribu-ted sugar beet (Beta vulgaris L.) yield reductions in Fig. 5. Soil concentrations of NH4±N, NO3±N and soluble carbon, pH and gravimetric moisture content at 0±10 cm depth at Boghall. Vertical
bars indicate the least significant difference (P< 0.05). LSDs are not given for the second sampling occasion when there were no significant differences in any property. The arrow at S points to the date of sowing of spring barley. The times indicated by the other arrows are explained in the key of Fig. 2.
compacted soil to nitrogen de®ciency caused by high denitri®cation activity and low mineralisation.
Our peak N2O emissions (Fig. 1) are considerably
lower than those of up to 500 g N haÿ1
dÿ1
reported by Douglas and Crawford (1993) for compacted grass-land and by Ball et al. (1999) for spring barley established by no-tillage on grassland. Cereals and oil-seed rape are considered to emit less N2O on a
cumulative yearly basis than grassland (Skiba et al., 1996). Nevertheless, our data show that wet soil and residues after harvest can produce substantial losses of N2O even though losses earlier in the season were
low (Table 1). High residual levels of nitrate, ammo-nium and reduced pH (Table 3) combine to make the soil likely to give high emissions of N2O on wetting
immediately after harvest, particularly in warm soil, as occurred at North Berwick.
4.2. Production and consumption of N2O in soil
Under winter barley, the concentration of N2O in
the pro®le below 15 cm depth may either be due to diffusion from the layer above or to the compact, wet soil layer at 0±15 cm restricting the escape of gas produced below. The latter is likely because gas diffusivity and air permeability were lowest at 5± 10 cm depth (Ball and Ritchie, 1999). Santruckova et al. (1993) found that compaction shifted the max-imum microbial biomass and its `potential activity' from near the surface to 20±30 cm depth. Thus sig-ni®cant N2O production at this depth is likely.
Nitrous oxide can be temporarily stored in the lower soil pro®le and may be further reduced to N2prior to
ef¯ux from the soil surface (Burton et al., 1997) or it may dissolve in the subsoil solution and be outgassed in drainage water (Beauchamp, 1997). Hence, our cumulative N2O ¯uxes (Table 1) are likely to be
underestimates. Haunz et al. (1992) found that, in a wet year, the poor structure in compacted soil led to a loss of up to 20% of the fertiliser N by denitri®cation. Loosening of the heavy compaction treatment improved crop growth and soil aeration (Ball and Ritchie, 1999) but did not consistently reduce cumu-lative N2O emissions. Indeed, at Boghall, this
treat-ment emitted most N2O post-harvest (Table 1).
However, light compaction gave consistently lower cumulative emissions. Thus residual compaction below 10 cm was important in stimulating ¯uxes,
but light compaction of the top 10 cm layer may help to mitigate emissions. Separation of the effects of compaction, crop residues and soil wetness on N2O
emission, and the fate of N2O accumulated at the
topsoil/subsoil interface, are important aspects for further study.
5. Conclusions
The in¯uence of compaction on N2O emissions
appeared to act mainly through increased topsoil wetness, resulting in greater anaerobic conditions associated with reduced gas diffusivity and increased cone resistance. These combined effects were parti-cularly important because they limited crop growth, as occurred in the winter barley. Wet post-harvest con-ditions also provided concon-ditions for high N2O ¯uxes
from oil-seed rape residues and from residual soil N. Manipulation of topsoil compaction status such that compaction is light and con®ned to near the soil surface may help in mitigating both surface ¯uxes of N2O and losses to the subsoil.
Acknowledgements
We are grateful to our colleagues R.M. Ritchie and I.J. Crichton for technical assistance and to Dr. G.W. Horgan, Biomathematics and Statistics, Scotland, for statistical advice. The project was supported by the Scottish Of®ce Agriculture, Environment and Fish-eries Department.
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