Soil and residue management effects on arable
cropping conditions and nitrous oxide ¯uxes
under controlled traf®c in Scotland
1. Soil and crop responses
B.C. Ball
*, R.M. Ritchie
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
Soil compaction can affect crop growth and greenhouse gas emission and information is required of how both these aspects are affected by compaction intensity and weather. In this paper we describe treatments of compaction intensity and their effects on soil physical conditions and crop growth in loam to sandy loam cambisol soils. Soil conditions and crop performance were measured over three seasons in a ®eld experiment on soil compacted by wheels on freshly ploughed seedbeds. Ploughing buried the chopped residues of the previous crop. After ploughing, traf®c was controlled such that the experimental plots received wheel traf®c only as treatments. The overall objective was to discover how the intensity and distribution of soil compaction just before sowing in¯uenced crop performance, soil conditions and emissions of nitrous oxide. Compaction treatments were zero, light compaction by roller (up to 1 Mg mÿ1
) and heavy compaction by loaded tractor, (up to 4.2 Mg). The experiment was located at Boghall, near Edinburgh (860 mm average annual rainfall) for the ®rst two seasons under spring and winter barley (Hordeum vulgareL.) and in a drier area at North Berwick (610 mm average annual rainfall) for the third season under winter oil-seed rape (Brassica napusL.). Heavy compaction in dry soil conditions had little effect on crop growth. However, in wet conditions heavy compaction reduced air porosity, air permeability and gas diffusivity, increased cone resistance and limited winter barley growth and grain yield. Heavy compaction in wet conditions reduced winter barley yields to 7.1 Mg haÿ1, in comparison to 8.8 Mg haÿ1 in the zero compaction treatment. The compaction status of the top 15 cm of soil seemed to be particularly important. Loosening of the top 10 cm of soil immediately after heavy compaction restored soil conditions for crop growth. However, zero seed bed compaction gave patchy and uneven crop emergence in dry conditions. Both zero and light compaction to a target depth of 10 cm gave similar crop productivity. Maintenance of a correct compaction level near the soil surface is particularly important for establishment and overwintering of barley and oil seed rape.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Compaction; Residues; Barley; Soil
*Corresponding author. Tel.:44-131-535-4392; fax.:44-131-667-2601
E-mail address: [email protected] (B.C. Ball)
1. Introduction
Considerable effort has been devoted to evaluating the effects of wheel traf®c on soil properties and crop growth (Soane and van Ouwerkerk, 1994). Manage-ment systems which completely eliminate traf®c or which permit a reduction in ground pressure on the crop growing area give more favourable soil condi-tions and, often, better crop growth and yield than conventional systems (Chamen et al., 1992; Dickson and Ritchie, 1996). Extensive compaction before sow-ing is particularly damagsow-ing to soil structure and subsequent crop growth (Campbell et al., 1986; Bak-ken et al., 1987; McAfee et al., 1989). Tractor traf®c on wet soil can also increase denitri®cation by a factor of 3±4 (Bakken et al., 1987). The presence of straw residues, particularly in compacted soil, can also contribute to waterlogging, nitrous oxide (N2O) pro-duction and reduced crop survival over winter (Ball and Robertson, 1990). Reduced or no-tillage systems can also increase N2O production (Aulakh et al., 1984), particularly in the presence of straw residues (Ball and Robertson, 1990). Nitrous oxide production represents a loss of nutrient from the system and contributes to global warming and the destruction of stratospheric ozone (Crutzen, 1970). Indeed, the signi®cance of arable agricultural soils as a source of N2O may have been underestimated (Beauchamp, 1997).
Recent work on the in¯uence of tillage and com-paction in Scotland under arable cropping and con-trolled traf®c conditions has involved chisel ploughing and reduced or no-tillage (Campbell et al., 1986; Dickson and Ritchie, 1996). These techniques are rarely used in practice and the experiments conducted did not include measurements of gaseous losses of nitrogen. There is a need to consider how normal soil management techniques involving mouldboard ploughing can create soil conditions which suit crop growth but which also minimise losses of nitrogen, particularly as N2O. In the UK, crop residues are increasingly incorporated in the soil, partly due to legal restrictions of straw burning. 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 nitrous oxide emission require assessment.
Our objective was to determine which soil proper-ties in¯uenced crop growth in response to different compaction intensities and depths. This then permits identi®cation of the optimum level of seedbed com-paction (including zero) after normal ploughing of residues, which gives the best cropping conditions whilst minimising the losses of nitrogen as N2O. No-tillage was included as a reference treatment. This paper reports the treatments and the soil and crop responses. A subsequent paper reports on the nitrous oxide emissions and the soil nitrogen status (Ball et al., 1999).
2. Methods and materials
2.1. Location, soil and weather
The ®eld experiment was located at two sites. The ®rst, at Boghall, 10 km south of Edinburgh, contained spring barley in 1995 and winter barley in 1995/6. The second site, 3 km south of North Berwick, contained winter oil-seed rape in 1996/7. The average annual rainfall near Boghall is 880 mm and near North Berwick is 611 mm. In addition, monthly rainfalls at weather stations close to each site are given in Table 1. The ®rst site was on Macmerry series (Cam-bisol in FAO classi®cation) and the second site was on Kilmarnock series (Gleyic Cambisol). Macmerry ser-ies vary in texture between loam and sandy loam whereas Kilmarnock series are mostly loams. Typical topsoil organic matter levels are 51 g kgÿ1
for Mac-merry and 38 g kgÿ1
for Kilmarnock. Both have been described as imperfectly drained brown forest soils (Ragg and Futty, 1967). Both soil types are in the British land use capability class 2, where soil wetness is a minor limitation to the choice of crops and cultivations (Bibby et al., 1982).
2.2. Treatments, experimental design and nitrogen fertiliser applications
The experimental design was a randomised com-plete block with fourfold replication, replicates being aligned side by side. Plots were 24 m2.4 m. The plots were suf®ciently narrow to be straddled by wide wheel track machinery. The residues of the previous cereal crop had been chopped and spread over the
experimental site during harvesting. The soil was mouldboard ploughed to 25 cm depth the day before treatment application, thereby incorporating these residues. Wide-wheel track machinery was used for all management operations after ploughing, which ensured that wheel compaction was con®ned between each plot. All machinery operated along the long axis of the plots ensuring that most of the variability of soil compaction and residue incorporation would be across the plots. Treatments were (1) zero compaction, (2) light compaction (target depth of 10 cm) using a heavy roller (up to 1 Mg per metre length), (3) heavy com-paction (target depth of 25 cm) using a laden tractor (up to 4.2 Mg), and (4) heavy compaction with the soil subsequently loosened down to 10 cm depth with a rotary cultivator. Details of the machinery used for the compaction treatments are given in Table 2. The soil was compacted before and after sowing except for the ®rst heavy compaction treatment at Boghall. The soil was rotary harrowed immediately before treatment application and before sowing in order to prepare a uniform surface. All plots were rolled immediately after sowing (Table 2).
Ploughing and treatment application occurred twice in 1995 at Boghall, before sowing spring barley in April and winter barley in September, and once in August 1996 at North Berwick, before sowing oil-seed rape. An additional treatment of no-tillage was included at Boghall. Due to management constraints,
this could not be included at North Berwick. At Boghall, nitrogen fertiliser was applied to the spring barley at sowing at 120 kg N haÿ1
on 12 April. For the 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.
2.3. Soil and crop measurements
Cone resistance to 30 cm depth (10 penetrations/ plot) was measured before and after ploughing and treatment application with a hand-held digital penet-rometer linked to an electronic data collector (O'Sul-livan et al., 1983). Bulk density and porosity were measured in October 1995 at Boghall and in March 1997 at North Berwick from 0±25 cm depth in cores of 7.3 cm diameter at increments of 5 cm. One core per depth was taken from plots in replicates 2 and 3 only. A pit was dug in each plot and cores taken at staggered depth intervals. Relative gas diffusivity and air perme-ability were measured on the same samples at ®eld water content, using the methods of Ball et al. (1981). Soil gas diffusivity was also measured in situ at Boghall in May 1995, November 1995 and June 1996 to give a direct measure of the ability of the soils to exchange gases with minimal soil disturbance.
Table 1
Monthly rainfall (mm) at weather stations near the Boghall and North Berwick sites
Boghall North Berwick
February 106 52 61 26 63 35
March 52 26 77 17 20 44
September 150 23 81 15 34 60
October 115 132 88 58 24 59
November 73 126 82 64 66 58
December 44 109 82 88 93 49
Table 2
Details of machinery used in the application of the compaction treatments at the Boghall and North Berwick sites
Site, crop and date of treatment Light compaction by roller Heavy compaction by tractor Compaction by 1 pass of roller before application
Weight (Mg mÿ1)
Passesa Weight
(Mg)
Front tyres Rear tyres Passesa sowing (all plots): Weight (Mg m
ÿ1)
Size Inflation pressure (kPa)
Size Inflation pressure (kPa)
Boghall, spring barley, 12 April 1995 1 2 3.05 7.5±16 220 13.6R 36 80 1 0.33 Boghall, winter barley, 27 September 1995 1 2 4.22 7.5±16 220 13.6R 36 80 2b 0.33
North Berwick, winter oil-seed rape, 27 August 1996
0.66 3 3.59 7.5±16 220 13.6R 36 80 3 0.54
It was measured by injecting Freon into a chamber enclosing the soil surface and measuring its subse-quent rate of escape into the soil as the decrease in Freon concentration within the chamber (Ball et al., 1997a). In order to calculate diffusivity, it is necessary ®rst to simulate diffusion numerically using Fick's equation. The time axis of the simulation is expanded or contracted until it matches the observed decrease in concentration [see Ball et al. (1994)].
Using a gouge auger, samples for measurement of gravimetric water content were regularly taken from 0±20 cm in 5 cm intervals at treatment application and through the growing season. For the winter crops, these were converted to volumetric water contents and air-®lled porosities using the bulk densities mea-sured in the core samples. Particle size distribution was measured using the pipette method at Boghall in winter 1995 from 0±30 cm depth in 10 cm intervals. Soil temperature was recorded at 90-min intervals using thermistor probes inserted at 2.5, 7.5 and 15 cm depth in selected plots at Boghall. Probes were at two locations per depth and were connected to a ®eld logger which was downloaded weekly.
Grain yield and grain nitrogen content were mea-sured at Boghall. Plot yields were assessed on a crop area of about 50 m2using a plot combine. The yield of oil-seed rape could not be assessed because the wet weather in the month prior to harvesting had caused the crop to lodge and tangle suf®ciently for it to require windrowing to allow harvesting. No machin-ery small enough to windrow individual plots was available. However, the nitrogen content of the oil-seed was determined just before harvest.
3. Results
3.1. Soil measurements
Cone resistances measured before ®rst ploughing did not differ signi®cantly between treatments (data not shown). However, after ploughing and treatment application, cone resistances (Fig. 1) differed signi®-cantly between some treatments on all occasions. At Boghall in April 1995, the effect of heavy compaction on cone resistance was small. Subsequently this treat-ment was made more effective by increasing the weight of the tractor and applying the treatment both
before and after sowing the winter crops (Table 2). For the winter barley, this increased cone resistance throughout the topsoil, but particularly near the sur-face where the soil was wetter (270 g kgÿ1
) than at the spring treatment application (190 g kgÿ1
). Cone resis-tance under no-tillage was similar on both occasions indicating the persistence of soil compaction from spring 1995. At Boghall, on both occasions of treat-ment application and cone resistance measuretreat-ment, water contents deeper in the topsoil were similar, between 260 and 300 g kgÿ1
. At North Berwick, the top 30 cm of soil was considerably drier (150 g kgÿ1
) when the treatments were applied. Despite two passes of both compaction treatments before sowing, cone resistance (Fig. 1) in the top 8 cm soil layer was low, though below this depth it was considerably greater with a peak at 14±16 cm depth. However, part of this effect may have resulted from the dryness of the topsoil (170 g kgÿ1
) increasing strength.
Dry bulk densities (Fig. 2) at most depths were ranked in order of intensity of compaction, viz. hea-vy > light > zero. Density in the heahea-vy compaction treatment subsequently loosened to 10 cm varied more between depths than in the heavy compaction treat-ment. Topsoil maximum bulk densities measured at nearby sites (D.J. Campbell, 1999, unpublished data), using the Proctor test were, on average, 1.59 Mg mÿ3 for Macmerry series and 1.73 Mg mÿ3
for Kilmarnock series. The highest ®eld dry bulk densities at Boghall (Fig. 2) were more uniformly distributed through the topsoil and were closer to the theoretical (Proctor) maximum than at North Berwick. At Boghall, the highest ®eld bulk densities under heavy compaction were 89% of the theoretical (Proctor) maximum and under light compaction were 82% of the maximum.
Core relative diffusivities (Fig. 3) and air perme-abilities (Fig. 4) covered a wider range and showed larger treatment effects at Boghall than at North Berwick. The in situ diffusivities measured in Novem-ber 1995 at Boghall (Table 3) ranked the treatments similarly to the cores. In situ diffusivities were also strongly in¯uenced by changes in soil water content throughout the season (Table 3).
Under zero compaction, at 0±10 cm depth, most air-®lled porosities (Fig. 5) were about 0.1 m3mÿ3
winter which were established by the high rainfalls in September and October (Table 1). In the next season, the lower rainfalls maintained air-®lled porosities about 0.05 m3mÿ3
higher at North Berwick than at Boghall. However, the rainfall at North Berwick in June 1997 was suf®ciently high to reduce air-®lled porosities almost to winter levels.
At Boghall, particle-size distribution (Fig. 6) varied between experimental plots with differences in coarse sand and clay contents of up to 60 and 70 g kgÿ1
. The
soil texture became coarser from replicate 1 through to replicate 4. This may have contributed to differences between replicates in some measurements, notably, soil water content, cone resistance and crop yield. The soil in replicate 1 was wetter, on average, by 30± 60 g kgÿ1
compared with the other three replicates. However, when coarse sand or clay content was included as a covariate within analyses of variance of these properties, the effect never reached signi®-cance atP< 0.05.
Fig. 2. Dry bulk density at Boghall, October 1995 and at North Berwick, March 1997. Error bars represent the average range of the two replicates.
Table 3
In situ Freon diffusivity and soil moisture content at 0±5 cm depth at Boghall site
Compaction treatment Diffusivity (mm2sÿ1) Soil water content (g kgÿ1)
May 1995 November 1995
June 1996 May 1995 November 1995
June 1996
Zero 2.55 1.59 3.46 192 295 124
Light 1.91 1.11 4.19 219 294 119
Heavy 1.66 0.51 0.67 210 298 134
Heavy loosened to 100 mm 2.29 1.46 2.71 224 335 148
No-tillage 0.98 1.85 3.84 255 295 112
SEDa 0.73 1.97 55.4
Significance * ns ns * ns **
3.2. Crop performance and measurements
The growth of the spring barley and winter oil-seed rape appeared not to be impaired by the compaction treatments. Indeed, under zero compaction spring barley emergence was slightly delayed and winter oil-seed rape emergence was patchy. The oil-seed rape on the compacted plots overwintered better, resulting in a more uniform and greater crop cover than under zero compaction. However, subsequent growth compensated for variations in emergence and overwintering with spring barley yields, grain and oil seed rape nitrogen contents (Table 4) little affected by compaction. Under spring barley,
compac-tion appeared to give a small, non-signi®cant yield increase (Table 4). Under winter barley, the heavy compaction and no-tillage treatments reduced and delayed crop emergence. By November 1995, when the soil was very wet, plants in the heavily compacted treatment became small and upright, with yellowing of the outer leaves and the development of ¯eshy roots. These symptoms were attributed to transient water-logging damage as a result of the poor soil structure produced by the treatments. The poor, stunted growth and appearance of the heavily compacted and no-tilled plots persisted until harvest when both treat-ments gave signi®cantly lower yields (Table 4) than the others.
Fig. 3. Core gas diffusivity at Boghall, October 1995 and at North Berwick, March 1997. Relative diffusivity is soil gas diffusivity expressed as a fraction of diffusivity in free air. Error bars represent the average range of the two replicates.
Fig. 4. Air permeability at Boghall, October 1995 and at North Berwick, March 1997. Error bars represent the average range of the two replicates.
Table 4
Grain yield and grain nitrogen contents at Boghall (spring and winter barley) and oil-seed nitrogen contents at North Berwick
Compaction treatment Grain yield (Mg haÿ1) Nitrogen content (g kgÿ1)
Spring barley 1995
Winter barley 1996
Spring barley 1995
Winter barley 1996
Winter oil-seed rape 1997a
Zero 7.05 8.85 17.3 16.2 19.9
Light 7.19 8.92 17.6 16.9 20.8
Heavy 7.46 7.1 16.9 15.7 21.7
Heavy loosened to 100 mm 7.11 8.5 16.8 15.2 21.4
No-tillage 6.04 6.10 17.4 20.8 ±
SEDb 0.39 0.4 0.49 0.62 2
Significance ns *** ns *** ns
LSD0.87 LSD1.36
4. Discussion
4.1. Soil responses
Cone resistance was used as the main indicator of the effectiveness of the compaction treatments, mainly because it can indicate layers of differential compac-tion (Campbell and O'Sullivan, 1991). Although core dry bulk densities revealed that the compaction treat-ments were more effective at Boghall than at North Berwick, the pro®les of cone resistance proved to be more sensitive indicators of the soil physical changes
among the compaction treatments. Blunden et al. (1994) came to a similar conclusion when comparing traf®cked and untraf®cked soil.
At Boghall, heavy compaction gave particularly low gas diffusivities at 10±15 cm depth and low air permeabilities between 0 and 15 cm depth. Air perme-ability is very sensitive to the size of the largest air-®lled pores (Ball, 1981). This indicates that heavy compaction (under wet conditions) was particularly effective in reducing the size and continuity of the pores. The greater effectiveness of the treatments under the winter crop was also shown by the greater
differences in gas diffusivities between treatments under winter barley (November 1995 and June 1996) than under spring barley. Compaction of moist soils is important when air-®lled porosity is reduced to below the commonly quoted critical level for satisfac-tory aeration of 0.1 m3mÿ3
. This occurred after heavy compaction under winter barley. Etana and HaÊkansson (1996), who also found this effect, attributed it to the high mass of the compacting vehicle. However, Bak-ken et al. (1987) found little effect of the weight of the tractor (1800 kg vs 4800 kg) on soil properties in a similar experiment on compaction of seedbeds.
4.2. Crop responses
Our heavy compaction treatment only caused pro-blems with crop growth when applied to wet soil that remained wet during early growth. This resulted in air-®lled porosities of generally less than 0.1 m3mÿ3
, and cone resistances and bulk densities of nearly 2 MPa and 1.4 Mg mÿ3
throughout the topsoil. The latter are close to the critical values for barley growth of 2.5 MPa and 1.41 Mg mÿ3
on the same soil type (Ball and O'Sullivan, 1982). A critical value of bulk density of 1.48 Mg mÿ3
was quoted by Styk and Sochaj (1992) using a similar experimental approach to ours but on a Polish loess soil. Compaction of the top 15 cm soil layer appeared to be the most important in terms of restricting crop growth. Campbell et al. (1986)
attributed the restriction in crop growth and yield after heavy compaction to a combination of seed bed water-logging and high soil strength below sowing depth. They found soil strength, measured as cone resistance at 15 cm depth, correlated well with winter barley yield. Similarly, Dickson and Ritchie (1996) attributed much of the variability of barley yield associated with compaction to cone resistance between 0 and 27 cm depth.
The satisfactory winter barley growth and yield of the treatment of heavy compaction loosened to 10 cm (Table 4) revealed the importance of shallow tillage for ameliorating compaction. The top 10 cm of soil is particularly important in controlling gas and water movement and drainage (Ball et al., 1997b). Air permeability showed a marked minimum at this depth with values of <10mm2. In zero-tilled soils of the same
type, Ball and Robertson (1994) found values of <10mm2atÿ6 kPa soil matric potential. Air perme-ability is related to macropore size, number and con-tinuity so these low values may represent low macroporosity. Aura (1983) attributed reductions in wheat yields to a reduction in the volume of macro-pores >300mm diameter.
A zero level of compaction, however, may not be the optimum for crop growth under this ploughing regime. Most of the work on ®eld compaction in this part of Scotland has indicated that crop productivity and soil structural qualities were best preserved when
®eld traf®c was eliminated (Ball et al., 1997b). How-ever, Boone and Veen (1994) found that elimination of ®eld traf®c can leave the soil too loose so that unsa-turated hydraulic conductivity and root±soil contact are too small. They also found that light compaction after ploughing often decreased soil porosity suf®-ciently to approach the optimum for silage maize (Zea maysL.) in the Netherlands. This helps explain why our light compaction treatment gave about the same crop yield as zero compaction.
Under no-tillage, the decrease in spring and winter barley yields in comparison to the optimum treatment (light compaction) were attributed partly to compac-tion in the 10±15 cm layer and partly to the presence of grass weeds. Grass weeds are considered to be a major constraint to the use of zero-tillage in southern Scot-land (Ball and Davies, 1996).
5. Conclusions
Compaction near the soil surface, but particularly below the seedbed at 10±15 cm depth had an impor-tant in¯uence on crop growth in wet conditions. Much of the effect was associated with restricted aeration and waterlogging resulting from structural deteriora-tion restricting the size, continuity and volume of the porosity. However cone resistance and soil bulk den-sities were also close to critical levels for crop growth. Cone resistance is a good indicator of treatment differences in soil physical conditions relevant to crop growth particularly in combination with air perme-ability or air-®lled porosity. Loosening of heavy com-paction in the top 10 cm partly restored adequate conditions for crop growth. Maintenance of ®rm, but not overcompact conditions near the soil surface is particularly important for the establishment and overwintering of barley and oil-seed rape.
Acknowledgements
We are grateful to E.A.G. Robertson and W. Chap-man for technical help, to Dr G.W. Horgan, Biomathe-matics and Statistics, Scotland, for statistical advice and to A. Scott for advice on data presentation. The project was supported by the Scottish Of®ce Agricul-ture, Environment and Fisheries Department.
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