Effects of soil water hysteresis and the direction of sampling on aeration and pore function in relation to soil compaction and tillage

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sou & I l l [age . I (esearcn E LS EV I E R Soil & Tillage Research 32 (1994) 51-60 Effects of soil water hysteresis and the direction of sampling on aeration and pore function in relation to soil compaction and tillage B.C. BalP'*, E.A.G. Robertson b aSoil Science Department, SAC, West Mains Road, Edinburgh, EH9 3JG, UK bScottish Centre of Agricultural Engineering, SAC, Bush Estate, Penicuik, Midlothian, EH26 OPH, UK Accepted 23 March 1994 Abst rac t Cores of intact soil were taken in vertical and horizontal directions in two tillage exper- iments from horizons subjected to compaction. Cores from below the plough layer were equilibrated at a range of matric potentials by drainage. Cores from near the soil surface were subjected to hysteresis by drainage to -2 kPa from saturation, further drainage to - 10 kPa and finally re-wetting to - 2 kPa. Air-filled porosity, relative diffusivity, diffu- sion time delay and air permeability were measured at each potential, and pore continuity and pore organisation indices were calculated. Diffusion time delay is a measure of the time taken for gas to diffuse across a core at the beginning of a measurement of diffusion. Hysteresis influenced both air-filled porosity and the relationships between air-filled porosity and gas diffusivity and air permeability. Soil aeration was generally more favour- able after wetting to - 2 kPa from - 10 kPa than after drainage to - 2 kPa from saturation. Hysteresis effects were greater in ploughed than in direct drilled soils. Direction of sam- pling influenced diffusion and flow properties, mainly in compact direct drilled soil from near the surface. These effects were partly attributed to the sampling method but gave evidence of greater vertical than horizontal orientation of macropores in ploughed soil and greater horizontal than vertical orientation of macropores in direct drilled soil. Keywords: Macropore orientation; Field cores; Gas diffusivity; Air permeability; Pore continuity * Corresponding author. 0167-1987/94/$07.00 1994 Elsevier Science B.V. All rights reserved SSDI 0167-1987 ( 94 ) 00405-4 52 B.C. Ball, E.A.G. Robertson / Soil & Tillage Research 32 (1994) 51- 60 1. Introduction In moist climates in the United Kingdom, many arable crops are sown in the autumn and spend the winter in soil close to field capacity. Soil aeration status may influence crop survival in such wet soil, particularly if the soil has also been compacted (Campbell et al., 1986; Ball and Robertson, 1990). In such soils, the presence of a sufficient volume of continuous macropores is particularly impor- tant for soil aeration (Blackwell et al., 1986; Ball and Robertson, 1994). These macropores may show some preferential orientation. For example, in compact direct drilled soils with lenticular or laminar structure, horizontally oriented planar voids may occur between structural units (Bullock et al., 1985) whereas worm holes penetrating the structural units may be vertically oriented (Ehlers, 1975 ). Thus pore type or continuity may be anisotropic. In order to detect such direc- tional effects, measurements need to be made both horizontally and vertically at the same depth, which can be achieved by taking core samples in different orientations. The matric potential of the soil water at field capacity is approximately -6 kPa in south-east Scotland (Duncan, 1979). During autumn and winter, the soil is subjected to wetting and drying cycles in a limited range around this potential. At any given matric potential within this range, the soil water content is likely to be influenced by hysteresis (Mualem, 1984). Since soil water hysteresis involves the differential blockage of pores by air during wetting and drying (Mualem, 1984),air-filled macropore continuity is also likely to be affected by hysteresis. Soil water hysteresis is thus likely to influence soil aeration. Core measurements of gas diffusivity and permeability provide useful indicators of soil aeration and, provided that air-filled porosity is known, macropore continuity indices can be assessed from measurements of diffusion (Ball et al., 1988 ) and air permeability (Blackwell et al., 1990). The objective of this study was to discover whether relative diffusivity, air permeability and macropore continuity are influenced by the direction of sam- piing and by soil water hysteresis in wet soils where soil structure has been influ- enced by tillage and compaction. 2. Theory Relative gas diffusivity and air permeability were measured in soil cores using the methods of Ball et al. ( 1981 ). Diffusivity was determined by measuring the diffusion of the tracer gas 85Kr between gas chambers attached to both ends of a core sample. In this procedure, 85Kr is injected into one chamber and the concen- tration change in both chambers is determined by regular counting offl radiation at a plastic scintillator-photomultiplier tube assembly. The counts are used in a non-linear regression technique to estimate diffusivity. Diffusivities are ex- pressed relative to those in free air adjusted to the temperature of measurement using a formula cited by Currie (1960). Diffusion time delay, the time before B.C. Ball, E.A.G. Robertson / Soil & Tillage Research 32 (1994) 51-60 53 first appearance of the gas in the second chamber, is estimated by extrapolating the concentrations in that chamber to zero. Air permeability was measured in the same cores using the same apparatus. From the relative diffusivity and air permeability measurements, we calculated indices of pore continuity and pore organisation. These indices relate porosity to gas conductivity. Pore continuity is determined by the overall length and degree of interconnection of the conducting paths (Ball et al., 1988 ). The pore continu- ity index is the relative diffusivity divided by the air-filled porosity (Ball et al., 1988). Pore organisation is the air permeability divided by macro-porosity (Blackwell et al., 1990). Pore organisation is effectively a measure of the conti- nuity of the macroporosity. To ensure that only the macroporosity was conduct- ing, Blackwell et al. (1990) recommended measurement of air permeability and air-filled porosity at - 6 kPa matric potential, when macropores of effective di- ameter > 0.05 mm are drained. Here we measured air-filled porosity at both - 2 and - 10 kPa when macropores of effective diameter > 0.15 mm and > 0.03 mm respectively were drained. The relationship between pore organisation and air- filled porosity is a characteristic which can be used to describe changes to the macroporosity of soils caused by management practices or biological activity (Blackwell et al., 1990). Although air permeability cannot be shown in the data points of the characteristic, isolines for air permeability can be shown, thereby allowing the value for each data point to be estimated. In the functional model of porosity proposed by Arah and Ball (1994) pores are assumed to be either arterial, peripheral or isolated. Diffusion through soil occurs primarily through arterial pores. Peripheral pores consist of a series of dead-ends and loops lying off the arterial pores. Arah and Ball (1994) suggest that relative diffusivity is influenced by the arterial pores whereas diffusion time delay is influenced by both arterial and peripheral porosity, and that isolated po- res have no influence on diffusion. The pore continuity index takes account of the influence of air-filled porosity; thus, in the model, the pore continuity index is determined by the arterial porosity. Changes in diffusion time delay indepen- dent of those in pore continuity index indicate changes in the content of periph- eral porosity. 3. Materials and methods Direct drilled and ploughed treatments were sampled in long-term arable field experiments of the former Letcombe Laboratory and of the Scottish Agricultural College (SAC). The Letcombe experiment was located in Berkshire, England on an Orthic Luvisol (FAO-UNESCO) of the Hamble series, a silt loam containing 16% clay (Jarvis et al., 1984). The SAC experiment was located in Midlothian, Scotland on a Gleysol (FAO-UNESCO) which is a Winton-Macmerry complex, a loam containing 18% clay (Ragg and Futty, 1967 ). The Letcombe experiment was described by Ellis et al. (1982) and the SAC experiment by Ball et al. ( 1989 ). Tests were made on intact cores removed from the field with minimal distur- 54 B.C Ball, E.A.G. Robertson ~Soil & Tillage Research 32 (1994) 51-60 bance. Cores were retained within sampling cylinders 50 mm long and 73 mm diameter during subsequent treatments and measurements. The cores were taken 2 to 3 months after drilling of winter cereals and stored, where necessary, at 4 C before testing. On the Luvisol, the increase in bulk density attributable to cultivation was greatest at just below the plough depth of 200-250 mm (Douglas et al., 1986). Five samples per tillage treatment were taken from just below this depth, hori- zontally from 240-310 mm and vertically from 250-300 mm. Horizontal sam- ples were taken parallel to the crop rows. Samples were equilibrated to - 2, - 10, and -20 kPa matric potential on sand suction tables and to - 100 kPa on pres- sure plates. The equilibration times for the successively decreasing potentials were approximately 6,10, 14 and 21 days. No hysteresis treatment was applied to these samples. Two samples per tillage treatment were also taken vertically from 30- 80 mm depth. These cores were tested for soil water hysteresis as described below for the Gleysol. On the Gleysol, a direct drilled plot was chosen where crop growth appeared to suffer from adverse soil physical conditions, particularly in the season prior to sampling (Ball and Robertson, 1994). In the adjacent ploughed plot growth was normal. Three samples per plot were taken horizontally from 35-105 mm depth and vertically from 45-95 mm depth. In testing for the existence of hysteresis, cores were drained from saturation by equilibration to - 2 kPa matric potential. The cores were then equilibrated to - 10 kPa, slightly drier than field capacity. The cores were then rewetted by equilibration on the tension table at -2 kPa. Re-wetting was slower than drainage, so that a longer period ( 14 days) was given for equilibration. The target potential of -2 kPa was chosen rather than field capacity to ensure that only the larger macropores ( > 0.15 mm effective diame- ter) were available to conduct air and that soil aeration was likely to be limited by inadequate porosity. The influence of soil water hysteresis was determined from a comparison of measured values of air-filled porosity, relative diffusivity and air permeability at - 2 kPa after re-wetting from - 10 kPa, with values at - 2 kPa after initial drainage from saturation. Data were analysed using analysis of variance. In cores taken from near the soil surface, differences in measured values of air-filled porosity, relative diffusivity and air permeability between the ploughed and direct drilled treatments were large enough for the treatments to be considered as separate populations. In the Luvisol cores taken from below plough depth, measurements differed little be- tween ploughing and direct drilling and thus were averaged over cultivation treat- ments. In the analyses, the effects of direction of sampling and matric potential were treated as two factors. 4. Results The distributions of air-filled porosities within the sample populations were normal. Some relative diffusivity and most of the air permeability populations B.C. Ball, E.A. G. Robertson ~Soil & Tillage Research 32 (1994) 51-60 55 were log-normally distributed. For Luvisol cores taken from below the plough depth, the direction of sampling did not significantly influence the relationships between relative diffusivity or air permeability and air-filled porosity during drainage (Fig. 1 ). However, relative diffusivity and air permeability at -2 and - 10 kPa were both consistently greater in the vertical than in the horizontal di- rection in spite of air-filled porosity being greater in the horizontally oriented cores. In cores taken from near the soil surface, large and highly significant differences were found in measured properties between the direct drilled and ploughed soils (Figs. 2 and 3) indicating a marked difference in structure, associated with dif- ferences in bulk density. Means for the deeper samples of the Luvisol at - 2 kPa matric potential only are also included in Fig. 3(A). In direct drilled soil, air- filled porosities, relative diffusivities and air permeabilities were significantly greater in horizontal than in vertical samples. In ploughed soil, relative diffusiv- ity was the only property to differ significantly between vertical and horizontal samples. The influence of direction of sampling on air permeability in ploughed 0.02 0.016 >. ~ 0.012 o.oo8 0.004 0 0 -I00 -100 J _ 2 /20d/ ,~ 10 i-~ 0.05 0.1 0.15 012 Ai r - f i l l ed porosity {m3/m 3) ver t i ca l o horizontal 0'.25 100 10 1 0.1 2 _ ~ -100 i i L i 0.05 0.] 0.15 0.2 i 0.25 Ai r - f i l l ed porosity (m3/m 3) Fig. 1. Effect of sampling direction and water release on relative diffusivity, air permeability and air- filled porosity in Luvisol cores from below plough depth, sampled vertically or horizontally. The bars represent standard errors for the sampling direction X matric potential interaction means. In this and in subsequent figures, the numbers beside data points refer to the matric potential to which the cores were equilibrated. 56 B.C. Ball, E.A.G. Robertson /Soil & Tillage Research 32 (1994) 51-60 >, > n - (A.) 0.025 0.02 0.015 0.01 0.005 0 -0.005 0 Direct drilled Ploughed j / t / ' ' 11-2 . . . . re-wetted - - drained r . /~-1o -2"2 ~ vertical o horizontal i i i r 0105 0.1 01.15 012 0.25 Air-filled porosity (m3/m 3) >, ,g :1= :'5 n- (B.) 0.05 0.04 0.03 0.02 0.01 0 -0.01 Direct drilled Ploughed -2 / I I _ 2 ~ I0 i oio5 o.1 o115 Air-filled porosity (m3/m a) 012 0125 Fig. 2. Effect of sampling direction and hysteresis on mean relative diffusivity and air-filled porosity in cores taken from near the surface of the Luvisol (A) or the Gleysol (B). The bars represent average values of the range (A) or standard errors (B). soil (Fig. 3 (B)) was similar to that reported by Janse and Bolt (1960) who found that vertical air permeabilities were double the horizontal values and that air- filled porosity was greater in the vertical samples. They attributed these differ- ences, in part, to greater compaction of the horizontal samples during sampling. Such compaction is unlikely to have occurred during the sampling of the horizon- tal direct drilled cores since these were relatively compact. Direction of sampling had little effect on diffusion time delay independent of pore continuity index (Fig. 4). The differences associated with drainage and wetting are assumed to be inde- pendent of swelling and shrinkage. Shrinkage, defined as soil movement away from the core sleeve, was not visible during drainage. Re-wetting of all soils re- suited in only a partial return of air-filled porosity, relative diffusivity, air perme- ability and diffusion time delay (Figs. 2-4) to values measured after initial drainage, revealing the influence of soil water hysteresis. However, none of these differences was significant. Air permeability and relative diffusivity returned closer to their original values after re-wetting in direct drilled soil than in ploughed B.C. Ball, E.A.G. Robertson / Soil & Tillage Research 32 (1994) 51-60 57 o~ . (A.} 10000 1000 100 10 1 0 Direct drilled Ploughed t i Ka = 1000g m2 K a 100p.m 2 _ j - - . = ! ~ Ka = 10pm 2 2 K a = l~tm 2 0.115 ' J 0.05 0.1 0.2 0.25 Air-filled porosity (m3/m 3) . . . . re-wetted - - drained vertical o horizontal E: 8~ e o (&) 100000 Direct drilled Ploughed 10000 Ka = 1000prn 2 1000 ~ - 2 o" '~ , K a = lO0~m 2 100 m _~- ' : ;~=~-~ ~0 0.'05 0'., 01~5 o'.2 01~5 Air-filled porosity (m3/m 3) Fig. 3. Effect of sampling direction and hysteresis on pore organisation versus air-filled porosity in cores taken from near the surface of the Luvisol (A) or the Gleysol (B). Pore organisation is air permeability divided by air-filled porosity. In (A), means for the deeper samples are shown as trian- gles. Air permeability (K,) isolines are shown. The bars represent average values of the range (A) or standard errors (B). 100000 10000 1000 100 10 1 E rr,.,ct d P,oug.ed - ?o '~~o . . . . re-wetted - - drained vertical o horizontal o.~5 oi~ o.~s o'.2 Pore continuity index Fig. 4. Effect of sampling direction and hysteresis on diffusion time delay and pore continuity index in cores taken from near the surface of the Gleysol. The bars represent standard errors. 58 B.C. Ball, E.A.G. Robertson / Soil & Tillage Research 32 (1994) 51-60 soil. Indeed, air permeability in many ploughed samples taken vertically in- creased on re-wetting. This may have been caused by evaporation of water from the surface of the samples during the relatively long re-wetting period. Pore con- tinuity indices in the ploughed soil and diffusion time delays in the direct drilled soil were markedly influenced by soil water hysteresis (Fig. 4). 5. Discussion 5.1. Anisotropy Since air-filled porosity is a scalar quantity, its value for a fixed soil volume element must be independent of sampling direction. Here, although samples were centred on the same depth, the horizontal samples covered a greater depth range (73 mm) than the vertical samples (50 mm) owing to the use of cylindrical co- res. In addition, the circular vertical cross-section of the horizontal samples caused the sampled volume to change with depth within the sample whereas this volume was uniform within the vertically oriented samples. These factors may help to explain the differences in air-filled porosity apparently associated with sampling direction. The likely pore structural properties can be interpreted from the relative posi- tions of the data points in Fig. 3 (Blackwell et al., 1990) which show that air permeability is mainly determined by air-filled porosity. Pore continuity index (Fig. 4) differed between directions of sampling, particularly at - 2 kPa, where only the largest macropores (those with neck diameters > 150 am) are drained. Thus, near the surface, more macropores were oriented vertically than horizon- tally in the ploughed soil. In the direct drilled soil the reverse was true, this obser- vation being supported in another recent study (Ball and Robertson, 1994). 5.2. Hysteresis Soil water hysteresis influenced the relationships between air-filled porosity, relative diffusivity and air permeability and this contrasts with the findings of Shearer et al. ( 1966 ). They found hysteresis to affect gas diffusivities as a result of changes in air-filled porosities but not the relationship between diffusivity and air-filled porosity. They attributed this to the type of medium (cemented sand) and to the lack of observations at air-filled porosities < 0.15 m3/m 3. Hysteresis had a greater influence on diffusion and air flow in ploughed than in direct drilled soil and this may relate to differences in pore structure and func- tion. The opening of conducting channels by drainage from -2 to - l0 kPa is indicated from the changes in pore organisation and air-filled porosity and from the increases in pore continuity index. These changes were reversed to a greater extent in the direct drilled than in the ploughed Gleysol samples upon re-wetting. Interconnected macropores of widely varying diameter which remain drained are indicated by the persistence of pore continuity in the ploughed soil. However, in B.C Ball, E.A.G. Robertson /Soil & Tillage Research 32 (1994) 51-60 59 the direct drilled soil, macropores of a more uniform diameter would be indi- cated. Diffusion time delay was also smaller after rewetting than after drainage to - 2 kPa which suggests that there were fewer peripheral pores after re-wetting than after drainage owing to the incomplete re-entry of water (Arah and Ball, 1994). 6. Conclusions Soil water hysteresis in wet, undisturbed cores influenced air-filled porosity, relative diffusivity and air permeability. Hysteresis also affected the relationships between air-filled porosity and gas diffusion and air permeability. These effects were more marked in ploughed than in direct drilled soils indicating differences in pore structure and function. The direction of sampling influenced relative dif- fusivity and air permeability in samples from near the surface, particularly where these were compact. Although the effects were fairly small, there was some evi- dence that more macropores were oriented vertically than horizontally in the ploughed soil and this was the reverse of the situation in the direct drilled soil. References Arah, J.R.M. and Ball, B.C., 1994. A functional model of soil porosity based on measurements of gas diffusion. Eur. J. Soil Sci., 45 (in press). Ball, B.C. and Robertson, E.A.G., 1990. Straw incorporation and tillage methods: straw decomposi- tion, denitrification and growth and yield of winter barley. J. Agric. Eng. ReS., 46: 223-243. Ball, B.C. and Robertson, E.A.G., 1994. Soil structural and transport properties associated with poor growth of oil seed rape in soil direct drilled when wet. Soil Tillage Res., 31: 119-133. Ball, B.C., Harris, W. and Burford, J.R., 1981. A laboratory method to measure gas diffusion and flow in soil and other porous materials. J. Soil Sci., 32: 323-333. Ball, B.C., O'Sullivan, M.F. and Hunter, R., 1988. Gas diffusion, fluid flow and derived pore conti- nuity indices in relation to vehicle traffic and tillage. J. Soil Sci., 39: 327-339. Ball, B.C., Lang, R.W., O'Sullivan, M.F. and Franklin, M.F., 1989. Cultivation and nitrogen require- ments for continuous winter barley on a gleysol and a cambisol. Soil Tillage Res., 13: 333-352. Blackwell, P.S., Graham, J.P., Armstrong, J.V., Ward, M.A., Howse, K.R., Dawson, C.J. and Butler, A.R., 1986. Compaction of a silt loam soil by wheeled agricultural vehicles. 1. Effects upon soil conditions. Soil Tillage Res., 7: 97-116. BlackweU, P.S., Ringrose-Voase, A.J., Jayawardane, N.S., Olsson, K.A., McKenzie, D.C. and Mason, W.K., 1990. The use of air-filled porosity and intrinsic permeability to characterise macropore structure and saturated hydraulic conductivity of clay soils. J. Soil Sci., 41:215-228. Bullock, P., Newman, A.C.D. and Thomasson, A.J., 1985. Porosity aspects of the regeneration of soil structure after compaction. Soil Tillage Res., 5: 325-341. Campbell, D.J., Dickson, J.W., Ball, B.C. and Hunter, R., 1986. Controlled seedbed traffic after ploughing or direct drilling under winter barley in Scotland, 1980-1984. Soil Tillage Res., 8: 3-28. Currie, J.A., 1960. Gaseous diffusion in porous media. Part 1. A non-steady state method. Brit. J. Appl. Phys., 11: 314-317. Douglas, J.T., Jarvis, M.G., Howse, K.R. and Goss, M.J., 1986. Structure of a silty soil in relation to management. J. Soil Sci., 37:137-151. 60 B.C Ball, E.A.G. Robertson ~Soil & Tillage Research 32 (1994) 51-60 Duncan, N.A., 1979. The moisture regime of six soil series of the central lowlands of Scotland. J. Soil Sci., 30: 215-223. Ehlers, W., 1975. Observations on earthworm channels and infiltration on tilled and untilled loess soil. Soil Sci., 199: 242-249. Ellis, F.B., Christian, D.G. and Cannell, R.Q., 1982. Direct drilling, shallow tine-cultivation and ploughing on a silt loam soil, 1974-1980. Soil Tillage Res., 2." 115-130. Janse, A.R.P. and Bolt, G.H., 1960. The determination of the air permeability of soils. Neth. J. Agric. Sci., 8: 124-131. Jarvis, M.G., Allen, R.H., Fordham, S.J., Hazelden, J., Moffat, A.J. and Sturdy, R.G., 1984. Soils and their use in South East England. Bulletin of the Soil Survey of England and Wales, Harpenden, UK. Mualem, Y., 1984. A modified dependent domain theory of hysteresis. Soil Sci., 137:283-29 I. Ragg, J.M and Futty, D.W., 1967. The soils of the country round Haddington and Eyemouth. Her Majesty's Stationery Office, Edinburgh, 310 pp. Shearer, R.C., Millington, R.J. and Quirk, J.P., 1966. Oxygen diffusion through sands in relation to capillary hysteresis: 2. Quasi-steady state diffusion of oxygen through partly saturated sands. Soil Sci., I01: 432-436.

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