13 Gas Movement and Air-Filled Porosity Bruce C. Ball Scottish Agricultural College, Edinburgh, Scotland Keith A. Smith University of Edinburgh, Edinburgh, Scotland I. INTRODUCTION The movement of gases through the pore space of the soil is important in several respects. It plays a vital role in soil biological processes, in which the supply of oxygen, O 2 , to respiring roots and microorganisms from the atmosphere above the soil is balanced by the outward flow of carbon dioxide, CO 2 . Impedance of this gas exchange is frequently damaging to plant growth, due to deficiency in O 2 supply to the roots. Such conditions also give rise to emissions to the atmosphere of the microbially produced gases methane, CH 4 , and nitrous oxide, N 2 O, which, like CO 2 , contribute to the greenhouse effect (Houghton et al., 1996); conversely, part of the methane in the atmosphere is removed by diffusion into well aerated soils, where it is oxidized by microorganisms. Soil fumigation to control diseases of horticultural crops depends on movement of the fumigant in the vapor phase; emissions of methyl bromide, the most widely used fumigant, contribute to strato- spheric ozone depletion (as does N 2 O). In a very different context, emissions of the radioactive gas radon into buildings, following the decay of radium present in underlying soils, may be sufficient to constitute a health hazard in some localities. The mechanisms responsible for the transport of all these gases are diffu- sion, resulting in a net movement of gas from a zone of higher concentration to one of lower concentration, and mass flow, where the whole gas mixture moves in response to a pressure gradient. Most gas movement is by diffusion; mass flow is important only when pressure differences develop because of changes in barometric pressure, temperature, or soil water content. The movement occurs Copyright © 2000 Marcel Dekker, Inc. overwhelmingly in the air-filled pores, because diffusion in the gas phase is about four orders of magnitude greater than through water. As air-filled porosity varies with soil water content and soil structure, these factors have a major effect on the rate of gas movement in soils. To measure this movement we need to identify the boundary conditions (i.e., soil depth, compactness, and water content), and take into account factors that might cause errors, such as temperature, matric potential gradients, soil res- piration, and changes in absolute pressure at the soil surface. Establishing bound- ary conditions in the field is difficult; soil structure, bulk density, water content, and temperature can vary over only a few cm, and in particular may differ mark- edly between soil horizons. We consider here techniques for measuring diffusion and flow of gases and air-filled porosity in the laboratory and in the field, and the relationships of diffusion and flow to air-filled porosity. These methods include both direct measurements and indirect assessments from models. We also consider the applications of these techniques to the characterization of soil aeration and the impact on it of tillage and traffic, the study of trace gas exchange, and the inves- tigation of the movement of radon and fumigants. II. BASIC CONCEPTS A. Air-Filled Porosity Since gases move almost exclusively in the air-filled pores, measurement of po- rosity is vital to the understanding of gas movement in soil. Air-filled porosity is often used as an indicator of the likely aeration status of the soil and its ability to conduct and store gases. Air-filled porosity (e A ) is that fraction of the total soil volume that is occu- pied by air. Total porosity e T is the percentage of soil volume not occupied by solids. e A and e T are equal only in dry soils. e A is less than e T in moist soils because a fraction of the total porosity is occupied by soil water, this fraction being called the volumetric water content (u). Thus air-filled porosity is e ϭ e Ϫ u (1) AT e T may be calculated from the dry bulk density of the sample, r b , and from the particle density of the soil, r p , as follows (Hall et al., 1975): r b e ϭ 1 Ϫ (2) ͩͪ T r p e T in undisturbed soil cores may also be estimated as the volumetric water content at saturation (u S ). Air-filled porosity is most useful when determined at a given water potential. This can be readily achieved by equilibrating on tension tables as used for the determination of pore size distribution (see Chap. 3). Alternatively, 500 Ball and Smith Copyright © 2000 Marcel Dekker, Inc. samples may be taken at field moisture content. This is best taken as field capacity when e A is the ‘‘air capacity’’ and corresponds to the soil drainable porosity or macroporosity (Hall et al., 1975). Air-filled porosity may also be determined by use of an air pycnometer. This apparatus uses the principle of Boyle’s Law. The volume of air in a soil sample is measured by observing the resulting pressure when a gas at a measured volume and pressure expands into a larger volume, which includes the sample. This method excludes pores whose entrances are blocked by water films unless they are compressed by the change in pressure, when part of the volume is measured and is used to calculate this volume of trapped air (Stonestrom and Rubin, 1989). Air-filled porosity can be divided into three functional categories (arterial, marginal, and remote), using a simple model to interpret the results of a series of gas diffusion measurements in soils (Arah and Ball, 1994). Diffusion along the axis of a sample occurs through arterial pores, marginal pores do not contribute to axial diffusion, and remote pores are isolated from gas transport. Estimates of the three functional pore fractions were made by optimizing the fit between real and simulated data collected, using the technique described in Sec. IV.B. B. Gas Diffusion Gaseous molecules exhibit random movement as a result of their thermal energy. Where a gradient of partial pressure or concentration of a gas occurs, this random movement results in a net transfer, or flux, of gas along this gradient. This is the process of gas diffusion. In soils, gas diffusion is ‘‘counter-current’’; that is, a flux of one gas is matched by a flux of another gas in the opposite direction. Oxygen is required by root cells for the metabolism involved in root growth and nutrient and water uptake, and also by microorganisms. The resulting consumption of oxy- gen causes a fall in concentration and the consequent creation of a concentration gradient between the soil and the atmosphere above. This is responsible for a net diffusive flux of oxygen into the soil. Carbon dioxide respired by the roots and microorganisms increases the soil concentration above that of the atmosphere, and, correspondingly, an outward flux occurs. Diffusion is also involved in the transfer of water vapor and soil gases (e.g., methane, nitrous oxide) produced under anaerobic conditions. The diffusion coefficient of a particular gas is usually determined in the presence of another gas, commonly air. Kirkham and Powers (1972) cited a method for measuring the countercurrent diffusion coefficients of oxygen and ni- trogen. Pritchard and Currie (1982) described a method to measure the counter- current diffusion coefficients (D 0 ) of soil gases in air and gave values for carbon dioxide, nitrous oxide, ethylene, and ethane. The coefficient D 0 depends on abso- lute temperature and pressure, and can be calculated at the required values using the Boltzmann equation (see, e.g., Pritchard and Currie, 1982). Gas Movement and Air-Filled Porosity 501 Copyright © 2000 Marcel Dekker, Inc. In soil, gas diffusion coefficients are considerably less than in free air be- cause of obstruction by soil particles and water. Water effectively blocks gas dif- fusion, since the diffusion coefficients of the two gases of main interest in soils, oxygen and carbon dioxide, are nearly 10,000 times greater in air than in water (Grable, 1966). One-dimensional steady-state diffusion in soil is generally de- scribed by use of Fick’s first law: dC q ϭϪD (3) xS dx where q x is the mass transfer rate of gas per unit area (ML Ϫ2 T Ϫ1 ), D s is the effec- tive diffusion coefficient (L 2 T Ϫ1 ), C is the gas concentration (ML Ϫ3 ), and x is the distance along the line of transfer (L). D s is related to the diffusion coefficient in free air. Currie (1960) proposed the relationship D ϭ aeD (4) s0 where e is the air-filled cross-sectional area of soil (equal to the air-filled porosity) and a is a factor to account for the reduction in the effectiveness of e for diffusion because of deviations in pore direction from the overall direction of gas move- ment (tortuosity) and roughness of the pore surfaces. This aspect is considered in greater detail in Sec. V. Field soil is generally aggregated and contains roots; as pointed out by Currie (1961), it cannot be regarded as homogeneous with ran- domly distributed pores. Currie suggested that soil contains two pore phases, the large pores between structural units (the intercrumb pores) and the small pores within the units (intracrumb pores). The diffusion coefficient within crumbs is considerably smaller than that between crumbs because of the greater complexity of the pore space within crumbs. Thus diffusion in the soil profile as a whole consists of contributions from diffusion in crumbs, between crumbs, through the water films surrounding roots, and through the plant roots themselves (Glinski and Stepniewski, 1985). Uncertainty of boundary conditions makes the choice of appropriate diffu- sion solutions to Fick’s law uncertain. However, field methods can give useful indications of gaseous exchange. Laboratory methods of measurement of gas dif- fusion offer the advantages that boundary conditions can be chosen, controlled, and specified and that sample size and volume can be chosen to represent the soil layer(s) of interest. The main disadvantages are soil disturbance during sampling and the problems associated with relating measurements to field conditions. Both laboratory and field methods rely on solving Fick’s first law. However, the application of this law to gases is empirical (Jaynes and Rogowski, 1983), and only under special circumstances is the diffusion coefficient contained in Fick’s law a constant, independent of the mole fraction and the diffusive fluxes of other gases. In an atmosphere composed of O 2 ,CO 2 , and N 2 , and where the concentra- 502 Ball and Smith Copyright © 2000 Marcel Dekker, Inc. tion of N 2 is constant (a system similar to the soil atmosphere), variations of about 10% from the tracer value of the diffusion coefficient of O 2 and CO 2 are possible with variations in the mole fraction (Jaynes and Rogowski, 1983). Nevertheless, Fick’s law is almost universally used, and several solutions of it for different con- ditions were presented by Kirkham and Powers (1972). Techniques for measurement of diffusion in the gaseous state are discussed in Sec. IV.B. Methods of measurement of oxygen diffusion rate (ODR) that use platinum electrodes (Stolzy and Letey, 1964) and relate to the rate of supply of oxygen through water films, such as those that occur at a root surface, are also dealt with in Sec. IV.B. C. Mass Flow Mass flow is the movement of molecules in response to a pressure gradient. Thus mass flow can cause gas exchange between the atmosphere and the soil air only if there are changes in temperature, barometric pressure, wind, or water content (Henderson and Patrick, 1982). Mass flow is a less important mechanism than diffusion (Evans, 1965) and accounts for only a few percent of the normal gas exchange (Henderson and Patrick, 1982). However, air permeability has been shown recently to be more relevant to gas exchange through its contribution to nitrous oxide flux (Ball et al., 1997a), to soil venting (Stylianon and De Vantier, 1995), and to soil vapor extraction (Poulsen et al., 1998). Since mass flow, unlike diffusion, is sensitive to the size of individual pores, measurements of air perme- ability are more relevant to the characterization of soil structure. They provide an alternative to hydraulic conductivity measurements to describe soil structure, since air flow at low pressure differences causes negligible sample disturbance (Janse and Bolt, 1960). The flow of gases through soil is comparable to that of water, with certain restrictions. Darcy’s law applies if flow is laminar or viscous, as it is when the flow rates are relatively small (Janse and Bolt, 1960): Kdp q ϭ (5) ͩͪͩͪ h dx where q is flow rate, p is pressure, x is distance, K is gas permeability, and h is viscosity. In a tube of radius r and length L, flow rate can be calculated from Poiseuille’s law: 4 pr Dp q ϭ (6) 8hL where Dp is the difference in pressure between the ends of the tube. It follows that flow rate, hence permeability in soils, depends on the fourth power of the pore radius, whereas diffusion depends only on the square of the radius. Flow is less Gas Movement and Air-Filled Porosity 503 Copyright © 2000 Marcel Dekker, Inc. subject than diffusion to changes due to small temperature differences, though ambient temperature, pressure, and humidity affect flow by their influence on gas viscosity (Grover, 1955). Deviations from laminar or viscous flow occur when large pressure differences are applied to samples containing pores of large enough radius to give flow velocities sufficiently high for a Reynolds number of about 2000 or greater; under these conditions, flow becomes turbulent. (For an expla- nation of the Reynolds number concept, see a fluid mechanics or physics text, e.g., Denny, 1993). Alternatively, gas slippage (i.e., gas moving along pore surfaces) may occur in very small pores. As for gas diffusion, air flow is blocked by water- filled pores, so that air permeability decreases as soil water content increases. Field and laboratory techniques are available involving either steady-state or non-steady-state flow. Steady-state measurements of gas permeability are more generally applied than the non-steady-state variety; this is the reverse of the situ- ation relating to measurements of diffusion. Field techniques are often rather in- conclusive, because the variability of soil structure in the upper layers is large and is nonnormally distributed. Thus laboratory methods are preferable and are dis- cussed here. III. SAMPLING AND DIRECT MEASUREMENT A. Sampling The choice of soil sample size, the degree of replication, and the extent of pre- treatment depend on the objective of the experiment and will guide the choice of measurement technique. Where it is appropriate to use disturbed soil for a diffu- sion measurement, sufficient is needed to fill a small cell (say 20 –30 mm diam. and 20 –30 mm long). However, when minimally disturbed samples are required that are representative of field conditions, the choice of sample size and dimen- sions is difficult. Ideally, the sample volume should be equal to or greater than the representative elementary volume (REV), i.e., the smallest volume that con- tains a representative packing of particles that is repeated throughout the porous region (Youngs, 1983). Bouma (1983) recommended that a representative sample should contain at least 20 peds and that the REV should be increased as the tex- ture becomes finer and the structure becomes coarser. The REV classes suggested by Bouma for the sphere of influence relevant to individual plants are given in Table 1. Sampling techniques relevant to the laboratory determination of e A using minimally disturbed cores are discussed in Chap. 3 (see also McIntyre, 1974; Hall et al., 1975; Hodgson, 1976). Sample size and sampling intensity can be less for e A than for assessment of gas movement, because porosity and bulk density vary less than flow and diffusion properties within a soil horizon, since porosity does 504 Ball and Smith Copyright © 2000 Marcel Dekker, Inc. not depend on pore continuity. The Soil Survey of England and Wales recom- mended triplicate sampling of individual horizons (Hodgson, 1976). Guidance for the construction of sampling equipment and for collection and preparation of minimally disturbed samples is given by McIntyre (1974). Solu- tions for the diffusion coefficient and equations for the calculation of air perme- ability generally include sample volume and length. Thus these variables should be kept as constant as possible, to minimize error. The sample size for most reported diffusion measurements (100 –300 cm 3 ) is smaller than the typical REV of 10 3 cm 3 assumed for a soil structure made up of small peds (Table 1). Thus the use of larger samples, as reported by de Jong et al. (1983), is desirable, even though changing or measuring the temperature or water content of such large samples is difficult. In such cases, a field method of diffusion measurement (discussed below) may be more suitable. Many reported measurements of diffusion in minimally disturbed samples (Table 2) relate to the description of tillage treatments on specific soil layers, for example, those around a germinating seed. Where these layers are narrow and well-defined, samples can be relatively small, e.g., 35–75 mm deep (Bakker and Hidding, 1970; Ball et al., 1981). However, the great sensitivity of air permeability to pore diameter means that sample disturbance such as cracking or shrinking from the sides of the holder has a greater effect on this parameter than on mea- surement of diffusion. This sensitivity to pore and crack size also demands a greater requirement than for diffusion measurements for samples to be as large as the representative elementary volume (Bouma, 1983). The use of smaller samples can be justified if the largest channels, such as those produced by earthworms and cracks, are avoided (Ball, 1982; Groenevelt et al., 1984), provided that these are not required in the assessment. The variation of air permeability among samples is large, with standard er- rors of replicated data often greater than the means. Ball (1982) attributed this Gas Movement and Air-Filled Porosity 505 Table 1 Four Hypothetical Classes of Representative Elementary Volumes of Samples Relative to Soil Texture and Structure Class Texture Structure Hypothetical REV (cm 3 ) 1 Sandy No peds 10 2 2 Loamy, silty Small peds 10 3 3 Clayey Medium peds, continuous macropores 10 4 4 Clayey Large peds, continuous macropores 10 5 Source: Bouma (1983). Copyright © 2000 Marcel Dekker, Inc. Table 2 Relationships Between Relative Diffusivity and Air-Filled Porosity in Minimally Disturbed Soil Cores Soil texture Soil origin/ management /depth Soil moisture content Range of e A Relationship between D S /D 0 and e A Source Silt loam Compacted layer 4 cm deep at the surface of grass field Field content or equilibrated to 50 m bar tension 0 –0.4 0.27 e A Bruckler et al. (1989) Range Range of distinct structures Varied by changing soil water tension 0.05–0.35 ϳ0.5 e A Ayres et al. (1972) Sandy loams and silt loams Arable topsoils Field content 0.04 –0.3 0.85 e A 2 (non-puddled) 2.0 e A 2 (puddled) Bakker and Hidding (1970) Range Range: 5–140 cm depth Varied by changing soil water tension 0 –0.55 1.5–3 ϳe A Flu¨hler (1972) Clay loam Arable cultivation ex- periment, top 18 cm Field content 0.02–0.1 (scattered) 2.5 ϳe A Boone et al. (1976) Sand, silt, clay loams Arable cultivation ex- periment, top 20 cm Field content 0 –0.3 0.17 e A or e A 2 Richter and Grossge- bauer (1978) Silt loam, clay loam Arable cultivation ex- periment, top 20 cm Varied by changing soil water tension 0 –0.5 (for ZL) 2–2.5 e A (for CL) 2.1–4.5 e A Ball (1982) Loam Compaction experiment Varied by changing soil water tension 0 –0.28 0.28–0.5 2.75 3 e A 0.34 e A Glinski and Stepniewski (1985) Sandy clay loam Compaction and tillage experiments Field capacity 0.05–0.3 1.7–2.6 0.3–2.1 e A Ball et al. (1988) Sandy loams Arable and woodland topsoils Field content 0.05–0.5 1.9 0.69 e A Ball et al. (1997b) Sand Forest Varied by changing soil water tension 0.05–0.35 and 2.3 (e /e ) AAM 1.33 2.3 e (e /e ) TAT Moldrup et al. (1996) Sand, loamy sand, sandy loam Forest and arable Field content and varied by changing soil water tension 0 –0.5 0.45 e A 3 /e T 2 Poulsen et al. (1998) ZL: silt loam; CL: clay loam; e AM is maximum measured air-filled porosity, corresponding to maximum measured D S . e A , e T are defined in the text. Copyright © 2000 Marcel Dekker, Inc. variability to the great variation among replicates of the radius, length, and conti- nuity of the largest air-filled pores. Thus a relatively large number of samples, usually 15 to 30 per treatment, is required for adequate assessment of air perme- ability (Kirkham et al., 1958; Janse and Bolt, 1960; Ball, 1982). Significant scale dependence is also found with air permeability. Garberi et al. (1996) found that air permeability increased dramatically with sampling scale and that standard methods of air permeability assessment could underestimate advective transport of gas phase contaminants in soils. Sampling distributions of relative diffusivities and air permeabilities may be skewed rather than normal. In such cases, conventional parametric statistics do not strictly apply. Coefficients of variation of replicated relative diffusivities can be up to twice as great as those of the air-filled porosities measured on the same sample using conventional water release calculations (Ball, 1982). Thus a greater number of samples may be required than for, say, assessment of soil water release. Laboratory treatment of samples may also influence choice of size. If the matric potentials of samples have to be adjusted (e.g., if the intracrumb pores have to be blocked by water by wetting to field capacity to assess diffusion in the inter- crumb pores), the time for attainment of equilibrium throughout the sample in- creases with sample length. We commonly use samples 50 mm long for such ex- periments (Ball, 1982). If a soil core in a sample holder is dried in stages, and diffusion or air per- meability measurements are made at each stage (see, for example, Ball, 1982), then shrinkage may occur from the walls of the holder. In such cases the gap can be filled with paraffin wax and the sample diameter remeasured; or, as suggested by de Jong et al. (1983), samples can be cut from larger blocks and the nondiffus- ing surfaces coated with wax. B. Measurement of Air-Filled Porosity To measure air-filled porosity at a specific water potential, the samples require equilibration on tension tables, as discussed in Chap. 3 (see also Ball and Hunter, 1988). Samples can also be used for subsequent measurement of diffusion and air permeability. To minimize equilibration times, sample lengths no greater than 50 mm are recommended (Hall et al., 1975). Methods of measurement of r p and r b , necessary for assessment of e T and thence e A , are given by McIntyre (1974) and Vomocil (1965). In cores of known volume, r b is easily calculated from the weight of the soil core. In soils containing significant quantities of organic matter, the estimation of r b by liquid pycnometry may overestimate the soil particle den- sity because organic matter is destroyed in this technique. In such cases, a better estimate of e T may be the volumetric water content at saturation, u S . This may be determined after saturation either by capillary wetting and immersion or under vacuum (McIntyre, 1974). The first method may leave air trapped in the sample, Gas Movement and Air-Filled Porosity 507 Copyright © 2000 Marcel Dekker, Inc. thereby underestimating u S , and the second method may give structural break- down and slaking in soils that are structurally unstable, as trapped air is rapidly released from aggregates. Ball and Hunter (1988) found that in the laboratory u S agreed best with e T after saturation by capillary wetting and subsequent estimation of u S by weighing the sample immersed in water. The calculation steps for air- filled porosity are described by Carter and Ball (1993). Field assessment of air-filled porosity is best achieved by using the gamma probe to measure bulk density and then making one or more assessments of water content by time domain reflectometry, neutron moisture meter, or gravimetric measurement on samples taken with an auger (see Chaps. 1, 8). Separate measure- ment of particle density is required. C. Measurement of Gas Diffusion 1. Laboratory Methods Methods in current use involve non-steady-state diffusion where the concentration gradient and the flux of molecules change with time. The method recommended by Rolston (1986) involves measurement of the mutual diffusivity of argon (Ar) and nitrogen (N 2 ) but also applies to other gases of interest. Argon is used because it is relatively unreactive and has approximately the same values for gas diffusivity and solubility in water as O 2 . In this method (Fig. 1), Ar is used to displace most of the air from a diffusion vessel that is initially isolated. The initial concentration of N 2 in the vessel is C 0 . The diffusion vessel is slid under the soil sample and lines up with its open lower face, so that nitrogen in the air above the sample and the argon in the diffusion vessel can counterdiffuse through the soil. The change in N 2 concentration, C, in the diffusion vessel is monitored regularly by taking samples and analyzing them in a gas chromatograph. In this method, diffusion is in the unsteady state and is described by Fick’s second law as dc ddc e ϭ D (7) ͫͬ s dt dx dx Rolston (1986) solved this equation for e remaining constant in space and time, and for soil that was uniform with respect to diffusivity. His solution (with slight amendment to the symbols originally used) was 2 C Ϫ C 2h exp(ϪD a t/e) 0s1 ϭ (8) 22 C Ϫ C l(a ϩ h ) ϩ h 0s 1 where h ϭ e/(ae c ), e c is the air content of the chamber, a is the chamber height, l is the length of the soil sample, a 1 is the first root of a 1 l tan a 1 ϭ hl (values are tabulated by Rolston, 1986), and C s is the concentration of N 2 in the atmosphere. To calculate D s , ln[(C Ϫ C s )/(C 0 Ϫ C s )] is plotted vs. time, t. This is a straight 508 Ball and Smith Copyright © 2000 Marcel Dekker, Inc. [...]... M., and B C Ball 1994 A functional model of soil porosity used to interpret measurements of gas diffusion Eur J Soil Sci 45 : 135 –144 Armstrong, W., and E J Wright 1976 A polarographic assembly for multiple sampling of soil oxygen flux in the field J Appl Ecol 13 : 849 – 856 Copyright © 2000 Marcel Dekker, Inc 532 Ball and Smith Ayres, K W., R G Button, and E de Jong 1972 Soil morphology and soil physical. .. accompanied by studies of the effect on the balance of soil water content and/ or bulk density, which affect gas diffusivity in the soil Examples include temperate forest and agricultural land (Lessard et al., 1994; Ball et al., 1997b); humid tropical forest and pasture soils (Keller and Reiners, 1994); a temperate wetland (Melloh and Crill, 1996); and a tropical soil permeated by termite galleries (MacDonald... boundaries between topsoil and subsoil and in the upper subsoil after zero-tillage (20 –35 cm depth, Fig 9) The latter effect was attributed to compaction below plow depth and to the disruption of the continuity of channel-type macropores Air permeabilities have been found to be greater in general in plowed than in zero-tilled topsoil but similar or less after plowing than in zero-tilled soil below plowing... physical properties I Soil aeration Can J Soil Sci 52 : 311–321 Bakker, J W., and A P Hidding 1970 The influence of soil structure and air content on gas diffusion in soils Neth J Agric Sci 18 : 37– 48 Ball, B C 1982 Pore characteristics of soils from two cultivation experiments as shown by gas diffusivities and permeabilities and air-filled porosities J Soil Sci 32 : 483 – 498 Ball, B C., and R Hunter 1988... Measurement of soil gas diffusivity in situ Eur J Soil Sci 45 : 3 13 Ball, B C., W Harris, and J R Burford 1981 A laboratory method to measure gas diffusion in soil and other porous materials J Soil Sci 32 : 323 –333 Ball, B C., G W Horgan, H Clayton, and J P Parker 1997a Spatial variability of nitrous oxide fluxes and controlling soil and topographic properties J Environ Qual 26 : 139 9 –1409 Ball,... a soil under permanent grass: Seasonal and diurnal fluctuations as influenced by manuring and fertilisation Soil Biol Biochem 15 : 531–536 Clayton, H., I P McTaggart, J Parker, L Swan, and K A Smith 1997 Nitrous oxide emissions from fertilised grassland: A two-year study of the effects of N fertiliser form and environmental conditions Biol Fertil Soils 25 : 252 –260 Conen, F., and K A Smith 1998 A re-examination... Grant, C D., and P H Groenevelt 1993 Air permeability In: Soil Sampling and Methods of Analysis (M R Carter, ed.) Boca Raton, FL: Lewis, pp 645 – 650 Green, R D., and S J Fordham 1975 A field method for determining air permeability in soil In: Soil Physical Conditions and Crop Production Tech Bull No 29 London: HMSO, pp 273 –288 Groenvelt, P H., B D Kay, and C D Grant 1984 Physical assessment of soil with... waterlogged soils: Some improvements of techniques and their application to experiments using lysimeters J Soil Sci 34 : 271–285 Bogner, J., K Spokas, E Burton, R Sweeney, and V Corona 1995 Landfills as atmospheric methane sources and sinks Chemosphere 31 : 4119 – 4130 Boone, F R., S Slager, R Miedema, and R Eleveld 1976 Some influences of zero-tillage on the structure and stability of a fine-textured river... T Douglas, and M J Goss 1983 Gaseous diffusion in shrinking soils Soil Sci 136 : 10 –18 Denny, M W 1993 Air and Water Princeton, NJ: Princeton Univ Press Desjardins, R L 1985 Carbon dioxide budget of maize Agric For Meteor 36 : 29 – 41 Domby, C W., and H Kohnke 1956 The influence of soil crusts on gaseous diffusion Soil Sci Soc Am Proc 20 : 1–5 Dorr, H., and K O Munnich 1990 222 Rn flux and soil air concentration... soils Soil Sci 134 : 149 –156 Campbell, G S 1974 A simple method for determining unsaturated conductivity from moisture retention data Soil Sci 117 : 311–314 Campbell, D J., J W Dickson, B C Ball, and R Hunter 1986 Controlled seedbed traffic after ploughing or direct drilling under winter barley in Scotland Soil Till Res 8 : 3 –28 Carter, M R., and B C Ball 1993 Soil porosity In: Soil Sampling and Methods . particular may differ mark- edly between soil horizons. We consider here techniques for measuring diffusion and flow of gases and air-filled porosity in the laboratory and in the field, and the relationships. characterization of soil aeration and the impact on it of tillage and traffic, the study of trace gas exchange, and the inves- tigation of the movement of radon and fumigants. II. BASIC CONCEPTS A. Air-Filled. air-filled pores, measurement of po- rosity is vital to the understanding of gas movement in soil. Air-filled porosity is often used as an indicator of the likely aeration status of the soil and