Soil and Environmental Analysis: Physical Methods - Chapter 13 ppt

Most gas movement is by diffusion; massflow is important only when pressure differences develop because of changes inbarometric pressure, temperature, or soil water content.. As air-filled

of the microbially produced gases methane, CH4, and nitrous oxide, N2O, which,like CO2, contribute to the greenhouse effect (Houghton et al., 1996); conversely,part of the methane in the atmosphere is removed by diffusion into well aeratedsoils, 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 N2O) In a very different context, emissions ofthe radioactive gas radon into buildings, following the decay of radium present inunderlying 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; massflow is important only when pressure differences develop because of changes inbarometric pressure, temperature, or soil water content The movement occurs

overwhelmingly in the air-filled pores, because diffusion in the gas phase is aboutfour orders of magnitude greater than through water As air-filled porosity varieswith soil water content and soil structure, these factors have a major effect on therate 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 factorsthat 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 diffusionand flow of gases and air-filled porosity in the laboratory and in the field, and therelationships of diffusion and flow to air-filled porosity These methods includeboth direct measurements and indirect assessments from models We also considerthe applications of these techniques to the characterization of soil aeration and theimpact on it of tillage and traffic, the study of trace gas exchange, and the inves-tigation of the movement of radon and fumigants

A Air-Filled Porosity

Since gases move almost exclusively in the air-filled pores, measurement of rosity is vital to the understanding of gas movement in soil Air-filled porosity isoften used as an indicator of the likely aeration status of the soil and its ability toconduct and store gases

po-Air-filled porosity (eA) is that fraction of the total soil volume that is pied by air Total porosityeTis the percentage of soil volume not occupied bysolids eAand eT are equal only in dry soils eAis less than eT in moist soilsbecause a fraction of the total porosity is occupied by soil water, this fractionbeing called the volumetric water content (u) Thus air-filled porosity is

eTin undisturbed soil cores may also be estimated as the volumetric water content

at saturation (uS) Air-filled porosity is most useful when determined at a givenwater potential This can be readily achieved by equilibrating on tension tables asused for the determination of pore size distribution (seeChap 3) Alternatively,

samples may be taken at field moisture content This is best taken as field capacitywheneAis the ‘‘air capacity’’ and corresponds to the soil drainable porosity ormacroporosity (Hall et al., 1975).

Air-filled porosity may also be determined by use of an air pycnometer Thisapparatus uses the principle of Boyle’s Law The volume of air in a soil sample ismeasured by observing the resulting pressure when a gas at a measured volumeand pressure expands into a larger volume, which includes the sample Thismethod excludes pores whose entrances are blocked by water films unless theyare compressed by the change in pressure, when part of the volume is measuredand 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 ofgas diffusion measurements in soils (Arah and Ball, 1994) Diffusion along theaxis of a sample occurs through arterial pores, marginal pores do not contribute toaxial diffusion, and remote pores are isolated from gas transport Estimates of thethree functional pore fractions were made by optimizing the fit between real andsimulated data collected, using the technique described in Sec IV.B

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 nutrientand water uptake, and also by microorganisms The resulting consumption of oxy-gen causes a fall in concentration and the consequent creation of a concentrationgradient between the soil and the atmosphere above This is responsible for a netdiffusive flux of oxygen into the soil Carbon dioxide respired by the roots andmicroorganisms increases the soil concentration above that of the atmosphere,and, correspondingly, an outward flux occurs Diffusion is also involved in thetransfer of water vapor and soil gases (e.g., methane, nitrous oxide) producedunder anaerobic conditions

The diffusion coefficient of a particular gas is usually determined in thepresence of another gas, commonly air Kirkham and Powers (1972) cited amethod 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 (D0) of soil gases in air and gave values for carbon

dioxide, nitrous oxide, ethylene, and ethane The coefficient D0depends on lute temperature and pressure, and can be calculated at the required values usingthe Boltzmann equation (see, e.g., Pritchard and Currie, 1982)

In soil, gas diffusion coefficients are considerably less than in free air 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:

be-dC

dx where qxis the mass transfer rate of gas per unit area (ML⫺2T⫺1), Dsis the effec-tive diffusion coefficient (L2T⫺1), C is the gas concentration (ML⫺3), and x is the distance along the line of transfer (L) Dsis related to the diffusion coefficient infree air

Currie (1960) proposed the relationship

wheree 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 ofe for diffusionbecause of deviations in pore direction from the overall direction of gas move-ment (tortuosity) and roughness of the pore surfaces This aspect is considered ingreater detail in Sec V Field soil is generally aggregated and contains roots; aspointed out by Currie (1961), it cannot be regarded as homogeneous with ran-domly distributed pores Currie suggested that soil contains two pore phases, thelarge pores between structural units (the intercrumb pores) and the small poreswithin the units (intracrumb pores) The diffusion coefficient within crumbs isconsiderably smaller than that between crumbs because of the greater complexity

of the pore space within crumbs Thus diffusion in the soil profile as a wholeconsists of contributions from diffusion in crumbs, between crumbs, through thewater films surrounding roots, and through the plant roots themselves (Glinski andStepniewski, 1985)

Uncertainty of boundary conditions makes the choice of appropriate sion solutions to Fick’s law uncertain However, field methods can give usefulindications 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 soillayer(s) of interest The main disadvantages are soil disturbance during samplingand 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), andonly under special circumstances is the diffusion coefficient contained in Fick’slaw a constant, independent of the mole fraction and the diffusive fluxes of othergases In an atmosphere composed of O2, CO2, and N2, and where the concentra-

diffu-tion of N2is constant (a system similar to the soil atmosphere), variations of about10% from the tracer value of the diffusion coefficient of O2and CO2are possiblewith 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 useplatinum electrodes (Stolzy and Letey, 1964) and relate to the rate of supply ofoxygen through water films, such as those that occur at a root surface, are alsodealt with in Sec IV.B

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 certainrestrictions Darcy’s law applies if flow is laminar or viscous, as it is when theflow rates are relatively small (Janse and Bolt, 1960):

where q is flow rate, p is pressure, x is distance, K is gas permeability, andh is

viscosity In a tube of radius r and length L, flow rate can be calculated from

whereDp 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 poreradius, whereas diffusion depends only on the square of the radius Flow is less

subject than diffusion to changes due to small temperature differences, thoughambient temperature, pressure, and humidity affect flow by their influence on gasviscosity (Grover, 1955) Deviations from laminar or viscous flow occur whenlarge pressure differences are applied to samples containing pores of large enoughradius to give flow velocities sufficiently high for a Reynolds number of about

2000 or greater; under these conditions, flow becomes turbulent (For an 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

expla-or non-steady-state flow Steady-state measurements of gas permeability are mexpla-oregenerally 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 cussed here

The choice of soil sample size, the degree of replication, and the extent of treatment depend on the objective of the experiment and will guide the choice ofmeasurement 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 requiredthat 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 thanthe representative elementary volume (REV), i.e., the smallest volume that con-tains a representative packing of particles that is repeated throughout the porousregion (Youngs, 1983) Bouma (1983) recommended that a representative sampleshould 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

pre-by Bouma for the sphere of influence relevant to individual plants are given inTable 1

Sampling techniques relevant to the laboratory determination ofeAusingminimally disturbed cores are discussed inChap 3(see also McIntyre, 1974; Hall

et al., 1975; Hodgson, 1976) Sample size and sampling intensity can be less for

eAthan for assessment of gas movement, because porosity and bulk density varyless than flow and diffusion properties within a soil horizon, since porosity does

not depend on pore continuity The Soil Survey of England and Wales mended triplicate sampling of individual horizons (Hodgson, 1976).

recom-Guidance for the construction of sampling equipment and for collection andpreparation 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 cm3)

is smaller than the typical REV of 103cm3assumed 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 orwater content of such large samples is difficult In such cases, a field method ofdiffusion 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, forexample, those around a germinating seed Where these layers are narrow andwell-defined, samples can be relatively small, e.g., 35 –75 mm deep (Bakker andHidding, 1970; Ball et al., 1981) However, the great sensitivity of air permeability

to pore diameter means that sample disturbance such as cracking or shrinkingfrom 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 agreater requirement than for diffusion measurements for samples to be as large asthe representative elementary volume (Bouma, 1983) The use of smaller samplescan be justified if the largest channels, such as those produced by earthworms andcracks, are avoided (Ball, 1982; Groenevelt et al., 1984), provided that these arenot required in the assessment

The variation of air permeability among samples is large, with standard rors of replicated data often greater than the means Ball (1982) attributed this

Table 1 Four Hypothetical Classes of Representative Elementary

Volumes of Samples Relative to Soil Texture and Structure

HypotheticalREV (cm3)

deep at the surface ofgrass field

Field content or equilibrated

tension

0 – 0.280.28 – 0.5

2.75

3eA0.34eA

Glinski and Stepniewski(1985)

Sandy clay loam Compaction and tillage

experiments

Field capacity 0.05 – 0.3 0.3–2.1eA 1.7–2.6 Ball et al (1988)Sandy loams Arable and woodland

topsoils

Field content 0.05 – 0.5 0.69eA 1.9 Ball et al (1997b)

sandy loam

Forest and arable Field content and varied by

changing soil water tension

0 – 0.5 0.45eA3/eT2 Poulsen et al (1998)

ZL: silt loam; CL: clay loam; eAMis maximum measured air-filled porosity, corresponding to maximum measured DS eA, eTare defined in the text.

variability to the great variation among replicates of the radius, length, and 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 scaledependence is also found with air permeability Garberi et al (1996) found thatair permeability increased dramatically with sampling scale and that standardmethods of air permeability assessment could underestimate advective transport

conti-of gas phase contaminants in soils

Sampling distributions of relative diffusivities and air permeabilities may beskewed rather than normal In such cases, conventional parametric statistics donot 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 samesample using conventional water release calculations (Ball, 1982) Thus a greaternumber of samples may be required than for, say, assessment of soil water release.Laboratory treatment of samples may also influence choice of size If thematric 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 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)

inter-If a soil core in a sample holder is dried in stages, and diffusion or air 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

per-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 ing surfaces coated with wax

nondiffus-B Measurement of Air-Filled Porosity

To measure air-filled porosity at a specific water potential, the samples requireequilibration on tension tables, as discussed inChap 3(see also Ball and Hunter,1988) Samples can also be used for subsequent measurement of diffusion and airpermeability To minimize equilibration times, sample lengths no greater than

50 mm are recommended (Hall et al., 1975) Methods of measurement ofrpand

rb, necessary for assessment ofeTand thenceeA, are given by McIntyre (1974)and Vomocil (1965) In cores of known volume,rbis easily calculated from theweight of the soil core In soils containing significant quantities of organic matter,the estimation ofrbby liquid pycnometry may overestimate the soil particle den-sity because organic matter is destroyed in this technique In such cases, a betterestimate ofeTmay be the volumetric water content at saturation,uS This may bedetermined after saturation either by capillary wetting and immersion or undervacuum (McIntyre, 1974) The first method may leave air trapped in the sample,

thereby underestimatinguS, and the second method may give structural down and slaking in soils that are structurally unstable, as trapped air is rapidlyreleased from aggregates Ball and Hunter (1988) found that in the laboratoryuSagreed best witheTafter saturation by capillary wetting and subsequent estimation

break-of uSby weighing the sample immersed in water The calculation steps for filled porosity are described by Carter and Ball (1993)

air-Field assessment of air-filled porosity is best achieved by using the gammaprobe to measure bulk density and then making one or more assessments of watercontent by time domain reflectometry, neutron moisture meter, or gravimetricmeasurement on samples taken with an auger (seeChaps 1,8) Separate measure-ment of particle density is required

C Measurement of Gas Diffusion

of the air from a diffusion vessel that is initially isolated The initial concentration

of N2in the vessel is C0 The diffusion vessel is slid under the soil sample andlines up with its open lower face, so that nitrogen in the air above the sample andthe argon in the diffusion vessel can counterdiffuse through the soil The change

in N2concentration, 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

2

C ⫺ C0 2h exp(⫺D a t/e)s 1

C0 ⫺ Cs l(a1 ⫹ h ) ⫹ h

where h ⫽ e/(aec),ecis the air content of the chamber, a is the chamber height,l

is the length of the soil sample,a1is the first root ofa1l tan a1⫽ hl (values are tabulated by Rolston, 1986), and Csis the concentration of N2in the atmosphere

To calculate Ds, ln[(C ⫺ Cs)/(C0⫺ Cs)] is plotted vs time, t This is a straight

line with slope⫺D a /es 2 1 for sufficiently large t A problem with this system is the

need to disturb and change the diffusion system by the withdrawal of samples forgas analysis

Several variants on this method have been produced, monitoring structively the changes in gas concentration (commonly O2) in the diffusion ves-sel In that of Schjønning (1985a), the chamber is initially filled with N2 andcontains an electrode to monitor O2concentration Up to 12 samples are run si-multaneously, with automatic data-logging In undisturbed core samples the error

nonde-in the determnonde-ination of gas diffusivity due to consumption of O2 is generally

⬍0.5% but may be greater if the soil is recently disturbed or amended with organicmatter subject to rapid microbial turnover (P Schjønning, pers comm., 1999).One problem with such a system, identified by Rust et al (1956), is thatearly measurements fail to take into account mass flow when the diffusivities ofthe two counterdiffusing gases differ significantly In addition, such systems have

Fig 1 Apparatus for measurement of gas diffusion in soil, using argon and nitrogen ascounterdiffusing gases (From Rolston, 1986.)

one face of the sample open to the atmosphere, so that uniform boundary tions of concentration, temperature, and pressure are difficult to maintain Othermethods overcome these problems by enclosing the sample between two gas-filledchambers and by using gases at trace concentrations as the diffusing species, toovercome the problem of mass flow Such systems allow precise control of ex-perimental conditions and can give accurate measurements of diffusion coeffi-cients in soils of very low air-filled porosities (e.g., those that are nearly saturatedand in which soil aeration is likely to limit plant growth) Three such methodsused85Kr (Ball et al., 1981), sulfur hexafluoride (SF6) (Reible and Shair, 1982),and freon-12 (CCl2F2) (Sallam et al., 1984; Jin and Jury, 1996), respectively, astracers All these gases have low solubility in water and are neither strongly ad-sorbed on soil surfaces nor consumed by soil microorganisms In two of thesemethods (Ball et al., 1981; Reible and Shair, 1982), pressure differences be-tween the end faces of the sample, which could cause mass flow, are monitored

condi-by a micromanometer capable of detecting differences as small as 0.01 Pa Inthe method of Reible and Shair (1982), syringe samples of a SF6–air mixture aretaken from each chamber at regular intervals and analyzed for their SF6concen-tration, using an electron capture gas chromatograph Samples of relatively small(2.54 cm) diameter are tested The air –freon mixture is sampled at the beginningand at the end of the diffusion measurement In the method of Sallam et al (1984),the size of the chambers enclosing the sample varies according to the expectedsample porosity

The method of Ball et al (1981) assesses trace gas concentration structively and was designed for the use of minimally disturbed field samples held

nonde-in their samplnonde-ing cylnonde-inders Samples 76 mm diameter and 50 mm long, or 150 mmdiameter and 100 mm long, can be inserted directly into the apparatus in the field-moist condition or after equilibration to a given matric potential This method,with its self-contained apparatus, which is relatively quick and easy to use, isbriefly described below; fuller details are given in Ball et al (1981)

In the apparatus (Fig 2), two cylindrical gas chambers with scintillator disksand photomultipliers attached are sealed on to the ends of the stainless steelsample holder A mixture of air and radioactive85Kr with an activity of the order

of 400 GBq m⫺3(⬃1 Ci m⫺3) is injected into one gas cell and diffuses throughthe soil until an equilibrium concentration is achieved throughout the apparatus.The concentration of tracer is measured by regular counting ofb radiation de-tected at each photomultiplier In the latest version of this equipment, the countingdata are recorded and analyzed by a PC

It is assumed that after a short initial period (⬍5 min), the relationship tween the count rates in the two gas chambers is given by

be-⫺kt

where CIand CRare the concentrations of gas in the injection and receiving gas

chambers, respectively, t is time (s), Ceis the concentration in each gas chamber

at equilibrium, and k is given by

2D As s

VLs

whence Dsmay be found Asand Lsare the area and length of the sample, and V

is the volume of the gas cell

In practice, to speed up the measurements, diffusion is usually monitored

only halfway to equilibrium Values of CI⫺ CRand t, excluding those detected in the first 5 minutes, are fitted to Eq 7, and k and Ceare estimated by exponentialregression This modification allows the making of diffusion measurement onsamples at or below field capacity, typically in under an hour Samples wetter thanfield capacity, particularly if they are compact or fine-textured, may require up to

15 hours for significant diffusion to occur and are best measured overnight.Interest has recently increased in the movement of volatile organic com-pounds in soils Batterman et al (1996) reviewed the theory and methods for mea-surement of the diffusion of volatile organic components in the laboratory Theypresented a novel one-flow sorbent-based technique The system maintains a con-

Fig 2 Apparatus for measurement of gas diffusion and permeability, using85Kr as tracergas (From Ball et al., 1981, with slight adaptation.)

stant concentration gradient across a soil column using a test gas flow at one sideand a high-capacity sorbent at the other The diffusion coefficient of trichloroethy-lene was estimated using the difference between the inlet and outlet concentra-tions The measurement of the transport of reactive gases which hydrolyze in wa-ter, such as SO2and CO2, is problematic, and dependent on soil structure and soilsolution pH (Rasmuson et al., 1990) The assessment of gas diffusivity in suchsoils may best be assessed using modeling (see Sec IV.A).

2 Field Methods

a Large Scale

Field methods of diffusion measurement overcome some of the soil disturbanceproblems of sampling, but have their own complexities Methods involve with-drawal and analysis of gases injected into the soil or nondestructive sampling oftrace gases

McIntyre and Philip (1964) pointed out that early methods allowed no orous analysis because the geometry of the diffusion path was irregular and theboundary conditions not known They developed a technique that measured soilsurface gas exchange A thin-walled brass cylinder was driven into the soil, thesoil in the cylinder was flushed with air to give a known concentration initially,and then oxygen from a chamber placed on the cylinder was allowed to diffusethrough the soil The oxygen concentration in the chamber was measured with

rig-a membrrig-ane-covered oxygen crig-athode This method trig-akes into rig-account errors due

to temperature, relative humidity, and changes in soil porosity, but it suffers fromproblems of oxygen storage and consumption However, the approach of mea-surement of the diffusion of gases across the soil water interface is of great rele-vance to water evaporation, soil aeration, loss of nitrogen and volatile organiccompounds Rolston et al (1991) modified the technique to use freon-13 (CClF3)

as a tracer and proposed an analytical solution for the diffusion coefficient and

a thorough appraisal of the boundary conditions, including comparison with corevalues of diffusivity

A smaller-scale technique was proposed by Lai et al (1976) The method isbased on the theory of radial diffusion of a finite quantity of a gas into a semi-infinite medium Oxygen is injected through a needle inserted into the soil, smallaliquots of soil air are withdrawn at regular intervals, and oxygen and nitrogenconcentrations are measured using a portable gas chromatograph Two principaladvantages were claimed for their method: there is no removal or alteration of thesoil from its natural state, and minimal instrumentation is required at the site ofmeasurement In a modification of the method of Lai et al (1976), Jellick andSchnabel (1986) used a numerical finite-difference model to allow the initial con-centration profile within the sphere of injected gas to vary, based on experimental

data The diffusion coefficients compared favorably with those determined onminimally disturbed core samples (Fig 3).

These techniques suffer from the need to take samples from the site of fusion, thereby changing the concentration and pressure of the tracer solution.Also the concentration of samples of the gas may change before analysis due toleakages Further, such tests give no indication of the likely magnitude of thediffusion coefficient to help determine the frequency and duration of sampling.Ball et al (1994) developed an apparatus that can be used either as a buried res-ervoir capable of sampling several soil layers in succession or as a surface cham-ber (Fig 4) The method initially used85Kr as the diffusing gas, which was moni-tored continuously and nondestructively as it diffused from a cell surrounding aGeiger–Mu¨ller tube at the base of a probe (Ball et al., 1994) Later, in response

dif-to the inconvenience of satisfying radiological protection procedures, freon-22(CHClF2) was used as the diffusing gas and was monitored nondestructively at anelectrical sensor (Ball et al., 1997b) In the buried reservoir mode, the probe isinserted within a hole augered in the soil and is used to measure diffusion at soildepths below about 150 mm In the surface chamber mode the probe is locatedwithin a chamber enclosing the soil surface, and the system measures the rate ofdiffusion into the surface In both modes the gas cell containing the detector isisolated from the probe above it by lightly inflating the rubber membrane abovethe gas cell In use,85Kr or freon-22 (or freon-23) is injected into the gas cell so

as to form a cylindrical source, and the decrease in concentration is monitoredregularly (usually at intervals of 15 s) until it has decreased to – of its original12 13value In order to calculate diffusivity, it is necessary first to simulate diffusion

Fig 3 Relationship between Ds/D0and volumetric air content for lawn soils.●: sion measurements with the injection method using the numerical model;䡩: measurementswith the core method (From Jellick and Schnabel, 1986.)

diffu-numerically using Fick’s equation The time axis of the simulation is expanded orcontracted until it matches the observed decrease in concentration (Ball et al.,1994) The advantage of this numerical system is that an exact volume of tracerneed not be injected; the initial concentration can vary, provided it is known.

In both laboratory and field measurements of diffusivity, the temperatureshould be stated when the results are reported The following equation, cited byRolston (1986) allows the gas diffusivity measured at any temperature to be cal-culated for any other temperature:

Fig 4. Equipment to measure gas diffusion in soils in situ Diagram shows use in eitherburied reservoir mode or surface chamber mode (From Ball et al., 1997b.)

determine the oxygen content of gaseous samples and that the analysis was morerapid and required less expensive equipment than for gas chromatography.Soil oxygen flux and redox potential can be measured in waterlogged soilusing polarographic techniques These methods are based on the reduction of oxy-gen at a platinum wire cathode buried in the soil This is linked to a calomel orsilver–silver chloride anode placed in electrical contact with the soil (Fig 5).Redox potential is a measure of the intensity of reduction in soils containing no

Fig 5 Structure of anodes and cathodes used for measurements of oxygen flux with bareplatinum electrodes Key: a, Platinum wire; b, epoxy resin; c, crimped and soldered joint(Pt wire in hole at end of conductor); d, mild steel conductor; e, heat-shrinkable insulatingsleeve; f, soldered connection; g, wire connected to cathode plug; h, self-amalgamatinginsulation tape; i, porous pot (air entry pressure approximately 100 kPa); j, saturated KClsolution; k, flexible connecting tube filled with solution; l, prepolarized silver sheet, surfacearea approximately 25 cm2; m, crimped and soldered joint; n, silicone rubber cement; o,wire connected to anode plug; p, tap for bleeding air from solution; q, syringe body (FromBlackwell, 1983.)

molecular oxygen Oxygen flux (also termed oxygen diffusion rate, ODR) to acathode is a measure of the rate of supply of oxygen from the air through thesurrounding soil through a film of soil water This flux is comparable to the maxi-mum required by roots respiring in moist soil (Blackwell, 1983) In measurement

of ODR, Armstrong and Wright (1976) recommended that the relationship tween current and voltage be established (polarogram) Where a plateau is reached

be-on the polarogram, current is related to the flux of oxygen to the electrode.Both redox potential and ODR can be measured with the same pair of elec-trodes Blackwell (1983) showed that platinum cathodes can be left in the soil andremain functional for several months without removal for cleaning These tech-niques work best in wet soil In unsaturated soil, variations in pH and aerationstatus alter the shape of polarograms, and it may be necessary to measure soilelectrical resistance before ODR can be calculated (Callebaut et al., 1982) Re-views of the principles and the conditions under which this equipment can beused, and detailed descriptions of electrodes and electronic instrumentation re-quired for multiple assessments of both measurements for a lysimeter installationand for field use, can be found in Armstrong and Wright (1976), Callebaut et al.(1982), and Blackwell (1983)

D Measurement of Mass Flow

Several methods are based on the steady-state method proposed by Grover (1955).Grover devised a permeameter with a float, a thin-walled cylinder that can besuspended to keep it centered (Janse and Bolt, 1960) The float is open only at thebottom and forms an air chamber that fits over an annular water reservoir (Fig 6).The air pressure can be increased by adding weights to the reservoir The air isdisplaced directly into field soil (Grover, 1955) or through a core sample sealed

on the bottom (Janse and Bolt, 1960) Bowen (1985) proposed improvements tothis apparatus, to incorporate a sensitive flowmeter and manometer, the latter read-ing to a maximum of 0.5 kPa The direct reading of flow and pressure considerablyspeeds up measurements, since it is otherwise necessary to time the fall of thefloat for a given distance to be able to calculate permeability The main advantage

of this technique is that constant low pressures (0.03 –1 kPa) can be applied ham (1946) discussed in some detail the errors and the assumptions involved inair permeability measurement, particularly that of neglecting gas compressibility

Kirk-He integrated Eq 6 into a form applicable to most air permeability measurementtechniques:

K DP As

hLswhere qvis the volumetric flow rate [L3T⫺1], K is air permeability [L2],DP is

the pressure difference across the sample [M L⫺1T⫺2], Asis the cross-sectional

area [L2], Lsis the length of the sample [L], andh is the dynamic gas viscosity[M L⫺1T⫺1] corrected for temperature.

Permeability in topsoil layers is likely to be anisotropic Janse and Bolt(1960) measured air permeability on undisturbed cores using a Grover-type per-meameter and found that ‘‘vertical’’ samples were about twice as permeable as

‘‘horizontal’’ samples, part of the effect being attributed to greater compressionduring horizontal sampling than during vertical sampling Conversely, McCarthyand Brown (1992) found the horizontal air permeabilities were greater than thevertical, an effect they attributed to the alluvial origins of their soils

Fig 6 Schematic diagram of air permeameter with mercury seal to allow quick ment and good seal between permeameter and soil insert See Grover (1955) for construc-tion details

attach-Due to the increasing interest in soil structure, steady state methods ing soil cores have developed further The method of Corey (1986) allows forcontrol of matric potential and volumetric water content by hydraulic contactaround the circumference of the soil sample This method is only applicable to dis-turbed, repacked samples However, Roseberg and McCoy (1990) modified themethod to allow the use of intact cores 77 mm long and 70 mm diameter Theirequipment allowed simultaneous measurement of air permeability and the soil wa-ter potential in the 0 to⫺6 kPa matric potential range This was accomplished bycounteracting the effects of the gravitational potential gradient within the sampleusing controlled air pressure This enabled air flow measurements at or near satu-ration, where only macropores conduct air, and assessment of macroporecontinuity.

involv-Ball et al (1981) devised a method for intact samples that requires the sametwo-chamber apparatus as that used for the measurement of diffusion (Fig 2).This method applies a constant pressure difference and measures the resultantflow A differential micromanometer sensitive to pressure differences as small as0.01 Pa is connected across the two gas chambers, and compressed air from abottle is fed via a regulator and a flow controller to one gas chamber Exhaust air

is piped from the other gas chamber into a soap-film bubble meter or ball flowmeter To preserve laminar flow, the pressure differences applied are keptsmall (0.15 –300 Pa), as are the resultant rates of flow (0.15 – 6 cm3s⫺1) For eachsample, flow is measured at two or more pressure differences This permits inves-tigators to check their proportionality, giving two or more permeabilities, using

suspended-Eq 12 An advantage of this technique is that air permeability can be measuredimmediately after a diffusion measurement in 2 –3 min without disturbing thesample In addition, flow and pressure differences are measured with high accu-racy Other similar methods, using constant flow and measuring the resultant pres-sure difference with a water manometer, were proposed by McCarthy and Brown(1992) and Grant and Groenevelt (1993)

The statistical distribution of air permeabilities from a given depth and ment within replicated field experiments is generally nonnormal The distribu-tions are skewed and usually log-normal (Kirkham et al., 1958; Ball, 1982;Groenevelt et al., 1984) In such cases either nonparametric tests should be applied

treat-to reduce the data and get an indicatreat-tor of statistical degree of spread, such asstatistical rank analysis (Kirkham et al., 1958) or the Mann –Whitney U Test(Groenevelt et al., 1984), or parametric tests should be applied to log-transformeddata (Ball, 1982)

E Soil–Atmosphere Trace Gas Exchange

Much attention has been given in recent years to the measurement of fluxes of