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Ann. Bot. 43 285-293 1979

On the Use of the Katharometerfor the Estimation of Carbon Dioxide

Concentrations in SoilsL. W. POEL

Department o f Plant Biology, University of Birmingham, P.O. Box 3 63, Birmingham BIS 2TTAccepted: 17 November 1977

A B S T R A C TThe method depends upon the difference in thermal conductivity between carbon dioxide and othercomponents of the soil atmosphere.The circuit calibration and mode of operation of the instrument are fully described together withan investigation of the effect of differences in ambient temperature.Values of carbon dioxide concentration o btained in trials on culture and field soils are given. The valueslie within ranges reported by earlier workers using gas analysis of samples of soil air.Key words: katharometer, CO2 concentration, soil atmosphere.

I N T R O D U C T I O NIn 1915, G. A . Shakespear, of the D epartm ent of Physics in the University of Birmingham,was approached by the British Admiralty to devise a method of detecting dangerousconcentrations of hydrogen arising in hangars during the filling of airships. Theinstrument that he produced, and to which he gave the name 'kath aro m ete r', dependedupon the difference in thermal conductivity between hydrogen and air. Over the yearsthe method was developed by the Cambridge Instrument Company Limited for theestimation of a wide range of gases and found application in many diverse branches ofScience and Industry (Barron, 1951). In the sphere of plant metabolism, early forms ofthe instrument were used in studies of the carbon dioxide exchange of leaves (Waller,1926) and of plant respiration (Stiles and Leach, 1931; Leach, 1932). More recently, thekatharometer has been employed extensively as a detector in gas analysis by gaschromatography.

Carbon dioxide concentrations in soils have almost always been estimated by gasanalysis of samples of the soil atmosphere obtained either by suction or by buryinga porous thimble and allowing its contents to attain equilibrium with the soil air (e.g.Russell and Appleyard, 1915). Jensen, Van Gundy and Stolzy (1965) used a mem brane-covered glass electrode to record in situ the partial pressure of carbon dioxide in soilcultures of tomato and found good agreement with gas chromatographic analysis ofwithdrawn samples. However, one would anticipate that such an electrode would provetoo fragile for routine work in the field, a disadvantage that the rugged katharometerdescribed in the present article does not have.

0305-7364/79/030285 +1 0 $02.00/0 1979 Ann als of Botany Com pany

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28 6 Poel - Estimation of Carbon Dioxide Concentrations in SoilsPRINCIPL E S AND GE NE RAL FE AT URE S

The katharometer consists of a cylindrical block of copper or brass in which two holesare drilled longitudinally. One of these holes passes completely through the block whereasthe oth er does n ot. A fine platinum spiral or loo p of approx ima tely 10 Q resistance isinserted into each hole. In some instruments, both leads from each spiral are insulatedfrom the block whereas in others, one lead is insulated and brought out, the second beingconnected directly to the block so that the block itself forms part of the circuit. Thespirals consti tute two adjacent arms of a W heatstone bridge, the othe r two arms beingmanganin resistors each of about 10 O.. The latter are not connected directly to eachoth er in the bridge but a re l inked by a circular sl ide-wire of mang anin or constantan , thusena bling balancing of the bridge to be carried o ut. Th is arran gem ent and other featuresof the basic circuit are shown in Fig. 1.

Heating currentresistancekatharometer block

FIG. 1. Basic kathar om eter bridge circuit.If the variable resistance is adjusted to furnish only a small current in the bridge asindicated by the milliammeter, if all the resistances in the bridge circuit are of preciselythe same value and, further, if all are at the same temperature, then the bridge will be 'inbalance' , giving a reading of zero on the galvanometer. Suppose now that the currentfrom the battery is increased to such a level that the platinum spirals are heated. Theirresistance will fall but, if both spirals are in contact with the same gas mixture, the bridgewill remain in balance for the resistances of the spirals will still be equal although lowerthan when cold. However, if the lower end of the katharometer block is brought intocontact with air enriched with carbon dioxide, then the two spirals will no longer beexposed to the same gas mixture; the 'closed' spiral will be in contact with ordinary airas before but the 'open' spiral will be accessible to carbon-dioxide-enriched air. Owingto the high thermal conductivity of carbon dioxide, more heat will be lost from the openspiral than from the closed one. Hence, the resistance of the former will increase inproportion to the concentration of carbon dioxide and the bridge will drift out of balanceto a corresponding degree, with a lag of about 5 min for the attainment of equilibrium


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Poel - Estimation of Carbon Dioxide Concentrations in Soils 287There are two ways to record quantitatively the effect of exposure of the open spiralto a concentration of carbo n dioxide above that in air. Either the out-of-balance currentcan be read o n the galv anom eter o r, if it is grad uated with a scale of suitable divisions, theslide-wire dial can be adjusted to restore balance, the number of divisions of rotationrequired to achieve this being recorded. In either case, the instrument is calibrated againstgas mixtures of known carbon dioxide contents. An unknown concentration can then bederived by interpolation on a calibration curve.

D ESCR IPTIO N O F TH E IN STRU M EN TPlate 1 shows the complete carbon dioxide analyser. The prob e, now sh rouded ininsulating materials against variations in external temperature, is the actual one depictedin Stiles and Leach (1931, p. 469). Recent calibration of this instrument (Fig. 3) showsthat i t has not deteriorated over the years.The detailed circuit of the meter is shown in Fig. 2. Stiles and Leach (1931) calibratedand operated their instruments on the out-of-balance bridge current principle. This calledfor a highly sensitive and hence delicate galvanometer. Clearly, such a galvanometercannot be employed in an instrument designed for use in the field. The Pye Unicam tautsuspension galvanometer installed in the present meter is insufficiently sensitive formeasuring out-of-balance bridge curre nt nor for providing other than a very appro ximateindication of null-point in a balanced bridge mode of operation . How ever, measurem entof the potential difference across the galvanometer substantially raises the effective sen-sitivity of the meter. For this purpose, a Comark D.C. microvoltmeter (portable model)is employed. This would be equally suitable for use in either mode of operation of thebridge, but, since the null-point method depends not only on the sensitivity of the nulldetector but also on that of the slide-wire adjustment, the potential drop across thegalvanometer corresponding to the out-of-balance current is measured.

It may be wondered why, as the potential difference across the Pye galvanometer isbeing measured, the latter instrument is retained in the circuit since it could be replacedfor this purpose by an equivalent resistor. In practice, the galvanometer serves a valuablefunction in indicating approximately the degree to which the bridge is put of balance,so allowing the app rop riate rang e of the m icrovoltmeter to be selected before switching on.The lower end of the katharometer block is protected against direct contact with thesoil (and the consequ ent risk of soil reaching the open sp iral) by a tub ula r brass shr oud ing.To the upper end of the block, a length of copper tubing is attached and this containsthe fixed resistors of the bridg e. As men tioned previously, all the external m etal surfaces ofthe p rob e are sleeved with insula ting m aterials so as to minimize the influence of changesin ambient temperature. The five-core cable is taped to the probe to avoid movement ofthe cable being transm itted to the fixed resistors and their soldered jo in ts ; such m ovem entcauses instability of readings. The cable terminates in a six-pin plug socket and at thisend also, there is provision for securing the cable to the side of the case to precludevariation in contact resistance at the panel connexion arising from movement of the plug.

The k atharom eter used is operated a t 102 mA , derived from a 2-4 V, 15 AH D.E .A.C .Type 2/SD15 sealed nickel cadmium accumulator. In order to obtain this current withinthe range of adjustment of the variable resistors Rl and R2 (coarse and fine adjustmentsrespectively) further resistance in the circuit is necessary and this is provided by a com-bination of fixed resistors on the positive side of the battery. Because the instrument isinherently highly sensitive to changes in heating current, the milliammeter is fitted witha magnifying lens to aid precise setting of the current.The resistance of the manganin slide-wire is low (0-1 Q) and this would limit the rangeof carbon dioxide concentrations th at could be handled by the instrument. M oreover, thefour resistances used in the construction of a katharometer probe are unlikely all to be

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28 8 Poel - Estimation of Carbon Dioxide Concentrations in Soils3

Katharometer blockconnections

oTo microvoltmeter

FIG. 2. Detailed circuit of the soil carbon dioxide analyser.of precisely the same value so that there may be a weighting in favour of the 'openside' or the 'closed side'. Such imbalance would reduce the effective range of adjustmentof the slide-wire and may even be sufficient in magnitude to render the slide-wire controltotally ineffective. For these reasons, a chain of four resistances (made by hand fromconstantan wire) is fitted. Each resistance is of approximately 0-1 Q. and they can beadded stepwise to either end of the slide-wire as necessary by means of the contactselector S3 and the rotary changeover switch S4.The Pye Unicam galvanom eter is broug ht into circuit by opening the switch S2 which,in the closed posit ion, short-circuits the galvanometer and thus damps i ts movement forprotection during carriage of the meter. S5 is a rotary changeover switch, useful if eithera centre-zero microvoltmeter is not available or one wishes to utilize the greater range


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Poel - Estimation of Carbon Dioxide Concentrations in Soils 289of a left-hand zero instrument. Since measurements are made at the microvolt level, it isimperative that all switches be of low and reproducible contact resistance.

CALIBRATIONA glass chamber having inlet and outlet tubes was fitted to the probe and through thiscarbon-dioxide-free air (derived from a small compressor via a soda lime tube) waspassed for 30 min after switching on the instrument, adjusting the heating current to102 mA and bringing the galvanometer and microvoltmeter readings approximately tozero deflexion. At the end of this period, a null reading on the microvoltmeter was setprecisely. The carbon-dioxide-free airstream was replaced by a series of known con-centrations of carbon dioxide in air delivered by a gas-mixing assembly. For each carbondioxide concentration, the mixture was allowed to pass for 10 min followed by carbon-dioxide-free air for 10 min. At the end of each of these 10 min periods the out-of-balancepotential difference was measured and, after recording the difference between the tworeadings, the bridge was brought to balance again.

All gas streams, whether carbon-dioxide-free or otherwise, were moistened sincea calibration based on dry gases would not be valid for the humid conditions prevailingin the soil atmosphere. Water has a very high thermal conductivity (about seven timesthat of carbon dioxide) and hence any instrument depending upon differential thermalconductivity will be profoundly affected by variations in water content of the medium,be it gaseous or solid, into which its probe is inserted. Indeed, this has furnished a methodfor estimating in situ the water content of soils (De Jager and Charles-Edwards, 1969.)However, the katharometer as used for soil carbon dioxide estimation is in contact notwith the liquid phase of the soil but with the soil atmosphere containing water vapour.Although the water content of the soil may vary widely ' the air between the soil particlesis generally nearly saturated with water vapour' (Nobel, 1974, p. 388). Thus constancy ofwater vapour content can probably be assumed for practical purposes, except perhapsin dry, sandy soils. In passing, the carbon dioxide katharometer clearly cannot be usedin exceptionally wet soils since water would be liable to flood into the open chamber andreach the platinum spiral.The calibration curve obtained is shown in Fig. 3. Each point represents the mean ofnot fewer than three completely separate determinations. Only in the case of ordinaryair, the carbon dioxide content of which is close to the resolution of the instrument, didthe standard deviation of replicates exceed 1 -74 per cent of the mean. The slope of thecurve indicates a sensitivity of 0-0258 per cent carbon dioxide per microvolt.The above calibration was carried out under uniform conditions of ambient tempera-ture in the laboratory. In the field, soil temperature and atmospheric temperature willvary both individually and relative to each other. Stiles and Leach (1931) found thattemperature had an appreciable effect on the katharom eter readings in their studies andtherefore a laboratory investigation of the temperature factor was conducted with thepresent system.An attachment for the supply of humidified carbon-dioxide-free air was constructed(Plate 1). This comprised a coil of copper tubing and a cylindrical brass chamber inseries preceded by a soda lime tube, a U-tube containing moistened cotton wool anda hand pump. The copper tubing and brass chamber were immersed in a water bath filledwith cold water. The attachment was fitted to the probe and the instrument having beenswitched on and stabilized for 30 min, carbon-dioxide-free air at the temperature of thewater bath (12-5 C, measured with a Comark digital thermometer) was pumped throughintermittently for sufficient time to achieve a steady null reading. By judicious manipu-lation of the water bath thermostat, the temperature of the water bath was raised one ortwo degrees at a time. At each temperature, air was pumped to the katharometer probe


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290 Poel - Estimation of Carbon Dioxide Concentrations in Soils300

1 2 3 4 5 6 7Carbon dioxide concentration(% by volume)FIG. 3. Calibration curve for the Stiles and Leach (Shakespear) katharometer.

Temperature (C)20 30

- 2 0

FIG. 4. Effect of air temperature on katharometer readings. The bridge was balanced againstcarbon-dioxide-free air pumped intermittently through a metal coil and cylinder immersed ina w ater bath at 12-5 C. Increasing imbalance as the water-bath tem perature was raised is shownas negative values, the effect of temperature being opposite in direction to that of carbon dioxide.



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Poel - Estimation of Carbon Dioxide Concentrations in Soils 291and the out-of-balance potential difference recorded at equilibrium. The results areplotted in Fig 4.As shown by Stiles and Leach (1931), the effect of temperature is opposite in directionto that of carbon dioxide and for this reason, the potential differences are shown inFig. 4 as negative values. Fro m 12-5 to 25 C , the relationship is a linear on e and the slopeindicates a difference of 1-05/JV for a change of 1 C which is equivalent to 0-0271 percent carb on dio xide. This is in good agreem ent with the value of 0-025 pe r cent quo ted b yStiles and Leach (1931) for th e tem per atur e range 17 C to 25 C. Abo ve 25 C theinfluence of tem pera ture p rogressively declines up to 40 C, the highest tem pera turereached in the investigation.Clearly, temperature differences of no more than a few degrees could introduceappreciable errors particularly at low levels of soil carbon dioxide. Such differences arelikely to occur between the soil atmosphere and the air above the soil which is used(when freed of carbon dioxide) for setting the zero. The solution to this problem is tobring the temperature of the carbon dioxide-free air reaching the probe as close aspossible to that of the soil. In achieving this, use is made of the metal coil and cylinderemployed in the laboratory investigation described above. On arrival at a site, theassembly is buried in the soil to the same depth as that to which the katharometerprobe will be inserted. Providing that pumping is gentle, the carbon-dioxide-free air willbe at or close to the soil temperature. Discrepancies in soil temperature between sitesand also at a given site on different occasions will often exist and are allowed for byadjustment of the carbon dioxide concentration data to an arbitrarily chosen referencetemperature. Finally, many other components of the meter circuit are susceptible todrifting resistance as a result of variation in external temperature. Accordingly, stepsmust be taken when necessary to shield the entire system from direct insolation andstrong winds.

LABORATORY TRIALFor this purpose, a quantity of soil taken from a field adjoining the Biology Building waspacked into a tall litre beaker and moistened. A number of barley grains was sownsuperficially in the soil. The beaker was placed in an illuminated growth cabinet at 25 Cfo r 1 week at which stage the leaves were ab ou t 7 cm long. The beak er was the n takenfrom the growth cabinet and time allowed for its contents to return to laboratorytem per ature . A soil core was removed by me ans of a no. 18 cork b ore r, leaving a hole6-5 cm deep for insertion of the katharo me ter pr obe. The data obtained are presentedT A B L E 1. Analyses of carbon dioxide concentration in a soil culture of seedling barley

Potential

Soil106261615

difference acrossCarbondioxide-freeair

- 5- 1 01020

galvanometer (fiV)

Difference1516161415

Equivalentpercentage(by volume)carbon dioxide0-250-270-270-230-25

Mean 0-254S.D. 0017

Heating current = 102 m A; readings taken after lO m in; microvoltmeter brou ght to zero after eachcarbon dioxide-free air reading.


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292 Poel - Estimation of Carbon Dioxide Concentrations in Soilsin Table 1. The level of carbon dioxide concentration indicated appears reasonable forthe conditions of the trial and the individual readings show a remarkable degree ofuniformity.

FIELD TRIALThe area selected for preliminary testing of the instrument in the field lies to the north ofthe Biology Building, extending from the Worcester and Birmingham Canal to theboundary of the University Tennis Courts. From the overgrown towpath the land risesgently to the crest of the canal emb ankm ent and th en falls steeply (gradient 1 in 0-35)to m ore or less level groun d abo ut 3 m below. On the canal em bankm ent, characteristicplant species are Rubus fruticosus, Urtica dioica, Festuca rubra. The vegetation of thelevel area, cut over during the growing season, is coarse grassland of which the mainconstituents are Agropyron repens, Agrostis tenuis, Holcus lanatus, Dactylis glom erataand Plantago lanceolata. The soil throughout the area is a brown loam, much looser intexture on the canal embankment than on the level part.

In October 1977 seven sites for estimation of soil carbon dioxide concentration werechosen, three located on the canal embankment and four on the level area. At each site,three or in some instances four separate estimations were made. Soil temperature was14 C at all sites. As an analysis of varia nce revealed no significant difference betweenthe means for the sites on the canal embankment and likewise for those on the levelground, the raw data for the sites in each area were pooled. The means for these pooleddata are presented in Table 2. The difference of means proved highly significant (byStudent 's / test) at better than P = 0 0 0 1 .T A B L E 2. Mean soil carbon dioxide concentrations on a canal embankment anda level grassland area below

Canalemb ankm ent Level areaN o. of estimations 9 13Mean (% by volume) 0-246 0-566Range of values 010-0-38 0-29-100Standard deviation 010 5 01 85

Since gase ous diffusion from the soil to the atm os ph ere is facilitated by a loose soiltexture, one would expect to find a lower equilibrium concentration of carbon dioxidein the embankment soil than in that of the level grassland. That this was actually the casein the trial insp ires confidence in the ins trum en t for u se in field inves tigations , a confidencewhich is strengthened by the fact that the carbon dioxide concentrations recorded fallwithin ranges of values encountered by Russell and Appleyard (1915) in their classicstudy of the soil atmosphere of Rothamsted fields.A CK N O WLED G EMEN TS

T he auth or is grateful to M iss V. B. A. W illington for carryin g ou t the field trial, and toMiss A. J. Cox for the photographic work.


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POEL - Estimation of Carbon Dioxide Co ncentrations in Soils

P L A T E I

Ann. Bol. 43, 285-293, 1979 (Facing p. 293)



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Poel - Estimation of Carbon Dioxide Concentrations in Soils 293LITERA TU RE CITED

BARRON, S. L., 1951. The Development of the Thermal C onductivity Method of G as Analysis. CambridgeMonograph No. 3. Cambridge Instrument Company Ltd, London.D E JAGER, J. M. and CHARLES-EDWARDS, J., 1969. Thermal conductivity probe for soil-moisture deter-minations. J. exp. Bot. 20 , 46-51.JENSEN, C. R., VAN GUNDY, S. D. and STOLZY, L. H. 1965. Recording CO2 in soil-root systems witha potentiometric membrane electrode. Proc. Am. Soil Sci. Soc. 29,631-3.LEACH, W., 1932. Further experimental methods in connexion with the use of the katharometer for themeasurement of respiration. Ann. Bot. (Old Series) 46, 583-96.NOBEL, P. S., 1974. Introduction toBiophysical Plant Physiology. W. H. Freeman, San Francisco.RUSSELL, E. J. and APPLEYARD, A., 1915. The atmosphere of the soil: its composition and the causes ofvariation. / . agric. Sci., Camb. 7, 1-48.STILES, W. and LEACH, W., 1931. On the use of the katharometer for the measurement of respiration.Ann. Bot. (Old Series) 45, 461 -88.WALLER, J. C, 1926. The k atharom eter as an instrument for measuring theoutput and intake of carbondioxide by leaves. New Phytol. 25, 109-18.

E X P L A N A T I O N OF P L A T E 1The soil carbon dioxide analyser showing the bridge control unit, the katharometer probe, the battery,the portable microvoltmeter and the carbon-dioxide-free air attachment.


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