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Sampling air from dung pats by silicone rubber diffusion chambers
Sod Bd. Buxhem. Vol. 22. No. 7. pp. 995-997. Printed in Great Britam. All n&s reser~cd
0038-071790 53.00 + 0.00 Copyright C 1990 Pergamon Press plc
19%
SHORT COMMUNICATION SAMPLING AIR FROM DUNG PATS BY SILICONE RUBBER DIFFUSION CHAMBERS HOLTER
PEI-ER Institute
of Population
Biology. University 2100 Copenhagen (Accepted
Universitetsparken
15,
5 June 1990)
Dung pats often contain an extremely rich fauna of insects which have important roles affecting carbon and nutrient release from the material. The composition of the air inhaled by these animals in their special habitat is unknown. Stevenson and Dindal (1987) measured the redox potential (Eh) of dung in laboratory microcosms. but this is an indirect measure in relation to air-breathing animals. In addition. reliable measurements of Eh are only feasible in wet substrate. whereas many of the insects may inhabit rather dry dung. Hence. direct sampling and analysis of the air within dung pats is desirable. Essentially two methods are applied in the related field of soil air sampling. One method uses capillary tubing placed in the soil: samples are drawn directly from the soil pores through the tubing (e.g. Hack, 1956; Tackctt, 1968; Roulicr ef ul., 1974). However. wet dung will block a capillary tube almost immediately. The other method requires a chamber of considerable volume (typically at least SO-100 ml) which is installed in the soil and allowed to equilibrate by diffusion with the soil atmosphere through holes of suitable size (e.g. Taylor and Abrahams. 1953; Tackett. 1968; Patrick, 1977). Small air samples can then be exlrdcted from the chamber without causing undesirable mass flow into the system (in porous soils) or vacuum (in impermeable soil). Chambers of SO-100 ml are clearly unsuitable in small habitats like dung pats. In addition, there are problems with vigorous fungal growth within such chambers. The method to be presented here utilizes small (2 ml), completely closed. diffusion chambers made of ordinary silicone rubber tubing which are placed in the dung pats. Since silicone rubber is highly permeable to various gases (Rogers er ~1.. 1972; Larsen and Nilsson. 1985). the air in a small, closed tube will rapidly equilibrate with the surrounding atmosphere. The air in tubes extracted from dung is sampled in the field, stored and transported to the laboratory where the concentrations of 0:. CO1 and CH, are determined by gas chromatography. After a description of the practical procedure, two important aspects of the method will be examined: (I) the rate at which gaseous equilibration takes place between tubes and the surrounding atmosphere is quantified; this rate has important implications for the sampling procedure; (2) the storage of air samples necessary for transport between field site and laboratory is briefly discussed.
PRACl-lCAL
of Copenhagen, 0, Denmark
PROCEDURE
Pieces of silicone rubber tubing, 9 cm in length with I mm wall thickness and 6 mm i.d. (Asicomo A/S. Fat-urn, Denmark), were closed at each end by rubber stoppers and placed in an experimental dung pat while it was laid out in
the field. Tubes to be placed along the edge of the pat contained a length of thin stainless steel wire to maintain their curvature. On sampling. each tube was withdrawn from the dung and a 1.5 ml sample was taken from the tube with a hypodermic syringe (the tube was gently compressed to avoid sampling against a vacuum). The gas sample was then injected into a 3.5 ml Venoject blood collecting tube (Terumo Corp.), previously filled with pure Nr at atmospheric pressure. The samples were returned to the laboratory and analysed in a gas chromatograph with thermal conductivity detector (Mikrolab Aarhus) at 12OC. using a Poropak Q column for CO? and a Molesieve 5A for 0, and CH,; both columns were 2 m x 2 mm bore, maintained at 30°C. The carrier gas was He with a flow rate of 25ml min-t (Poropak) or i5ml mitt-’ (Molesieve). Results were corrected for the sample dilution by 3.5ml N,.
RATE OF GASEOUS
EQUILIBRATION
The rate of diffusive equilibration of gas concentrations between the inside of tubes and the surrounding atmosphere is crucial to the applicability of the method. Equilibration must be sufficiently rapid for the gas within a tube to follow changing external atmospheric conditions fairly closely. On the other hand, a rapid equilibration might lead to changes of gas concentrations in a tube during the inevitable time lapse between extraction of the tube from the dung and withdrawal of a gas sample. Equilibration rates for Oz. CH, and CO? were determined at 10.5 and 22’C (4 measurements at each temperature). For each measurement, 10 tubes were kept for at least I day in a 5OOml bottle (air-tight lid with a rubber septum) with a mixture (similar to the dung atmosphere) of 0, (0.8-1.5%). CH, (9-19%). CO: (7-13%) and N, at atmospheric pressure. Tubes were then removed from the jar and suspended in air, sampled individually at every 6 min and the I.5 ml samples stored in Venojects until analysis. To express the rate of equilibration from these mcasurements we assume: (I) identical barometric pressure inside and outside the tube; (2) that the diffusive properties of the gases (N,, 0,. CO,, CH,) are similar so that the principles of binary diffusion apply (c/ Paganelli. 1980); and (3) constant composition of the external atmosphere. If C, and C, are the concentrations outside and inside the tube. we define
O=lC,-Ccl. 995
of a particular
gas
336
Short Communications
0
f
s
Table 2. Relatrvc changes m concentratton of CO:. CH, and 0: rn silicone rubber tubes olaced in armosoheric ax
3’
I? g 2 2 =
,L
Q(
Z. I
Ratmsafter
CO,(
A0.994
f
co:
rC0.999 )
_,’
2
2
Imual gas cont. I” nJbe(O.J
Gas CHJ
,!
;
PaO.996 1
30
: u
20
30 Time
40 50 (mitts)
0,
IO 0.1 05
CH. CO1
IO.5 22 10.5 21 10.5 22
Table 3. ERect of prolonged storage (21 C) of a gas mtxture in Venojccts on concentrations of 0:. CH, and CO: Durauon of storattc fdavs) 0.143 7
taken dung.
k tmin “) 0.0 I77 2 0.OOOs.l 0.0216 =00OlJO 0.0273 i o.ooo7.t 0.0345 2 0.0OO83 O.O6t7_~000O61 0.0715 5 O.tlOl06
IO12
t,427 I ofix 10’1
Ratios arc concentration after exposure m atmosphenc air inmat conccntrafion. Calculations based on k-values at 22 C (Table I).
T~hic 1. R~rc cxwst.tnrr (k) for the cxponenttal decrease of gtts concentrafmn diffcrenccs bcrwccn the inside of silicone rubber tubes .tnd the surrounding atmosphere
*SE (n = 4)
I min 0 933 0.931 0.966 0.966 5 465 I.876
60
where D,, and D, arc the vahres of D at times 0 and 1. respectively. Consequently. a plot of InD against I should gave a straight line, slope -X-. Figure I shows that straight hnrs were indeed obtained, and so this simple model seems satisfactory for pmctical purposes. Values of k determined by rcgrcssion (rr 0.986-0.99Y; P
n.
0.982 0.9YI 0.991 2. I’6 1.221 LIOX I.017 I O06
I
In D, = In 0, - kl
( Cl
10
0.966 0 965 0.Yt.Q 0.983 3 245 I.440 I.215 1.035
I
From the laws of diffusion. the rate constant k (where kD = - dD;dt) must be proportional to the permeability of the tube wall and to the ratro of tube area to tube volume. By integration:
Tcmpcraturc
0.5 RI,”
0.983
5 IO
Fig. I. Gas concentration differences between air in the silicone rubber tubes and atmospheric air as a function of exposure ttme of tubes to atmospheric air. Initial gas concentrations in the tubes were 1.2% Or. 9.6% CH,. 72% CO: and 82% N:.
G.l\
0 25 mm
I
C%
g-2 L "
an exposure of
r,,(h) 0.65 0.53 0.42 0.33 0.19 0 16
r,)o$(h) 2.X? 2.3 I 1.83 I .4s O.YI 0.70
The umec at which conccntrauon dtffercnccs arc 50% (fu ,) and 5% (r,,,jq) of their initial value arc also given.
within
Conccntratlon (96) after storage _+SE tn = Xl ____-.0.
CH,
co.
1.532 0.0206 I .5 I i_0.0072
33.1 to.29 335+0.19
19,5*0.13 171i.016
I5 s after
the tube was extracted
from
the
Storage of samples is necessary between field sampling and a convenient time for amtlysis in the laboratory. Table 3 shows that concentrations of Or and CfI, were practically unchanged by 7 days of storage at room temperature in VVenojects, whcrcds cu 1X6 of the initial CO, escaped by difTusion through the stopper. Assuming exponential decrease with time, this corresponds to a relative CO,-Ioss of cu 2% day _I. f fence. sforagc should not exceed a few days if CO! dctcrmtnations arc important. The method presented here works for sampling the gases 0,. CO, and CH,, provided that:
(I)
the tubes are extracted quickly from the substratum; (2) samples are taken immediately upon extraction of tubes; (3) that the concentration of a gas to be measured is at least I-?% of that in the atmosphere.
ff nder these conditions, this simple method avoids some of the problems connected with either direct gas sampling through capillary tubing or with conv~ntion~if diKusion chambers. The method may also be of use in other habitats such as leaf litter or loose surface soil.
REFERENCES Hack H. R. B. (1956) An application of a method of gas microanalysis to the study of soil air. S<~il Science 82, 217-231. Larsen J. and Nilsson J. R. (1985) A new principle of no air-liquid interface but gas celt cultivation: exchange across a synthetic membrane. Proroplusrrtu 125, 214-218. Paganelli C. V. (1980) The physics of gas exchange across the avian eggshell. .&nericun Zoo/@! 20, 329-338. Patrick W. H. (1977) Oxygen content of soil air by a held method. Soil Science Socier_r of Anwricu fournul 41. 65l-652. Rogers C. E., Fcls M. and Li N. N. (1972) Separation by permeation through polymeric membranes. In Recent Decck~pmenrs in Sepamion Science. Vol. 2 (N. N. Li, Ed.), pp. 107-155. CRC Press, Cleveland.
Short Communications Roulier .M. H.. Stolzy L. H. and Szuszkiewcz T. E. (1974) An improved procedure for sampling the atmosphere of field soils. Soil Science Sociery of America Proceedings 38, 687-689. Stevenson B. G. and Dindal D. L. (1987) Insect effects on decomposition of cow dung in microcosms. Pedobiologia 30. 81-92.
997
Tackett J. L. (1968) Theory and application of gas chromatography in soil aeration research. Soil Science Society of America Proceedings 32, 346350. Taylor G. S. and Abrahams J. H. (1953) A diffusionequilibrium method for obtaining soil Bases under field conditions. Soil Science Society of America Proceedings 17, 201-206.
0038-071790 53.00 + 0.00 Copyright C 1990 Pergamon Press plc
19%
SHORT COMMUNICATION SAMPLING AIR FROM DUNG PATS BY SILICONE RUBBER DIFFUSION CHAMBERS HOLTER
PEI-ER Institute
of Population
Biology. University 2100 Copenhagen (Accepted
Universitetsparken
15,
5 June 1990)
Dung pats often contain an extremely rich fauna of insects which have important roles affecting carbon and nutrient release from the material. The composition of the air inhaled by these animals in their special habitat is unknown. Stevenson and Dindal (1987) measured the redox potential (Eh) of dung in laboratory microcosms. but this is an indirect measure in relation to air-breathing animals. In addition. reliable measurements of Eh are only feasible in wet substrate. whereas many of the insects may inhabit rather dry dung. Hence. direct sampling and analysis of the air within dung pats is desirable. Essentially two methods are applied in the related field of soil air sampling. One method uses capillary tubing placed in the soil: samples are drawn directly from the soil pores through the tubing (e.g. Hack, 1956; Tackctt, 1968; Roulicr ef ul., 1974). However. wet dung will block a capillary tube almost immediately. The other method requires a chamber of considerable volume (typically at least SO-100 ml) which is installed in the soil and allowed to equilibrate by diffusion with the soil atmosphere through holes of suitable size (e.g. Taylor and Abrahams. 1953; Tackett. 1968; Patrick, 1977). Small air samples can then be exlrdcted from the chamber without causing undesirable mass flow into the system (in porous soils) or vacuum (in impermeable soil). Chambers of SO-100 ml are clearly unsuitable in small habitats like dung pats. In addition, there are problems with vigorous fungal growth within such chambers. The method to be presented here utilizes small (2 ml), completely closed. diffusion chambers made of ordinary silicone rubber tubing which are placed in the dung pats. Since silicone rubber is highly permeable to various gases (Rogers er ~1.. 1972; Larsen and Nilsson. 1985). the air in a small, closed tube will rapidly equilibrate with the surrounding atmosphere. The air in tubes extracted from dung is sampled in the field, stored and transported to the laboratory where the concentrations of 0:. CO1 and CH, are determined by gas chromatography. After a description of the practical procedure, two important aspects of the method will be examined: (I) the rate at which gaseous equilibration takes place between tubes and the surrounding atmosphere is quantified; this rate has important implications for the sampling procedure; (2) the storage of air samples necessary for transport between field site and laboratory is briefly discussed.
PRACl-lCAL
of Copenhagen, 0, Denmark
PROCEDURE
Pieces of silicone rubber tubing, 9 cm in length with I mm wall thickness and 6 mm i.d. (Asicomo A/S. Fat-urn, Denmark), were closed at each end by rubber stoppers and placed in an experimental dung pat while it was laid out in
the field. Tubes to be placed along the edge of the pat contained a length of thin stainless steel wire to maintain their curvature. On sampling. each tube was withdrawn from the dung and a 1.5 ml sample was taken from the tube with a hypodermic syringe (the tube was gently compressed to avoid sampling against a vacuum). The gas sample was then injected into a 3.5 ml Venoject blood collecting tube (Terumo Corp.), previously filled with pure Nr at atmospheric pressure. The samples were returned to the laboratory and analysed in a gas chromatograph with thermal conductivity detector (Mikrolab Aarhus) at 12OC. using a Poropak Q column for CO? and a Molesieve 5A for 0, and CH,; both columns were 2 m x 2 mm bore, maintained at 30°C. The carrier gas was He with a flow rate of 25ml min-t (Poropak) or i5ml mitt-’ (Molesieve). Results were corrected for the sample dilution by 3.5ml N,.
RATE OF GASEOUS
EQUILIBRATION
The rate of diffusive equilibration of gas concentrations between the inside of tubes and the surrounding atmosphere is crucial to the applicability of the method. Equilibration must be sufficiently rapid for the gas within a tube to follow changing external atmospheric conditions fairly closely. On the other hand, a rapid equilibration might lead to changes of gas concentrations in a tube during the inevitable time lapse between extraction of the tube from the dung and withdrawal of a gas sample. Equilibration rates for Oz. CH, and CO? were determined at 10.5 and 22’C (4 measurements at each temperature). For each measurement, 10 tubes were kept for at least I day in a 5OOml bottle (air-tight lid with a rubber septum) with a mixture (similar to the dung atmosphere) of 0, (0.8-1.5%). CH, (9-19%). CO: (7-13%) and N, at atmospheric pressure. Tubes were then removed from the jar and suspended in air, sampled individually at every 6 min and the I.5 ml samples stored in Venojects until analysis. To express the rate of equilibration from these mcasurements we assume: (I) identical barometric pressure inside and outside the tube; (2) that the diffusive properties of the gases (N,, 0,. CO,, CH,) are similar so that the principles of binary diffusion apply (c/ Paganelli. 1980); and (3) constant composition of the external atmosphere. If C, and C, are the concentrations outside and inside the tube. we define
O=lC,-Ccl. 995
of a particular
gas
336
Short Communications
0
f
s
Table 2. Relatrvc changes m concentratton of CO:. CH, and 0: rn silicone rubber tubes olaced in armosoheric ax
3’
I? g 2 2 =
,L
Q(
Z. I
Ratmsafter
CO,(
A0.994
f
co:
rC0.999 )
_,’
2
2
Imual gas cont. I” nJbe(O.J
Gas CHJ
,!
;
PaO.996 1
30
: u
20
30 Time
40 50 (mitts)
0,
IO 0.1 05
CH. CO1
IO.5 22 10.5 21 10.5 22
Table 3. ERect of prolonged storage (21 C) of a gas mtxture in Venojccts on concentrations of 0:. CH, and CO: Durauon of storattc fdavs) 0.143 7
taken dung.
k tmin “) 0.0 I77 2 0.OOOs.l 0.0216 =00OlJO 0.0273 i o.ooo7.t 0.0345 2 0.0OO83 O.O6t7_~000O61 0.0715 5 O.tlOl06
IO12
t,427 I ofix 10’1
Ratios arc concentration after exposure m atmosphenc air inmat conccntrafion. Calculations based on k-values at 22 C (Table I).
T~hic 1. R~rc cxwst.tnrr (k) for the cxponenttal decrease of gtts concentrafmn diffcrenccs bcrwccn the inside of silicone rubber tubes .tnd the surrounding atmosphere
*SE (n = 4)
I min 0 933 0.931 0.966 0.966 5 465 I.876
60
where D,, and D, arc the vahres of D at times 0 and 1. respectively. Consequently. a plot of InD against I should gave a straight line, slope -X-. Figure I shows that straight hnrs were indeed obtained, and so this simple model seems satisfactory for pmctical purposes. Values of k determined by rcgrcssion (rr 0.986-0.99Y; P
n.
0.982 0.9YI 0.991 2. I’6 1.221 LIOX I.017 I O06
I
In D, = In 0, - kl
( Cl
10
0.966 0 965 0.Yt.Q 0.983 3 245 I.440 I.215 1.035
I
From the laws of diffusion. the rate constant k (where kD = - dD;dt) must be proportional to the permeability of the tube wall and to the ratro of tube area to tube volume. By integration:
Tcmpcraturc
0.5 RI,”
0.983
5 IO
Fig. I. Gas concentration differences between air in the silicone rubber tubes and atmospheric air as a function of exposure ttme of tubes to atmospheric air. Initial gas concentrations in the tubes were 1.2% Or. 9.6% CH,. 72% CO: and 82% N:.
G.l\
0 25 mm
I
C%
g-2 L "
an exposure of
r,,(h) 0.65 0.53 0.42 0.33 0.19 0 16
r,)o$(h) 2.X? 2.3 I 1.83 I .4s O.YI 0.70
The umec at which conccntrauon dtffercnccs arc 50% (fu ,) and 5% (r,,,jq) of their initial value arc also given.
within
Conccntratlon (96) after storage _+SE tn = Xl ____-.0.
CH,
co.
1.532 0.0206 I .5 I i_0.0072
33.1 to.29 335+0.19
19,5*0.13 171i.016
I5 s after
the tube was extracted
from
the
Storage of samples is necessary between field sampling and a convenient time for amtlysis in the laboratory. Table 3 shows that concentrations of Or and CfI, were practically unchanged by 7 days of storage at room temperature in VVenojects, whcrcds cu 1X6 of the initial CO, escaped by difTusion through the stopper. Assuming exponential decrease with time, this corresponds to a relative CO,-Ioss of cu 2% day _I. f fence. sforagc should not exceed a few days if CO! dctcrmtnations arc important. The method presented here works for sampling the gases 0,. CO, and CH,, provided that:
(I)
the tubes are extracted quickly from the substratum; (2) samples are taken immediately upon extraction of tubes; (3) that the concentration of a gas to be measured is at least I-?% of that in the atmosphere.
ff nder these conditions, this simple method avoids some of the problems connected with either direct gas sampling through capillary tubing or with conv~ntion~if diKusion chambers. The method may also be of use in other habitats such as leaf litter or loose surface soil.
REFERENCES Hack H. R. B. (1956) An application of a method of gas microanalysis to the study of soil air. S<~il Science 82, 217-231. Larsen J. and Nilsson J. R. (1985) A new principle of no air-liquid interface but gas celt cultivation: exchange across a synthetic membrane. Proroplusrrtu 125, 214-218. Paganelli C. V. (1980) The physics of gas exchange across the avian eggshell. .&nericun Zoo/@! 20, 329-338. Patrick W. H. (1977) Oxygen content of soil air by a held method. Soil Science Socier_r of Anwricu fournul 41. 65l-652. Rogers C. E., Fcls M. and Li N. N. (1972) Separation by permeation through polymeric membranes. In Recent Decck~pmenrs in Sepamion Science. Vol. 2 (N. N. Li, Ed.), pp. 107-155. CRC Press, Cleveland.
Short Communications Roulier .M. H.. Stolzy L. H. and Szuszkiewcz T. E. (1974) An improved procedure for sampling the atmosphere of field soils. Soil Science Sociery of America Proceedings 38, 687-689. Stevenson B. G. and Dindal D. L. (1987) Insect effects on decomposition of cow dung in microcosms. Pedobiologia 30. 81-92.
997
Tackett J. L. (1968) Theory and application of gas chromatography in soil aeration research. Soil Science Society of America Proceedings 32, 346350. Taylor G. S. and Abrahams J. H. (1953) A diffusionequilibrium method for obtaining soil Bases under field conditions. Soil Science Society of America Proceedings 17, 201-206.