diffusion probe for gas sampling in undisturbed soil

European Journal of Soil Science, September 2014, 65, 663–671 doi: 10.1111/ejss.12170

Diffusion probe for gas sampling in undisturbed soil

S . O . P e t e r s e nDepartment of Agroecology, Aarhus University, PO Box 50, DK8830 Tjele, Denmark

Summary

Soil-atmosphere fluxes of trace gases such as methane (CH4) and nitrous oxide (N2O) are determined by complexinteractions between biological activity and soil conditions. Soil gas concentration profiles may, in combinationwith other information about soil conditions, help us to understand emission controls. This paper describes asimple and robust diffusion probe for soil gas sampling as part of flux monitoring programmes. It can be deployedwith minimum disturbance of in-situ conditions, and also at sites with a high or fluctuating water table. Separateprobes are used for each sampling depth, in this study ranging from 5 to 100 cm. The probe has a 10-ml diffusioncell with a 3-mm diameter opening covered by a 0.5-mm silicone membrane. At sampling the diffusion cell isflushed with 10 ml N2 containing 50 μl l−1 ethylene (C2H4) as a tracer; tracer recovery is used to calculate sampleconcentrations. Ethylene is immediately removed by flushing with unamended N2. Equations are presented tocorrect for dead volumes of connecting tubing and valves. Laboratory tests evaluated recovery of CH4, N2O andcarbon dioxide (CO2), removal of C2H4 and equilibration of CH4, N2O and CO2 in air and water. Field tests onpeat soils used for grazing showed soil gas concentrations of CH4 and N2O as influenced by topography, siteconditions and season. The applicability of the diffusion probe for trace gas monitoring is discussed.

Introduction

Terrestrial ecosystems are important net sources of atmosphericmethane (CH4) and nitrous oxide (N2O) (Denman et al., 2007), butit is still not well understood how fluxes are influenced by man-agement practices such as drainage, cultivation and fertilizer appli-cation, or indirectly through climate change (Zimov et al., 2006).The direction and magnitude of greenhouse gas fluxes are deter-mined by a complex balance of reaction and transport processes.Production of CH4 is normally associated with methanogenic activ-ity in saturated soil, although production above the water table hasalso been reported (Knorr et al., 2009; Schäfer et al., 2012). Unsat-urated soil can be an important sink for CH4, depending on factorssuch as nutrient status (Hütsch, 1998), physical disturbance (Prieméet al., 1997), gas diffusivity (Estop-Aragonés et al., 2012) and veg-etation (Couwenberg et al., 2011). Nitrous oxide, as a free interme-diate of biologicial denitrification, may also be metabolized duringtransport between sites of production and the soil surface (Cloughet al., 2005). For both CH4 and N2O, soil gas concentration profiles,and changes in isotopic composition, may reveal information aboutzones of production or removal (Scheutz et al., 2009; Park et al.,2011; Clough et al., 2013), which, in turn, can help us to understandthe regulation of atmospheric emissions.

A wide range of techniques have been developed for measurementof soil gas concentrations. Measurements may involve extraction of

Correspondence: S. O. Petersen. E-mail: [email protected]

Received 26 February 2014; revised version accepted 24 June 2014

gas from sub-surface sampling positions via nylon tubing (Rolstonet al., 1976), stainless steel tubing (Park et al., 2011; Petersen et al.,2011) or more advanced probe designs (Nauer et al., 2013). Distur-bance of soil gas distribution during sampling can be avoided bysampling from a diffusion cell in contact with the surrounding soilwith a gas-permeable membrane of polytetrafluoroethylene (Mag-nusson, 1989), polyethylene (DeSutter et al., 2006), polypropylene(Gut et al., 1998) or silicone (Dörr & Münnich, 1980; Thomas &Lloyd, 1995; Panikov et al., 2007). Gas from the diffusion cell istransported via gas-impermeable tubing to an analytical unit or sam-pling port, typically by intermittent flushing with an inert gas withor without recirculation of the sample gas (Gut et al., 1998; Panikovet al., 2007; Deppe et al., 2010). Not all procedures assume equi-librium before measurement, but rely on calibration of the detectorresponse (Rothfuss & Conrad, 1994; Panikov et al., 2007). With adiffusion cell approach, equivalent gas phase concentration profilescan be monitored also at sites with a high or fluctuating water table.

In the past, diffusion cell probes have been used in the field afterhorizontal installation from a trench or pit (Davidson et al., 2007;Estop-Aragonés et al., 2012), or after burial (Gut et al., 1998), orthey have been installed in excavated soil columns (Deppe et al.,2010). Manipulation of the soil during installation can changein-situ soil conditions, and natural fluctuations of the water tableare not easily reproduced in excavated soil cores. Also, withlarge spatial variability in source strength under field conditions,the number of sampling positions may be significant, requiring asimple and flexible method for soil gas sampling.

© 2014 British Society of Soil Science 663

664 S. O. Petersen

Ø 13,9

(a)

Ø 3

SS-tubing Ø 16 x 12

(b)

Hole for handle

Stopper

120

30

Soil gas sampler (6red)

Shrinkable sleeving

Silicone disc Ø 12 x 0,5 mm

(c)

Luer Lock needle 18 G

Ø 6

50

M 14 x 1

M 14 x 1

soldered

soldered

25

RS 397-944

(d)

Figure 1 A schematic illustration of the diffusion probe. (a) Open-ended diffusion cell with 3-mm diameter central opening and a 30-mm long section withreduced diameter. A silicone membrane over the opening is held in place by heat-shrinkable sleeving. (b) Hollow shaft with thread connecting the diffusioncell to the soil surface. It is equipped with two custom-made syringes with a 90∘ bend near the top, one of which extends to near the centre of the diffusioncell. Also near the top is an opening for a crossbar. (c) Conical plug of hardened PVC with thread. (d) The assembled probe; shaft length is varied dependingon sampling depth.

In this paper, a diffusion probe that may be installed near

flux sampling points at depths ranging from 5 to 100 cm with

minimum disturbance of the soil profile is described. It has a

10-ml diffusion cell that is flushed with N2 during sampling,

enabling sample collection and analysis as an integrated part of flux

monitoring programmes. The flushing gas contains 50 μl l−1 C2H4,

which enables calculation of equivalent gas phase concentrations in

the undiluted sample. Laboratory tests evaluated the performance

of the probe for analysis of CH4, N2O and CO2, and preliminary

field data from grassland sites on peat are presented.

Materials and methods

Probe design

The gas sampling probe described below was produced by Mikro-

lab (Højbjerg, Denmark). The basic design includes an open-ended

diffusion cell (Figure 1a) and a variable-length shaft extend-

ing to the soil surface (Figure 1b), both prepared from 16 mm

outer× 12 mm inner diameter corrosion-resistant steel tubing

(grade EN 10216-5), and a conical tip extending 30 mm below

the diffusion cell when mounted (Figure 1c). Probe tips were

© 2014 British Society of Soil Science, European Journal of Soil Science, 65, 663–671

Diffusion probe for soil gas sampling 665

(a)

(b)

Figure 2 (a) A set of diffusion probes corresponding to depths of 5, 10, 20, 50 and 100 cm. The lower edge of the red tape marks insertion depth. Each probeis assigned a unique code for tracing any abnormalities. (b) An example of probes installed between paired sampling points within a grazed pasture.

initially prepared from brass but, to eliminate the risk of galvaniccorrosion under wet soil conditions, these are now replaced by tipsof hardened PVC. The assembled probe is depicted in Figure 1(d).

The diffusion cell diameter is reduced to 14 mm over a 30-mmlong section, where a central 3-mm diameter opening is cov-ered by a 12× 0.5 mm2 silicone disc held in place by 25-mmlong heat-shrinkable sleeving (RS Components, Copenhagen, Den-mark). This design helps protect the silicone disc during instal-lation. The variable-length shaft (Figure 1b) is fitted with twotubes connected to the diffusion cell that are prepared fromcustom-made cut-off needles of stainless steel (18G, inner diameter0.8–0.875 mm) with female Luer fittings. The inlet tube extends tothe centre of the diffusion cell, whereas the outlet tube opens at thetop of the cell; both are embedded in a soldered plug with an exter-nal thread for assembly with the diffusion cell. Both syringes arebent to 90∘ and protrude on opposite sides near the top of the shaft(Figure 1b), where there is also a separate hole for a crossbar usedwhen removing the probe and a brass stopper at the top. The threeparts are assembled around 11.9× 1.78 mm2 neoprene O-rings toensure gas- and waterproof connections.

Field installation and sampling

The length of the probe shaft can be varied to meet the requiredsampling depth; in the present study probes corresponding tosampling depths of 5, 10, 20, 50 and 100 cm below the soilsurface were used. Sampling depth is defined by the position ofthe 3-mm diameter opening of the diffusion cell. To facilitate probe

installation and reduce compaction around the diffusion cell, a holemay be pre-made with an auger to near the final position of the3-mm diameter opening. A set of probes are shown in Figure 2(a),and an example of side-by-side installation of probes to five depthsin Figure 2(b).

For gas sampling, 3-l aluminum foil gas sample bags (SKC Ltd,Blansford, UK) with N2 and N2 + 50 μl l−1 C2H4 (AGA, Enköbing,Sweden) were taken to the field site. A three-way valve is mountedon the outlet tube. This is fitted with a 10-ml glass syringe (Poulten& Graf Fortuna, Frederiksen, Ølgod, Denmark) in the horizontalposition, and a 6-ml pre-evacuated Exetainer (Labco Ltd, Lampeter,UK) in the vertical position. Then 10 ml N2 +C2H4 is injected intothe inlet tube using a plastic syringe. This forces the (now partlydiluted) gas sample into the glass syringe, where the displacementof the piston serves as a check for leakages. Changing the three-wayvalve position enables transfer of the gas sample from the glasssyringe to the Exetainer; the piston is actively forced back to thezero position in order to pressurize the vial. The N2 +C2H4 gas isalso transferred directly to Exetainers (n= 3) at the time of samplingfor later analysis together with soil gas samples. Following gassampling, the diffusion cell is flushed with 2× 60 ml N2 for removalof residual C2H4. Both Luer fittings are closed between samplings.

Gas analysis

An Agilent 7890 gas chromatograph (GC) equipped with a CTCCombiPal auto-sampler and dual inlets (Agilent, Nærum, Denmark)was used to analyse CH4, N2O and CO2 in a single 2-ml sample


666 S. O. Petersen

split between the two inlets during injection. One channel had a 2-mback-flushed pre-column with Hayesep P connected to a 2-m maincolumn with Poropak Q. From the main column, the gas streamwas diverted either to an electron capture detector (ECD) for N2Oanalysis, or to a flame ionization detector (FID) for CH4 analysis.The carrier of this channel was N2 at a flow rate of 45 ml minute−1.For the ECD, Ar-CH4 (95%/5%) at 40 ml minute−1 was usedas make-up gas. The FID was supplied with 45 ml minute−1 H2,450 ml minute−1 air and 20 ml minute−1 N2. Temperatures of theinjection port, columns, ECD and FID were 80, 80, 325 and 200∘C,respectively. The second channel was equipped with a 3 m PoropakQ column and used He at 42 ml minute−1 both as carrier and asreference for the thermo-couple detector (TCD). Temperatures ofthe inlet, column and TCD were 80, 80 and 250∘C, respectively,and with He at 7 ml minute−1 as make-up gas. Concentrations werequantified with reference to synthetic air and a calibration mixturecontaining CH4, N2O and CO2 at dry mixing ratios of, respectively,9.97, 1.994 and 1999 μl l−1.

Ethylene was also analysed by gas chromatography, but elution ofC2H4 could not be accommodated with the valve and temperatureprogramming needed for analysis of the other three gases. Instead,a separate injection of 1-ml sample gas to channel 1, with a run timeof 6 minutes, was used for analysis of C2H4. All GC settings were asdescribed earlier, except that gas from the main column was alwaysdirected to the FID because C2H4 was the only analyte.

Correction for dilution

The concentration of a gas, cS, may be corrected for dilution by:

cS = cm∕[

1 −am

aF

], (1)

where cm is the concentration observed, am is the concentration oftracer measured, and aF is the concentration of tracer in the flushinggas. However, the inlet and outlet tube volumes, din and dout, preventfull mixing of the sample and flushing gas. If the volume of flushinggas is V , then only (V − din) enters the diffusion cell, and recoveryof C2H4 should be related to this fraction:

a′F =

V − din

VaF . (2)

It may be assumed that the contents of dout are quantitativelyrecovered in the sample and not mixed with the flushing gas. Toaccount for this, am should therefore be corrected by:

a′m =

V + dout

Vam. (3)

The gas sample collected will include contributions from both thediffusion cell and dead volumes:

Vcm = dincin + doutcout +(V − din − dout

)c′m, (4)

where cin and cout are the concentrations of the analyte in the inletand outlet tubes, and c′m is the corrected (true) concentration in

the measurement cell. Hence, the adjusted concentration of a givensample gas becomes:

cS = c′m∕[

1 −a′

m

a′F

]= c′m∕

[1 −

(V + dout

)am(

V − din

)aF

], (5)

where c′m = cm in the case of complete equilibrium between thediffusion cell and the outlet tube at sampling (thus cin = cout = c′m),and c′m =

[V∕

(V − din − dout

)]cm if there is no exchange between

the diffusion cell and tubes between samplings. The degree ofequilibration as a function of time is not known, but is discussedlater with reference to these assumptions.

Laboratory tests

The recoveries of N2O, CH4 and CO2 by the sampling proceduredescribed above were determined in a test, where probes corre-sponding to sampling depths of 5, 10 and 100 cm were flushed with2× 60 ml of the calibration mixture. Each probe was then flushedwith 10 ml N2 +C2H4 and collected gas samples analysed for con-centrations of N2O, CH4, CO2 and C2H4.

Then the effectiveness of C2H4 removal following gas samplingwas evaluated by first injecting 10 ml N2 +C2H4 into a 100-cmprobe to simulate a sampling situation. Subsequently, the probe wasflushed with 120 ml N2, but in 10-ml portions that were collectedseparately in 6-ml Exetainers in accordance with the gas samplingprocedure described earlier.

Next, the time course of equilibration with air was evaluated in atest where three sets of probes representing all five sampling depthswere all flushed with 60 ml of the calibration mixture, and then leftat room temperature. After 1, 4, 7, 10 and 14 days, three probes wererandomly selected and the diffusion cell flushed for collection of agas sample as described above.

Finally, the time course of equilibration was examined withprobes immersed 10 cm (relative to the 3-mm opening) into tapwater at atmospheric pressure and temperature, and with gentlestirring of the water to ensure constant boundary conditions. Fivesets of probes corresponding to sampling depths of 10, 20 and 50 cmwere sampled after, respectively, 1, 3, 7, 10 and 17 days.

Field tests

Spatial variability with respect to soil gas concentration profileswas examined in a trial where probes corresponding to gas sam-pling depths of 5, 10, 20, 50 and 100 cm were installed next tothree paired sampling positions at a drained fen peat site usedfor grazing until 2 years earlier. Previously, CH4 and N2O fluxeshad been monitored at this site (referred to as E-PG in Petersenet al., 2012). Several characteristics of the soil horizons at eachsampling position have been described by Schäfer et al. (2012).The three paired sampling positions (blocks) were separated byapproximately 10 m, with block 1 on a ridge and block 2 in adepression. The water tables of blocks 2 and 3 were consistently45 and 35 cm below the level at block 1 (unpublished data). Theprobes were installed in September 2009, and the concentration



Table 1 Percentage recovery (±SD) of N2O, CH4 and CO2 from selected probe lengths when sampled immediately after flushing with a calibration gas mixturecontaining 9.97 μl l−1 CH4, 1.994 μl l−1 N2O and 1999 μl l−1 CO2

N2O CH4 CO2

Probe (cm) Equation (1) Equation (5) Equation (1) Equation (5) Equation (1) Equation (5)

5 94.5 (2.3) 96.1 (2.3) 92.9 (2.2) 94.5 (2.2) 95.8 (1.7) 97.4 (1.8)10 95.8 (0.9) 97.6 (0.9) 93.5 (1.2) 95.3 (1.1) 96.3 (4.5) 98.3 (4.7)100 98.6 (2.0) 102.9 (2.2) 96.6 (1.9) 100.9 (2.1) 95.0 (1.5) 97.9 (3.0)

Concentrations were calculated from the dilution of 50 μl l−1 C2H4 according to Equation (1) (no correction for dead volumes) or Equation (5) (correction fordead volumes).

profiles presented here were measured on 5 May 2010, after theprobes had over-wintered in the field.

The performance of the probes during long-term deploymentwas examined by monitoring seasonal dynamics of soil N2Oconcentration profiles in three contrasting grassland sites on peatwith a history of grazing. The sites will be referred to as Mørke(56.38∘N, 10.40∘E), Torsager (56.45∘N, 9.61∘E) and Fussingø(56.48∘N, 9.83∘E). One of the three sites, Torsager, had not beengrazed for several years prior to this study, and at all sites livestockwere excluded from the sampling area. Torsager was located ina river valley that is regularly flooded during winter. A set of gassampling probes (five depths) was installed at each site in earlyMay 2010, and all probes were then sampled in connection withfield campaigns during weeks 17, 25, 27, 32 and 36 (Mørke) or 39(Fussingø). A PEH piezometer with a screen at around 1-m depthwas installed within 1 m from the position of soil gas samplingprobes, where water table depth was (except in week 27) recordedduring sampling.

Results and discussion

Recovery of N2O, CH4 and CO2

Displacing the contents of the diffusion cell with an inert gas gavea variable (typically 10–25%) dilution of the original sample, and atracer was therefore added to the flushing gas to enable calculationof gas concentrations in the original sample. In the first test toexamine recovery of N2O, CH4 and CO2, probes were flushed with aknown calibration mixture and then immediately sampled (Table 1).The amounts of C2H4 corresponded to 16.4–20.3% dilution in thistest. Recoveries were calculated using both Equations (1) and (5),that is, without or with correction for dead volumes; full equilibriumbetween diffusion cell and connecting tubing could be assumed inthis test. Corrections for dead volumes din and dout, which bothrepresented around 0.85 ml for the longest probe, are not trivial,as indicated by the differences between recovery estimates usingEquation (1) or (5). With Equation (5), the calculated recovery was96–103, 95–101 and 97–98% for N2O, CH4 and CO2, respectively.

Removal of C2H4 following sampling

In preliminary tests the tracer was sulphur hexafluoride (SF6),which could be determined together with N2O. However, SF6 was

difficult to flush out of the probes, resulting in carryover, and wastherefore replaced by C2H4, which is readily removed. Ethylene is aplant hormone that can affect root development (Sharp & LeNoble,2002), and should therefore not be released to the soil environment.Any diffusion loss across the silicone membrane was expected tobe negligible provided C2H4 was effectively removed following the< 5-minute gas-sampling procedure. A test was therefore conductedto examine the effectiveness of C2H4 removal by flushing the probewith 2× 60 ml N2. In this test the flushing gas was collected in 12fractions of 10 ml each, and concentrations of C2H4 in each fractiondetermined (data not shown). The cumulated recovery of C2H4

corresponded to a 23.6% dilution of the original sample, whichwas comparable to recoveries obtained during regular use. Ethylenecould not be detected in fractions 7–12, showing that the fieldsampling procedure with 2× 60 ml N2 effectively removes C2H4.

Equilibration in air and water

Gas sampling probes were first flushed with a calibration mixtureand left surrounded by air until sampled (n= 3). Laboratory airwas also collected at each sampling. Recovery was calculatedwith Equation (5), but full cm correction was not possible becauseof the unknown extent of equilibration with dead volumes. Theshaded areas in Figure 3 represent the potential range of recoveriesassuming full equilibrium or no equilibration, respectively, betweenthe diffusion cell and dead volumes. In this test, no equilibrationimplied that dead volumes contained the calibration mixture,whereas under field conditions dead volumes would contain N2.The equilibration of N2O and CO2 with laboratory air was nearlycomplete within 48 hours (Figure 3), whereas CH4 in the diffusioncell equilibrated more slowly and only reached the concentrationof surrounding air at the last sampling. The delay in CH4 exchangerelative to N2O was in accordance with previous observations withsilicone tubing (Petersen et al., 1998). The agreement betweenobserved equilibrium concentrations, calculated with Equation (5),and background concentrations of surrounding air indicates thatgas phase concentrations can be reliably estimated with the tracerapproach.

The calculated uncertainty ranges were smaller for the test ofequilibration in water (Figure 4). There was little dilution of sampleair in this test, around 10% in most cases, presumably becauseturbulence during sampling was kept to a minimum. Equilibration


668 S. O. Petersen

(a) (b) (c)

Figure 3 Diffusion probes were flushed with a calibrationmixture containing 9.97 μl l−1 CH4 (b), 1.994 μl l−1 N2O(a) and 1999 μl l−1 CO2 (c) and left at room temperatureto equilibrate with atmospheric air. Background concen-trations of CH4, N2O and CO2 in the laboratory air duringthe experiment were 2.38, 0.407 and 565 μl l−1, respec-tively. Numbers represent average± SD (n= 3). Shadedareas represent the possible range of recovery assumingfull equilibrium or no equilibration, respectively.

(a) (b) (c)

Figure 4 Diffusion probes were flushed with a calibrationmixture containing 9.97 μl l−1 CH4 (b), 1.994 μl l−1 N2O(a) and 1999 μl l−1 CO2 (c) and immersed in water at roomtemperature. Numbers represent average±SD (n= 3).Shaded areas represent the possible range of recoveryassuming full equilibrium or no equilibration, respectively.

of N2O in water was delayed substantially compared with air. Incontrast, little difference was observed in CO2 equilibration in watercompared with air, possibly because the carbonate system mediatedgas exchange with the atmosphere. Similar CO2 exchange rates inair and water were also observed by Panikov et al. (2007). Theequilibration of CH4 in water was, as in air, delayed relative to theother gases, although temporal dynamics were somewhat irregular.In saturated soil, equilibration of the diffusion cell will be furtherdelayed by the presence of a solid phase; this is discussed later.

Effects of spatial heterogeneity

Profiles of N2O and CH4 were measured in early May in threepositions at a peat-land pasture site with variable topography. Gasphase concentrations are reported here as mixing ratios, becauseinformation about water-filled porosity could not be obtained at thetime, but commercial equipment for measurement of volumetricwater content at multiple depths with minimum disturbance isavailable (Nauer et al., 2013). Table 2 reports actual samplingdepths relative to the soil surface (Di), as well as ‘absolute’ samplingdepths (Dabs) relative to the highest (ridge) position.

For N2O, the maximum soil concentration was observed aroundDabs = 50 cm in blocks 1 and 3, whereas in block 2 concentrationswere generally small. Methane concentration profiles were verydifferent between blocks, but all sampling positions showed CH4

accumulation below Dabs = 50 cm, showing that CH4 accumulated

Table 2 Soil concentration profiles of N2O and CH4 were determined on 5May 2010 at three positions within a grazed pasture with some topographicvariability. Three positions were selected adjacent to flux sampling points ina block design previously used for a GHG monitoring study (Petersen et al.,2012). In addition to actual installation depths (Di), the absolute depths(Dabs) were calculated from observed differences in WT depth across theseason

Di Dabs N2O CH4

Position / cm / cm / μl l−1 / μl l−1

Block 1 (ridge) 5 5 0.60 2.010 10 2.07 1.620 20 2.55 1.650 50 58.8 6.1

100 100 1.28 4035Block 2 (depression) 5 50 0.41 34.5

10 55 0.34 25620 65 1.99 22650 95 2.08 10.0

100 145 0.57 2.0Block 3 5 40 0.61 1.9

10 45 0.44 1.820 55 3.91 2.050 85 0.63 25.2

100 135 0.18 49.5



at < 10 cm from the soil surface at the depression (block 2). Theseobservations would be consistent with a WT depth of approximately50 cm (Dabs), and with very different potentials for emission ofN2O and CH4 at the three positions. Indeed, a previous monitoringprogramme (Petersen et al., 2012) consistently found N2Oemissions at the ridge position, and CH4 emissions at the depres-sion where emissions appeared to be mediated by soft rush (Juncuseffusus L.). Kutzbach et al. (2004) described similar effects oftopography on (plant-mediated) CH4 emissions from a tundra soil.

Seasonal dynamics of N2O

During a 5-month period in 2010, soil concentration profilesof N2O to 1-m depth were recorded at 2- to 6-week intervalsin three pastures on peat with a history of grazing (Figure 5).Concentrations of CH4 were also recorded, but will be presentedelsewhere (Henneberg et al., in preparation). Except for a campaignat all three sites on 6 July, WT depth was recorded at the time ofsampling. The Torsager site was flooded by the time of sampling inSeptember, and no results could be obtained.

The dynamics of N2O in the soil profile were very different atthe three sites. In Mørke, little N2O accumulated during spring, andthere was even evidence of reduction of N2O to below atmosphericconcentration during summer. In September, the accumulation ofN2O extended to 50-cm depth, and thus well below the water tablewhere anoxic conditions presumably dominated. There may havebeen a downward flux of N2O produced in the capillary fringe, orN2O could have been produced below the water table after reductionof NO3

− accumulating during summer drainage. Goldberg et al.(2010) reported that N2O emissions from a minero-trophic fenduring the first 2 weeks after a WT rise accounted for 20–40% oftotal emissions in a > 70-week monitoring period.

At the Torsager site, N2O concentrations of up to 25 μl l−1 wererecorded at 5-cm depth in late May. The WT was recorded at120-cm depth at this time; hence N2O production was not a resultof recent soil saturation. The Torsager site is flooded by a nearbystream during winter and this could have damaged the vegetation,resulting in net N mineralization in the root zone during spring. Thezone of maximum N2O accumulation moved downward in the soilprofile during summer.

Finally, at Fussingø there was little evidence of N2O productionin the soil profile, but N2O reduction below the water table wasindicated. Water-table depths at this fen peat site were shallow,20–40 cm, and it is likely that capillary rise saturated large partsof the soil volume above the water table, preventing nitrification(Bloemen, 1983; Deppe et al., 2010).

General discussion

Gas exchange rates between soil and a diffusion cell will partlydepend on membrane properties. DeSutter et al. (2006) reportedthat the equilibration time for CO2 over different silicone tubingmaterials (wall thickness of 1.6–3.2 mm) was 3.9–9.7 hours. Thediffusion probe presented in our paper has a silicone membrane

that is 0.5 mm thick, indicating that equilibration time for CO2

across this membrane will be shorter. There are contrasting reportsabout the degradation of silicone during its use (Rothfuss & Conrad,1994; Panikov et al., 2007). All materials tested by DeSutteret al. (2006) tolerated field deployment for 183 days. The diffusionprobes described here remained functional after both winter andsummer use for 5–6 months.

As well as membrane properties, equilibration is a function ofarea and volume of the diffusion cell, which define, respectively,spatial and temporal resolution. The probe design described herehas a large spatial resolution with an orifice of only 3-mm diameter,but a large diffusion cell volume of 10 ml. The latter was chosen toallow for sample collection and gas chromatographic analysis withthe same configuration used for determination of fluxes with staticchambers. This implies that equilibrium between the diffusion celland surrounding soil should then be achieved within the time rangenormally used in flux monitoring programmes.

The diffusion cell equilibrates with a soil volume that is deter-mined by porosity, water content and gas solubility, as well asdiffusion cell volume. The following example considers equilibra-tion of CH4 in saturated soil as a worst case. With a solubility ofCH4 in water of 0.04 ml ml−1 at 10∘C (Yamamoto et al., 1976),CH4 in the 10-ml diffusion cell will correspond to CH4 dissolvedin 250 ml pore water. From information about soil conditions atthe Mørke site (Schäfer et al., 2012), and assuming a particledensity of 1.4 and 2.65 g cm−3 for soil organic matter and minerals,respectively (Davidson et al., 2007), the porosity below the watertable was estimated to be about 0.8 cm3 cm−3 at both ridge anddepression positions. Following Thomas & Lloyd (1995), theaverage diffusion distance as a function of time may be calculatedas: x=

√(qtDs), where x is the distance (cm), q is a constant (2

for one-dimensional transport), t is time (s) and Ds is the effectivediffusion coefficient (cm2 s−1) estimated as described by Thomas& Lloyd (1995); for the saturated peat Ds was estimated to be0.75× 10−5 cm2 s−1. The average diffusion range within 1, 7 and14 days would then be, respectively, 1.2, 3.2 and 4.6 cm, corre-sponding to spheric pore water volumes of 6, 115 and 324 ml. It istherefore not unrealistic to expect a 10-ml diffusion cell to reachequilibrium with dissolved CH4 within 1–2 weeks in saturated soileven in the absence of de novo production. Gas sampling will createconcentration gradients around the probe, but this should lead toreplenishment of the sampled gases from greater distances. Giventhe dynamics of changes in water table depth and temperaturein most soils, even a bi-weekly to monthly sampling frequencymay contribute useful information about seasonal variations indistribution of trace gases in the soil profile (Davidson et al., 2007;Petersen et al., 2012). This is also a typical sampling frequency forflux monitoring programmes that rely on manually operated staticchambers. Compared with CH4, the solubility of N2O is muchgreater (Weiss & Price, 1980), and the corresponding diffusionrange and equilibration time are therefore also shorter. For bothgases, the presence of air-filled pores will substantially shorten theequilibration time. Sampling intervals for N2O and CO2 as short as2–3 days may be possible in well-drained soil (see Figure 3).


670 S. O. Petersen

Figure 5 Diffusion probes were installed at three sites, Mørke (a), Torsager (b) and Fussingø (c), with a history of grazing. Sampling took place five timesduring the growing season, except at Torsager, where the site was flooded from September. The plots show concentrations of N2O, and dotted lines indicaterecorded WT levels at sampling. Only three sets of probes were available at the time, hence no replication was possible.

Conclusion

The basic design principles of the diffusion probe described in thispaper are not new, but novel features were the use of a tracer gas forcalculating in situ soil gas concentrations, and equations to accountfor dead volumes. The diffusion probe was developed for use inconnection with greenhouse gas monitoring and should be readilyintegrated into existing gas sampling and analysis routines. As nosoil excavation or trench is needed for installation, probes can beplaced near flux measurement positions, or even within permanentchamber supports. The experimental data presented in this paper,and most literature cited, relate to peat soil, but the diffusion probe

has also been used recently in studies on mineral soil (Görres et al.,2013; Baral et al., 2014); hence the range of potential applicationsappears to be wide.

Acknowledgements

I wish to thank Vagn Aagaard, Mikrolab A/S, for valuable dis-cussions about probe design. The skilled technical assistance ofBodil Stensgaard, Lene Mejer Rosenlund and Jørgen M. Nielsenis gratefully acknowledged. Construction and testing of the probewas partly funded by the Danish Ministry of the Environment, andpartly by the Department of Agroecology, Aarhus University.



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diffusion probe for gas sampling in undisturbed soil

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