diurnal oscillations in gas production (o2, co2, ch4, and n2) in soil monoliths

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Diurnal Oscillations in GasProduction (O2, CO2, CH4, and N2)in Soil MonolithsS.K. Sheppard & D. LloydPublished online: 09 Aug 2010.

To cite this article: S.K. Sheppard & D. Lloyd (2002) Diurnal Oscillations in Gas Production(O2, CO2, CH4, and N2) in Soil Monoliths, Biological Rhythm Research, 33:5, 577-591

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Diurnal Oscillations in Gas Production (O2, CO2, CH4,and N2) in Soil Monoliths

S.K. Sheppard and D. Lloyd

Microbiology Group, Cardiff School of Biosciences, Cardiff University, Cardiff

Abstract

Soil cores (35cm long, 7cm diameter) from the Macaulay Land Use Research Insti-tute’s Sourhope Research Station in the Scottish Borders were kept and monitored atconstant temperature (18 ± 1°C) for gas production using a 1.6mm diameter stain-less steel probe fitted with a membrane inlet and connected to a quadrupole massspectrometer. This provided a novel method for on-line, real time monitoring of soilgas dynamics.

In closed-system headspace experiments, O2 and CO2 (measured at m/z values 32and 44, respectively) showed anti-phase diurnal fluctuations in low-intensity simu-lated daylight and under a light-dark (LD, 12:12h) regime. O2 increased duringperiods of illumination and decreased in the dark. The inverse was true for CO2

production. Ar (m/z = 40) concentration and temperature (°C) remained constantthroughout the experiments.

The same phase-related oscillations, in CO2 and O2 concentrations, were observedat 2 and 5cm depth in soil cores. The O2 concentration did not oscillate diurnally at10cm depth. In below-ground experiments, CH4 (m/z = 15) concentration showeddiurnal cycles at 2, 5 and 10cm depth. The CH4 production had the same diurnalphase cycle as CO2 but with lower amplitude. Evidence of below-ground diurnaloscillations in N2 (m/z = 28) concentration was provided at 5cm depth.

The scale of production and consumption of gases associated with soil-atmosphereinteractions and below-ground processes, are shown to be a multifaceted output ofseveral variables. These include light, circadian-controlled physiological rhythms ofplants and microbes, and the interactions between these organisms.

Keywords: CO2, O2, CH4, N2, soil, diurnal oscillations, circadian rhythms.

Address correspondence to: Prof. D. Lloyd, Microbiology Group, Cardiff School of Biosciences, CardiffUniversity, P.O. Box 915, Cardiff, CF10 3TL Wales, UK. Tel.: + 44 (0) 2920 874772; Fax: +44 (0) 2920874305; E-mail: [email protected]

Biological Rhythm Research 0165-0424/3305-577$16.002002, Vol. 33 No. 5. pp. 577–591 © Swets & Zeitlinger

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Introduction

Gaseous exchange between land and atmosphere is a fundamental step in the bio-geochemical processes that determine climate homeostasis and environmental change(Schimel, 1995). The earth’s atmospheric chemistry is partly determined by processesin the terrestrial component of the biosphere. Of particular significance are the activ-ities of microorganisms in soils and sediments (Lloyd, 1995; Conrad, 1995). Thesource and sink concentrations of gases in these ecosystems are largely responsiblefor maintaining atmospheric composition, and thus mediation of global warming(Conrad, 1996).

The soil atmosphere is composed of a number of, sometimes transitory, gases exist-ing in dynamic equilibrium governed by abiotic and biotic processes. Primarily N2,O2 and CO2, these gases also include NH3, H2, CO, NOx, SO2, H2S, CH4, C2H4 andmiscellaneous volatile organics. The soil atmosphere generally has a lower O2 andhigher CO2 concentration than the above-ground atmosphere, due to the respirationof plant roots and soil microbes. These two gases are maintained in equilibrium andthe decrease in soil O2 is proportional to increase in soil CO2 (Paul & Clark, 1996).This balance is maintained by the low diffusivity gradient (1–2%) required to moveO2 and CO2, which have similar diffusion constants, between soil and atmosphere(Paul & Clark, 1996).

Soil aeration is determined by two major physical factors and is inversely propor-tional to the bulk density and moisture content of the soil (Paul & Clark, 1996). AtO2 concentrations of < 1%, the ecosystem becomes almost anaerobic and may becharacterised by the predominance of anaerobic metabolism, such as methanogene-sis and dentrification. These processes also occur in anaerobic microsites of manyaerobic soils (Wachinger et al., 2000). Although, as one may expect, anaerobic pro-cesses may dominate at greater depths in soils where O2 concentrations are lowest(Sheppard & Lloyd, 2002a,b), some studies have shown that much greater numbers(¥ 10) of anaerobic bacteria reside in the upper three centimetres of soils (Paul &Clark, 1996). One possible reason for this is the preparatory role of aerobic microbesin creating and maintaining anaerobic conditions through O2 consumption. Also,anaerobic organisms can persist in an inactive state in aerobic soil environments whileconditions remain unfavourable for their growth (Paul & Clark, 1996).

Soil microbial processes and associated gas dynamics, vary between soil samplesand with the depth at which readings are taken (Sheppard & Lloyd, 2002a,b). Spatialvariations in soil physical, chemical and biological properties are not, however, the only factor influencing soil gas dynamics. Temporal variations in gas productionalso occur. Thus, for example, terrestrial ecosystems that are usually CO2 sinks, maybecome net sources of CO2 during winter months as reduced solar energy limits pho-tosynthetic uptake and thus microbial and plant respiration out-balances photosyn-thetic incorporation of CO2 (Whiting & Chanton, 1993). Modelling the gas dynamicsof soils is, therefore, difficult and must incorporate a number of parameters. Wheregas concentrations are likely to be different at depth and diffusion processes are a sig-nificant factor, concentration equations for gases can be constructed (Cripps, 1996).The following equation is adapted from the Clements and Wilkening (1974) one-

578 S.K. Sheppard and D. Lloyd

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dimensional transport equation for radon concentration in soils, and attempts to char-acterise gas concentrations by considering numerous parameters:

Where C = concentration of gas (molm-3), D = bulk diffusion coefficient of gas insoil (m2 s-1), t = time (s), e = soil porosity, z = spatial direction (m), v = velocity ofsoil gas, or fluid volume current density (ms-1, defined as positive upwards), l =consumption of gas (molm-3 s-1), and F = production of gas (molm-3 s-1).

As long as the values in such an equation remain constant, a solution can be cal-culated relatively easily (Cripps, 1996). Conclusions on gas concentrations will there-fore, by definition, be valid only for a given time. This is not an ideal situation at all,as the gas concentration of soils may change dramatically from hour to hour, as naturalenvironmental factors influence the system. Accurate measures of the daily fluctua-tions in soil gases are vital if models of gas consumption and production in soils areto be achieved. Traditionally such analytical results were difficult to obtain, but recentdevelopments in mass spectrometry enable the continuous measurement of multiplegas species at various depths throughout soil cores. Membrane inlet mass spectrom-etry (MIMS) has been used in the ‘on-line’ analysis of peat cores to observe diurnaloscillations in gas production of CO2, CH4 and O2 (Thomas et al., 1998; Beckmann& Lloyd, 2001). In this study, the initial aim is to apply similar techniques to characterise the dynamics of diurnal variation in gas production in an upland soil andthus add to the understanding of the functional diversity of associated microbial communities.

Materials and Methods

Sample site and maintenance of cores

Sample cores were taken to represent a pristine upland grassland ecosystem, from theMacaulay Land Use Research Institute’s Sourhope Research Station in the ScottishBorders, National Grid Reference: NT854196. Sourhope is positioned at the head ofthe Bowmont valley, 15 miles south of Kelso on the western slope of the Cheviothills. The bedrock of the site area is glacial till, derived from andesitic lavas of theOld Red Sandstone Age. The major soil subgroup was brown forest soil with mod-erate but variable soil drainage. Detailed descriptions of Sourhope soil physico-chemical characteristics and soil horizons are provided by Sheppard and Lloyd(2002a).

Soil cores were extracted using PVC cylinders (35cm long, 7cm diameter), sealedat the base, and maintained in constant temperature environmental growth chambers(12h light/dark cycle). Illumination was achieved using a bank of twelve 58W fluo-rescent tubes (Thorn White Pluslux 3500), suspended 50cm from the soil surface.Photosynthetically active radiation (PAR) was measured at 110 mmol s-1 m-2, using anIntegrated/Quantum/Radiometer Phomometer (LI-188B equipped with a LI-190SB

∂∂ e

∂∂ e

∂∂

lC

t

D C

z

vC

zC= ◊ - ◊

( )- +

2

2

1F

Diurnal Oscillations in Soil Gases 579

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Quantum Sensor, LI-COR Inc.). All subsequent experiments were carried out in a controlled-temperature chest freezer (18 ± 1°C) with a small orifice drilled in thelid for insertion of the MIMS probe. The watering regime was carefully monitored(70ml per week, reverse osmosis purified) reflecting natural rainfall levels from February 2, 1999, to June 30, 2000 (2.5mm/day). Cores received water one day prior to commencement of each experiment.

Membrane inlet mass spectrometry (MIMS)

Constant Depth profiles of gas concentrations were obtained using membrane inletmass spectrometry (Srinivasan et al., 1997). Gases were monitored using a VG-quadrupole mass spectrometry system (SX 200, Vacuum Generator, Middlewich, UK)through a membrane inlet probe. This method is readily reproducible and causesminimum disturbance to the system under study (Boddy & Lloyd, 1990; Lloyd et al.,1986). The mass-to-charge (m/z) ratios 15(CH4), 28(N2), 32(O2), 40(Ar) and 44(CO2),were monitored at a total pressure of 1 ¥ 10-6 mbar (Lloyd & Scott, 1985). Partialpressure data were acquired on a PC in multiple ion monitoring (MIM) mode (SpectraLab V6, European Spectrometry Systems, Northwich, UK). One meter ofstainless steel tubing (1.6mm O.D. and 0.5mm I.D.) was used as a probe. A 0.3mmorifice was drilled 7mm from the sealed end of the tube. The opening was coveredwith a 0.5cm length of gas permeable, silicone membrane (O.D. 1mm, I.D. 0.5mm).Details of membrane inlet probes are provided by Benstead and Lloyd (1994), Thomasand Lloyd (1995), Lloyd et al. (1996), and Cowie and Lloyd (1999). Modificationsto the membrane inlet probe were necessary for insertion into firm terrestrial soilsand involved the welding of stainless steel flanges above and below the inlet pore(Sheppard & Lloyd, 2002a). The calibration of the gas concentration in soil porespaces was conducted by direct introduction of known concentrations of each of thegases under study, into sterile (Autoclaved) soil (Sheppard & Lloyd, 2002a). Fromthese data, mass spectrometer output was related directly to gas concentrations at fixedtemperatures (Wilhelm et al., 1997). Analysis of diurnal oscillations and headspaceemission commenced 3–20 days after sampling from the field site.

Sub-surface and head space measurements

In below-ground gas analysis, the core surface remained uncovered (open system)and measurements were made by careful insertion of the probe to the precise depthin the core, measured by 0.5cm markings on the probe exterior. Calibration of theMIMS system was carried out for each gas species under consideration as describedby Sheppard and Lloyd (2002b). Three depths were selected for sub-surface gas analy-sis of untreated soils. These were 2cm, 5cm and 10cm and were selected to repre-sent the H, Ah and B soil horizons (Green et al., 1990). Three replicate cores wereused for analysis at each depth. In below-ground diurnal oscillation experiments, oneexample is included, as natural variation limited the usefulness of calculating means.Each experiment commenced at 9 am (time = 0h) and continued for approximately3 days (76h). The number of readings per hour varied between experiments.


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Headspace measurements were made in cores with an inverted funnel sealed tothe surface of the PVC cylinder (closed system). A thermometer and the MIMS probewere inserted through the funnel orifice, which was not sealed (Fig. 1). Partial pres-sures were monitored using the same probe as for other experiments, but calibrationof the MIMS system was carried out in gas phase prior to each experiment. Thevolume of the headspace was 120cm3 and gas concentrations are given in mM.

Accumulating gas concentrations in the headspace of the soils during these exper-iments may influence gas exchange rates. CO2 levels, which are greater than ambientvalues, accumulate relatively quickly and may cause increased plant growth rates and photosynthetic activity. Anaerobic microbial processes may also be stimulated.To limit this problem, the head-space was flushed daily to restore ambient gas con-centrations. Experiments were, therefore, carried out over shorter time periods (1 day)than for those for below-ground experiments. Three replicate cores were analysed forboth day and night experiments and one example is included.


Figure 1. Experimental set-up for headspace gas analysis from soil monoliths. Light inten-sity was 18.7mmols-1 m-2 (PAR) at the core surface.

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Results

Below-ground gas dynamics

Typical data on diurnal oscillations in sub-surface soil gas concentrations are pro-vided (Figs. 2, 3 and 4). The probe was maintained at 2, 5 and 10cm in the H, Ahand B horizons, respectively, and gas concentrations and temperature were monitoredthroughout the light/dark cycles of three days. The temperature remained constantthroughout the core, 18 ± 1°C. Soil argon concentration reflected the soil consistencyand hence effective porosity, with approximate values of 15.2, 10 and 4.6 mM at 2, 5 and 10cm depth, respectively, in the examples provided. Argon concentrationremained relatively constant throughout the experiments, indicating that experimental


Figure 2. Diurnal cycles in below-ground gas concentrations (N2, O2, Ar, CO2, and CH4) mea-sured directly at 2cm depth (H horizon). Trend lines associated with different gases are labelledon the graph. Temperature is given in °C. Gas concentrations are measured in mM, or mM forCO2. Light intensity was 110 mmols-1 m-2 (PAR).

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conditions of temperature and instrument sensitivity did not vary. This results fromthe biologically inert nature of this gas; variations reflect only physical changes in,for example, temperature, instrument sensitivity and diffusion coefficients. O2, CO2

and CH4 concentrations oscillated with a diurnal pattern at 2cm depth (Fig. 2). O2

concentration increased from approximately 220mM to 330mM, during the period ofillumination. The mean amplitude (peak-trough expressed as % of their minimalvalues) was 50%. CO2 and CH4 displayed the opposite pattern, both decreasing duringthe period of illumination: amplitudes of approximately 104 and 130%, respectively,were observed. N2 fluctuated to some extent, but a clear diurnal cycle was not evidentin any of the sample cores at 2cm depth. In experiments conducted at 5cm depth,concentrations of O2, CO2, CH4 and N2 oscillated with a diurnal pattern (Fig. 3), from


Figure 3. Diurnal cycles in below-ground gas concentrations (N2, O2, Ar, CO2, and CH4) mea-sured directly at 5cm depth (Ah horizon). Trend lines associated with different gases arelabelled on the graph. Temperature is given in °C. Gas concentrations are measured in mM, ormM for CO2. Light intensity was 110 mmols-1 m-2 (PAR).

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between values of approximately 300mM, 1.25mM, 2.2mM and 525mM, respectively,in the light, to 150 mM, 4mM, 3mM and 730mM, respectively, during the dark. As at2cm depth, oxygen concentration increased, with a minimum amplitude of 100%,throughout the period of illumination, and decreased to an equal extent in the dark.Other measured gas concentrations, CO2, CH4 and N2, showed inverse changes witha minimum amplitude of 220, 36 and 39%, respectively, throughout the period ofdark. In the third set of sub-surface gas analysis experiments, conducted at 10cmdepth, diurnal oscillations were not observed in O2 or N2 concentrations in any of thecores analysed (Fig. 4). The concentration of these gases, and Ar, was also consider-ably lower than in the A-soil horizon. This reflects a change in the soil physical prop-erties, decreased aeration and a tendency towards anaerobic conditions (Sheppard & Lloyd, 2002a,b). Diurnal oscillations were, however, observed in CO2 and CH4


Figure 4. Diurnal cycles in below-ground gas concentrations (N2, O2, Ar, CO2, and CH4) mea-sured directly at 10cm depth (B horizon). Trend lines associated with different gases arelabelled on the graph. Temperature is given in °C. Gas concentrations are measured in mM, ormM for CO2. Light intensity was 110 mmols-1 m-2 (PAR).

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concentrations with maximum values of 3.5mM and 2.5 mM, and minimum values of2.6mM and 1.5, respectively. The mean peak-trough amplitudes for CO2 and CH4

were 35 and 67%, respectively. The greatly reduced amplitude of the CO2 oscillationand absence of O2 oscillation in the B horizon, compared to the A horizon, reflectsthe reduced impact of surface plant photosynthesis on gas dynamics at this depth.Regular cycles, as presented in Figures 2, 3, and 4, were observed in all three repli-cate cores for each depth experiment and no desynchronization was observed, as wasthe case in similar studies on peat cores (Thomas et al., 1998).

Headspace gas dynamics

Figures 5 and 6 show typical trends in N2, O2, Ar, CO2 and CH4 in the headspace inone of three replicate soil cores maintained with artificial 12h light/dark cycle. The


Figure 5. Diurnal cycles in headspace gas concentrations (N2, O2, Ar, CO2, and CH4). Trendlines associated with different gases are labelled on the graph. Temperature is given in °C. Gasconcentrations are measured in mM, or mM for CO2. Light intensity was 110 (18.7 at coresurface) mmols-1 m-2 (PAR) and illumination ceased at time 12 (h).

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head space was flushed every 24h. Figure 5 shows trends observed over 0–12h ofdark, and Figure 6 shows trends over 0–12 hours of light and 12–18h dark. Underthe glass funnel the light intensity was low and was measured as 18.7 mmolesm-2 s-1

PAR at the soil vegetation surface. The headspace temperature was continuously mon-itored and showed little fluctuation (17.7 ± 0.1°C). N2, Ar and CH4 concentrationsremained almost unchanged throughout experiments and there was no evidence ofdiurnal rhythmicity associated with these gases in the headspace of soil cores. O2 andCO2 concentrations followed similar diurnal patterns to those observed in sub-surfacesoil experiments. O2 decreased over the period of dark from 333 mM to 243mM after12h. Following a 1-h period of equilibration, CO2 concentration demonstrated the opposite trend increasing, over the 12-h period from 0.23mM to a maximum of0.87mM. The mean amplitudes (peak-trough expressed as % of their mean values)


Figure 6. Diurnal cycles in headspace gas concentrations (N2, O2, Ar, CO2, and CH4). Trendlines associated with different gases are labelled on the graph. Temperature is given in °C. Gasconcentrations are measured in mM, or mM for CO2. Light intensity was 110 (18.7 at coresurface) mmols-1 m-2 (PAR) and illumination commenced at time 12h.

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of O2 and CO2 were 37 and 239%, respectively (Fig. 5). Throughout the period ofillumination O2 concentration increased from 231mM to 350mM, with a mean ampli-tude of 52%. CO2 concentration decreased during the period of illumination from0.78mM to 0.57mM, with a mean amplitude of 37%. Similar results were obtainedin all three replicate cores.

A table listing the changes in soil gas concentrations (M) and the amplitudes ofdiurnal oscillations (%) in soil headspace and at 2, 5 and 10cm depth, is included(Table 1).

Discussion

A useful method for continuously monitoring variations in production and consump-tion of various gases in soils over days, weeks or months, is provided by membraneinlet mass spectrometry (MIMS). In this study, methods are applied for the first timeto intact and minimally perturbed upland soil microcosms; studies with cores fromwetland peat bogs have previously been reported (Thomas et al., 1998, Beckmann &Lloyd, 2001). Phase-related diurnal oscillations are observed in headspace gas con-centrations of O2 and CO2 (Figs. 5 and 6). O2 concentration in the headspace increases


Table 1. Typical changes in soil gas concentration (N2, O2, CO2 and CH4) throughout 12-h light/dark cylces, expressed as mol l-1 (M) and mean amplitude of diurnal oscillations(peak-trough expressed as % of their minimal value), in Sourhope soil headspace and at 2, 5and 10cm depth. Soil Ar concentration is not included as no oscillations were observed in anyexperiment.

Gas N2 (mM) O2 (mM) CO2 (mM) CH4 (mM)

Head space 1 Dark-phase – 243 0.78 –(Fig. 5) Light-phase – 333 0.23 –

Amplitude – 37% 239% –

Head space 2 Dark-phase – 231 0.57 –(Fig. 6) Light-phase – 350 0.78 –

Amplitude – 52% 37% –

2cm depth Dark-phase – 220 4.6 2.3(Fig. 2) Light-phase – 330 2.25 1

Amplitude – 50% 104% 130%

5cm depth Dark-phase 730 150 4 3(Fig. 3) Light-phase 525 300 1.25 2.2

Amplitude 39% 100% 220% 36%

10cm depth Dark-phase – – 3.5 2.5(Fig. 4) Light-phase – – 2.6 1.5

Amplitude – – 35% 67%

– = no oscillation observed.

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during periods of illumination. This indicates that, even in indoor experiments withrelatively low light intensity, photosynthetic activity compensates for the plant andmicrobial respiratory O2 demand. In similar experiments on changing peat core head-space gas compositions, the O2 concentration was seen to decrease in the light phaseat low light intensities (Thomas et al., 1998). In such experiments the respiratorydemand of the soil microbes is not achieved by photosynthetic O2 production, possi-bly due to the stimulation of microbes by plant secretion of photosynthates (Rovira,1969). CO2 showed an inverse relationship with O2 production, being consumedduring periods of illumination (Figs. 5 and 6). This is evidence for the dominance ofphotosynthetic CO2 fixation in the light phases.

Sub-surface analysis of gas concentrations in the H, Ah and B horizons wasachieved by positioning the probe at 2, 5 and 10cm depth. At all three depths, diurnalchanges in gas concentrations can be explained as secondary consequences of photosynthetic plant processes. In aerated upland soils, O2 concentrations remain sufficiently high to maintain aerobic conditions at 10cm depth (Sheppard & Lloyd,2002a,b). There is however considerable evidence to suggest the presence of anaero-bic microbial communities throughout the soil monolith. Production of CH4 and N2

provides evidence for the presence of methanogens and denitrifiers, respectively. Inall cores examined, where increased production of these gases was observed, it wasassociated with increased production of CO2 and a corresponding decrease in the O2

concentration. It is, therefore, reasonable to assert that it is the generation of anaer-obic conditions and a reduction in the inhibitory effect of O2 concentrations thatenhances methanogenesis and denitrification. Conversely, as O2 concentrationsincrease, aerobic microbial processes are enhanced, hence the consumption of CH4

by methanotrophic bacteria.The trends in gas production in sub-surface soils may be associated simply with

the diffusion paths of gas products from microbial processes and photosynthesis toand from the soil surface. This is certainly feasible at 2cm and 5cm depth in suchaerated soils. It is likely, however, that gas movement is facilitated by plant rootsystems. Specialized aerenchyma cells in some vascular plants provide a continuousconduit system, and enable mass transport of gases from deep root systems to leavesand out through stomata (Thomas et al., 1996; Beckmann & Lloyd, 2001). The routeof least resistance will be preferentially selected by gases diffusing through soil cores.The presence of vascular plants has been shown to effectively enhance Ar diffusioncoefficients through peat cores by an order of magnitude, when compared to diffu-sion through water (Stephen et al., 1997). Such dramatic facilitation of gas transportin peat cores is usually associated with large vascular plants such as Eriophorum andCarex (Thomas et al., 1996). It is, however, important to note that diurnal oscillationsin CO2 and CH4 concentrations, as observed in this study (Fig. 4), have also beendemonstrated in peat cores in the absence of such plants (Beckmann & Lloyd, 2001).Clearly, if diffusion of gases is possible through a semi-aquatic system such as peat,it will occur through aerobic soils without augmentation from large vascular plants.

Increasing CO2 and CH4 concentrations, at 2, 5 and 10cm, and N2 concentrationat 5cm — all in the dark — may be the result of decreased plant-controlled conduc-tion of these gases to the soil surface. Equally, decreased O2 conduction down to oxic


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zones surrounding plant roots (Thomas et al., 1998) may result in the observedincreases in CO2, CH4 and N2 as fermentation processes, methanogenesis and deni-trification in the rhizosphere would be enhanced. Diurnal cycles in CO2 and CH4 con-centrations were observed at 10cm depth without a corresponding oscillation in O2

concentration. Despite the phase relationship with light/dark, this does not necessar-ily support the idea that below-ground oscillations, in gases associated with micro-bial metabolism, are a simple secondary consequence of plant-driven processes. Dailyrhythms may be influenced by ‘clock-controlled’ cycles associated with plants ormicroorganisms. Self-sustained, free-running, endogenous rhythms are defined asoscillations that are perpetuated for a large number of cycles while external environ-mental conditions remain constant (Bünning, 1964). Further experiments are neces-sary to determine if the observed oscillations, for example in CO2 and CH4 at 10cm,are free-running in isolation from the influence of green-plant mediated circadiancycles.

The scale of production and consumption of gases associated with soil-atmosphereinteractions and below-ground processes has been shown to be a multifaceted outputof several variables. These include light, circadian-controlled physiological rhythmsof plants and microbes, and the interactions between these organisms. Diurnal cyclesin gas production, similar to those observed in monoliths from wetlands (Thomas et al., 1998; Beckmann & Lloyd, 2001), have been demonstrated in soil cores. Thismethodology provides a basis for the modelling of gas dynamics and functional diver-sity of soil ecosystems. The experiments highlight the importance of measuring gasproduction at comparable times of day. Continuous monitoring over extended time-scales (weeks, months, years) is feasible using MIMS; the filament life is more thansix months, and automated change-over to a second filament is possible. Inter- andintra-annual variability of flux rates of biogenic trace gases, including those of CH4,(Khalil & Rasmussen, 1983), CO2, N2O and NO (Potter & Klooster, 1998) have beendocumented. Whether the measurements were routinely taken at a fixed circadian timeis not clear from the publications. It is important to understand the differences inshort-term trends and long-term variations in biosphere-atmosphere interactions andto make a distinction between them. Quantification of seasonal variations should, there-fore, take account of diurnal changes such as those presented in this study. Automatedcontinuous readout of the type initiated here will provide more definitive studies.

Acknowledgements

SKS held a Natural Environment Research Council Studentship Dr. Stuart Davies andDr. Manfred Beckmann provided expertise.

References

Beckmann M, Lloyd D (2001): Mass spectrometric monitoring of gases (CO2, CH4, O2)in mesotrophic peat core from Kopparas Mire, Sweden. Glob Change Biol 7: 1–10.


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diurnal oscillations in gas production (o2, co2, ch4, and n2) in soil monoliths

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