FEMS Microbiology Ecology 102 (1993) 185-195
© 1993 Federation of European Microbiological Societies 0168-6496/93/$06.00
Published by Elsevier
185
FEMSEC 00434
Methane emission and methane oxidation
in land-fill cover soil
H i l a r y A. J o n e s 1 a n d D a v i d B. N e d w e l l
Department of Biology, University of Essex, Colchester, UK
Received 13 March 1992
Revision received 12 November 1992
Accepted 18 November 1992
Key words: Landfill; Methane oxidation; Methanotroph; Greenhouse effect
1. S U M M A R Y
The vertical profiles of methane and oxygen
concentrations were measured in the cover soil at
four sites in a restored and covered landfill. At
sites 2 and 3 within the landfill area methane was
detectable even to the soil surface and emission
of methane occurred at these two sites. Measured
methane emission rates varied seasonally and appeared to be most influenced by soil water content. On an annual basis methane emissions at
these two sites were 495 and 909 mol methane
m - 2 y - l , respectively. At sites 1 and 4 methane
was detected in the cover soil but was not present
in the immediate subsurface layer, and emission
of methane did not occur. Oxidation of methane
by bacteria Within the soil profile at these two
sites appeared to prevent methane emission from
the surface.
Correspondence to: David B. Nedwell, Department of Biology,
University of Essex, Colcester CO4 3SQ, UK.
I Present address:,~Warren Spring Laboratory, Gunnels Wood
Rd, St~venage, Herts SG1 2BX, UK.
A methane-oxidising microflora had been enriched in the soils of all four landfill sites, as
shown by counts of methanotrophs and methylotrophs which were greater than in controls of
garden soil not subjected to elevated methane.
Counts of methanotrophs and methylotrophs were
generally higher in those soil strata where
methane concentrations were greatest.
Methane oxidation rates were maximum at soil
depths where gradients of methane and oxygen
overlapped, usually 10-30 cm depth. The depth
integrated rates of methane oxidation were very
high at sites 2 and 3, the sites also where methane
was emitted from the soil surface. A maximum
oxidation rate of 450 mmol C H 4 m - 2 d -1 was
measured at site 3.
The data suggested that the microflora in the
soil above landfill adapted to the presence of
elevated methane concentrations by selection of a
more methanotrophic community which was able
to rapidly oxidise methane. Optimisation of microbial oxidation of methane by bacteria in landfill cover soil may provide a cheap m a n a g e m e n t
strategy to minimise the emissions of methane to
the atmosphere from landfill.
186
2. INTRODUCTION
The degradation of organic matter in landfill
sites generates methane which may be sufficient
to be economically collected and used [1-3].
However, in many, particularly older or small
sites, it is often uneconomical. In the absence of
suitable preventative measures, landfill gas may
migrate considerable distances both vertically and
horizontally. Where a site is completed and
capped with soil vertical migration brings the gas
from the anaerobic region of the landfill where it
is formed into an aerobic environment where it
may be subjected to microbiological oxidation by
methanotrophic bacteria. The ability of microorganisms to oxidise methane has been long recognised but it is only recently that the significance
of its oxidation above landfill has been examined
[4,5]. Whalen et al. [4] recently reported the high
capacity for methane oxidation in the cover soil
above a sanitary landfill site near San Francisco;
rates of 45 g m-2 d - ~ being the highest reported
for any environment. Even at high methane concentrations the soil's capacity to oxidise methane
was not saturated with methane, but waterlogging
of the soil restricted oxidation rates because of
reduced vertical transport of the gas.
Although carbon dioxide is presently the most
significant greenhouse gas, atmospheric methane
is increasing annually by about 0.9% compared to
carbon dioxide increasing by 0.4% [6]. Moreover,
over a 100 year span methane has a global warming potential 21 times that of carbon dioxide
because of its stronger molar absorption coefficient for infra red radiation and longer residence
time in the atmosphere. Methane is formed by
bacteria within anoxic environments such as
aquatic sediments, wetlands, paddy fields, anaerobic digesters and the animal rumen [7]. Emissions of methane to the atmosphere from different environments have been measured [8,9] but
the relative contributions of each environment to
net atmospheric accumulation is poorly defined.
Moreover, it has become apparent that considerable microbial oxidation of methane can occur
within soil and sedimentary environments before
it is released to the atmosphere. In freshwater
sediments and wetland soil between 15 to > 90%
of methane has been reported to be oxidised
[10-12] and 80-95% of methane production in
peatbogs [8]. Whalen & Reeburgh [14] reported
that the activity of methane oxidising bacteria in
tundra soil resulted in a net consumption of atmospheric methane, and other environments have
been reported to be net sinks of methane [15].
Clearly, therefore, where methane production is
high its impact upon the atmospheric environment may besignificantly reduced by the activity
of methane oxidation in soil before its release to
the atmosphere.
The present work was undertaken to investigate the rates of methane emission, methane
concentration profiles within soil, and activity of
methane oxidising bacteria in the cover soil above
a completed and capped landfill site.
3. MATERIALS AND METHODS
3.1. Site
The landfill site studied was the completed
and restored area of Martin's Farm Landfill Site
in Essex, UK. (National Grid Reference TM
117177). Although some of the site is still operational, the area studied was completed in 1980. A
thin restoration soil cover (approximately 40-60
cm maximum) was applied with a sealing layer of
clay over some parts of the site.
3.2. Sampling strategy
A contour profile of the site is shown in Fig. 1
and four sites were studied along this transect.
On the basis of preliminary measurements, the
sites were selected to represent the range of
distribution of CH 4 and 0 2 within the landfill
soil. At each site a marking peg was permanently
inserted. Samples were subsequently taken at
each site with a 10 x 10 m grid around the peg
using randomly selected coordinates on the grid
for each sample. In order to study the extent of
C H 4 oxidation on the site a variety of measurements were made. Some preliminary data have
been described [5].
3.3. Concentration profiles of gases in soil
Vertical concentration profiles of CH 4 and 0 2
187
PlQn
C H 4 and 0 2 analysed by gas chromatography
within 3 h of recovery of the stakes. (Preliminary
work had shown that there was no significant
exchange of gases from the chambers of the stakes
within this time). Average concentrations of the
gases at each depth were then calculated and the
vertical gas concentration profiles drawn for each
site.
Active lQndfiU.
1
t_.______~e
Pr 0f i
Creekf---.
~_ ______
2
~
Se_o leye_l. . . . . . . .
Fig. 1. Plan and vertical profile of the Martin's F a r m landfill
site, Essex, UK.
in cover soil were measured with diffusion samplers. These were steel stakes machined with
chambers (1 cm diameter by 1 cm deep at 1.5 cm
depth intervals) covered with polyethylene membrane. The design of these stakes was derived
from samplers described by Hesslein [16] and
used in aquatic sediments [17]. They are described in detail by Jones & Nedwell [5]. The
stakes permitted the soil gas atmosphere to be
ar~alysed at 1.5 cm intervals over the top 36 cm of
soil depth.
From March 1988 to May 1989, triplicate stakes
were randomly inserted at each of the four selected sites and left for 48 h for the soil gas
atmosphere at each depth to equilibrate across
the membrane. (Equilibration was usually complete within 24 h [12]). The stakes were then
removed, immediately sealed with gas-impermeable plastic tape, and returned to the laboratory.
Gas samples were withdrawn from each chamber
with a microsyringe through the membrane and
3.4. Gas analysis
A gas chromatograph (Carlo-Erba Fractovap
model 4200) was used to measure CH 4 and O 2.
For CH 4 analyses samples (50/~1) were injected
onto a glass column (3 m x 3 mm i.d) packed with
silica gel (80/100 mesh); N 2 carrier gas at 15 ml
min-X; column temperature 100°C; flame ionisation detector. Limit of detection was 0.1 nmol
ml -x. For 0 2 analyses samples (50 /zl) were injected onto a stainless steel column (3 m x 3 mm
i.d.) packed with Carbonsieve S (Phase Separations Ltd., UK); He carrier gas at 25 ml min-1;
column temperature 50°C; thermal conductivity
detector. Limit of detection was 0.5 nmol ml" ~.
(N.B. The higher limits of detection given previously by Jones & Nedwell [5] were incorrect.)
3.5. Emission of methane from soil surface
Each month triplicate polyethylene containers
(21.5 × 24.5 × 7.5 cm) were randomly placed onto
the soil surface at each site and pressed into the
surface of the soil. The containers were shaded to
prevent direct insolation. A Subaseal (Gallenkamp Ltd., UK) through the upper surface of
each container allowed access to the enclosed gas
and samples were withdrawn over a period of 3 h
with hypodermic syringes and sealed for subsequent analysis of C H 4. Increase in the CH 4 concentration with time was linear and permitted
calculation of the emission rate of CH 4 from the
surface of the soil.
3.6. Temperature and water content of the land-fill
cover soil
When each month's samples were taken the
soil temperature at 10 cm depth was measured
with a thermometer. Samples of surface soil were
also removed and dried at 105°C to determine
the soil water content.
188
3. 7. Organic content of soil
The organic content of surface soil from the
four study sites was determined by measuring
weight loss of triplicate samples of soil heated at
650°C in a muffle furnace for 2 h.
After the medium set the plates were autoclaved at 121°C for 20 min, cooled and dried in a
laminar flow hood to maintain sterility, and then
stored in foil until used.
3.9. Preparation of counts
3.8. Counts of methanotrophs and methylotrophs
Counts of methanotrophic and methylotrophic
bacteria were made with methane or methanol as
substrate. Initial work used a basal mineral salts
medium [18] containing methanol (0.2% w/v)
solidified with 1% (w/v) bacteriological agar (No.
1, Oxoid, UK). However, high background counts
were obtained on control plates containing no
methanol, the bacteria apparently growing on organic impurities in the agar. Repeated tests with
other 'high purity' agars, and the polysaccharide
gelling agent Gelrite (Merck & Co., Kelco Division, USA) still gave unacceptably high background counts. Many bacteria hydrolysed Gelrite.
With agarose (BDH Ltd., UK) background counts
were usually acceptably low but agarose is prohibitively expensive for routine counting. However, a number of counts were carried out using
methanol agar solidified with agarose (1% w/v).
Finally silica gel was used as the solidifying
agent for media in order to avoid introducing
traces of available organic substrates, which gave
false-positives for methanotrophs or methylotrophs during counting and isolation procedures.
A 5% sodium silicate solution (BDH Ltd, UK)
containing silicon dioxide (SiO 2) and sodium
monoxide (Na20) was passed through a strongly
acid cation exchange column (Dowex 50W, hydrogen form) to remove excess sodium hydroxide
[19]. The resulting solution was collected and
mixed with phosphate buffer (pH 7) and mineral
solutions to give (per Petri plate):
5% sodium silicate solution 10 ml
phosphate buffer (pH 7, 0.2 M) 9.2 ml
0.1% CaCI 2 solution 0.2 ml
0.05% FeSO 4 solution 0.2 ml
2.0% MgSO 4 solution 0.2 ml
mineral salts solution 0.2 ml
Mineral salts solution. (per 100 ml) K H z P O 4
5g; K2HPO 4 5g; NHnNO 3 5g; trace element solution (20) 10 ml.
Decimal dilutions of weighed amounts of soil
were prepared in sterile mineral salts solution
containing Tween 80 (0.1%) to aid removal of
cells from soil particles. Soil suspensions were
agitated for 1 min with a whirlimixer before samples were removed. Inocula were spread onto
triplicate plates at each dilution and incubated at
22°C until there were no further increases in
colony counts. Plates were incubated in chambers
containing 50% CH 4 in air and the atmosphere
was renewed every 48 h. High purity CH 4 (British
Oxygen Company, UK) was used as normal laboratory grade C H 4 may contain trace gases inhibitory to methanotrophs. Plates inoculated with
dilutions giving 30-300 discrete colonies were
selected and used to calculate the number of
methanotrophs in the soils. Control plates incubated in the absence of CH 4 always gave negligible counts demonstrating that there was no significant growth of microorganisms in the absence of
methane.
On a number of occasions samples of surface
soil were taken aseptically from each o f the four
sites at Martin's Farm and numbers of methylotrophs or methanotrophs counted, either with
methanol or with methane provided as substrate
(see Results). During July, 1989, a soil auger was
used to remove soil samples from different depths
over the soil profiles at each site. Triplicate soil
samples were taken from the undisturbed centre
of the soil core at a number of different depths
and counts of methanotrophs carried out as described previously.
3.10. Measurements of methanotrophic and methylotrophic activity
Two approaches were used in an attempt to
determine activity in soils.
[14C]methanol oxidation. All methanotrophs
oxidise methanol, although not all methylotrophs
oxidise methane [21]. Methanol oxidation was
used initially as a general index of potential
189
methylotrophic activity. To soil samples (2 g) in a
flask a small volume (2 ml) of methanol solution
(0.1%, v / v ) was added to provide a high and
constant methanol concentration. A [lac]methanol solution (250/zl; 1.5 kBq ml-1; Specific Activity 2 GBq mmol-1; Amersham International, UK)
was then added to each flask, which was agitated
to evenly distribute the solutions. The flasks were
incubated at field temperature for known times.
The incubation was ended by adding 5 ml
hydrochloric acid (50%, w / v ) and each flask was
gassed for 30 min with an air stream from which
carbon dioxide had been removed previously by
passage through soda lime. The air stream was
then bubbled through 2 ml ethoxyethanol/ethanolamine (7 : 1, v / v ) which absorbs CO z. Recovery
of added [14C]bicarbonate standards was > 95%.
Scintillant (2 ml Optiphase Safe; Packard Instrument B.V., Holland) was added to each trap and
radioactivity in the recovered CO 2 was counted in
a scintillation counter (Rackbeta, LKB, Sweden).
14CH4 oxidation. Radiolabelled 14CH4 was
prepared by using a culture of methanogenic bacteria grown in the medium of Zeikus et al. [22]
without acetate under H z / C O z (80:20%, v/v).
To the actively growing culture 1 ml of H14CO3
(1 mCi; 37 MBg) was added and the culture was
incubated until the H z was completely removed.
Residual CO 2 was then removed by making the
culture alkaline and allowing CO z to absorb for
48 h. Specific Activity of the 14CH4 was determined by passing a known amount of the gas in
an airstream through copper oxide in a tube
furnace at 950°C. (Carbon dioxide was removed
from the airstream with soda lime) after passage
through the column 14CO2 was absorbed as described above and c o u n t e d in a scintillation
counter. The gas used in these experiments had a
specific activity of 48 kBq mmo1-1 (13 /zCi
mmol - ~).
For each measurement of 14CH4 oxidation,
triplicate samples of soil from each sample site
were taken, and multiple subsamp!es (2 g) were
subsequently taken from each and placed in conical flasks (20 ml) sealed with subaseals. An aliquot
(2 ml) of CH 4 was injected into each flask followed by 500/zl of 14CH4, giving a concentration
of 12.5% methane in the headspace. The flasks
were incubated for 6 h at 22°C, which was the
temperature of surface soil at this time of the
year. Three replicate flasks were removed at
known intervals and the radiolabel present as
14CO2 was measured as described previously.
3.11. Experimental
In the first survey in April 1989, turnover of
[14C]methanol in surface soil from all four sites at
Martin's Farm was measured. Controls of normal
garden soil from the Wivenhoe Park campus were
also measured to provide comparison with soil
not exposed to elevated concentrations of
methane.
In September, 1989, the turnover of 14CH4 in
surface soil from the four sites was carried out,
followed later in the month by a complete survey
of CH 4 oxidation profiles with depth at the four
sites. Controls of garden soil were again included.
4. RESULTS
4.1. Soil organic matter
Samples of surface soil from the four sites
yielded organic contents (% weight loss at 650°C)
of 4.95 (SE + 1.35), 2.38 (SE + 1.01), 8.07 (SE +
2.08), 3.29 (SE + 1.36) at sites 1 through 4, respectively.
4.2. Soil temperature, and water content
Soil temperature at 10 cm depth varied only
between 5-15°C at sites 1, 2 and 4, but at site 3
the temperature rose to 25°C in the summer. This
site was definitely a 'hot spot' and the high temperature could be felt at the surface. The soil
moisture content in the surface soil at each site
(Fig. 2) typically varied between low values near
5% during summer to as great as 35% during
winter when the soil was saturated.
4.3. Emission of methane
Methane emission (Fig. 2) was never detected
at sites 1 and 4, but was detected at sites 2 and 3
where the methane concentration profiles extended up to the surface of the soil. Emission
rates were maximum during summer at these two
190
t,0
within the soil profiles at the four sites (Fig. 3).
Comparison of the CH 4 profiles showed that at
all four sites 0 2 penetrated down into the top 36
cm of soil, decreasing in concentration with depth.
However, at sites 2 and 3 the 0 2 concentration in
the soil atmosphere in the top 0-1 cm slice was
always less than that expected by equilibration
with air suggesting that 0 2 was being rapidly
removed in the surface soil at these two sites. In
contrast, CH 4 concentration profiles decreased in
concentration towards the surface.
Methane was never present within the surface
10 cm of soil at sites 1 and 4 and was often not
detectable at all within the 36 cm deep profile
examined. At sites 2 and 3 CH 4 was always detectable within the soil profile and usually extended up to the surface. Comparisons of the
profiles in winter and summer suggested that at
sites 2 and 3 CH 4 concentrations in the surface
layers of soil were always greater during the summer than in the winter.
o
o
o
o
o
o
3c
o
o=
o
20
o
o
•
o
I
I
•
m
I
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.
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,
,
,
,
,
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=Z 06
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t.J
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ol
~
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o',.,
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÷
-
.....
.
M.o.~.o,
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MQy Jun JuL Aug Sep Oct Nov Dec Jan Feb Mar Apr Mny
4.5. Counts of methylotrophs and methanotrophs
Fig. 2. Soil water content, and emission rates of methane from
soil, at four sites at Martin's Farm landfill. • = site 1; ~ = site
2; • = site 3; o = site 4. Bars indicate standard errors. There
were no emissions of m e t h a n e from sites 1 and 4.
The results of counts of methylotrophs and
methanotrophs are shown in Tables 1-2. Replicated counts from each site gave reproducible
results with acceptably small standard errors.
Control counts with garden soil from Wivenhoe
Park were always at least an order of magnitude
smaller than those obtained from soil from the
landfill site. This indicated that the landfill soil
microflora had adapted to the presence of
methane by selection of methanotrophs, as suggested by other workers [4,23].
The counts of methylotrophs and methanotrophs in surface samples of soil consistently
showed greatest values at site 3. It was at this site
that C H 4 concentrations were greatest in the
sites but decreased markedly during winter when
the soils were saturated with water [5]. Whalen et
al. [4] have previously demonstrated that transport of methane in the aqueous phase is greatly
reduced compared to transport in the gas phase.
Annual emission from site 2 was 495 mol
CH4.m-2.y -1 and from site 3 was 909 mol CH 4
m-2
y-1.
4. 4. Vertical profiles of gases
The gas sampling stakes revealed significant
differences in the vertical profiles of C H 4 and 0 2
Table 1
Counts of methylotrophic microorganisms in surface soil at four sites at Martin's Farm landfill site. Numbers are per g soil
( x 106) -t- standard error (n = 3)
Date
Substrate
Jun. 1987
May1988
Dec. 1988
Jun. 1989
methanol
methanol
methane
methane
Site
1
2
2.2 +0.16
0.13 + 0.06
3.6 +0.44
6.5
3.3
0.11
7.9
+0.22
+0.71
+ 0.05
+1.85
3
4
4.9 + 0.44
24.0 + 2.70
0.32 + 0.06
20.0 +10.9
1.3 +0.59
0.14 + 0.01
3.9 + 1.00
191
surface layer of soil, and where highest emission
of C H 4 from the soil surface occurred [5]. It was
also the site with the best developed soil and the
highest soil organic matter content. In general,
sites 1 and 4, where C H 4 did not extend to the
soil surface, showed the lowest counts although
still greater than in garden soil. Site 2 was intermediate.
The vertical profiles of counts of methanotrophs made in July 1989 (Table 2) corroborated
the data from the surface soil counts. Counts
were again consistently higher than in garden soil
by an order of magnitude, although appreciable
numbers of methanotrophs were present in the
control garden soil. At sites 1 and 4 counts were
relatively constant throughout the depth profile
examined, suggesting C H 4 oxidation throughout
the 0-36 cm horizon examined. At sites 2 and 3
maximum counts were at the surface and declined with depth, consistent with the hypothesis
that at these sites, where CH 4 was emitted from
the soil surface, maximum CH 4 oxidation also
was near the soil surface. In this context it should
be noted that below the 0-25 cm horizon at these
sites O 2 declined markedly and this suggested
that maximum aerobic CH4 oxidation was in the
top 0 - 2 0 cm of soil. Comparison of the counts
during December 1988 and June 1989 showed
that numbers of methanotrophs at all sites were
lower in the winter than in the summer (Table 1),
correlating with our observation that CH 4 concentrations in the surface soil, and CH 4 emission
rates from the soil surface declined during winter
[5]. It would appear that the decrease in CH 4
concentrations near the soil surface were reflected in lower counts of methanotrophs at that
time.
4.6. Methylotrophic activity
The data for radioactivity recovered a s 14CO2
from oxidation of 14C-labelled methanol or CH 4
was analysed by linear regression. The plots of In
dpm versus time always conformed to a straight
line ( P < 0.05) suggesting that the reaction was
first order despite the high concentration of
methanol or methane added. (Bender and Conrad [23] have recently reported oxidation rates of
methane in soils preincubated in the presence of
methane. After 100 h preincubation soils developed a larger methanotroph community, and a
high capacity for methane oxidation which was
not saturated even at 20% methane in the
headspace). The turnover constants for both
lnCH3OH and 14CH4 in surface soils are shown
in Table 3. On both occasions, with either
[14C]methanol or inCH4, the turnover at the surface was greatest at site 3 (where C H 4 emission
from the surface and counts of methylotrophs in
surface soil were also greatest). At the other
three landfill sites turnover rates were lower than
at site 3, but were lowest in the controls of
garden soil.
5. D I S C U S S I O N
Our counts of methanotrophs and methylotrophs (Tables 1 and 2) were very similar to
Table 2
Counts of methanotrophic microorganismsdown soil profile at Martin's Farm, and in a control garden soil, in July, 1989. Numbers
are per g soil (× 106)-4-standard error (n = 3)
Depth
(cm)
Site
1
2
3
4
0-5
5-10
10-15
15-20
~-25
4-30
30-35
2.4±0.3
2.9±0.1
3.4±0.1
4.8±0.9
3.2±1.2
2.9±1.1
1.5±0.5
4.9±0.2
3.9±0.3
3.1±0.1
2.9±0.1
2.3±0.1
1.8±0.1
1.1±0.1
26.1 ±8.0
34.0 ±4.0
18.0 ±9.0
13.0 ±4.0
6.8 ±0.9
2.2 ±1.1
0.77±0.03
2.1 ±0.5
2.7 ±0.1
1.2 ±0.1
1.3 ±0.6
1.0 ±0.2
0.96±0.17
0.83±0.08
Control
0.34±0.01
0.37±0.01
0.41±0.03
0.3 ±0.~
0.27±0.03
0.27±0.~
0.27±0.04
192
Table 3
methane concentrations, of a methanotrophic
community with a high affinity for methane; but a
second methanotrophic community, with a low
affinity for methane but high capacity for methane
oxidation, developed when the soils were exposed
to elevated CH 4. In general, our counts of
methanotrophs coincided with activity measurements, maximum counts being found where oxidation of CH 4 was also greatest. Plate counts may
not be truly representative of the in situ active
methanotroph biomass, but the data at least show
the relative abundance of methanotrophic and
methylotrophic types which are present in landfill
cover soil. The highest counts were detected at
site 3 which also had the highest turnover constants for methane (Table 3).
All of the isolates were initially grown on silica
gel plates in the absence of organic matter and
were therefore all capable of methanotrophic
growth. However, all isolates were subsequently
maintained, and grew better, on organic medium
in the absence of C H 4 o r methanol. This would
seem to suggest that where some organic matter
is available in landfill cover soil facultative, possi-
Turnover constants for [14C]methanol and 14CH4 in surface
(0-1 cm) soils from four sites at Martin's Farm, and in a
control garden soil (+ standard errors, n = 3)
Site
14cn3on turnover
constant (h- t)
14CH4turnover
constant (h- 1)
1
2
3
4
0.031 5:0.011
0.030 5:0.005
0.120 + 0.021
0.032 5:0.006
0.003 5:0.000
0.003 5:0.001
0.010 + 0.001
0.003 5:0.000
Garden soil
0.015 5:0.014
0.001 5:0.000
those of Bender and Conrad [23] who detected
2 x 105-4 x 10 6 cells g-1 dry weight soil in soils
not exposed to elevated CH4, but 3 x 106-2 x 107
cells g - t dry weight in soils exposed to elevated
C H 4. In particular, the relatively large number of
methanotrophs present even in control soils exposed only to ambient C H 4 concentrations suggested a background methanotrophic microflora
even at very low CH 4 concentrations. Bender and
Conrad [23] reported the presence in a variety of
soils exposed only to atmospheric, ambient,
Ln number methenotrophslg-lsoit)
13.5
lt,.5
15.5
16.5
17.5
Me t hQne or oxygen (/..tmo't.mt-lgas "1
0
5
10
15
20
,I,
|
S.
EIO
Ln number methanotrophs (g'lsoit).
135
lt, S
15.5
16-5
17.5
Methane or oxygen (~umot.mtlgas).
0
10
15
20
l
',
/,
........... / "
~ 2o :£%
.'.~
•
*o
30
e
Site 1
~0
I
,z~
I
I
~
Site 2
!
I
I
!
e~l o
>(
~2o
30
40
S
~-'~--
~!<.
!
2:5
5
/S
Site 3
Sife /,
!
7'.5
Methane
10
o
2'.5
5'
7'.5
10
'
oxidation rate (xlO-2,~umo[ [H¢.mt-lsoit. h-1)
Fig. 3. Profiles of methane oxidation rates (o), methane concentration (rn), oxygen concentration (11) and number of methaneoxidising microorganisms( zx) at the four study sites at Martin's Farm during July, 1989.
193
ble mixotrophic, methanotrophs are competitively
more effective than obligate methanotrophs.
However, further work is necessary to confirm
this point. Whalen and Reeburgh [14] have suggested that the ability of methanotrophs in tun~
dra soil to oxidise C H 4 a t concentrations less
than a t m o s p h e r i c may be associated with
mixotrophic growth.
The magnitude of the rate constants for
turnover of methane or methanol may be regarded as indicators of the relative size and activity of the methanotrophic community present in a
soil sample. Although C H 4 (and methanol)
turnover in the surface soils during September
1989 had previously been shown (Table 3) to be
greatest at site 3, in the soil profiles the fastest
turnover constants for C H 4 oxidation were measured between 5-25 cm at site 1, At this site C H 4
was never emitted from the soil surface and these
substantially faster turnover rates of C H 4 oxidation at depth in this site strongly suggested that
the lack of emission was the result of complete
oxidation of C H 4 within the soil profile as it
diffused up from below.
The first order turnover constants for 14CH4
oxidation measured down the depth profiles at
the various sites permitted estimates of in situ
rates of C H 4 oxidation if it was assumed that the
measured in situ C H 4 concentration profiles represented steady states. The C H 4 oxidation rate at
each depth could then be calculated from the
turnover constant multiplied by the C H 4 concentration. These profiles of C H 4 oxidation rates are
shown in Fig 3. In site 1 the maximum C H 4
oxidation rates occurred in that region of the
depth profile where concentration profiles of C H 4
and O 2 overlapped. Methane oxidation rates were
negligible at depths < 12 cm because C H 4 w a s
not present in the soil profile. Maximum counts
of methanotrophs at site 1 also were detected
between 10-25 cm depth (Table 2).
At site 2 also there were maximum oxidation
rates of C H 4 between 5-25 cm depth, although
the methane turnover constants were lower than
at site 1. The highest rates of C H 4 oxidation were
at site 3, despite the methane turnover constants
being moderate, and were the consequence of the
high in situ C H 4 concentrations at this site. De-
Table 4
Daily rates Of methane oxidation during July, 1989, integrated
ov¢r lh0 0-32 ¢m depth of cover soil at each site
Site
Rate of methane oxidation
(mmol CH 4 m -2 d -1)
1
2
3
4
12.5
156.7
449.6
0.02
spite this, emission of C H 4 from the surface
occurred at this site. The highest plate counts of
methanotrophs were in the soil profile at site 3,
although these high counts did not coincide with
the highest values for the methane turnover constants which were at site 1. In contrast to site 1,
at site 4 the C H 4 turnover constants were low but
nonetheless the C H 4 concentrations within the
soil profile remained small and the calculated
rates of C H 4 oxidation negligible.
Each profile of C H 4 oxidation rates was integrated over the 0 - 3 2 cm depth interval by calculating the area beneath each profile by a simple
trapezium method. The integrated daily rates of
C H 4 oxidation per m E of soil surface are shown
in Table 4. The greatest rate was at site 3 with a
rate of 450 mmol CH4.m-E.d -1. This compared
to a maximum rate of C H 4 oxidation of 45 g C H 4
m - 2 d -1 (2.81 mol C H 4 m -2 d - 1 ) reported by
Whalen et al. [4] for the microbial community in
landfill cover soil from Berkely North Waterfront
Park, San Francisco, Cal. It should be noted that
our measurements are likely to be underestimates
of C H 4 oxidation rates as we only measured
14CH4 oxidised to 14CO2 and were unable to
measure the 14CH4 w h i c h was incorporated into
microbial biomass. The latter component can be
a significant proportion of the CH4 oxidised 69% reported in the study by Whalen et al. [4].
The recovery of oxidised 14CH4 as 14CO 2 in our
study was usually about 2 0 - 3 0 % which also indicated carbon assimilation of the order of 70%.
Allowing for this difference, the integrated rate
calculated for site 3 must approach the maximum
rate reported by Whalen et al. [4].
Emission of C H 4 from the soil surface ce lsed
during winter even at sites 2 and 3 (Fig. 2),
194
coinciding with the p e r i o d o f low t e m p e r a t u r e
a n d high w a t e r c o n t e n t w h e n t h e soils w e r e w a t e r
saturated. During the winter the CH 4 content of
the soil was lower t h a n in t h e s u m m e r a n d t h e
n u m b e r o f m e t h a n o t r o p h s was also lower in surface soil d u r i n g w i n t e r ( D e c e m b e r 1988) t h a n in
s u m m e r (June 1989; see T a b l e 1). W h a l e n et al.
[4] have previously p o i n t e d o u t t h a t C H 4 transp o r t in t h e gas p h a s e is 10 4 t i m e s m o r e r a p i d
t h a n w h e n dissolved, a n d C H 4 o x i d a t i o n was inh i b i t e d in w a t e r s a t u r a t e d soil. (It was also inhibited by soil w a t e r c o n t e n t < 11% p r e s u m a b l y
b e c a u s e o f d e s s i c a t i o n o f t h e m e t h y l o t r o p h i c microflora.) O u r c o u n t s suggest s e a s o n a l changes,
possibly in r e s p o n s e to s e a s o n a l c h a n g e s o f
m e t h a n e availability in t h e soil.
R e c e n t w o r k has e s t a b l i s h e d t h a t in m a n y natural e n v i r o n m e n t s w h e r e C H 4 is p r e s e n t t h e r a t e s
of a e r o b i c C H 4 o x i d a t i o n a r e sufficient to oxidise
the m a j o r i t y o f C H 4 b e f o r e it is r e l e a s e d to t h e
a t m o s p h e r e [10,24]. O x i d a t i o n o f C H 4 over l a n d fill sites has b e e n r e p o r t e d b e f o r e [18,25] a n d
r e c e n t l y W h a l e n et al. [4] have r e p o r t e d r a p i d
o x i d a t i o n o f C H 4 in landfill cover soil. O u r d a t a
f r o m M a r t i n ' s F a r m landfill site illustrates t h a t
s e l e c t i o n of a m e t h a n o t r o p h i c c o m m u n i t y occurs
in landfill cover soil a n d t h e c a p a c i t y for m i c r o bial o x i d a t i o n o f c n 4 is c o n s i d e r a b l e b e c a u s e ,
unlike aquatic environments, transport of CH 4
within t h e soil a t m o s p h e r e is rapid. This c a p a c i t y
for C H 4 o x i d a t i o n is o f c o n s i d e r a b l e ecological
i m p o r t a n c e as on a m o l a r basis, over a 20 year,
p e r i o d C H 4 is s o m e 21 times m o r e p o t e n t a
g r e e n h o u s e gas t h a n C O 2 [6] a n d C H 4 o x i d a t i o n
b e f o r e emission to the a t m o s p h e r e will a m e l i o r a t e t h e g r e e n h o u s e effect. M a n a g e m e n t s t r a t e gies for landfill at p r e s e n t t e n d to use relatively
i m p e r m e a b l e clay c a p s to seal t h e site. I n o r d e r to
s t i m u l a t e m e t h a n e oxidation, a n d h e n c e r e d u c e
emissions, t h e use o f p e r m e a b l e soil caps m a y
offer e n v i r o n m e n t a l l y b e n e f i c i a l alternatives.
ACKNOWLEDGEMENT
This r e s e a r c h was s u p p o r t e d by t h e D e p a r t m e n t of t h e E n v i r o n m e n t , U.K. t h r o u g h c o n t r a c t
PECD 7110/96. The opinions expressed are those
of the authors and do not reflect those of the
D e p a r t m e n t o f t h e E n v i r o n m e n t . T h e results m a y
b e u s e d in t h e f o r m u l a t i o n o f G o v e r n m e n t policy
b u t at this s t a g e d o n o t r e p r e s e n t G o v e r n m e n t
Policy. W e wish to t h a n k D r J a n e t G r o n o w for
h e r h e l p a n d e n c o u r a g e m e n t , a n d Essex C o u n t y
Council for p e r m i s s i o n to w o r k on t h e i r M a r t i n ' s
F a r m landfill site.
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