OIKOS 66: 269-278. Copenhagen 1993
Decay potential of hummock and hollow Sphagnum peats at
different depths in a Swedish raised bog
Edward H. Hogg
Hogg, E. H. 1993. Decay potential of hummock and hollow Sphagnum peats at
different depths in a Swedish raised bog. - Oikos 66: 269--278.
Samples of organic material derived from two hummock-forming species (Sphagnum
fuscum and S. rubellum) and two hollow species (Sphagnum cuspidatum and S.
magellanicum) were collected from several depths in a southern Swedish raised bog
and rates of CO 2 emission were measured in the laboratory under aerated conditions
~t 18°C. The rate of CO 2 release from the most .recently deposited Sphagnum feat
litter from the 2.5-5 cm depth layer was nearly tWice as high (0.66 mg CO 2 g-l d- ) as
that from older organic material from the 10--12.5 cm depth layer (0.35 mg CO 2 g-l
d- 1) under common conditions of temperature, moisture and aeration. No consistent
differences in CO 2 emission rates were detected among the four Sphagnum species
for the shallow organic material tested. However, when deeper (50--100 cm) Sphagnum-dominated peat samples were exposed to the same aerated conditions, peat
formed by the hummock-forming species released CO 2 at a slower rate (0.23 mg g-l
d- 1) than the two hollow species (0.29 mg g-l d- 1). The results suggest that the
amount of previous exposure of Sphagnum peat to decay processes may be more
important than the species composition in determining subsequent rates of decay and
carbon loss.
The observed rates of CO 2 emission were used to estimate the annual mass loss from
aerobically decaying organic material under the thermal regime at the study site.
Based on these estimates, Sphagnum litter loses about 5% of its dry mass annually
during the initial stages of decay, but the decay rate decreases to 2-3% annually for
deeper Sphagnum peat exposed to aerobic conditions.
E. H. Hogg, Dept of Ecology, Plant Ecology, Univ. of Lund, Ostra Vallgatan 14,
S-223 61 Lund, Sweden (present address: Northern Forestry Centre, 5320--122 Street,
Edmonton, Alberta, Canada T6H 355).
Peatlands are characterized by slow rates of decomposition which lead to the long-term accumulation of organic material. For many types of organic material, the
rate of decomposition (k) tends to decrease over time as
the more easily decomposed fractions are removed by
microbial respiration (Minderman 1968, Heal et al.
1978, Morris and Lajtha 1986, Fyles and McGill 1987).
The development of decay resistant characteristics of
older, more decomposed organic material tends to reduce the rate of carbon loss even under environmental
conditions favourable to microbial decay (Farrish and
Grigal 1988, Hogg et al. 1992). Thus increased decay
resistance may contribute to the peat accumulation pro-
cess and is likely to limit the rate of decay in deeper
peats during periods of low water levels in peatlands
when aeration is improved (Hogg et al. 1992).
Sphagnum moss is one of the dominant components
of organic deposits in peatlands, but change in the decay
potential during the decomposition of Sphagnum peat
has received little attention. Heal et al. (1978) documented the decrease in decay potential of several vascular species on a British bog in their measurements of
oxygen consumption by litter that had been exposed to
field conditions for periods of up to five years. However, because of the intrinsically much slower decay rate
in Sphagnum litter compared to most vascular plant
Accepted 9 June 1992
© OIKOS
OIKOS 66:2 (1993)
269
litter (Reader and Stewart 1972, Coulson and Butterfield 1978) a much longer period would be needed for
such an experiment using Sphagnum. An alternative
method is to rely on the concept that because Sphagnum
grows from the apex, deeper Sphagnum peat is older
and under normal conditions, more decomposed than
the overlying shallower peat. The first objective of this
study was to use this concept to test the hypothesis that
Sphagnum peat becomes more decay resistant as it decomposes, by comparing the rate of CO 2 release under
constant laboratory conditions from peats collected at
different depths.
Conditions for microbial decay of organic material in
peatlands are much better in the well-aerated acrotelm
layer, which is usually restricted to within 40 cm of the
peatland surface, than in the anoxic catotelm layer below (Ingram 1978, Clymo 1984). In most peatlands
there are microtopographical features referred to as
hummocks and hollows. Hummocks are small, often
elongated mounds that typically rise 20-50 cm above
intervening hollows, or wet depressions. The thickness
of the acrotelm, defined approximately as the peat
thickness above the summer water table (Clymo 1978),
is generally much greater in hummocks than in hollows
(Wallen 1987, Maimer 1988). In hummocks with a
30-40 cm thick acrotelm, Sphagnum peat may be exposed to aerobic conditions for> 100 years and lose up
to 80% of its original carbon before it is added to the
slowly decaying catotelm layer (Maimer and Holm
1984). In hollows, total carbon losses from aerobic decay processes should be much lower because of the
much thinner « 10 cm) acrotelm layer and a shorter
residence time of the organic material in this layer before it is added to the catotelm.
If peat becomes more resistant to further decay as it
decomposes, then deep hummock Sphagnum peat
which has already lost a high fraction of its initial mass
should decay more slowly than deep hollow Sphagnum
peat, when both are held under common environmental
conditions. The second objective of this study was to
test the hypothesis that hummock peat becomes more
decay resistant than hollow peat during its passage
through the acrotelm layer.
Several studies of Sphagnum decomposition using thelitter bag method have found differences in the rate of
mass loss among Sphagnum species (Clymo 1965, Rosswall et al. 1975, Rochefort et al. 1990). Johnson and
Damman (1991) found a slower rate of mass loss in the
hummock species Sphagnum fuscum than in the hollow
species Sphagnum cuspidatum and suggested that these
differences contribute to the maintenance of hummocks
and hollows in ombrotrophic peatlands. However, the
litter bag method may not be sensitive enough to detect
the low rate of mass loss in decaying Sphagnum litter
after the first one or two years (Rochefort et al. 1990).
The third objective of the present study was to use the
measurement of CO 2 emission rates as a more sensitive
method to examine the influence of Sphagnum species
270
composition on the decay of organic material from different depths.
Models of peat accumulation in Sphagnum-dominated systems (Clymo 1978, 1983, 1984) require estimates of the rate of decay of organic material at different depths under field conditions. Thus the fourth objective of this study was to use laboratory measurements
of CO 2 emission rates to estimate the annual rate of
mass loss in the well-aerated surface layers of Sphagnum bogs in southern Sweden.
Lowered water tables in peatlands caused either by
global warming or drainage may lead to a greater degree of aeration and increased decomposition of
deeper, anoxic peat strata (Armentano and Menges
1986, Moore and Knowles 1989, Gorham 1991). The
final objective of this study was to determine potential
rates of CO 2 emission and obtain crude estimates of
mass loss from deeper, anoxic peats after short-term
exposure to aerobic conditions.
Methods
Study site
Peat samples were collected from Akhult Mire, an ombrotrophic, raised bog with an area of 1.1 km 2 , located
in southern Sweden (57°06'N, 14°32'E). Vegetation, water chemistry and hydrology of the mire and the regional climate have been thoroughly described by Maimer (1962). All collections were made from within a 1
ha area near the centre of the southwestern portion of
the mire that is isolated from groundwater influence.
The sampling area is treeless except for a few individuals of Pinus sylvestris L. The bog surface has a hummock-hollow microtopography with a microrelief of
about 30 cm. Seasonal variation in water level is about
16 cm (Maimer 1962).
The vegetation shows a vertical zonation in the hummock-hollow complexes that is typical of bogs throughout the region (Maimer 1962, Hornets 1988, Wallen et
al. 1988). Hummocks are dominated by Sphagnum fuscum (Schimp.) Klinggr. and S. rubellum Wils. In hollows, S. cuspidatum Ehrh. ex Hoffm. is dominant in the
wettest sites, while S. magellanicum Brid. predominates
near the edges of hollows or in relatively flat areas
referred to as lawns. The ericaceous dwarf shrub Calluna vulgaris (L.) Hull often predominates on hummocks while Eriophorum vaginatum L. is abundant on
low hummocks and lawns.
Experiment 1
A total of 16 cores measuring 30-40 cm in length were
collected from Akhult mire in October 1990 using a
lO-cm diameter cylindrical corer with a sharp, waveshaped cutting edge. Four cores were collected from
OIKOS 66:2 (1993)
sites uniformly dominated by each of following species:
Sphagnum fuscum, S. rubellum, S. magellanicum, and
S. cuspidatum. Cores showing a change in peat type or
excessive amounts (> 10%) of vascular plant material
within the uppermost 15 cm were rejected. Cores were
placed into a three-sided box with slits for cutting the
peat into 2.5 cm depth sections. The peat sections were
then stored in plastic bags at 4°C in the dark before the
experiment, which was initiated 6 wk after cores were
collected.
The holes where cores had been taken were used to
measure the depth of the water level on four subsequent
occasions (19 October and 4 November 1990, 23 April
and 13 June 1991). The mean water level differed by
only 3-5 cm among these four dates and was located an
average of 2, 4, 12, and 21 cm below the surface of the
S. cuspidatum, S. magellanicum, S. rubellum and S.
fuscum sites respectively (n = 4 sites per species).
From each core, the depth sections 2, 3 and 5 (Sphagnum litter from the 2.5-5 cm, 5-7.5 cm and 10-12.5 cm
depths respectively) were selected for a laboratory experiment on CO 2 evolution rate. Green or living Sphagnum was occasionally present in the uppermost portion
of section 2, but was excluded from samples taken from
this layer. A minimum of one additional section was
selected from each core, including the deepest possible
core section of pure Sphagnum peat derived from the
same species as that present at the surface.
From each section, two equal subsamples, each with a
fresh mass of 5-10 g were prepared after removing
visible living roots, notably the fine roots of Calluna
vulgaris, and other material derived from vascular
plants. One subsample was oven-dried at 80°C for determinations of dry mass and moisture content. The
second subsample was placed into a 300 ml glass jar with
a tight-fitting lid which had a 12 mm diameter hole fitted
with a butyl septum. The remaining organic material
from each section was also oven dried (80°C) and total
dry mass including subsamples was used to calculate
peat bulk density (dry mass / fresh peat volume).
After all samples had been prepared, septa were removed and each jar was weighed before being placed in
a large, covered plastic basin in a dark, constant-temperature room at 18°C. This temperature is near the
summer maximum expected in acrotelm peat at Akhult
Mire (Wallen and Hogg, unpubl.). Water was added to
the basin to maintain a high humidity and reduce the
rate of moisture loss from the samples. Evaporative
losses from the jars were periodically replaced with
sufficient distilled water to restore samples to their original moisture content, as determined gravimetrically.
The position of samples was randomized weekly. The
same set of samples was exposed the following sequence
of four experimental treatments (A-D), which were designed to allow comparisons of CO 2 emission rates
among species and depths under different environmental conditions:
Treatment A: The rate of CO 2 production was meaOIKOS 66:2 (1993)
sured from each sample while it was near its original
field moisture content. Gas sampling using the methods
described below was conducted on all samples after 1
and 2 wk.
Treatment B: Deionized water was then added to
each sample where necessary to achieve a similar gravimetric moisture content of 2500%, and CO 2 emissions
were measured after an additional 1 and 4 wk. This
moisture content should have been sufficient to ensure
that moisture was not limiting decay rates and that all
hyaline cells were filled with water (Clymo and Hayward 1982). The small size of peat samples, with a high
surface area to volume ratio, should have also ensured
that aeration was not limiting the decay rate. Microbial
decay is impeded only when oxygen is lowered to exceptionally low levels of ca 5 x 10- 6 M (Greenwood 1961,
Wood and Greenwood 1971).
Treatment C: The effect of lowered temperature was
examined by measuring CO 2 emission rates of samples
after being held at 10°C for one wk.
Treatment D: The effect of drought and re-wetting
was then examined by allowing samples to air dry at
18°C for three wk, and then adding sufficient deionized
water to restore their moisture content to 2500%. CO 2
emission rates of the rewetted samples were determined
after 1 and 4 wk.
The total experiment lasted 14 wk, i.e. the experiment was ended 20 wk after samples were collected
from the field.
The potential carbon loss from the living Sphagnum
layer in the upper 2.5 cm section of each core was also
examined using the above methods, but with a few
modifications. Samples were prepared by collecting the
1.5 cm length of each living Sphagnum shoot from immediately below the capitulum. CO 2 emissions were
determined after 1 and 4 wk of incubation at 18°C in the
dark at a constant moisture content of 2500%. Samples
were then air dried for 3 wk at 18°C, rewetted to 2500%
moisture, and CO 2 emission rates were again measured
after 1 and 4 wk.
Two-way analysis of variance (ANOVA) was used to
examine the effect of depth and species on CO 2 emission rates from shallow organic material from depth
layers 2, 3, and 5. The core of sample origin was included as a third factor nested within each species. A
logarithmic transformation was used because of heteroscedasticity and an approximately constant coefficient
of variation among the three depth layers. A separate
one-way ANOVA using untransformed data was used
to examine differences in CO 2 emission rates from the
living Sphagnum (dark respiration) from layer 1.
Experiment 2
In this experiment, deeper peat samples from the catotelm layer were exposed to aerated, laboratory conditions to examine the influence of peat composition on
271
Table 1. Results of two-way ANOVA (with core as nested variable) for CO 2 emission rates from organic material for four
Sphagnum species from three depth layers (2.5-5 cm, 5-7.5 cm and 10--12.5 cm). Analyses shown are for the four portions of the
experiment conducted sequentially on the same set of samples. Asterisk denotes significant (p < 0.05) factor effects.
Experimental conditions
Temperature(°C)
Moisture treatment
Moisture content (%)
Source of variation
Depth layer
Species
Depth layer x species
Core within species
Residual
(df= 2)
(df= 3)
(df= 6)
(df=12)
(df=24)
18
field
780-2530
18
constant
2500
10
constant
2500
18
air-dried, rewetted
2500
p<O.OOI*
p=0.016*
p=0.281
p=0.041*
p<O.OOI*
p=0.285
p=0.185
p=0.092
p<O.OOI*
p=0.086
p=0.049*
p=0.026*
p<O.OOI*
p=O.OOI*
p=0.095
p=0.026*
SPECIES OF SPHAGNUM
cuspid. magell. rubell. fuscum
1.0
A
0.8
0.6
0.4
0.2
0.0
1.0
B
0.8
0.6
0.4
Zl
o
(f)
(f)
~
W
N
o
U
"0
0.2
0.0
1.0
C
0.8
0.6
0.4
0.2
-----.-J----........---'-......
O.O.L.-I.......
1.0
LJ
D
0.8
0.6
0.4
potential carbon loss following lowered water levels. In
November 1990, 24 cores were collected from the 50100 cm depth layer using a Russian corer with an internal diameter of 5 cm. Sampling locations included to
hummock sites, 7 lawn sites and 7 hollows. Samples
were cut into to-cm depth sections in the field, transported to the laboratory the same day, and stored at
4°C. Sections from the 50-60, 70-80 and the 90-toO cm
depth layers were selected for analysis.
Peat sections were classified according to their botanical composition, and humification was assessed by the
von Post H scale (Stanek and Silc 1977). The proportions of colloidal (amorphous) matter and Sphagnum
material were each visually estimated under 10-30x
magnification. Higher magnification was used to differentiate Acutifolia (S. fuscum and S. rubellum) peat
from that derived from small-leaved species in section
Cuspidata (S. baLticum and S. tenellum). Samples were
grouped according to the dominant Sphagnum species
but the two Acutifolia species were combined into a
single category because they could not always be differentiated (Svensson 1986).
In a pilot study it was found that deep peat (50-100
cm) showed a gradual increase in CO 2 release rates
before reaching stable levels during a 6 wk exposure to
air. Thus peat sections were allowed to equilibrate for 3
months in air-filled plastic bags at 4°C before the experiment. Samples weighing 10-20 g were prepared from
each section and incubated (18°C) at their original field
moisture content. Determinations of CO 2 release rate
were made after 5 and 8 wk.
0.2
0.0
235
235 235
DEPTH LAYER
Fig. 1. Rates of CO 2 emission from three depth layers of
shallow organic material produced by four species of Sphagnum under four experimental conditions in the laboratory: A,
18°C at field moisture content (range 780%-2530%); B, 18°C at
constant moisture (2500%); C, l00C at constant moisture
(2500%); D, 18°C with samples air dried and rewetted to
constant moisture (2500%). Layers 2, 3 and 5 refer to the 2.5-5
cm, 5-7.5 cm and 10--12.5 cm depth intervals respectively.
272
Determination of CO2 emission rates
On each sampling date, septa were inserted into the
12-mm diameter holes and gas samples were collected
for CO 2 analysis after 5-to min and again after a 2-3 d
period. Gas samples of 1 ml were injected into a Varian
3700 gas chromatograph equipped with a thermal conductivity detector and a Hewlett-Packard 3390A integrator. Separations were made at 33°C with helium
carrier gas on a 2 m x 3 mm stainless steel column
OIKOS 66:2 (1993)
w
!;(
oc
z
o
60.0 . - - - - - - - - - - - - - : S - .-=F::-U-=S-=C-:U-:M:---"'-'
40.0
Vi
(f)
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o
o
S. RUBELL. •
S. MAGELL. 0
S. CUSPID. \l
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<I:
I
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-40.0+----+---+--...--+---+---~
500
1000
1500
2000
2500
3000
FIELD MOISTURE CONTENT (%)
Fig. 2. Percentage change in the rate of CO 2 emission from
samples from layers 2, 3 and 5 after moisture content was
adjusted from field levels of 780-2530% to a constant level of
2500% (as shown by arrow). There was no overall relationship
between % change in CO 2 emission rate and initial field moisture content (r2 = 0.0059, df = 46, P > 0.5).
packed with Porapak R (80/100 mesh). Readings were
standardized against calibration gases containing CO 2
concentrations in the range of observed values (991 and
1981 ppm). The rate of CO 2 evolution was expressed as
mg CO 2 per g dry mass of sample per day. A pilot study
indicated that measured CO 2 emissions from duplicate
samples (n = 30 pairs) using this method differed by an
average of 9.7%.
Annual mass loss estimates
The results of CO 2 losses were also used to estimate the
rate of mass loss of organic material over a one-year
period at the study site under aerated conditions. It was
assumed that 0.68 g of organic material is lost per g of
CO 2 released (Hogg et a!. 1992). The thermal regime of
surface peat at the study site could be approximated as a
double sine function (Clymo 1984) with a mean of 6°C,
a 7°C amplitude of annual variation, and a 2°C amplitude of daily variation (Wallen and Hogg, unpub!.). It
was assumed that the rate of CO 2 emission (R) increases
exponentially with increased temperature (T) according
to the equation R = k x eO. ll3T , giving a 010 value of 3.1.
This equation is based on a laboratory experiment of
CO 2 release from Sphagnum peat at ten different temperatures between -2°C and 20°C (Wallen and Hogg,
unpub!.). Using these relationships, the amount of mass
loss after one year of decomposition under field conditions was estimated to be equivalent to the mass loss
after 112 d of sample incubation at 18°C.
OIKOS 66:2 (\993)
Results
CO2 emissions from shallow cores
The rates of CO 2 emission differed significantly (p <
0.001) among the three layers from the 2.5-5.0,5.0-7.5
and 10.0-12.5 cm depths (Table 1). Peat samples from
each of the four species showed a consistent decrease in
CO 2 emissions with increased depth under all four experimental conditions used (Fig. 1). However, within a
given depth layer, CO 2 emission rates were generally
similar among samples formed by different species.
In the first portion of the experiment, when samples
were near their original field moisture content, differences in CO 2 emission rates were detected statistically
among species (p = 0.016, Table 1) and in depth layers 3
and 5, were highest for the two hummock species
(Sphagnum fuscum and S. rubellum, Fig. 1A). However, in the second and third portions of the experiment, when the moisture content of all samples was
held at a constant level of 2500%, no differences in CO 2
emission rates were detected among species (Table 1,
Figs 1B and C). In the final stage of the experiment, the
air-dried and rewetted samples again showed greater
CO 2 emission rates from the two hummock species,
particularly from depth layers 3 and 5 (Fig. 1D).
At the beginning of the experiment there was considerable variability in moisture content among samples,
ranging from 780-2530%. Average moisture content (n
= 12 per species) was 1090%, 1630%, 1750% and
1880% for Sphagnum fuscum, S. rubellum, S. magellanicum and S. cuspidatum respectively. However, it appeared that these differences in moisture were having a
very minor effect on CO 2 emissions. This is based on the
observation that when the moisture content of all samples was raised to 2500% (± 50%), the percentage
change in CO 2 emissions showed no overall relationship
to initial field moisture content (Fig. 2). The greatest
change in CO 2 emissions occurred in the Sphagnum
rubellum samples, which decreased by an average of
15% between the first and second portions of the experiment.
When the temperature was reduced to 10°C in the
third part of the experiment, CO 2 emissions decreased
to an average of only 39% of those recorded at 18°C,
based on measurements at 2500% moisture (Fig. 1C).
The proportional decrease was not significantly different among species or depths, based on a two-way
ANOVA of the ratios for each sample. The observed
CO 2 emission rates at 10°C as a percentage of those at
18°C (39%) was very similar to the 40% expected using
the exponential relationship of CO 2 emission rates to
temperature with a 010 of 3.1 (Wallen and Hogg, unpub!,; see Methods).
In the fourth part of the experiment (Fig. 1D), mean
CO 2 emission rates from the air-dried and rewetted
samples showed only a 3% overall decrease from the
mean rate in the second part of the experiment before
273
15
SPHAGNUM CUSPIDATUM
10
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I-
<:
4
4
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0.4
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0.4
3
0.3
2
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enl
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o
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0
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A ........
1.0
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15
20
SPHAGNUM MAGELLANICUM
10
\::~
1.0
0.4
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5
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15
PEAT DEPTH (em)
20
0.6
3
0.4
2
0.3
1.5
0.2
25
5
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15
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25
SPHAGNUM FUSCUM
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6
0.6
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SPHAGNUM RUBELLUM
1.0
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w.,S
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15
>w 0
w
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6
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1.5 ~
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20
25
PEAT DEPTH (em)
Fig. 3. Rates of COl emission (logarithmic transformation, left axis) and estimated long term rate of mass loss (right axis, see
Methods) from layers of.organic material up to 25 cm deep from four species of Sphagnum. Samples were held at 18°C under dark
laboratory conditions with constant moisture content (2500%), corresponding to Fig. lB. Similar symbols connected by solid lines
denote results from samples taken from the same core (n = 4 cores per species). The uppermost samples (0-2.5 cm depth) from
each core consisted of green living Sphagnum.
rewetting (Fig. lB). However, CO 2 emissions from the depth sections had therefore been situated immediately
rewetted S. rubellum samples had increased by 23% above the transition point to a different, and often much
while the other species showed decreases of 6-14%.
more humified peat type beneath.
The greatest decrease in CO 2 emission rates with
A comparison of CO 2 emissions (18°C, 2500% moisture) over the full range of depths in each core (Fig. 3) depth between 2.5 and 12.5 cm was observed in the S.
showed that the rates of CO 2 emission were greatest cuspidatum cores (Fig. 3). However, in this depth infrom green, living Sphagnum (0.9-2.1 mg CO 2 g-I d- 1)
terval, bulk density (dry mass/fresh volume) was greater
in the 0-2.5 cm depth layer. Within each core, rates of for the organic material formed by S. cuspidatum (41.6
CO 2 emission were always much less « 0.2~. 9 mg CO 2 • ± 2.1 g dm- 3) than that formed by S. magellanicum, S.
g-I d- ' ) from the decomposing organic material below, rubel/um and S. fuscum (28.0 ± 2.7, 34.5 ± 2.0 and 31.7
except in one of the S. cuspidatum samples from the ± 2.5 g dm- 3 respectively). Thus the organic material at
2.5-5 cm layer. Below the living layer, a strong decrease a given depth in the S. cuspidatum cores could be conin CO 2 emission rates with increased depth below the sidered to be "deeper" than in cores of the other species
living layer was evident for all species except S. rube/- because it had a greater amount of cumulative dry mass
above that depth.
fum. This decrease occurred only within the uppermost
Based on the observed CO 2 emission rates, the rate of
12.5 cm in the S. cuspidatum and S. magellanicum
cores, but CO 2 emission rates continued to decrease mass loss of organic material under well-aerated field
over a wider range of depths (up to 22.5 cm depth) in conditions could be estimated, as described in the Methone of the S. fuscum cores. In the deeper sections of ods. The estimated average mass loss is 10.7% per year
some cores there was a tendency for CO 2 emissions to for the green, living Sphagnum, and 4.8%, 3.5% and
increase slightly with increased depth. The deepest sec- 2.6% per year for organic material in the 2.5-5 cm,
tions from each core represented the lowermost pos- 5-7.5 cm and 10-12.5 cm depth layers respectively
sible depth layer containing the same type of organic (Fig. 3).
Rates of CO 2 emission from the dark respiration of
material as that being formed at the surface. These
274
OIKOS 66:2 (1993)
Table 2. Rates of CO 2 emission from deep (50-100 cm) peat samples after 5 and 8 wk exposure to warm, aerobic conditions at 18°C
in the laboratory (mg CO 2 g-I d- I). Means and standard errors were calculated on a core basis by pooling the results from sections
obtained from the same core.
Rates of CO 2 emission
Sample size
5 wk
8wk
cores
sections
Peat type
Poorly humified:
S. cuspidatum
S. magellanicum
S. Sect. Acutifolia
0.311 ± 0.015
0.326 ± 0.D18
0.239 ± 0.012
0.263 ± 0.006
0.252 ± 0.012
0.201 ± 0.009
4
12
10
6
12
II
Well-humified:
S. Sect. Acutifolia
0.343 ± 0.023
0.263 ± 0.012
8
10
fresh, living Sphagnum in the uppermost layer showed
no difference among species (one-way ANaYA, df =
3,12, F = 2.38, p > 0.1). However, when these samples
were air dried and rewetted, significant differences
among species were noted (F = 7.31, P < 0.005). After
rewetting, CO 2 emissions had decreased in all species
but were higher in S. cuspidatum (mean ± SE = 1.33 ±
0.09 mg CO 2 g-I d- I, n = 4) than in S. magellanicum, S.
rubellum and S. fuscum (0.82 ± 0.10,0.98 ± 0.08 and
0.92 ± 0.05 mg CO 2 g-I d- I respectively).
CO2 emissions from deep cores
Examination of the composition of deep core sections
indicated that peat from the 50-60 and 70-80 cm depths
was dominated by S. magellanicum in 14 of the 24 cores,
while at the 9{}'-100 cm depth, Acutifolia peat was present in 18 of the 24 cores. To avoid confounding effects
of depth on CO 2 emissions from the different peat
types, sections from the three depth layers were selected so as to give a similar mean depth of73-78 cm for
each peat type.
All of the S. magellanicum and S. cuspidatum deep
peat samples were poorly humified (von Post H2-H4),
with a high (60-95%) estimated proportion of visible
Sphagnum and a low « 35%) proportion of amorphous, colloidal material. In contrast, about half of the
Acutifolia samples were more strongly humified (von
Post H5-H8), with an estimated 25-55% proportion of
Sphagnum, > 30% amorphous material, and often contained considerable amounts of dead Calluna roots. The
CO 2 emission rates from these humified Acutifolia samples are presented in a separate peat category in Table
2.
Differences in the depth of origin for samples used in
this experiment had no significant overall effect on CO 2
emission rates (r2 = 0.066, df = 37, p > 0.10). In
addition, no significant relationships with depth were
detected within each peat type for these deep samples.
The average rate of CO 2 emission for all samples
showed a decrease from 0.302 to 0.241 mg CO 2 g-l d- I
between 5 and 8 wk after exposure to aerobic conditions
OIKOS 66:2 (1993)
at 18°C, and this decrease occurred in each of the four
peat categories (Table 2). On both sampling dates, the
rates of CO 2 emission from the poorly humified samples
were very similar (within 5%) between the S. cuspidatum and S. magellanicum samples. However, the rates
were consistently 2{}'-26% lower from the poorly humified hummock (Acutifolia) samples than from the hollow (S. cuspidatum and S. magellanicum) samples
(Table 2).
The rates of CO 2 emission from the well-humified
hummock peat were higher than from the poorly humified hummock samples and were similar to those recorded from the hollow samples (Table 2). CO 2 emissions from the hummock samples taken as a whole
showed a positive correlation with degree of humification by the von Post H scale (r2 = 0.309, df = 19, T =
2.91, P < 0.01). Within these samples, the estimated
percentage of visible Sphagnum showed a negative correlation (r2 = 0.559) with humification. Thus the rates
of CO 2 emission were negatively correlated with the
percentage of visible Sphagnum (r2 = 0.331, P < 0.01).
When humification was included as a covariate in a
one-way ANaYA of all deep peat samples, mean rates
of CO 2 emission from the hummock peats were significantly lower than from hollow peats (Fl.36 = 11.8, P =
0.001).
Discussion
In the present study there was evidence that organic
material produced by Sphagnum becomes more resistant to further decay as it decomposes. Under constant
conditions of temperature and moisture, mean losses of
CO 2 were nearly twice as high from the 2.5-5.0 cm layer
(0.66 mg CO 2 g-I d- I) as from the 1O.{}'-12.5 cm layer
(0.35 mg CO 2 g-I d- I). In several other studies it has
been documented that deeper peats respire more slowly
than surface peats when incubated under common environmental conditions (Lahde 1969, Ivarson 1977, Farrish and Griga11988, Hogg et al. 1992). Thus the rate of
decay should decrease with depth even in the upper
275
acrotelm layer of hummocks where microenvironmental
conditions may be equally favourable for decay in the
uppermost 15-20 cm (Farrish and Grigal 1988).
The negative exponential model is commonly used as
a mathematical description of the mass loss of decaying
organic material under constant environmental conditions (Clymo 1983). This model assumes that the rate of
decay remains constant over time, when calculated as a
proportion of the remaining organic material. However,
the observed decrease in potential decay rate of organic
material with depth in peatlands indicates that this
model could be highly unrealistic. Among the decay
models that have been used, the choice of model is
unlikely to be important in describing the course of
aerobic decay until at least 60% of the original mass has
been lost (Clymo 1984, 1991). However, the choice of
model can influence predictions of the rate of organic
matter deposition into the catotelm layer (Clymo 1991)
and also affects predictions of decay in deeper peats
following lowered water levels or the removal of surface
material by fire (Hogg et al. 1992). The use of alternative models, such as the two-component exponential
decay model (Clymo 1983, 1984, Fyles and McGill
1987), which give a decrease in decay rate as organic
matter decomposes are thus likely to improve the results of computer simulations of peat growth, even in
situations where there is only one dominant, peat-forming species.
The hypothesis that hummock peat becomes more
decay resistant than hollow peat because of a greater
residence time in the acrotelm layer was not supported
by the results from the shallow Sphagnum cores in the
present study. The expected response was for hummock
cores to show a more rapid and sustained decrease in
the potential rate of CO 2 emission with depth when
compared to hollow cores. However, the species which
did not show a decrease with depth was not a hollow
species, but a hummock species (S. rubellum). The high
rates of CO 2 emission in the deeper portions of some of
the hummock cores could have been caused by the
presence of residual, recently living fine roots of Calluna vulgaris, which have a diameter of < 0.05 mm.
These fine roots were often abundant and difficult to
extract, especially in the deeper portions of the S. rubellum cores, and were presumably releasing CO 2 at a
much faster rate than the surrounding Sphagnum material (Chapman 1979, Wallen 1986). In hollows, the
strong decrease in decay potential with depth within the
uppermost 12.5 cm may suggest a rapid rate of decay in
this zone, coupled with the removal of organic components most easily metabolized by microbial decomposers. However, a rapid aerobic decay rate would not be
expected because except for the uppermost 2-4 cm,
hollow peat was located below the water table and was
thus poorly aerated. These conditions would have led to
anaerobic decay and the production of methane, which
usually occurs at a much slower rate than aerobic decay
processes, particularly in bog peats (Svensson and Ross276
wall 1984, Moore and Knowles 1989). An alternative
explanation for the rapid decrease in decay potential
with depth in the hollow cores could be that in the
deeper organic material, the presence of organic byproducts of anaerobic decay may have inhibited the rate
of CO 2 emission even after exposure to warm, aerated
conditions in the laboratory.
In contrast to the shallow cores, CO 2 emissions from
the deep (50-100 cm) cores provided partial support for
the hypothesis that deep hummock peats have a greater
resistance to decay than hollow peats. When deep,
poorly-humified samples from the 50-100 cm deep anaerobic zone were held under aerated conditions at
18°C, the hummock peats released CO 2 more slowly
(0.23 mg g-l d- l) than hollow peats (0.29 mg CO 2 g-l
d- 1). Unlike the shallow organic material, these deep
peats contained no living Calluna roots that could have
inflated estimates of the CO 2 emissions produced by
microbial decay processes. However, some of the deep
hummock samples were more strongly humified and
released carbon at a rate very similar (0.30 mg CO 2 g-l
d- 1) to the deep hollow samples. These humified hummock samples contained large amounts of amorphous,
colloidal material but its origin was uncertain. If this
colloidal material was predominantly derived from
Sphagnum, then the higher degree of humification
could be interpreted as a more advanced state of Sphagnum decomposition. Alternatively, the colloidal material may have been derived mainly from vascular plants,
particularly the fine roots of Calluna which presumably
decay at a much faster rate than the Sphagnum. This
explanation is consistent with the higher rate of CO 2
emission from the humified samples, because more
strongly decomposed Sphagnum would have been expected to have a greater resistance to decay and a lower
rate of CO 2 emission. This would imply that "humification" of samples per se is a poor indicator of the
degree of decay in the Sphagnum component.
Litter bag studies have shown differences in the rate
of mass loss among species during the first year (Clymo
1965, Rochefort et al. 1990). Johnson and Damman
(1991) found that S. euspidatum lost dry mass about
twice as rapidly as S. [useum over a two-year period
when these species were placed in litter bags in the same
microhabitat. In the present study, air-dried and rewetted green S. euspidatum released carbon more rapidly
than S. [useum, but there was little evidence of species
differences in the decay rate of older organic material.
Within the uppermost 12.5 cm of organic material,
depth was much more important than species in determining the rate of CO 2 emission under common environmental conditions. A high amount of variability in
CO 2 emission rates was noted among cores within the
same species and depth interval and thus small differences in decay rate among species would not have been
detected statistically.
The rate of CO2 emission from the shallow Sphagnum
organic material in the present study corresponded to
OIKOS 66:2 (1993)
an estimated mass loss rate of 2.6-4.8% under wellaerated conditions in the acrotelm layer at the field site.
These estimated rates are similar to the 3.2-6.2% annual decay rate estimated by Clymo (1978) for S. magellanicum lawns at a high-altitude bog in Britain. Much
higher decay rates of 9-25% have been reported for
Sphagnum species during the first year of litter bag
studies (Clymo 1965, Rochefort et al. 1990, Johnson
and Damman 1991), but the rate of decay has usually
been found to decrease very sharply after the first year.
Studies by Skre and Oechel (1981) indicate that rewetted, living Sphagnum subsecundum Nees. is capable of
photosynthesis after 8 d of desiccation at 15°C and can
undergo a very high initial rate of CO 2 emission of ca 12
mg CO 2 g-I d- ' as it respires in the dark. If such a high
rate were sustained over a one-week period it would
lead to the disappearance of ca 5-6% of the original
plant dry mass. Thus plant respiration may cause a high
initial rate of mass loss in litter bag studies if green,
air-dried Sphagnum is used.
There is some evidence that the decay rate of Sphagnum in the acrotelm layer of hummocks decreases to
values even lower than the estimated 2-5% rate of
annual mass loss in the present study. Studies by MaImer and Holm (1984) and by Johnson et al. (1990)
indicate that at the base of the acrotelm, Sphagnum
peat in Swedish bog hummocks is at least 100--150 years
old and has lost 60--80% of its original carbon. This
would correspond to an overall exponential decay rate
in the acrotelm of about 1% per year or less. The lower
rate of decay in the deeper portion of the acrotelm
appears to be caused by a combination of 1) poor environmental conditions for microbial activity, including
lowered oxygen and redox potential, and 2) decay-resistant characteristics of the more strongly decayed organic material (Farrish and Grigal 1988, Johnson et al.
1990).
In the deep, waterlogged catotelm layer of raised
bogs, the annual rate of mass loss must have been very
low, probably less than 0.01 %, for peat accumulation to
have occurred (Clymo 1983, 1984). However, the present study indicates that the exposure of this deep,
anoxic peat to aerated conditions is likely to cause more
than a 200-fold increase in the rate of CO 2 emission and
mass loss from this material. The average observed rate
of CO 2 emission from 50--100 cm deep peat exposed to
aerated conditions at 18°C after 5-8 wk was 0.27 mg
CO 2 g-I d- ' , which, if extrapolated to field conditions,
would correspond to a rate of mass loss of about 2% per
year. This rate is a very rough estimate because it is
based on short-term studies, but is only slightly less than
the 2.6% mean estimated mass loss for aerated, shallower peat from the 10--12.5 cm depth layer. If the
decay rate of anoxic peat undergoes a sustained, longterm increase from < 0.01 % to 2% following exposure
to aerated conditions, then even a slight lowering of
water levels could have a strong effect on peatland
carbon balance. For example, a lowering of water tables
OIKOS 66:2 (1993)
by 10 cm in a low bulk density (0.05 g cm- 3) peat deposit
would cause the additional breakdown of 100 g m- 2 of
organic material each year. Similar estimates have been
made by Silvola (1986) for Finnish peatlands following
drainage. Since the net accumulation rate of organic
material in peatlands is generally < 100 g m -2 yr- I
(Clymo 1984, Maimer and Holm 1984, Zoltai 1991),
such a lowering of water tables in a peatland would
cause it to become a chronic exporter of carbon dioxide.
Acknowledgements - I am grateful to N. Maimer for providing
both hospitality and research support during my stay in Sweden. This study was supported by grants to N. Maimer from the
Swedish Natural Science Research Council (NFR program for
guest researchers) and the Royal Swedish Academy of Sciences, and a scholarship to E.H. Hogg from the Swedish Institute. I thank N. Maimer, B. Wallen, R.S. Clymo and L.c.
Johnson for useful discussions about this work. Field and laboratory assistance were provided by B. Wallen and c.l. Hogg.
and N. Maimer provided helpful comments on the manuscript.
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