We measured methane ebullition from a patterned boreal bog situated in the
Siikaneva wetland complex in southern Finland. Measurements were conducted
on water (W) and bare peat surfaces (BP) in three growing seasons (2014–2016)
using floating gas traps. The volume of the trapped gas was measured weekly,
and methane and carbon dioxide (CO2) concentrations of bubbles were
analysed from fresh bubble samples that were collected separately. We applied a mixed-effect model to quantify the effect of the environmental controlling
factors on the ebullition.
Ebullition was higher from W than from BP, and more bubbles were released
from open water (OW) than from the water's edge (EW). On average, ebullition
rate was the highest in the wettest year (2016) and ranged between 0 and 253 mg m-2 d-1 with a median of 2 mg m-2 d-1, 0 and 147 mg m-2 d-1 with a median of 3 mg m-2 d-1, and 0 and 186 mg m-2 d-1 with a median of 28 mg m-2 d-1 in 2014, 2015, and 2016,
respectively. Ebullition increased together with increasing peat
temperature, weekly air temperature sum and atmospheric pressure, and
decreasing water table (WT). Methane concentration in the bubbles released
from W was 15–20 times higher than the
CO2 concentration, and from BP it was 10 times higher. The proportion of ebullition fluxes upscaled to
ecosystem level for the peak season was 2 %–8 % and 2 %–5 % of the total
flux measured with eddy covariance technique and with chambers and gas
traps, respectively. Thus, the contribution of methane ebullition from wet
non-vegetated surfaces of the bog to the total ecosystem-scale methane
emission appeared to be small.
Introduction
Historically, bogs were commonly feared, as people saw mysterious lights
that gave rise to the tales of the “will o' the wisps” that lure
travellers from their paths to sink into bog holes (Meredith, 2002).
Nowadays, these lights are thought to be the spontaneous combustion of peatland
gases, such as methane, bubbling into the atmosphere, rather than deceptive
fairies. However, the widespread folklore indicates that the phenomenon is
well known around the world in peatland-rich areas. Although currently
peatlands are more known for their climate-cooling impact as small carbon
sinks and the storage of a third of the global soil carbon stock (Strack,
2008), they are also a major natural source of methane, a potent climate
warming greenhouse gas (IPCC, 2014). The same high water table (WT)
conditions that support accumulation of organic material as peat by slowing
down aerobic decomposition also favour methane production by anaerobic
microbes, methanogens (Archaea) (Hanson and Hanson, 1996). It has been
predicted that carbon dioxide (CO2) uptake typically offsets sustained
methane emissions in natural ecosystems in the long term (i.e. several
centuries), albeit with large spatiotemporal variability (Petrescu et al.,
2015).
Methane is emitted from peatlands into the atmosphere via three routes: by
diffusion from peat, transport through aerenchymatous vascular plants and by
episodic bubble release, i.e. ebullition (LeMer and Roger, 2001;
Raghoebarsing et al., 2005). A large part of the produced methane is oxidised
by methanotrophic bacteria in the aerobic peat layer above water level
(Hanson and Hanson, 1996; LeMer and Roger, 2001; Larmola et al., 2010), and
thus methane flux rate of a peatland depends on the rates of methane
production and consumption, in addition to transportation within the peat to
the atmosphere. It is known that part of methane can also be oxidised in
plants, such as rice (Bosse and Frenzel, 1997), but so far significant
methane oxidation has not been detected in bog plants, such as Eriophorum spp. (Frenzel
and Rudolph, 1998). As methane emitted through vascular plants or by
ebullition bypasses the oxidation in the aerobic peat layer, these pathways
can potentially release high amounts of methane into the atmosphere.
Diffusion through peat and vascular plants have been regarded as being the
dominant pathways of methane emissions and those emission pathways have been
largely targeted with chamber measurements (e.g. Bubier et al., 2005;
Ström et al., 2005; Turetsky et al., 2014). Alternatively, the eddy
covariance (EC) technique is used to estimate the integrated ecosystem-scale methane flux (e.g. Brown et al., 2014; Rinne et al., 2018) but is
unable to differentiate the emission pathways.
Current models of the global methane budget are still uncertain due to limited
knowledge of the relative contribution of different factors controlling
methane fluxes (Riley et al., 2011). The largest source of uncertainty is
the quantity of methane emissions from natural wetlands, such as peatlands
(Riley et al., 2011; Melton et al., 2013). Within peatland emissions, the
largest uncertainty is related to the magnitude of ebullition (Peltola et
al., 2018). We are aware of only few studies that have directly measured
ebullition from boreal peatlands with gas traps. In the first one, Hamilton
et al. (1994) carried out measurements over 24 h and found no bubbles.
In three other studies conducted in a fen (Strack et al., 2005; Strack
and Waddington, 2008) or a bog (Stamp et al., 2013) ebullition fluxes
between 7 and 96 mg CH4 m-2 d-1 were detected, but the
importance of ebullition for the ecosystem flux remained unrevealed.
Ebullition has also been measured in the field by separating peak methane
releases from steady chamber flux (Riutta et al., 2007; Tokida et al., 2007;
Goodrich et al., 2011) with emissions varying from 49–152 mg CH4 m-2 d-1 (Goodrich et al., 2011) to 48–1440 mg CH4 m-2 d-1 (Tokida et al., 2007). These studies show contrasting results in relation to
the contribution of ebullition to the total emission. While Riutta et al. (2007) estimated the role of ebullition to be small in the two study years,
Tokida et al. (2007) (with two sample plots) found that the proportion of
ebullition may constitute up to 50 % of the total flux. Results on
mesocosm studies in laboratory conditions are similarly disparate as they
show that the proportion of ebullition in the total emission varies from
3 % (Green and Baird, 2013) to 50 % (Christensen et al., 2003).
Similar to chamber and EC measurements (Rinne et al., 2007, 2018;
Jackowicz-Krczyński et al., 2010; Turetsky et al., 2014; Mikhaylov et
al., 2015), direct ebullition studies have connected the
rate of methane emission to peat temperature (Strack et al., 2005) related
to increasing microbial activity (Conrad et al., 1997). It is noteworthy that the
incoming energy flux has been shown to primarily control the methane
production and ebullition in shallow subarctic lakes (Wik et al., 2014) that
could be contrasted to peatland pools. Ebullition in peatlands has
additionally been linked to decreasing WT and falling atmospheric pressure:
the decrease in hydrostatic pressure increases the volume of the gas phase
of methane in peat and releases it into the atmosphere (Tokida et al.,
2007). Also, an increase in atmospheric pressure can trigger ebullition by
decreasing the bubble size, due to compression, and thus increasing the bubble
mobility in shallow peat (Comas et al., 2011; Chen and Slater, 2015).
Furthermore, peat structure has been shown to affect bubble sizes and
determine whether ebullition is steady or erratic (Ramirez et al., 2016).
However, the importance of these factors for ebullition is still based on
only a few studies, of which the longest covers two growing seasons (Strack and
Waddington, 2008).
In this study, we measured methane ebullition from open water pools (W) and
bare peat surfaces (BP) with gas traps in three consecutive growing seasons
(2014–2016) in a boreal bog where methane fluxes were also measured with EC
and static chamber techniques. We aimed to (1) quantify the spatial and
temporal variation in methane ebullition from wet bog surfaces, (2) study
the controlling factors and (3) assess the contribution of ebullition from
wet surfaces to the ecosystem-level emission.
Materials and methods
The study was conducted in the ombrotrophic bog that is part of Siikaneva
peatland complex situated in southern Finland (61∘50′ N,
24∘12′ E, 160 m a.s.l.), within the southern boreal vegetation
zone (Ahti et al., 1968). Annual rainfall in the area is 707 mm, the snow
depth in March (with the thickest snow cover) is 36 cm, the annual
cumulative temperature is 1318, the length of growing season is 168 d,
the average annual temperature is 4.2 ∘C, and the average
temperatures in January and July are -7.2 and 17.1 ∘C, respectively (30-year averages from the nearby Juupajoki–Hyytiälä
weather station, except snow depth, which uses a 20-year average). The microtopography
of the studied bog site varies from W and BP to hollows, lawns and hummocks.
W and BP together cover approximately one-fourth of the site (W 11.6 % and
BP 15.3 % within a 30 m radius from the EC tower of the site). The bottom
layer is formed by Sphagnum mosses, except in W and BP that are devoid of moss.
Sedges are the dominant vascular plants in hollows and lawns, whereas
vascular plant vegetation on hummocks is dominated by dwarf shrubs. In BP, Rhynchospora alba is often the only plant species (Korrensalo et al., 2018a).
In order to measure methane ebullition from the studied bog, floating gas
traps were placed in W and BP in three consecutive years (2014–2016). Only W
and BP microforms were chosen because we expected high ebullition from these
waterlogged surfaces that have almost no vegetation, and the
sampling method required gas traps to be easily filled with water. BP are
patches of visible peat that have WT at or near the surface. For example, in
2014, WT in BP was on average -1.8 cm. W are without a clear bottom but have
on average 1 m of water over very loose peat slurry and their water
area starts directly from the edge of the surrounding moss cover. As it is
difficult to determine what is the bottom of the pools, we did not measure
the water depth or temperature in the bottom of the W.
The gas traps were constructed from inverted plastic funnels with diameters
ranging between 14.3 and 24.5 cm (Fig. 1). A syringe with a three-way
stopcock was attached to the narrow end of each funnel, and the joint was
covered with sealant to make it airtight. A piece of metallic netting coated
with filter fabric was glued inside the funnels to prevent litter and small
animals from entering the gas traps in the open water pools. The gas traps
on W were attached to a floating styrofoam raft and placed in the pools in
lines of two or three gas traps, anchored to the opposing shores of the pool
with string (Fig. 1). To study the potential difference in availability of
substrate for methanogenesis, some gas traps were anchored further away from
the surrounding moss cover at the centre of the pools (open water, OW),
while the other gas traps were anchored at the water's edge (EW) right next
to the moss (Fig. 2). The gas traps on BP were placed next to boardwalks at
the study site (Fig. 2). The air was sucked out of the gas traps with an
extra syringe until they were filled with water. The rate of ebullition was
measured weekly by sampling the gas volume that had replaced water in each
gas trap.
Floating gas traps in an open water peatland pool (OW).
Aerial photo of the study site in Siikaneva bog. Red lines with
dots mark the floating gas traps in open water (OW) and at the water's edge (EW).
Red circles mark the area where the gas traps were placed on bare peat
surfaces (BP), which are seen as brownish-grey in the photo. The eddy
covariance (EC) raft is marked with the red ×.
A total of 16 gas traps were used (11 in W and 5 in BP) from 3 June to 25 September in
2014, 20 gas traps (13 in W and 7 in BP) were used from 13 May to 24 September in
2015 and 18 gas traps (12 in W and 6 in BP) were used from 27 May to 9 September in
2016.
Methane concentrations of the gas caught in the gas traps during the weekly
sampling periods were measured in 2014 and compared with methane
concentration of fresh ebullition samples. We found methane concentrations
in the gas traps to be clearly lower than in the fresh ebullition samples
(Table A1 in the Appendix), and thus methane concentration of the gas caught in the traps
was assumed to dilute during the weekly sampling periods due to diffusion.
Therefore, methane concentration of the releasing bubbles was not measured
from the weekly samples but instead by collecting fresh ebullition samples
from W without disturbing the gas traps and from BP that had no gas traps.
Ebullition was triggered manually from the sampled surfaces and the formed
bubbles were caught in an extra gas trap, from where 20 mL samples were
taken into vacuumed glass vials. The samples were analysed with an Agilent
Technologies HP 8690 gas chromatograph at the Natural Resources Institute
Finland (LUKE), Vantaa. Fresh ebullition samples were collected 4 times
during the measurement season in 2014 and 2016 and 13 times in 2015.
Average methane concentration was interpolated linearly from the fresh
ebullition samples for each weekly measurement day.
Average methane emission by ebullition as mL m-2 d-1 was
calculated based on the area of the gas trap, number of days and volume of
gas collected in each measurement period and the average methane
concentration of each measurement period. In order to convert the emissions
to mg m-2 d-1, methane density in each measurement period was
calculated based on the average air temperature of the measurement period in
degrees Celsius and the standard atmospheric air pressure, 101 325 Pa.
Average methane emission (mg m-2 d-1) was calculated separately
for ebullition from OW, EW and BP.
In order to compare the ebullition fluxes to EC and chamber measurements
(Korrensalo et al., 2018b), the ebullition flux was upscaled to ecosystem
level by linearly interpolating the total average ebullition that was
calculated as a sum of average ebullition fluxes from W and BP weighted with
their relative surface areas.
Air pressure and temperature data from 2014 to 2016 were received from the
Juupajoki–Hyytiälä weather station that is situated about 6 km from
the study site in Siikaneva. The data on WT, water temperature and peat
temperatures at the depths of 5, 20 and 50 cm were received from data
loggers installed in a lawn about 1.5 m away from the EC raft.
Photosynthetically active radiation (PAR) data was measured at the site.
Linear mixed-effect models were used to analyse the effect of measured
environmental variables (peat temperature in different depths, WT,
atmospheric pressure and cumulative PAR, and effective temperature sum of a
measurement period as variables of incoming energy flux) on log-transformed
ebullition flux rates. The gas trap was included as a random effect in the
model. We also tested which of the four peat temperature variables explained
the variation in ebullition fluxes the best. The data were analysed with the
function lme of the package nlme of the R software (version 3.3.2).
Results
Among the three studied years, the year 2014 was the warmest, driest and
had the highest amount of cumulative photosynthetically active radiation
(PAR) (Finnish Meteorological Institute open data) (Table 1). It was also
warmer than the 30-year average. The year 2015 was the coolest, with a lowered annual
rainfall and PAR, while 2016 was the wettest and the cloudiest year (Table 1). All 3 years were significantly drier than the average (Table 1).
Effective temperature sum of the growing season, annual rainfall
and the cumulative photosynthetically active radiation (PAR) in the three
studied years (2014–2016), compared to the 30-year averages of the area.
Data for the Hyytiälä weather station are from Finnish Meteorological
Institute open data.
Measured methane ebullition ranges were 0–253, 0–147 and 0–186 mg m-2 d-1 with medians 2, 3 and 28 mg m-2 d-1 in 2014, 2015 and 2016, respectively (Fig. A1 in the Appendix). Weekly
medians of individual gas traps were 0–57 mg m-2 d-1 in 2014,
0–33 mg m-2 d-1 in 2015 and 10–67 mg m-2 d-1 in 2016.
The 3 years differed (degrees of freedom, DF = 2, 746; p<0.0001) as slightly higher ebullition fluxes were generally obtained in 2015
than 2014, while, on average, the ebullition fluxes were at their highest in
the wettest year (2016).
Higher ebullition was observed on W than on BP (Fig. 3) (DF = 1, 746; p<0.0001). Ebullition from OW was significantly higher than
ebullition from EW, except in the middle of the growing season 2015 (Fig. 3). Although BP showed lower ebullition with fewer peaks than W, all the
surfaces had the same seasonal ebullition pattern each year, with highest
fluxes observed in August (Fig. 3). However, in 2015 the highest ebullition
was measured later than in other years after relatively low ebullition in
late summer (Fig. 3).
Mean methane ebullition measured weekly in Siikaneva bog over three
consecutive years (a) 2014, (b) 2015 and (c) 2016 over different surfaces: bare
peat, open water and the water's edge.
Ebullition increased with increasing average peat temperature at the depth
of 5 cm (DF = 1, 746; p<0.0001) that explained ebullition better
than the other peat temperature variables measured. The seasonal pattern of
ebullition followed the temperature in each year (Fig. 4). Higher ebullition
rates were also explained with decreasing average WT (DF = 1, 746; p=0.0001). The highest ebullition peaks were associated with the lowest WT in
each year (Fig. 4). A prolonged depression of WT further explained the late
peak of ebullition in 2015, as well as the increase in ebullition in the
autumn 2016 (Fig. 4). Change in atmospheric pressure during the measurement
period further explained ebullition: more bubbles were released with a higher
increase in pressure (DF = 1, 746; p=0.001). Some events of ebullition
might be directly related to decreasing atmospheric pressure, such as the
small peak in ebullition in mid-August 2014 that appears to be better
explained by the long decrease in atmospheric pressure than by peat temperature
or WT (Fig. 5). After including peat temperature, WT and change in
atmospheric pressure, the effective temperature sum of a measurement period
still had a positive effect on ebullition (DF = 1, 746; p=0.0351.
Finally, the cumulative PAR had no significant effect on ebullition and was
excluded from the final model.
Mean weekly methane ebullition with standard error of the means
from all surfaces compared to water table (WT) (a, c, e), and air, water,
and peat temperatures at the depths of 5, 25 and 50 cm (b, d, f)
measured in Siikaneva bog in the years (a–b) 2014, (c–d) 2015 and (e–f) 2016.
Mean weekly methane ebullition from all surfaces compared to
atmospheric pressure measured in Siikaneva bog in (a) 2014, (b) 2015 and (c) 2016.
Fresh ebullition sample analyses showed that the released gas bubbles
contained more methane than CO2. Methane concentration of bubbles
released from W was 15–20 times higher than their CO2 concentration,
while bubbles from BP had a methane concentration tenfold higher than their CO2 concentration
(Table 2).
Average methane (CH4) and carbon dioxide (CO2)
concentrations (mL L-1), with the standard deviation (SD) of gas-releasing bubbles
from pools (W) and bare peat surfaces (BP) in the three studied years
(2014–2016).
Monthly cumulative methane fluxes (mg m-2 month-1), measured
as ebullition and with the eddy covariance (EC) technique for June–August in
the three studied years (2014–2016).
The average ebullition flux upscaled to ecosystem level was an order of a
magnitude lower than the net methane flux measured by EC in each year (Fig. 6). The sum of ebullition and upscaled chamber flux in 2014 was higher than
the one measured with EC, but the two estimates followed the same seasonal
trend (Fig. 6). The contribution of ebullition to the total methane flux
measured with chambers and bubble traps during the peak season in 2014 was
2 %, 3 % and 5 % in June, July and August, respectively (Table 3). The
contribution of ebullition to EC flux during the peak season varied from 2 % in June 2014 to 8 % in August 2015 (Table 3).
Ecosystem-level methane fluxes measured with the eddy covariance
(EC) technique and upscaled from ebullition measurements in (a) 2014, (b) 2015
and (c) 2016. In 2014, ecosystem-level methane fluxes are also compared to
upscaled chamber fluxes.
DiscussionThe magnitude of ebullition
The methane ebullition measured in this study ranged from 0 to 253 mg m-2 d-1 and the seasonal weekly median of ebullition for different
surfaces ranged from 0 (measured from BP in 2014) to 37 mg m-2 d-1
(measured from OW in 2016). Our results are of the same magnitude as
ebullition fluxes previously measured in boreal peatlands with gas traps,
ranging from 7 to 96 mg m-2 d-1 (Strack et al., 2005; Strack and
Waddington, 2008; Stamp et al., 2013), and with automatic chambers, ranging
from 9 to 152 mg m-2 d-1 (Goodrich et al., 2011). In addition to
field measurements, some of the laboratory-based experiments have shown
similar ebullition flux rates in the range of 0–270 mg m-2 d-1
(Christensen et al., 2003; Kellner et al., 2006; Yu et al., 2014) but also
higher fluxes up to 784 mg m-2 d-1 (Green and Baird, 2012). Some
laboratory studies have even shown potential for much higher ebullition
rates up to 33 000 mg m-2 d-1 (Sphagnum surface samples from bog in Tokida
et al., 2005; fen lawn samples in Waddington et al., 2009). So far, only
Tokida et al. (2007) have estimated ebullition fluxes reaching 1440 mg m-2 d-1 in the field based on methane fluxes measured with the
static chamber method from two sample plots showing high episodic fluxes
during 30 min measurements. Generally, there is a difference in temporal
resolution between the two methods as chamber measurements usually cover
only short time periods (from minutes to hours), while gas traps show
estimates of cumulative bubble flux over several days.
The fact that the ebullition rates measured with gas traps are lower than in
laboratory studies might be partly explained by the process of bubbles
stacking in the gas traps instead of automatically gathering in the
headspace. In this study, we tried to overcome this potential error source
by gently shaking and tapping the gas traps before sampling, simultaneously
trying to avoid causing more ebullition from this disturbance. However,
methane ebullition fluxes of up to 1683 mg m-2 d-1 have been
previously measured with the same method from subarctic lakes (Wik et al.,
2013), which shows the potential of this method to also measure higher
ebullition fluxes.
Temporal and spatial variation
Our study conducted over three growing seasons showed inter-annual variation.
The highest ebullition on average was measured in 2016, whereas the average
flux rates of 2014 and 2015 did not differ significantly from each other.
More ebullition was measured from BP in 2016 especially, which was the wettest
year with the highest WT. This indicates that despite higher WT increasing hydrostatic pressure in peat, wetter conditions in BP facilitate gas release
as bubbles. Although 2015 was almost as wet a year as 2016, it was much
cooler, which decreases methane production. The warmest year (2014) again was
much drier than 2016, and although there was high ebullition with sharp drop
in WT during the peak season, the general ebullition level from BP was low.
The only other peatland study with gas traps covering more than one growing
season (Strack and Waddington, 2008) also found the ebullition level to
differ between the study years. Similarly, Wik et al. (2013) found
differences in bubble methane concentrations and fluxes in subarctic lakes
among the four summers studied. These results point out the need for
multi-year studies in order to include inter-annual variation in ebullition
fluxes in methane models. Furthermore, the higher ebullition rate from W
than from BP in our study indicates that balanced sampling in a bog should
cover microform variability, although in some studies no spatial variation
in ebullition were found (Green and Baird, 2012, 2013; Stamp et al.,
2013). However, drier and wetter conditions can change the proportions of
water and bare peat surfaces, and, according to our results, such changes may
have an impact on ebullition.
Controlling factors and their importance
The measured ebullition rates increased together with peat temperature as
also shown earlier (Strack and Waddington, 2008). Increasing temperature
generally increases the activity of methanogens, and thus more methane is
produced in the peat when it gets warmer, until the temperature optimum of
the microbes around 20–30 ∘C is reached (Dunfield et al., 1993).
Peat temperature affects also the solubility of methane, according to Henry's
law, as gas solubility decreases with increasing temperature (Strack et al., 2005).
Thus, increasing peat temperature may lead to transfer of methane from
aqueous to gaseous phase, which increases bubble formation (Strack et al., 2005).
In our study, the peat temperature at the depth of 5 cm showed the highest
correlation with ebullition but temperature at all depths was highly
intercorrelated. The effect of peat temperature was reflected in the
seasonal pattern of ebullition.
As expected, ebullition fluxes increased when WT decreased, as found in
previous studies (Strack and Waddington, 2008). Bubbles may accumulate in
peat under barriers, such as pieces of wood, and they are suppressed by high
hydrostatic or air pressure (Rosenberry et al., 2003; Strack and Waddington,
2008; Chen and Slater, 2015). Decreasing WT lowers the hydrostatic pressure,
releasing newly formed and the accumulated bubbles. Many studies have also
shown that the falling atmospheric pressure can trigger high rates of
ebullition (Tokida et al., 2005, 2007). Although some weeks
showed higher ebullition rates when atmospheric pressure was falling, this
pattern was not consistent as increasing ebullition rates were also measured during periods of rising atmospheric pressure. After including WT as
explanatory variable, we still found the weekly change in atmospheric
pressure to significantly affect ebullition, as bigger increase in weekly
pressure was related to more ebullition. Previously, Comas et al. (2011)
used ground penetrating radar (GPR) to study the vertical distribution of
free-phase gas in a northern peatland and found that increasing atmospheric
pressure caused rapid ebullition by releasing gas from shallow peat, whereas
decreasing pressure released gas from deeper peat to shallow layers. Also,
Chen and Slater (2015) showed that increasing pressure can trigger
ebullition as it increases the bubble mobility in peat.
Furthermore, higher ebullition rates were measured with a higher effective
temperature sum of the measurement period. This indicates the importance of
energy input as a driver of methane production and release as shown by Wik
et al. (2014). They found strong positive correlations between seasonal
bubble methane flux from subarctic lakes and four proxies of energy flux,
such as average short-wave radiation and maximum water sediment temperature
(Wik et al., 2014). We tried also to compare cumulative PAR (i.e. short-wave
radiation) to seasonal cumulative ebullition fluxes but could not find clear
correlation between the two in the three study years. However, the positive
effect of the measurement period temperature sum on ebullition shows that
increasing energy input can increase ebullitive methane flux rates at the
studied bog site.
Importance for the ecosystem-level flux
When measured ebullition fluxes were upscaled to the ecosystem level, they
showed much lower methane emissions than measured with chamber and EC
techniques. In our previous study (Korrensalo et al., 2018b), we measured
diffusive methane fluxes with the static chamber technique from six
different plant community types, including BP, at the same bog site in 2014.
We found higher methane fluxes from BP than from high hummocks (HHU), but
otherwise all the studied plant community types had similar methane fluxes.
When chamber fluxes were upscaled to ecosystem level, they were similar to
the EC flux (Korrensalo et al., 2018b). Although laboratory incubation
studies have shown that the contribution of ebullition to the total methane
flux may reach up to 50 % (Christensen et al., 2013; Tokida et al.,
2007), the ebullition contribution in this study was only 3 %–5 % during
the peak season of 2014. Here, ebullition is only considered from
waterlogged surfaces, as we did not measure ebullition from vegetated
surfaces. Previously, Riutta et al. (2007) measured methane fluxes from
different plant communities with static chambers at the Siikaneva fen site,
situated 1.3 km south-east of our studied bog site, and calculated results
for both diffusive and ebullition fluxes. They found ebullition from all
communities but showed that its contribution to the total flux (diffusive
flux plus ebullition) was negligible or very small (Riutta et al., 2007). We
estimated ebullition to occur only twice in 210 measurements on moss-covered
surfaces, i.e. in 0.8 % of our 2014 chamber data. Therefore, we assume
that ebullition from vegetated surfaces would not greatly contribute to the
total flux at the bog site either. Earlier, similar to our study, Green
and Baird (2013) found ebullition to contribute less than 3.3 % of
total methane fluxes when incubating peat samples collected from hollows and
lawns from two raised bogs in laboratory study. As the measured bubble
methane fluxes in our study were of the same magnitude in each year,
ebullition did not contribute significantly to the ecosystem methane
emissions in any studied growing season, as seen in the comparison with the
EC flux. While the same seasonal trend and peaks can be seen in both fluxes
in each year, the total flux measured with EC is constantly at least an
order of magnitude higher than the ebullition flux rate.
Conclusions
More methane ebullition was found from W than from BP, and within the pools
more bubbles were released from OW than from EW. We also found variation
between the three studied growing seasons, as ebullition rate was generally
higher in the wettest year (2016). Due to this spatial and temporal variation,
differences between years in wet or dry conditions may have an effect on
ebullition. As expected, ebullition increased together with increasing peat
temperature, which facilitates methane production, and with decreasing WT,
which reduces hydrostatic pressure on peat. Additionally, more bubbles were
released with a bigger weekly increase in atmospheric pressure, which is
related to rapid ebullition from shallow peat. Furthermore, a higher weekly
temperature sum had a positive effect on ebullition, which shows that
increasing energy input can increase ebullitive methane flux rates at the
studied bog site. Therefore, the growing season lengthening and the increase in
the average temperatures due to climate change may increase the methane
emissions in the peatland ecosystem, as long as waterlogged anoxic conditions
in the peat for methane production persist. Ebullition flux upscaled to the
ecosystem level showed a similar seasonal pattern to methane fluxes measured
with EC and chamber techniques but was an order of magnitude lower and had a
very small contribution to the total ecosystem flux. Our study only includes
ebullition from the waterlogged surfaces, as we did not expect
ebullition from all the plant community types to be substantial based on the
previous study at the nearby fen site and our chamber measurements in 2014.
However, estimating the amount of ebullition from all the plant community
types would be needed to fully understand the spatial variation in
ebullition in the future. In addition, measurements with, e.g. time-lapse
cameras, are needed to study the short-term temporal variation in ebullition
and to estimate the frequency and magnitude of rapid ebullition events that
may contribute to the total ecosystem flux.
Data availability
The data used in this study are available upon request from the corresponding author.
Mean methane concentrations of the gas caught in the gas traps and
of fresh ebullition samples in 2014. Concentration samples were collected
four times from gas traps on water (W) and bare peat (BP) surfaces and by
triggering fresh ebullition from similar surfaces.
DateMean CH4 concentration mL L-1W gas trapBP gas trapW freshBP fresh10 Jun174529326816 Jul6124542031313 Aug861523991725 Sep4963379221
Numbers in italics indicate the concentration in a single measured gas trap, as opposed to a mean of many gas traps.
Frequency distribution of methane ebullition (mg m-2 d-1) per gas trap from open water pools (OW), the water's edge (EW) and
bare peat surfaces (BP) in (a–b) 2014, (c–d) 2015 and (e–f) 2016. Note the
differences in scales between the years.
Author contributions
AK, EST, PA and TV came up with the idea and design. AK and EM conducted the
ebullition measurements and processed the data. Eddy covariance data was
collected and analysed by PA and IM. EM fitted the mixed-effect models. The
manuscript was written by EM, AK and EST and commented on by all the other
authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank
Hyytiälä Forest Research Station and its staff for research
facilities and Pauli Karppinen from Natural Resources Institute Finland
(LUKE) for analysing methane concentration of the fresh bubble samples. We
also thank Salli Uljas, Janne Sormunen, Franziska Rossocha and Laura Kettunen for the help in the field.
Financial support
This research has been supported by the Academy of Finland (grant no. 287039), the Finnish Cultural Foundation (grant no. 00170743), the Academy research project CLIMOSS (grant no. 41007-00086900), the Strategic Research Council research project SOMPA (grant no. 41007-00114600), the National Centre of Excellence (grant no. 272041), the ICOS-Finland (grant no. 281255), the Academy professor project funded by the Academy of Finland, AtMath funded by the University of Helsinki (grant no. 284701), and the Postdoctoral Researcher project funded by the Academy of Finland (grant no. 315424).
Review statement
This paper was edited by Alexey V. Eliseev and reviewed by two anonymous referees.
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