Diurnal variations of methane and
carbon dioxide fluxes at Thorne Moors
Dooling, Gemma P.1*, Chapman, P.J.1, Baird, A.J.1, Shepherd, M.J.2 and Kohler, T.2
1School
of Geography, University of Leeds
2Natural
1. Rationale and aim
Fluxes of methane (CH4) and carbon dioxide (CO2) from peatlands are mainly
controlled by temperature, water table position, vegetation cover, levels of
photosynthetically-active radiation (PAR) and nutrient availability1.
England
*[email protected]
@GPDooling
4. Diurnal methane variation
Many studies examining gaseous carbon cycling within peatlands only conduct
field studies during daylight hours. However, both temperature and PAR levels
change diurnally. The temperature-dependencies of both CH 4 production and
consumption are well-documented. Carbon dioxide fluxes have also been strongly
linked to PAR levels. Therefore, fluxes of CH4 and CO2 may be different during the
night when PAR levels have reduced to zero, and temperatures are lower.
Collar
The aim of this study is to examine if fluxes of CH4 and CO2 vary diurnally, and if
so, to also ascertain which environmental variables exert the greatest control on
the variation.
Figure 4: Methane fluxes over 24 hours. Positive values = release to the atmosphere.
2. Field site and data collection
This study was conducted at Thorne Moors, a lowland raised bog undergoing
restoration management in eastern England that forms part of the Humberhead
Levels. Figure 1 shows the area where the study occurred. The study was
completed during a 24 hour period on July 25th-26th 2012.
Static closed chambers, as shown in Figure
2, were used to measure gaseous fluxes
every 90 minutes during the 24 hours. Four
collars were used simultaneously across the
site during each test.
Collars 1 and 2 had vegetation dominated
by Eriophorum vaginatum. Collars 3 and 4
had vegetation dominated by Eriophorum
angustifolium.
There is no clear overall diurnal variation within the CH4 fluxes from the four
collars, as shown in Figure 4. Fluxes ranged over almost 60 mg CH 4 m-2 day-1. The
lowest flux of 35.61 mg CH4 m-2 day-1 occurs at 01:30 from collar 3. The highest
flux of 94.89 mg CH4 m-2 day-1 occurs at 03:00 from collar 2.
Eriophorum vaginatum
Eriophorum angustifolium
Collar
Figure 1: Study site at Thorne Moors.
A dipwell was located adjacent to each
collar. As the site was inundated during the
study period, depth of surface inundation
was measured at each test.
Figure 5: Methane fluxes per collar over 24 hours. Positive values = release to the atmosphere.
Soil temperature and PAR levels were
measured adjacent to one collar during
each test in rotation.
Meteorological variables were also
collected from a nearby automatic weather Figure 2: Static closed chamber sitting
in a collar with a water-filled gutter.
station.
However, when the results from each collar are examined separately, as shown in
Figure 5, then some patterns do emerge. The highest flux from each collar occurs
during the night. Except for collar 3, the lowest fluxes from each collar occur
during the day. The highest and lowest fluxes from collars 1 and 2 occur at the
same times (03:00 and 16:30 respectively), although the values and surrounding
patterns are different.
3. Diurnal carbon dioxide variation
Collar
p <0.05
Collar
p <0.05
p >0.05
p <0.05
Figure 6: Correlations of CH4 fluxes and air temperature per collar with R2 and p-values.
Figure 3: Carbon dioxide fluxes and PAR levels over 24 hours. Positive flux values = release to
the atmosphere. Negative flux values = uptake from the atmosphere.
A clear diurnal variation in CO2 fluxes can be seen in Figure 3, with all four collars
behaving similarly. All of the CO2 uptake occurs during daylight hours, with the
highest uptake of -10315.9 mg CO2 m-2 day-1 at 12:00 from collar 4. The highest
rates of CO2 release occurs during the night, with the highest flux of 43369.78 mg
CO2 m-2 day-1 at 03:00 from collar 1.
Figure 3 also shows the corresponding levels of PAR at each time point during
the 24 hours. The highest PAR level (620 µmol m-2 s-1) occurs during the first
sampling time point at 13:30. Yet the highest CO2 uptake occurs at a lower PAR
level (390 µmol m-2 s-1 ) at 12:00.
Overall, there is a net loss of CO2, despite the uptake in the peak daylight hours.
Conditions were average for a summer day; however, there was full cloud cover.
So the net CO2 loss suggests that only on days with bright sunshine would there
be any net CO2 gain.
Significant correlations are found with the CH4 fluxes from collars 1, 2 and 4 and air
temperature, as shown in Figure 6. The only significant correlation with CH 4 flux
and PAR is from collar 4 (R2 = 0.33, p <0.05, negative correlation). There are no
significant correlations between CH4 flux and depth of surface inundation.
5. Summary
The analysis of these results is still on-going; however, early results indicate that
vegetation type may play a role in controlling diurnal CH4 flux patterns. If fluxes
from collars dominated by Eriophorum vaginatum are greater during the night,
then fluxes measured during the daytime could be an underestimation.
The peaks in CH4 fluxes all occur at least seven hours after the last recorded carbon
fixation by photosynthesis,. This suggests a lag time effect within the system, which
will be investigated further.
Some other studies have found CH4 fluxes to be greater during the night than the
day, some have found no pattern, but most have found CH4 fluxes to be greater
during the day than the night2.
Reference: 1Benstead, J. and Lloyd, D., 1996. Spatial and temporal variations of dissolved gases (CH 4, CO2, and O2) in peat cores. Microbial Ecology, 31, pp57-66 .
2Please
see hand-out for reference table.