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.
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