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Greenhouse gas flux at a temperate peatland : a comparison of the eddy covariance method and the flux-gradient method

Karlsson, Kristofer LU (2017) In Student thesis series INES NGEM01 20161
Dept of Physical Geography and Ecosystem Science
Abstract
Northern peatlands have accumulated carbon for thousands of years and currently hold around one third of all soil carbon on earth. Large uncertainties are associated with predictions of how northern peatlands will react to climate change. This is because the cold, wet and anaerobic conditions lead to two opposing forces from a radiative forcing perspective – CO2 sequestration from photosynthesis being greater than respiration, and release of the potent greenhouse gas CH4. Accurately measuring fluxes of CO2 and CH4 is therefore key in order to understand the underlying processes and thereby implementing them in the global climate models.

In this thesis, the commonly used eddy covariance (EC)... (More)
Northern peatlands have accumulated carbon for thousands of years and currently hold around one third of all soil carbon on earth. Large uncertainties are associated with predictions of how northern peatlands will react to climate change. This is because the cold, wet and anaerobic conditions lead to two opposing forces from a radiative forcing perspective – CO2 sequestration from photosynthesis being greater than respiration, and release of the potent greenhouse gas CH4. Accurately measuring fluxes of CO2 and CH4 is therefore key in order to understand the underlying processes and thereby implementing them in the global climate models.

In this thesis, the commonly used eddy covariance (EC) technique is compared to the less applied flux-gradient (FG) technique in order to assess the applicability of the use of CH4 estimates by the FG method. The study took place at a temperate peatland in Southern Sweden, Fäjemyr, February-November 2015. CO2 flux were estimated for each method and scalar similarity was assumed (with CO2 as a reference) in order to estimate CH4 flux using the FG method.

In general, there was good agreement between the CO2 flux for both methods. Greatest agreement was found during daytime conditions with relatively high flux magnitudes (e.g. summer days) whereas worst agreement was found during nighttime and during periods of low flux magnitudes (e.g. winter and early spring). Since the agreement was satisfactory, the CH4 flux by the FG method were deemed reliable. Furthermore, the CH4 flux followed similar dependencies on temperature and water table depth as found for similar studies using other measurement techniques.

Cumulative CH4 flux for the study period was estimated to 6.8 g C m-2, which is middle of the range for similar sites, but also significantly higher than a previous study at Fäjemyr during a much drier year, indicating the importance of wetland hydrology on the carbon budget.

In conclusion, the FG technique is a good alternative to the EC method for northern peatlands and it is especially suitable for CH4 measurements since the method is most reliable during the summer, when most CH4 flux takes place. (Less)
Popular Abstract (Swedish)
Kol har lagrats is torvmarker i tusentals år och idag innehåller torvmarkerna ca en tredjedel av all markkol på jorden. Det råder stor osäkerhet hur torvmarker kommer att reagera på klimatförändringar. Detta beror på att de kalla, blöta och syrefattiga förhållandena leder till motsatta effekter för global uppvärmning; dels lagras koldioxid eftersom fotosyntesen är större än respirationen (kylande effekt) och dels släpper torvmarker ut den starka växthusgasen metan (värmande effekt). Därför är det viktigt att noggrant kunna mäta flöden av koldioxid och metan för att förstå de underliggande processerna och därmed kunna implementera dem i globala klimatmodeller.

I denna studie jämförs den vanligare metoden eddy covariance (EC) med en... (More)
Kol har lagrats is torvmarker i tusentals år och idag innehåller torvmarkerna ca en tredjedel av all markkol på jorden. Det råder stor osäkerhet hur torvmarker kommer att reagera på klimatförändringar. Detta beror på att de kalla, blöta och syrefattiga förhållandena leder till motsatta effekter för global uppvärmning; dels lagras koldioxid eftersom fotosyntesen är större än respirationen (kylande effekt) och dels släpper torvmarker ut den starka växthusgasen metan (värmande effekt). Därför är det viktigt att noggrant kunna mäta flöden av koldioxid och metan för att förstå de underliggande processerna och därmed kunna implementera dem i globala klimatmodeller.

I denna studie jämförs den vanligare metoden eddy covariance (EC) med en mindre använd metod, flux-gradient (FG), för att utvärdera möjligheten att använda FG för att beräkna metanflöden. Experimentet utfördes på en myr i södra Sverige, Fäjemyr, under februari- november 2015. Bägge metoderna användes för att beräkna koldioxidflöden, medan enbart FG-metoden användes för att beräkna metanflöden. Antagandet att transport av skalärer (t.ex. koldioxid och metan) sker likvärdigt genom turbulenta rörelser i atmosfären möjliggjorde sedan utvärdering av FG-metodens användningsbarhet för mätningar av metanflöden.

Generellt sett överensstämde koldioxidflödena väl mellan metoderna. Bäst överensstämmelse fanns under dagtid då flödena var höga (t.ex. sommardagar) och sämst överensstämmelse fanns under nätter och under perioder med låga flöden (t.ex. vinter och tidig vår). Eftersom överensstämmelsen var tillfredsställande ansågs metanflödena från FG-metoden också vara pålitliga. Dessutom överensstämde förhållanden mellan metanflöden och temperatur respektive grundvattennivån med de förhållanden som andra studier funnit med hjälp av andra mätmetoder.

Totala flöden av metan under experimentet uppskattades till 6.8 g C m-2. Jämfört med andra studier av metanflöden från torvmarker placerar detta Fäjemyr i mitten av spannet. Å andra sidan är metanflödet betydligt högre än vad som uppskattats av en tidigare studie på Fäjemyr under ett extremt torrt år, vilket påvisar hydrologins betydelse för torvmarkers kolbudget.

Sammanfattningsvis är FG-metoden ett bra alternativ till EC-metoden för torvmarker och metoden är särskilt användbar för mätningar av metan eftersom mätningarna är mest pålitliga under sommaren då majoriteten av metanflödena sker. (Less)
Please use this url to cite or link to this publication:
author
Karlsson, Kristofer LU
supervisor
organization
course
NGEM01 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Physical Geography and Ecosystem analysis, micrometeorology, eddy covariance, flux-gradient method, methane, carbon dioxide, flux, peatland
publication/series
Student thesis series INES
report number
414
language
English
id
8908170
date added to LUP
2017-05-23 08:57:47
date last changed
2017-05-23 12:14:06
@misc{8908170,
  abstract     = {Northern peatlands have accumulated carbon for thousands of years and currently hold around one third of all soil carbon on earth. Large uncertainties are associated with predictions of how northern peatlands will react to climate change. This is because the cold, wet and anaerobic conditions lead to two opposing forces from a radiative forcing perspective – CO[sub]2[/sub] sequestration from photosynthesis being greater than respiration, and release of the potent greenhouse gas CH[sub]4[/sub]. Accurately measuring fluxes of CO[sub]2[/sub] and CH[sub]4[/sub] is therefore key in order to understand the underlying processes and thereby implementing them in the global climate models.

In this thesis, the commonly used eddy covariance (EC) technique is compared to the less applied flux-gradient (FG) technique in order to assess the applicability of the use of CH[sub]4[/sub] estimates by the FG method. The study took place at a temperate peatland in Southern Sweden, Fäjemyr, February-November 2015. CO[sub]2[/sub] flux were estimated for each method and scalar similarity was assumed (with CO[sub]2[/sub] as a reference) in order to estimate CH[sub]4[/sub] flux using the FG method.

In general, there was good agreement between the CO[sub]2[/sub] flux for both methods. Greatest agreement was found during daytime conditions with relatively high flux magnitudes (e.g. summer days) whereas worst agreement was found during nighttime and during periods of low flux magnitudes (e.g. winter and early spring). Since the agreement was satisfactory, the CH[sub]4[/sub] flux by the FG method were deemed reliable. Furthermore, the CH[sub]4[/sub] flux followed similar dependencies on temperature and water table depth as found for similar studies using other measurement techniques.

Cumulative CH[sub]4[/sub] flux for the study period was estimated to 6.8 g C m-2, which is middle of the range for similar sites, but also significantly higher than a previous study at Fäjemyr during a much drier year, indicating the importance of wetland hydrology on the carbon budget.

In conclusion, the FG technique is a good alternative to the EC method for northern peatlands and it is especially suitable for CH[sub]4[/sub] measurements since the method is most reliable during the summer, when most CH[sub]4[/sub] flux takes place.},
  author       = {Karlsson, Kristofer},
  keyword      = {Physical Geography and Ecosystem analysis,micrometeorology,eddy covariance,flux-gradient method,methane,carbon dioxide,flux,peatland},
  language     = {eng},
  note         = {Student Paper},
  series       = {Student thesis series INES},
  title        = {Greenhouse gas flux at a temperate peatland : a comparison of the eddy covariance method and the flux-gradient method},
  year         = {2017},
}