Diurnal variation in methane flux in a low-arctic fen in Southwest Greenland

Abstract

Popular science: A Day in the Life of a Methane Bog
Methane gas is continuously released from wetlands around the world. In Kobbefjord,
Greenland the release of methane is low during the cold of winter and higher in warm
summer months. But the amount released also varies from day to night; a pattern that
has to do with both temperature and light.
Methane is a waste product from bacteria living in the soil below the groundwater table. The
bacteria producing methane are only able to live where no oxygen is present. Their production
of methane is affected by plants growing at the ground surface above them, as the plants
release sugars and oxygen into the soil through their roots. The bacteria consume sugars from
the plants which helps them to decompose organic matter in the soil. On the other hand,
oxygen released into the soil by plants causes methane production to drop, as no methane is
produced where oxygen is present.
How much methane is released from the wetland into the atmosphere depends partly on the
amount produced, but also on how it is transported to the surface. In the same way that plants
are able to transport oxygen from the air into the soil, methane in the soil can pass to the
atmosphere through the roots, stems and leaves of plants. This is a relatively fast mode of
transport. Alternatively, methane can move up slowly through the soil, but near the ground
surface there are bacteria which consume methane as it rises up from below; therefore part of
it never makes it to the surface.
Both the bacteria that produce and those that consume methane are more active at warm than
at cold temperatures. The same is true for plants, but these are also more active in light than in
darkness. As a result, the plants release more sugars into the soil and usually transport more
oxygen and methane during the day when it is light and warm. The methane producing
bacteria often live at a depth in the soil where there is only little temperature difference
between day and night. But the methane consuming bacteria near the surface are more active
by day than at night because of the higher temperature. The amount of methane released at the
surface may vary from time to time and from place to place depending on factors such as
light, temperature, groundwater table depth, plant species, soil type etc.
At the study site in Kobbefjord, the amount of methane released was in some periods higher
by day and in others higher at night. Analyses showed that temperature and light both had a
role to play in this, but there was not enough information available to explain which
mechanisms were responsible. Some of the factors mentioned above may well explain the
change, but not all were measured at the time of the study. Most probably, plants are
important for the short term variations in methane patterns.

Abstract (Danish)

Der forelægges et studie af døgnvariationen i methanflux fra et kær i Kobbefjord i det
sydvestlige Grønland. Studiet omhandler fire adskilte perioder, hver på en uge, fra
vækstsæsonerne i 2009 og 2010. Methanfluxdata blev sammenholdt med overfladetemperatur,
jordtemperatur og fotosyntetisk aktiv stråling (PAR). Regressiontests af overfladetemperatur og
methanflux gav i august 2009 oftest stærkt signifikante negative korrelationer (p<0,01) og
positive korrelationer med tilsvarende signifikans i august 2010. Korrelationerne mellem PAR
og methanflux fulgte et lignende mønster med lidt lavere signifikans. Korrelationerne mellem
jordtemperatur og methanflux var mindre tydelige. Udover analysen af de naturlige fluxdata
indgik et eksperiment hvor vegetationen blev mørklagt i fem minutter ad gangen. T-tests viste at
fluxniveauerne var signifikant højere i mørke (p<0,05) i fire ud af fem tilfælde. De negative
korrelationer for metanflux mod overfladetemperatur og PAR kan skyldes begrænset ilttilførsel
til de øvre jordlag eller til rhizosfæren. De tilgængelige data er utilstrækkelige til at forklare,
hvorfor både positive og negative korrelationer kunne ses i de samme områder.

Abstract (Swedish)

Scientific abstract: The study set out to investigate diurnal patterns in methane flux in a fen in Kobbefjord,
Southwest Greenland and to link these to physical and biological parameters in the
environment. Methane flux was measured using six chambers which recorded the flux
every ten minutes from spring to autumn. Methane flux data from 2009 and 2010 were
compared to photosynthetically active radiation (PAR), surface temperature and soil
temperature at 5 cm depth.
Methane is produced by microorganisms under anaerobic conditions in the soil. The primary
source of carbon for methane production is fresh organic substrate released by vascular plants
into the root zone through their roots. Vascular plants also transport oxygen from the
atmosphere into the root zone which creates an aerobic area where no methane production
occurs. In this way vascular plants may both increase or decrease the rate of methane
production. The net effect varies from case to case.
Methane produced below ground is transported to the surface via three different pathways;
diffusion, in bubbles or through the roots, stems and leaves of vascular plants. As methane
diffuses to the surface it must pass through the aerobic zone in the upper layers of the soil
where part of it may be oxidised by microorganisms. When transported in bubbles or through
vascular plants the methane is to a lesser degree subject to oxidation. Furthermore, vascular
plants growing in wetlands may influence methane emissions through the rate of substrate and
oxygen supply. Gas transport and substrate supply both tend to increase with PAR and
temperature. The rate of methane oxidation in the upper layers of the soil tends to increase
with temperature.
In August 2009 at the Kobbefjord site, highly significant negative correlations were observed
for surface temperature and PAR against methane flux. A year later, in August 2010,
comparisons of surface temperature and PAR against methane flux gave highly significant
positive correlations. During both periods, correlations between methane flux and soil
temperature were less evident than for surface temperature and PAR.
Negative correlations indicate that the methane flux is higher at night than by day, while
positive correlations indicate the opposite. Negative correlations may relate to increased
oxygen supply to the root zone during daytime or to decreased methane oxidation in the upper
soil layers due to lower night time temperatures. Positive correlations, in turn, may be
explained by increased substrate supply into the root zone as well as plant mediated methane
transport during day. From the available data it was not possible to explain why both positive
and negative correlations occur at different times at the same sites.

Abstract

A study of diurnal variations in methane flux in a fen in Kobbefjord in Southwest Greenland is presented. Four separate periods, each of one week, from the growing seasons of 2009 and 2010 were chosen. Methane flux data were compared to surface temperature, soil temperature and photosynthetically active radiation (PAR). Regression tests between surface temperature and methane flux produced highly significant negative correlations (p<0.01) in the majority of cases in August 2009, and positive correlations at similar significance levels in August 2010. Correlations between PAR and methane flux showed a similar pattern at slightly lower significance. Correlations between soil temperature and methane flux were less evident. In addition to the analysis of the ambient flux data, an experiment was conducted in which the vegetation was darkened for five minutes at a time. In four out of five cases T-tests showed that flux levels were significantly higher in darkness (p<0.05). The observed negative correlations of methane flux to surface temperature and PAR may relate to reduced oxygen availability in the upper soil layers or in the rhizosphere. The available data are insufficient to explain why both positive and negative correlations were found at the same sites.

• author: Lyshede, Bjarne Munk
• supervisor: Magnus Lund; Torben Christensen
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20112
• year: 2012
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: geography, physical geography and ecosystem analysis, diurnal, methane, wetland, Arctic, Greenland
• publication/series: Student thesis series INES
• report number: 232
• language: English
• id: 2430852
• date added to LUP: 2012-04-10 16:25:54
• date last changed: 2012-04-12 15:51:06