Greenhouse gas flux measurements on peatlands

CO₂ dynamics in peatlands can be divided in three components:1) carbon dioxide absorption by the vegetation from which it produces tissues through photosynthesis using solar energy, 2) autotrophic respiration, where CO₂ is released by the vegetation when light is missing, 3) heterotrophic respiration where CO₂ is released to the atmosphere from decomposition of the dead plant material forming the peat. The combined result of these exchanges is called Net Ecosystem Exchange (NEE). NEE can be measured using a static chamber installed on the peat surface (Figure 1). Because photosynthesis and NEE are function of light intensity or Photosynthetic Photon Flux Density (PPFD) received by the vegetation, it is possible to simulate light conditions by placing shrouds on the static chamber at time of measurement. Overall the peatlands release CO₂ towards the atmosphere when light is not available, (e.g. during the night, PPFD = 0), and absorb CO₂ as a function of PPFD (Figure 2). Annually, the amount of carbon absorbed by the peatlands is greater than the amount released towards the atmosphere which makes peatlands sinks for carbon (CO₂ sinks). This can seem paradoxical as the growing season in boreal region is short, but the cold peat temperatures combined with the fact that peat is saturated with water slows the decomposition process and therefore the release of CO₂ to the atmosphere. The measurements done in the Eastmain-1 peatlands show absorption rates similar to other peatlands of the same type with values of absorption ranging from -0.7 to -1.6 grams of CO₂ per square meter per day (g CO₂ m-² d-¹).

Figure 2 : Relationship between net ecosystem exchange and light intensity (PPFD- photosynthetic photon flux density) in a peatland located in the Eastmain river watershed. Negative CO₂ values represent absorption by the peatland.

Peatlands are important sources of atmospheric methane (CH₄). CH₄ is a greenhouse gas 20 times more powerful CO₂. Emissions of this gas from peatlands can be measured using dark static chamber installed on the peat surface (Figure 3). CH₄ is produced in peatlands through fermentation by bacteria (methanogenic bacteria) under anoxic condition, where there is no oxygen. Close to the surface water table and the unlimited organic matter availability favors methane production. Part of the methane produced through peat decomposition under anoxic conditions does not reach the atmosphere as it is oxidize to CO₂ by methanotrophic bacteria in the oxic zone while moving towards the atmosphere in the peat layer above water table depth. The position and thickness of the zones with (oxic) and without oxygen (anoxic) are controlled by the water table depth (wtd). Therefore, methane fluxes are generally well correlated with wtd, fluxes being more important where water table is closer to the peat surface (Figure 4). Results for the Eastmain-1 area peatlands show mean annual emissions rates of 41 to 71 milligrams CH₄ square meter per day (mg m-² d-¹).

Figure 4 : Relationship between methane (CH₄) fluxes and water table depth in a peatland located in the Eastmain river watershed. Negative water table depth values represent below the peat surface values.