Methane Cycle in Permafrost Soils

The carbon pool estimates for permafrost soils vary between 4 and 110 kg C m-2 (Schell and Ziemann 1983; Tarnocai and Smith 1992; Michaelson et al. 1996). These large variations can be attributed to different soil types (from mineral to peaty soils) and varying depths of measurement (from the upper few cm to 1 m depth). Permafrost soils can function as both a source and a sink for carbon dioxide and methane (Fig. 15.2). Under anaerobic conditions, caused by flooding of the permafrost soils and the effect of backwater above the permafrost table, the mineralization of organic matter can only be realized stepwise by specialized microorganisms of the so-called anaerobic food chain (Schink and Stams 2006). Important intermediates of the organic matter decomposition are hydrogen, carbon dioxide and acetate, which can be further reduced to methane (methanogenesis) by metha-nogenic archaea (see Sect. 15.3.1). The fermentation of carbon by microorganisms takes place much more slowly than oxidative respiration. As a result of the prolonged anaerobic conditions and low in situ temperatures of permafrost soils organic matter accumulates (peat formation) in these environments.

Nervertheless, the quantity of organic matter provides no information on its quality. This, however, determines the availability of organic compounds as energy

Organic Matter Cycle

Fig. 15.2 The carbon cycle in permafrost soils. Permafrost soils can be both a source and a sink for CO2 and CH4. Under aerobic conditions soil organic matter (SOM) is respired to CO2, whereas under anaerobic conditions SOM is decomposed via a sequence of microbial processes to CH4. Methane fluxes from anaerobic soil horizons to the atmosphere result from diffusion (slow), ebullition (fast), and through plant-mediated transport (bypassing the oxic soil layer). Therefore, the method of transport determines the amount of methane that is re-oxidized by microorganisms in aerobic soil horizons. Photosynthesis provides an important sink for CO2 in permafrost environments. Thereby, biomass is produced. In contrast, the consumption of atmospheric methane (negative methane flux) in the upper surface layer of the soils plays only a minor role for the methane budget. The thickness of the arrows reflects the importance of the above processes

Fig. 15.2 The carbon cycle in permafrost soils. Permafrost soils can be both a source and a sink for CO2 and CH4. Under aerobic conditions soil organic matter (SOM) is respired to CO2, whereas under anaerobic conditions SOM is decomposed via a sequence of microbial processes to CH4. Methane fluxes from anaerobic soil horizons to the atmosphere result from diffusion (slow), ebullition (fast), and through plant-mediated transport (bypassing the oxic soil layer). Therefore, the method of transport determines the amount of methane that is re-oxidized by microorganisms in aerobic soil horizons. Photosynthesis provides an important sink for CO2 in permafrost environments. Thereby, biomass is produced. In contrast, the consumption of atmospheric methane (negative methane flux) in the upper surface layer of the soils plays only a minor role for the methane budget. The thickness of the arrows reflects the importance of the above processes and carbon sources for microorganisms (Hogg 1993; Bergman et al. 2000; see also Chap. 16). For this purpose, the humification index (HIX, dimensionless), for instance, is a criterion for organic matter quality and can, therefore, give suitable information with regard to microbial metabolism (Zsolnay 2003). It has been demonstrated that the availability of organic carbon in permafrost soils decreased with increasing HIX (Wagner et al. 2005). It has further been shown that the HIX increased continuously with depth in Holocene permafrost sediments (Wagner et al. 2007). This indicates that the organic carbon is less available for microorganisms with increasing depth because of the higher degree of humification. Therefore, in addition to the quantity, the quality of soil organic matter should also be taken into account with regard to permafrost environments as a huge carbon reservoir.

Wherever oxygen is present in permafrost habitats (upper oxic soil horizons, rhizosphere), methane can be oxidized to carbon dioxide by aerobic methane-oxidizing bacteria (see Sect. 15.3.2). Between 76% and up to more than 90% of the methane produced in wetlands is oxidized by these specialists before reaching the atmosphere (Roslev and King 1996; Le Mer and Roger 2001). Hence, the biological oxidation of methane represents the major sink for methane in Arctic permafrost environments.

Vegetation is another important factor occupying a central position for microbial processes and the transport of methane. Plants can have both enhancing and attenuating effects on methane emission. Through the aerenchyma of vascular plants, oxygen is transported from the atmosphere to the rhizosphere, thus stimulating methane oxidation in otherwise anoxic soil horizons (Van der Nat and Middelburg 1998; Popp et al. 2000). In the opposite direction, the aerenchyma is a major pathway for methane transport from the anoxic horizons to the atmosphere, bypassing the oxic/anoxic interface in the soil, where methane oxidation is most prominent. It has been shown that up to 68% of the total methane release from wet permafrost soils is transported through sedges like Carex aquatilis (Kutzbach et al. 2004). Furthermore, the vegetation provides the substrates for methanogenesis such as decaying plant material and fresh root exudates (Whiting and Chanton 1992; Joabsson et al. 1999).

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