Introduction

A better understanding of the global terrestrial carbon cycle has become a policy imperative, both nationally and worldwide. The Kyoto Protocol recognizes the role of terrestrial systems as carbon sinks and sources. Terrestrial and sub-marine permafrost is identified as one of the most vulnerable carbon pools of the Earth system (Osterkamp 2001; Zimov et al. 2006). About one third of the global soil carbon is preserved in northern latitudes (Gorham 1991), mainly in huge layers of frozen ground, which underlay around 24% of the exposed land area of the northern hemisphere (Zhang et al. 1999). This carbon reservoir is of global climatic importance, in particular due to the currently observed climate changes in the Arctic (IPCC 2007; see Chap. 1 and Sect. 15.4).

Thawing of permafrost could release large quantities of greenhouse gases into the atmosphere, thus further increasing global warming and transforming the Arctic tundra ecosystems from a carbon sink to a carbon source (Oechel et al. 1993). Trace gas fluxes from permafrost ecosystems are influenced by a number of biotic and abiotic parameters (Fig. 15.1). The decomposition of soil organic matter and the generation of greenhouse gases result from microbial activity, which is affected by habitat characteristics (soil parameters) and by climate-related properties (forcing parameters). The method of gas transport determines the ratio between methane and carbon dioxide emission to the atmosphere. However, the processes of carbon release, their spatial distribution and their climate dependency are not yet adequately quantified and understood.

The world-wide wetland area has a size of about 5.5 x 106 km2 (Aselmann and Crutzen 1989). About half of it is located in high latitudes of the northern hemisphere (> 50°N). The atmospheric input of methane from tundra soils of this region has been estimated to vary between 17 and 42 Tg CH4 yr-1 (Whalen and Reeburgh 1992; Cao et al 1996; Joabsson and Christensen 2001), corresponding to about 25% of the methane emission from natural sources (Fung et al. 1991).

Dirk Wagner

Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A45, 14473 Potsdam, Germany e-mail: [email protected]

R. Margesin (ed.) Permafrost Soils, Soil Biology 16,

DOI: 10.1007/978-3-540-69371-0, © Springer-Verlag Berlin Heidelberg 2009

Fig. 15.1 Schematic view of the process variables influencing the formation, transport, and release of climate-relevant trace gases in permafrost soils

In the last decades, numerous studies on methane fluxes have been focused on tundra environments in Northern America and Scandinavia (Svensson and Rosswall 1984; Whalen and Reeburgh 1988; Bartlett et al. 1992; Liblik et al. 1997; Reeburgh et al. 1998; Christensen et al. 2000). Since the political changes in the former Soviet Union in the early 1990s, the large permafrost areas of Russia have been integrated into the circum-Arctic flux studies (Christensen et al. 1995; Samarkin et al. 1999; Panikov and Dedysh 2000; Tsuyuzaki et al. 2001; Wagner et al. 2003; Corradi et al. 2005; Kutzbach et al. 2007; Wille et al. 2008). All these studies revealed temporal and spatial variability of methane fluxes, ranging between -1.9 and 360 mg CH4 m-2 per day. To understand these dramatic fluctuations, some studies focused on the environmental conditions and soil characteristics, comprising the water table position, soil moisture and temperature, type of substrate and vegetation as well as availability of organic carbon (Torn and Chapin 1993; Vourlitis et al. 1993; Bubier et al. 1995; Oberbauer et al. 1998; Joabsson et al. 1999; Yavitt et al. 2000). These factors influence the methane dynamics of tundra environments. Although 80-90% of total methane emissions originate from microbial activity (Ehhalt and Schmidt 1978), only a few investigations dealt with methane production and methane oxidation caused by microbiological processes in the course of carbon dynamics (Slobodkin et al. 1992; Vecherskaya et al. 1993; Samarkin et al. 1994; Schimel and Gulledge 1998; Segers 1998; Frenzel and Karofeld 2000; H0j et al. 2005; Wagner et al. 2005; Liebner and Wagner 2007; Metje and Frenzel 2007).

This review first examines the processes of the methane cycle in permafrost soils. It then describes the methane-cycling microorganisms, including possible impacts of global warming on their structure and function.

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