Description of natural terrestrial methane sources

3.2.1

Processes

Wetland environments have long been known to be significant contributors to atmospheric methane through microbial breakdown (decomposition) of organic material in saturated soils (Ehhalt 1974; Fung et al. 1991; Bartlett and Harriss 1993).

In wet, anaerobic (oxygen-free) environments, methane is formed through the microbial process of methanogenesis.

Methane formation follows from a complex set of ecosystem processes that begins with the primary fermentation of organic macromolecules to acetic acid, other carboxylic acids, alcohols, carbon dioxide, and hydrogen. Primary fermentation is followed by secondary fermentation of the alcohols and carboxylic acids to acetate, hydrogen_, and carbon dioxide, which are fully converted to methane by methanogenic bacteria (Cicerone and Oremland 1988; Conrad 1996). Many factors affect this sequence of events, including temperature, the persistence of anaerobic conditions, gas transport by vascular plants, changes in microbial community composition, and supply of easily decomposable organic substrates (Whalen and Reebugh 1992;

Davidson and Schimel 1995; Joabsson and Christensen 2001;

Ström et al. 2003). Also, substances such as nitrate and, in particular, sulfate, may competitively inhibit methanogenesis and support anaerobic methane oxidation. Figure 3.1 shows the variety of controls on methane formation rates at different spatial and temporal scales.

The net release of methane from wetland soils is the result of transport and the competing soil processes of methane production and methane consumption. While methane is produced in anaerobic soils, it is consumed (oxidized) in aerobic parts of the soil. This oxidation takes place through the microbial process of methanotrophy, which can even take place in dry soils by bacteria oxidizing methane transported from the atmosphere (Whalen and Reeburgh 1992; Moosavi and Crill 1997; Christensen et al. 1999). Microbial methane oxidation in soils can represent a terrestrial sink for atmospheric methane and a process that to some extent can counterbalance net methane production in areas where dry tundra landscapes dominate (Emmerton et al. 2014). Nevertheless, the total estimated soil sink of such dry landscapes, even globally, remains small compared with the overwhelming importance of hydroxyl radical (OH) oxidation in the atmosphere (Ch. 6) but also compared with the large wetland and freshwater emissions (Kirschke et al. 2013). However, methanotrophy is responsible for the oxidation of an estimated 50% of the methane produced at depth in the soil column in wet areas with net emissions Box 3.1 Key terminology

Active layer The top part of a permafrost soil which thaws annually in summer and refreezes in winter.

Anaerobic environment Living environment with no free oxygen available. Often found in sediments and wetland soils.

Cryoturbation (frost churning)

Refers to the mixing of materials from various horizons of the soil due to freezing- and thawing-induced expansion and contraction of soil.

Drained thermokarst lake basin

Lake that has been drained naturally by contact with a river system or other lower-lying lakes.

Ebullition In this context, associated with the bubbles that cause sudden and erratic release of gas (with a large content of methane) from wet soils and sediments.

Peatland Can be both dry and wet ecosystems but characterized by a substantial accumulation of organic material, peat. This means peatlands must in a past or present perspective have been considered as mires where there is a surplus of organic matter produced that accumulated in the ground. According to the International Peat Society the definition of a peatland is simply

“an area with or without vegetation with a naturally accumulated peat layer at the surface”.

Permafrost table The top of the frozen soil horizons below the active layer.

Taiga A biome characterized by being dominated by coniferous forest consisting mostly of spruce, pine and larch.

Talik A layer of year-round unfrozen ground that lies in permafrost areas. In regions of continuous permafrost, taliks often occur underneath lakes and rivers, where the deep water does not freeze in winter, and thus the soil underneath will not freeze either.

Thermokarst Subsidence and erosion processes created by thawing of ice-rich permafrost.

Thermokarst lake Lake created by subsidence of the soil by thawing of permafrost that is oversaturated with ice.

Tundra, wet and dry Tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes from the Sami word tuntuuri meaning ‘treeless mountain tract’. Tundra is present in vast areas of the Arctic as wet tundra overlapping with the global wetland category. Medium-wet (mesic) and dry tundra comprise the rest of the biome.

Wetland ‘Wetland’ is a very broad characterization of an ecosystem where the vegetation has adapted to constant inundation. According to the RAMSAR Convention a wetland is “a land area that is saturated with water, either permanently or seasonally, such that it takes on the characteristics of a distinct ecosystem”. The main wetland types include swamps, marshes, bogs and fens. Tundra wetlands may include all types.

Yedoma deposits Ice-rich Pleistocene loess deposits of mixed origin with labile but frozen organic carbon that may have a total ice volume content of 30–90%.

(Reeburgh et al. 1994) and, in terms of controlling net methane emissions, is as important a process as methanogenesis.

The anaerobic process of methanogenesis (methane production) is far more responsive to changes in temperature than the aerobic process of methanotrophy (methane consumption/oxidation) (Conrad 2009). The mechanistic basis for this difference is not clear, but the ecosystem consequences are straightforward: soil warming in the absence of any other changes will accelerate methane emission (which is the difference between production and consumption), in spite of the simultaneous stimulation of the two opposing processes (Ridgwell et al. 1999). Therefore, in the absence of other changes, warming favors increasing production and net emission of methane on a short-term basis (Yvon-Durocher et al. 2014). Over the longer term, indirect effects of warming may result in changes in the water balance, vegetation, and overall soil carbon dynamics, making the overall outcome less certain. In permafrost environments undergoing thaw, in addition to the effect of temperature on the microbial processes themselves, warming has been shown to favor increasing emission through a combination of the stimulating effects of increased vascular plant coverage and the availability of thawing old organic material (Klapstein et al. 2014). This finding corresponds well with landscape-scale analysis of changes in Scandinavian permafrost wetlands over decades where increasing emissions have been documented due to changes in community structure following permafrost thaw (Johansson et al. 2006; Bosiö et al. 2012).

Analysis of growing-season methanefluxes for a large number of boreal sites across permafrost zones (Olefeldt et al. 2013) illustrates not only strong relationships between methane flux and water-table position, soil temperature, and vegetation composition but also their interacting effects on fluxes. For example, emissions from wetlands with water tables at or above the soil surface are more sensitive to variability in soil temperature than are drier ecosystems, whereas drier wetlands are more sensitive to changes in water-table position. Methane storage and transport issues may disturb the described picture of temperature, water table and plant mediated controls on net emission. At certain time scales, episodic releases of stored gases may be triggered by physical pressure build-up in permafrost soil when the active layer starts freezing from the top downwards toward the permanently frozen soil in the autumn (Mastepanov et al. 2008, 2013). There may also be sudden methane emissions during the growing season related to atmospheric pressure change (Klapstein et al. 2014). In a related ecosystem, but more

strictly freshwater setting, Wik et al. (2014) showed how the transport of methane to the atmosphere in bubbles of gas from subarctic lakes shows a highly predictable relationship with energy input, suggesting increasing emissions as the duration of lake ice cover diminishes. The bottom line is that ebullition (bubble emission) and storage/transport issues as well as microbial community shifts may complicate seasonal emission patterns such they do not always follow simple relationships with variations in temperature and plant productivity.

The controls on methane emissions are, therefore, a rather complex set of processes, often working in opposing directions.

Early empirical models of wetland methane exchanges suggested sensitivity to climate change (Roulet et al. 1992; Harriss et al.

1993). A simple mechanistic model of tundra methane emissions, including the combined effects of the driving parameters (temperature, moisture, and active layer depth), also suggested significant changes in methane emissions as a result of climate change (Christensen and Cox 1995). Since then, wetland methane emission models have grown in complexity (Panikov 1995; Cao et al. 1996; Christensen et al. 1996; Walter and Heimann 2000;

Granberg et al. 2001; Wania 2007; Riley et al. 2011; Zhang et al.

2012, 2013; Watts et al. 2014) as the mechanistic understanding of the most important processes controlling methane fluxes has improved. Autumn and winter processes have also been found to have a strong influence on net annual emissions of methane, in addition to summer/growing season processes (Panikov and Dedysh 2000; Mastepanov et al. 2008, 2013). In northern wetlands, variations in methane emission at the regional to global scale are found to be driven largely by temperature (Crill et al. 1992; Harriss et al. 1993), but with important modulating effects of vascular plant species composition superimposed (Christensen et al. 2003; Ström et al. 2003). Thus, from the perspective of empirical studies of northern wetlands, an initial warming is expected to lead to increased methane emissions, but the scale of this increase depends on associated changes in soil moisture conditions, and the secondary effects of changes in vegetation composition.

The highest tundra methane emissions are generally associated with wetland conditions combined with highly organic soils (often peat). Plant productivity can amplify the source strength of methane production, and this interaction has been studied at scales ranging from below-ground microbial investigations (Panikov 1995; Joabsson et al. 1999) to large-scale vegetation models linked to methane parameterizations (Cao et al. 1996;

Christensen et al. 1996; Walter and Heimann 2000; Zhuang et al.

Fig. 3.1 Major controls on the pathways to methane formation.

Distal (climate/environmental) and proximal (chemical) controlling parameters are indicated as well as a hierarchy of importance in a complex ecosystem context. Based on Schimel (2004).

2004; Sitch et al. 2007a; Zhang et al. 2013). Various studies have attributed the relationship between living plants and methane emissions to different mechanisms, such as: stimulation of methanogenesis by increasing C-substrate availability (input of organic substances to soil through root exudation and litter production); build-up of plant-derived peat deposits that retain water and provide an anaerobic soil environment; removal from the soil by plants of mineral nutrients such as nitrate and sulfate, which are competitive inhibitors of methanogenesis (competitive electron acceptors); and enhancement of gas transport from methanogenic soil layers to the atmosphere via root aerenchyma acting as gas conduits that bypass zones of potential methane oxidation in the soil. In addition to these stimulatory effects on net methane emissions, certain plants may also reduce emissions through actively oxidizing the root vicinity (rhizospheric oxidation), which can enhance methane consumption, while Sphagnum and other peat mosses can also possibly increase oxidation through a symbiotic relationship with methanotrophs (Kip et al. 2010; Parmentier et al. 2011).

Figure 3.2 summarizes the ways in which plants may affect methane emissions from wetlands.