Methane is produced in flooded, anaerobic soil by methanogenic archaea, and oxidized in aerobic soil by methanotrophic bacteria. Both groups are active in wetland environments such as peat bogs, rice paddies and lake sediments, but these ecosystems are usually net methane sources. On the other hand, comparatively dry, aerobic soils (frequently termed 'upland soils' in the literature) are net sinks for atmospheric methane. Methanotrophic bacteria in upland soils oxidize methane that diffuses into the soil from the overlying atmosphere, where it is present at a trace mixing ratio of ~1.75 ppmv. The term 'atmospheric methane' will be used throughout this chapter to denote methane at this mixing ratio.
The first observation of a soil acting as a net methane sink was made in the Great Dismal Swamp, in south-eastern Virginia and north-eastern North Carolina, USA, when the water table was low and there was a thick aerobic surface soil layer (Harriss et al., 1982). Since then, atmospheric methane uptake has been observed in a number of soil ecosystems, including forests (Keller et al., 1983; Steudler et al., 1989), savannah (Seiler et al., 1984), grasslands and meadows (Mosier et al., 1991; Neff et al., 1994), landfill cover soils (Whalen et al., 1990), desert (Striegl et al., 1992), tun dra (Whalen and Reeburgh, 1990), heathland (Kruse and Iversen, 1995), dryland rice soils (Singh et al., 1998) and other agricultural soils (Mosier et al., 1991; Hutsch et al., 1993). This sink is microbial, as autoclaved soil has no methane oxidation activity (Whalen et al., 1990; Yavitt et al., 1990; Bender and Conrad, 1992), the kinetics of the process is typical of an enzymatic reaction (Bender and Conrad, 1992) and 14CH4 is incorporated into micro-bial biomass (Roslev et al., 1997).
Although aerobic upland soils are generally considered methane sinks alone, methanogenesis also occurs. Periodic net methane emissions, or elevated mixing ratios of methane in soil matrix air, have been observed in many upland soils (e.g. Yavitt et al., 1990; Keller et al., 1993; Mosier et al., 1993; Savage et al., 1997; Castro et al., 2000; Maljanen et al., 2003). In some soils, methane production was localized in organic horizons (Sexstone and Mains, 1990; Adamsen and King, 1993; Yavitt et al., 1995; Amaral and Knowles, 1997; Saari et al., 1997); in others, methane production was located in water-saturated zones (Klemedtsson and Klemedtsson, 1997; Kammann et al., 2001). Methane emissions may be particularly strong during spring thaw in the temperate zone (Dunfield et al., 1995; Wang and Bettany, 1995). Isotope dilution techniques indicate that methanogenesis occurs simultaneously
©CAB International 2007. Greenhouse Gas Sinks (eds D.S. Reay, C.N. Hewitt, K.A. Smith and J. Grace)
with methane oxidation in some soils, even when only net methane uptake is measurable using flux techniques (Andersen et al., 1998; von Fischer and Hedin, 2002). Methane is also produced in the anaerobic guts of soil macrofauna, especially soil-feeding termites (Seiler et al., 1984). The soil methane sink should therefore be considered a net effect of methane consumption and production processes, which may occur either simultaneously or separated in time and space. Periodic methane emission rates in localized areas may be extremely high compared to methane uptake rates, and effectively negate the sink strength of a large area (Simpson et al., 1997), or cause a site that is usually a net sink to be a net source when considered over an entire year (Dunfield et al., 1995).
The magnitude of the soil methane sink is usually estimated to be 30-60 Tg/year. However, this estimate is based on extrapolations of data-sets that are geographically and temporally incomplete. In a review of published flux measurements, Smith et al. (2000) place a potential range of 7-100 Tg/ year on the sink strength. Considerable variability exists among different soil types, and many ecosystems are poorly studied. The highest methane oxidation rates have been measured in pristine forests, and the record is 13.7 mg/m2/day measured in tropical forests of India (Singh et al., 1997).
The microbial soil methane sink is much less in magnitude than the atmospheric sink mediated by OH radicals (see Shallcross et al., Chapter 11, this volume). However, there are several good reasons, besides scientific curiosity, to investigate it. Its magnitude is close to the present source-sink imbalance of the methane budget, and it is very sensitive to many types of anthropogenic disturbance, including conversion of forests to agriculture, fertilization, soil compaction, acidification and nitrogen deposition. Cultivated soils support very low rates of methane oxidation compared to native ecosystems. It is worthwhile to examine the mechanisms of disturbance, and determine whether the decline of the soil methane sink can be limited or reversed by proper management. The soil methane sink might also be better under stood, and more amenable to preservation, if we knew exactly which microorganisms were responsible for it. Although much is known about methanotrophic bacteria in general, evidence indicates that atmospheric methane oxidizers in upland soils are species that have not yet been isolated into pure culture. The following sections will provide a review of the biogeochemi-cal controls of the soil methane sink, and summarize what is known about the bacteria responsible.
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