Global Change Biology (1997) 3, 351–362 Responses of atmospheric methane consumption by soils to global climate change GARY M. KING Darling Marine Centre, University of Maine, Walpole, ME 04573, USA Abstract Soils consume about 40 Tg methane from the atmosphere annually. Thus, soils contribute significantly to the atmospheric methane budget. However, responses of atmospheric methane consumption to climate change are uncertain. Predicting these responses requires an understanding of the effect on methane consumption of specific variables (temperature and soil water content) as well as interactions among parameters (methane, ammonium, water content). Key considerations involve the limitations of diffusive transport and controls of methane diffusivity; limitation of methanotrophic activity by water stress; relatively slow growth rates of methane-oxidizing bacteria on atmospheric methane; ammonium toxicity. Interactions among these parameters may be particularly important, and lead to responses contrary to those predicted from changes in temperature and water content alone. Results from a number of analyses indicate that atmospheric methane consumption is especially sensitive to anthropogenic disturbances, which typically decrease activity. Continued increases in wet and dry ammonium deposition are likely to exacerbate inhibition resulting from changes in land use. Changes in hydrological regimes could further decrease activity if dry periods increase water stress at soil depths currently colonized by methanotrophs. Future trends in the soil methane sink are likely to lead to enhanced accumulation of atmospheric methane. Keywords: ammonium, atmosphere, forest, land use, methane, methanotroph, soil, water potential Introduction Biological methane oxidation is widely distributed in ability and temperature, all of which can change markedly terrestrial, marine, lacustrine and wetland ecosystems both among and within systems (King 1992, 1993; (King 1992; Conrad 1996; Hanson & Hanson 1996; Hoehler Mancinelli 1995; Hanson & Hanson 1996; King 1996b). & Alperin 1996). The process occurs under both oxic Other biotic controls may include competition (Graham and anoxic conditions, and over a range of methane et al. 1993) and bactivory. While the responses of both concentrations from sub-nanomolar to millimolar (King cultures and in situ methane oxidation to some of these 1996a). The microbiology and controls of methane oxida- parameters are known from manipulative experiments, tion are similarly varied. The relevant microorganisms and changes in some of these parameters can be predicted include freshwater and soil populations similar to with modest certainty at regional scales, reliable predic- methanotrophs extant in cultures (Hanson & Hanson tions of either the relative or absolute extent of future 1996); uncharacterized, presumably novel methanotrophs methanotrophic activity are not yet feasible. in soils (Bender & Conrad 1992; King 1992); largely Limitations for predicting the response of methane uncharacterized methanotrophs and perhaps ammonia- oxidation to global-scale changes in climate, atmospheric oxidizing bacteria in the marine water column (Sieburth composition, eutrophication and land use do not simply et al. 1987; Ward 1987; Lees et al. 1991); uncharacterized derive from a lack of understanding of the controls and novel sulfate-reducing bacteria or consortia of sulfate of methane oxidation. In a more fundamental sense, reducers and methanogens that are active in anaerobic estimates of the contemporary magnitude of global meth- methane oxidation (Hoehler & Alperin 1996). Controls ane oxidation remain uncertain (Reeburgh et al. 1993). of the activity of these varied organisms include methane, The integrated rate of atmospheric methane consumption oxygen and ammonium concentrations, pH, water avail- by soils appears constrained to a relatively small range of values, as are the rates of methane oxidation in anoxic Correspondence: fax +1/207–563–3119, e-mail gking@maine.edu marine sediments and in the water columns of lakes and © 1997 Blackwell Science Ltd. 351