Abstract. In this paper we present a model that can reconstruct water table position and soil temperature profiles to 3 m depth in boreal mixed mire systems using the readily available climate data on air temperature and precipitation as driving variables. The model simulates complete, multiple annual cycles including winter conditions and freeze-thaw processes. The major requisite for an accurate description of the soil heat flux in a mire is an accurate description of the water content of the profile because of the high porosity of the soil and the thermal properties of water. The soil moisture profile in this model is described as a function of water table position The necessity of correctly describing water content over time in modeling heat flows and ice formation in arctic soils was discussed by Waelbroeck [1993]. In peat-forming mires it is even more important to have a correct water content description for calculating heat fluxes because of the high water content in the saturated zone and the large variation of the water content in the unsaturated zone. Accurate water content de-
Natural wetlands form the largest source of methane (CH4) to the atmosphere. Emission of this powerful greenhouse gas from wetlands is known to depend on climate, with increasing temperature and rainfall both expected to increase methane emissions. This study, combining our field and controlled environment manipulation studies in Europe and North America, reveals an additional control: an emergent pattern of increasing suppression of methane (CH 4) A tmospheric methane (CH 4 ) is a powerful greenhouse gas (GHG) that is responsible for an estimated 22% of the present anthropogenically enhanced greenhouse effect (1). Natural (nonrice agriculture) wetlands are the world's largest single CH 4 source and are estimated to currently contribute between 110 and 260 Tg (Tg ϭ 10 12 g) to the global methane budget (2), of which one-third is derived from temperate and boreal northern wetlands (3). CH 4 emissions from wetlands are climatesensitive, responding positively to increases in temperature and rainfall as microbial activity and anaerobic conditions increase and negatively to cool temperatures and drought (4, 5). Like many other ecosystems, wetlands are also subject to the effects of aerial pollution and increasing CO 2 levels. The stimulatory effects of increased atmospheric CO 2 concentrations on CH 4 emission (by enhancement of net primary productivity) is well reported (6-8), although a similar stimulatory effect of nitrogen pollution on wetland CH 4 emission has not always been identified (8-10) because of differing effects nitrogen has on the ecosystem, e.g., plant species composition is an important factor in determining the effect of experimental N additions on CH 4 fluxes (10).CH 4 is produced by two different groups of methanogenic archaea (MA); one group obtains energy by the fermentation of simple organic compounds, such as acetate to CO 2 and CH 4 , and the other obtains energy by oxidizing molecular hydrogen to H 2 O by using CO 2 , which is reduced to CH 4 . Acetate-fermenting MA tend to dominate in more nutrient-rich peatlands and in summer, when the supply of labile organic carbon is relatively high. However, it has been recently demonstrated that climate, depth of the acrotelm, and acetate concentrations add a fair degree of plasticity over controls on acetate-fermenting MA (11). Both groups of microorganisms are strictly anaerobic, and both are suppressed by another group of anaerobic microorganisms, sulfate-reducing bacteria (SRB) (12).SRB have a higher affinity for both hydrogen and acetate than MA, which, under ideal conditions, enables them to maintain the pool of these substrates at concentrations too low for MA to use (13,14). In wetlands, however, the balance between sulfate reduction and methanogenesis is affected by factors such as the temperature [warmer temperatures favor methanogenesis (15)], the rate of SO 4 2Ϫ and acetate supply [lower concentrations of sulfate or higher concentrations of acetate reduce the intensity of competition (13)], and the availability of noncompetitive substra...
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