Figure 1. The landscape topography and mire surface level relative to the local ground water table determines mire hydrology. (a) Ombrogenous mires: the mire surface receives water and nutrients only through precipitation. However, depending on climatic conditions, the catotelm peat may either lose water to or receive water from the underlying mineral soil. (b) Topogenous mires: rely on stagnant ground water, resulting in low rates of nutrient supply. (c) Soligenous mires: are supplied with moving ground water resulting in relatively high rates of mineral nutrient supply.

Figure 2. Chemical constituents of peat, from boreal Swedish mires, of different botanical origin and degree of decomposition. LS, Sphagnum, low degree of humification; MS, Sphagnum, medium degree of humification; HS, Sphagnum, high degree of humification; Mixed, mixed Sphagnum and sedge peat; BC, mixed sedge and brown moss peat; LC, sedge peat, low degree of humification; MC, sedge peat, medium degree of humification; HC, sedge peat, high degree of humification. Based on data from Bergner et al. (1990).

Figure 3. The different microtopographical features have a very strong influence on biogeochemical processes. The level of the groundwater table can be taken as an acceptable indicator for the anoxic limit. In the hollow plant communities the groundwater table is close to the vegetation surface, resulting in a very restricted oxic zone. In the lawn plant communities the water table is often at a depth of up to 20 cm and in hummocks the groundwater table varies between 20 and 60 cm. These differences have pronounced effects on plant composition, carbon degradation and methane production and emission.

Figure 4. The way organic carbon from recently dead sources is incorporated into the mire has important implications for both peat accumulation and methane production. Litter from above-ground parts of vascular plants is subjected to aerobic decay. Moss litter in the hollow will be decomposed anaerobically virtually from the moment of death, while mosses growing on other microtopographical features will decompose aerobically for some time before they enter the anoxic zone. Plant roots represent the most important energy and carbon source for the anoxic environment, while a large proportion of the plant roots will be degraded only under anoxic conditions.

Figure 5. The rates of accumulation of carbon in peat have fluctuated during the Holocene, showing both long-term trends and more short-term variations. (a) Mesotrophic sedge mire. (b) Oligotrophic pine fen. The labels to the right (zone) designate dominating plant fragments and the superscripts designate different sphagna subsections: S, Sphagnum spp. (Ac, acutifolia; Pa, palustria; Cu, cuspidata; EuS, eutrophic Sphagnum spp.); B, brown mosses; C, sedge. From Klarqvist M, Bohlin E and Nilsson M, unpublished data.

Figure 6. Carbon flow in the mire ecosystem. (1) Photosynthesis; (2) aerobic decomposition; (3) anaerobic decomposition; (4) production of methane from anaerobic degradation products; (5) aerobic oxidation of methane to carbon dioxide; (6) photochemical oxidation of methane to carbon dioxide in the atmosphere. Root exudates have high nutritive value for the microorganisms and therefore they are very important for their activity.

Figure 7. The amount of methane emitted from the mire surface depends both on the amount anaerobically produced and the amount aerobically oxidized; thus, the amount emitted is normally highest from carpets or lawns and low, or even negative, from hummocks.