The Holocene 14,1 (2004) pp. 119–124 Mud-bottom hollows: exceptional features in carbon-accumulating bogs? Edgar Karofeld* (Institute of Ecology at Tallinn Pedagogical University, Kevade st. 2, Tallinn 10137, Estonia) Abstract: Mud-bottom hollows are depressions on the bog surface where Sphagnum mosses have died and peat accumulation is retarded or even replaced with loss by oxidation. Results of measurements carried out at Ma ¨nnikja ¨rve Bog, central Estonia, conrmed that peat accumulation has stopped and that in 1999–2001 the uppermost 2–3 cm thick surface layer of mud-bottom hollows became thinner by 1.95 6 0.75 mm yr 21 (n = 188). Different methods revealed the corresponding carbon loss from the mud-bottom hollows 25 mm thick surface layer as 50–60 g C m 22 yr 21 . As a result, the surface of mud-bottom hollows becomes lower as compared to surroundings with peat accumulation c. 1.5 mm yr 21 , and they are likely to have an important role in the differentiation of bog microtopography. Owing to the combination of a cessation in peat accumu- lation, carbon loss by oxidation and increased emission of decomposition gases, mud-bottom hollows could have an important inuence on the carbon budget of bogs. Key words: Bog, mire surface, carbon loss, peat oxidation and accumulation, mud-bottom hollows. Introduction In mires, owing to the combination of specic environmental con- ditions, up to 20% of the dead plant biomass remains undecom- posed and accumulates as peat (Tolonen and Turunen, 1996; Yu et al., 2001). Therefore mires have generally been treated as car- bon sinks in most studies of the global carbon cycle, although in reality they are perhaps better regarded as a store than a sink (see Chambers and Charman, this issue). Estimates of the amount of stored organic matter as peat range from 120–260 Gt (Franze ´n, 1994) to 450 Gt in northern peatlands (Gorham, 1991) in terms of carbon. This carbon pool is approximately one-third of the total world soil pool (Post et al., 1982) and therefore northern peatlands may become an important source for atmospheric C in a time of signicant climatic change. Indeed, Franze ´n (1994) introduced a theory that ice-age cycles are largely generated by the peat growth in temperate wetlands and its impact on the greenhouse effect. The dynamics of carbon accumulation is determined by the ratio between production and decomposition of organic matter and is highly variable over different time and space scales. There could be periods and areas in mires where carbon accumulation is retarded or even replaced with C loss by oxidation and emission of CH 4 and CO 2 . Depending on climatic conditions, some peat- forming ecosystems have been found to be potential net sources of CO 2 to the atmosphere (Waddington and Roulet, 1996; Caroll and Crill, 1997; Vourlitis and Oechel, 1997). Mires are also essen- tial natural sources of an important greenhouse gas – methane. It is not clear whether the warming potential of the conversion of CO 2 to CH 4 is less than or more than the cooling potential of the removal of CO 2 by mires as peat (Clymo, 1996). *Author for correspondence (e-mail edgar@eco.edu.ee) Ó Arnold 2004 10.1191/0959683604hl694rp Change of peatlands between carbon sources and sinks would affect signicantly the global carbon budget (Yu et al., 2001), but little is known about the variability of carbon uxes and seques- tration through time in mires, especially in response to climatic change (Moore et al., 1998). The response of mires to changing environmental conditions will partly be determined by the mire microtopography(Bridgham et al., 1995; Waddington and Roulet, 1996; Weltzin et al., 2001). Exceptional in terms of the carbon budget may be mud-bottom hollows in bogs where, owing to degeneration of Sphagnum mosses and vascular plants, peat accumulation is presumably retarded and replaced by an increased emission of decomposition gases, especially CH 4 (Crill et al., 1992; Frenzel and Karofeld, 2000). Because mud-bottom hollows may cover large areas of some bogs (Figure 1) and because they are more common in northern areas of the Boreal zone (Paasio, 1934), which may experience greater climatic changes, they may play an important role in the carbon budget of boreal bogs and also in the changing concentration of carbon gases in the atmosphere. The main aim of the present study was to reveal the peat- accumulation or oxidation rate in the mud-bottom hollows, and to estimate the potential organic matter and carbon loss from the surface layer of mud-bottom hollows. Briey dened, mud- bottom hollows with a diameter up to tens of metres are depressions on the bog surface where Sphagnum mosses have died and the surface is formed of bare peat or a dense algal mat with single vascular plants. During dry summers the surface layer becomes dry and cracks into polygons of 20–30 cm with upturned edges. The underneath of this dried surface remains green. Karofeld (1999) noted: ‘Different terms like mud-bottom hollow, mud-bottom, bare or naked peat area, ark, regressive complex, etc. have been used. The term mud-bottom hollow is employed