The present magnitude of soil organic matter accumulation is a function of the balance between organic matter production and decomposition. The decomposition rate generally increases with increased temperature but decreases with soil water saturation at a level where permanent or periodic anoxic conditions are created. On the other hand, net primary production appears to be less affected by anoxia because most plant species in the wettest ecosystems are adapted to anoxic conditions. For instance, the vascular plants have aerenchymatous tissues, which lead atmospheric air from the canopy to the belowground, inundated plant parts, and the plants thereby avoid oxygen depletion (Chapin et al., 1996). The strong constraints on decomposition, but lower constraints on plant productivity in wet tundra, together with high proportions of sphagna of low decomposability, appears to be the main factors explaining the accumulation of soil C in, e.g., wet sedge tundra and arctic mires.
In contrast, it appears that the rates of C incorporation and C loss are much more balanced in the drier ecosystems, leading to low C sequestration, except in low arctic semideserts which have an even higher SOM-to-NPP ratio than the wet ecosystems (Table 2). This shift to high accumulation in the semideserts could be because of drought-limited decomposition, as decomposition rates generally decline below about 200% soil water content (Heal et al., 1981). Furthermore, dry tundra is dominated by slowly growing, evergreen dwarf shrubs with sclerophyllous tissues, which may increase the constraints on decomposition further and lead to the high organic matter accumulation. Indeed, the most productive ecosystem types in tundra are in areas with surface or subsurface flowing water, such as low arctic, riverside willow scrubs in which the tall shrubs have high biomass and high net primary production rate and the soil is rapidly turned over by the decomposers (see Table 2). Hence, the rate of the C cycle in this part of the hydrological gradient is high through a combination of favorable hydrological conditions and lifeform properties of the vegetation, which interact to keep both plant production and microbial decomposition rates high.
This strong hydrologic control on ecosystem structure and function coincides with observations in laboratory experiments of increased soil C turnover with decreasing soil water content and increased water table depth in wet and moist tundra (Billings et al, 1982; Johnson et al, 1996). Indeed, microcosm experiments have shown that water conditions, rather than temperature, exert the main control on C exchange, and that it is ecosystem respiration rather than photosynthesis that is affected (Johnson et al, 1996). However, if the depth to the water table increases and the soil dries, temperature becomes increasingly important for the C balance with large net C losses with increasing temperatures (Billings et al, 1982; Shaver et al, 1998). Also, reported net losses of C from tussock tundra during a series of dry and warm years (Oechel et al., 1993) indicate that the C exchange in moist tundra is controlled most strongly by respiration. For this reason, a wetter climate may increase C sequestration in present mesic and wet systems depending on the balance between precipitation and évapotranspiration. The greater C accumulation rate in the western Siberian tundra then in the east (Christensen et al, 1999) suggests that a combination of moister and warmer conditions may lead to substantially increased C accumulation, particularly in the coldest tundra regions.
Dry tundra may be regulated differently because the low soil water content might limit decomposition (Ileal et al., 1981; Oberbauer et al. 1996). However, it is uncertain to what extent observed low rates of CO, fluxes in dry tundra are due to reduced respiration in microorganisms versus that in plants (Illeris and Jonasson, 1999). In spite of this uncertainty, it appears that predicted changed climatic conditions in the Arctic (IPCC, 1996) can lead to both decreased and increased C sequestration by different arctic ecosystems. It also appears that the same change of environmental conditions may have different effects across ecosystem types and could even lead to different directions of the changes in C balance between neighboring systems.
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