In northern boreal ecosystems, due to impeded microbial activity, organic carbon is mainly stored in the upper soil as peat or other plant debris of different decomposition stages. This highly labile organic C may greatly exceed the biomass of vegetation and is most vulnerable to climate change. Special attention has to be paid to SOC buried into permafrost and becoming bioavailable to decomposition as permafrost retreats. These carbon pools stored in high-latitude soils and peats represent the major ecosystem source of DOC (Aitkenhead-Peterson et al. 2005). On the basis of water-soluble organic matter extracted under laboratory conditions, DOC constitutes about 1% of total OC in organic soil layers (Fig. 16.1). Therefore, there is a significant
Fig. 16.1 Relationship between water extractable organic carbon (WEOC) and total organic carbon in forest floor of feather-moss dominated larch ecosystems in Central Siberia. Black dot represents the mean value for 50 cm deep peat of Sphagnum fuscum. All study sites are underlain by continuous permafrost
Fig. 16.1 Relationship between water extractable organic carbon (WEOC) and total organic carbon in forest floor of feather-moss dominated larch ecosystems in Central Siberia. Black dot represents the mean value for 50 cm deep peat of Sphagnum fuscum. All study sites are underlain by continuous permafrost pool of potentially mobile OM in topsoils of permafrost terrains, which is also supposed to be renewable along with SOC decomposition (Neff and Hooper 2002).
Permafrost degradation, nevertheless, may also increase the size and frequency of fires that are important controls of carbon storage in the taiga biome of Siberia (Conard et al. 2002). Combustion of organic layers greatly reduces the amount of mobile C fraction and export of DOC to the subsoil (Shibata et al. 2003). However, deeper soil thawing activates subsoil C-cycling after a fire event.
It has been reported that about 10-40 g DOC m-2 are translocated annually from the organic surface layer into the mineral soil horizons in temperate forests (summarized in Michalzik et al. 2001), with only slightly lower amounts (4-17 g DOC m-2) in the continuous permafrost zone of Siberia (Prokushkin et al. 2005). This means that about 10-25% of annual C input to the forest floor with litter is leached from the organic surface layers. Mobilization of organic matter in the dissolved state, driven by biotic and abiotic mechanisms of SOM degradation, is the major prerequisite for mineralization of SOM to CO2.
Increased production of DOC has been demonstrated abiotically in freeze/thaw and drying/rewetting cycles (Kalbitz et al. 2000; Billett et al. 2006), both of which are of high importance in high latitudes. Nevertheless, there is little or contradictory information about these effects on DOC mobilization in permafrost soils in situ. Our observations in Central Siberia demonstrated lowest concentrations of DOC in organic soil leachates after earlier spring rainfalls, and highest DOC concentrations in subsoil. This may be caused by precipitation of DOC when concentrated by freezing, but such a process has not been investigated so far.
Temperature has a more profound effect through an increasing decomposition of SOM and thus DOC production by enhancing microbial activity (Christ and David 1996). The temperature regime in permafrost terrains drives the depth and timing of permafrost thawing (Fig. 16.2) and controls soil microbial activity. There is strong evidence that DOC production and CO2 evolution in soils are coupled, and increases with rising temperatures (Neff and Hooper 2002). Increased content of DOC (Kawahigashi et al. 2004) and doubled DOC flux (Prokushkin et al. 2005) from organic soils of warm and deeper frost in south-facing slopes as compared to north-facing slopes and water-logged valleys in Central Siberia corroborates these findings. During the frost-free period, however, our previous data showed that DOC concentrations in forest floor leachates in areas with deeper frost declined with increasing litter layer temperature in the range of 7-13°C (Prokushkin et al. 2005, 2008). In contrast, DOC production in the forest floor of cooler north-facing slopes positively correlated with increasing temperatures. In addition, decomposition/oxidation of upper soil organic carbon, leading to the production of DOC in ecosystems limited by lower temperatures and soil moisture, would be enhanced in the drier and warmer climate. In particular, in Western Siberia, which stores at least 70.2 Pg C, an increase of the mean annual air temperature to values above -2°C is expected to produce a large increase of DOC export for watersheds containing 100% peat cover (Frey and Smith 2005).
Midsummer droughts may impede microbial activity in the upper soil of deeper frost areas, thereby reducing DOC export. In particular, the decline of the native
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Fig. 16.2 Dynamics of the (1) air temperature and (2) humification horizon at the north-facing slope in 2002. 3 Dynamics of active layer thickness is shown during June-September. The white points denote the moment when the soil surface transforms into a frozen state (Prokushkin and Guggenberger 2007). DOY day of year
1 31 61 91 121 151 181 211 241 271 301 331 361 DOY
Fig. 16.2 Dynamics of the (1) air temperature and (2) humification horizon at the north-facing slope in 2002. 3 Dynamics of active layer thickness is shown during June-September. The white points denote the moment when the soil surface transforms into a frozen state (Prokushkin and Guggenberger 2007). DOY day of year microbial communities within the permafrost zone of central Siberia has already been demonstrated at temperatures above 5°C (SantruCkova et al. 2003). Thus, complex interactions between soil temperature, hydrology, and microbial activity will result in specific local responses of DOC flux in permafrost soils to changes in climate.
Precipitation constrains the yearly amount of DOC transported from the organic layers to mineral soil. Despite the decrease of DOC concentrations in solutions percolated through organic layers at higher precipitation, overall DOC flux demonstrates significant positive correlations with the amount of seepage water (Fig. 16.3). This suggests that DOC export is mainly water-limited, not C-limited. Therefore, under wetter climate conditions more DOC can be translocated into subsoil, and the retention of DOC in mineral horizons is of great importance for the fate of DOC leached from upper organic soils.
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