Carbon stock estimation and changes associated with thermokarst activity, forest disturbance, and land use changes in Eastern Siberia

This study was conducted to evaluate the change in total soil carbon [TSC = litter carbon (LIC) + soil organic carbon (SOC) + soil carbonate carbon (SCC)] associated with recent thermokarst activity and forest disturbance induced by the climate change and land use change in the permafrost-affected soils of Eastern Siberia. TSC stock from 80 sites in three types of the ecosystem: forests, arable land, and thermokarst were analyzed. The SOC content was positively correlated with root C density in all ecosystems. In the forest and arable land ecosystems, soil texture strongly affected SOC content, but water content was the main factor in SOC accumulation in the thermokarst ecosystem. Although the composition ratio of TSC stock significantly differed among the ecosystem, SOC was the main component of the TSC in all ecosystems. SOC and TSC stocks in thermokarst ecosystem (137.1 ± 125.5 Mg SOC ha−1 30 cm−1 and 150.5 ± 123.9 Mg TSC ha−1 30 cm−1) were significantly higher than those in the forest (28.3 ± 13.5 Mg SOC ha−1 30 cm−1 and 51.4 ± 20.9 Mg TSC ha−1 30 cm−1) and arable land ecosystem (41.9 ± 16.9 Mg SOC ha−1 30 cm−1 and 54.6 ± 23.1 Mg TSC ha−1 30 cm−1), suggesting that thermokarst formation accumulates C. In the thermokarst ecosystem, significantly higher SOC stock was observed in moderately-wet and wet grassland than dry grassland suggesting that flooding increases C by suppressing organic matter decomposition. Among the forest ecosystem, forest disturbance significantly altered the C balance. Forest fire did not change the SOC and SCC stock but significantly burned the LIC stock at an average rate of 51.7 ± 65.0% of the LIC. Water damage to the forest significantly increased the SOC stock by increasing the water content, but 58.5 ± 62.4% of the LIC stock was dismissed. Forest cultivation significantly reduced TSC stock at an average rate of 37.3 ± 47.2% by mineralization of the LIC. On the other hands, abandonment of arable land significantly restored TSC stock at an average rate of 92.4 ± 83.5% by increasing C input. This study revealed that redistribution of the C stock has been occurred under the climate change and human activity.