Organic matter decomposition and stabilization in Siberian tundra soils affected by thermokarst processes

Thawing of permafrost like the wide spread  ice-rich Yedoma deposits in northern Siberia release large quantities of organic matter that may be decomposed to the greenhouse gases (GHG) CO₂ and CH₄. Since Yedoma deposits store up to 130 Pg of organic carbon (OC), the release of GHG from these thawing deposits might be of global relevance. The degradability of released organic matter is unclear. Current estimates on how fast the organic matter from thawing Yedoma may be transferred into CO₂ range between 66% in one summer thaw season and 15% in 100 years. To reduce uncertainties about the degradability of Yedoma organic matter and to quantify the carbon pool that rapidly may be released a CO₂, we incubated samples from different thermokarst affected soils and fractionated the organic matter by density fractionation. One set of soils originated from a vegetated thermokarst depression, the second set from a retrogressive thaw slump without vegetation. The total release of CO₂ after 500 days at 4°C was significantly higher from soils of the vegetated thermokarst depression (4.0 ± 4.1% of OC) than from the retrogressive thaw slump (2.1 ± 0.9 % of OC), likely due to the input of fresh organic matter by the vegetation. Most of the organic carbon was bound to the mineral fraction (45 ± 24%), while the free particulate organic matter (fPOM) and the occluded organic matter (oPOM) contributed almost equally (26.8 ± 20.9% and 27.8 ± 12.0% of OC, respectively). The amount of carbon in the mineral fraction did not correlate with the CO₂ formation, indicating stabilization of organic matter. Surprisingly, the oPOM fraction was stronger correlated with released CO₂ than the fPOM fraction. However, the strongest correlation was found between CO₂ production and the C/N ratio of total OC.