Typical thermokarst alters soil organic carbon fractions controlled by soil water content in the Qinghai–Tibet Plateau

The ongoing climate warming has accelerated permafrost thaw worldwide, resulting in rapid thermokarst development. However, a comprehensive quantification of the responses of soil organic carbon (SOC) fractions to thermokarst development is still lacking. This study evaluated the changes in SOC fraction concentrations in typical thermokarst landscapes. The results showed that thaw slumps, thermo-erosion gullies, and mature thermokarst lakes exhibited a drastic decrease in SOC concentration, whereas the single-site observation of developing thermokarst lake exhibited an increase of 23.47%. Particulate organic carbon (POC), rather than mineral-associated organic carbon (MAOC), dominated the SOC changes. POC decreased significantly from control stage to drape (of thaw slumps) / collapsing (of thermo-erosion gullies) / mid-lakeside (of mature thermokarst lakes) stages. Mature thermokarst lakes featured the reductions in POC from control stage (57.04 g/kg) to near-lakeside stage (33.72 g/kg), with a drastic decrease occurring in the mid-lakeside stage (30.91%). The developing thermokarst lake site exhibited a continuous increase in POC concentrations. Basic soil properties and microbial properties played dominant roles in explaining SOC variance during thermokarst development. Soil water content (SWC) was the most important factor controlling SOC concentrations in all thermokarst landscapes. SOC variations of thaw slumps might be positively correlated with precipitation, whereas those of thermo-erosion gullies might be negatively correlated with elevation. Path analysis confirmed that SWC controlled the dynamics of SOC through both direct (via organic matter transport) and indirect (microbially mediated) pathways. The preliminary order-of-magnitude estimate suggests a cumulative loss in SOC stock of up to 6.30 Tg C by 2100 due to the future expansion in thermokarst landscapes under climate change. This study provided new insights into potential dynamics of permafrost‑carbon feedbacks.