Asymmetric patterns of soil carbon mineralization during thaw slump recovery: Stable fast-pool size but rebounding decomposition rate

Abstract. In areas of thermokarst landslides on the Qinghai-Tibet Plateau, the decline in soil CO₂ emissions over time is accompanied by a decrease in the proportion of active carbon and an increase in the proportion of inert carbon, suggesting that the soil organic carbon pool capacity and its mineralization rate may have changed. The lack of quantitative analysis regarding how actual active carbon pool capacity and mineralization rates regulate CO₂ emissions has led to some uncertainty in CO₂ emission estimates. In light of this, this study utilized a publicly available dataset of a 23-year recovery sequence from thermokarst landslides in the Qilian Mountains. By introducing a dual-pool index model, the cumulative CO₂ release process was decoupled into fast-pool capacity (C_0,fast) and fast-pool decomposition rate constant (k_fast), and their driving factors were identified using hierarchical variance decomposition (LMG) and partial least squares structural equation modeling (PLS-SEM). The results indicate that over the 23-year recovery period, C_0,fast did not change significantly, while the turnover rate k_fast rebounded significantly. Furthermore, C_0,fast was primarily driven by total microbial biomass (r = 0.74, p = 0.003), whereas k_fast was influenced by the chemical quality of organic carbon (r = 0.60, p = 0.023). Both LMG and PLS-SEM confirmed that the chemical quality of organic carbon is the primary driver of variation in k_fast (contributing approximately 30 %), while microbial indicators contributed less than 15 %. Furthermore, as the number of recovery years increased, δ¹³C exhibited a significant negative trend (β = −0.761, p < 0.001), and both this trend and the subsequent recovery in pH were significantly correlated with k_fast (r = −0.53 and 0.51, respectively). Soil carbon mineralization during the recovery from thermokarst landslides exhibits an asymmetric pattern characterized by “stable pool capacity and a rebound in mineralization rate.” This finding provides new kinetic evidence for assessing carbon dynamics following permafrost disturbance.