Warming accelerates soil organic carbon mineralization in thawed seasonally frozen ground by reshaping carbon...

Seasonally frozen ground (SFG) strongly regulates soil organic carbon (SOC) dynamics, yet the vulnerability of thawed SFG soils to warming remains unclear. We conducted a 45-day laboratory incubation using alpine meadow soils from the Qinghai-Tibet Plateau under three temperature treatments (5, 15, and 25 ℃). We quantified SOC mineralization temperature sensitivity (Q₁₀), assessed changes in carbon fractions and extracellular enzyme activities, and applied random forest (RF) analysis and partial least squares path modeling (PLS-PM) to identify key drivers and pathways regulating SOC mineralization. The Q₁₀ values of thawed SFG soils ranged from 0.53 and 4.69 (1.72 ± 0.94). Warming significantly altered the size of the potentially mineralizable carbon pool and changed the contents of dissolved organic carbon (DOC) and particulate organic carbon (POC), with DOC decreasing by 94.91-97.88%, POC declining by up to 19.37%, and SOC decreasing by approximately 48% at 25 ℃. Warming increased the activities of β-D-cellobiosidase (CBH) and phenol oxidase (POX) and reshaped carbon-enzyme relationships. It weakened the coupling between DOC and labile carbon-degrading enzymes but strengthened the negative associations of CBH with DOC, MAOC, and SOC, suggesting a shift in microbial carbon acquisition from labile substrates toward more recalcitrant particulate and mineral-associated pools. RF identified SUC, DOC, CAT, CBH, and POX as the dominant predictors of SOC mineralization rate, while PLS-PM showed that warming promoted SOC mineralization both directly and indirectly through changes in carbon fractions. These results indicate that SOC mineralization in thawed SFG soils is governed by labile carbon availability. Continued warming may accelerate the conversion of SFG from a carbon sink to a carbon source, with implications for regional climate feedbacks.