Dissolved organic carbon-mediated controls dominate soil carbon mineralization in response to freeze-thaw cycles

Abstract. Soil freeze-thaw cycles (FTCs) exert substantial effects on the mineralization of soil organic carbon (SOC), particularly in high-altitude and -latitude cold regions. Ongoing climate change is altering FTC frequency and duration, yet the responses of SOC mineralization to such changes remain poorly understood, limiting our ability to predict carbon cycle-climate feedbacks. Here, we incubated soils from two depths across three sites to quantify how FTC regimes regulate SOC mineralization and explore underlying controls. Across all treatments, we observed a pronounced thaw-induced pulse of CO₂ release, but more frequent freeze-thaw cycles led to more cumulative CO₂ release, given the same length of cumulative thaw days. Across treatments, mineralization was most strongly correlated with DOC and hydrolytic/oxidative enzyme activities, while being suppressed by mineralogical (free and amorphous Fe/Al oxides) and physical (aggregate-protected carbon) constraints. Partial correlations and path analyses revealed that DOC was the single most consistent predictor of mineralization, retaining its influence even when enzymatic, substrate quality, or mineralogical variables were controlled. Subsoil SOC mineralization was additionally shaped by molecular carbon composition and mineral protection. These findings reveal a vertical shift from DOC-mediated substrate accessibility to molecularly and physically constrained decomposition. Accounting for these depth-specific mechanisms will improve prediction of SOC-climate feedbacks under FTC shifts due to climate change.