Vegetation and permafrost interactions shape soil moisture stratification in marginal permafrost zones

Permafrost degradation is reshaping soil moisture regimes in cold regions, yet the vertical structure and regulatory mechanisms of moisture remain underexplored in marginal permafrost zones. Here, we integrate 75 shallow profiles (0–3 m) and 15 boreholes (up to 20 m) across forests, grasslands, and wetlands in the Genhe Basin, Northeast China, to investigate how vegetation and permafrost jointly control volumetric water content (VWC). Vegetation—especially wetland cover—dominates the 0–0.5 m layer; a mid-depth coupling zone is evident where vegetation and permafrost jointly control VWC; and 1.5–2.0 m moisture maxima at permafrost sites reflect accumulation above the permafrost table. Machine learning and structural equation modeling reveal that surface characteristics (e.g., wetland cover, soil organic carbon) dominate shallow moisture, while bulk density and permafrost extent govern subsurface retention. Climatic effects operate indirectly through insulation and freeze–thaw buffering. These findings demonstrate a vertically stratified control system shaped by vegetation–permafrost interactions, underscoring the need to incorporate vertical coupling into process-based models.