Aspect-associated contrasts in soil thermal regimes and phase-sensitive dielectric responses during seasonal freeze–thaw periods

Seasonal freezing and thawing influence soil thermal conditions, water–ice phase partitioning, and spring liquid-water availability, but field evidence remains limited on how aspect-associated surface-energy contrasts are transmitted through semi-arid loess profiles. We monitored a south-facing slope and a north-facing slope from November 2024 to April 2025, measuring slope-parallel incoming solar radiation, snow depth, air temperature, soil temperature, and frequency-domain reflectometry (FDR)-derived phase-sensitive dielectric signals at 20, 40, 60, 80, and 100 cm. The south-facing slope received greater solar radiation, whereas the north-facing slope had greater snow depth on the December–February survey dates. These contrasting surface conditions were associated with earlier shallow thawing on the south-facing slope and longer subzero persistence on the north-facing slope. Seasonal freezing reached the 100 cm observation boundary on both slopes, so the actual maximum freezing depths could not be resolved. Exploratory analyses indicated differences in freezing start date and duration between the two monitored slopes and among depths. Depth interval had the largest effect on the initial downward propagation rate of the lower 0 °C boundary, whereas the monitored-slope effects were weaker. Air–soil temperature coupling, soil-temperature variability, and FDR-signal variability generally attenuated with depth. FDR observations were interpreted as phase-sensitive dielectric responses rather than direct measures of water storage or flux. Overall, the results distinguish an earlier and more variable shallow freeze–thaw response on the south-facing slope from a colder and more persistent profile response on the north-facing slope, providing field constraints for interpreting aspect-associated thermal and dielectric heterogeneity in seasonally frozen loess profiles.