Responses of physical properties of typical Mollisols to freeze–thaw cycles under simulated conditions

Freeze-thaw cycles (FTCs) extensively and intensely occur in cold regions, significantly affecting soil properties. However, quantifying the impacts of FTCs at different initial conditions on soil properties is challenging due to the complex interactive responses. In this study, porosity, bulk density, field capacity and saturated hydraulic conductivity (K_s) were measured to evaluate the responses of soil to FTCs. Eight FTCs (0, 1, 3, 5, 7, 10, 15 and 20 cycles), five initial soil mass water contents (10, 20, 30, 40 and 45 %), four eroded soils (original, degraded, deposited and parent), two initial bulk densities (1.2 and 1.3 g cm⁻³) and two freezing temperatures (−10 and −15 ℃) were employed to quantify the impacts of FTCs on physical properties. Results showed that repeated FTCs had a cumulative effect on soil physical properties, which generally entered a steady state after 10–15 FTCs. Changes in soil physical properties were highlighted after the initial 1–3 FTCs, and the effects of FTCs on physical properties diminished when the soil water content was below 20 %. Soil physical properties with different initial conditions responded differently to FTCs. Porosity increased with FTCs in increments ranging from about 0.2 to 8.7 %; however, the opposite result was observed when the initial soil water content exceeded 30 %. The bulk density decreased by 1.1–3.7 % with increasing FTCs, whereas the bulk density of the soils with high water content and severe soil erosion increased by 0.2–7.5 %. Compared to the initial state, the field capacity decreased by 0.1–15.3 % after 20 FTCs; however, the field capacity increased by 10.4 % under high bulk density. The K_s increased by 21.8–249.5 % with the increase of FTCs, whereas the K_s of the soils with 40 % and 45 % water content decreased by 28.7 % and 90.4 %, respectively. Overall, soil physical properties responded more strongly to FTCs at high water content, severe soil erosion, moderate bulk density and low freezing temperature. In comparison, the degree of soil erosion was the most critical factor influencing soil physical properties. These findings can help to improve the understanding of soil dynamic processes and provide new insight into mechanisms of erosion caused by seasonal FTCs.