Background In order to reveal the mechanism of the influence of freeze-thaw cycles on the physical properties of soil in the spoil heap of an open-cut coal mine during the spring thawing period, this study selected the soil in different reclamation vegetation types (Ulmus pumila forestland, Rhus typhina forestland, farmland and waste grassland) and different soil depths (0−20, 20−40 and 40−60 cm) in the spoil heap of Haizhou open-cut coal mine in Liaoning as the research object.

Methods In the standard sample plot, three sampling points were selected respectively, and 80 cm soil profile was excavated at each sampling point. At a depth of 0−60 cm, samples were taken at an interval of 20 cm from the soil layer, and 3 samples were taken from each layer. The samples were frozen at −15 ℃ for 12 h and then thawed at 12 ℃. Physical water parameters such as soil bulk density, capillary porosity, non-capillary porosity and soil moisture content before and after freeze-thaw were measured by ring knife method and drying method respectively. Through significance analysis, the effects of freeze-thaw cycle on soil moisture physical properties of different reclaimed vegetation types in the dump were discussed.

Results 1) After freezing and thawing, the average soil bulk density of U. pumila forestland, R. typhina forestland, farmland and waste grassland increased by 0.07, 0.06, 0.06 , and 0.06 g/cm³, respectively. The soil bulk density of the 20−40 cm soil layer in the R. typhina forestland and the 40−60 cm soil layer in the waste grassland were significantly different from that before freezing and thawing (P < 0.05), and the other soil layers of the reclaimed vegetation types were significantly different from that before freezing and thawing (P < 0.01). 2) The average soil moisture content of U. pumila forestland, R. typhina forestland, farmland and waste grassland decreased by 1.59%, 2.40%, 1.81% and 1.74%, respectively, except that the soil moisture content of 20−40 cm soil layer of U. pumila forestland and 0−20 cm soil layer of farmland had a very significant difference compared with that before freezing and thawing (P < 0.01). The soil layers of other reclaimed vegetation types were significantly different from those before freeze-thaw (P < 0.05). 3) The average non-capillary porosity decreased by 1.44%, 1.99%, 1.54% and 1.45% in U. pumila forestland, R. typhina forestland, farmland and waste grassland, except that the non-capillary porosity of 20−40 cm soil layer in U. pumila forestland and 0−20 cm soil layer in farmland were significantly different than that before freeze-thaw (P < 0.01). The soil layers of other reclaimed vegetation types showed significant differences compared with those before freeze-thaw (P < 0.05). 4) The average capillary porosity decreased by 3.87%, 6.04%, 4.44% and 4.38% in U. pumila forestland, R. typhina forestland, farmland and waste grassland, except that the capillary porosity of 20−40 cm soil layer in U. pumila forestland and 0−20 cm soil layer in farmland was significantly different than that before freeze-thaw (P < 0.01). There were significant differences in soil depth of other reclaimed vegetation types compared with that before freeze-thaw (P < 0.05).

Conclusions Freeze-thaw effect has a certain degree of irreversible damage to soil structure. After freeze-thaw cycle, the bulk soil density of reclaimed vegetation types in 0−60 cm soil layer increases, while the water physical parameters of other soils decrease. This can provide a basis for revealing the mechanism of freeze-thaw effect on soil water physical properties.