Evaluation of the impact of freeze-thaw cycles on the soil pore structure of alpine meadows using X-ray computed tomography

Freeze-thaw cycle (FTC) is an extremely complex soil surface process in cold regions, and quantifying the changes in soil pore structure during successive FTCs remains challenging. This study investigated response of soil pore structure to FTCs and revealed the underlying influence on soil water infiltration through an FTC simulation experiment (0, 1, 3, 5, and 10 cycles) and computed tomography (CT) scanning of undisturbed soil cores in an alpine meadow. One FTC involved soil freezing at -10 degrees C for 12 h, followed by thawing at 20 degrees C for 12 h. The results showed that CT-measured soil porosity presented a variation trend of increase-decrease during successive FTCs. With increasing number of FTCs, soil porosity first increased and then decreased in the 0-to-75-mm soil layer, decreased after five FTCs in the 75-to-150-mm soil layer, and gradually decreased in the 150-to-280-mm soil layer. Pore structures of different soil layers responded differently to FTCs, which was attributed to soil texture. The connectivity of soil pores and soil water infiltration rate decreased considerably after 10 FTCs. Therefore, FTCs changed soil pore structure of the alpine meadow, which was not conducive for soil water infiltration to deep soil layers.